Clay minerals in sediments of Portuguese reservoirs and their ...

Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531-009-0488-3

ORIGINAL PAPER

Clay minerals in sediments of Portuguese reservoirs and their significance as weathering products from over-eroded soils: a comparative study of the Maranhaõ, Monte Novo and Divor Reservoirs (South Portugal) Rita M. F. Fonseca • Fernando J. A. S. Barriga Patrıćia I. S. T. Conceiçaõ

•

Received: 18 August 2008 / Accepted: 10 October 2009 Ó Springer-Verlag 2009

Abstract The Southern region of Portugal is subjected to several forms of over-erosion. Most leached products, mainly composed of fine particles containing nutrients, metals or pesticides, are easily transported by river flows. When these are hindered by a physical barrier such as a dam, the particulate load accumulates on the bottom of the reservoirs, often leading to a pronounced decrease of water quality. Bottom sediments from three reservoirs were subjected to grain-size analysis and a study of clay minerals by X-ray diffraction. Most sediments contain a diverse set of clay minerals, mostly illites, smectites, chlorites and kaolinites. The nature of the clay minerals reflects the nature of the parent rocks. During the cycles of transport and temporary deposition, they may undergo significant chemical and physical transformations, which lead to an increase of expandable properties and therefore, to a higher cationic exchange capacity, determining its important role as vehicles of environmental pollutants. Keywords Clay minerals Dam reservoirs Over-erosion Sediments South Portugal

R. M. F. Fonseca (&) Department of Geosciences, University of E´vora, 7002-554 E´vora, Portugal e-mail: [email protected] R. M. F. Fonseca F. J. A. S. Barriga P. I. S. T. Conceiçaõ Creminer LA/ISR, Faculty of Sciences, University of Lisbon, Campo Grande, 1749-016 Lisbon, Portugal F. J. A. S. Barriga Department of Geology and Creminer LA/ISR, Faculty of Sciences, University of Lisbon, Campo Grande, 1749-016 Lisbon, Portugal

Introduction Continental aquatic environments are often dominated by fine-grained sediments, which have an important role due to the properties that their fine grain-size and chemistry impart to them (Hillier 1995). In an artificial lake such as a dam reservoir, most fine-grained particles are detrital, often recycled from one environment to another (Eberl 1984) and they are mainly supplied from weathered rocks and from soils in their drainage basins. These fine particles, mainly composed of clay minerals, represent the more susceptible units to weathering and they are easily washed away from soils and transported by river flows as suspended loads, eventually trapped and deposited in dam reservoirs. As a result of chemical and physical processes (e.g. ion exchange and fixation) that they may undergo during cycles of transport and temporary deposition, clay minerals may act as important vehicles and buffers of environmental pollutants, which tend to be strongly sorbed to clay minerals surfaces (Fo¨rstner et al. 1990). Sediment accumulation decreases the value of reservoirs themselves, as the water storage capacity decreases over time and the quality of water progressively decreases due to the continuous chemical and biological exchanges between bottom sediments and the water column. The finer particles are particularly easily washed out from soils used for agriculture, as a consequence of practises such as ploughing and prolonged land exposure because of plant cover removal. We call over-erosion to this human-generated excess production of eroded soil components, mainly related to agriculture. Fine particles contain the nutrition elements much needed for sustainable organic productivity. As a consequence of over-erosion, soils become deficient in a host of components, often evolving into arid soils.

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Int J Earth Sci (Geol Rundsch)

The southern part of Portugal, with its Mediterranean climate, is largely dependent of surface water storage. New dams have been constructed. Most leached products from over-eroded soils are transported by river flows and accumulate on the bottom of reservoirs. Considering (1) the accelerated over-erosion of fine-grained particles from soils and (2) the accumulation of these particles on the bottom of artificial lakes, where they generate a number of environmental problems, the purpose of the present study is to identify and characterize clays in bottom sediments from reservoirs of the Alto Alentejo region of South Portugal, in order to assess their significance as transporters of nutrients and pollutants. The Maranhaõ, Monte Novo and Divor reservoirs were selected. They are representative of distinct hydrological basins and very different geomorphologies and geological characteristics, age and uses. Additionally, we evaluate the significance of the studied sediments as weathering products from the drainage basins by relating the spatial and temporal distributions with the parent rocks of the source areas.

Geological setting of the reservoirs Regarding the paleogeographic and tectonic features, the drainage basins of the three studied reservoirs belong to the Ossa Morena Zone, one of the main sub-divisions of the Variscan Fold Belt in Iberia (Fig. 1). One of the most important aspects concerning the study of these reservoirs is the large variety of rocks in the catchment areas, which enhance a remarkable array of mineralogical, textural and geochemical characteristics of the accumulated sediments. This geological diversity is higher in the Maranhaõ dam, the oldest and largest surface water reserve of Alto Alentejo. Its drainage basin contains a Cenozoic sedimentary cover over Paleozoic and Precambrian formations of the Variscan Fold Belt (Fig. 2). This sedimentary cover is mainly detrital (Oliveira et al. 1992), consisting of gravels, pebbles, sandstones, conglomerates, clayey sandstones, shales and marls. The partly exposed Paleozoic and Precambrian basement includes a large diversity of metasediments (shales and pelitic schists, greywackes, quartzites, conglomerates and carbonate rocks), metavolcanic sequences ranging in composition from acid to basic rocks and an intrusive massif (granitic rocks with different geochemical features and mafic and ultramafic intrusive bodies). The drainage basins of Monte Novo and Divor, with smaller dimensions, have less geological diversity (Fig. 3), explaining the higher homogeneity regarding the texture, mineralogy and chemistry of sediments (Fonseca et al. 1993, 1998, 2003; Fonseca 2002). The two basins are contiguous and the geological setting nearly the same,

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Fig. 1 Location of the drainage basins of Maranhaõ, Monte Novo and Divor reservoirs in the tectono-stratigraphic units of the Ossa Morena Zone (OMZ) in Portugal (adapted from Arau´jo 1995)

Fig. 2 Simplified setting geology of the drainage basin of the Maranhaõ reservoir

Monte Novo displaying the largest variety. The lithologies that feed both reservoirs are mainly composed of (a) a volcano-sedimentary complex including schists ranging from sericitic–chloritic schists to quartz–feldspar-rich


problems, such as blue-algae blooms, leading to eutrophization and interdiction for domestic use. In the three reservoirs, the climate is classified as Mediterranean (Csa in Koppen classification), characterized by hot, dry summers and cool, wet winters. This region receives almost all of its yearly rainfall during the winter season, especially between October and March, and may have no significant precipitation between June and September. As a consequence, most sedimentation takes place from December to March and settling of the finer-grained particles occurs during the summer, when hydrodynamics are less intense.

Materials and analytical methods Fig. 3 Geological setting of the drainage basins of the Monte Novo and Divor reservoirs

schists, associated to basic and acid volcanites, (b) intrusive acid rocks (tonalitic, granitic and granodioritic rocks) and (c) scarce zones of Miocene cover (shales, conglomerates and carbonate rocks; Oliveira et al. 1992). Although the geological setting is identical, there occur significant differences concerning the formations that host the reservoirs: Monte Novo is set on sericitic–chloritic schists whereas Divor is hosted on mafic volcanites.

Characterization of the reservoirs The studied reservoirs belong to two distinct hydrological systems: Tagus River (Maranhaõ and Divor) and Guadiana River (Monte Novo). In spite of their geographical proximity and the nearly identical geological, topographical and climatic conditions, the reservoirs have distinct hydrological and morphometric characteristics (Table 1). Maranhaõ is an old dam, largely filled with sediment and is the largest surface water reserve of Alto Alentejo. Monte Novo is more recent and supplies water to E´vora, the largest city in the Alentejo region. However, there exist water quality problems, due to the excess of N and P, which has originated toxic compounds, e.g. trihalomethanes. Divor, the smallest of the studied reservoirs, was until a few years ago a supplier of water for domestic use to E´vora. However, the progressive nutrients enrichment in the water column and the related anoxic conditions encompassed a number of

The sediments were mapped in the reservoirs from a regular sampling net (60 points in Maranhaõ, 13 points in Monte Novo, 7 points in Divor), and they were collected with a Shipeck dredge and a modified Van Veen dredge on the most representative periods in the annual cycle (February and September), in order to define seasonal physical or mineralogical modifications, under distinct temperature and rainfall conditions. Grain-size analysis Grain-size classes (Wentworth Lane scale; Pettijohn 1975) were separated by wet sieving (gravel-sand-silt clay), dry sieving (grain-size sand distribution) and measurement with a laser sedimentometer (clay and silt distribution). Textural classification of sediments was based on the representation of clay, silt and sand proportions in a Shepard (1954) triangular diagram (Pettijohn 1975). Sediments were mapped according to grain-size distribution. Mineralogical analysis Clay minerals of sediments were examined on the clay fraction (\2 lm) by X-ray diffraction (XRD) using a Philips PW 1710 diffractometer with automatic slit and Cu–Ka radiation at 40–50 kV, 30–40 mA. The XRD patterns were collected in the 2–50°2h range and counts were recorded at 0.02°2h intervals. The clay fraction (\2 lm) was extracted by normal centrifugation techniques after removal of the organic

Table 1 Morphometric and hydrological data of Maranhaõ, Monte Novo and Divor reservoirs Reservoirs

Year

River

Basin

Surface area (km2)

Volume (m3)

Maximum depth (m)

Medium depth (m)

Main use

Maranhaõ

1957

Sorraia

Tagus

19.6

205 9 106

55

16

Irrigation

Monte Novo

1982

Degebe

Guadiana

2.8

15 9 106

30

5.5

Domestic

Divor

1965

Divor

Tagus

2.65

11.9 9 106

23

4.5

Irrigation

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Int J Earth Sci (Geol Rundsch) Fig. 4 Sampling sites and textural facies map of bottom sediments from the Maranhaõ, Monte Novo and Divor reservoirs (in February): 1 silty clay/clayey silt, 2 silt, 3 sand– silt-clay, 4 sandy silt, 5 silty sand, 6 sand, 7 coarse sandgravel/silty coarse sand and 8 silty gravel/gravel

matter with hydrogen peroxide. Separate aliquots were saturated with Mg, K and Li by methods adapted from Thorez (1976), Ransom et al. (1988) and Moore and Reynolds (1997) and oriented aggregates were produced by pipetting a clay suspension on glass slides to eliminate mineral segregation. The unsaturated and Mg–K-saturated samples were air-dried, saturated with ethylene glycol (EG) and heated to 550°C. The Li-saturated sample was submitted to the Green-Kelly (1955) test-heating to 300°C during 12 h followed by EG treatment. Clay minerals interpretation and semi-quantification were based on methods developed by Brindley and Brown (1980), Brindley (1981), Velde (1995), Moore and Reynolds (1997) and S. Hillier (1997, personal communication) and described by Fonseca et al. (2007). The procedures consisted of (1) determination of the precise position and intensity of the individual reflections and broad diffraction bands in °2h and (2) direct comparison of peaks position and intensity in patterns obtained from distinct treatments (air-dried, EG-solvation, heat treatment, Mg–K–Li-saturation). Semi-quantitative analysis was achieved by the estimation of the area of reference peaks in EG-solvated and Mg-saturated patterns (as proposed by Moore and Reynolds 1997). The relative proportions of each mineral phase, normalized to 100%, were estimated through the integrated intensity of its reflections. This parameter was calculated by dividing each peak area

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(product of the peak height times peak width at half-height) by intensity calibration factors extracted from Moore and Reynolds (1997) and adapted from S. Hillier (1997, personal communication). The mineral intensity factors (MIF) were based on the unit intensity for the I003 reflection selected as the base of normalization: S001 = 5,2; I001 = 1,3; I003 = 1,0; Ch001 = 0,96, Ch002 = 4,6; Ch004 = 4,6; K001 = 3,18; K002 = 2,18. Beyond the detailed XRD study of clay minerals, subsequent complementary infrared (FTIR) studies were also performed in KBr discs. Clay minerals interpretation was based on methods described by Russel and Fraser (1994) and Fraser (1997, personal communication).

Results and discussion Texture of sediments In all studied reservoirs, especially Maranhaõ, grain-size analysis (Fig. 4) suggests a remarkable sedimentary diversity, owing to (1) the large geological diversity of the drainage basins and (2) the spatial and seasonal fluctuations of the hydraulic flow, which produces distinct energetic conditions inside the lakes. The sedimentary distribution in the bottom coincides with the major contribution of fine material, as observed in other similar systems (Sly 1978;


Fig. 5 Variation of the proportions of sediment textural classes in the summer and winter periods, in the Monte Novo and Divor reservoirs. Maranhaõ has an identical seasonal variation when compared with Divor

Fig. 6 Average cumulative weight percentage curves of the sandy fraction of sediments from Maranhaõ, Monte Novo and Divor reservoirs and cumulative curves of the sandy sediments collected in

the winter campaign of Monte Novo reservoir. The black solid line represents the Normal distribution of Gauss

Keulder 1982; Duck 1986). Most sediments fall in the silty clay and clayey silt textural classes and are mostly deposited in the old watercourse-bed along the reservoirs which correspond to the larger depths. Average grain-size increases to (1) the marginal areas, where deposition of the coarser fraction predominantly takes place, mainly due to the active erosion processes occurring in the margins, and to (2) the entrance of waterways, due to the competency lost of the agents of transport. The wide-ranging sediments are composed of a mixture of several grain-size sub-populations which, according to McManus (1988), define distinct sources or distinct hydrodynamic environments (Figs. 5, 6). Occurrence of silts or clays in sandy and gravel deposits, observed in the uppermost sectors of Maranhaõ and Monte Novo, for example, suggests the development of deposits under more than one hydraulic regime. The variation of the source of clastic material transported to the reservoirs and of the energetic conditions inside the lakes and in the streams that feed them can be observed in the graphs of Fig. 6. These

graphs represent the average cumulative weight percentage curves of the sandy fraction of sediments from the overall systems and the cumulative curves of sandy sediments of Monte Novo, depicted as an example. The shape, slope and distance of these curves in relation to the Gaussian normal distribution suggest bi- and trimodal patterns in the generality of the studied sediments, low degree of selection of grain-size populations and introduction of coarser elements in relation to the populations of the modal class (negative asymmetry). Beside the spatial and seasonal fluctuations of the hydraulic flow inside the lakes, the grain-size distributions are clear evidence of the importance of the nature and distribution of parent rocks in the drainage basins. The variation of dimensional classes of the particles deposited in Maranhaõ denotes the existence of lithologies with coarser textures, as source of the material accumulated in the reservoir. In fact, the drainage basin of Maranhaõ is widely represented by igneous acid rocks and by a mainly detrital sedimentary cover, which, through weathering,

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produce mainly coarse-grained material. The uppermost sector of this reservoir exhibits the most clayey sediments, reflecting the fine grain-size of the setting geology, mainly composed of shales, pelitic schists, greywackes, lidites and amphibolites. Monte Novo and Divor depict a higher uniformity of lithofacies distribution and a higher contribution of finer particles with respect to Maranhaõ, reflecting the fine-grained characteristics of the geological formations that host the reservoirs, sericitic–chloritic schists in Monte Novo and mafic metavolcanites in Divor. Furthermore, in a region where climatic conditions are much contrasted throughout the annual cycle, the variations of the hydraulic flow led to distinct inputs of clastic sediments and enhanced the mixing, calibration and stirring of granulometric populations. The seasonal energetic conditions easily explain the textural differences observed in sediments collected in distinct periods, in each sampling site (Fig. 5). During the summer, the low hydrodynamic conditions and the chemical variance of the water column (generally associated to increased salts concentrations) greatly decrease the input of larger particles and generate the flocculation and precipitation of suspended clay particles. Thus, in the Maranhaõ and Divor reservoirs, between February and September, the distribution of sediments in the bottom is more homogeneous and there is a noteworthy decrease of the particles grain-size. A similar seasonal variation is not observed in Monte Novo; the low local internal hydrodynamism is probably unable to transport coarser particles and mix granulometric populations, inducing a higher textural homogeneity. These features are a likely explanation for the slight decrease of the clay fraction in the summer period and the slight increase of coarser particles in winter only along the main watercourse of Monte Novo (Fonseca 2002).

Also, following the same authors, the asymmetry and irrational spacings and breadth of most reflections denote that clay minerals often consist of complex juxtapositions of two or more individual species, corresponding largely to mixed-layered clay minerals. According to Righi and Meunier (1995), these interstratified structures often coincide with the finest fraction of sediments, and they usually correspond to mineral phases which may be formed or modified inside the reservoirs. As usual in sedimentary rocks and soils formed under stable conditions (Moore and Reynolds 1997; Righi et al. 1997), the most abundant mixed-layered structure found in reservoir sediments is illite–smectite (I–S) with various proportions of smectitic and illitic layers, followed by chlorite–smectite (Ch–S) and minor amounts of illite–chlorite (I–Ch). The XRD pattern of one sample from Monte Novo (Fig. 7) is representative of the changes produced by unsaturated sample treatment procedures (air-dried, solvated with EG) and heated to 550°C): (1) air-dried samples ˚ , followed by less produce a well-defined peak at 14–15 A ˚ , (2) in EG-solvated intense peaks at 10 and 7–7.2 A ˚ peak separates preparations, the superimposed 14–15 A ˚ ˚ . The into two, one at 17 A and another at the original 14 A ˚ reflections at 10 and 7–7.2 A are kept; (3) heating at 550°C

Identification and characterization of the clay minerals The clay minerals of most sediments studied exhibit strong first-order reflexions, often with asymmetrical shapes and a few associations of weak and broad diffraction bands or shoulders. Frequently, the same set of (00l) peaks shows different breadths (width of the diffraction peak at half its height above background) and irrational spacings. According to the qualitative identification procedures proposed by Brindley and Brown (1980), Brindley (1981), Velde (1995), Moore and Reynolds (1997) and S. Hillier (1997, personal communication) and described in Fonseca et al. (2007), the behaviour of minerals reflections under natural conditions (air-dried, ethylene glycolation and heating to 550°C) and Mg-, K- and Li-saturation, define a highly diverse set of clay minerals represented by illite, smectite, chlorite, kaolinite, and, in a few samples of Maranhaõ reservoir, by small amounts of vermiculite.

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Fig. 7 Diffraction patterns of one representative sample from Monte Novo reservoir, under air-dried (N), ethylene glycol-solvated (EG) and heated conditions (550°). Numbers on the top of each peak ˚ represents d-spacings in A


collapses all the referred peaks and largely increases the ˚. intensity of a peak near 10 A The Mg-, K- and Li-saturation of samples enhanced the distinction between smectite and other minerals having similar d-spacing, such as vermiculite and chlorite: upon glycolation of Mg-saturated samples, smectite expands to ˚ , while vermiculite and chlorite stay at 14 A ˚ and 17–18 A upon K-saturation at air-dried conditions or heating at ˚ , producing a diffraction 500°C, smectite collapses to 10 A pattern similar to that of illite, in contrast with chlorite, ˚. which stays at 14 A Comparing the three reservoirs, the clay minerals generally have basal reflections with similar basal spacing, differing only in the intensity of peaks and, in a few cases, in their breadth and shape. These data indicate high mineralogical homogeneity of the clay fraction of sediments, regardless of the provenance reservoir and the sampling season. The variability nevertheless present is minor, restricted to relative abundances, dimension of particles, crystallinity index and rare variations in the nature of the cations present.

3.

Smectite characterization Beyond the distinction of smectites from other clay minerals having identical d spacings, the Mg-, K-, and Li-saturation of samples are the bases of the definition of several chemical and structural characteristics. The occurrence of this mineral was also proved through the presence of two broad reflections of low intensity, (002) and (003), not always visible. S(002) appears as a small and wide peak or as a broad diffraction band at the base of the ˚ . The S(003) Ch(002) ? K(001) reflection, near 8,46 A reflection, most of the times of difficult distinction, turns up individualized as a weakened and broadened peak at ˚. 5,66 A 1.

2.

Ca is the main interlayer cation; this is denoted by the d-spacing values in air-dried and glycolated samples ˚ of S(001) upon Mgand by the shifting to 10 A saturation. This reflection shift means that as samples are saturated with MgCl2, Ca2? adsorbed or fixed in smectitic layers is total or partially replaced by Mg2?, with an ionic radius lower than that of Ca2?, decreasing the interlayer space of successive layers. Beyond the identification of high-charge smectites in a large number of samples, low-charge smectites or Fe– smectites were also identified, mostly in sediments collected in the summer. The contraction of the firstorder reflection after K-saturation in air-dried condi˚ , allows the tions, giving a broad band near 10–11 A identification of high-charge smectites (originated from the transformation of other minerals, namely

4.

illite). Upon the same treatments, the occurrence of a ˚ denotes the peak or broad reflection at 12–12.5 A presence of low-charge smectites or Fe–smectites (authigenic, formed from the weathering of augite, hornblende or feldspars; see Thorez 1975; Moore and Reynolds 1997). The proportions of both species differ among the three reservoirs and, in each reservoir, from one sampling site to another, denoting its provenance from different parent rocks. Montmorillonite is the most common variety, usually associated with nontronite, the most Fe-rich species. Montmorillonite was identified through the irreversible ˚ after Li-saturation, followed by collapse to 10 A heating to 300°C and EG-solvation (Green-Kelly test 1953); the appearance of a sharp, well-defined 17.5– ˚ (001) reflection upon glycolation on Mg17.7 A saturated samples indicates the presence of nontronite. The presence of Fe in a few smectites is also confirmed by infrared data. According to Russel and Fraser (1994), with increasing isomorphous substitution of Al3? by Fe3? in smectite, the AlFe3?OH deformation band shifts from 890 to 870 cm-1. The presence of a weak inflection at 876 cm-1 in the spectrum of Fig. 8 (Monte Novo sample) denotes the slightly ferric composition of a few smectitic minerals. However, the typical OH-stretching band at 3,622 cm-1 indicates the predominance of montmorillonite over other smectitic minerals. Smectite is often interstratified with illitic layers. An evaluation and characterization of the ‘‘crystallinity’’ of this mineral were attained through the Biscaye (1965) index and the ‘‘crystallinity’’ classes defined by Thorez (1976). Most smectites correspond to mediumor poorly crystallized minerals, in accordance with the enlarged shape of S001 reflection and the absence or

Fig. 8 Infrared spectrum of the clay fraction of a sediment from the Monte Novo reservoir

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occurrence of S002 as a shoulder or a poorly shaped reflection. Illite characterization The identification of the pure illitic phase was based on the position and shape of the reflections, taking as start point the slight asymmetry of the sharp tailing I(001) towards the low angle side of diffractograms and the absence of any profile modification upon glycolation or cations saturation, as suggested by Brindley and Brown (1980), Brindley (1981), Brattli (1997) and Moore and Reynolds (1997).The relative intensities of the basal (00l) series were used to recognize the chemical composition of the octahedral sheet as well as the occurrence of expandable layers: 1.

2.

A wide variation of the 002/001 intensity ratio suggests a variable chemical composition of the octahedral sheet, ranging from Al3?-dioctahedral illites (derived from muscovite transformation) to Fe3?–Al3? or Fe3?–trioctahedral illites (derived from biotite transformations). As found in the majority of soils, most illites fit in the aluminous dioctahedral group; Values upper than 1 for peak-height-intensity ratio (Ir) given by: Ir = [I001/I003]air-dried/[I001/I003]glycolated, followed by the development of asymmetry of both flanks of the (001) reflection, indicate the regular presence of expandable layers. Most mixed-layers built on illite are of smectitic nature, given illite–smectite (I–S).

Kaolinite characterization Minerals identified as kaolinites generally show various degrees of disorder and unless we can identify the polytype, we should use the name kaolin or kaolin-group mineral (Moore and Reynolds 1997). However, the observation of some samples by infrared spectrometry defined kaolinite as the major species, given the similarity of bands, number of peaks, frequency and position in the absorption spectra, in relation to the spectra defined by Russel and Fraser (1994). Kaolinite-group minerals were studied not only by FTIR techniques but also by XRD analysis. The mixture with chlorite, with its series of basal diffraction peaks superposed (or nearly so) on kaolinites, rendered more difficult the distinction between kaolinites and chlorites. A few methods described by Fonseca et al. (2007) and based on the suggestions of Brattli (1997), Moore and Reynolds (1997), Righi et al. (1997) and S. Hillier (1997, personal communication) were used to differentiate kaolinite from chlorite. As the asymmetry and intensity of the (00l) reflections decrease with the degree

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of disorder (Thorez 1976), through the height above background and the position, shape and breadth of the superposed K001 and K002 reflections and the individual K003, it was possible to determine the structural order– disorder of this mineral group. Based on these general principles, three discrete species were identified: (1) wellcrystallized kaolinite, (2) medium-crystallized kaolinite and (3) disordered kaolinite. Chlorite characterization The high geological diversity of the catchment areas, in addition to the wide possible range of ionic substitutions and the easy alteration by weathering provide, in all the reservoirs, a large variety of chlorites relative to their chemical and structural characteristics. The relative intensities of the chlorite (00l) series used for determining the total heavy metal content, and the distribution between the silicate and hydroxide octahedral sites, showed a wide variation on the Fe content and on the distribution of Fe with respect to Mg in the two layers (symmetry) (see Fig. 9; Hillier et al. 1996; Moore and Reynolds 1997; Bauluz et al. 1998). The occurrence of asymmetrical reflections and the slight change of position after heating to 550°C, EG-solvating or Mg- and K-saturating, denote the existence of random mixed-layers with expandable properties. Identification and characterization of mixed-layered clay minerals Mixed-layered species were identified in XRD patterns obtained from Mg-saturated, K-saturated and EG-solvated samples, based on the occurrence of a broad band or a ˚ . The asymmetrical, tailing reflection between 10 and 14 A poorly defined and enlarged peaks showing increased background towards one or the two sides of the main reflections, the irrational spacing and its position in the diffraction patterns, denote the lack of alternation between layers. These characteristics are representative of randomly interstratified structures. Following the methods described by Thorez (1976), Moore and Reynolds (1997), S´rodon´ (1984, 1999) and ˚ Brattli (1997), (1) the enlargement of the 5 and 10 A reflections reported to illite, (2) the asymmetry on the ˚, lower angle side developed on the flank of the I001 at 10 A showing an increase of the background, (3) the exhibition of some ‘‘swelling’’ upon glycolation and (4) the presence of a weak, broadened peak in an intermediate position ˚ and S003 at 5.54 A ˚ in glycolated difbetween I002 at 5 A fraction patterns proves the widespread occurrence of illite–smectite. In the sediments of all the reservoirs, this is the most abundant mixed-layered structure and shows

Int J Earth Sci (Geol Rundsch) Fig. 9 Relative intensities of the chlorite (00l) series in representative sediments from the three studied reservoirs, denoting different heavy metal contents and different distribution of Fe and Mg into the mineral structure: a Ch001 [ Ch003, enrichment of Fe in the octahedral layer and Ch002 \ Ch004, sediments from Maranhaõ; b Ch001 & Ch003 symmetrical chlorites as a function of Fe content and Ch002 \ Ch004, sediments from Monte Novo; c weak odd-order peaks and Ch002 [ Ch004–Ferich chlorites, sediments from Divor. All the patterns were obtained upon glycolation. Numbers on the top of each ˚ peak represents d-spacings in A

various proportions of illitic and smectitic layers. The positions of relevant peaks indicate the presence of two predominant transitional phases, as mixtures comprising near 90 and 50–60% of illite, respectively. Both phases often appear together in the same sample, with the (I–S 0.9/ 0.1) form always predominant. However, the occurrence of (1) some shoulders, displacements or slope change of the chlorite (001) slightly towards the low angle side of the diffractograms upon glycolation, (2) their shift to about ˚ by heating and (3) the increase of width and 12.3–13.5 A irregularity of Ch003 in glycolated diffraction patterns denotes the presence of chlorite–smectite with low contents of swelling layers. Though subordinate in abundance, we also have identified illite interstratified with chloritic layers ˚ or an (I–Ch), as evidenced by a weak reflection at 12 A asymmetry developed on the low angle side flank of the illite (001), upon air-died treatment in natural or Mg-saturated samples, which remain stable after glycolation or heating. Distribution of clay minerals in the sediments of the reservoirs The mineralogical composition of sediments is quite similar in the three cases and except for vermiculite, all the clay minerals are present, with variation of relative abundances only. The major clay component of sediments is illite, followed by smectite in Maranhaõ and by chlorite in Monte Novo and Divor (see Figs. 10, 12 and 14). Though mixed-layer clays were not quantified due to the difficulty on differentiating the related reflections from the discrete species, Maranhaõ shows the higher levels and higher

diversity of these structures, especially in the rainy season, when hydrodynamic activity is higher. In Divor, these structures are scarce. The large variety of clay minerals found in the sediments of these reservoirs is favoured by the remarkable variation of the lithology of the catchment areas, associated to (1) the Mediterranean climate of Southern Portugal which determines modest rock weathering, (2) various alteration states of the weathering products and (3) the mineralogical mixture resulting from transport and sedimentation. Irrespective of the reservoirs, clay minerals of sediments are in accordance with the weathering mineralogy of the altered rocks and soils of their drainage basins, studied by Abreu (1986), Vieira e Silva (1990) and Fonseca (2000) and discussed by Fonseca et al. (2007). Illite, the dominant component in all the studied sediments, is one of the most common alteration products of acid intrusive rocks, quartz schists, gneisses, marbles and detrital sedimentary cover, which represent prevailing lithologies in the drainage basin of the three reservoirs. The high contents of illite in reservoir sediments, compared to those of (proximal) weathering products of parent rocks and soils, could also outcome from aggradative transformations involving fixation of K? within the structure of some clay minerals (as discussed by Fonseca et al. 2007). The prevalence of smectite over chlorite in the Maranhaõ reservoir discloses the presence of mafic and ultramafic metavolcanites and intrusive rocks, shales and pelitic schists in the drainage area which, according to the same authors, produce mainly smectite and vermiculite by weathering. The absence or near absence of vermiculite in the studied sediments could

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Int J Earth Sci (Geol Rundsch) Fig. 10 Representation of the relative contents of the clay mineral groups occurring in the sediments of the Maranhaõ reservoir in two distinct sampling periods: February (winter) and September (summer)

be a consequence of transformation mechanisms leading to the acquisition of expandable layers which can occur, either in the soils or during the cycles of transport and deposition within the reservoir. Chlorite, the second most represented clay mineral in the sediments of Monte Novo and Divor, is the main alteration product from the mineral constituents of sericitic and chloritic schists and mafic volcanites, widely represented lithologies in the drainage basins of both systems. Each reservoir shows spatial and seasonal fluctuations of the distribution of levels and chemical and structural characteristics of each mineral group. Such variations could be explained by the following: (1) each mineral group could outcome from different lithological sources, which have a dissimilar distribution in the setting areas, (2) the parent rocks have diverse mineralogical compositions, which can generate distinct clay mineral assemblages, (3) minerals from the parent rocks or soils could undergo distinct alteration mechanisms and these weathered products, mainly composed of clays, could be diffused transported in suspension into the reservoirs (Bengtsson and Stevens 1998), (4) the spatial and seasonal fluctuations of the hydraulic flow produce distinct energetic conditions inside the lakes and (5) when each mineral species transported as suspended load reaches static water, it has a specific settling rate. These spatial and seasonal variations are clearly higher in chlorites, probably because chlorites undertake complex ionic substitutions in the tetrahedral or in the octahedral sheets (Thorez 1976; Hillier 1995), becoming more susceptible to any chemical modification of the water column. The spatial and seasonal variations of the clay minerals indicate that kaolinite and illite show less chemical variation among the reservoirs, in agreement with their higher chemical stability (Galań et al. 1999).

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Clay minerals in the Maranhaõ reservoir Illite and smectite are the main clay minerals in the Maranhaõ reservoir (see Fig. 10). The local distribution of minerals is not homogeneous, and there is only limited relation with the proximity of sources in the drainage basin or with the sedimentary texture. This heterogeneity is probably due to the internal hydraulic flow which easily transfers the finer particles to distinct areas of the reservoir. Actually, the mineralogical heterogeneity increases in winter, when hydrodynamic conditions inward are higher. However, the mineralogical variation observed between the summer and winter seasons correlates with the texture of the sediments: in the summer, when the deposition of finegrained particles preferentially takes place, the finer-particle mineral groups (smectites and chlorites) increase in abundance, whereas in the period of higher input of clastic material (winter), illites and kaolinites, which form larger particles, show a considerable augment. In this period, the mixed-layered structures and the degree of interstratification also increase. Illite Illite shows higher abundances not only in the period corresponding to a larger input and deposition of clastic sediments but also in areas under the influence of carbonate and acid and basic intrusive rocks which have, as major alteration products, illite and illite–smectite (Vieira e Silva 1990; Fonseca 2000). The studied illites show chemical compositions ranging from aluminous dioctahedral illites to ferrous trioctahedral illites, denoting the high geological diversity of the drainage basin. In winter, the prevalence of Fe–illites derived from phengites (I002/I001 = 0.34–0.38), in opposition to the


increase of illites derived from muscovites in summer (I002/ I001 = 0.41–0.91), denotes the enrichment in Fe concurrently to the larger contribution of clastic material. Two classes of illites have been identified, given the shape, thickness and position of the basal reflection I001 which is related to the crystallinity and the amount of K? fixed in the interlayer space: (1) illites with narrow, intense and symmetrical reflections with identical breadth and spacings, indicating a high crystallization degree—this group is better represented in summer samples and (2) illites with asymmetrical shape and enlarged foot, indicating a low crystallinity and random interstratification. This group is widely represented in winter samples. The displacements and change in position, intensity and asymmetry, exhibited by reflections upon glycolation, are indicative of mixed layers built on smectite, ranging from 0.6I–0.4S to 0.9I– 0.1S (the most frequent). In winter, beyond the larger number of samples with illite interlayered with smectite, the relative contents of these structures increase, as well as the proportions of the expandable layers (Fig. 11). These structural modifications over the annual cycle probably result from stripping of the interlayer K?, giving ‘‘open illites’’, in the period of higher stream load, followed by a period of higher stability after its deposition on the bottom of the reservoir, characterized by capture into the structure of K ions present in the aqueous environment. Smectite Highly represented in this reservoir, smectitic minerals are invariantly characterized by a prominent and symmetrical basal (001) reflection, and the higher-order reflections S002 ˚ and S003 at about 5.4 A ˚ are always detectable, at about 8 A

though relatively weak. Smectite distribution along the reservoir shows a slightly increase upstream, denoting the lithological nature of this sector, represented by granodiorites and gabbros, the latter having smectite associated to vermiculite–Fe and vermiculite–Al as main weathering products (Abreu 1986). The remarkable increase of smectite in summer samples can result from input of this mineral (or a precursor, like vermiculite) into the reservoir largely in the period of greater sediment input (winter); once in the water column, owing its very small particle size, it could be held in suspension until the summer, with its weaker hydrodynamic conditions, more adequate for deposition in the bottom. Like in the other studied reservoirs, Ca–smectite predominates and high-charge and low-charge smectites are associated. The latter, with a most ferrous composition (according to the higher Fe content inferred from the decrease of the intensity ratio of S002 over S003), has a larger occurrence in sediments collected in summer. In agreement with these observations, the position and intensity of the (001) basal reflection in patterns obtained from Mg-saturation ? glycolation and K-saturation in airdried conditions, and the Green-Kelly (1955) test yield the occurrence of montmorillonite, the most common variety, usually accompanied by nontronite. Both species have various proportions in any sampling period, with a slight increase of the most Fe-rich species (nontronite) in summer samples. Comparing the sediments overall, the smectites collected in winter show a lower crystallinity degree, as a consequence of the higher interstratification with illitic layers. Relative to the ‘‘crystallinity’’ classes defined by Thorez (1976), the shape and intensity of smectite reflections allow their classification as well- to medium-crystallized minerals. The occurrence of smectite having various chemical characteristics and random interstratification degrees involving illitic layers indicate distinct sources for this mineral group, as well as extensive chemical variations in the environment. Chlorite

Fig. 11 Seasonal variation of the diffraction patterns of illite and chlorite in a sediment from Maranhaõ. In the period of higher stream load (February) the higher asymmetry of peaks is noteworthy, indicative of a higher interstratification with expandable layers. In this sample and during this period, illite does not have individual reflections, denoting its nonexistence as a discrete mineral

Among all the clay minerals, chlorite, as a group, has the most homogeneous distribution in the reservoir. Chlorites are roughly as abundant as kaolinite in Maranhaõ. In both summer and winter samples, higher levels are found along river streams with higher hydraulic flow, which have, as major rock sources, shales, black shales and lidites. Studies performed in this region by Abreu (1986) defined chlorite and swelling chlorite as the main weathering products of these rocks. Chlorites may also result from the early stages of the weathering of biotite, a very common mineral in the drainage basin of the reservoir.

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Int J Earth Sci (Geol Rundsch) Fig. 12 Representation of the relative contents of the clay mineral groups occurring in the sediments of the Monte Novo reservoir in two distinct sampling periods: February (winter) and September (summer)

The relative intensities of chlorite series XRD peaks disclose the presence of a wide structural and chemical range of minerals, rather heterogeneously distributed along the reservoir, denoting again the high geological diversity of the drainage basin and the effect of distinct rock sources. From February to September (period of preferential selective deposition of fine-grained particles) beyond a general increase of chlorite contents, Fe concentration was enhanced (increase of even-order reflections relative to the odd-orders). Especially in winter, chlorites have a slightly expandable composition (10% of smectite layers or less) (see Fig. 11) and, in a few samples, they show a displacement of the high-order reflections in glycolated diffraction pat˚ upon K-satuterns and absence of the reflection at 14 A ration. These features mean that all chlorite is interstratified with smectitic layers. Its location in areas directly fed by shales agrees with the nature of products resulting from the weathering of shales, mainly composed by swelling chlorites (Abreu 1986; Vieira e Silva 1990). According to the proportion of interstratified layers, most chlorites are medium-crystallized. Kaolinite Kaolinite distribution in the reservoir is relatively uniform, showing higher values in areas under the influence of intrusive acid and basic rocks and carbonate rocks, which have kaolinite as a major weathering product, indicating a largely detrital origin for the reservoir kaolinite. The symmetrical shape of the individual reflection K003 at ˚ , associated to a low background in the diffracto2.39 A gram, characterized medium- to well-crystallized kaolinites. The widespread overprinting of nearly all the diffraction peaks of kaolinites by reflections belonging to

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other minerals did not allow study of some chemical and structural aspects, including the possible presence of interstratified layers. Vermiculite ˚ reflections in glycolated Comparative analysis of the 14 A and heated diffractograms evidenced the presence of vermiculite in almost all the sediments collected in winter. Its abundances, generally low, range from vestiges to 3–3.5%, with the higher values generally in winter samples. The scarcity of vermiculite in the sediments compared with its relative abundance in the weathering products of the parent rocks and soils (according to studies developed by Abreu 1986, Vieira e Silva 1990 and Fonseca 2000) suggests that in the last stages of weathering-transport-deposition, vermiculite could be replaced by more evolved clays, such as smectites. This could explain the high levels of smectite found in reservoir sediments. Clay minerals in the Monte Novo reservoir Illite prevails over the remaining clay minerals. Chlorite and kaolinite follow, in amounts comparable to each other (both high when compared with Maranhaõ and Divor). Mineral’s distribution (Fig. 12), though more homogeneous than in the Maranhaõ reservoir, does not show significant relation with proximity of sources in the drainage basin. This higher homogeneity may reflect limited geological diversity of the setting area, mainly represented by sericitic–chloritic shales, and the low energy of the hydraulic flow within the area, insufficient to the displacement of coarser particles and to the mixing of granulometric populations inside. Owing to the specific hydrodynamic conditions, which enhance the clay fraction


during the period of higher stream load (winter), the seasonal distribution patterns of the clay minerals are distinct from the other studied reservoirs: (1) chlorite and kaolinite show near constant contents in the annual cycle, (2) minerals characterized by small-sized, light particles, such as smectites, augment in winter, while minerals such as illites, made of larger particles, decrease. Illite The seasonal distribution suggests that the higher input of illite in the reservoir results from a wide period of mineral transport, ranging from winter to summer, due to a rainy spring. Transfer into the reservoir was mostly made through the streams having a stronger volume and higher velocity of flowing waters, where the higher illite contents ([40%) were detected. These river tributaries flow over granitic and acid metavolcanic rocks, in areas with steeper slopes, thus more susceptible to erosion processes. According to Vieira e Silva (1990), in this region, the rocks contain illite and illite–smectite as main weathering products from primary minerals such as muscovite, biotite or sericite. Reservoir illites may be largely detrital. The relative intensities of I002 and I001 reflections suggest that nearly all reservoir illites are aluminous dioctahedral illites derived from muscovites, denoting again the higher geological homogeneity of the catchment area. This geological homogeneity, coupled with the weak hydrodynamic conditions and the low sediment load inward, explains the invariance of the chemical composition of illites throughout the annual cycle. As observed in Maranhaõ, the shape and position of the I001 reflection show the presence of two types of illites as follows: (1) illites with narrow, intense and symmetrical reflections with identical breadth and spacings, indicating a high degree of crystallization (about 60% of samples) and (2) illites with asymmetrical shape and enlarged foot, indicating low crystallinity indices and random interstratification, mostly involving expandable layers (about 40% of samples). Also, considering the corresponding reflections in the diffractograms, the different width measured at half of the peak height above the background and the slight modification of the width and position upon glycolation support the occurrence of illite ? illite/smectite. This association and the presence of various proportions of smectitic layers built on illite underline the variety of the source material in the drainage area, responsible for illitic minerals with diverse structural characteristics. The number of samples having interstratified illites and the interstratification degree (Ir \ 1.4) are much lower than in Maranhaõ, with the mixed structures corresponding mostly to 0.9I–0.1S. The majority of illites exhibit seasonal

variations, especially an increase of expandable layers in winter. Smectite In Monte Novo, smectite distribution is more uniform relative to the other clay minerals and to the other reservoirs. In winter, the period of preferential input and deposition of this mineral, the higher levels are found in the main watercourse into the reservoir which has, as major setting lithologies, sericitic–chloritic schists and basic metavolcanic rocks. Previous studies conducted in this region (Vieira e Silva 1990; Fonseca 2000) concluded that those rocks have, as weathering products, smectite and mostly, vermiculite–Al and vermiculite–Fe. As vermiculites are undetected in the sediments, it is likely that they evolve towards higher expansibility minerals during the transport and deposition into the reservoir. Ca is the predominant cation in the interlayer space and the ratio S002/S003 increases in summer samples indicating a higher Fe content, as observed in Maranhaõ. The seasonal oscillation of Fe contents, common in all of the studied reservoirs, may suggest that after input and deposition of smectite or during its upholding in suspension in the water column, there occurs a structural modification followed by Fe enrichment in the octahedral sheet. Fe could exist in solution (in sediment interstitial fluids) or adsorbed on mineral and organic particles (in the water column). The comparison of the position and intensity of the S001 reflection in patterns obtained upon Mg-, K- and Li-saturation (Fig. 13) disclose the association of a Fe-rich species, nontronite, and a much common variety having a more aluminous composition—montmorillonite. Although the Fe content of smectites exhibited considerable variations throughout the year, as a consequence of possible structural modifications, nontronite and montmorillonite could have equal or distinct sources. They could both derive from the weathering of sericitic–chloritic schists and basic metavolcanic rocks, well represented in the drainage area; alternatively, the Fe-rich nontronite could be inherited from the Fe–Mg minerals, such as olivines and pyroxenes, while the Al-rich montmorillonite could outcome from structural and chemical modifications of other aluminosilicate minerals, such as plagioclases (according to Velde 1992; Moore and Reynolds 1997). Almost all smectitic minerals exhibit enlarged but symmetrical and well-defined S001 reflections. The higher order reflections are seldom detectable and, when they are, they appear as broad peaks or shoulders on the other basal reflections. The comparison of these characteristics with the ‘‘crystallinity’’ classes defined by Thorez (1976) assesses the majority of smectites as medium-crystallized minerals. As observed in Maranhaõ, the number of samples

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Fig. 13 Distinction of some smectitic species in representative sediments from Monte Novo and Divor, resulting from a Mgsaturation ? glycolation and b Li-saturation ? 250°C ? glycolation ˚ (Greene-Kelly test, 1955). In a the prominent reflection at about 17 A

results from the occurrence of montmorillonite and/or nontronite. In b ˚ while montmorillonite (associated to illite) collapses to 10 A ˚ . Numbers on the top of each peak nontronite remains at 17 A ˚ represents d-spacings in A

having a higher crystallinity degree increases in the period of preferential input of these minerals (February in Monte Novo; September in Maranhaõ), which may reflect a slight degradation of its structure following deposition. This structural degradation comes from a random interstratification involving mainly illitic layers.

reflections, and the set of (00l) peaks do not have the same breadths or rational spacing. However, reflections do shift their positions relative to the air-dried condition. This discloses the presence of both discrete chlorite and slightly expandable chlorite, with up to 10% smectite layers, chlorites–smectites (0.9Ch–0.1S).

Chlorite

Kaolinite

Chlorite has the second higher representation in reservoir sediments, as a consequence of the local geological setting, dominated by sericitic–chloritic schists, in which chlorite is the main weathering product (Vieira e Silva 1990). Thus, chlorite distribution in the reservoir is very uniform. The relative intensities of its reflections indicate that, relatively to the Fe content, chlorites from Monte Novo fall between those from Maranhaõ and Divor. The Ch004/Ch002 ratio is lower in February samples, increasing in September samples, which means that, in contrast to Maranhaõ, Monte Novo chlorites are more Fe-rich in winter samples. According to Velde (1992) and Hillier (1995) once deposited, chlorites may undergo structural modifications, namely a Fe3? reduction into Fe2?. This chemical form, more soluble, easily exchanges with interstitial fluids. This mechanism can explain the seasonal evolution of Fe in Maranhaõ chlorites (lower Fe in the period of lower chlorite input). In Monte Novo, Fe behaviour is more obscure, given the nearly constant abundance of chlorite throughout the annual. Chlorites show slight changes upon glycolation expressed by increase of asymmetry, irregularity and width of

This mineral has a heterogeneous distribution along the reservoir and its abundance increases slightly in February. The crystallinity degree, yielded by the shape of the individual K003 reflection and of the background, suggests that about 50–60% of the samples contain medium-crystallized kaolinites. In the remaining samples, kaolinites have sharper, symmetrical and stronger K003 reflections and a lower background, indicating a higher structural ordering. The number of samples having well-crystallized kaolinites is higher in February. This suggests that after its deposition on the bottom of the reservoir, kaolinites can lose some components to the water column.

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Clay minerals in the Divor reservoir Not surprisingly, given the predominantly tonalitic and granitic nature of the rocks in the drainage basin, Divor has the highest contents of illite and the lowest contents of smectite among the studied reservoirs (Fig. 14). Except for chlorite, the distribution of clay minerals is more homogeneous in winter samples, as indicated by the standard deviation values (Illite: September 8.3, February 4.9;

Int J Earth Sci (Geol Rundsch) Fig. 14 Representation of the relative contents of the clay mineral groups occurring in the sediments of the Divor reservoir in two distinct sampling periods: February (winter) and September (summer)

Smectite: September 4.1, February 1.9; Chlorite: September 3.2, February 3.5; Kaolinite. September 5.8; February 4.2) Grain-size distribution, as mentioned above, clearly evidences the similarity of sedimentary processes between the Divor and Maranhaõ reservoirs, in spite of the much lower hydrodynamic conditions of Divor, as a consequence of the lower depth and the lower dimension of the drainage area. Such processes may include (1) spatial and seasonal heterogeneities of the sediments distribution, (2) occurrence of variable hydraulic regimes during sedimentation, (3) higher input of clastic sediments in winter and (4) increase of the clay fraction in summer as the energetic conditions of the environment decrease. These sedimentary conditions are responsible for the remarkable illite gains (one of the coarser-grained clay minerals) in the season of higher detrital input (winter). The minor hydraulic oscillations throughout the annual cycle can perhaps explain the minor variations only of the remaining minerals (\3%) in both sampling periods, although with a slight increase of chlorite and kaolinite in summer samples. Comparison of Divor with Maranhaõ and Monte Novo leads to the following conclusions (see Fig. 15): (1) all mineral groups show higher Fe abundances, as a consequence of the predominantly basic (basic metavolcanics/ amphibolites) nature of host rocks underneath the reservoir and (2) all mineral groups show fewer interstratification with expandable layers. Illite The widespread occurrence of acid and intermediate volcanic rocks in the source area explains the high concentration of illite ([50%) in the generality of sediments, which agrees, as already mentioned, with the nature of the regional weathering mineralogy. The correlation of higher

Fig. 15 Comparison of the XRD patterns of the clay minerals from Divor (a), Monte Novo (b) and Maranhaõ (c) reservoirs. In Divor the absence of expandable layers built on illite is evident and the high-Fe composition of the generality of minerals (absence of S002 ? high ratio I001/I002 ? weakening of the chlorite odd-order peaks relative to the even-order peaks with intensity of Ch002 [ Ch004. Numbers on the ˚ top of each peak represents d-spacings in A

amounts of illite with areas of the reservoir directly feed by granites indicates this lithology as the major source of illites.

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The I002/I001 ratio, always lower than 1/3, denotes the occurrence of Fe–Mg-rich trioctahedral illites. In the majority of samples the ratio is as low as 1/6. This is taken as an indication of illite origin from micaceous minerals such as biotite and secondarily from muscovite, as shown by the sediments located in areas under the influence of tonalitic rocks. Iron contents in illites are very similar in both sampling periods, again indicating that these minerals are mainly inherited from weathered acid intrusive rocks with biotitic composition, with large representation in the catchment area. The relationship between the Fe–Mg contents of illite and the mineralogical composition of the drainage basin lithology is very clear in this reservoir, as the minerals showing higher I002/I001 ratios, thus more Al-rich, correspond to sediments located in areas directly influenced by granitic rocks which have biotite and also significant amounts of muscovite. Unlike in the other studied reservoirs, illite does not show significant interstratification with expandable layers, as indicated by the narrow and symmetrical reflections and the peak-height-intensity ratio (Ir) near 1. This mineral is mixed-layered in a few samples only (with low proportions of chlorite). Smectite The weathering mineralogy of the rocks with higher influence on the sedimentation input (the metavolcanites where the reservoir is setting in) is dominated by vermiculite, Fe–smectite, expandable chlorite and kaolinite (e.g. Velde 1992; Righi and Meunier 1995). We can attribute the main provenance of smectite in reservoir sediments to these lithologies and explain their variations in abundances from the higher or lower input of sedimentary load. Although having a roughly uniform distribution throughout the annual cycle, during the period of higher clay fraction deposition (winter), smectite deposition was also prevalent in the west sector, where most of the basic rocks outcrop. Ca is once again the predominant cation in the interlayer space but, in contrast with the observations in previously described reservoirs, the lower-order reflections, S002 and S003, are not detected, due to (1) the very low contents in sediments and (2) the Fe-rich nature, which decreases the intensity of the even-order reflections. This ferrous composition is also detected by the simultaneous occurrence of montmorilonite and nontronite (see Fig. 13) Also, according to the position and intensity of reflections upon Ksaturation, only low-charge smectites have been identified, which are mainly authigenic and derived from the weathering of Fe–Mg-rich minerals such as augite and horneblende (Thorez 1975). Concerning the shape of reflections, most sediments exhibit weak and broader or irregular, large ˚ under asymmetrical S001. The diffuse tail towards 10 A

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air-dried conditions denotes the occurrence of regular interstratification with illitic layers. The weakened and irregular shape of S001 peak and the lack of S002, characterize poorly crystallized smectites (class C after Thorez 1975). Chlorite Chlorite distribution in the reservoir is more uniform in the summer (typical standard deviation values near 3.2) and the gradual decrease of its contents from West to East reveal the preferential supply of chlorite, together with smectite, in the West sector of the catchment area. As already mentioned, this sector exhibits the larger extension of basic metavolcanic rocks associated to shales, which have both chlorite and smectite as major weathering products. The significant weakening of the odd-order reflections when compared with the even-order Ch002 and Ch004, denotes the presence of iron-rich chlorites. Another peculiar characteristic of the sediments from Divor is the wide diversity of chlorites, related to the presence or lack of interstratification and to the types and proportions of layers involved. In view of the geology of the drainage basin, this structural diversity is not certainly related with differences in source rock lithologies, as these are all basic volcanic rocks. However, the chlorite diversity could denote the occurrence of several and complex alteration processes in the basic rocks. It should also be emphasized that chlorites exhibit a wide range of possible isomorphic substitutions and are easily affected by weathering (Thorez 1976; Hillier 1995). On the basis of the intensity and shape of Ch001, the individualization of higher-order reflections and modifications of peaks upon glycolation, three distinct groups of chlorite have been identified, showing spatial and seasonal heterogeneities: (1) normal chlorite, (2) swelling chlorite and (3) interstratified (Ch–I). Swelling chlorites are indicative of the presence of smectitic interstratified layers, not exceeding 10% (0.9Ch– 0.1S). As observed in the other reservoirs, there is a remarkable increase of samples having expandable layers during the period of higher detrital contribution (winter); Interstratified chlorites with illitic layers (Ch–I) occur predominantly in the summer. Kaolinite Kaolinite is mainly inherited from weathering of the most representative lithologies of the drainage basin, granitic and tonalitic rocks, which occupy large areas, South of the strip of basic metavolcanites and also in the North sector of the drainage basin. This inheritance could explain the slight heterogeneities in the distribution along the reservoir, including a slightly more significant increase towards the dam wall, where the influence of the material transported


by a watercourse running over granites is more pronounced. The shape of the individual K003 reflections characterizes the bulk of the kaolinites as mediumcrystallized.

Conclusions Given the intimate linkage between the particles deposited in dam reservoirs and the over-erosion of soils from their drainage basins, this study aimed at identifying and characterizing the spatial and seasonal distributions of the most important fine particles of sediments, the clay minerals, in three reservoirs from Alto Alentejo, an important agricultural region in South Portugal. The selected systems, Maranhaõ, Monte Novo and Divor, belong to distinct hydrological basins and have remarkable geomorphological and geological differences. However, the mineralogical composition of sediments is quite similar in the three cases, with variation of relative abundances only, consisting of (1) potassium illites, with variable Al3?, Fe3? or Al3?/Fe3? in the octahedral sheet, (2) dioctahedral Ca–smectites, nontronite (a Fe3?-rich montmorillonite-like mineral, often associated with other Fe3?-rich minerals), (3) trioctahedral chlorites with various amounts of Fe and Mg, (4) medium to highly disordered kaolinites and (5) randomly interstratified structures (chlorite–smectite, illite–smectite, illite– chlorite). This complex clay assemblage is due to the Mediterranean climate of Southern Portugal (responsible for relatively weak rock weathering) and to the following additional factors: (1) remarkable variation of the lithologies in the drainage basins, (2) various transformation states of the weathering products and (3) mineralogical mixture after transport and sedimentation. In all three reservoirs, each mineral group exhibits spatial and seasonal fluctuations, reflecting the distribution of different sources in the setting area and to differences of the hydraulic flow, which produce distinct energetic conditions inside the lakes. The diverse and complex steps of the ‘‘clay cycle’’ (Hillier 1995) from its formation to its deposition in the bottom of the reservoirs render difficult a precise reconstruction of the sedimentary processes involved in their genesis. In the studied reservoirs, as in other sedimentation basins, although the clay minerals result from a variety of factors, they have probably a dominantly detrital origin, as clays are the component more susceptible to weathering both in the parent rocks and in soils overlying them. The nature of the clay minerals reflects the nature of the parent rocks which, according to Jones and Bowser (1978), is the most common situation in climates characterized by alteration processes not intense enough to mask the source composition effect. Nevertheless, previous comparisons (Fonseca et al. 2003, 2007) between the mineralogy of the

clay fraction of parent soils and of sediments deposited in these three reservoirs showed significant differences concerning the nature, chemical and structural characteristics of the clay minerals. Such differences, mainly involving an increase of illite, smectite and mixed-layered structures, could outcome from mineralogical transformations undertaken during the complex weathering-transport-deposition cycles. Furthermore, since dams represent barriers to the natural sediments transport cycle, the trapped clay minerals can also undergo structural and chemical modifications by exchanging elements with the water column. The eroded clay-sized minerals from the parent rocks, subject to the mentioned transformations inherent to the cycles of transport and temporary deposition, provide, in the reservoirs sediments, an array of clay minerals having more expandable and permanent high-charge structures. These characteristics, more pronounced in the sediments of the Maranhaõ reservoir, determine high cationic adsorption and exchange capacities of the clay fraction of sediments. Thus, nutrients, metals, pesticides and other kinds of pollutant elements, easily washed away from over-eroded soils and transported by river flows, are easily held by clays, which act as important transporters of environmental pollutants. In minerals having expansive structures, they can be strongly sorbed to surfaces, but the frequent occurrence of randomly interstratified structures and medium-disordered crystallization degrees favour the slow release of components from relatively loose crystal structures, leading to a marked decrease of the water quality. The study of clays in the sediments can thus be used not only to assess their significance and possible use as suppliers of agricultural nutrients but also to help predicting the characteristics of reservoir water. Acknowledgments This research was co-financed by FEDER (EU) and FCT (Portugal) through project SABRE (POCI/CTE-GEX/59277/ 2004). We would like to express our appreciation to Prof. F. Rocha and to Dr. Thomas Go¨tte for their helpful revision and comments on an earlier version of the manuscript.

References Abreu MM (1986) Aspectos do Comportamento do Ferro na Crusta de Meteorizaçaõ—Alto e Baixo Alentejo. PhD Thesis, Univ. Technique of Lisbon, Portugal, 250 pp (in Portuguese) Arau´jo A (1995) Estrutura de uma geotransversal entre Brinches e Mouraõ (Zona de Ossa Morena). Implicaço˜es na evoluçaõ geodinaˆmica da margem Sudoeste do Terreno Autoćtone Ibe´rico. PhD thesis, University of E´vora, Portugal, 200 pp (in Portuguese) Bauluz B, Fernańdez-Nieto C, Gonzale´z Lo´pez JM (1998) Diagenesis-very low grade metamorphism of clastic Cambrian and Ordovician sedimentary rocks in the Iberian Range (Spain). Clay Minerals 33:373–394 Bengtsson H, Stevens RL (1998) Source and grain-size influences upon the clay mineral distribution in the Skagerrak and northern Kattegat. Clay Miner 33:3–13

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Int J Earth Sci (Geol Rundsch) Biscaye PE (1965) Mineralogy and sedimentation of recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans. Geol Soc Am Bull 76:803–832 Brattli B (1997) A rectorite–pyrophyllite–chlorite–illite assemblage in pelitic rocks from Colombia. Clay Miner 32:425–434 Brindley GW (1981) X-ray identification (with ancillary techniques) of clay minerals. In: Longstaffe FJ (ed) Short course in clays and the resource geologist. Mineralogical Association of Canada, Canada, pp 22–35 Brindley GW, Brown G (1980) X-ray diffraction procedures for clay mineral identification. In: Brindley GW, Brown G (eds) Crystal structures of clay minerals and their x-ray identification, monograph 5. Mineralogical Society, London, pp 305–359 Duck RW (1986) Bottom sediments of Loch Tummel, Scotland. Sed Geol 47:293–315 Eberl DD (1984) Clay mineral formation and transformation in rocks and soils. Philos Trans Roy Soc Lond A 311:241–257 Fonseca M (2000) Solos Argiluviados pouco insaturados. Caracterizaçaõ fı´sico-quı´mica e mineralo´gica de pe´dons tı´picos de solos Mediterraˆneos Pardos de materiais naõ calca´rios. PhD Thesis, Centro de Estudos de Pedologia, Portugal, 181 pp (in Portuguese) Fonseca R (2002) As albufeiras como estaço˜es de traˆnsito na sedimentaçaõ. Estudo geolo´gico sobre a re-utilizaçaõ de sedimentos de sistemas portugueses e brasileiros. Unpublished. PhD Thesis, Univ. E´vora, Portugal, 649 p (in Portuguese with English abstract) Fonseca R, Barriga FJAS, Fyfe W (1993) Suitability for agricultural use of sediments from the Maranhaõ reservoir. In: Fragoso MAC, Van Beusichem ML (eds) Optimization and plant nutrition. Kluwer Academic Publishers, Netherland, pp 665–671 Fonseca R, Barriga FJAS, Fyfe WS (1998) Reversing desertification by using dam reservoir sediments as agriculture soils. Episodes 21:218–224 Fonseca R, Barriga FJAS, Fyfe W (2003) Dam reservoir sediments as fertilizers and artificial soils. Case studies from Portugal and Brazil. In: Tazaki K (ed) Proceedings, water and soil environments, biological and geological perspectives. International symposium of the Kanazawa University 21st century COE Program. Kanazawa, Japan, pp 55–62 Fonseca R, Barriga FJAS, Tazaki K (2007) Land erosion and associated evolution of clay minerals assemblages from soils to artificial lakes in two distinct climate regimes in Portugal and Brazil. Clay Miner 42:161–179 Fo¨rstner U, Ahlf W, Calmano W, Kersten M (1990) Sediment criteria development. Contributions for environmental geochemistry to water quality management. In: Heling D, Rothe P, Fo¨rstner U, Stoffers P (eds) Sediments and environmental geochemistry: selected aspects and case histories. Springer, Berlin, pp 311–338 Galań E, Carretero MI, Fernandez-Caliani JC (1999) Effects of acid mine drainage on clay minerals suspended in the Rio Tinto River (Rio Tinto, Spain). An experimental approach. Clay Miner 34(1):99–108 Green-Kelly R (1955) Dehydration of montmorillonite minerals. Miner Mag 30:604–615 Hillier S (1995) Erosion, sedimentation and sedimentary origin of clays. In: Velde B (ed) Origin and mineralogy of clays. Clays and the environment. Springer, Berlin, pp 162–219

123

Hillier S, Son BK, Velde B (1996) Effects of hydrothermal activity on clay mineral diagenesis in Miocene shales and sandsones from the Ulleung (Tsushima) Back-Ark basin, East Sea (Sea of Japan), Korea. Clay Miner 31:113–126 Jones BF, Bowser CJ (1978) The mineralogy and related chemistry of lake sediments. In: Lerman A (ed) Lakes: chemistry, geology, physics. Springer, Berlin, pp 179–235 Keulder PC (1982) Particle size distribution and chemical parameters of the sediments of a shallow turbid impoundment. Dr W. Junk Publishers, The Hague. Hydrobiologia 91:341–353 McManus J (1988) Grain size determination and interpretation. In: Tucker M (ed) Techniques in sedimentology. Blackwell Scientific Publications, Oxford, pp 63–85 Moore DM, Reynolds RC (1997) X-Ray diffraction and the identification and analysis of clay minerals. Oxford University Press, Oxford Oliveira JT, Pereira E, Piçarra JM, Young T, Romano M (1992) O Paleozoíco Inferior de Portugal: sıńtese da estratigrafia e da evoluçaõ paleogeogra´fica. In: Gutie´rrez-Marco JC, Saavedra J, Ra´bano I (eds) Paleozoíco Inferior de Ibero-Ame´rica. Universidad de Extremadura, pp 359–375 Pettijohn EJ (1975) Sedimentary rocks, 3rd edn. Harper International Edition. Harper & Row Publishers, New York Ransom MD, Bigham JM, Smeck NE, Jaynes WF (1988) Transitional vermiculite-smectite phases in Aqualfs of South-Western Ohio. Soil Sci Soc Am J 52:873–880 Righi D, Meunier A (1995) Origin of clays by rock weathering and soil formation. In: Velde B (ed) Origin and mineralogy of clays. Clays and the environment. Springer, Berlin, pp 43–161 Righi D, Raïsa¨nen L, Gillot F (1997) Clay mineral transformations in podzolized tills in central Finland. Clay Miner 32:531–544 Russel JD, Fraser A (1994) Infrared methods. In: Wilson MJ (ed) Clay mineralogy. Spectroscopic and chemical determinative methods, 1st edn. Chapman & Hall, London, pp 11–67 Shepard FP (1954) Nomenclature based on sand-silt-clay ratios. J Sed Petrol 24:151–158 Sly PG (1978) Sedimentary processes in lakes. In: Lerman A (ed) Lakes, chemistry, geology, physics. Springer, New York, pp 179–229 S´rodon´ J (1984) Precise identification of illite/smectite interstratifications by X-ray powder diffraction. Clays Clay Miner 28:401– 411 S´rodon´ J (1999) Use of clay minerals in reconstructing geological processes: recent advances and some perspectives. Clay Miner 34:27–38 Thorez J (1975) Phyllosilicates and clay minerals. In: Lelotte G (ed) A laboratory handbook for their X-ray difraction analysis, Dison, Belgium, 579 pp Thorez J (1976) Pratical identification of clay minerals. In: Lelotte G (ed) A handbook for teachers and students in clay mineralogy, Dison, Belgium, 70 pp Velde B (1992) Introduction to clay minerals. Chapman Hall, London Velde B (1995) Composition and mineralogy of clay minerals. In: Velde B (ed) Origin and mineralogy of clays. Clays and the environment. Springer, Berlin, pp 8–42 Vieira e Silva JMA (1990) Mecanismos de Alteraçaõ em Rochas Eruptivas do Baixo Alentejo. PhD Thesis, University of Lisbon, Portugal, 240 pp (in Portuguese)