Journal of Hydrology 273 (2003) 51–68 www.elsevier.com/locate/jhydrol
Soil and karst aquifer hydrological controls on the geochemical evolution of speleothem-forming drip waters, Crag Cave, southwest Ireland Anna F. Tootha,*,1, Ian J. Fairchildb,2 a
Institut fu¨r Geologie und Pala¨ontologie, Universita¨t Innsbruck, Innrain 52, 6020 Innsbruck, Austria b School of Earth Sciences and Geography, Keele University, Keele, Staffordshire ST5 5BG, UK Received 14 November 2001; revised 22 October 2002; accepted 25 October 2002
Abstract In recent years there has been increased interest in cave speleothems as archives of palaeoclimate. Monitoring of rainfall and soil and karst water chemistries was performed at Crag Cave, Castleisland, Co. Kerry, southwest Ireland, in August 1997 and January 1998 in order to understand temporal and spatial variations in karst water hydrology and chemistry and their implications for interpreting the potential palaeohydrological signal preserved by speleothems at this site. Temporal variations in karst water drip rates and geochemistry allow drips to be classified by hydrological response to rainfall and the associated processes of dilution, piston flow, source change and prior calcite precipitation during aquifer throughflow. Evolution from soil matrix and preferential flow solutions has also been determined to exert an important control on karst water chemistries. As a result of these findings we present hydrogeochemical models and plumbing diagrams that delineate the controls on karst water evolution at a number of sampling locations within the cave at this site. We propose that a palaeohydrological signal may be recorded by Crag Cave speleothems that may be interpreted via the study of Mg/Ca ratios in speleothems linked to monitoring of modern drip water chemistry. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Karst water; Hydrology; Geochemistry; Speleothems; Palaeoclimate
1. Introduction Existing literature on limestone hydrology and hydrogeochemistry focuses largely on the behaviour * Corresponding author. Address: The Environment Agency, Guildbourne House, Chatsworth Rd, Worthing, West Sussex BN11 1LD, UK. Fax: þ44-1903-832312. E-mail addresses:
[email protected] (A.F. Tooth), i.j.
[email protected] (I.J. Fairchild). 1 Formerly at School of Earth Sciences and Geography, Keele University, Keele, Staffordshire ST5 5BG, UK. 2 Fax: þ44-1782-715261.
of entire aquifers and of stream outputs (White, 1988; Plagnes and Bakalowicz, 2001) and pollutant transport (Vaute et al., 1997), whilst modeling considers chiefly the development of karst aquifers and flow rate calculations (Dreybrodt, 1988). Recently, there has been a new focus on the hydrology of individual karstic drips stimulated by palaeoclimatic studies on the speleothems (stalactites, soda straws, stalagmites) that they feed. Speleothems may record a high resolution record of palaeoclimate due to the occurrence of annual banding generated by the inclusion of impurities
0022-1694/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 1 6 9 4 ( 0 2 ) 0 0 3 4 9 - 9
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(Baker et al., 1993; Shopov et al., 1994), or thickening of the walls of soda straw stalactites (Huang et al., 2001), implying a link to seasonal climate fluctuations. Variations in speleothem trace element chemistry may reflect palaeohydrology (Fairchild et al., 2000, 2001). Previous work on karst hydrology provides a clearer understanding of the controls on drip discharge. Williams (1983) investigated the role of the subcutaneous zone and determined its importance in terms of storage and maintenance of baseflow. Smart and Friederich (1986) produced a flow classification scheme based on variations in rate and maximum discharge, from which they developed basic hydrological models of changes in flow rate with recharge in the unsaturated zone. Bottrell and Atkinson (1992) performed dye-tracing experiments to determine aquifer flow routes and the degree of flow dispersion, deducing links between residence time and the degree of interconnectivity of the flow path. Destombes et al. (1997) and Genty and Deflandre (1998) showed that variations in drip rate may be governed by changes in atmospheric pressure in addition to rainfall input. In the study of variations in drip rate with antecedent rainfall conditions Baker et al. (1997) determined that the hydrological behaviour at individual drip sites varies according to the overall volume of water in the aquifer. However, such advances in drip hydrology need to be supplemented with hydrogeochemical studies, requiring basic observations to produce qualitative models of aquifer behaviour. Theoretical models and some field observations suggest that Mg/Ca and Sr/Ca ratios can reflect palaeohydrology (Fairchild et al., 2000). In order to fully understand hydrogeochemical variations in karstic drip waters it is necessary to take a holistic approach to the karst system encompassing atmospheric, soil and aquifer zones. Although workers have previously studied variations in karst water chemistry with surface vegetation and soil cover (Smart et al., 1986), hydrogeochemistry needs to be interpreted in the light of all controlling factors on solution chemistry. Temperature is an important control due to its effect upon microbial CO 2 production in the soil zone, which promotes the evolution of aggressive waters that drive carbonate
mineral dissolution. However, hydrology exerts a dominant control on water chemistry in terms of both geometry and residence time, both of which determine the degree and characteristics of water/sediment and water/rock interaction. For example, the concentration of solutes in soil solutions is related to whether transport through the profile occurs via preferential or matrix flow routes (Trudgill et al., 1983a,b). Similarly, in the aquifer zone, the control of carbonate lithology on karst water Mg/Ca ratios via dissolution, selective leaching and precipitation is overprinted by temporal variations induced by differing residence and throughflow times (Fairchild et al., 1996). Therefore, field observations need to be made on rainfall input, soil and karst water chemistry and karst water drip rate. The goal of this study is to determine the hydrological and geochemical response to recharge at karst water drip sites in order to develop hydrogeochemical models that illustrate variations in chemistry and discharge with varying rainfall input. This information can then be used in the interpretation of the palaeohydrological signal preserved in speleothems at the study site.
2. Methods The near-surface Crag Cave site was selected in order to minimise complexities arising from mixing of waters within the karst aquifer, with data being collected in both summer and winter to facilitate an assessment of the seasonal influence upon hydrological behaviour. Monitoring was performed on a daily time-scale in August 1997 and January 1998 for periods of 20 and 8 days, respectively. This strategy was employed in order to perform a more rigorous investigation of hydrological responses to recharge. Such a high-resolution monitoring approach has been performed before (Destombes et al., 1997; Genty and Deflandre, 1998; Borsato, 1997) but has involved an assessment of hydrology alone, whilst workers supplementing findings with chemical analyses have employed lower resolution sampling strategies (Baker et al., 1997; Fairchild et al., 2000; Smart et al., 1986). In addition to subsurface monitoring, rainfall volumes were recorded in order to determine variations in surface water flux to the aquifer, whilst matrix soil
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water chemistry was ascertained in order to investigate the importance of matrix versus preferential flow in both soil and aquifer zones. Incident rainfall was collected in plastic bucket samplers whilst soil water was extracted via the employment of a Soil Moisture Equipment Corporation 2005G2 lysimeter and vacuum test hand pump. In order to allow assessment of the spatial variation in drip rates throughout the cave, sites were chosen from different localities along the cave passageways. Sampling points were selected from a range of speleothem forms (stalactites, soda straws and fracture openings) and flow rate regimes. Selection was also constrained by the practicality of sampling from the greatest number of sites possible within the time available for each sampling session. Drip rates were noted as the average of five consecutive measurements of the time between drips. Continuous flow was collected in calibrated vessels and the volume per unit time recorded. Flow volumes were converted into drip rates using the weight of single drips collected during periods of lower flow. The pH and EC of freshly collected water samples was measured in situ in all solutions where sufficient fluid was available. For Ca2þ and trace element analysis two 15 ml aliquots of each sample were taken, one aliquot being acidified with 0.1 ml of 5% Analar nitric acid in order to prevent calcite precipitation from saturated solutions. All solutions were stored in acid-cleaned, chemically inert, plastic sample bottles and kept in a cool bag until the end of the monitoring period prior to return to the laboratory for analysis. Calcium and trace elemental composition of solutions were analysed by Dionex 100 instrument (Ca2þ, Mg2þ, SO22 4 ) and Atomic Absorption Spectrometry– Graphite Furnace (Sr2þ). All ion values are well above detection limits and precision within each sampling period is better than the normally quoted ^ 5% error of dilute solution analysis (Fairchild et al., 1994). Solution alkalinity (used in the calculation of saturation indices and PCO2) was determined using a Hach Digital Titrator and 0.16N H2SO4, Gran titration, or charge balance calculations. As an assessment of the reliability of water chemistry analyses, field measurements of EC were compared with ‘synthetic’ EC values determined from the ionic analyses using the method of Rossum (1975) as
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modified by Hughes et al. (1994), and found to be in good agreement. Saturation indices for calcite and dolomite along with solution PCO2 were calculated using the geochemical program MIX4 as in Fairchild et al. (2000) with values being identical within 0.01 with those of PHREEQM (Appelo and Postma, 1994).
3. Site description The cave area is overlain by a glacial till deposit of 1 to . 2 m thickness (see Fig. 1) which is thought to have been deposited during the Munsterian (Warthe, pre isotope stage 5) glaciation, derived predominantly from siliciclastic Namurian sediments to the north, supplemented by limestone debris in the lower 1 m depth above underlying carbonate bedrock. The till is clay-dominated and of low permeability and matrix flow to the underlying aquifer is slow. However, there is evidence that the till has a well-developed fracture network that permits rapid transport of infiltrating water. Cracks were not directly observed during the excavation of soil pits, but this is likely to be a function of the mode of excavation (due to the use of spades and/or a JCB which smoothed the till surface). Their presence is required by the observation that many karst waters exhibited rapid responses to surface rainfall events. In addition, crack and fissure networks have been reported to be typical of glacial clay deposits elsewhere (McGown and Radwan, 1975), and would be enhanced by periglacial expansion and contraction processes which would have affected the area during both Munsterian retreat and the later Midlandian (Weichselian) glaciation (the last glacial cycle). Finally, soil processes dictate that all soils must contain some macropores (Beven and Germann, 1982). Therefore, the till is regarded as a dual porosity system with rapid water throughput occurring through cracks whilst the large storage capacity of the till micropores facilitates the constant delivery of matrix flow to the underlying aquifer. The cave has developed in Lower Carboniferous limestones, specifically the Cracoean Reef Member of the Cloonagh Limestone Formation (Hudson et al., 1966; GSI, 1996; Tooth, 2000). Thin section analysis shows that the bedrock comprises predominantly finegrained calcite, with local dolomite crystals and veins and quartz grains. The cave passageways have a total
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Fig. 1. Crag Cave site map with (a) plan view (modified from Gunn, 1983), and (b) cross-section diagrams. The cross-section shows the location of surveyed floors of cave passageways which are typically 2–3 m in height.
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Fig. 2. Crag Cave karst water drip site discharge. The location of sites A to Z is shown in Fig. 1.
known length of 2272 m (Gunn, 1983), the outer 300 m of which are used as a touristic cave. The cave is developed on two main levels with the maximum depth from the ground surface being approximately 20 m (see Fig. 1). A total of 14 karst water sampling sites were monitored within the undisturbed inner section of the cave. The hydrological characteristics of each site are illustrated in Fig. 2. The site is approximately 20 km from the Atlantic coast and, therefore, the climate is mild and wet. McDermott et al. (1999) stated that mean annual precipitation at the nearby Reenagown (altitude 187 m) and Glountaine (altitude 241 m) meteorological stations is 1465 ^ 292 and 1485 ^ 284 mm, respectively (45 year averages ^ 2s). The antecedent rainfall conditions prior to monitoring in summer and winter sessions differed in that the winter was wetter (see Fig. 3). The average total rainfall received at Glountaine and Reenagown meteorological stations in the three months prior to monitoring in August 1997 and January 1998 was 400 and 594 mm, respectively. However, since August 1997 was the wettest summer month, seasonal differences in hydrological response may be masked. In August 1997 rain fell during 8 of the 20 days of monitoring, with four significant rainfall events occurring, the largest of which had an interpolated incident rainfall volume of at least 20 mm (see Fig. 4a). No rain fell during monitoring in January 1998. Comparison of the field data sets with reported meteorological station information indicates that they are representative of the prevailing climatic conditions of the region. However, the lack of rainfall
noted during monitoring in January 1998 is clearly atypical of the general seasonal precipitation pattern.
4. Karst water hydrology 4.1. Results Initial drip rates at all karst water sites in August 1997 and January 1998 monitoring periods are influenced by the occurrence of rainfall events on the day prior to the commencement of each session (see Fig. 3). Sites can be separated into four groups in terms of hydrological response to recharge: (i)
Rapid response without time-lag (sites B, F and J). For example, at site B (see Fig. 4a) the drip rate was initially relatively high and decreased over the first four days of monitoring, with the small rainfall events of 09 and 10/08/97 preventing the decrease from being more marked. Coincident with the large rainfall input of 11/08/97 the drip rate increased to the maximum discharge observed during August 1997, followed by marked decrease over the following days in the absence of surface water input. Each subsequent smaller rainfall event occurring between 14 and 21/08/ 97 maintained flow at quasi-constant levels for 2– 3 days, after which a decrease in discharge occurred. Sites F and J exhibited a similar hydrological response to rainfall.
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Fig. 3. Antecedent rainfall at Glountaine and Reenagown meteorological stations prior to (a) August 1997, and (b) January 1998 monitoring periods.
(ii) Rapid response with associated time-lag (sites A, D, E and I). At site A (see Fig. 4a) the drip rate was initially high, remaining at a constant discharge for 4 days in response to the rainfall event prior to monitoring. The small rainfall events of 09 and 10/08/97 presumably prevented a more marked decrease in drip rate occurring over the next three days. In response to the large rainfall event of 11/08/97 discharge became constant after a 24 h timelag. Subsequent rainfall events maintained flow at constant levels for periods of 1– 3 days with similar time-lags, after which decreases in drip rate occurred. Sites E, D and I all exhibited similar patterns in hydrological behaviour. (iii) Intermittent response (sites C, G and H). At site G discharge was generally relatively low. The only increase in drip rate occurred
coincident with the largest rainfall event (see Fig. 4a), suggesting that a particular threshold of water input must occur before flow becomes preferentially routed to site G. Site H was subject to underflow (see Fig. 4a): when peak rainfall input raised the aquifer water pressure head above a certain threshold flow was diverted to greater capacity vadose routes (Smart and Friederich, 1986), leading to a decrease in discharge relative to baseflow. Site C exhibited a similar intermittent underflow response to recharge. (iv) No response (sites K, R, X and Z). Although some variation in drip rate was observed, the overall variation in discharge at each site was minimal (e.g. see site K, Fig. 4a) and no apparent relationship was observed between oscillations in flow rate and rainfall input.
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Fig. 4. Temporal variation in (a) karst water drip rate, rainfall volume and till saturation in August 1997, and (b) karst water drip rate in January 1998. Percentage till saturation figures are averages of 10, 20 and 30 cm depth measurements (the till is water-logged below ca. 40 cm). No discharge data are available for karst water site K in January 1998.
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In January 1998 the general trend observed was of a decrease in drip rate throughout the monitoring period due to the absence of rainfall following that of the day before monitoring commenced, with underflow sites (such as site H) exhibiting an increase in discharge (see Fig. 4b). Overall, karst water sites exhibited similar hydrological behaviour in both summer and winter. Although drip rate data sets for the two periods cannot be directly compared in terms of the degree of variation and absolute discharge rate due to the difference in the time-frame of monitoring and the lack of repeated rainfall events in January 1998, it can be observed that decreases in drip rates in the absence of rainfall input in January were much more gradual than those of August 1997. This suggests that there was more storage water held within the aquifer in the winter, coincident with the rainfall maximum.
and Z, which exhibited no response to rainfall, are presumably dominated by seepage flow along a tortuous flow path of poor interconnectivity. The moisture content of the till increased in summer in response to rainfall after time-lags of 24– 48 h (see Fig. 4a). This indicates that increases in karst water discharge that were coincident with rainfall could not be related to the influx of matrix soil water into the underlying aquifer. Two possibilities for the source of rapidly increased flow exist: (i)
increase in discharge was related to receipt of a storm water component sourced via rapid preferential flow through soil macropores (direct inflow); (ii) increase in discharge arose due to the throughput of aquifer storage due to receipt of preferential flow waters from the base of the soil zone (piston flow).
4.2. Discussion The high degree of spatial variation in discharge, discharge variation and the nature of the drip rate response to rainfall at Crag Cave karst water sites is consistent with the findings of studies at other cave sites (Baker and Genty, 1999; Baker et al., 2001). Time-lags between the receipt of rainfall at the ground surface and increase in Crag Cave drip water discharge were also temporally and spatially variable, as was the duration of elevated discharge in response to rainfall receipt. Such variations may be explained by the switching of flow routes as a result of rainfall events of varying quantity and intensity, which may also be responsible for the intermittent responses of sites C, H, and G. That is, the underflow response at sites C and H would only be instigated above a certain pressure head threshold in response to large rainfall events. Similar thresholds presumably govern the intermittent response at site G. Sites which generally exhibited high maximum discharge (for example B, F, H and J) and short duration responses were not subject to time-lags, whereas lower discharge sites were subject to lags of between 1 and 3 days (for example, A, D E and I). This indicates that flow routes to the rapid response sites (without time-lags) have a greater fracture flow component during recharge and are of a higher degree of interconnectivity. The flow routes to sites K, R, X
Therefore, although karst water sites can be simply classified in terms of the discharge response to surface rainfall input, information on karst water chemistry is necessary in order to determine a more rigorous classification of hydrological behaviour.
5. Karst water hydrogeochemistry 5.1. Results From the August 1997 data set four major trends in variation in karst water chemistry with discharge can be deduced as illustrated by the behaviour of sites B, G, H and K in Fig. 5: (i)
At site B increase in drip rate was accompanied by a decrease in ion concentrations and Mg/Ca ratio. This behaviour was also noted at sites A, D, F, I and J with the degree of such a signal differing between sites depending on the magnitude of drip rate variation. (ii) At site G, Ca2þ increased with increasing drip rate at low flow, although a duality of behaviour was exhibited with Ca2þ concentrations decreasing with greater drip rate at periods of high flow. Trends of decreasing Ca2þ concentrations were accompanied by increasing Mg/Ca and Sr/Ca
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ratios. Similar behaviour was noted at site E. (iii) At site H change in drip rate caused little overall change in ion concentrations or Mg/Ca and Sr/Ca ratios). This behaviour was also noted at site C. (iv) At site K an overall change in ion concentrations and Mg/Ca and Sr/Ca ratios in the absence of any notable change in drip rate occurred. This behaviour was also observed at sites R, X and Z. Most sites exhibited a similar geochemical response in winter to that observed in the summer as illustrated for sites G and H in Fig. 5 (no data are available for site K). (Full details of total solute compositions of Crag Cave karst waters and their sources are given in Tooth, 2000). 5.2. Discussion
Fig. 5. Variation in Crag Cave karst water Ca2þ , SO22 4 concentrations and Mg/Ca and Sr/Ca ratios with drip rate in August 1997 (filled symbols) and January 1998 (open symbols). Median matrix soil water (SWS) Ca2þ concentrations for both monitoring periods are also added (along with the standard deviation) at arbitrary drip rates.
Variations in karst water chemistry with discharge can be used to determine the hydrogeochemical processes occurring in the aquifer (dilution, source change, prior calcite precipitation) and also whether increases in drip rate due to recharge are a result of direct inflow of storm water from soil macropores or due to augmentation from aquifer storage due to piston flow. The simplest hydrogeochemical process potentially occurring during aquifer throughflow is dilution. Since variations in Ca2þ concentrations may be influenced by carbonate dissolution and precipitation, SO22 was used to test for the 4 occurrence of dilution. Sulphate is derived from the slow oxidation of pyrite in the aquifer, which requires weeks to months to attain significant levels (Fairchild et al., 1999; Plagnes and Bakalowicz, 2001); therefore low levels of SO22 indicate 4 dilution by younger water (either new rainwater or water of a relatively low aquifer residence time). Pearson bivariant linear regression calculations were applied to drip rate and SO22 concentration data 4 with dilution being assumed whenever the correlation was statistically significant to the 0.01 level. Dilution during increased discharge was determined to occur at sites B, F and J in both summer and winter, at site E in August 1997 and sites G and I in January 1998 (see Table 1). However, dilutionrelated decreases in SO22 levels are generally not 4 marked (up to 50% at site B but only up to 15% at
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Table 1 Summary of Crag Cave karst water hydrogeochemical responses to recharge and details of associated speleothem forms at each site Site
Date
Dilution?
Prior calcite precipitation?
Mg/Ca
Source change?
Floor speleothem form
A
August January
No No
No No
Variable Stable
Yes (piston flow) No
Stalagmite (CC3)
B
August
Yes
No
Yes (dilution)
Extensive series of gour pools
January
Yes
No
Increases with decreasing discharge Increases with decreasing discharge
C
August January
No No
No No
Stable Stable
No No
Stalagmite
D
Aug January
No No
No No
Variable Stable
Yes (piston flow) No
None
E
August
Yes (low flow) Yes
Increases with decreasing discharge Increases with decreasing discharge
Yes (dilution at high flow) No
Crystalline pool
January
Yes (high flow) No
August
Yes
No
Yes (dilution)
None
January
Yes
No
Increases with decreasing discharge Stable
August
No
Yes (low flow)
January
Yes (high flow)
Yes (low flow)
H
August January
No No
No No
Stable Stable
No No
None
I
August
No
No
Variable
Yes (piston flow)
Series of active and relict gour pools.
January
Yes
No
Stable
Yes (dilution)
J
August January
Yes Yes
No No
Stable Increases with decreasing discharge
Yes (dilution) Yes (dilution)
None
K R X Z
August August August August
No No No No
Yes Yes Yes Yes
Variable Variable Variable Variable
No No No No
Relict flowstone Stalagmite Stalagmite Stalagmite
F
G
Increases with increasing discharge at high flow, increases with decreasing discharge at low flow Increases with increasing discharge at high flow, increases with decreasing discharge at low flow
other sites) and Ca2þ does not consistently co-vary with SO22 levels, suggesting that simple dilution 4 did not occur. Therefore, incoming diluting waters are thought to have entrained solutions from auxiliary feeding reservoirs with the dilution being associated with a source change component.
Yes (dilution)
Yes (dilution) Yes (piston flow at high flow)
None
Yes (dilution at high flow)
Variations in Ca2þ and Mg/Ca and Sr/Ca ratios in the absence of any dilution component may be the result of prior calcite precipitation. A study of carbon isotopes performed at this site shows that CO2 is constantly entrained downwards from the soil zone to the underlying aquifer by flowing water (Tooth,
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Table 2 Carbonate system parameters for soil and karst zones at Crag Cave in August 1997 and January 1998 Karst water August 1997
Soil water January 1998
August 1997
January 1998
pH
Median n
7.5 198
7.5 67
7.1 16
7.0 5
Calcite saturation index
Median n
0.4 134
0.4 66
0.2 10
20.2 5
Dolomite saturation index
Median n
20.5 134
20.6 66
21.0 10
21.8 58
2 log PCO2
Median n
2.1 134
2.1 66
1.6 10
1.6 5
2000). The upper aquifer is therefore subject to open system conditions, with incoming high PCO2 soil waters dissolving calcite to reach equilibrium with the lower PCO2 conditions of the aquifer. This supposition is supported by the fact that karst waters were generally supersaturated for calcite whilst soil waters were typically at the point of saturation (see Table 2). Further changes in carbonate chemistry may occur in the lower aquifer where closed system conditions predominate if solutions flow through isolated air pockets. Ventilated air pockets which are not initially in equilibrium with incoming karst water may drive degassing of CO2 from solution, causing precipitation of calcite along the flow route. Pure calcite will preferentially be precipitated, resulting in solutions depleted in Ca2þ and enriched in Mg2þ and Sr2þ, with commensurately increased Mg/Ca and Sr/Ca ratios. Therefore, the maximum Ca 2þ concentrations observed at karst water sampling sites may reflect the actual PCO2 contacted during dissolution in the upper aquifer and/or the degree of ventilated, air-filled lower aquifer porosity. In order to test for the occurrence of prior calcite precipitation in Crag Cave karst waters, calculated lines of prior calcite precipitation were added to Fig. 6 (after Fairchild et al., 2000) along with data for sites B, G, H and K in summer and winter. Data plotting parallel to the trend lines in both Mg/Ca and Sr/Ca plots indicate the occurrence of prior calcite precipitation. Each site illustrates a different trend: site K is constantly subject to prior calcite precipitation, site B and H are not and site
G is only subject to prior calcite precipitation during low flow (cf. Ca2þ plot Fig. 5). Prior calcite precipitation was also determined to occur at sites E, R, X and Z (see Table 1). Source changes due to piston flow during recharge must be responsible for variations in ion levels at all sites where drip rates change but neither dilution nor prior calcite precipitation occur, with increases in discharge being related to the receipt of aquifer storage waters (site D, sites A and I in summer, and site G in summer during maximum flow; see Table 1). Piston flow effects must be related to direct inflow from soil macropores in order for a response to recharge to be elicited, forcing storage waters through auxiliary feeding reservoirs. Trends in karst water Mg/Ca ratios vary according to the hydrogeochemical processes occurring at particular sites (see Table 1). Where recharge response is rapid and discharge high (sites B, F and J) dilution tends to be more extreme and karst water Mg/Ca ratios often decrease with increasing flow rate, perhaps indicating a relatively high storm water component. Alternatively, Mg/Ca ratios seem to be stable at some sites affected by dilution (e.g. site I in winter), indicating that the effects of storm waters are offset by entrainment of auxiliary storage waters from relatively high Mg/Ca ratio reservoirs. Where chemistry is constant and drip rate steady (sites A and D in winter) or subject to underflow (sites C and H) Mg/Ca ratios are stable. Where piston flow induces source changes (sites A, D and I in summer) and at sites where
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6. Soil and aquifer zone flow route hydrogeochemical models
Fig. 6. Prior calcite precipitation effects in Crag Cave karst waters as indicated by trends in Mg/Ca and Sr/Ca ratios in August 1997 (filled symbols) and in January 1998 (open symbols). Arrows indicate the occurrence of prior calcite precipitation at specified sites. Partition coefficients (K ) of 0.016 and 0.15 for Mg2þ and Sr2þ, respectively, derived from Huang and Fairchild (2001) were used (although K has only a slight effect on ratios).
drip rate remains constant and prior calcite precipitation occurs (sites K, R, X and Z), Mg/Ca ratios are variable. This is a result of temporal variations in the size of the piston flow component or the amount of prior calcite precipitation. At sites E and G, hydrogeochemical behaviour is more complex, with differing processes dominating Mg/ Ca variations at high and low flow regimes. For example, at site G, Mg/Ca increases with decreasing drip rate during relatively low discharge, due to prior calcite precipitation causing enrichment in Mg2þ. At relatively high discharge Mg/Ca increases with rising drip rate due to flow being augmented by a source change component with relatively high Mg/Ca.
The Crag Cave till is thought to comprise both matrix and preferential macropore flow routes, meaning that two soil water signals will develop coevally, with more concentrated matrix flow constantly feeding the underlying aquifer and dilute preferential flow becoming important during recharge. In the model presented in Fig. 7 the soil and aquifer are divided into two zones with regards to CO2, with the soil zone and upper aquifer being subject to open system conditions with carbonate dissolution dominating, and the lower aquifer being subject to closed system conditions with the exception of isolated ventilated air pockets in which calcite precipitation is the dominant process. All karst waters are assumed to be fed from matrix soil water flow in the absence of recharge. Although karst waters may actually be fed from aquifer storage during baseflow, this assumption is valid since the base of the soil zone at this site (which contains numerous carbonate fragments mixed in with the till) behaves in a similar way to the upper aquifer in terms of air supply (i.e. open system conditions), dissolution and median Ca2þ concentrations (see Fig. 5), and has a high storage capacity. Therefore, the lower soil zone and upper aquifer are treated as one unit in terms of the supply of long residence time water to the lower aquifer in order to reduce complexity. Karst water response to recharge is dictated by the flow route taken through the soil zone. The absence of a hydrological response suggests evolution from soil matrix flow (Fig. 7a) alone (sites K, R, X and Z). Where recharge response is consistently rapid during each rainfall event, flow through well-connected soil macropores (Fig. 7b) is inferred (e.g. sites B, F and J), whilst at intermittent response sites soil flow occurs through macropores of reduced interconnectivity (Fig. 7c), which only become linked together during higher magnitude rainfall events (sites C, G and H). In the aquifer, flow rates are illustrated as being either relatively fast (Fig. 7e) or relatively slow (Fig. 7f). All routes carrying feeding waters sourced from the soil zone matrix are assumed to have a relatively slow flow rate since the lower soil zone/ upper aquifer represents the area from which baseflow is maintained at individual karst water sites
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Fig. 7. Model illustrating the potential soil and aquifer zone flow pathways and conditions which may control karst water evolution.
during the absence of rainfall. Drip rates fall markedly at most sites in the absence of recharge, confirming that the soil matrix baseflow component has a slow aquifer throughflow rate. The response to recharge at individual karst water sites is illustrated by the flow rate depicted between the primary aquifer feeding reservoir and the cave output point. Primary aquifer feeding reservoirs are illustrated as having either fixed (Fig. 7g) or variable (Fig. 7h) head. Prior calcite precipitation is modelled as only occurring in feeding reservoirs that are in contact with a ventilated gas phase (Fig. 7l). Dilution with an auxiliary source change component is illustrated as
the capture of a secondary soil matrix flow route by incoming soil preferential flow waters in the aquifer (e.g. sites B, F and J). Direct macropore inflow may also cause aquifer underflow (Fig. 7i; sites C and H). Piston flow (Fig. 7d) is modelled as direct macropore inflow forcing storage through auxiliary reservoirs (e.g. sites A, and D). Full mixing of inflow and storage occurs over time. Karst water feeding reservoirs can be illustrated as having stable, relatively low, relatively high or variable Mg/Ca ratios (Fig. 7m –p, respectively) depending on the recharge response. The hydrological characteristics at sites G, H and K are modelled in Fig. 8 to illustrate the range of
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Fig. 8. Plumbing diagrams illustrating hydrological and hydrogeochemical response to rainfall, soil and aquifer evolutionary pathways, and the control of these on karst water Mg/Ca ratios at (a) sites K, (b) site H, and (c) site G.
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behaviours observed in response to recharge at Crag Cave. The simplest model is that of site K (see Fig. 8a) at which there is no hydrological response to surface recharge and flow is slow through both soil and aquifer. The feeding aquifer reservoir is kept at a constant head and is in contact with a ventilated gas phase that drives prior calcite precipitation, causing variable Mg/Ca ratios. The intermittent hydrological response site H is shown to be subject to underflow during large rainfall input due to soil preferential flow through poorly connected macropores (see Fig. 8b). Since this additional flow bypasses site H (the rapid flow aquifer passageway becoming the path of least resistance during peak recharge), Mg/Ca is constant in both dry and wet periods. Flow from the primary aquifer feeding reservoir to the cave output point is illustrated as being relatively fast as although response to recharge is intermittent, it is rapid. The absence of prior calcite precipitation at site H is presumably a result of a lack of ventilated air pockets in this area of the aquifer. The complex behaviour at site G is modelled in Fig. 8c. In August 1997, piston flow driven source change predominated during wet weather, whilst in January 1998 dilution with associated source change was the dominant process during high flow, indicating a change in hydrological pathways in winter when the aquifer has a greater overall water-filled volume. Flow from the primary aquifer feeding reservoir to the cave output point at site G is illustrated as being relatively fast since intermittent responses are always rapid and maximum drip rates high. During high discharge, Mg/Ca ratios increased with drip rate, implying an auxiliary feeding source with a relatively high Mg/Ca ratio. For wet periods, Mg/Ca ratios are modelled as being relatively high when the feeding reservoir water level is high (i.e. peak discharge) and relatively low at lesser drip rates. In dry periods, when prior calcite precipitation occurred, Mg/Ca ratios increased with decreasing drip rate suggesting that waters were in contact with a greater proportion of ventilated air during lower reservoir volumes. Variation in Mg/Ca ratios with falling water level during dry periods is modelled as the converse of that illustrated during periods of peak recharge.
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The recharge response behaviour of most other Crag Cave sites can be modelled by using the information given in Table 1 to identify and combine the relevant ‘building blocks’ given in Fig. 7. Not all hydrogeochemical behaviour may be as simple as the models presented since a number of processes may act at a karstic drip water site at any one time, each overprinting the other, making the chemical signal difficult to interpret and model.
7. Relevance to the palaeoclimatic signal preserved by speleothems at Crag Cave The investigation of karst water hydrogeochemistry at Crag Cave indicates that speleothems at this site may record a signal of palaeohydrology, which may be determined by variations in Mg/Ca ratios. Since Sr/Ca ratios in speleothems are also affected by crystallographic effects (Huang and Fairchild, 2001) they are not as robust a tool for identifying palaeohydrological signals. At sites where source change due to dilution occurs (e.g. B, F and J) calcite with low Mg/Ca ratios may be precipitated at high flow during wet periods, with high Mg/Ca characterising dry periods. The signal preserved by speleothems at sites where piston flow causes source change (e.g. A, D and E) must be interpreted with care since Mg/Ca may be either higher or lower during wet periods, depending on the chemistry of the auxiliary storage component. However, it is likely that Mg/Ca will generally be greater at low discharge when base flow is maintained by long residence time storage waters. At sites E, G, K, R, X and Z, where prior precipitation of calcite along the aquifer flow path is a dominant control on karst water chemistry, differences in Mg/Ca ratios may occur between relatively wet and dry periods. Prior precipitation of calcite may be greater during dry periods and reduced during wetter periods when the proportion of air-filled aquifer porosity is low due to an overall increase in aquifer water volume (Fairchild et al., 2000). Since the rainfall maximum occurs during winter at Crag Cave, speleothem Mg/Ca ratios may be higher in summer. In winter, thicker soda straw macroscopic growth bands may precipitate since karst water saturation would not be reduced en route through
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the aquifer and relatively greater calcite would be precipitated within the cave. Such a seasonal pattern in karst water Mg/Ca ratios between project monitoring periods may not have been observed because of relatively high rainfall in August, which was the wettest summer month in 1997. If August had been as dry as June and July 1997 (see Fig. 3), a more marked seasonal difference in hydrogeochemistry may have been noted. Reduced recharge would have led to a reduction in karst water drip rates, dilution and auxiliary storage flow components, meaning that solution chemistries may have been more constant. Slower aquifer throughput and greater ventilation would cause increased prior calcite precipitation and relatively greater dolomite dissolution at sites at which this mineral is thought to be present along the flow path (Tooth, 2000). Both of these processes would lead to higher karst water and speleothem Mg/ Ca ratios. In the assessment of palaeoclimatic signals, it is the convention to study variations in growth bands and chemistry in stalagmites and flowstones (Lauritzen and Lundberg, 1999). In Crag Cave, stalagmites have developed at the slow-dripping sites A, C, K, R, X and Z, of which that at site A (CC3) has yielded a Holocene climate record (Baker et al., 2000; McDermott et al., 1999; McDermott et al., 2001). Of these stalagmites, only that of site A is fed by karst waters which displayed a hydrological and geochemical response to rainfall, but this short-term signal was very muted. At site C, no short-term signal would be recorded due to the constancy of the corresponding karst water geochemistry. However, since dolomite is thought to be present along the flow path (Tooth, 2000), a long-term palaeohydrological signal may be recorded during periods of prolonged wetness or dryness, with enhanced dolomite dissolution during slower aquifer throughflow times (Fairchild et al., 2000) causing an increase in Mg/Ca ratios in the karst water and stalagmite. Although sites K, R, X and Z exhibited no hydrological response to rainfall input, a long term palaeohydrological signal may be preserved by the Mg/Ca ratios of the corresponding stalagmites via the mechanisms put forward earlier. Ion microprobe analysis of the chemical composition of soda straw R confirms that
Mg/Ca ratios reflect relative wetness/dryness (Fairchild et al., 2001). The Holocene record of stalagmite CC3 from site A shows extreme variations in carbon isotopes (McDermott et al., 1999) and trace elements (Tooth, 2000). Also the geochemical record for the last five years for the site R soda straw indicates more extreme variations than implied by monitored changes in water chemistry (Fairchild et al., 2001). These anomalies may be a result of the failure to perform monitoring over a prolonged dry period during which peaks in Mg/Ca might be expected. The principles developed at Crag Cave can be applied generally, with modifications being made for the effects of site-specific conditions such as soil and aquifer mineral composition, structure and hydrology.
8. Conclusions Soil matrix and preferential flow both influence karst water chemistry at Crag Cave. Soil matrix flow is the dominant karst water source during dry periods, whilst preferential flow through soil macropores is important during recharge. Monitoring of rainfall volumes and drip rates indicates that karst waters can be classified into four differing hydrological response categories: rapid response, rapid response with associated time-lag, intermittent response and no response. Interpretation of karst water Ca2þ, SO22 4 and Mg/Ca and Sr/Ca ratios shows that prior calcite precipitation, piston flow of storage and dilution control variation in chemistry. Dilution is never extreme and always augmented by an auxiliary flow component. At high capacity sites recharge may cause a decrease in Mg/Ca with increased discharge, potentially indicating a relatively high direct inflow component. At sites affected by piston flow, Mg/Ca reflects the composition of the storage that augments baseflow during recharge. Prior calcite precipitation occurs at lower flow rates and causes co-varying trends in Mg/Ca and Sr/Ca. The occurrence of prior calcite precipitation is controlled by the degree of ventilation in lower aquifer air pockets. Speleothem Mg/Ca may be used to interpret palaeohydrology. Since stalagmites develop preferentially at slow-dripping sites at Crag Cave, palaeohydrological signals will be related to prior
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calcite precipitation. Greater prior calcite precipitation and Mg/Ca ratios may occur during prolonged dry weather when ventilated air-filled aquifer porosity is higher and throughflow slower. During prolonged wet periods Mg/Ca will be lower and lesser precipitation of calcite during aquifer throughflow may lead to thicker stalagmite growth bands being precipitated. Since the annual rainfall maximum generally occurs in winter in temperate climates, Mg/Ca ratios may also potentially be used as a proxy for relative temperature in the interpretation of short term soda straw records. In terms of sampling strategy, although the shortterm daily monitoring performed at Crag Cave provided insight into the karst water hydrogeochemical response to rainfall, monthly monitoring may provide greater detail on absolute seasonal variations in soil and aquifer zone cation yields. Some combination of the two time-scales would comprise a better strategy.
Acknowledgements We thank Mrs M. Geaney for allowing us access to Crag Cave, Ian Wilshaw for technical support at Keele, Frank McDermott and Baruch Spiro for ancillary data and discussion, Cara Ward and Finbarr Quinn for assistance in the field, and the reviewers Paul Williams and Andy Baker whose comments greatly improved the manuscript. This work was performed with funding from NERC grant GR3/10801 awarded to Fairchild and a Keele University studentship awarded to Tooth. During completion of the manuscript Tooth was funded by Lise Meitner Scholarship M618, awarded by the Austrian Science Fund.
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