Hydro-ecological parameters for sustainable groundwater

Hydrological Basis ofEcokmcauy Sound Mana&ment of Soil and Groundwater (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 202, 1991.

Hydro-ecological parameters for sustainable groundwater management in the region of Kennemerland, The Netherlands A. BARENDREGT Department of Environmental Studies, University of Utrecht, P.O.Box 80.115, 3508 TC Utrecht, The Netherlands J.W. NIEUWENHUIS Department for Environment and Water, Province of NoordHolland, P.O.Box 3088, 2001 DB Haarlem, The Netherlands

ABSTRACT A method has been developed to identify the ecological parameters of a hydrological system in the Province of Noord-Holland in The Netherlands. The regional hydrology of Kennemerland is characterized by infiltration in a dune area and seepage into the adjacent polders. The pattern of aquatic ecosystems corresponds with subregional hydrological boundaries. Within the regional system local hydrological systems can be ecologically dominant. Application of the ecological parameters together with hydrological models illustrates the importance of groundwater flow as a conditioning factor in aquatic ecosystems. This knowledge provides a sound basis for the design of a program for sustainable groundwater management, which is incorporated in the policy of the province.

INTRODUCTION The ecological function of unpolluted aquatic ecosystems can deteriorate with changes in the hydrological system. Both the hydrological and biotic parameters characterizing these changes have to be determined. The public supports measures to preserve nature, to keep water from being polluted and to maintain a sufficient supply of water for both agriculture and nature. To design a program for sustainable ecosystem management, more insight is required into the conditioning factors posed by the interaction of the groundwater system and the surface water system. More information is needed about the relation of these systems with their with biotic elements as well. This knowledge can be derived from the detection of landscape ecological relations. Investigations of biotic and hydrological patterns and processes in a defined area generate complementary information. Integration of the results from both kinds of analysis highlights the ecological relations. This exercise can provide management tools for the responsible authorities. The province of Noord-Holland comprises regional hydrological systems. The ecological parameters in these systems that postulated to correlate with the landscape ecological relations. The parameters are tested at a local scale and re-adjusted if necessary. This sequence of investigations is illustrated by a description of the dune area and the adjacent polder area between the towns of Castricum and Bergen (Kennemerland), in the western part of The Netherlands (Fig. 1).

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DESCRIPTION OF THE STUDY AREA In the west of the Region Kennemerland, bordering the North Sea, lies an area (2 to 4 km wide) with sandy dunes up to 40 m above mean sea level (NAP, reference level). These dunes have a natural vegetation of dry grasslands and shrubs; in some places, pine trees have been planted. The groundwater is used for the drinking water supply. Near Bergen, 2 10° m 3 year -1 is pumped; near Castricum, 4 10° m 3 year" . Behind the dunes, a transition zone of sandy soils, just above mean sea level, is used for agriculture, mostly bulb growing and dairy farming. To the east of this zone, dairy pasture continues on the land lying below sea level. This is holocene peat and clay with scattered local sand deposits. These dairy pastures are located in polders, where the surface water level is artificially maintained throughout the year. In wet periods, superfluous water is pumped out of these polders into a system of main canals. In this way, agricultural land is drained. During dry summers, water from the main canals is used to supply dairy pasture with water. The origin of the water in the main canals is the river Rhine. This water is eutrophicated and slightly brackish.

FIG. 1 Location of the study area with the cross section X-X'. S=sea, D=dunes, Z=transition zone, P=polder, C=canal, T=town; NAP=mean sea level.

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REGIONAL HYDROLOGY: QUANTITY, QUALITY AND AQUATIC ECOSYSTEMS The driving forces in the hydrology of the Kennemerland are I) the higher piezometric head in the area of the dunes inducing a groundwater flow towards the lower polders and the sea, and II) the level of the sea inducing a deep groundwater flows towards the polders. The phreatic head in the dunes is 2 to 5 m above NAP; in the polders it is 0.7 to 1.5 m below NAP (ICW, 1982; Stuyfzand, 1985, 1989a). The subsoil consists mainly of sandy material separated by aquitards (clay). The hydrological base of the system lies 150 m below NAP (ICW, 1982; IWACO, 1989). Unlike the northern part of the study area, the southern part has no aquitard 20 to 30 m below NAP (Fig. 2 ) . In the northern part, the groundwater consists of three aquifers. The first aquifer is the system of the holocene dunes down to the BergenFormation: 10 m clay with a vertical flow resistance of c-30 000 to 70 000 days. There is an aquitard about 40 m below NAP with clay from the Eem-Formation (3 m, c-1 000 to 30 000 days). In the southern subregion most parts have a small aquitard 15 m below NAP and a 3 m-thick loamy aquitard from the Drente-Formation 40 m below NAP. These differences in geology cause differences in hydrology. In the north, the regional groundwater flow into the polders consists of a direct flow from the dunes into the first aquifer and a flow from the second into the first aquitard. In the southern part, there is a direct flow from three aquifers into a large polder area. This results in a pattern of infiltration (recharge) and seepage areas (discharge of groundwater) at the surface: the dunes form the regional infiltration area; the polders eastwards comprise the regional seepage area, several kilometers wide (Stuyfzand, 1989a). Extensive man-made aquatic systems occur in the polders where the ditch spacing is 50 to 100 m. These ditches are 1 to 3 m wide and contain 30 to 60 cm of surface water. This water system provides good conditions for submerged and emergent vegetation, as do the banks for

FIG. 2 Regional geology and groundwater flow. A) distribution of clay layer 20 to 30 m -NAP; B) cross section YY* of the northern subregion; C) cross section 1-1' of the southern subregion. After Stuyfzand (1985,1989a).

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Type A:

Potamogeton densus Nasturtium officinale Elodea canadensis Veronica beccabunga Apium nodiflorum Equisetum fluviatile

Type B:

Sparganium emersum Potamogeton trichoides Sagittaria sagittifolia Potamogeton natans Hippuris vulgaris Oenanthe fistulosa

Type C:

Zannichellia palustris Potamogeton pectinatus Ceratophyllum demersum Berula erecta

FIG. 3 Distribution of the vegetation types in the area, with identification of characteristic species.

phreatic plant species. The great ecological importance of the seepage area behind the dunes is clearly demonstrated by the map of the whole province showing the distribution of plant species that indicate seepage of groundwater (Nieuwenhuis et al., 1991). These indicators were identified by research on a provincial scale (3000 k m 2 ) . Various types of aquatic vegetation occurring in the ditches of Noord-Holland have been defined on a landscape ecological scale (Nieuwenhuis et al•, 1991). These types are indicators of different conditions of soil, dimension of the system and hydrology. Three different vegetation types are present in the area (Fig. 3 ) . Type A is found adjacent to the dune area. The other types corresponds to hydrological division of the area: Type B is found in the southern subregion; Type C occurs in the northern. The boundary between brackish and fresh groundwater indicates the extension of the dune system (dashed line in Fig. 4 ) . The chemical composition of infiltrating rainwater (fresh NaCl type, classification according to Stuyfzand (1989b)) changes rapidly, as the holocene sand layer is rich in lime (Eisma, 1968). Dune water without external influences is of a fresh CaHCOg type (Stuyfzand, 1985, 1989a). In the dune area, this type is present in the first aquifer (Fig. 4 ) . A saline NaCl type indicates current or subfossil influence of the sea. It is present in the second or third aquifer of the infiltration area. The contact zone between these types is of an NaHCOg type, which indicates desalination (Stuyfzand, 1989a). In the polder area of the northern subregion the groundwater is of the brackish NaCl and NaMix types; in the southern subregion of the fresh CaHCOg and CaMix types. The map of surface water chemistry indicates that the area directly adjacent to the dunes is of the CaHCOg type. Surface water two kilometer east of the dunes in the northern subregion water is.of a brackish NaCl, NaMix or an almost fresh NaHCOg type. In between the dunes and the center of the polder, as well as in the total southern subregion different types of fresh water are present (CaHCOg, CaMix).

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Hydro-ecological parameters for sustainable groundwater management m +20NAP-20-40-

-60-

FIG. 4. Chemistry of groundwater in southern subregion (right).

northern (left) and

REGIONAL HYDROLOGY: A SYNTHESIS In the regional hydrological system, the northern and southern subregions show important differences. In the southern subregion, fresh CaHCOg water can infiltrate to a depth of about 70 m and the regional seepage zone with this water has a width of several kilometers. In the northern subregion, infiltration is hampered by an important aquitard located 20 m -NAP. Therefore, the CaHCOg type tends to flow laterally towards a narrow zone at the foot of the dunes. At a greater distance from the dunes, regional brackish groundwater from a deeper aquifer (NaCl, NaMix type) seeps upward in the polders. This pattern is reflected in the distribution of vegetation types B and C (Fig. 3 ) . Seepage indicating species are encountered in all polders in the study area. This corresponds to the hydrology of the region. Plant species found in ditches are related to water chemistry (Nieuwenhuis et al. , 1991). The species of types B and C indicate a different chemistry: more saline and eutrophicated in the northern subregion; fresh and rich in C a + + in the southern subregion. The occurance of these species confirms the different water chemistry in those areas. The direct relation between the distribution of the two types of vegetation and the differences in hydrology can only be explained by one factor. That is the fact that the chemical composition of surface water in the ditches is determined by upward seepage of groundwater with a different chemical composition.

LOCAL HYDROLOGY: ANALYSIS OF THE GRADIENT IN THE HYDRAULIC HEAD The regional hydrological system is internally differentiated. Detailed investigation of this region al system reveals a gradient from the dunes to the polders. The area of Type A vegetation covers an important gradient: between the polde rs and the dunes, there exists a separate zone of aquatic systems (Sc huurkes et al•, 1989). This zone, with its man-made ditches, is situât ed above mean sea level outside the polders (west of the dashed line in Fig. 3 ) . A seasonal fluctuation occurs in this zone. In winter and spring, the ditches contain running water, like brooklet s; the water is mostly 10 cm deep and the chemistry is of the CaHC0 3 type. In summer many of these ditches fall dry. In contrast, the surface water in the polders is kept at a constant level. There is no visible flow of surface water,

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and the main canals supply water in dry periods. In the polder with Type A vegetation, a very fresh CaHC0 3 type of water is present in the area receiving runoff from the elevated area with type A vegetation. With a short gradual transition, Type A borders to an area with Type C vegetation, with brackish water, rich in nutrients. In this area, the seepage of groundwater is less prominent whereas the influence of eutrophic water from the main canals is evident. Eutraphent plant species are found here. To test the described relations, research was carried out at the local level for two cross sections of the northern subregion (A-A' and B-B' in Fig. 1). This was supplemented by detailed (1:5000) information on the distribution of species. In addition data were collected on piezometric heads and on the chemistry of surface and groundwater. Biological information on the ecology of seepage indicating species is also incorporated in this local research. At the provincial scale, the 17 seepage indicating species are tested for a correlation with the chemistry of surface water. These species turned out to belong to three ecological species groups (Nieuwenhuis et al. , 1991). The first group (1) of two species is fully dependent on fresh CaHCOo water (Potamogeton densus, P. obtusifolius). The second group (2) has a slight tolerance to the other types of water (Equisetum fluviatile, Veronica beccabunga). The third group (3) of seven species

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FIG. 5 Cross sections of geology and hydrology; presence of seepage indicating species at cross sections A-A' and B-B' (see Fig. 1) in the summer.

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shows greater tolerance water of different chemical composition. The study of cross section A-A' (Fig. 5) revealed that most of the important seepage-indicating species are restricted to a narrow zone. This zone coincides with the extent of local seepage. Evidently, the regional groundwater flow does not determine the presence of seepageindicating species, because local seepage of phreatic groundwater from the dunes is far more prominent. The cross section B-B* lies inside the regional infiltration area. The distribution of seepage-indicating species demonstrates that local hydrological circumstances are the conditioning factor. This is confirmed by data on piezometric heads and water chemistry. Two local seepage areas are found. One is directly at the foot of the dunes, where lateral groundwater flow is prominent. Within the regional infiltration area, a second local seepage zone exists. This area lies in the lowest parts of the transition zone, where the water infiltrating from the dunes and the transition zone seeps up. In this cross section the local hydrological systems determine the occurrence of aquatic and phreatic plant species.

LOCAL HYDROLOGY: A SYNTHESIS Hydro-ecological relations in the research area show two different scales. On the larger scale, these relations are dominated by the effect of the regional groundwater flow system. However, on a smaller scale, several local systems have great impact on ecological differentiation. Inside the regional infiltration area of the transition zone, local groundwater flow systems cause local seepage. This is concluded from data on piezometric heads, water chemistry and the presence of species indicative of Type A. Tha indicative species are present in local seepage areas with fresh CaHCOg water; these species are absent in local infiltration areas. An illustrative case is the anomaly that Type A species are present in a poldsr area with regional seepage of NaCl water. This location receives runoff with CaHCOo water from an upstream local seepage zone.

APPLICATION OF THE ECOLOGICAL PARAMETERS The relations detected at regional and local scale have been applied in the southern subregion (cross section C-C in Figs 1 and 6). In this cross section the species are also arranged in a specific pattern (Fig. 6b). The same groups of seepage-indicating species are used as in the previous section; Number 4 is the species Triglochin palustre, indicating slightly brackish and eutrophicated conditions. In the transition zone from the dunes to the polders, Group 1 constitutes the center, surrounded by Groups 2 and 3. Group 4 is restricted to the polders where water supplied from the main canals dominates. The option for the authorities is to formulate a scheme for sound management of surface and groundwater, whereby Group 1 (accompanied by 2 and 3) can be stimulated and the eutrophication indicative Group 4 can be reduced. Increasing seepage of groundwater instead of the present supply from the eutrophicated canals offers this opportunity. The question is how to achieve this. In the dune area, the abstraction of water can be reduced or even stopped. Hydrological research would

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have to determine the resulting water balance in the polders. The hydrology is modelled with the help of the computer program TRIWACO (Van der Wal, 1991). This model is based on a finite element method and is quasi-three-dimensional. The elements are 1000 by 500 m for Aquifers 1A, IB and 2. The model is applied regionally (Van der Wal, 1991) with the geohydrological outline adopted from Stuyfzand (1985). The calibration is carried out by steady-state application with mean data of 1981; data of 1984 are used for verification. This regional model is also specified locally for a restricted area of 1 by 4 km, giving more detailed information about the soil surface (screen C-C in Fig. 1). The present hydrology is given in Fig. 6c. Drinking water wells are located in Sectors I and III. Precipitation infiltrates in the dunes (Sectors I—IV). Prominent seepage is calculated for the actual situation, particularly in Sector VI, mainly originating from Aquifer IB. This sector is also the location of species of Group 1, indicating seepage of fresh groundwater. The first option in management is to reduce the local pumping of drinking water by 200 000 m 3 year -1 ; the second option is to close all wells in this region. The modelling of the first option (Fig. 6d) indicates that in the dunes the piezometric head of the first aquifer will rise 15 cm; 5 cm in the important Sector VI in the polder area. The quantity of seeping groundwater appeared to be low: local reduction does not influence the main regional groundwater flow. The

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FIG. 6 Cross section C-C (see Fig. 1). A) altitude; B) presence of species groups; C) modelling of the hydrology in present situation with three aquifers; D) differences in first aquifer of piezometric head and flow, in the case of local reduction of pumping; E) idem, closing all wells.

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TABLE 1 Probability of encountering seven species, calculated with ICHORS. l=intensive seepage; 2=moderate seepage; 3=half seepage, half infiltration; 4=infiltrat ion with water from the main canals. 1 Sparganium emersum Equisetum fluviatile Veronica beccabunga Potamogeton natans Elodea canadensis Zannichellia palustris Triglochin palustris

0.16 0.79 0.23 0.00 0.07 0.01 0.04

2

0.03 0.69 0.31 0.27 0.09 0.05 0.13

3

4

0.00 0.12 0.01 0.08 0.10 0.19 0.42

0.00 0.00 0.00 0.03 0.03 0.37 0.71

second option (Fig. 6e) results in considerable improvement: the piezometric head will increase 75 cm in the dunes, 37.5 cm in Sector V, and 25 cm in Sector VI. The transition zone will expand into the polder area and the dunes. Moreover, the quantity of seeping groundwater will double in Sector V, so that seepage water instead of supply water can flow through the ditches into the polder. This difference in groundwater flow results in ecological changes in the polders. With the rise of the piezometric heads the problems of parching of soil and mineralization of organic material will vanish. The water balance in the polder will become more positive. Consequently, less external supply water will be needed, and surface water will change from Mix types to the CaHCOg type. Groundwater poor in nutrients replaces water rich in nutrients, whereby also the problem of eutrophication will be reduced. The important Types A and B vegetation are stimulated by this management option. Physically, the transition zone with runoff of groundwater will expand and the brooklets will contain water all year long. Chemically, surface water will be fresh and mostly of the CaHCOg type. These are the conditions (Nieuwenhuis et al•, 1991) for continued existence of Type A vegetation that are incorporated in the policy (Provincie NoordHolland, 1991). The quantitative and qualitative hydrological conditions that occur in the present situation and after closure of the wells are tested (Table 1) with the hydro-ecological model ICHORS (Barendregt & Wassen, 1989). The response of the characteristic species to these environmental circumstances can only be explained by an increasing influence of fresh and unpolluted water of the CaHCOg type. This is, the suggested management option.

CONCLUSION The identification of the hydrological and biotic parameters to formulate a sound management of ecosystems is an iterative research process. Detecting the important relations requires regional and local investigations on quantity and quality of both surface and groundwater. At the same time, this goal requires research on the spatial distribution of biotic types and their relations with nonbiotic factors. All parameters are integrated at an interface: the

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ecological relations at a landscape scale. In a correct analysis, both hydrological and biotic research are complementary and can help to describe the regulatory mechanisms. This study shows that regional hydrological conditions define the presence of three main vegetation types. But the findings about regional seepage or infiltration areas have to be tested at the land surface. That is because local hydrology can determine the direction of groundwater flow and thus also the water chemistry. The presence of species with specific ecological demands is rather determined by local hydrology than by regional hydrology. The ecological importance of a hierarchical arrangement of water systems (Engelen & Jones, 1986) makes it necessary to investigate an area at different scales. Knowledge about processes and the relative importance of scale is needed to understand the quantity and quality of the available water at the surface and, consequently, in the root zone of the vegetation. In this case, the characteristic ecosystems can only be restored by increasing the groundwater seepage. Regional aquatic vegetation types depend on the quality of surface water. Surface water quality, in turn, depends on the quality and quantity of local groundwater seepage, which is influenced by regional hydrology. There is a direct (first-order) relation between vegetation, local water chemistry and hydrology.

REFERENCES Barendregt, A. & Wassen, M.J. (1989) Het hydro-ecologisch model ICHORS (versies 2.0 en 3.0). Interfac. Vakgroep Milieukunde, Utrecht. Eisma, D. (1968) Composition, origin and distribution of Dutch coastal sands between Hoek van Holland and the island of Vlieland. Neth. J. Sea Research 4 (2), 123-267. Engelen, G.B. & Jones, G.P. (1986) Developments in the analysis of groundwater flow systems. IAHS Publication 163. ICW (1982) Kwantiteit en kwaliteit van grond- en oppervlaktewater in Noord-Holland benoorden het IJ. Werkgroep Noord-Holland. Régionale studies 16. ICW, Wageningen. IWACO (1989) Hydrologische systeemanalyse-, Ontwikkeling meetnet grondwaterkwaliteit in de provincie Noord-Holland. Rotterdam. Nieuwenhuis, J.W., Siffels, J.W. & Barendregt, A. (1991) Hydroecological research for water management in the province of NoordHolland, The Netherlands. In: Proc. Int. Symp. Hydr. Basis of Ecol. Sound Management of Soil and Groundwater, IAHS Publication. Provincie Noord-Holland (1991) Natuurlijk, Water. Waterhuishoudingsplan. Rapport Provincie Noord-Holland, Haarlem. Schuurkes, J.A.A.R., Buntsma, J.J. & Nieuwenhuis, J.W. (1989) Duinrellen in Noord-Holland; betekenis, beheer en beleid. Noordhollands Landschap '89 (3), 68-71. Stuyfzand, P.J. (1985) Hydrochemie en hydrology van het duingebied tussen Egmond en Wijk aan Zee. Rapport KIWA N.V., Rijswijk. Stuyfzand, P.J. (1989a) Hydrochemie en hydrologie van duinen en aangrenzende polders tussen Egmond aan Zee en Petten. Rapport KIWA N.V., Nieuwegein. Stuyfzand, P.J. (1989b) A new hydrochemical classification of water types. IAHS Publication 182, 89-98. Van der Wal, B.J. (1991) Geohydrologisch modelonderzoek in het NoordHollands Duinreservaat. Rapport Provincie Noord-Holland, Haarlem.