The fresh water of coastal dunes is of great importance for their biological and ecological features. It defines to a certain degree patterns of vegetation, geomorphology and even geology. However, nowadays the hydrological conditions in most dune areas are strongly influenced by man and natural conditions hardly can be found (see: Natural hydrological conditions; Dune fringe nature development). Especially in dune areas with a low lying hinterland like in The Netherlands, the fresh water under the dunes is of great importance as a source of drinking water for the people living in the coastal zone. But groundwater extraction, artificial recharge and artificial drainage have all had undesirable effects on dune systems (see: Water abstraction and groundwater-level restoration; Water quality and artificial water infiltration) (1, 3).
 

 With respect to the geohydrology of dunes, three situations, which are common along the European coast, can be distinguished: (1) dunes with a low lying hinterland in humid areas (quite common in the lowlands of Belgium, Denmark, Germany, Poland and The Netherlands), (2) dunes with a high-lying hinterland in humid areas (common on rocky coasts in parts of Europe) and (3) dunes in arid areas (e.g. dunes of the Coto Doñana in southern Spain). Fresh water percolates into the sand dune body by precipitation and, in the case of a low lying hinterland, a flow of groundwater can be maintained because of the more or less permanent input. Extensive dune systems which have developed on a sedimentary plain or on low pleistocene deposits like barrier dunes and spit dunes, can develop a freshwater lense. This development determines dune hydrology and groundwater regime rather independent from the salt water environment. The fresh water in the dune bodies is of great importance for their biological and ecological features, its role in creating the wet slack environment of dunes must be highlighted (see: dune slacks on the Wadden Sea Islands, The Netherlands; The Zwanenwater, The Netherlands) (1).

   2 - Water abstraction and groundwater-level restoration

Water abstraction in dunes strongly influences the hydrological conditions. A dramatic fall of the groundwaterlevel is in almost all cases the first consequence. The Dutch coastal dunes have been used to supply drinking water since 1880. Already in 1900 the water table in wells had dropped off 3-4 m.
 Dune slacks, dune pools and seepages used to occur in 30% of the Dutch dunes. Wet dune slacks are depressions in dune systems which are flooded during winter and most of spring. Many rare and protected plant species occur in nutrient-poor, moist to wet slacks, where the soil pH is buffered around neutral and dune slack pioneer vegetation has a great conservation value (4). The threat to wet dune slacks, that are among the most seriously threatened natural habitats in The Netherlands, is of special significance. Lowering of the water levels and/or over-stabilisation of the dune can lead to the slacks becoming dry and progressively invaded by woody plants such as Salix repens and eventually scrub and woodland (2). There are several examples to restore this important part of a dune system by raising the groundwater table by a reduction of groundwater abstraction. In the Netherlands, in the Amsterdam Water Supply Dunes (The Netherlands) a regeneration project has been started to restore the dune slack landscape. Therefore the water winning is reduced and groundwater level has risen, thereby developing wet dune slacks. So far the project has been successful. Other examples are given for the dunes of Goeree, the Meijendel dunes and the North-Holland Dune Reserve (all in The Netherlands). Microbial mats may assist to select suitable sites for the restoration of dune slacks, as they may extending the life span of early pioneer stages during dune slack succession (see: Schiermonnikoog, The Netherlands).
 The natural state of the phreatic groundwater is low in nutrients. Whether or not a raising of groundwater level will result in a regeneration of original characteristic dune slack vegetation will therefore depend on the groundwater quality (see: North-Holland Dune Reserve and dune slacks on the Dutch Wadden Sea Islands). The groundwater may influence the soil processes like pH, dehydrogenase activity and nutrient availability (e.g. ammonium, nitrate and phosphate concentrations) due to its contents (5).

   3 - Water quality and artificial water infiltration

The increase in demand of drinking water since the beginning of water abstraction in the Dutch dunes caused shrinkage in the volume of freshwater and brackish sea water began to reach the abstraction wells. To raise the groundwater level, artificial recharge with surface water in dune valleys has been applied in The Netherlands since the early 1950s with various negative side-effects, e.g. unnatural water table fluctuations, disturbance in the direction and volume of groundwater flow, and input of nutrients (2, 3). Infiltration of (pre-purified) eutrophic water from rivers or lakes in so-called infiltration areas has led to eutrophication of the former oligotrophic groundwater and the development of unnatural, ruderal vegetation alongside the infiltration area and surrounding dune slacks. The vegetation in wet dune slack is determined by a poor nutrient supply of the soil. This results in colonisation by pioneer communities characterised by a high species diversity.
 All water supply companies have significantly improved the prepurification, e.g. in Berkheide – Meijendel, the NV Duinwaterbedrijf Zuid-Holland is currently reducing the infiltration area.

   4 - Dune fringe nature development

The landward fringes of the dunes are a transition zone towards the older dunes. Because of the flowing character of groundwater, geohydrology is a component of the landscape which links landscapes together. Human activities in one area may have (severe) consequences in another. When the waterlevel in the main dune area is changed, the groundwatertable in the zone next to the inner dune fringe will change also. This is particularly serious in dunes with a low-lying hinterland (1). Restoration projects may include purchase of agricultural land, eco-hydrological rehabilitation or improved management. These projects may aim at the restoration and development of dune seepage zones, brooks, grassland, wooded banks, coppice and other valuable landscape features.

References:

1. Bakker, T.W., P.D. Jungerius & J.A. Klijn (1990): Dunes of the European Coasts. Catena Supplement 18. Germany. (BCD90).
2. Doody (2000; in press): Coastal conservation and management. An ecological perspective.
3. Nordstrom, K.F. (2000): Beaches and Dunes of Developed Coasts. Camebridge University Press. (PC2000a).
4. Sival, F.P. 1997. Dune soil acidification threatening rare plant species. Thesis Groningen University. (18.4.13).
5. van Beckhoven, K. (1995): Rewetting of coastal dune slacks: effects on plant growth and soil processes. Thesis Vrije Universiteit Amsterdam. The Netherlands. (33BCd95).

Each case can be found via geographical maps and via thematical texts putting the cases in an order of six interesting topics:

	seashore dynamics
	sand mobility
	hydrology and water management
	conservation management
	management of forests
	management in relation to recreation and tourism