Calcium concentrations in wetland groundwater in relation to water sources and soil conditions in the recharge area

Abstract

Over the past several decades, vegetation communities in lowland wetlands in Western Europe have changed. Species-rich, low-productivity (i.e. low primary biomass production) communities characteristic of rich fens (fens characterized by microphilous species) have declined and are being replaced by vegetation communities characteristic of high-productivity rich fens, poor fens (fens characterized by ombrophilous species) and bogs. High concentrations of calcium in groundwater are generally believed to be an important controlling factor on the vegetation of rich fens Management for the protection of low-productivity rich fens therefore generally aims at conservation of calcereous groundwater conditions in the root zone of lowland wetlands. This paper relates the occurrence of calcium-rich groundwater in lowland wetlands of the Central Netherlands to water sources and soil conditions in the groundwater recharge area. The chemical composition of the following groundwater types is compared: groundwater recharged at a sandy ridge adjacent to the wetland area (ridge water); groundwater recharged by precipitation within the wetland (peat water); groundwater recharged by surface water in the wetland. Objective hydrological criteria are used to classify groundwater analyses according to recharge area. Oxygen-18 proved an essential criterion for the distinction between genetic groundwater types. It is therefore recommended to make oxygen-18 analysis standard procedure in geohydrological wetland research. Calcium concentrations are generally not suitable as tracers of different types of recharge areas. All groundwater types contain calcium. In contrast to what is generally assumed, lowest concentrations are observed in ridge water. Higher calcium concentrations are found in groundwater recharged in wetlands, especially in infiltrated surface water. Nutrient concentrations in both ridge and peat water are low, while those in infiltrated surface water are significantly higher. The main conclusion for nature conservation is that calcium-rich and mesotrophic conditions in the root zone can be realised not only by the seepage of groundwater from upland sandy recharge areas, as is generally thought, but also by the seepage of peat water. This is important as a severe deterioration in the quality of ridge water seeping upward on the river plain may be anticipated in the near future, which forms a threat to low-productivity rich fens. Peat water may offer an alternative water source for the conservation or regeneration of low-productivity rich fens.