The dynamic form of rainwater lenses in drained fens

Abstract

Inflow of nutrient poor alkaline groundwater is generally seen as a prerequisite for the development of species rich fen vegetation. Drainage and groundwater abstraction may lead to the development of so-called rainwater lenses in the upper groundwater of fens, which prevent upward seeping alkaline groundwater from reaching the fen root zone. This threatens the conservation of the species rich vegetation. To provide insight in rainwater lens dynamics as a basis for conservation and restoration of fen biodiversity, numerical groundwater flow model simulations were performed for a hypothetical case area in the Netherlands. The simulations indicate that drainage canal surface water levels mainly affect the phreatic level in the fens, while groundwater inflow fluxes mainly affect the thickness of the rainwater lens. Increasing thickness of a semi-confining surface layer results in a rise in phreatic level. As the phreatic level reaches the soil surface, the rainwater lens may split in two as a result of groundwater discharge windows, which come into existence in the centre of the parcel. The effect of seasonal recharge fluctuations on the rainwater lens appears small. Spells of dry and wet years have a considerably larger effect especially on the lower boundary of the rainwater lens, resulting in thinner lenses during spells of dry years. Most simulations show rainwater lens thicknesses well exceeding rooting depths of fen plants. This is inconsistent with the fact that fen vegetation is found in nature reserves for which the simulations are considered representative. Chemical buffer processes in the soil may explain this inconsistency, masking an acidification process under way. As buffer capacities become exhausted over time, acidification and fen deterioration will occur at a certain future moment. To arrive at more definite conclusions about this hypothesis, our simulations of convective groundwater flow need to be supplemented by modelling of hydrochemical processes in rainwater lenses, followed by verification at actual field locations.