Effect of groundwater abstraction on fen ecosystems

Abstract

Quantifying the effects of groundwater abstraction on fen ecosystems located in discharge areas can be complicated. The water level in fens is close to the terrain surface most of the year and it is controlled by a relatively constant groundwater exfiltration. It is difficult to measure the exfiltration fluxes and thus water level data is typically used to evaluate if the ecosystem is affected. The paper presents collected data and analysis from a case study, where the hydrological effect of groundwater abstraction on rich fens and springs in a Danish river valley has been studied. The natural hydrological conditions have been monitored intensively since March 2007. In the early spring 2009 two full scale pumping test were conducted in the regional high yield limestone aquifer. The evaluation focuses on three isolated fens covering an area of approximately 5000 m2 each and two natural springs all within a distance of 1.5 km to a planned well field. In the river valley the interaction between groundwater and surface water is strongly affected by low permeable sediments. These sediments reduce the direct discharge to the river and have a large impact on the functioning and presence of the rich fen and spring habitats. Continuous water level data from deep and shallow wells in the fens reflect the hydrological conditions. Large differences in water level drawdown during dry summer periods have been observed in the monitored fens and it is hypothesised, that these differences are directly related to the groundwater seepage rate. Two pumping tests with duration of 3 weeks were performed at two different locations and the effects were monitored with 20 pressure transducers in monitoring wells and by continuous flow logging in springs in the area. A significant reduction of the flow in the springs has been measured while the on the water level in the fens is difficult to distinguish from the natural variations. It is concluded, that hydrological modelling is a necessary tool in order to predict the long term effects of pumping in the fen areas. The collected data gives valuable knowledge on the hydrology of the monitored ecosystems and the response towards groundwater abstraction and forms a solid foundation for hydrological modelling.