Characterizing groundwater-stream interaction at catchment scale

Holtum stream is a low-land gaining stream situated in the Western part of Denmark and subjected to various investigations with the purpose of understanding the aquifer-stream interaction. The objectives have been; (i) to investigate the spatial and temporal variability in groundwater discharge along the stream network using differential gauging, temperature, and other tracers, (ii) to characterize the groundwater age distribution along the stream reach using CFCs, and (iii) relate differences in the groundwater age distribution to the observed groundwater discharge patterns. The investigations occur over a number of different spatial and temporal scales; (i) Plot investigations focusing on the daily and seasonal changes in groundwater discharge, (ii) spatial heterogeneity in groundwater discharge along stream reaches, and (iii) how groundwater outcrops to the stream network at the regional scale. We combine these field investigations with numerical flow modeling based on geophysical measurements to improve our geological models of the stream valley aquifers. Temperature sensors placed in the top/bottom of the water column and differential hydraulic head measurements along the stream network have identified groundwater discharge areas. The preliminary results show that the rate of discharge to the stream is very heterogeneous at all scales. At the regional scale there are stream reaches with significant discharge and even within these areas discharge may vary by an order of magnitude. In some cases this heterogeneity can by explained by local heterogeneity in stream bed characteristics, but not always. The landscape and regional hydrogeology play an important role which is delineated by the groundwater age distribution. The geological/geomorphological models are included in the numerical model to investigate these controlling factors of the aquifer-stream interaction at catchment scale.