Groundwater dynamics and channel activity as major controls of in-stream nitrate concentrations in a lowland catchment system?

Abstract

Summary In-stream nitrogen concentrations of a small agricultural lowland study catchment in Saxony-Anhalt, Germany, show high seasonal dynamics. Field data lead to the hypothesis that these dynamics are controlled by groundwater level fluctuations and the associated falling dry and rewetting of intermittent channels and not by seasonal variations of soil nitrogen leaching processes. To test this hypothesis a hypothetical model experiment was carried out using MODFLOW and MT3DMS. We simulated steady state and transient flow and nitrate transport in a two-dimensional model transect, driven by average and monthly lysimeter data of recharge and nitrate leaching. Falling dry and rewetting of open channels was accounted for. The transient groundwater flow model can capture the supposed mechanisms quite well. For conservative transport, the resulting concentration changes are in the order of 25 mg/l NO 3 -N, for the reactive transport simulation, the concentration ranges was still in the order of 12 mg/l NO 3 -N. It can therefore be concluded, that the dynamics of groundwater flow considerably control the dynamics of in-stream nitrate concentrations. The results of the steady state simulation based on averaged inflows are not equivalent to the averaged outflows given by the transient simulation, reflected in different loads. In such a dynamic system the steady state flow approach is not an acceptable representation of the system. Observed data and simulations showed similar relations of in-stream nitrate concentrations to groundwater level and discharge. Minor differences were explained by the simplified two-dimensional modelling concept in contrast to a spatially distributed, three-dimensional real-world catchment system. In a real-world system further processes superpose the assumed hydrological controls, such as in-stream or hyporheic zone denitrification, contribution of tile drains and observed fertilization practice.