Groundwater flow and transport of nutrients through a riparian meadow - Field data and modelling

Groundwater flow and nutrient transport were studied in a riparian meadow during a three-year period. The meadow is situated along a first order stream in the River Gjern catchment area, Jutland, Denmark. Field data included measurements of hydraulic head, hydraulic conductivity and soil characteristics. Groundwater sampled from piezometers was analysed for nitrate, ammonium and phosphate. Nitrogen and phosphorus contents in above-ground plant biomass were also measured. For the interpretation of our data we developed a one-dimensional hydraulic-transport model for the lateral groundwater flow, transport of nitrate, and nitrate removal in the meadow. The model is based on Darcy's equation, and input data are horizontal and vertical distances, hydraulic heads, hydraulic conductivities, and nitrate concentrations. We also developed a scheme for evaluating uncertainties of the modeling results. Annual removal of nitrate in the saturated zone of the riparian meadow was 326, 340, and 119 kg NO3--N ha(-1) y(-1) (59-68% of groundwater input) through the three-year period. The largest nitrate removal took place outside the growing seasons. Net loss of ammonium from the saturated zone was 0.4, 6.7, and 10.3 kg NH4+-N ha(-1) y(-1). In two of the years this was counter-balanced by atmospheric nitrogen deposition. Phosphate was not retained during the first two years but lost at rates of 0.88 and 0.36 kg P ha(-1) y(-1). In year 3 phosphate retention was 0.47 kg P ha(-1) y(-1). These data show how a riparian ecotone along a first order stream can reduces nitrogen pollution from agricultural areas. Also, the pronounced year to year variations in our nutrient budgets show that shorter studies, for example based on one year of observations, should be interpreted cautiously as representing a general picture of nutrient pathways. (c) 2006 Elsevier B.V. All rights reserved.