A concept for estimating depth of the redox interface for catchment-scale nitrate modelling in a till area in Denmark

A concept was developed for distributing the depth of the redox interface at catchment scale in tills for use in nitrate modelling. The hypothesis was that the redox interface has been developing since the beginning of the Holocene due to oxidation of inherent reduced compounds in the sediment by oxygen in recharging water. Key to the concept is estimation of the spatial pattern of the redox interface, inferred from variability in groundwater recharge and the amount of reduced compounds in the sediments. The concept was tested for the 101-km ² Norsminde fjord catchment in Denmark using a catchment-scale nitrate model, where two parameters controlling the estimated redox interface were calibrated based on the observed amount of nitrate transported to the fjord. Estimated depths of the redox interface across the catchment enabled reproduction of the cumulative distribution of measured depths across the catchment, but they did not fit well with the observations at individual wells. It is therefore concluded that the concept may be useful for simulation of nitrate transport and reduction in the saturated zone at catchment scale, but it is not able to correctly predict nitrate reduction at point scale.