Large scale modelling of groundwater contamination from nitrate leaching

Groundwater pollution from non-point sources, such as nitrate from agricult ural activities, is a problem of increasing concern. Comprehensive modellin g tools of the physically based type are well proven for small-scale applic ations with good data availability, such as plots or small experimental cat chments. The two key problems related to large-scale simulation are data av ailability at the large scale and model upscaling/aggregation to represent conditions at larger scale. This paper presents a methodology and two case studies for large-scale simulation of aquifer contamination due to nitrate leaching. Readily available data from standard European level databases suc h as GISCO, EUROSTAT and the European Environment Agency (EEA) have been us ed as the basis of modelling. These data were supplemented by selected read ily available data from national sources. The model parameters were all ass essed from these data by use of various transfer functions, and no model ca libration was carried out. The adopted upscaling procedure combines upscali ng from point to field scale using effective parameters with a statisticall y based aggregation procedure from field to catchment scale, preserving the areal distribution of soil types, vegetation types and agricultural practi ces on a catchment basis. The methodology was tested on two Danish catchmen ts with good simulation results on water balance and nitrate concentration distributions in groundwater. The upscaling/aggregation procedure appears t o be applicable in many areas with regard to root zone processes such as ru noff generation and nitrate leaching, while it has important limitations wi th regard to hydrograph shape due to its lack of accounting for scale effec ts in relation to stream aquifer interaction. (C) 1999 Elsevier Science B.V . All rights reserved.