MODELING OF NITRATE LEACHING FROM ARABLE LAND INTO UNSATURATED SOIL AND CHALK - 1 - DEVELOPMENT OF A MANAGEMENT MODEL FOR APPLICATIONS OF SEWAGE-SLUDGE AND FERTILIZER

A mathematical model has been produced to examine the impact of sewage sludge and fertilizer application to arable land and the effect of di fferent crop regimes on the amount of nitrate leached to chalk groundw ater. Previous work on nitrate leaching has concentrated on either a s oil science or a hydrogeological approach with little overlap between the two, This study considered both fields to obtain an overall pictur e of the nitrate leaching process, IMPACT is a layered deterministic N -leaching model which predicts the nitrogen loads entering groundwater daily from arable land, and can be used as a management tool in devel opment of sludge application and agricultural policy. The model relate s nitrogen species movement resulting from the application of sewage s ludge and fertilizer to differing vegetation-soil-hydrogeological cond itions. Field data collected at three sites on the unconfined chalk aq uifer of East Anglia, England over a two and a half year period was us ed to produce an initial conceptual model and to constrain the mathema tical model during development. IMPACT simulates nitrogen and transpor t processes in the soil and unsaturated zone of the chalk. The nitroge n processes include: mineralisation of soil organic-N and sewage sludg e organic-N, nitrification; crop uptake; volatilization; denitrificati on; and N inputs from fertilizers and precipitation. A mixing cell met hod is used to model solute transport in both the soil and chalk. Matr ix flow and combined fissure-matrix Bow are considered for the chalk. The model enables examination of the relationship between the arable/h ydrogeological systems and the environmental implications of sludge ap plication and of different arable regimes. Results are of use in devel oping strategies for arable farming and sludge application in areas se nsitive to nitrate leaching. This Part 1 paper describes the model dev elopment approach. Results of associated modelling scenarios are prese nted separately in the associated Part 2 paper. (C) 1997 Elsevier Scie nce B.V.