Stochastic analyses of field‐scale pesticide leaching risk as influenced by spatial variability in physical and biochemical parameters

Abstract

Field‐scale pesticide leaching risk assessments were performed by incorporating a numerical, one‐dimensional, water and pesticide transport and fate model into the two‐step stochastic modeling approach by Loll and Moldrup [1998]. The numerical model included first‐order pesticide degradation, linear equilibrium adsorption, and plant uptake of water and pesticide. Simazine was used as a model pesticide, and leaching risk was expressed as the cumulative mass fraction of applied pesticide leached below 100 cm after 1 year. Spatial variability in soil physical and biochemical data, as well as measured meteorological data from an average and a relatively wet year, was considered for two Danish field sites: (1) a coarse sandy soil, with relatively small variability in hydraulic properties, and (2) a sandy loam, with large variability in hydraulic properties. The two‐step stochastic modeling approach was used to investigate the relative impact of spatial variability in saturated hydraulic conductivity Ks, soil‐water retention through the Campbell [974] soil‐water retention parameter b, and pesticide sorption through the organic carbon content (OC). For the coarse sandy soil, field‐scale spatial variability in OC was the single most important parameter influencing leaching risk, whereas for the sandy loam, Ks was found more important than OC. The relative impact of field‐scale spatial variability in these parameters was found independent of the meteorological conditions, whereas the absolute level of leaching risk was highly dependent on the meteorological conditions. Assuming a linear dependency between pesticide half‐life and OC, a unified approach to modeling simultaneous field‐scale variability in biodegradation and adsorption was proposed. Leaching risk assessments based on this approach showed that the parts of the field with both low biological activity and low adsorption capacity contributed with a dramatic increase in leaching risk, and suggested that field‐scale spatial variability in biochemical processes can be of similar or larger importance than both hydraulic properties and meteorological conditions.