Use of geostatistics to determine spatial variation in pesticide leaching - Preliminary findings

The risk of pesticides leaching into ground water depends on the characteristics of the soil that affect their adsorption and degradation, and on water and solute fluxes in the soil. The study site is in a groundwater protection zone in the Thames Valley (England) where the soil is a sandy loam overlying gravel. Studies have shown that there is a strong linear relation between soil organic carbon content and the adsorption characteristics of atrazine. Using existing data sets for hydrodynamic properties and organic C measured at the same site we have used geostatistical techniques to interpolate and extrapolate to unsampled locations within a field. Soil organic matter (SOM) was measured by loss on ignition for the entire field and mean pore water velocity (V) and the solute dispersion coefficient (D) were measured in a 48 cm by 48 cm area that was adjacent. There were 64 drainage collectors in the latter area and each had a support of 6 cm by 6 cm. Variograms were computed for the three variables which suggested that the variation in leaching can be both very local, over distances of less than a metre, and can occur over much longer distances of hundreds of metres. The variograms were used with the data to produce kriged estimates of V, D and SOM for mapping. Conditional Gaussian sequential simulation of V and D, using the variogram models and the available data, was used to obtain information from a larger area which could be used as an input to the LEACHP model. Assuming a relation between the hydrodynamic properties of the soil and SOM these results suggest that there are at least two important scales of spatial variation in the pattern of leaching. This has implications for future sampling to take account of leaching at these very different spatial scales.