Transport of Agricultural Contaminants in Sand Aquifers Affected by Drainage Ditches

Abstract: The objectives of the project described in this report were to develop an improved understanding of the hydrologic and chemical factors affecting the fate of agricultural contaminants in sand aquifers and to evaluate the effectiveness of drainage ditches as passive barriers to contaminant migration. These objectives were addressed through detailed field studies conducted in the vicinity of an agricultural drainage ditch. The field studies included extensive sampling and hydraulic testing to characterize hydrogeologic and chemical properties, a conservative tracer experiment to delineate flow paths and groundwater velocities, and a reactive tracer experiment to evaluate the rate of in-situ denitrification. The field studies were accompanied by evaluations of existing models of ditch capture depth and aquifer dispersivity. At the field site, variations in hydraulic conductivity appear to generate stratification of flow and of groundwater chemistry. Comparison of hydraulic conductivity measured by slug tests with grain size distributions determined for vibracore and auger samples provides the basis for evaluating magnitudes of hydraulic conductivity variations within the aquifer. These variations can be used to estimate the anisotropy ratio. Chemical signatures, particularly the calcium/magnesium ratio, may be useful as “natural tracers” for mapping flow lines at the field site.