HETEROGENEITIES IN GROUNDWATER GEOCHEMISTRY IN A SAND AQUIFER BENEATHAN IRRIGATED FIELD

The contamination of shallow aquifers by elevated nitrate concentratio ns is a common problem in many rural regions of the world. Aquifers un der irrigated land are especially susceptible to this type of contamin ation. An intensive three-dimensional investigation of water chemistry was undertaken in a shallow unconfined sand aquifer in an area of int ensive irrigation in Mason County, Illinois, in order to investigate p rocesses affecting water quality. Results reveal considerable heteroge neity in the aqueous chemistry in three spatial dimensions and tempora lly. Recharge is rapid in this system and the water chemistry of the r echarge water is variable both spatially and temporally, being especia lly influenced by agricultural practices. Nitrate concentrations are e levated in a zone between about 6 and 10 m beneath the surface, althou gh in certain areas and at certain times this zone was not found. The maximum nitrate concentrations in this zone were slightly greater than 20 mg 1(-1) as N, well. above the US Environmental Protection Agency' s maximum contaminant level (MCL) of 10 ms 1(-1). Nitrate was generall y absent both above and below this depth in the aquifer. Water relativ ely depleted in nitrate recharges the aquifer from the surface at the site, producing a zone of dilute water near tile water table. Beneath the plume. denitrification reactions are responsible for removing nitr ate from solution. probably mainly coupled to oxidation of sulfide min erals; tritium data suggest that vertical movement of solutes is rapid and thus there has been enough time to transport surface-applied fert ilizer to depths in excess of 30 m in the aquifer. This rapid vertical movement is almost certainly enhanced by intensive irrigation in the county. A number of aqueous species and chemical parameters (Ca, Mg, S r, Fe, Si, dissolved inorganic carbon (DIG), dissolved oxygen, total d issolved solids. and pH) are correlated with nitrate concentrations, p rimarily because, like nitrate, they are either a significant fraction of fertilizers of are redox-sensitive. Drinking water quality is gene rally not degraded by fertilizer applications in this area, because al most all drinking-water wells are screened well below the zone of elev ated nitrate concentrations.