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Publication Abstract

Impact of Irrigation on the Dynamics of Nitrate Movement in a Shallow Sand Aquifer. Kelly, Walton R., and Chittaranjan Ray, 1999  Illinois State Water Survey, Champaign, IL,  ISWS RR-128    Full Text Available
A field-scale project in Mason County, Illinois, was performed to monitor the movement of nitrate in ground water beneath an irrigated field. Chemical tracers were used to assess the migration of solutes both laterally and vertically under the influence of an irrigation well and to determine the amount of recycling at a site due to irrigation pumpage and the amount of off-site transport of nitrate due to regional ground-water flow.

Water samples from the sand aquifer at the site reveal considerable spatial and temporal heterogeneity in aqueous chemistry. Recharge is rapid in this system, and it is probable that the water chemistry of the recharge water also is variable spatially and temporally; it is especially influenced by agricultural practices. Nitrate (NO3-) concentrations are elevated in a zone between approximately 15 and 30 feet (ft) beneath the surface, although this zone was not persistent laterally or with time. The maximum nitrate concentrations in this zone were slightly greater than 20 milligrams per liter (mg/L) as nitrogen, well above the drinking water standard of 10 mg/L. Nitrate was generally absent below 30 ft in the aquifer, probably due to denitrification reactions. The tritium data suggest that vertical movement of solutes in the aquifer is rapid, and that there has been enough time to transport solutes from the surface or soil zone to depths in excess of 100 ft. Because drinking-water wells generally are screened well below the zone of elevated nitrate concentrations in this area, it appears that fertilizer applications do not have a negative effect on drinking-water quality for most homeowners.

From the results of tracer tests, the effects of irrigation pumping on solute transport are measurable but not substantial. Tracer movement both horizontally and vertically was slight under pumping conditions, less than 10 ft horizontally and between 1 and 2 ft vertically about 100 ft from the irrigation well after three days of pumping. The vast majority of nitrate applied in this area is not being recycled through the irrigation wells.

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