AGRICULTURAL NITROGEN CONTAMINATION IN DELAWARE’S COLUMBIA AQUIFER

Excessive nutrient loads derived from anthropogenic activities including domestic, municipal, industrial, and agricultural land uses continue to threaten the health of bays and estuaries worldwide. Delaware’s Inland Bays are subjected to high nitrogen loads originating in large part from agricultural activities. Because southern Delaware is underlain by predominately sandy, porous sediments, precipitation rapidly permeates the subsurface, carrying with it agriculturally-derived nitrogen. Consequently, groundwater exerts a powerful influence on the health of the Inland Bays.

Using lysimeters and piezometers, we collected groundwater samples from the vadose zone and surface of the Columbia aquifer beneath an agricultural field and beneath the shoreline of adjacent Indian River estuary. Samples were taken during the cultivations of corn, soybeans, wheat, and winter wheat; each receives a different fertilizer regimen. Nitrate and dissolved oxygen concentrations, oxidation reduction potentials, and the isotopic signatures of nitrogen and oxygen in nitrate were analyzed to determine nitrogen loading from the field to the estuary, nitrogen losses via denitrification, and to confirm the agricultural nitrogen source.

Groundwater nitrate concentrations beneath the field and shoreline ranged from 113 to 1030 μM with no indication of losses through the shallow aquifer. Dissolved oxygen concentrations and oxidation reduction potentials were uniformly too high to promote denitrification – on average 7 mg/L and 175 mV respectively. Isotopic signatures in all shallow aquifer samples indicated a synthetic fertilizer nitrogen source. Our data suggest that the shallow aquifer behaves like a pipe, channeling nitrogen recharged through the agricultural field into the estuary with no attenuation via denitrification. Additional samples collected from the deep aquifer however, yielded nitrate concentrations, dissolved oxygen concentrations, oxidation reduction potentials, and isotopic signatures indicative of denitrification.