Effects of sludge disposal on groundwater nitrate concentrations

More than 100 groundwater samples were collectd and analyzed for nitrate-nitrogen, δ¹⁵N of the nitrate, dissolved organic carbon (DOC), and chloride. Multilevel samplers and nested monitoring wells were located beneath and down-gradient from an irrigated cornfield on which human waste sludge was injected. The sampling delineated a 1.3 km × 0.3 km plume of nitrate contamination. Both the nitrate-nitrogen concentrations and the δ¹⁵N values within the plume's centroid were homogeneous. The levels were 34 ± 3 mg 1^-1 and + 13.4 ± 1.2%, respectively. A retarding zone of clayey silt split the plume and separated the oxic water from the deeper anoxic water. Nitrate levels were lower in the anoxic water and declined rapidly with depth. The significant association (r = - 0.91) between increasing δ¹⁵N values and decreasing nitrate concentrations indicated that the nitrate was denitrified. High chloride concentrations in the anoxic zone beneath the retarding layer are thought to originate from the sludge storage lagoon and/or the sludge compost piles. Tritium and atrazine levels confirm that this is recent recharge water. Denitrification has utilized most of the original nitrate and DOC in the plume.