THE IMPACT OF SWINE LAGOON SEEPAGE ON SHALLOW GROUNDWATER QUALITY : GROUNDWATER REMEDIATION THROUGH LAGOON CLOSURE AND PUMPING
Anaerobic lagoons have been used extensively since about 1960 to temporarily store and partially treat livestock waste from concentrated animal production facilities. An earlier study characterized an unlined swine lagoon located in the Middle Coastal Plain of North Carolina and detected elevated mineral nitrogen concentrations (nitrogen as both nitrate and total ammoniacal nitrogen) 38 m down gradient from the lagoon. As this site was shown to be a potential source of nonpoint-source pollution, further study was needed to characterize the aerial extent of the seepage plume and remediate the contaminated groundwater. Hydrologic and water quality monitoring of the site was conducted from March 1999 to September 2004, a period during which the lagoon went from being semiactive to completely closed out. Analysis of data collected prior to lagoon closure indicated that seepage from the lagoon traveled to a nearby channelized stream. Total ammoniacal nitrogen (TAN = NH3-N + NH4-N) concentrations from monitoring wells installed between the lagoon and the channelized stream averaged 121 mg L-1 of TAN, with the highest concentrations exceeding 170 mg L-1. TAN concentrations in the stream 150 m down gradient from the lagoon ranged from 10 to 25 mg L-1. Monitoring of groundwater levels and subsurface water quality continued following the closure of the lagoon in March 2001, as well as through the installation of a groundwater pumping system and construction of a wetland that received and treated the contaminated groundwater. The average hydraulic gradient decreased from 0.0033 m m-1, prior to the activation of a groundwater pumping system (Mar. 1999 to Jan. 2000) to 0.0010 m m-1 after pumping was initiated (Sept. 2000 to Sept. 2004). Although the direction of groundwater flow was unaffected by the addition of the pumping system and water control structure, the average hydraulic gradient was reduced by 70%. Since the installation of the pumping system, approximately 17,900 m3 of subsurface water has been pumped, removing 1200 kg of TN from the groundwater plume. TAN concentrations steadily declined from 120 mg L-1 at the onset of pumping in September 2000 to less than 6 mg L-1 by September 2004.