Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/29/2005
Publication Date: 10/9/2005
Citation: Israel, D.W., Showers, W.J., Fountain, M.O., Fountain, J. 2005. Nitrate movement in shallow ground water from swine-lagoon-effluent spray fields managed under current application regulations. Journal of Environmental Quality 34:1828-1842. Interpretive Summary: Movement of NO3 from lagoon-effluent spray fields managed under .0200 regulations to an adjacent first order stream in shallow groundwater was evaluated. Six transects of ground water monitoring wells were installed from the sprayfields fields through the riparian zone to the stream edge. Nitrate concentrations and 15N natural abundance of NO3 in groundwater and in the stream were measured over a 30 month period. These measurements showed that 6 years after initiating application of lagoon effluent to the sprayfields 15N natural abundance of NO3 in streamside wells was similar to 15N natural abundance of N in the effluent in four of six transects. 15N natural abundance of NO3 in the stream (16.8 +/- 3.6 per mil) was higher than expected for fertilizer N (<8 per mil)and approaching the values (20 to 30 per mil) indicative of effluent N. Nitrate concentrations in streamside wells of these transects ranged from 3 to 15 mg/L while NO3 concentrations in the stream averaged 2.0 +/- 1.2 mg/L . These results document that effluent N does move from sprayfields managed according to strict regulations to an adjacent first order stream in the shallow groundwater system. However, the large decrease in NO3 concentration between stream edge groundwater and the stream indicate attenuation of nitrate concentration by processes within the stream. This study represents the first assessment of the impact of .002 regulations on NO3 movement from fields receiving swine lagoon effluent and should help environmental regulatory agencies make rational assessments and/or modifications of regulations.
Technical Abstract: Movement of NO3 from lagoon-effluent spray fields managed under .0200 regulations to an adjacent first order stream in shallow groundwater was evaluated. The study area was a 260 hectare watershed in which swine production increased from 3,750 to 43,000 animals per year between 1993 and 1998. Transects of monitoring wells were installed and sampled in two spray fields and in riparian areas between these fields and an adjacent stream. The spray fields received 300 kg of plant available N /ha/yr from effluent for four years. Receiver crops were coastal bermudagrass either cut for hay or grazed. Wells were sampled monthly for 17 months and then quarterly for an additional 16 months. Nitrate comprised more than 95% of the inorganic N in groundwater under spray fields and riparian areas. Nitrate concentrations averaged 15 mg/L in groundwater from all wells in the spray fields. Nitrate concentrations averaged 30 mg/L at the top of the water table and decreased to an average of 8 mg/L at 1.2 to 2.4 m into the water table. Nitrate levels measured in the uppermost part of water table were about 2 fold higher than those measured in shallow groundwater under coastal plain fields in row crop production. For the six transects, NO3 concentrations in groundwater from riparian wells screened at 1 to 1.8 meters into the water table averaged 10 mg/L. In three of six transects, NO3 concentrations in groundwater at the edge of the stream decreased 92% compared to that under the spray fields and were less than 1.7 mg/L. In three transects, groundwater NO3 concentrations at the stream edge decreased only 17% relative concentrations in field wells and ranged from 3 to 15 mg/L. 15N natural abundances of NO3 from fertilizer and soil sources range from -5 to 8 per mil and that of N in lagoon effluent ranges from 20 to 30 per mil. Thus, measurement of 15N natural abundances can be used to determine which N sources contribute NO3 to groundwater beneath the riparian zone and to the stream if denitrification in the system is negligible. The correlation between 18O and 15N natural abundances in NO3 from shallow groundwater wells throughout the spray field-riparian-stream system was not significant. This indicates that denitrification did not contribute to the 15N enrichment of NO3 as it moved through the shallow groundwater system. Delta 15N values > 10 per mil showed that N from applied effluent was moving through the riparian zone in shallow groundwater at rates of 4.5 to 16 m/yr. At 30 month sampling, delta 15 N values for NO3 in groundwater from stream side wells of four transects were similar to that for NO3 in groundwater beneath the spray fields (20 to 25 per mil) indicating that most of the NO3 in these wells was derived from effluent N. Delta 15N values for NO3 in stream samples ranged from 12 to 20 per mil. These values are within the range expected for NO3 derived from effluent applied to adjacent fields. Therefore, after six years of application effluent N has accumulated in the stream. Nitrate concentrations in the stream during the 30-month period averaged 2.0 +/- 1.2 mg/L. Therefore, processes within in the stream appear to be attenuating the concentrations of nitrate as concentrations are much lower than concentrations in the shallow ground water.