Evaluation of groundwater denitrification at a biosolids disposal site

A study was conducted at a sanitary sewage sludge (biosolids) disposal site in Springfield, Illinois, U.S.A. to determine if biological denitrification played a significant factor in attenuation of ground water nitrate values. The site selected for this study is a 23 ha (57 acre) dedicated biosolids disposal facility located adjacent to a 75.7 million liter per day (20 million gallons per day) municipal treatment plant that uses anaerobic solids stabilization for treatment of generated biosolids material. Biosolids have been disposed of by fixed-point spray applicators at the site since 1976, which has caused ground water nitrate levels to increase significantly above background levels. A method was developed using a conservative chemical tracer to simulate the biosolids application process and monitor the ground water directly beneath the simulated disposal site. Results demonstrated a net decline of nitrates that could not be attributed to dilution alone. While the monitoring methodology developed for this study did not directly estimate the denitrification rate, a rate for overall nitrate reduction was calculated that could be considered to take into account all transport and reduction mechanisms such as denitrification, advection, dispersion and dilution.