Enhanced in situ denitrification for a municipal well

In 37% of small community systems in Nebraska at least one sample exceeded the drinking water standard for nitrate of 10 mg N L−1 during the period from 1982 to 1998 (US Bureau of Reclamation, US Department of the Interior, 1999). In this experiment a daisy well system was designed to promote denitrification in the radial capture zone of a municipal well with nitrate-N levels>10 mg L−1, and thereby bring the nitrate concentration into compliance. The remediation design consisted of eight 15 cm diameter outer perimeter reduction wells and eight 5 cm diameter inner perimeter oxidation wells which are located roughly 18 and 9 m, respectively, from the municipal well, which serves as the extraction well. Endemic microbes are stimulated by pulsing separate injections of acetate-C and nitrate contaminated water (C:N=1.2) to enhance denitrification in the capture zone. Water was extracted from the municipal well at 6.6 L s−1 (liters per second). A 45% nitrate reduction occurred in municipal well samples when the total acetate-C input was increased by lengthening the acetate pulse from 1.0 to 1.5 h (C:N=1.8). Nitrate concentration stabilized at about 6.3 mg NO3-N L−1 for two weeks during alternating acetate pulse lengths. The in situ denitrification process was sustained for three months without evidence of clogging. Results from this experiment indicated that the extracted water was in compliance with respect to nitrate, nitrite, trihalomethanes, turbidity, and total and fecal coliforms; however, the total plate count exceeded the maximum permissible limit (500 cfu/mL).