, New Mexico Environment Dept. and Salvatore Misseri, M.A., P.E., New Mexico Environment Department
In oxic conditions, the nitrogen in septic tank effluent can oxidize to nitrate and contaminate ground water. Anoxic ground-water systems, however, occur along the inner
Rio Grande valley and in other areas of
New Mexico. High levels of ground-water nitrate, nitrite, ammonium, or total kjeldahl nitrogen are typically not detected in these areas, even after decades of septic tank discharges on small lots with shallow water tables. Elevated chloride, total dissolved solids, and dissolved organic carbon (DOC), however, demonstrate that septic tank effluents are impacting these anoxic aquifers. Potential subsurface nitrogen attenuation mechanisms include nitrification-denitrification, anaerobic ammonium oxidiation (anammox), ammonium sorption, and ammonia volatilization. Dissolved nitrogen gas, in excess of concentrations in equilibrium with the atmosphere, has been detected in some areas, suggesting that denitrification (anammox?) has occurred. Anoxic ground water impacted by septic tank effluent often contains dissolved ammonium-N in the range of 0.1 to 2.5 mg/L, and significant amounts of ammonium are believed to be partitioned, largely by cation exchange, to soil in these areas.
DOC transported to ground water in septic tank effluent can be oxidized by bacteria, which will increase the demand for, and consumption of, electron acceptors for respiration. Ground-water bacteria will preferentially respirate, and chemically reduce, dissolved oxygen, nitrate, geologic manganese and iron oxide minerals, sulfate and carbon, in that order, based on decreasing energy yield to the organism. Reduction of manganese and iron oxides releases soluble metal ions into ground water, and sulfate reduction creates hydrogen sulfide gas. These anaerobic respiration byproducts (ARBs) can create aesthetic and economic problems, and high levels of manganese in drinking water may present neurological risks.
Many regulatory programs for sewage discharges focus on protecting ground water from nitrate. Consideration also should be given to protecting ground water from ARBs if anoxic conditions are present.
