Submitted to: Microbial Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/3/2004
Publication Date: 8/1/2006
Citation: Smith, R.L., Baumgartner, L.K., Miller, D.N., Repeter, D.A., Bohlke, J.K. 2006. Assessment of nitrification potential in ground water using short term, single-well injection experiments. Microbial Ecology (51:22-35) Interpretive Summary: The conversion of ammonium to nitrate (nitrification) may affect the movement of nitrogen through aquifers. Oxygen and ammonia concentrations at a site on Cape Cod, MA, indicates that nitrification may be occurring within a narrow zone in a wastewater-derived contaminant plume. A series of short-duration tracer tests reinjecting groundwater and added constituents (ammonium, nitrite, oxygen, or difluoromethane) at the putative nitrification zone showed that nitrification was occurring in that zone. The potential rates of nitrification were higher than expected, which was likely due to enhanced groundwater mixing during injection.
Technical Abstract: Nitrification was measured within a sand and gravel aquifer on Cape Cod, MA, using a series of single-well injection tests. The aquifer contained a wastewater-derived contaminant plume, the core of which was anoxic and contained ammonium. The study was conducted near the downgradient end of the ammonium zone, which was characterized by inversely trending vertical gradients of oxygen (270 to 0 'M) and ammonium (19 to 625 'M) and appeared to be a potentially active zone for nitrification. The tests were conducted by injecting a tracer solution (ambient ground water + added constituents) into selected locations within the gradients using multilevel samplers. After injection, the tracers moved by natural ground water flow and were sampled with time from the injection port. Rates of nitrification were determined from changes in nitrate and nitrite concentration relative to bromide. Initial tests were conducted with 15N-enriched ammonium; subsequent tests examined the effect of adding ammonium, nitrite, or oxygen above background concentrations and of adding difluoromethane, a nitrification inhibitor. In situ net nitrate production exceeded net nitrite production by 3- to 6- fold and production rates of both decreased in the presence of difluoromethane. Nitrification rates were 0.02–0.28 'mol (L aquifer)'1 h'1 with in situ oxygen concentrations and up to 0.81 'mol (L aquifer)'1 h'1 with non-limiting substrate concentrations. Geochemical considerations indicate that the rates derived from single-well injection tests yielded overestimates of in situ rates, possibly because the injections promoted small-scale mixing within a transport-limited reaction zone. Nonetheless, these tests were useful for characterizing ground water nitrification in situ and for comparing potential rates of activity when the tracer cloud included non-limiting ammonium and oxygen concentrations.