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ARS Home » Plains Area » Lincoln, Nebraska » Agroecosystem Management Research » Research » Publications at this Location » Publication #199494
Title: Ammonium transport and reaction in contaminated groundwater: Application of isotope tracers and isotope fractionation studies

Author
item BOHLKE, J - U.S. GEO SUR/RESTON VA
item SMITH, RICHARD - U.S.GEO SURV/BOULDER CO
item Miller, Daniel

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2006
Publication Date: 8/11/2006
Citation: Bohlke, J.K., Smith, R.L., Miller, D.N. 2006. Ammonium transport and reaction in contaminated groundwater: Application of isotope tracers and isotope fractionation studies. Water Resources Research. 42:W05411.

Interpretive Summary: The movement of nitrogen through aquifers in contaminated groundwaters is complex and hard to predict due to its potential to transform into a variety of compounds which interact to varying degrees with aquifer substrate. In this report, the movement of nitrate, ammonium, and nitrogen gas along with the abundance of natural N-isotopes and the results of an isotopic tracer test with ammonium are used to describe the long-term movement and fate of nitrogen in a sewage-contaminated aquifer. Ammonium within the core of the plume was unreactive and moved a quarter of the rate of groundwater flow. At the upper interface of the ammonium plume where oxygenated water was present, there was evidence for a slow conversion of ammonium to nitrate, which subsequently moved at the same rate as groundwater flow. Without intervention, the bulk of the ammonium would eventually discharge with little conversion to nitrate or nitrogen gas.

Technical Abstract: Ammonium (NH4 +) is a major constituent of many contaminated groundwaters, but its movement through aquifers is complex and poorly documented. In this study, processes affecting NH4 + movement in a treated wastewater plume were studied by a combination of techniques including large-scale monitoring of NH4 + distribution; isotopic analyses of coexisting aqueous NH4 +, NO3 ', N2, and sorbed NH4 +; and in situ natural gradient 15NH4 + tracer tests with numerical simulations of 15NH4 +, 15NO3 ', and 15N2 breakthrough data. Combined results indicate that the main mass of NH4 + was moving downgradient at a rate about 0.25 times the groundwater velocity. Retardation factors and groundwater ages indicate that much of the NH4 + in the plume was recharged early in the history of the wastewater disposal. NO3 ' and excess N2 gas, which were related to each other by denitrification near the plume source, were moving downgradient more rapidly and were largely unrelated to coexisting NH4 +. The '15N data indicate areas of the plume affected by nitrification (substantial isotope fractionation) and sorption (no isotope fractionation). There was no conclusive evidence for NH4 +-consuming reactions (nitrification or anammox) in the anoxic core of the plume. Nitrification occurred along the upper boundary of the plume but was limited by a low rate of transverse dispersive mixing of wastewater NH4 + and O2 from overlying uncontaminated groundwater. Without induced vertical mixing or displacement of plume water with oxic groundwater from upgradient sources, the main mass of NH4 + could reach a discharge area without substantial reaction long after the more mobile wastewater constituents are gone. Multiple approaches including in situ isotopic tracers and fractionation studies provided critical information about processes affecting NH4 + movement and N speciation.