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WATER RESOURCES RESEARCH,
VOL. 42,
W05411,
doi:10.1029/2005WR004349,
2006
Ammonium transport and reaction in contaminated groundwater: Application of isotope tracers and isotope fractionation studies
J. K. Böhlke
U.S. Geological Survey, Reston, Virginia, USA
Richard L. Smith
U.S. Geological Survey, Boulder, Colorado, USA
Daniel N. Miller
U.S. Geological Survey, Boulder, Colorado, USA
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 δ15N 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.
Received 12
June
2005;
accepted 23
January
2006;
published 9
May
2006.
Keywords: ammonium;
sorption;
nitrification;
denitrification;
anammox;
N isotopes.
Index Terms: 0412 Biogeosciences: Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912); 0454 Biogeosciences: Isotopic composition and chemistry (1041, 4870); 0469 Biogeosciences: Nitrogen cycling; 1803 Hydrology: Anthropogenic effects (4802, 4902); 1831 Hydrology: Groundwater quality.
Read Full Article (file size: 1895896 bytes) Cited by
Citation: Böhlke, J. K., R. L. Smith, and D. N. Miller
(2006),
Ammonium transport and reaction in contaminated groundwater: Application of isotope tracers and isotope fractionation studies,
Water Resour. Res.,
42,
W05411,
doi:10.1029/2005WR004349.
This paper is not subject to U.S. copyright. Published in 2006 by the
American Geophysical Union.
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