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Rate‐limited U(VI) desorption during a small‐scale tracer test in a heterogeneous uranium‐contaminated aquifer
Author(s) -
Fox Patricia M.,
Davis James A.,
Hay Michael B.,
Conrad Mark E.,
Campbell Kate M.,
Williams Kenneth H.,
Long Philip E.
Publication year - 2012
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2011wr011472
Subject(s) - tracer , alkalinity , desorption , aquifer , groundwater , bicarbonate , chemistry , mass transfer , soil science , contamination , environmental chemistry , hydrology (agriculture) , adsorption , geology , mineralogy , environmental science , chromatography , geotechnical engineering , organic chemistry , ecology , physics , nuclear physics , biology
A tracer test was performed at the Rifle Integrated Field Research Challenge site to assess the effect of addition of bicarbonate on U(VI) desorption from contaminated sediments in the aquifer and to compare equilibrium and rate‐limited reactive transport model descriptions of mass transfer limitations on desorption. The tracer test consisted of injection of a 37 mM NaHCO 3 solution containing conservative tracers followed by down‐gradient sampling of groundwater at various elevations and distances from the point of injection. Breakthrough curves show that dissolved U(VI) concentrations increased 1.2–2.6‐fold above background levels, resulting from increases in bicarbonate alkalinity (from injectate solution) and Ca concentrations (from cation exchange). In general, more U(VI) was mobilized in shallower zones of the aquifer, where finer‐grained sediments and higher solid phase U content were found compared to deeper zones. An equilibrium‐based reactive transport model incorporating a laboratory‐based surface complexation model derived from the same location predicted the general trends in dissolved U(VI) during the tracer test but greatly overpredicted the concentrations of U(VI), indicating that the system was not at equilibrium. Inclusion of a multirate mass transfer model successfully simulated the nonequilibrium desorption behavior of U(VI). Local sediment properties such as sediment texture (weight percent <2 mm), surface area, cation exchange capacity, and adsorbed U(VI) were heterogeneous at the meter scale, and it was important to incorporate these values into model parameters in order to produce accurate simulations.