Multispecies Reactive Transport in an Aquifer with Spatially Variable Chemical Conditions: Dispersion of Bromide and Nickel Tracers

By Kathryn M. Hess, James A. Davis, Jennifer A. Coston, and Douglas B. Kent

ABSTRACT

Dispersion of ground-water solutes was investigated as part of a multispecies reactive tracer test conducted under spatially varying chemical conditions in an unconfined, sewage-contaminated, sand-and-gravel aquifer on Cape Cod, Massachusetts. The fate and transport of two tracers, nonreactive bromide and reactive nickel (complexed with ethylenediaminetetraacetic acid (EDTA)), are reported here. About 14 percent of the aqueous nickel mass was lost throughout the test, probably due to reversible adsorption of the nickel-EDTA complex onto the iron and aluminum oxyhydroxide minerals coating the aquifer sediments. The two tracers traveled through the aquifer at constant velocities, but the nickel was retarded relative to the bromide (retardation factor, 1.2). The tracers showed little dispersion transverse to the direction of flow (vertical dispersivity, 0.5-3.8 millimeters; lateral dispersivity, 1.4-1.5 centimeters) and great dispersion in the direction of flow (longitudinal dispersivity, 1.1-2.2 meters). The nickel tracer cloud showed smaller longitudinal dispersion than the bromide cloud, possibly due to nonlinear adsorption that led to proportionally greater removal of mass at the edges of the tracer cloud where concentrations were lower. This smaller dispersion also may have resulted from greater adsorption in the shallow uncontaminated part of the aquifer that coincidentally had a higher flow velocity than the deep part of the aquifer. The bromide dispersion results are similar to those obtained in a test conducted previously in this aquifer. The similarity suggests that physical dispersion in this aquifer was spatially stationary at the scale of about 300 meters covered by these two overlapping tests.