AbstractMultispecies Reactive Tracer Test in a Sand-and-Gravel Aquifer, Cape Cod, Massachusetts (EPA/600/R-01/007) March 2001
Part 1. Experimental Design and Transport of Bromide and Nickel-EDTA Traceres
Part 2. Transport of Chromium (VI) and Lead-, Copper-, and Zinc-EDTA Tracers
In this report, we summarize a portion of the results of a large-scale tracer test conducted at the U. S. Geological Survey research site on Cape Cod, Massachusetts. The site is located on a large sand and gravel glacial outwash plain in an unconfined aquifer. In April 1993, about 10,000 liters of groundwater from the site were injected into the shallow aquifer with bromide (Br), chromate (Cr(VI)), and four metals (lead (Pb), copper (Cu), nickel (Ni), zinc (Zn)) complexed with ethylenediaminetetraacetic acid (EDTA) added as tracers. At the time of the tracer test, shallow groundwater at the site was contaminated by secondary sewage effluent that was discharged since 1936 onto infiltration beds located about 240 m (meters) upgradient of the injection wells. Vertical concentration gradients in dissolved oxygen, pH, dissolved Zn, and other water quality parameters are caused by the mixing of the sewage effluent with ambient groundwater (Kent et al., 1994). The experiment was carried out such that the water with tracers would be injected across these vertical gradients in water quality. Thus, the mobility of the tracers under variable chemical conditions could be compared and the fate of the tracers in response to different chemical processes could be contrasted. The main objectives of the experiment were:
- to demonstrate the importance of chemical reactions, aqueous speciation, and variable background chemical conditions on the transport of selected toxic elements (Cr, Pb, Cu, Zn, Ni) in groundwater;
- to compare the rates and extents of chemical reactions observed in a field study with those measured under similar conditions in laboratory experiments;
- to examine the relationships between chemical and hydrologic processes in solute transport;
- to examine the importance of the spatial variability of geochemical properties and its impact on reactive transport;
- to investigate the problem of modeling flow coupled with chemical reactions in the groundwater environment with a view towards simplification of the geochemical reaction network;
- to contribute a well-characterized but chemically complex data set to the literature that would spur the development and/or application of hydrogeochemical transport models of flow coupled with chemical reactions; and,
- to examine the methods of parameter estimation used for describing reactive chemical processes during transport and to compare such parameters with those determined from laboratory, small-scale field, and modeling investigations.
This report discusses the experimental design and execution of the test and the quantitative description of bromide, EDTA, and nickel-EDTA transport during the test. Experimental results for the transport of the remaining tracers (Cr, Pb, Cu, and Zn) are presented in a companion report (Davis et al., 2000).