Environ. Sci. Technol. 2000, 34, 1679-1686
Comparison of Batch and Column Methods for Determining Strontium Distribution Coefficients for Unsaturated Transport in Basalt INDREK PORRO,* MEREDITH E. NEWMAN,† AND FRANK M. DUNNIVANT‡ Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, Idaho 83415-2107
Contaminant distribution coefficients determined under saturated conditions are often used to model transport under unsaturated conditions. Although the distribution coefficients are assumed to be consistent under different moisture conditions, this is rarely tested. Column and batch adsorption tests were used to determine strontium distribution coefficients in crushed basalt. Column tests were conducted at saturated and unsaturated moisture contents. Batch tests were conducted at several solid/liquid ratios. Preliminary column tests using bromide as a conservative tracer indicated the presence of immobile water in the column and pointed toward the use of a two-region model to determine the Sr distribution coefficients. Use of a single-region model (no immobile water), however, resulted in an average value (3.09 mL/g) not significantly different than the average value determined using the two-region model (3.41 mL/g). Moisture content had no significant effect on Sr distribution coefficients determined by applying either model to the column test data. The batch test distribution coefficient determined at the recommended standard solid/liquid ratio (0.25 g/mL) was less than the column test values and decreased significantly with increasing solid/liquid ratio. The results indicate that Kds determined with this method will not accurately reflect Sr transport in unsaturated or saturated basalt.
Introduction Wastewater containing strontium-90 has been discharged to onsite infiltration ponds and disposal wells at the Idaho National Engineering and Environmental Laboratory (INEEL) since the early 1950s. Water samples collected from selected wells penetrating the Snake River Plain (SRP) aquifer underlying the site show strontium-90 concentrations exceeding the maximum contaminant level (MCL) of 8 pCi/L (1, 2). Numerous wells are used to track the strontium-90 plume in the aquifer, but tracking the strontium-90 in the vadose zone is more difficult. The vadose zone at the INEEL ranges in thickness from 61 to 275 m and comprises numerous basalt layers and sedimentary interbeds. In view of this * To whom correspondence should be addressed. E-mail:
[email protected]; phone: (208) 526-0906; fax: (208) 526-0875. † Present address: Department of Chemistry, Hartwick College, Oneonta, NY 13820. ‡ Present address: Whitman College, Chemistry Department, Boyer Avenue, Walla Walla, WA 99362. 10.1021/es9901361 CCC: $19.00 Published on Web 03/17/2000
2000 American Chemical Society
difficulty, modeling is often used to determine the behavior of strontium-90 in the vadose zone. Transport models used to simulate solute movement in porous media often use the distribution coefficient, Kd (mL/g), to describe the partitioning of the solute between solid and liquid phases. Although the accuracy of the distribution coefficient as a means for describing the interaction of a chemical species with subsurface material has been a subject of intense debate (3-8), it is still the primary method in use today. Of the many questions surrounding the use of the distribution coefficient, the one that has received the least attention may be the effect of soil moisture content. Previous work in this area has dealt primarily with flow model development or sorption of organic contaminants (9-14). Varying soil moisture content may influence the distribution coefficient of cations, such as strontium, in a variety of ways. For example, at low moisture content, transport of the species to the porous media surface by diffusion may be slower and thus more important than the sorption rate. For heterogeneous surfaces, the fact that the portion of the surface available for sorption is reduced at low moisture contents may also alter the type of surface sites available for sorption (clay, iron oxide, organic matter, etc.). Finally, micropores and other limited flow regions may cause concentration gradients which, in turn, may alter the speciation of the contaminant. Distribution coefficients are determined in the laboratory most commonly using either column (15) or batch (16) techniques. The column technique involves packing a small column with the relevant geologic material and allowing water with a contaminant pulse to flow through the column under controlled flow (usually saturated) conditions. Analysis of the resulting contaminant breakthrough curve (BTC) leads to determination of the Kd. The batch technique involves adding a known amount of adsorbate in an aqueous solution to an adsorbent and agitating the mixture until equilibrium is achieved. The mixture is centrifuged and the concentration of the contaminant in the supernatant is determined. Any adsorbate missing from the supernatant is assumed to be adsorbed. A simple calculation then leads to the determination of the Kd. Column tests have the advantage of being conducted under conditions approximating those observed in the field. Specifically, (1) columns may be packed to have bulk densities and porosities similar to those observed in the field, (2) column tests are conducted under advective and dispersive flow conditions similar to those in the field, and (3) column tests may be conducted under a variety of different moisture contents. Column tests have the disadvantage of being quite time consuming and of commingling chemical and physical transport processes. Batch tests have the advantage of being relatively simple and fast. They are generally the method of choice for strongly sorbing contaminants due to the extremely long time requirements of column tests for such contaminants. Batch methods are currently the only methods available for determining sorption rate constants, because chemical processes cannot be completely separated from physical processes in column tests. Batch tests have the disadvantage of being conducted under conditions far removed from those observed in the field. Specifically, they are carried out under static conditions (no advective flow) and at solid/liquid ratios much lower than present in the field. Strontium (Sr) Kds have been determined for sediments (17-20) and basalt (19) at the INEEL. Strontium Kds in basalt ranged from 1.1 to 2.7 mL/g. Strontium Kds have also been determined for other geologic media at other sites (21-26). VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1679
TABLE 1. Physical and Chemical Characteristics of the Crushed Basalt characteristic
value
NH4Oac extractable Mn (ppm) total CEC (meq/100 g) particle size used (mm) major minerals
1.0 6.05 0.25-2.0 pyroxene (30-50%) plagioclase (30-50%) olivine (15-30%) magnetite (
