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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 K ds determined with this method will not accurately reflect Sr transport in unsaturated or saturated basalt.

In a now classic study, Zinke (1962) showed that a single Pinus contorta tree growing on a sand dune along the coast of California modified the chemistry of the soil underneath its crown. He found distinct patterns of pH, exchangeable cations and nitrogen (N) content moving from the bole outward to the crown drip zone, because the acidic bark and stemflow were concentrated around the bole (Zinke 1962). Subsequent studies in temperate forests have also found tree species to affect soil chemical properties such as pH, organic carbon (C) and rates of N mineralization (Boerner & Koslowsky 1989, Boettcher & Kalisz 1990, Finzi et al. 1998). Presumably, these species-specific effects are caused by inter-specific differences in organic acid exudation, nutrient uptake, litter quality or quantity, decomposition rates or nutrient outputs (Binkley & Giardina 1998, Knops et al. 2002, Rhoades 1997). Regardless of the causes, species-generated soil heterogeneity has implications for stand-level estimates of biogeochemical processes such as soil C storage and N-cycling as well as implications for plant diversity and regeneration (Finzi et al. 1998). Although a number of studies have demonstrated that tree species modify soil environments in temperate forests or monospecific tree plantations in the tropics (Fisher 1995, Rhoades 1997), few studies have investigated these processes in species-rich tropical forests (but see Rhoades et al. 1994).

A coupled field‐scale aquifer pumping and water infiltration test was conducted at the Idaho National Engineering and Environmental Laboratory in order to evaluate subsurface water and contaminant transport processes in a heterogeneous flow system. The test included an aquifer pumping test to determine the storage properties of the aquifer and the state of confinement of the aquifer (∼190 m below land surface), and a vadose zone infiltration test to determine vertical moisture and radioactive tracer migration rates. Pump test results indicated that the Snake River Plain Aquifer was locally unconfined with a transmissivity ranging from 5.57 × 105 to 9.29 × 104 m2day. Moisture monitoring with neutron probes indicated that infiltrating water was initially transported vertically through the upper basalt layer of the vadose zone, primarily through fractures and rubble zones, at an average rate of 5 m/day (based on vertical distance traveled and first arrival of water at the monitoring points). Analysis of breakthrough curves for a conservative tracer allowed estimation of the arrival of the peak concentration and yielded an average velocity of 1 m/day. The migration velocities from the neutron probe and tracer tests are in good agreement given the scale of the test and difference in analysis techniques. None of the data sets showed a correlation between migration velocity (arrival time) and distance from the point source, but they strongly indicate preferential flow through discrete fractures. Upon reaching the first continuous sedimentary interbed layer in the basalt formation, water flow was diverted laterally along the interbed surface where it spread outward in primarily three areas corresponding to topographic lows on the interbed surface, and slowly infiltrated into the interbed. The nonpredictable movement of water and tracer through specific fractures underlying the site suggests that a priori prediction of trans‐missive fractures in this media is not possible. Results do suggest that the continuous sedimentary interbed layers, in general, impede vertical water flow and contaminant migration.