Solute Transport as Related to Soil Structure in Unsaturated Intact Soil Blocks

Concern about soil and groundwater pollution has resulted in numerous studies focused on solute transport. The objectives of our study were to investigate the effect of soil type and land-use management on solute movement. Transport of water and Cl− were measured through intact blocks of Maury (fine, mixed, semiactive, mesic Typic Paleudalf) and Cecil (fine, kaolinitic, thermic Typic Kanhapludult) soils, under steady-state, unsaturated flow conditions. Three replicate blocks for the Maury soil and two replicate blocks for the Cecil soil were studied per land-use treatment. The land-use treatments were conventional-till corn (Zea mays L.) production and long-term grass pasture. Individual blocks were instrumented with time domain reflectometry (TDR) probes at the 5-, 15-, and 25-cm depths. The effluent Cl− and TDR breakthrough curves were fitted using the convection dispersion equation (CDE); the estimated parameters were pore water velocity (v), dispersion coefficient (D), and, for the TDR breakthrough curves, maximum bulk electrical conductivity (BECmax). The CDE fitted the data very well, with model R 2 values ranging from 0.971 to 0.999. Volumetric water content (θ), total porosity, the soil water retention curve, and saturated hydraulic conductivity were determined on the same blocks. Volumetric water content increased (R2 = 0.25) as the slope of the water retention curve decreased. Increasing θ resulted in decreasing v (R2 =0.20) and thus, because of the linear relationship between D and v(R2 = 0.26), decreasing D Structural controls on solute dispersion in this study were mainly indirect, and related to variations in water content produced by differences in pore-size distribution

Solute and Bacterial Transport through Partially-Saturated Intact Soil Blocks

Steady-state transport of water, chloride and bacteria was measured through intact blocks of Maury and Cecil soils, under partially saturated conditions. Major objectives were to determine if transport occurs uniformly or via preferential flow paths, and if soil physical properties could be used to predict breakthrough. The blocks were instrumented with TDR probes and mounted on a vacuum chamber containing 100 cells that collected eflluent. After each experiment the blocks were sampled for soil physical properties. The fluxes showed no spatial autocorrelation and the eflluent variance was not statistically different between soils. Less than 3% of the influent bacteria appeared in the effluent. Maximum bacterial breakthrough occurred after 0.25 water-filled pore volumes had been leached, and was greater for Cecil soil than for Maury soil. The chloride breakthrough curves were fitted to the convection dispersion equation. The best predictor of dispersivity was volumetric water content (R2 = 0.28, P \u3c 0.01), with dispersivity increasing with decreasing water content. Lower water contents lead to more tortuous flow paths and thus, a broadening of the velocity distribution. Soil structural controls on solute dispersion under partially saturated conditions are likely to be indirect, and related to differences in water content at given flux produced by differences in pore-size distribution

Caged Layer Management.

20 p