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 break-through 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 (theta), total porosity, the soil water retention curve, and saturated hydraulic conductivity were determined on the same blocks. Volumetric water content increased (R-2 = 0.25) as the slope of the water retention curve decreased. Increasing theta resulted in decreasing v (R-2 = 0.20) and thus, because of the linear relationship between D and v (R-2 = 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.