Solute transport as related to soil structure in unsaturated intact soil blocks

Concern about soil and groundwater pollution has resulted in numerous studi es focused on solute transport. The objectives of our study were to investi gate the effect of soil type and land-use management on solute movement. Tr ansport of water and Cl- were measured through intact blocks of Maury (fine , mixed, semiactive, mesic Typic Paleudalf) and Cecil (fine, kaolinitic, th ermic Typic Kanhapludult) soils, under steady-state, unsaturated flow condi tions. Three replicate blocks for the Maury soil and two replicate blocks f or the Cecil soil were studied per land-use treatment. The land-use treatme nts were conventional-till corn (Zea mays L.) production and long-term gras s pasture. Individual blocks were instrumented with time domain reflectomet ry (TDR) probes at the 5-, 15-, and 25-cm depths. The effluent Cl- and TDR breakthrough curves were fitted using the convection dispersion equation (C DE); the estimated parameters were pore water velocity (v), dispersion coef ficient (D), and, for the TDR break-through curves, maximum bulk electrical conductivity (BECmax). The CDE fitted the data very well, with model R-2 v alues ranging from 0.971 to 0.999. Volumetric water content (theta), total porosity, the soil water retention curve, and saturated hydraulic conductiv ity were determined on the same blocks. Volumetric water content increased (R-2 = 0.25) as the slope of the water retention curve decreased. Increasin g theta resulted in decreasing v (R-2 = 0.20) and thus, because of the line ar relationship between D and v (R-2 = 0.26), decreasing D. Structural cont rols on solute dispersion in this study were mainly indirect, and related t o variations in water content produced by differences in pore-size distribu tion.