Measurement of field soil hydraulic and solute transport parameters

Agricultural chemical presence in groundwater has drawn attention toward transport processes occurring in soil. Hydraulic conductivity (K) and water-holding capacity of a soil have great influence on water now and solute transport. However, much of the chemical transport to groundwater ran occur through preferential flow pathways. The simplified, preferential now, mobile-immobile model partitions the water content (theta) into mobile (theta(m)) and immobile (theta(im)) domains, with solute exchange between the domains characterized by the mass-exchange coefficient (alpha). In this study a sequential tracer application technique was used and K, theta, theta(im), and alpha were estimated for a series of pressure heads (H = 10, -30, -60, and -150 mm). This method uses a tension infiltrometer to measure both hydraulic and solute transport parameters in situ. The study took place in a no-till corn (Zea mays L.) held mapped as a Harps series soil (fine-loamy, mixed, mesic Typic Calciaquoll). Unsaturated values of theta and K were distinct from the saturated values. Similarly, though less clear cut, distinctions between saturated and unsaturated values of theta(im), immobile water fraction (theta(im)/theta), and alpha were observed. The medians of theta for the sequence of decreasing H values were 0.40, 0.31, 0.34, and 0.33 m(3) m(-3) The median K values for the same sequence of H were 108, 1.69, 1.51, and 0.72 mu m s(-1). The median theta(im)/theta values for the H sequence were 0.40, 0.28, 0.25, and 0.39. The median values of alpha for the H sequence were 0.59, 0.015, 0.0028, and 0.0029 h(-1). A strong correlation between alpha and H suggests a velocity dependence of alpha.