Effects of Infiltration on Chemical Transport into Overland Flow

Abstract

ABSTRACT THE effect of raindrop impact and infiltration on the mixing process between surface runoff and tracer dye in a saturated porous medium (Ottawa sand) was studied in the laboratory. Experiments were conducted in a tilting plexiglass flume 10 cm wide, 200 cm long, and 30 cm deep. Infiltration and rainfall rate and energy could be closely controlled, and the mixing process at the surface was easily observed. Samples of pore fluid were obtained by withdrawing fluid through rubber septa with a microsyringe, and concentrations were measured by fluorometry. A complete description of the apparatus is presented, along with the results of 21 simulated rainstorm events. The mass of dye removed in overland flow was positively related to slope of the flume and negatively related to infiltration rate. The solute concentration profiles in the porous media, with zero slope and no infiltration, were convex upward. With infiltration, the profiles assumed a characteristic s shape. These observations suggest that the complete mixing model may be appropriate when infiltration rates are high, but an incomplete mixing model, or a convective dispersion model, would provide better approximations at low infiltration rates. Unexpected patterns of infiltration and exfiltration were observed at the surface of the porous medium when the flume was tilted. These patterns were probably due to heterogeniety of sand packing and high permeability of the material.