EFFECT OF SIMULATED RAINFALL ON THE TRANSPORT OF METALS, ORGANIC MATTER, AND COLLOIDS IN A METAL-CONTAMINATED FLOOD PLAIN SOIL

Facilitated transport of contaminant metals by organic matter and inorganic colloids in the vadose zone may contaminate shallow groundwaters. Our laboratory study sought to determine the extent of metal, organic matter, and inorganic colloid mobilization and transport generated by simulated rainfall in unsaturated, intact soil cores. The soil cores were collected from the flood plain of the Arkansas River in central Colorado. The flood plain soils were contaminated by mining wastes. To do this, we tested the effect of rainfall intensity on the mobilization of organic matter and colloids and expected interactions between metal cations and organic matter and colloids by evaluating the influence of chemical factors such as pH, calcium concentration, and organic matter concentration on the extent of metal-colloid associations. We also examined the influence of water flow and colloid size on metal transport. During two rainfall simulations spaced 14 d apart, only about 20% of the zinc, copper, and lead present the soil was leached. On average, the colloid-bound fraction of mobile zinc was 20%, copper was 55%, and lead was 90%. The fraction of colloid-bound zinc and lead increased with increasing colloid concentrations. Colloid size, pH, and calcium concentrations did not correlate as expected with partition coefficients (K_d) for zinc, copper, or lead. Flow field flow fractionation techniques suggested that most of the colloidal material was organic matter and that the extent of metal binding was not impacted by the size of the colloid or the rainfall intensity.