Water-dispersible soil particles and the transport of nonpoint-source pollutants in the lower Rio Grande Valley

Abstract

The transport of nonpoint-source pollutants in surface runoff may be enhanced through sorption to mobile soil particles, a process known as particle-mediated transport. In order to predict the potential importance of this process, the major geochemical and mineralogical factors controlling particle dispersion and pollutant sorption must be identified. These factors were determined through characterization of water-dispersible clay (WDC) assumed to be an analog of natural mobile particles. WDC were obtained from three soils representative of the lower Rio Grande Valley by dispersion in water. WDC content of the three soils varied between 5 to 15%. WDC was proportional to clay content and inversely proportional to CaCO3 content. Relative to the bulk soils characteristics, WDC was enriched in organic matter (OM), CaCO3, and Fe oxides. The presence of amorphous coatings of OM, silica, and carbonates influenced the surface chemistry and dispersion of phyrosilicate minerals in WDC. Sequential extraction of WDC, using Na-acetate (pH=5), H202 and citrate-dithionate-bicarbonate extractants, generated particles of higher surface area (an increase from 63 to 1 18 and 127 m2/g, respectively), more negative electrophoretic mobility (an increase from-2.5 to-3.5 and-4.2 um/s/m/V, respectively), and higher critical coagulation concentration (an increase from 8 to 12 and 14 meq/L, respectively). An increase in particle dispersivity upon action of the extractants was visible on Transmission Electron Microscope micrographs. Batch sorption experiments were conducted using bulk soils samples and WDC (untreated, OM removed, OM and Fe oxides removed) reacting with Zn and Cu (model metals) and pyrene (model hydrophobic organic). Higher amounts of metals were sorbed by WDC than bulk soils, the maximum enrichment ratios were 1.45 and 3.3 for Cu and Zn, respectively. The removal of OM, Fe oxides and amorphous coatings rendered the WDC material less reactive towards Zn and Cu. Metal sorption was controlled by solution pH, cation exchange capacity of the mineral phases, and the OM content. Sorption of pyrene (Koc=24290) was controlled by organic matter and followed a linear isotherm.