Water dispersible soil colloid properties and their role in the adsorption and transport of Escherichia coli through intact soil columns

Animal waste contains soluble and insoluble constituents, the latter ranging in size from colloidal to coarse particulate. Colloids have been implicated in co-transport of several chemical pollutants to groundwater. The co-transport of insoluble plant nutrients with colloids from manure amended soils would be expected, not least because the nutrients are likely incorporated within the colloidal compounds. However, the possibility of manure amended soils accelerating the transmission of fecal bacteria via colloidal co-transport has not been investigated. This study investigated the effects of poultry litter decay on water dispersible soil colloid properties, from three Kentucky soils, and their role in the adsorption and transport of E. coli through intact soil columns. Regardless of significant soil, litter treatment, and temporal variations in the soil properties of reported importance to clay dispersion, dispersion was affected neither by soil type nor by litter amendment and there were no significant temporal variations in dispersion. The most important variable explaining variation in clay dispersion was silt. Colloids at circumneutral pH significantly adsorbed more E. coli compared to colloids at moderately acid pH. Colloids from litter-amended soils also significantly adsorbed more E. coli than colloids from unamended soils. Higher adsorption at circumneutral pH and on colloids from litter-amended soils was presumably due to hydrophobic rather than electrostatic interactions. Colloid and E. coli transport parameters were evaluated by a moment method for analyzing breakthrough curves of step inputs. After two pore volumes of leaching, computed retardation factors (R) indicated that colloids from unamended soils were transported significantly faster (P ≤ 0.1) than colloids from litter-amended soils. Transport enhancement was probably due to pore size exclusion of large colloid flocs rather than to electrostatic repulsion between flocs and pore surfaces. Therefore, increased transport partially depended on suspension-solution physicochemical properties that control floc size. Colloids significantly (P ≤ 0.05) facilitated E. coli transport. Increases in dissolved organic carbon and divalent cations, especially Ca, appeared to impede colloidal mobility. It is, therefore, likely that lime and organic matter application could reduce colloid mediated E. coli transport.