Rating Soils for Buffering the Quality of Drainage Water.

Water bodies are potentially threatened by chemicals that contaminate drainage water from farmlands. The vulnerability of groundwater to pollution is affected by properties of the regolith, including soil, in the vadose zone. The effect of routinely measured soil properties on the retention of contaminants is well known but it is difficult to rank soils consistently in relation to their capacity to preserve water quality because a number of interacting factors affect contaminant mobility. These include soil pH, the quantity and composition of the colloidal fraction (humus, metal oxides and silicate clays), and the nature of the contaminant, especially whether it be cationic, anionic, or non-polar organic. Some 200 soil samples from different horizons of 76 profiles were equilibrated with several solution concentrations of CuCl₂ or NaH₂PO₄ and the sorption of Cu and P by each soil at an equilibrium solution concentration of 1 mg L^-1 was determined interpolatively from the resulting van Bemmelen-Freundlich isotherm. Plots of cation (Cu) or anion (P) sorption against soil pH (CaCl₂), clay content, dithionite-extractable Fe and Al, organic C and sum of exchangeable bases were used to establish boundary line equations, by quantile regression, in terms of which the maximum possible sorption could be calculated for any specified value of the determinant variable. This then formed the basis for determining which of the six variables would have the lowest maximum sorption potential associated with it, and therefore would be most critical in dictating the potential for Cu or phosphate discharge in drainage water. The potential thus calculated was used to give each soil material a quantitative rating in terms of groundwater vulnerability to contamination by Cu (and similar metals) or PO₄ (and similar ligands). Organic C content was used to rank the soils in terms of their retention capacity for non-polar organic compounds such as pesticides. The process was completed by assigning, for each class of contaminant, the vulnerability rating of the most retentive layer to the vadose zone as a whole. This rating scheme is novel because it identifies, from known values of commonly determined soil properties, the potential maximum mobility of broad classes of contaminants and therefore classifies soils in terms of the relative risk (maximum = 1) of pollutants escaping in harmful concentrations. In principle it can be applied universally.