Chemical conditions conducive to the release of mobile colloids from ultisol profiles

Dispersed colloidal clay particles may enhance the transport of environmental contaminants adsorbed to the colloid surfaces. The objective of this study was to determine how soil pH (4.3-6.9), low total electrolyte concentration (TEC, 0.4-2.8 mol(c) m(-3)), and low Na adsorption ratios (SAR, 0.3-1.8 [mol(c) m(-3)](0.5)) affected the concentration of mobile colloids released from 13.5-m(3) reconstructed Ultisol profiles. Critical flocculation concentration (CFC, the minimum TEC to induce flocculation of a clay suspension) tests were conducted using the water-dispersible clay fraction of the Ap horizon of these profiles, which is the primary source for the mobile colloids. Surface soil pH, TEC, and SAR levels of the profiles that released high concentrations of colloids fell within the dispersion domain identified by the laboratory CFC tests. The effect of SAR on dispersion appears to be a continuum that approaches zero, and this effect is significantly (P less than or equal to 0.01) dependent on TEC. The dispersible nature of these soils is in part attributed to their very low TEC, generally < 2 mol(c) m(-3), which results from low concentrations of weatherable minerals. The TEC of 10 of the 13 profiles were < 2 mol(c) m(-3), the CFC for SAR = 1 (mol(c) m(-3))(0.5), indicating that if the pH is not limiting (pH greater than approximate to 6.0), colloidal dispersion will occur. Thus, these weathered soils can be highly dispersive within limited chemical conditions, and only small increases in SAR or pH may induce dispersion. By identifying these limits, the risk of enhancing subsurface contaminant transport by soil-derived mobile colloids can be reduced.