ESTIMATING THE RETARDATION COEFFICIENT OF TRICHLOROETHENE FOR A SAND AQUIFER LOW IN SEDIMENT ORGANIC-CARBON - A COMPARISON OF METHODS

Significant groundwater contamination by trichloroethene (TCE) has bee n identified in the Spearwood Sands aquifer, Perth Western Australia. The organic carbon content of the aquifer sediment is very low and rep orted retardation coefficients for TCE in low organic carbon subsurfac e environments an rare. Carbon-based correlation equations were report ed to underestimate the retardation of nonionic hydrophobic organic co mpounds in aquifers characterised by low organic carbon contents. Esti mating the retardation coefficient for TCE in the Spearwood Sands aqui fer, therefore, is difficult. Various methods to estimate the retardat ion coefficient for TCE in the Spearwood Sands aquifer were investigat ed. Estimates obtained by commonly used correlation equations were com pared to retardation coefficients calculated based on laboratory batch and column studies. An in situ transport experiment using deuterium l abelled TCE (TCEd1) within a TCE contaminated zone was carried out to assess the reliability of the various estimation techniques. The field transport experiment showed that TCEd1 was not retarded in the Spearw ood Sands aquifer. Carbon-based correlations overestimated the retarda tion coefficient for TCE. The equations are shown to be limited by the lack of reliable estimates of the organic carbon content for the Spea rwood Sands. Laboratory batch studies resulted in more reliable estima tes of TCE retardation. Retardation coefficients from the batch studie s, however, were still overestimated since Teflon-coated silicone line rs used in the experiments show considerably higher sorption capacity for TCE than does the aquifer sediment. Laboratory column studies were accurate in assessing the sorption behaviour of TCE in the Spearwood Sands. Column studies eliminated the additional uncertainty inherent i n determining the input concentration for TCE, by assessing the retard ation coefficient in relation to an inorganic tracer. Column studies a lso avoided the experimental difficulties experienced with the batch s tudies and proved to be more cost effective. It is shown, however, tha t also with batch experiments an accurate prediction of the field reta rdation for TCE can be obtained, if naphthalene is used in the batch s tudies to determine the sorptive properties of the sediment. (C) 1998 Elsevier Science B.V.