Determination of Solute Organic Concentration in Contaminated Soils Using a Chemical-equilibrium Soil Column System

Groundwater risk assessment of contaminated soils implies determination of the solute concentration leaching out of the soil. Determination based on estimation techniques or simple experimental batch approach has proven inadequate. Two chemical equilibrium soil column leaching tests for determination of solute concentration in a contaminated soil were developed; (1) a chemical Equilibrium and Recirculation column test for Volatile organic chemicals (ER-V) and (2) a chemical Equilibrium and Recirculation column test for Hydrophobic organic chemicals (ER-H). The two test systems were evaluated using two soils with different content of organic carbon (f_oc of 1.5 and 6.5%, respectively). A quadruple blind test of the ER-V system using glass beads in stead of soil showed an acceptable recovery (65–85%) of all of the 11 VOCs tested. Only for the most volatile compound (heptane, K_H～80) an unacceptable recovery was found (9%). The contact time needed for obtaining chemical equilibrium was tested in the ER-H system by performing five test with different duration (1, 2, 4, 7 and 19 days) using the low organic carbon soil. Seven days of contact time appeared sufficient for achieving a solute equilibrium concentration. The repeatability of both test systems (evaluated by performing five identical tests) was considered acceptable (8–16% and 7–28% for the ER-V and ER-H system, respectively). Comparing determined solute concentration in both systems for volatile and hydrophobic chemicals with estimated concentrations using an K_ow–K_oc relation and determined total soil concentrations, large differences between measured and estimated solute concentrations were observed, especially for the hydrophobic chemicals (PAHs). This clearly illustrates the need for a reliable method to measure solute phase concentration of PAHs in contaminated soils. Overall a reliable and reproducable system for determining solute concentration of a wide range of organic compounds in contaminated soils has been developed.