Prediction of organic chemical leachate concentrations from soil samples

Volatile organic compound (VOC) contamination of soil is common in the United States.  Once soil becomes contaminated, precipitation may infiltrate through the soil and carry contaminants downward into the groundwater.  Understanding VOC leaching from soil is important because groundwater is the primary source of drinking water for about 50 percent of the United States and VOCs are known to negatively affect human health.  Methods are needed to assess how VOCs leach from soil.  In order to model mass transport of VOCs at sites where subsurface contamination is present, their desorption processes must be understood.

Quantification of desorption rates will allow use of the advection dispersion equation with a time varying influent boundary condition and other simple models.  An empirical leach rate relation is developed based upon basic soil parameters.  It can be used to evaluate VOCs leaching from a body of soil.

Constant head reservoirs were developed and used to conduct column desorption studies using four VOCs – toluene, methylene chloride, trichloroethylene, and m-xylene – and three soils – topsoil, sand, and clay.  In addition, batch isotherm studies were performed on the same soils and chemicals.  A gas chromatrograph was used to analyze samples.

An empirical leach rate constant relation was statistically developed from curve fits to the column data.  The leach rate curve fits were not continuous functions and reached asymptotic concentrations about zero.  Long term and short term leach rates had distinctively different rates.  The difference in the long and short term leach rates and the lack of a single continuous function describing leach rates would suggest different rate limiting process control desorbtion over time.

The empirical leach rate relation will help consultants and regulators assess the impact of VOC contaminated soil on groundwater quality, and quantify potential human health risks.