Modeling tracers and contaminant flux in heterogeneous aquifers

Abstract

The results of this research have a potential impact on two major areas of current groundwater research and policy. First, a complete methodology is now available for analyzing the accuracy of dense nonaqueous phase liquid (DNAPL) volume derived from partitioning interwell tracer test (PITT) data with more confidence than any other measurement method current available in the field. Second, the benefit to groundwater quality of reducing DNAPL mass in source zones has been shown to be very substantial following high mass removal similar to what has actually been demonstrated in the field with surfactant flooding at two typical sites. It is hoped that this research provides an impetus to the further use of careful and accurate flow and transport simulation both to better understand the key issues such as flux reduction and to promote more widespread cleanup of DNAPL source zones. The effect of permeability heterogeneity on the tracer response was investigated using the flow and transport simulator UTCHEM. This study shows the important contribution of the geology of the aquifer to inefficient tracer sweep of the DNAPL source zone. Sensitivity of the results was examined for geostatistical parameters such as the standard deviation of the permeability and the correlation length. An investigation was conducted to study the use of multilevel samplers for estimating vertical DNAPL distribution in an aquifer. The limitations of accurately estimating vertical DNAPL saturation profile were presented for a field aquifer in Hill Air Force Base (Hill AFB). The error introduced due to exponential extrapolation of tracer data was investigated as a function of tracer detection limit and the heterogeneity of the aquifer. The error in estimating oil volume increased as the tracer detection limit was increased and the heterogeneity was increased. Alternate approaches to extrapolation of tracer data have been presented and discussed. A systematic and thorough study to evaluate the benefits of source zone reduction in heterogeneous aquifers was performed. A methodology was developed to quantify the effectiveness of SEAR by calculating downstream contaminant mass flux across a control plane. The results show that partial DNAPL removal is beneficial in improving downstream groundwater quality. Results from modeling a portion of the DNAPL source zone at Hill AFB showed a trichloroethylene (TCE) mass flux reduction of greater than 99.98% for a mass reduction of 94%. Results from modeling of the Camp Lejeune aquifer showed that a 79% perchloroethylene (PCE) mass reduction achieved a 99% mass flux reduction downstream. The effect of contaminant diffusion from a low permeability zone to a high permeability zone at Camp Lejeune was found to be negligible.

The results of this research have a potential impact on two major areas of current groundwater research and policy. First, a complete methodology is now available for analyzing the accuracy of dense nonaqueous phase liquid (DNAPL) volume derived from partitioning interwell tracer test (PITT) data with more confidence than any other measurement method current available in the field. Second, the benefit to groundwater quality of reducing DNAPL mass in source zones has been shown to be very substantial following high mass removal similar to what has actually been demonstrated in the field with surfactant flooding at two typical sites. It is hoped that this research provides an impetus to the further use of careful and accurate flow and transport simulation both to better understand the key issues such as flux reduction and to promote more widespread cleanup of DNAPL source zones. The effect of permeability heterogeneity on the tracer response was investigated using the flow and transport simulator UTCHEM. This study shows the important contribution of the geology of the aquifer to inefficient tracer sweep of the DNAPL source zone. Sensitivity of the results was examined for geostatistical parameters such as the standard deviation of the permeability and the correlation length. An investigation was conducted to study the use of multilevel samplers for estimating vertical DNAPL distribution in an aquifer. The limitations of accurately estimating vertical DNAPL saturation profile were presented for a field aquifer in Hill Air Force Base (Hill AFB). The error introduced due to exponential extrapolation of tracer data was investigated as a function of tracer detection limit and the heterogeneity of the aquifer. The error in estimating oil volume increased as the tracer detection limit was increased and the heterogeneity was increased. Alternate approaches to extrapolation of tracer data have been presented and discussed. A systematic and thorough study to evaluate the benefits of source zone reduction in heterogeneous aquifers was performed. A methodology was developed to quantify the effectiveness of SEAR by calculating downstream contaminant mass flux across a control plane. The results show that partial DNAPL removal is beneficial in improving downstream groundwater quality. Results from modeling a portion of the DNAPL source zone at Hill AFB showed a trichloroethylene (TCE) mass flux reduction of greater than 99.98% for a mass reduction of 94%. Results from modeling of the Camp Lejeune aquifer showed that a 79% perchloroethylene (PCE) mass reduction achieved a 99% mass flux reduction downstream. The effect of contaminant diffusion from a low permeability zone to a high permeability zone at Camp Lejeune was found to be negligible.