Dense nonaqueous phase liquids (DNAPLs) are prevalent at a large number of sites throughout the world. The variable release history and geologic heterogeneity make the spatial distribution of DNAPLs in the source zone complex. This causes difficulties in cleanup and can contribute to long-term groundwater contamination for decades to centuries. The objectives of this project were: 1) to develop algorithms that fused different types of information using a stochastic approach to provide a cost-effective characterization, monitoring, and predictive technology for the DNAPL source zone, 2) to conduct laboratory experiments to test and verify the proposed technology, and 3) to distribute the results of the research through a web-based virtual tomography laboratory to assist scientists, engineers, and managers to solve DNAPL contamination problems.
A cost-effective technology that images DNAPL source zones in 3-D without extensive invasive sampling was developed. This new technology based on stochastic methods, assimilates results of hydraulic and partitioning tracer tomography surveys to derive the best estimate of the DNAPL distribution and its uncertainty. Specifically, it first analyzes the information derived from hydraulic tomography to identify the 3-D heterogeneity in hydraulic conductivity (K) and specific storage (Ss) of the aquifer. The knowledge of heterogeneity is then used to design conservative tracer and partitioning tracer tomography tests to accurately depict the spatial distribution of DNAPL residual saturation (SN) in the source zone.
Building on the concept of scan technologies developed in medical sciences and geophysics, this effort developed a subsurface characterization tool comprised of three “fused” field methodologies: Hydraulic Tomography (HT), Conservative Tracer Tomography (CT), and Partitioning Tracer Tomography (PTT). The results of these methodologies were fused via an innovative algorithm that provided an improved, more accurate depiction of subsurface heterogeneities, subsequently allowing greater certainty in identifying and treating contaminant source zones.
Preliminary calculations suggested that the fused tomography technology becomes markedly more cost effective over conventional characterization approaches at sites with suspected investigation areas larger than 2,500 sq. ft. The degree of cost savings increased dramatically in conjunction with the increasing size of the area being characterized. While the resolution of the heterogeneity patterns were dependent on the density of the monitoring well network, the developed algorithm still yielded improved estimates of K,Ss, and SN in comparison to traditional interpretive techniques.
The developed technology was found to be superior to traditional characterization techniques as it required less invasive sampling and far fewer boreholes, resulting in substantial cost savings while obtaining the same level of accuracy as traditional methods. The cost savings were found to increase in larger and deeper DNAPL source zones where drilling becomes very expensive. It was also found that the technique was non-destructive and could be applied repeatedly (e.g., pre- and post-remediation). Most importantly, the new technology provided uncertainty estimates that could facilitate better decision making.