Spatial Variability of Organic Carbon in a Fractured Mudstone and Its Effect on the Retention and Release of Trichloroethene (TCE)
Abstract
Although fractures are responsible for the majority of groundwater flow in consolidated rock, the longevity of contamination is largely dependent on the retention and release of contaminants in the primary porosity of the rock (rock matrix). Contaminants that have diffused into the rock matrix or sorbed onto solid surfaces act as a pervasive source that can diffuse back into fractures, adversely affecting groundwater quality for extended periods. Organic carbon in the rock matrix acts as sorption sites for organic contaminants, which can retard diffusion and extend the longevity of contamination further. The fraction of organic carbon (foc) in the rock matrix is commonly estimated by taking the average of analyses conducted on a small number of samples. A detailed investigation of the spatial variability of foc was conducted in the fractured mudstone underlying the Naval Air Warfare Center, West Trenton, NJ, which is a site of groundwater contamination with TCE. Approximately 400 core samples from 7 closely spaced boreholes were analyzed. foc varied over two orders of magnitude over distances of tens of centimeters, translating into order of magnitude variability in the retardation applied to TCE. Because of the large variability in foc, assigning values based on a small number of samples may not be appropriate in evaluating contaminant retention in the rock matrix. This investigation shows that foc depends on the depositional characteristics of the mudstone layers. Deep water depositional environments are most conducive to the retention of organic carbon in comparison to shallow water depositional environments. The deep water depositional environments were also the most fissile layers containing fractures that formed bedding plane partings responsible for most of the groundwater flow. Understanding the spatial distribution of foc in the rock matrix relative to fractures is critical in evaluating the longevity of contamination and the success of remediation strategies.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.H32D..07S
- Keywords:
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- 1831 Groundwater quality;
- HYDROLOGYDE: 1832 Groundwater transport;
- HYDROLOGYDE: 1847 Modeling;
- HYDROLOGYDE: 1875 Vadose zone;
- HYDROLOGY