Conference proceeding
Temporal evolution of a NAPL source zone up-gradient of a pump & treat system in a fractured sandstone aquifer
American Geophysical Union Fall Meeting, Vol.2018
American Geophysical Union 2018 fall meeting
12/2018
Abstract
Performance of NAPL remediation technologies rely on effectively characterizing source zone architectures spatially and temporally. Few published studies have used field-focused methods at fractured rock sites whose hydrogeologic conditions display large spatial heterogeneity and complex NAPL distribution. Our study site is located in south-central Wisconsin and consists of a multicomponent DNAPL (chlorinated compounds, ketones, BTEX) released prior to 1970 that has created a 2.8 km long plume 48 to 56 mbgs in the Tunnel City (TC) sandstones. Mobile DNAPL remains in the TC sandstones and residual DNAPL in the overlying rock. 3 pumping wells screened in the TC sandstones located 1 km downgradient of the source zone have been operating since 2003. We characterize the temporal variability of the in situ mass using high resolution rock core sampling at 5 locations along a transect oriented perpendicular to ground water flow and 35 m downgradient of the source zone. One of these locations includes 3 co-located coreholes drilled in 2003, 2014 and 2017. Complementary groundwater samples were collected from depth-discrete multilevel systems installed in the 3 cored holes. We also examine mass discharge and concentration data from the pumping wells and compare the results to the changes in the rock matrix upgradient. We report a 77% mass reduction in rock matrix VOC mass between 2003 and 2014 and a further 6% by 2017, representing an exponential mass reduction with time. Analysis of mole fractions with depth show a 57% decrease in chlorinated compounds and a corresponding increase in ketones and BTEX. Within the TC sandstones, concentrations of groundwater are higher than in the rock core pore water consistent with upgradient DNAPL. The pumping wells extracted 23,693 kg of VOCs whose composition matches that lost from the source zone. However, a substantial change in mass and composition occurs in the upper rock rather than in TC sandstones where the pump & treat system is screened due to higher microbial activity in the upper rock evidenced by an increase in metabolic intermediates (ketones) and transformation from parent to daughter products. This study provides insights on designing the depth-targeted remediation systems while evaluating microbial degradation and contaminant composition changes within the source zone.
Details
- Title: Subtitle
- Temporal evolution of a NAPL source zone up-gradient of a pump & treat system in a fractured sandstone aquifer
- Creators
- Brian Ndirangu - Instituto Superior Tecnico PRT PortugalBeth L Parker - University of GuelphJessica R Meyer - University of Iowa
- Resource Type
- Conference proceeding
- Publication Details
- American Geophysical Union Fall Meeting, Vol.2018
- Conference
- American Geophysical Union 2018 fall meeting
- Publisher
- American Geophysical Union
- Alternative title
- AGU 2018 fall meeting
- Language
- English
- Date published
- 12/2018
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984240895802771
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