Natural Attenuation Progress, Plume Movement, and Source Reduction for the SRS CMP Pits VOC Plume - 18384
- South Carolina State University (United States)
- Savannah River Nuclear Solutions (United States)
Monitoring the effectiveness of natural attenuation for remediating the Savannah River Site (SRS) Chemical, Metals, and Pesticides (CMP) Pits plume fringe has focused intensive sampling of shallow groundwater for different reaches of Pen Branch from 2005 to 2017. Volatile organic compounds (VOCs) were analyzed for the critical hyporheic zone beneath Pen Branch and adjacent floodplain. These sampling efforts assisted larger-scale compliance monitoring in accordance with US Environmental Protection Agency (EPA) guidelines. When applicable, EPA has found natural attenuation to be an acceptable mode of achieving regulatory compliance. While natural attenuation can include any ecosystem function that reduces contaminant loads, it is focused on microbial biochemical pathways. The study plume originated from CMP Pits, a waste disposal area in operation from 1971 to 1979, when the pits were closed and backfilled. In 1984, excavations of source material and contaminated soils were removed up to a depth of 7.6 m below the trenches. However, monitoring well data indicated that perchloroethylene (PCE) and trichloroethylene (TCE) had seeped beneath the vadose zone and formed groundwater plumes reaching the Pen Branch valley below. Although 'hot spots' of TCE were found within the Pen Branch floodplain, it was unknown just how the flow pattern was entering Pen Branch and to what degree natural attenuation was reducing the contaminant load. For this reason, our stations were located along the Pen Branch reach bordering the modelled plume fringe. In general, VOCs were highest at station piezometer holes in the middle study zone. Moderate VOC levels were detected at upstream stations and lowest VOC levels were found at downstream stations. Electrical resistance heating (ERH) with soil vapor extraction (SVE) conducted by Savannah River Nuclear Solutions (SRNS) from 2008 to 2009 at the filled CMP Pits helped to remove contaminants trapped in clay horizons beneath the CMP Pits and to reduce overall new plume concentrations. Collectively PCE, TCE, and their degradation products displayed a favorable yearly trend of contaminant natural attenuation. Additionally, phytoremediation actions by native bamboo, Arundinaria gigantea, added to VOC removal from wetlands soils. Total VOC concentrations at some stations declined by >25% and concentrations of vinyl chloride (VC), a nearly-final stage degradation product increased from 0.0 ppb to >20 ppb. Temporal patterns displayed a continued reduction of VOCs, while spatial patterns indicated a possible slow progression of the plume downstream along Pen Branch. Conclusions about plume export into surface waters must address the seasonal and geological complexities of this system. Hydrological patterns are complex and plumes do not directly outcrop into Pen Branch surface waters, but emerge into the stream through pathways in the porous hyporheic zone. For this reason, continued sampling must identify critical depths and downstream locations where plume flows may be more intense. During drought periods with no surface flow in Pen Branch, subsurface plume flows may be flowing downslope in the hyporheic zone. (authors)
- Research Organization:
- WM Symposia, Inc., PO Box 27646, 85285-7646 Tempe, AZ (United States)
- OSTI ID:
- 22977688
- Report Number(s):
- INIS-US-20-WM-18384; TRN: US21V0318017733
- Resource Relation:
- Conference: WM2018: 44. Annual Waste Management Conference, Phoenix, AZ (United States), 18-22 Mar 2018; Other Information: Country of input: France; 9 refs.; Available online at: https://www.xcdsystem.com/wmsym/2018/index.html
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
54 ENVIRONMENTAL SCIENCES
BACKFILLING
BAMBOO
BIOCHEMISTRY
BIOREMEDIATION
DECONTAMINATION
ECOLOGICAL CONCENTRATION
ELECTRIC CONDUCTIVITY
ENVIRONMENTAL PROTECTION
GROUND WATER
HYDROLOGY
NATURAL ATTENUATION
PIEZOMETRY
PLUMES
SAFETY STANDARDS
SAMPLING
SAVANNAH RIVER PLANT
SOILS
SURFACE CONTAMINATION
SURFACE WATERS
US EPA
VINYL CHLORIDE
VOLATILE MATTER
WATER POLLUTION
WETLANDS