By Philip T. Harte
U.S. Geological Survey Scientific Investigations Report 2004-5176
In cooperation with the
New Hampshire Department of Environmental Services and the
U.S. Environmental Protection Agency, Region 1
The Savage Municipal Well Superfund site, named after the former municipal water-supply well for the town of Milford, is underlain by a 0.5-square mile plume of volatile organic compounds (VOCs), primarily tetrachloroethylene (PCE). The plume occurs mostly within a highly transmissive sand-and-gravel unit, but also extends to an underlying till and bedrock unit. The plume logistically is divided into two areas termed Operable Unit No. 1 (OU1), which contains the primary source area, and Operable Unit No. 2 (OU2), which is the extended plume area.
PCE concentrations in excess of 100,000 parts per billion (ppb) had been detected in the OU1 area in 1995, indicating a likely Dense Non-Aqueous Phase Liquid (DNAPL) source. In the fall of 1998, the New Hampshire Department of Environmental Services (NHDES) and the U.S. Environmental Protection Agency (USEPA) installed a remedial system in OU1. The OU1 remedial system includes a low-permeability barrier that encircles the highest detected concentrations of PCE, and a series of injection and extraction wells. The barrier primarily sits atop bedrock and penetrates the full thickness of the sand and gravel; and in some places, the full thickness of the underlying basal till. The sand and gravel unit and the till comprise the aquifer termed the Milford-Souhegan glacial-drift aquifer (MSGD).
Two-dimensional and three-dimensional finite-difference solute-transport models of the unconsolidated sediments (MSGD aquifer) were constructed to help evaluate solute-transport processes, assess the effectiveness of remedial activities in OU1, and to help design remedial strategies in OU2. The solute-transport models simulate PCE concentrations, and model results were compared to observed concentrations of PCE. Simulations were grouped into the following three time periods: an historical calibration of the distribution of PCE from the initial input (circa 1960) of PCE into the subsurface to the 1990s, a pre-remedial calibration from 1995 to 1998, and a remedial (post-barrier wall) calibration from 1998 to 1999. Model results also were checked against observed PCE concentrations from May and June 2000 as a post-audit of model performance.
Results of the simulations of the two-dimensional model for the historical calibration indicate that the model-computed length of the plume is affected by the retardation factor (retardation). Values of retardation greater than 3 caused the longitudinal length of the computed plume to be too short compared to the observed plume. A retardation of 2-2.5 produced a reasonable comparison between computed and observed PCE concentrations. Testing of different starting times and rates of mass input of PCE indicated that the plume reaches a quasi steady-state distribution in about 20 years regardless of the rate of mass input or values of the solute-transport parameters (retardation, dispersion, and irreversible reaction) assigned the model.
Results of the simulations of the three-dimensional model for the pre-remedial (1995-98) calibration of PCE for the OU2 area identified some spatial biases in computed concentrations that generally were unaffected by changes in retardation. The computed PCE concentrations exceeded observed concentrations along the northern part of the plume in OU2, where PCE increases were observed in a bedrock well. These results indicate that some PCE in this area may be entering the bedrock, which is not simulated in the model. Conversely, computed PCE concentrations were less than observed concentrations along the southern part of the plume in OU2. Because testing of high (above 4) values of retardation did little to reduce residuals, it is concluded that the low computed PCE concentrations along the southern flank are likely the result of an underestimation of the initial PCE mass in this area or an unaccounted source of PCE.
Results of the simulations of the three-dimensional model for the remedial calibration period (1998-99) and post-audit period (May-June 2000) showed a decline in concentration at the OU1/OU2 boundary comparable to that observed in the field. In September 1999, computed PCE concentrations decreased by 6 percent from initial concentrations in December 1998, and observed PCE concentrations decreased by 10 percent. In May 2000, decreases were 26 and 29 percent from initial concentrations for computed and observed PCE, respectively.
TABLE OF CONTENTS
Purpose and Scope
Description of Study Area
Description and Trends of the Savage Site Volatile Organic Compound Plume
Description of Processes Affecting Solute Transport of Tetrachloroethylene
Simulation of Solute Transport of Tetrachloroethylene
Construction of Solute-Transport Models
Numerical Solvers and Accuracy
Evaluation of Solute-Transport Models
Apparent Age of Ground Water
Comparison with Model-Computed Ground-Water Ages
Analysis of Extended Time Period
Enhancements to the Remedial Model
Calibration of Final Solute-Transport Model
Summary and Conclusions
Plates accompanying SIR 2004-5176 will need to be printed on a large plotter
Plate 1 (146 KB)
Plate 2 (106 KB)
Plate 3 (131 KB)
Plate 4 (513 KB)
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Pembroke, NH 03275
Web site http://nh.water.usgs.gov
Philip Harte, firstname.lastname@example.org (603) 226-7813
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