Changes in groundwater flow and volatile organic compound concentrations at the Fischer and Porter Superfund Site, Warminster Township, Bucks County, Pennsylvania, 1993-2009

Abstract

The 38-acre Fischer and Porter Company Superfund Site is in Warminster Township, Bucks County, Pa. Historically, as part of the manufacturing process, trichloroethylene (TCE) degreasers were used for parts cleaning. In 1979, the Bucks County Health Department detected TCE and other volatile organic compounds (VOCs) in water from the Fischer and Porter on-site supply wells and nearby public-supply wells. The Fischer and Porter Site was designated as a Superfund Site and placed on the National Priorities List in September 1983. A 1984 Record of Decision for the site required the Fischer and Porter Company to pump and treat groundwater contaminated by VOCs from three on-site wells at a combined rate of 75 gallons per minute to contain groundwater contamination on the property. Additionally, the Record of Decision recognized the need for treatment of the water from two nearby privately owned supply wells operated by the Warminster Heights Home Ownership Association. In 2004, the Warminster Heights Home Ownership Association sold its water distribution system, and both wells were taken out of service. The report describes changes in groundwater levels and contaminant concentrations and migration caused by the shutdown of the Warminster Heights supply wells and presents a delineation of the off-site groundwater-contamination plume. The U.S. Geological Survey (USGS) conducted this study (2006-09) in cooperation with the U.S. Environmental Protection Agency (USEPA). The Fischer and Porter Site and surrounding area are underlain by sedimentary rocks of the Stockton Formation of Late Triassic age. The rocks are chiefly interbedded arkosic sandstone and siltstone. The Stockton aquifer system is comprised of a series of gently dipping lithologic units with different hydraulic properties. A three-dimensional lithostratigraphic model was developed for the site on the basis of rock cores and borehole geophysical logs. The model was simplified by combining individual lithologic units into generalized units representing upward fining sedimentary cycles capped by a siltstone bed. These cycles were labeled units 1 through 8 and are called stratigraphic units in this report. Groundwater in the unweathered zone mainly moves through a network of interconnecting secondary openings--bedding-plane fractures and joints. Groundwater generally is unconfined in the shallower part of the aquifer and confined or semiconfined in the deeper part of the aquifer. The migration of VOCs from the Fischer and Porter Site source area is influenced by geologic and hydrologic controls. The hydrologic controls have changed with time. Stratigraphic units 2 and 3 crop out beneath the former Fischer and Porter plant. VOCs originating at the plant source area entered these stratigraphic units and moved downdip to the northwest. When the wells at and in the vicinity of the site were initially sampled in 1979-80, three public-supply wells (BK-366, BK-367, MG-946) and three industrial-supply wells (BK-368, BK-370, and BK-371) were pumping. Groundwater contaminated with VOCs flowed downdip and then northeast along strike toward well BK-366, downdip toward well BK-368, and downdip and then west along strike toward well MG-946. The long axis of the TCE plume is oriented about N. 18° W. in the direction of dip. In 1979-80, the leading edge of the plume was about 3,500 feet wide. With the cessation of pumping of the supply wells in 2004, the size of the plume has decreased. In 2007-09, the plume was approximately 2,000 feet long and 2,000 feet wide at the leading edge. On the western side of the site, TCE and tetrachloroethylene (PCE) appear to be moving downdip though stratigraphic unit 3. The downdip extent of TCE and PCE migration extended approximately 550 feet off-site to the northwest and 750 feet off-site to the north. TCE concentrations in water samples from wells at the western site boundary increased from 1996 to 2007. On the northern side of the site, TCE and PCE appeared to be moving downward and laterally though stratigraphic units 2, 3, and 4. Groundwater-flow directions shifted to the northwest in the intermediate and deep zones after cessation of pumping of well BK-366 in 2004. The shutdown of the Warminster Heights wells had little effect on the direction of groundwater flow in the shallow zone. In 2007, TCE concentrations measured in water samples from the three remediation wells by the USGS ranged from less than 340 to 3,000 µg/L, and PCE concentrations ranged from less than 8.4 to 51 µg/L. TCE concentrations in water samples from the source-area remediation wells have decreased with time but remain highly variable. From 2001 to 2008, the TCE and PCE concentrations in water samples from wells BK-370 and BK-371 showed a linear decreasing trend. TCE and PCE concentrations in water samples from well BK-1324 showed an exponentially decreasing trend. In 2007, TCE concentrations measured in water samples from shallow wells ranged from less than 0.1 to 14,000 µg/L, and PCE concentrations ranged from less than 0.1 to 340 µg/L. The TCE and PCE plumes followed the hydraulic gradient in the shallow zone. In 2007, TCE concentrations measured in water samples from on-site intermediate-depth monitor wells ranged from less than 0.1 to 500 µg/L, and PCE concentrations ranged from 1.3 to 28 µg/L. The TCE and PCE plumes followed the hydraulic gradient in the intermediate zone and extended off-site to the north and northwest of the source area. Concentrations of TCE in water samples north and west of the source area increased from 1996 to 2007. In 2007, the TCE concentrations measured in water samples from on-site monitor wells in the deep zone ranged from 1.1 to 86 µg/L, and PCE concentrations ranged from less than 0.1 to 8.4 µg/L. The TCE and PCE plumes generally followed the hydraulic gradient in the deep zone and extended off-site to the northwest of the source area. In general, concentrations of TCE in water samples from monitor wells outside the source area increased between 1996 and 2005 and decreased between 2005 and 2007; concentrations were less in 2007 than in 1996.