ABSTRACT

Several shutdown-rebound tests have been conducted at the Henderson Road Superfund Site, which has been on the U.S. Environmental Protection Agency’s National Priorities List since 1984. For a given test, the extraction wells are turned off, and water samples are collected from selected monitor wells at regular intervals before and during cessation of pumping to monitor for changes in chemical concentrations. A long-term shutdown-rebound test began on July 17, 2006. In support of this test, the U.S. Geological Survey conducted this study to determine the effects of shutting down on-site extraction wells on concentrations of selected contaminants and water levels. Concentrations were compared to ARARs (applicable relevant and appropriate requirements), which were set as remediation goals in the Henderson Road Site Record of Decision.

Water from 10 wells in and near the source area and to the north, northeast, and northwest of the source area sampled in 2008 exceeded the 5.52 µg/L (micrograms per liter) ARAR for benzene. The greatest changes in benzene concentration between pre-shutdown samples collected in July 2006 and samples collected in February and March 2008 (19 months after the shutdown) were for wells in and north of the source area; increases in benzene concentration ranged from 1.5 to 164 µg/L.

Water from five wells in the source area and to the north and northwest of the source area sampled in 2008 exceeded the 60 µg/L ARAR for chlorobenzene. The greatest changes in chlorobenzene concentration between pre-shutdown samples collected in July 2006 and samples collected in February and March 2008 were for wells north of the source area; increases in chlorobenzene concentration ranged from 6.9 to 99 µg/L. The highest concentrations of chlorobenzene were near or outside the northern site boundary, indicating chlorobenzene may have moved north away from the source area; however, no monitor well clusters are on the northern side of the Pennsylvania Turnpike, which is about 190 feet north of the source area. A much larger area was affected by chlorobenzene than benzene. Chlorobenzene concentrations decreased in the source area and increased at and beyond the site boundary.

Water from four wells in and northeast of the source area sampled in 2008 exceeded the 5.06 µg/L ARAR for 1,1-dichloroethane (1,1-DCA). Increases in 1,1-DCA concentration between pre-shutdown samples collected in July 2006 and samples collected in February 2008 ranged from 0.4 to 20 µg/L. Water from two wells in the source area sampled in 2008 exceeded the 175 µg/L ARAR for total xylene. The 1,1-DCA and xylene plumes appear to extend in an east-northeast direction from the source area.

Large drawdowns in the Upper Merion Reservoir during droughts in 1998 and 2001 affected water levels in the Chester Valley and at the Henderson Road Site, except for well HR-17-170. After the drought of 2001, water levels in the Chester Valley showed a protracted recovery lasting from September 2001 until June 2005 (46 months).

Water-level data were evaluated temporally for 1997–2008 and spatially for (1) June 16, 2003, when the extraction wells were pumping at the full rate prior to the start of the June 2003 shutdown test; (2) July 10, 2006, during the period of reduced pumping after the June 2003 shutdown test; and (3) February 25–29, 2008, when the extraction wells were not pumping. Except for well HR-5-195, wells were categorized as shallow, intermediate-depth, and deep wells. The potentiometric surface for shallow wells did not appear to be affected by pumping of the extraction wells. The general direction of ground-water flow was to the north. The potentiometric surface for intermediate-depth wells showed a cone of depression when the extraction wells were pumping at the full rate but did not show a cone of depression when the extraction wells were pumping at the reduced rate. The ground-water-flow direction was toward the north and northeast, similar to that when the extraction wells were pumping.

Water-level data were available for only four on-site deep wells when the extraction wells were pumping. The available water-level data showed a flat hydraulic gradient. When the wells were pumping at the full rate, the ground-water-flow direction was to the Henderson Road Site from the northwest; when the wells were pumping at the reduced rate, the ground-water-flow direction was reversed, and flow was away from the Henderson Road Site to the northwest. When the extraction wells were not pumping, the ground-water-flow direction was away from the Henderson Road Site toward the north and northeast.

Benzene concentrations measured in water samples from shallow wells in February and March 2008 were above the ARAR only in the source area. Benzene concentrations measured in water samples from intermediate-depth wells were above the ARAR in the source area and beyond the northern site boundary. Benzene concentrations measured in water samples from deep wells were above the ARAR in the source area and at and beyond the northern site boundary. Benzene appears to have moved to the north away from the source area in the intermediate and deep zones.

Chlorobenzene concentrations measured in water samples from shallow wells collected in February and March 2008 were above the ARAR beyond the northern site boundary northwest of the source area. Chlorobenzene concentrations measured in water samples from intermediate-depth wells were above the ARAR in the source area and beyond the northern site boundary northwest of the source area. Chlorobenzene concentrations measured in water samples from deep wells collected in February and March 2008 were above the ARAR at and beyond the northern site boundary. Chlorobenzene appears to have moved to the north away from the source area in the shallow, intermediate, and deep zones.

Off-site wells were drilled to monitor the possible movement of chemicals of concern from the Henderson Road Site. Concentrations of benzene and chlorobenzene above the ARARs were measured in water samples from well cluster HR-7 at the northern site boundary and well cluster HR-6 beyond the northern site boundary on the southern side of the Pennsylvania Turnpike. No off-site monitor wells are located directly north of these well clusters on the northern side of the Pennsylvania Turnpike. Potentiometric-surface maps indicate the direction of ground-water flow from these well clusters is to the north. Additional monitor well clusters north of well clusters HR-6 and HR-7 on the northern side of the Pennsylvania Turnpike could help determine how far benzene or chlorobenzene have moved offsite and could be used to better define ground-water-flow directions north of the site.

First posted July 1, 2009