Groundwater Residue Sampling Design - American Chemical Society

Chapter 14

Water Quality Sampling Program at Low-Level Radioactive Groundwater Contamination Site Wood River Junction, Rhode Island 1

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Downloaded by NORTH CAROLINA STATE UNIV on May 13, 2018 | https://pubs.acs.org Publication Date: June 20, 1991 | doi: 10.1021/bk-1991-0465.ch014

Barbara J. Ryan and Denis F. Healy 1

U.S. Geological Survey, Washington, DC 20240 U.S. Geological Survey, Hartford, CT 06103 2

The U.S. Geological Survey conducted a three-year research study of ground-water contamination at a low-level radioactive waste site in southern Rhode Island. One goal of the study was to collect water samples that accurately represented water-quality conditions in the aquifer, while minimizing the variability due to sampling method, the potential for cross -contamination, and the time required for sample collection. The water -quality-sampling program consisted of establishing an observation well network, adopting and standardizing sampling procedures, and determining an optimum sampling frequency. A network of 150 observation wells was used to determine direction of ground-water flow and spatial variations (horizontal and vertical) in ground -water quality. The sampling procedures involved the following three steps for each observation well: (1) insertion of a piece of 0.9 cm (inside diameter) polyvinylchloride suction tubing into the well and position its intake 0.6 to 0.9 m below the water level; (2) evacuation of approximately three times the volume of water in the well with either a centrifugal or peristaltic pump until steady-state conditions (stable specific conductance) were reached; and (3) attachment of a smaller variable-speed peristaltic pump to the tubing for sample collection and field measurements. From April 1981 through January 1984, a bimonthly sampling frequency for 30 to 75 observation wells was used to collect a total of 1,000 samples. Spatial variations (horizontal and vertical) of gross-beta concentrations over very short distances suggest that samples collected correctly reflected water-quality conditions in the aquifer. The major drawback of the system was lack of control of pumping rates.

This chapter not subject to U.S. copyright Published 1991 American Chemical Society

Nash and Leslie; Groundwater Residue Sampling Design ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


14.

RYAN & HEALY

Sampling at Radioactive Contamination Site

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The collection of water samples representative of the water quality in an aquifer is an important part of ground-water-contamination studies. As part of a 3-year research study by the U.S. Geological Survey of ground-water contamination at a low-level radioactive-waste site in southern Rhode bland, ground-water sampling procedures were adopted and standardized to ensure that samples accurately represented chemical conditions in the aquifer while minimizing the variability due to sampling method, the potential for cross contamination, and the time required for sample collection. From April 1981 through January 1984, a bimonthly sampling frequency for 30 to 75 observation wells was used to collect a total of 1,000 samples. The purpose of this paper is to describe the methodologies and equipment used at this site for the collection of ground-water samples for radiological analyses. The methods presented here may have applicability to other similar ground-water contamination studies. From 1964 through 1980, an enriched uranium cold-scrap recovery plant was operated near Wood River Junction, Rhode Island (Figure 1). The recovery process involved digestion of the scrap with hydrofluoric and nitric acids, and organic separation with tributyl phosphate and kerosene. Solid wastes from the process were shipped offsite for disposal. Liquid wastes were discharged to the Pawcatuck River through a drain pipe from 1964 through 1966, and to polyethylene- and polyvinyl-chloride (PVC)-lined evaporation ponds and trenches from 1966 through 1980. Overflow of the existing ponds due to precipitation runoff and high disposal flow rates led to periodic construction of additional ponds and trenches, which eventually encompassed approximately 2,300 m . Liquid contaminants from these ponds and trenches percolated to the water table and formed a plume of contaminated ground water. 2

Site Description The study area, located within the lower Pawcatuck River basin, is approximately 3 km east of the confluence of the Pawcatuck and Wood Rivers. The aquifer is composed of unconsolidated sands and gravels of Pleistocene age. Sediments consist of predominantly medium to coarse sands and gravels to about 24 m below land surface and mostly fine sands and silts below a depth of 24 m. The water table slopes westward from the plant site at an average gradient of 14 m/km. Ground-water-flow velocities are estimated to range from 0.6 to 0.8 m/d (1). During low-flow periods (late summer and early autumn), the water table ranged from 0.6 m below land surface in a swamp at the western edge of the Pawcatuck River to approximately 8 m below land surface at the eastern side of the Pawcatuck River; water levels were higher during the rest of the year. Well yields ranged from 1 L/min in wells screened in fine sands and silts to approximately 20 L/min in wells screened in coarse sands and gravels. The plume of contaminated ground water extends a total of 700 m (Figure 2). From the source area, it extends northwestward approximately 460 m to the Pawcatuck River where it turns southwestward, extending approximately 240 m in a downstream direction through the swampy area west of the river. The plume is approximately 90 m wide and is confined to


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RYAN & HEALY



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GROUNDWATER RESIDUE SAMPLING DESIGN

the upper 24 m of saturated thickness of sediments that consist of medium to coarse sand and gravel (Figure 3). East of the Pawcatuck River the top of the contamination plume is depressed below the water table, and its depth increases away from the source area. The maximum depth of the plume (24 m below land surface) is present 425 to 460 m from the source area. Contaminantsriseto land surface in the discharge area at the river and adjacent swamp. Chemical and radiochemical constituents in contaminated ground water include nitrate, 5 to 1,200 mg/L; boron, 20 to 1,000 ug/L; potassium, 3 to 26 mg/L; strontium-90, 4 to 290 pCi/L (picocuries per liter); and technetium-99, 75 to 1,350 pCi/L. Concentration of gross-beta emitters in contaminated water range from 5 to 1,600 pCi/L, and specific conductance ranges from 150 to 5,400 uS/cm (microsiemens per centimeter at 25 degrees Celsius). Concentrations of chemical and radiochemical constituents in uncontaminated water at the site are commonly below the following detection levels: Nitrate,

Groundwater Residue Sampling Design - American Chemical Society

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