USE OF LOW-COST PLASTIC LINERS TO TEMPORARILY SEAL, COLLECT WATER SAMPLES FROM, AND MEASURE WATER LEVELS IN BOREHOLES OPEN TO BEDROCK

摘要

Temporary barriers can be used to limit the vertical flow of water and contaminants in open boreholes completed in fractured bedrock during idle periods between drilling, geophysical and hydraulic testing, and sampling. Available pneumatic packers or flexible vinyl liners are highly effective temporary barriers, but are relatively expensive. An alternate method was developed by the U.S. Geological Survey (USGS ) in cooperation with the U.S. Environmental Protection Agency (USEPA) in which lightweight, 6-mil-thick, low-density polyethylene (LDPE) tubing and readily available hardware were used to create flexible liners to seal open boreholes to depths of up to 250 feet. The cost of materials, including the liner material (USD0.10 per foot), rope, and hardware, needed to seal a 200- foot open borehole was about USD70. Thirteen liners were installed in six boreholes to depths of 90 to 250 feet. Leaks prevented a perfect seal in most cases, but use of the liners limited the vertical movement of contaminated water within the borehole at relatively low cost. As an alternative to the use of straddle packers, the liners were modified to allow the collection of water samples and measurement of water levels in discrete zones. A plastic mesh (designed to act as a screen) was installed at pre- selected intervals on the outside of the liner to create an annulus connected to a specific water-producing zone but isolated from the rest of the borehole. Diffusion-bag samplers, plastic tubes, and pressure transducers were placed outside the liner in these screened intervals. Difficulties emplacing the liner/screen combination once samplers were installed prevented full deployment in the boreholes in which this procedure was attempted; however, trichloroethylene (TCE) or tetrachloroethylene (PCE) concentrations in samples from diffusion bags in two boreholes were the same or nearly the same as those collected with conventional packer-testing methods, indicating that this technique may be useful as a reconnaissance tool. Installation of diffusion-bag samplers and a liner in a third borehole allowed the collection of water-quality samples from isolated, discrete depths that were otherwise unattainable because the hole was inaccessible to the packer-testing rig, showing that the liners may be a valuable tool at sites with restricted access. Water samples were collected from this borehole using a peristaltic pump connected to small-diameter tubes extending from the surface to depths of 60, 77, and 95 feet. TCE concentrations in these samples were 7, 12, and 63 micrograms per liter, respectively. Water-level differences between zones were detected in three of the boreholes, indicating that vertical gradients can be measured with the liners in place. Further development of low-cost liners with screens and comparison of this method to the traditional packer-testing method for collecting water-quality samples and measuring water levels in discrete zones in open boreholes is planned.