HR: 13:55h
AN: H13L-02    [Abstracts]
TI: Borehole Flow and Contaminant Distribution in Sedimentary-Bedrock Recharge Areas
AU: * Williams, J H
EM: jhwillia@usgs.gov
AF: U. S. Geological Survey, 425 Jordan Road, Troy, NY 12180 United States
AB: Borehole flow and contaminant distribution has been investigated at a series of VOC-impacted sites in New York, New Jersey, and California that are recharge areas for sandstone, mudstone, and carbonate-bedrock aquifers. At the investigated sites, downward flow occurs through open boreholes that intersect shallow and deep fracture zones having differing hydraulic heads. Detected flows span the dynamic measurement range of 0.03 to 30 liters per minute for high-resolution flowmeters that employ heat-pulse and electromagnetic technologies. Estimated head differences between shallow and deep zones from model simulation of ambient and stressed flow profiles range from one to more than 10 meters, and are in close agreement with differences measured between zones isolated by straddle packers or by multiple-depth well completions. The presence of boreholes that are open temporarily during installation or completed as open holes impacts contaminant distribution and adversely affects the results of ground-water quality sampling programs. In boreholes open to shallow contaminated zones and deep non-contaminated zones, ambient flow diverts dissolved VOCs from the shallow zones down the borehole and into the deep zones. Samples obtained from such boreholes at purge and sampling rates less than the ambient downward flow represent the water quality of the shallow zones regardless of the depths where samples are collected. This can lead to the erroneous interpretation of a deep contaminant plume. Sampling by use of straddle-packer systems with low purge and sampling rates and short-term zone isolation also may indicate deep contamination because of inadequate flushing. Even in deep zones isolated by multiple-depth completions, elevated VOC concentrations linger for many months because of contaminant diffusion into and out of the bedrock matrix. Conversely, in boreholes open to shallow non-contaminated zones and deep contaminated zones, samples collected at rates less than the downward flow represent the water quality of the shallow zones and the presence of a deep plume is masked.
DE: 1828 Groundwater hydraulics
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: Fall Meeting 2005