THE SCALE AND FREQUENCY OF SAMPLING: RETHINKING A CONCEPTUAL MODEL

CORLEY, Helen P., and MARTIN, Robert L. Ogden Environmental and Engineering Services, Inc. 300 S. Old Statesville Rd., Huntersville, NC 28078 and MACKLIN, Roberta L., Avery Dennison Corporation, 409 Fortune Blvd., Milford, MA 01757

Natural attenuation and monitoring had been selected as the appropriate response to the release of non-chlorinated solvents into ground water from aboveground storage tanks and/or ancillary structures at a printing facility in western South Carolina. The site, underlain by approximately 10.5 meters of saprolite, consists of sandy silt and sandy clay resulting from chemical weathering of granite and granitic gneiss. Commonly accepted investigative techniques proved inadequate to define the plume geometry or provide accurate contaminant transport characteristics to predict off-site risks. A comprehensive site characterization was recently implemented, and significant spatial and temporal variability of VOC concentrations and hydrogeologic properties was discovered. The investigation utilized the Waterloo Profiler, a specialized direct push ground-water sampling tool developed by the University of Waterloo, along with two gas chromatographs to provide near real-time data for use in assessing the 3-D distribution of solutes on a very small scale. The Waterloo Profiler was also used to provide a continuous record of the relative distribution of hydraulic conductivity to identify high and low conductivity zones within the saprolite. The small-scale sampling resulted in the following new information: the hydraulic gradient at the small scale of measurement was 35 degrees different from the regional gradient; multiple high concentration plume cores (sometimes plunging) were identified within zones of relatively high hydraulic conductivity; and, significant concentration gradients existed both horizontally and vertically.

The VOC temporal distribution has also varied greatly in the monitor wells sampled from 1995 to 1998. The control on the temporal distribution appears to be the significantly fluctuating water table elevation. When the water table was lower and thereby closer to the top of screen, the VOC plume appeared more extensive and concentrated. When the water table was higher, the VOC plume appeared to shrink and decline in concentration.

Natural attenuation and monitoring is again recommended but currently far more defensible due to the increased level of understanding of the hydrogeologic heterogeneities.