Investigating a groundwater solute plume in three dimensions

Abstract

Groundwater contaminant plumes are often studied in two dimensions, but in some cases it is essential to characterize them in all three dimensions. The depth of a plume can be investigated by several methods, including completing wells at multiple depths at the same location; using inflatable packers to isolate an interval of borehole for sampling; and sampling a well at multiple depths without use of packers. A contaminant plume in the Triassic Culpeper Basin in northern Virginia was investigated using these methods. It was found to be about 12 000 feet long, up to 3000 feet wide, and as much as 1000 feet deep. Because of surface recharge and pumping from a deep well at the downgradient end of the plume, its center descended from the surface at the upgradient end, near a possible source area, to a depth of about 400 feet at the pumping well. Characterizing the plume in three dimensions greatly increased the confidence that all of the contaminant had been found, and provided valuable guidance for the design of possible remedial systems. Shallow wells installed early in the overall investigation would have given a misleading picture of the plume, since they showed much less than the actual maximum concentration at that point, if they had not been supplemented by information obtained deeper in the plume.