Identification of layers of recalcitrant PCE were mapped with repeated vertical profiling and with repeated well sampling. In the absence of a source term, rates of PCE decline are governed by attenuation processes affecting the PCE. Results from the vertical profiling show high variability in rates of PCE declines. PCE declines can be described with a first order rate constant of -0.29 to -0.71 per year over parts of the aquifer; however, some layers show negligible changes over several years, indicating a potential source is present.

The distribution of the recalcitrant layers was mapped vertically by analyzing borehole gamma logs in conjunction with PCE concentrations. Borehole gamma logs measure radioactive decay of several elements and can indicate sediment grain size. At several locations, recalcitrant PCE concentrations correlated with large changes in gamma signals, suggesting the presence of stratigraphic contacts. Lithologic data collected during test drilling corroborated that gamma signal changes are due to lithologic variation. The highest PCE concentrations occurred within well sorted sands and gravels situated above relatively poorly sorted, less permeable sediments. The occurrence of the highest PCE concentrations within the more permeable layers suggests that either a residual PCE source is located above the less permeable layers or residual PCE is vertically migrating into the permeable layer from the less permeable layers.