INVESTIGATING THE HYDRAULIC INTEGRITY OF AN AQUITARD, CONFINING UNIT 1, COLUMBIA SOUTH SHORE WELL FIELD, CITY OF PORTLAND, OREGON

The City of Portland, Oregon, owns and operates the Columbia South Shore Well Field (CSSW) in northeast Portland, which has a peak capacity of approximately 100 million gallons a day (mgd). The CSSW acts as the supplementary and emergency drinking water supply for the City of Portland. Dense non-aqueous phase liquid (DNAPL) contamination exists, from historic releases, in the uppermost layers of the CSSW, which are named the overbank deposits (OD) and the Troutdale gravel aquifer (TGA). Confining unit 1 (CU1), a heterogeneous fine-grained aquitard, underlies the TGA and may act as a barrier for the downward vertical migration of DNAPL chemicals into the lower Troutdale sand aquifer (TSA). The TGA is a productive aquifer for the City – seven production wells with a combined peak capacity of approximately 20mgd – so they are interested in keeping it clean. A three-dimensional model of the lithology of CU1 has been correlated from well log data within the CSSW. The mean vertical hydraulic conductivity and conductance of CU1 have been calculated, throughout the CSSW, using the type and thickness of correlated lithologic layers. These calculations have been plotted in plain view to examine the spatial heterogeneity of the estimated hydraulic parameters of CU1 across the CSSW. The results of this work have been compared to groundwater elevation data recorded in multiple hydrogeologic units in the CSSW over the past ten years. The comparison supports the results of the correlation in many areas, but suggests inaccuracy in the correlation in other areas. The heterogeneity of CU1 is extremely complex and some of the assumptions made during the correlation processes may have been invalid. However, this study will support the next step of the City’s on-going effort to manage the historic plumes in the CSSW by indentifying areas where more data is needed. Also, the locations of the higher conductance zones, suggested by the model, are the most helpful results. Especially, those zones of weakness that have been supported by the hydraulic head fluctuations. The City can view these areas as possible conduits for water and DNAPL contamination to leak across CU1 and into Portland’s drinking water supply.