Factors Affecting Hydraulic Conductivity of High pH, High Silica Groundwater at the Former Occidental Chemical Corporation Facility

Abstract

Pollution of water and sediments in and along the Port of Tacoma, WA, has resulted from the historic discharge of chlorinated solvents and caustic materials from the former Occidental Chemical Corporation (OCC) Facility. A pump-and-treat (P&T) system is in operation, yet the extraction of contaminated groundwater with elevated pH is being avoided because doing so has previously fouled well screens. This study used batch and column tests to identify factors that affect hydraulic conductivity of OCC site soil in response to high pH groundwater. High pH groundwater at the OCC site has elevated concentrations of Si and Al. These constituents dissolved from OCC site soil historically, though whether dissolution is still active in the OCC subsurface remains to be ascertained. It is however clear that precipitation of Si-based solids causes the soil to plug, reducing hydraulic conductivity. Soil plugging is due to the interactions between the soil and Si and Al in the groundwater, though Al has a greater effect. This study shows that Darcy velocity of groundwater and specific groundwater-soil interactions have the largest impacts on hydraulic conductivity of OCC site soil when high pH, high Si groundwater passes through it. When the Darcy velocity exceeds a specific threshold, hydraulic conductivity reduces over time, though soil plugging does not occur below this threshold. OCC site soil that has been exposed to high pH, high Si groundwater also has a higher Darcy velocity threshold for soil plugging than soil that has not been exposed to this groundwater. Therefore, P&T efforts can be designed to explore options that may ultimately enable long-term extraction and ex-situ treatment of high pH, high Si groundwater.