Exploring the Treatability of 1,4-dioxane and PFAS in Marana, Arizona Groundwater

Abstract

Despite phasing out production decades ago, toxic pollutants such as per-and polyfluoroalkyl substances (PFAS) and 1,4-dioxane persist in the environment and contaminate drinking water supplies across the United States. Tucson Water’s (TW) isolated Silverbell West water system has been shut down due to the increasing public health risk from the migrating groundwater plumes of these compounds. The groundwater wells were shut down due to violation of Tucson Water’s self-imposed regulatory limits for PFAS and 1,4-dioxane: 18 ppt and 0.35 ppb, respectively. To guide the decision-making process, rapid small scale column tests (RSSCT), point of use (POU) device bench studies, and ultra-violet/advanced oxidation process (UV/AOP) testing were conducted to determine the appropriate treatment solution for this site. The RSSCT experiments revealed that ion exchange (IX) media, Evoqua and Purofine, reduced contaminant concentrations below TW operations targets until nearly 100k bed volumes (BVs) of spiked groundwater passed through the column. When granulated activated carbon (GAC) was the adsorption media, PFAS broke through above TW’s target at nearly 50k BVs. POU bench tests supported commercial devices' claims in removing PFAS to at least their filtration cartridge life. However, the POU device selected for 1,4-dioxane removal failed to remove the compound past 10% of its expected cartridge life. For UV/AOP, it was determined that using chlorine as an oxidant may be a possible solution for degrading 1,4-dioxane to regulatory levels. Two treatment trains have been designed: 1) UV/AOP and GAC and 2) IX, UV/AOP, and GAC to remove the two compounds. The two treatment train costs were normalized to three years of operation or 21 million gallons of treated water produced. Treatment option 1 is the more suitable choice for Silverbell West remediation site, due to lower construction and operation and maintenance costs.