Determination of alternate concentration limits for groundwater at the Midvale Slag superfund site

Historic smelting and ore refining operations at the Midvale Slag Superfund Site Operable Unit 2 (OU2), located in Salt Lake County, Utah, have resulted in contamination of the water table aquifer (the shallow unconfined aquifer). Concentrations of arsenic, cadmium, selenium and antimony in groundwater exceed remediation goals (MCL or MCLG), and no practicable remedial alternatives were identified to reduce the observed concentrations to acceptable levels within a reasonable time frame. In the Superfund program, alternate concentration limits (ACLs) may be established as cleanup levels in lieu of those otherwise applicable (e.g. MCLs) if the conditions specified in the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) Section 121(d)(2)(B)(ü) are met, and, according to the NCP, active restoration of groundwater is not practicable. Conditions at the Midvale Slag Site were evaluated and determined to satisfy the general requirements for establishing ACLs according to CERCLA. Arsenic, the primary chemical of concern (COC), is utilized to illustrate the ACL determination methodology. The ACL value is determined by examining the present and predicted distribution in the aquifer then calculating an allowable groundwater concentration for two scenarios, (a) assuming that the applicable surface water quality criteria cannot be exceeded at low flow, and (b) assuming that the groundwater discharge will not result in a statistically significant increase in the surface water concentration under average surface water flow conditions. The ACL value calculated for arsenic is 7,000 µg/L.