Trace Metal and Phosphorus Loading from Groundwater Seepage into South Fork Coeur d'Alene River After Remediation at the Bunker Hill Superfund Site, Northern Idaho, 2022.

Widely dispersed waste products from historical mining in northern Idaho's Coeur d'Alene mining district have long been a concern in the Coeur d'Alene River Basin in northern Idaho. The Central Impoundment Area (CIA), an unlined mining waste repository that is part of the Bunker Hill Superfund Site designated in 1983, is adjacent to the South Fork Coeur d'Alene River between Kellogg and Smelterville, Idaho. Previous studies, including a pre-remediation seepage study completed by the U.S. Geological Survey (USGS) in 2017, have identified groundwater seepage from beneath the CIA as a major contributor to trace-metal and nutrient loads (including zinc, cadmium, and phosphorus) in the South Fork Coeur d'Alene River. A major remediation project, led by the U.S. Environmental Protection Agency from late 2017 to 2021, specifically aimed to reduce groundwater loading to the river via a groundwater collection system (GWCS) at the CIA. In 2022, the USGS completed a post-remediation seepage study to quantify zinc, cadmium, and phosphorus loading from groundwater to the South Fork Coeur d'Alene River in the same reach as the 2017 pre-remediation study. Like in the previous USGS study, discharge measurements and water-quality samples were collected during base-flow conditions in the South Fork Coeur d'Alene River between Kellogg and Smelterville as well as in surface-water inputs to the reach. Results of this study show a reduction in groundwater loads of dissolved zinc, dissolved cadmium, and total phosphorus entering the South Fork Coeur d'Alene River compared to 2017. The largest reductions in groundwater loading to the South Fork Coeur d'Alene River occurred in a discrete section (the middle section) of the reach adjacent to the CIA where the GWCS was expected to have the biggest impact. In the South Fork Coeur d'Alene River middle section, loads from groundwater (presented as a mean plus or minus [±] standard deviation) of dissolved zinc decreased from 85 ± 9.3 kilograms per day (kg/d) in 2017 to 11.6 ± 19.2 kg/d in 2022 (86-percent reduction), dissolved cadmium decreased from 0.59 ± 0.10 kg/d in 2017 to 0.11 ± 0.06 kg/d in 2022 (81-percent reduction), and total phosphorus decreased from 6.5 ± 0.45 kg/d in 2017 to 0.79 ± 0.97 kg/d in 2022 (88-percent reduction). In addition to reduced groundwater loading, lower concentrations of dissolved zinc, dissolved cadmium, and total phosphorus were observed at the site farthest downstream from the GWCS. Furthermore, the ambient water-quality-criteria ratios decreased at all river monitoring sites in 2022, although zinc and cadmium concentrations still exceeded the site-specific criteria designated to protect aquatic life. This post-remediation study indicates that the GWCS at the CIA has reduced groundwater loading of trace metals and phosphorus to the South Fork Coeur d'Alene River. This reduction in trace metals and phosphorus in South Fork Coeur d'Alene River also has implications for water quality downstream in the main-stem Coeur d'Alene River and in Coeur d'Alene Lake. [ABSTRACT FROM AUTHOR]