Trace metal and nutrient loads from groundwater seepage into the South Fork Coeur d’Alene River near Smelterville, northern Idaho, 2017

The Coeur d’Alene mining district in northern Idaho historically was a globally important source of lead, zinc, and silver, but over 100 years of mining has left a legacy of metals contamination in the Coeur d’Alene River valley. Previous studies by the U.S. Geological Survey (USGS) and others have indicated that groundwater discharging into the South Fork Coeur d’Alene River between Kellogg and Smelterville, Idaho, is a substantial source of dissolved zinc, dissolved cadmium, and total phosphorus. As part of its ongoing cleanup efforts, the U.S. Environmental Protection Agency is constructing a groundwater collection and treatment system to intercept and treat this contaminated water before it reaches the river.

To establish conditions prior to construction, the USGS conducted a seepage study in September 2017 to quantify the rate and quality of groundwater discharging into the South Fork Coeur d’Alene River between Kellogg and Smelterville. Repeated measurements of streamflow were taken at multiple locations in the river and tributaries, and water-quality samples were collected and analyzed for trace metals and nutrients. Results showed consistent increases in streamflow (5.8 ± 1.3 cubic feet per second); and in dissolved zinc (85 ± 9.3 kilograms per day [kg/d]), dissolved cadmium (0.58 ± 0.10 kg/d) and total phosphorus (6.3 ± 0.45 kg/d) loads in a discrete segment of the reach. These gains exceeded tributary inputs, thereby implicating groundwater discharge as the main source of loading. Zinc and cadmium loads from groundwater in 2017 were less than those measured in 1999 but comparable to those measured from 2003 to 2008. This suggests that remedial actions in the late 1990s and early 2000s decreased trace-metal loading from 1999 to 2003, but that conditions remained similar from 2003 to 2017. A second seepage study will be conducted after construction and treatment plant system optimization are complete; this second study will evaluate changes in groundwater discharge to and water quality in the South Fork Coeur d’Alene River compared to the pre-construction conditions.