Pollutant loads have been increasing over time in the Elk River, B.C. due to coal mining operations and runoff from associate spoil piles. The Elk River is a tributary to the Kootenay/Kootenai River and Lake Koocanusa. Extensive water chemistry monitoring has been conducted in Lake Koocanusa to assess the impacts from the Elk River, however, this is not the case for the Kootenai River downstream of Lake Koocanusa, downstream of Libby Dam (http://deq.mt.gov/DEQAdmin/LakeKoocanusa). This study generated data on selenium and nutrient concentrations and loads in the Kootenai River (Libby Dam to Canadian border), which will help to differentiate between local loads and loads transported via Lake Koocanusa. Working in cooperation with the U.S. Environmental Protection Agency (USEPA), the U.S. Geological Survey (USGS) collected and analyzed water-column samples in the Kootenai River watershed downstream of Libby Dam. Field data collection also included discharge measurements. The following are key design components of water quality sample collection in the Kootenai River watershed downstream of Libby Dam: Sampling locations: (1) USGS gages on the mainstem Kootenai River: Kootenai River below Libby Dam, MT (USGS site identification number 12301933), Kootenai River at Leonia, ID (12305000), Kootenai River at Tribal Hatchery near Bonners Ferry, ID (12310100), and Kootenai River at Porthill, ID (12322000); (2) USGS gages on tributaries to the Kootenai River: Fisher River near Libby MT (12302055), Yaak River near Troy MT (12304500); (3) Ungaged tributaries: Moyie River near the mouth (12307750). Water quality analyses: (1) Low-level nutrients, total water (3 sampling events) (2) Total selenium, dissolved water (3 sampling events) (3) Selenium speciation, dissolved water (2 events): selenite, Se(IV); selenate, Se(VI); selenocyanate, SeCN; selenomethionine, SeMet; and methylseleninic acid, MeSe(IV).
Total dissolved selenium and selenium speciations analyses were performed by Brooks Applied Labs, Bothell, WA. The nutrient samples were analyzed by the USGS National Water Quality Laboratory (NWQL), Lakewood, CO. Secondary, total dissolved selenium analyses were also conducted by the NWQL for a subset of samples for interlaboratory comparisons. The three sampling events were: (1) September 2018, a period of low and stable flows in both the Kootenai River and tributaries; (2) December 2018, a period of low, stable winter flows in the tributaries but unstable and increasing flows in the Kootenai River due to dam releases in preparation for spring runoff (3) May 2019, a period close to annual peak runoff from snowmelt in tributaries plus high dam releases of snowmelt originating in the Canadian Rockies. Results: Results of the selenium sampling are compiled and summarized in this data release. These data and the nutrient data are separately published through the USGS National Water Information System (https://doi.org/10.5066/F7P55KJN). No samples exceeded USEPA’s national recommended aquatic life criteria for selenium, expressed as a water concentration, of 3.1 µg/L. Selenium concentrations in the four mainstem Kootenai River sites had low variability across sites and sampling events, ranging from 0.628 to 1.17 µg/L for the 12 samples, for an average concentration of 0.91 µg/L (± 0.17 µg/L, standard deviation). In all measured concentrations, selenate was the dominant chemical species, accounting for 87% of the total selenium, on average (range 79-94%). Selenium concentrations were less than the detection limit of 0.037 µg/L in all of the tributary samples, indicating that selenium concentrations in the Kootenai River cannot be attributed to tributary sources. Daily average selenium loads in the Kootenai River were estimated by assuming that the width and depth integrated water samples collected were representative of average conditions for that day and were multiplied by daily average discharge volumes of the Kootenai River at those locations. In the September 2018 low flow sampling, estimated daily average selenium loads were similar at the 4 mainstem sampling locations in the Kootenai River from Libby Dam to the Idaho/British Columbia border, ranging from about 12.1 to 14.5 kilograms per day (kg/d). In the December 2018 winter dam release sampling, calculated selenium loads declined from 63.7 kg/d at Libby Dam to 42.3 kg/d at Porthill, ID near the Canada/US border. However, flows were rapidly changing during that sampling event, giving uncertainty to these December 2018 daily load estimates. In the May 2019 high flow sampling, estimated loads were similar throughout the 4 mainstem Kootenai River sites, ranging from about 48.1 to 53.4 kg/d.