Multi-Media Assessment of Dissolved Metal Contamination in Park Lake, Montana Following the 2024 Arctic Grayling Fish Kill

Abstract

In July 2024, Park Lake in the Lewis and Clark National Forest near Helena, Montana experienced a mass fish kill event that resulted in the death of approximately 500–800 Arctic grayling. Experts speculated zinc, copper, lead, arsenic and cadmium may have contributed to the mortality of Arctic grayling by metal toxicosis. Although prior investigations may have been conducted following the event, results were not made publicly available. To address this gap, our team undertook an independent comprehensive analysis of the metal contamination across the Park Lake ecosystem and systematically compared our findings to regulatory toxicity limits. This study quantified dissolved metals in Park Lake to assess whether metal contamination was consistent with the observed fish kill. Water, sediment, vegetation, and fish tissue samples were collected from up to eight sites across the lake in two sampling campaigns: an initial response days after reporting of the 2024 fish kill, and an expanded survey in May 2025. Samples were analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES) for dissolved metal concentration of ten environmentally relevant metals (Ag, Al, As, Cd, Fe, Pb, Ni, Se, and Zn). Water quality parameters including pH, dissolved oxygen (DO), and temperature were also measured in the field. Measured metal concentrations were compared to Montana Department of Environmental Quality (DEQ-7) water quality standards for acute and chronic thresholds. Testing in late summer 2024 indicated that four out of the ten metals exceeded the toxicity thresholds by 3.4- to 22.7-fold. But testing in summer of 2025 showed that eight out of the ten metals exceeded the toxicity thresholds, now as high as 147-fold. Spatial analysis identified two inlets as primary contamination pathways as metal concentrations were up to 3.5x higher at inlets versus the lake body. Fish tissue exceeded permissible limits for four of the ten metals tested. These results indicate that multi-metal contamination at Park Lake is consistent with acute metal toxicosis as a primary contributor to the 2024 fish kill, and that contamination was ongoing via identifiable inlets. A third sampling campaign is planned for summer 2026 to assess whether these trends continue.