COMPARISON OF MERCURY MASS LOADING IN STREAMS TO ATMOSPHERIC DEPOSITION IN SELECTED WATERSHEDS IN THE WESTERN UNITED STATES: EVIDENCE FOR NON-ATMOSPHERIC MERCURY SOURCES

Many streams in the western United States (US) are listed as impaired by mercury (Hg) under the Clean Water Act, and it is necessary to understand the magnitude of various sources to implement management strategies. Wet and dry atmospheric deposition can be major contributors of aquatic Hg contamination, along with mine wastes and natural sources such as rock weathering. After atmospheric Hg deposition, subsequent transport is mostly dependent on erosive processes mobilizing particulate Hg. Prior studies in the eastern US showed that streams typically deliver less than 50% of atmospherically deposited Hg annually. We compared annual river Hg loads at 25 watersheds in the western US to the sum of measured wet and modeled dry Hg deposition. Data from the Mercury Deposition Network was used to estimate Hg loading from precipitation. Dry deposition of Hg was estimated using the Community Multi-scale Air Quality model. Watershed area ranged from less than 1 to 70,000 km². Land cover included agricultural, urban, and forested categories, plus localized mining of Hg and precious metals that used Hg amalgamation. This allowed comparison of natural and anthropogenic influences. Annual loads of Hg in streams were obtained from the literature or modeled using Hg concentration and discharge data. At a pristine location in the Rocky Mountains, the ratio of stream Hg load to atmospheric deposition was 0.2 during a year of average precipitation. In contrast, at Greenhorn Creek in the Sierra Nevada foothills of California, with known Hg contamination from historical placer gold mining, the average ratio was 70 over three years. Other watersheds with historical gold, silver, or mercury mines that were less than 500 km² in area had ratios greater than 1.0. The lower Sacramento River (agriculture, urban, natural, and mining land uses) was the largest watershed for which Hg riverine loads were available. The ratio of stream Hg load to atmospheric deposition was 0.10. Although Hg was used and lost to the environment in upstream historical mining operations in the Sacramento River watershed, downstream river Hg loading is mitigated by reservoirs, which trap sediment. This study represented the first compilation of riverine Hg loads in comparison to atmospheric Hg deposition on a regional scale; the approach may be useful in assessing the relative importance of atmospheric and non-atmospheric Hg sources.