Mercury Loading Estimates for McPhee and Narraguinnep TMDL Development

The adjacent McPhee and Narraguinnep Reservoirs in southwest Colorado have been included on the 1998 303(d) list due to the presence of elevated fish tissue concentrations of mercury. Ambient water quality criteria for concentrations of mercury in water have not been exceeded; however, the physical and chemical characteristics of the lakes have led to bioaccumulation of mercury in fish tissue to levels that exceed fish consumption guidelines. A source characterization and total load estimation was required for support of a mercury Total Maximum Daily Load (TMDL) which was developed using the Dynamic - Mercury Cycling Model. Within the upland watersheds that contribute flow to the reservoirs, there are no permitted point sources of mercury discharge. There are, however, significant loads of mercury to McPhee Reservoir that derive from past mining activities. In addition, there are nonpoint or diffuse loads of mercury to both reservoirs that derive from naturally occurring background sources such as local geology and atmospheric deposition. Watershed mercury loads were considered to be primarily associated with sediment delivered to the reservoirs during storm scour events, as well as dissolved load present at lower flows. Extensive mercury sampling using ultra clean methods was undertaken throughout the watersheds in 1999 to estimate water column and sediment concentrations. A series of Generalized Watershed Loading Function (GWLF) models using meteorology data for the period from 1980 to 1999 was developed to estimate annual flow and sediment load from the eighteen subwatersheds draining to the reservoirs. Enrichment factors and water column concentrations developed from monitoring collected in 1999 at locations throughout the watersheds were linked to the GWLF results to estimate total sediment-sorbed and dissolved mercury loads. Atmospheric loading was also evaluated as a direct source to the reservoir surface. These atmospheric loads which derive from both distant and nearby sources may be enhanced by emissions from coal-fired power plants located south and west of the. Additional work is ongoing to further refine loading estimates including mercury accumulation in winter snowpack, surrogate surface sampling, monitor new Mercury Deposition Network station near the watershed, and plan for additional watershed sampling and characterization.