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The Arkansas and Louisiana 1998 Section 303(d) Lists included five segments and three subsegments in the Ouachita River basin that were impaired due to excess concentrations of mercury in fish. While there have been no known violations of the numeric mercury water quality standard and fishable designated use for the listed waterbodies, they are not meeting the narrative water quality standard and designated uses of fishable water bodies. A basin-wide approach was used in this Total Maximum Daily Load (TMDL) study because the waterbodies had similar ecoregion and watershed characteristics, and similar causative factors such as atmospheric and geologic mercury contributions.

The upper Ouachita River basin is in the Ouachita Mountain, South Central Plain, and Mississippi Alluvial Plain ecoregions in Arkansas and northern Louisiana. The basin has gently rolling topography, with hilly uplands, flat wooded uplands, terraces, and floodplains. Land use in the basin is 71% forest and 13% in wetlands. There is one permitted point source with permit mercury limits in the basin. There are 373 mercury air emission point sources that contribute to mercury deposition in the basin. The geology of the Ouachita Mountains contains rocks with relatively high naturally occurring mercury concentrations. The soils in the basin reflect this geology and atmospheric mercury deposition.

Both Arkansas and Louisiana have numeric mercury water quality standards of 0.012 μg/L. There have been no known violations of the numeric water quality standards, but clean sampling procedures and ultra-trace level analyses have not been used. There are fish consumption advisories in the Ouachita River basin in both Arkansas and Louisiana due to mercury contamination of fish. The Action Level for fish consumption advisories in Arkansas is 1.0 mg/kg. While Louisiana does not have an established Action Level, fish tissue mercury concentrations of approximately 0.5 mg/kg have historically triggered fish consumption advisories. Target fish tissue mercury concentrations used for all fish species in this TMDL were 0.8 mg/kg in Arkansas and 0.4 mg/kg in Louisiana, using a 20% Margin of Safety for the Action Levels.

The TMDL was developed using a two step approach. The first step was estimating the mercury loads from the facility with a permit mercury limit, municipal wastewater treatment facilities, local emission point sources, atmospheric deposition, watershed nonpoint sources, and natural background. Annual loads were used because mercury bioaccumulation occurs over a period of years. In the second step, average fish tissue mercury concentrations measured in the basin were used to estimate the reduction in fish tissue mercury needed to achieve the target fish tissue mercury concentrations. A linear relationship was assumed between mercury in fish and mercury loading to the basin. This reduction to achieve target fish tissue mercury concentrations was then used to determine the reduction needed in mercury loading.

The predominant sources of mercury loading to the Ouachita River basin are atmospheric deposition and watershed nonpoint source and background loads. Less than 1% of the load came from the point source wasteloads. A reduction factor of 1.5 (i.e., reduction to 67% of current total mercury load) would achieve target fish tissue mercury concentrations in Arkansas, and a reduction factor of 1.8 would achieve target fish tissue mercury concentrations in Louisiana. The TMDL for mercury loading for Arkansas to achieve the target fish tissue mercury concentrations is 292,376 g/year. The TMDL for total mercury loading for Louisiana to achieve the target fish tissue mercury concentrations is 243,647 g/year. Because of regional and global sources of atmospheric deposition, and natural and previously deposited mercury loads from the watershed, it is likely to be decades before this load reduction can be achieved.

Document Type: Research Article


Publication date: 2003-01-01

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