Spatial and Temporal Distribution of Mercury in Caballo and Elephant Butte Reservoirs, Sierra County, New Mexico

Caballo and Elephant Butte reservoirs are located in south-central New Mexico on the Rio Grande. The reservoirs are managed together for flood control and irrigation. As a result, Caballo Reservoir undergoes seasonal water volume fluctuations creating large littoral or shallow areas. Water and sediment samples were collected monthly for one year (July 1995 to June 1996) in Caballo Reservoir to examine spatial and temporal variability of total mercury (THg) and monomethylmercury (MMHg). Concentrations of THg and MMHg were greatest in water and sediments at the site located in the seasonally flooded area (Palomas) compared to five sites in deep water. In July 1995, the concentration of dissolved MMHg from the Palomas site was 0.230 ng/L compared to 0.061-0.087 ng/L in five sites within deep water. Except for July and August 1995, MMHg concentrations in sediments (6.7-11.1 ng/g, dry weight) were greatest at the Palomas site. In contrast, concentrations of MMHg from the five sites were at or below 1.0 ng/g. The percentages of THg in the MMHg form was greatest in sediment collected from the Palomas site from September 1995 to June 1996 (5.4-33.8%) compared to sediment from the five sites in deep water. By October 1995, a site above Caballo Reservoir in the Rio Grande had greater concentrations of dissolved MMHg (0.508 ng/L) than the Palomas site (0.411 ng/L). The presence of a potential source of contamination upriver, in addition to a series of unrelated events (fire and late summer rains), precipitated a second study from July 1996 to June 1997. Thus, the second year was initiated to determine the sources of THg and MMHg entering Caballo Reservoir. Collection of water and sediment samples continued monthly from two sites in Caballo Reservoir (rather than six) and one site on the Rio Grande. Two sites were added upstream and downstream of the dam at Elephant Butte Reservoir. From July 1996 to October 1996, Elephant Butte Reservoir thermally and chemically stratified. Concentrations of dissolved MMHg downstream of Elephant Butte Dam increased 14.4 fold from 0.070 ng/L in July 1996 to 1.008 ng/L in October 1996. In contrast, concentrations of dissolved MMHg in water collected from Elephant Butte Reservoir were less than the detection limits (0.018 ng/L) from July 1996 to September 1996. By November 1996, when the Reservoir had undergone mixing, concentrations of dissolved MMHg had peaked at 0.074 ng/L while concentrations of total MMHg (unfiltered water representing both the bound and free fractions of MMHg) peaked at 0.262 ng/L. Coincidentally, stratification and turnover of Elephant Butte Reservoir were accompanied by the shutdown of hydroelectric power at the dam. The relationship between concentrations of MMHg, fall turnover, and hydroelectric generation are discussed.