EXURBAN LAKES ON THE SOUTHERN CUMBERLAND PLATEAU OF TENNESSEE PART 2: METAL STRATIFICATION AND IMPLICATIONS FOR WATER SUPPLY

The watersheds of the southern Cumberland Plateau support some of the most biologically diverse aquatic ecosystems in North America. Historically, exurban development in this region has been dependent upon water supplied by the impoundment of streams to create reservoirs. Underlying these man-made lakes are Pennsylvanian sandstones, shales, and conglomerates rich in iron and manganese oxides. Studies of meromictic lakes demonstrate that high concentrations of dissolved iron and manganese in stagnant layers can influence the turbidity, color and odor of water. In addition to the economic cost associated with treating this lower quality water for consumer use, the presence of metal contaminants can adversely affect the flora and fauna downstream from the reservoirs. The objective of our study was to examine the chemical gradation of iron and manganese in reservoirs on the southern Cumberland Plateau to determine how it affects both municipal water supplies and downstream aquatic ecosystems. From May through November 2006, we monitored concentrations of iron and manganese, as well as pH, dissolved oxygen and temperature, at one-meter depth intervals, throughout the vertical extent of four reservoirs in the vicinity of Sewanee, TN. In three lakes, we documented a strong increase in total metal content with a concomitant decrease in dissolved oxygen at a critical depth of 4 to 5 meters. Benthic sediments with high concentrations of reduced iron and manganese are the most likely source of metal species in the water column. These chemoclines appeared to increase from June through September, but were eliminated with seasonal lake mixing in October. With current water treatment capacity, we estimate that the presence of chemoclines in these reservoirs reduces treatable water volume by 10-25 percent (depending upon the lake) during the months most prone to drought in a region facing an accelerating rate of exurban growth. We will also discuss the impact of iron and manganese enrichment on downstream aquatic habitats.