URBAN WATER SUPPLY VULNERABILITY

In the United States 68% of the population relies on surface water (streams, reservoirs) for drinking water. 36% of large cities water supplies are vulnerable to failure in the future from overuse, and this number will increase to 44% in 2040 due to population growth. Raleigh and the surrounding metropolitan areas are rapidly expanding and rely upon Falls and Jordan Lakes for drinking water. These lakes develop anoxic bottom waters during the summer stratification periods. Depth profiles show that an oxycline develops in early spring and persists until late fall in the lower portions of Falls and of Jordan lakes above the dam. Above the oxycline, a Chl_a and BGA maximum develops, and below the oxycline DOM (dissolved organic matter) concentrations increase.

We have investigated this lower anoxic layer with a GOPRO anchor-cam and found bacterial mats on the floor of the lake, and “lake snow” or floating bacterial colonies in the anoxic bottom layer. We have quantified the numbers of floating colonies and found that they are absent in highly turbid shallow bottom water areas and increase in size and density in the lower portions of the reservoirs above the dam where the DOM increases. This is not a problem in Jordan Lake because drinking water is taken from the shallow middle portion of the lake but can be a problem in Falls Lake because the drinking water is taken from 15 feet in the lower portion of the lake where DOM and "lake snow" is concentrated.