Case Study for Customer Location CA124:

Reservoir or Lake Use:
The lake is a flood control reservoir that was previously a large gravel pit. There is heavy recreational use of the lake, including swimming and fishing.

System Overview and Reservoir:
Lake Almaden has a surface area of 25 acres. Due to gravel extraction activity prior to filling, the reservoir bottom is uneven with a gravel berm separating two relatively deep holes of 43 ft and 36 ft, respectively.

Reported Problem Before SolarBee Installation:
The lake has a long history of mercury contamination due to mercury mining operations in the surrounding watershed from 1850-1972. Lake monitoring data showed that the seasonal production of unfiltered methyl mercury (the biologically available form of mercury) in the lake is greatest during summer stratification in anoxic bottom waters with low oxidation-reduction potentials (ORP). Bioaccumulation and magnification of methyl mercury up the food chain (see figure on second page) has resulted in average concentrations in adult fish of 2.3 mg/kg, or many times the safe level for human consumption. The lake discharges into the San Francisco Bay, adversely affecting existing beneficial uses including sportfishing, protection of rare and endangered species, and wildlife habitat.

SolarBee Installation:
Date: May 2006, installed one SB10000v12 unit, deployed for hypolimnetic oxygenation in the deepest hole of the lake. March 2007, installed one SB5000v12 unit in the second deep hole, also for hypolimnetic oxygenation. The goal is to transport dissolved oxygen (DO) down to bottom waters to raise the ORP sufficiently to prevent the methylation of mercury in the water column.

Results:
Results from both 2006 and 2007 indicated significant reductions in methyl mercury concentrations in the bottom waters affected by SolarBee hypolimnetic oxygenation. In 2006, systematic monitoring documented up to a 96% reduction of methyl mercury (as compared to 2005) in the deep basin treated by the first circulator, while methyl mercury concentrations in the untreated portion of the lake were similar to 2005 values (see summary graph on page 2). In 2007, there was up to a 95% reduction in methyl mercury (as compared to 2005) in both treated portions of the lake. Increases in hypolimnetic ORP measurements due to deep circulation of DO were apparently responsible for reducing methyl mercury concentrations. The water district is very happy with the significant and sustainable reduction of methyl mercury concentrations in the lake using only solar-powered long-distance circulation.