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Addendum on Anomalous Lake Merced Levels David Dawdy, Hydrologist January 12, 2006 At the Lake Merced Task Force meeting on January 11 some analyses by SFPUC were distributed. They showed the January 2 lake levels for 2002-2006, which demonstrate the anomalous four feet rise in lake levels which you described. They also show the decreased rate of decline in the lake level in 2005. They report: “Closest to Lake Merced, groundwater levels in Daly City recovered by about 40 feet over the three-year period, but they remain about 75 feet below sea level, or about 90 feet below the surface of Lake Merced. Daly City also initiated a recycled water project with three local golf courses around Lake Merced in late 2004 that partially replaced groundwater pumping with recycled water for irrigation. As a result of the decreased pumping for irrigation in 2005, groundwater levels beneath Lake Merced did not experience the seasonal fluctuations of about 15 feet that have characterized previous years and they rose about 5 to 8 feet in the deep aquifer beneath the Lake; however they remain 20 to 30 feet below Lake Merced. The combined benefit of the conjunctive use demonstration and the recycled water irrigation supply has been a generally reduced gradient for groundwater flow from the vicinity of Lake Merced toward the areas of reduced groundwater levels beneath Daly City, South San Francisco, and San Bruno.” The change in water levels is the result of rebound as a result of the cessation or reduction of pumping. However, the cause for the reduced gradient is not explained. However, pressure effects are transferred almost immediately in an aquifer. Part of the reduced rate of decline in Lake Merced may result from reduced evaporation, as developed earlier. However a reduced gradient in the deep aquifer may well be transferred to the upper aquifer, and that would more definitely reduce the rate of outflow from Lake Merced by reducing the gradient in the upper aquifer. Decreased percolation through the confining layer to the lower aquifer may account for some of this reduced gradient, but that amount probably is small in relation to the total flow in the aquifer. However, the pressure can be transferred almost immediately. Therefore, it is highly likely that there are paths of transfer of pressure from the lower aquifer to the upper aquifer. This could be caused by fingers of sand in the aquitard. These would allow easier transmission of flow and pressure than a solid clay layer. The Westside aquifer has many intermingled sand and clay lenses. In fact, toward the south the lenses increase in number and the aquitard seems to disappear. If this gradient reduction is permanent rather than transient, and if the result is a reduced gradient in the upper aquifer, much less water would need to be added in order to maintain the level of Lake Merced. If the recession rate of Lake Merced is similar in 2006 to 2005, then the gradient change may be permanent and the cessation of pumping may have a greater impact than has been hypothesized. Further, if the reduced gradient in the upper aquifer, if any, is permanent, the balance between inflow and outflow will have been changed and Lake Merced may stabilize at a higher level than before. Jan. 12, 2006  |