Further Thoughts on the Anomalous Lake Merced Levels
David R. Dawdy, Hydrologist
January 19, 2006
The anomalous lake levels at Lake Merced can be explained only partially by phenomena outside the effects of the cessation of pumping by golf clubs and Daly City. The reduction of the gradient downstream can explain both the slower decline of Lake Merced than in previous years and the higher elevations than expected based on the data of previous years. In my previous addendum I stated:
“ 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.”
In fact, the clay layer seems to disappear fairly soon in a down gradient direction. Referring to Cross-Section F-F’ in Figure II-8 of “Conceptualization of the Lake-Aquifer System” by Luhdorff and Scalmanini in March 2002 one can see that the confining clay layer seems to have disappeared by Well LMCC #3 (Lake Merced Country Club), which is approximately a mile south of Lake Merced. The Daly City pumping wells are some 4,000 feet farther south, whereas the Olympic Club well is a hundred or so feet south of Lake Merced. Therefore all three of these sources of drawdown have a ready path from the lower to the upper aquifer according to Luhdorff and Scalmanini. Thus, the rebounding pressure level resulting from the cessation of pumping can be transferred directly into a reduced gradient in the upper aquifer. Lake Merced, as we know, is a surface expression of the upper aquifer.
The present condition of Lake Merced and the gradient downstream is a transient, resulting from the recent commencement of the use of recycled water on the golf courses and Hetch Hetchy water as a supplemental supply for Daly City. Eventually a new quasi-steady state condition will establish itself, with a new gradient for the upper as well as the lower aquifer and a new stable elevation for Lake Merced. This may result in a higher or, perhaps, a lower level for Lake Merced, but the reduced gradient, whatever it may be, will require less water to be supplied to Lake Merced in order to raise it to a given level. Thus, the previous amounts of estimated additions based on previous studies may well be incorrect if they are based on models which do not include the close coupling and interconnection of the upper with the lower aquifer.
This direct reaction of the upper aquifer and Lake Merced to changes in the lower aquifer seem to best explain the anomalous results demonstrated by Lake Merced elevations this past year.
January 19, 2006