Task 3: Initiative to Raise and Maintain Lake Level
And Improve Water Quality
EDAW & Talavera Richardson
August 2003
Comments from John Plummer
Friends of Lake Merced
I will detail comments on specific items in this report below. First, however, some general observations:
This report is based on these unsupported assumptions:
As a consequence, viable options have not been considered:1) A sustainable program for maintaining the level of Lake Merced at any desired level can be accomplished through additions of imported water.
2) As the water rises we have no option but to observe the impacts passively.
3) These basic premises can be accepted without consideration of any alternates.
These points were raised at the one public meeting conducted to discuss this study that was held last September, and have been repeated on several occasions. That they have received no consideration again suggests that the PUC’s approach to “Public Review and Comment,” as the Task 3 Technical Memorandum is described on its cover, is simply a pro forma exercise, intended to check off one more activity in compliance with some unexplained standard. There is no evident intent to actually change something in response to public input, nor is it necessary to explain why nothing has been considered.1) The effect of programs now underway and proposed to restore the health of the supporting Westside Basin aquifer are not evaluated.
2) No actions are considered that might mitigate harmful effects of rising lake levels in terms of habitat protection, preserving biodiversity and assuring water quality.
One is forced to wonder, why bother. Rather I think we should now make a concerted effort to raise the political profile of the PUC’s lack of responsiveness, and attempt even more vigorously to have the full range of potentially viable alternatives evaluated.
Now to the specifics:
Executive Summary
“The lake levels considered in the adapted model are from plus two to plus eight feet (City Datum) above the October 2002 water level.”What does City Datum have to do with the amount of increase?“For one-year simulations, water volume requirements were calculated to range from 500 acre-feet for a two-foot year-round addition to 4200 acre-feet for an eight-foot year-round addition.”
Explain. Why is more than 8 times as much water needed to raise the lake 4 times as high when the areal increase is less than 15%?“The annual fluctuation in water level for the seasonal addition scenario ranges from 1.83 to 2.8 feet for a 2-foot and 8-foot increase respectively.”
How determined?
Alternative Water Sources:Recycled water: Is there any precedent for direct potable reuse in California? If not, what reason do we have to be optimistic this will be allowed here?Stormwater: To what degree does adding water during the rainy season, when stormwater is available, exacerbate the problem of excessive lake level fluctuation?Page 7: “Lake Merced’s original watershed was approximately 6350 acres extending up to the Twin Peaks area. Today, the water shed is roughly 600 acres including the 300 acre lake and is approximately defined by the adjacent roadways.”
SFPUC System water: Will this option survive a best-use test for allocation of scarce potable water?
Groundwater: Ditto prior point.
Why has the potentially best solution, restoring the Westside Basin aquifer, been ignored in this analysis?
The report prepared by Katie Pilat for the Neighborhood Parks Council, Leaving a Lake Legacy, observes “Lake Merced’s decline is tied to the same root cause as that at Pine Lake -- the regional decline of groundwater levels in the Westside Basin aquifer.” The aquifer in turn is supported by infiltration of rainwater, not by runoff. Katie Pilat’s report continues: “However, even prior to urbanization, little surface runoff reached Lake Merced (LS, 2002a). Instead, rainfall quickly infiltrated into the sandy soils surrounding the lake and arrived at the lake as groundwater inflow.” The permeable area in the described watershed has been maintained at least at 50% of its natural state, with a much higher proportion of permeable area in the immediate vicinity of Lake Merced.
Other reports confirm that a large multiple in runoff into Lake Merced would have relatively little impact on the overall water budget.Pages 13-14: “These data confirm that golf course pumping from the deep aquifer does not directly affect water levels in the shallow aquifer of the lakes on a daily basis. . . It is likely that ground-water in the shallow aquifer currently flows southwestward in response to perennially depressed ground-water levels caused by withdrawals from the deep aquifer.”
What is the relevance of the observation regarding daily response? Of course there is none, but so what? The important point is the long-term effect of pumping on the condition of both the deep and shallow aquifers. That is not adequately evaluated in this report.Page 14: Urbanization around Lake Merced has reduced the recharge capacity of the watershed., decreasing ground-water inflow into Lake Merced and creating lower water levels and a flatter ground-water gradient in the shallow aquifer.”
This is undoubtedly true. However, I have seen no attempt to estimate the effect of this factor. Were there no pumping from the aquifer would the lake level have gone down as a result of reduced recharge? Or would the flow of water into the ocean simply been reduced? The fact that lake levels were in large part maintained for 35 years after urbanization was essentially complete suggests that the effect of reduced recharge capacity may well have been quite small.
In addition, there is potential for increasing the recharge from its current levels, again using conservation approaches suggested in Katie Pilat’s report. Examples include the use of permeable surfaces for parking lots and the diversion of rooftop runoff into localized aquifer recharge systems. Why have those not been considered here?Page 15: “ Lake water levels rise and fall about two to three feet seasonally due to rainfall, evaporation and seepage.”
This suggests that the seasonal variation in lake level may actually be reduced while undertaking a seasonal program of water additions to maintain the lake at 8’ above the October 2002 level. (See estimated fluctuation of 1.83 to 2.8 feet above.) That’s hard to believe.Page 15: “Efforts to further understand groundwater conditions and their interaction with Lake Merced have led to ongoing groundwater studies.”
Full stop? What is being learned from these studies? Why aren’t those results presented here?Page 20: Lake Merced (South and North Lakes) for the most part experience primarily isothermal conditions with infrequent, weak stratification.”
Would a depth of 26’ significantly increase the amount of temperature stratification, improving habitat for cold-water fish species?Page 30: “The seasonal supplementation schedule has the ecological benefit of being correlated to seasonal rainfall inputs, and attempts to mimic natural conditions with all inputs occurring between December and April. However, one drawback might be the ability of vegetation to inhabit regions of shoreline with highly variable hydroperiods. A year-round supplementation schedule would thus be designed to provide a shoreline habitat with a relatively consistent hydroperiod.”
Is the year-round program simply the seasonal program with additions off-season to maintain lake level? If so, is this the basis for estimating the relatively low (1.83’ to 2.8’) rate of fluctuation? And if that is so, why is this attributed to the seasonal program rather than the year-round program? I admit I’m confused.Page 32: “The model (GeoResource ’93) contains assumptions that limit its predictive ability.”
No kidding! Is there any model that doesn’t? The question is, does the model have sufficient predictive accuracy to evaluate policy decisions? What is the status of the new model being developed by Gus Yates? If the GeoResource model is severely limited wouldn’t it make sense to wait until the improved model is available before attempting these estimates?Page 37: Figures 4-5 and 4-6 appear to answer the questions just raised. However, the basis for these Figures deserves better support. It seems unlikely that adding 6’ of imported water in addition to the normal 2’ of rainfall would not produce an increase in the seasonal decline if no additional water is added during the summer months.
Perhaps more important, Figure 4-6 suggests that once the lake is filled to any given level we can expect that level to be maintained through normal seasonal variation. That is, the normal rise of two to three feet, as reported above, is sufficient to compensate for the estimated decline of 1.83 to 2.8 feet that occurs when no water is added during the months May through October. Unfortunately, the record shows that isn’t going to happen.
Finally, these Figures suggest that the baseline level would be maintained with no additions of water to the lake. Again, that is observably not the case, as the lake level has declined about one foot a year during the ‘90s. That should be the baseline, with the effect of added pressures on lake seepage resulting from greater volumes of water evaluated to determine the incremental annual loss resulting from those additions.Page 50: Response of vegetation to rising water levels.
This section describes an approach to evaluating what will happen if? It says nothing about actions that might be taken to mitigate negative effects, in terms of lost habitat or reduced water quality. Shorelines may be contoured so as to maintain adequate areas hospitable to bulrush and other wetland species. Species that are not tolerant to inundation that have moved into areas traditionally under water can be removed before the water reaches that level so as to reduce the amount of vegetation decay affecting the water quality.In short, the two primary points of emphasis raised previously continue to be ignored:
1) What actions can be taken to restore the Westside Basin aquifer? What beneficial impact will those actions have on the level of Lake Merced?
2) What mitigating actions need to be taken to preserve habitat, biodiversity, and water quality as the lake level rises?Unless and until those issues are addressed we do not have a satisfactory analysis.
John Plummer
September 4, 2003