The San Francisco Public Utilities Commission (SF-PUC), working together with the City of Daly City Department of Water and Wastewater Management (DC-DWWM), is planning a series of tests where water from the Vista Grande Storm Canal will be added to Lake Merced. These tests are intended to evaluate the potential for using this water as a source of maintenance water to stabilize lake level.
Unlike San Francisco, Daly City maintains separate storm water and waste water systems. Storm water that runs off city streets, sidewalks, rooftops, and open spaces may contain significant contaminants, e.g., oils, heavy metals, debris of various sorts, but it is not expected to contain fecal coliform, also called E-coli, a family of bacteria found in animal waste. For more information on the nature of coliforms generally, and fecal coliform especially, go to http://www.epa.gov/enviro/html/icr/gloss_path.html.
Unfortunately, large quantities of E-coli have been found in the Vista Grande Storm Canal. A series of seven tests conducted last winter (12/15/02 – 2/27/03) produced the following results:
Total coliform – average MPN[1] 188,878
E-coli – average MPN 35,242
Coliform levels in Lake Merced were measured by samples taken from the nearby shoreline concurrently with the Vista Grande tests. These results were:
Total coliform – average MPN 9,610
E-coli – average MPN 1,754
While these are substantially lower than the levels found in the Vista Grande Canal, they exceed safe limits for recreational use such as boating and fishing. The current USEPA recommendations for body-contact recreation is fewer than 200 colonies/100 mL; for fishing and boating, fewer than 1000 colonies/100 mL; and for domestic water supply, for treatment, fewer than 2000 colonies/100 mL. These are all designated beneficial uses for Lake Merced.
Two questions are suggested by this data:
1) Is the
level of coliform found in Lake Merced normally this high?
2) How
badly will the safety of Lake Merced be impaired by adding Vista Grande
water to the lake?
Normal lefels of colioform in
Lake MercedThe SF-PUC water quality control chemists have been monitoring coliform levels in Lake Merced at roughly quarterly intervals beginning in March 2002. Since that time, through July 2003, a total of eight samples were taken at two locations in South Lake. The results of those samples are as follows:
Average Maximum
Total coliform – MPN 811.2 1,414.0
E-coliform – MPN 47.2 336.0
The maximum reading for E-coli occurred in October 2002. The second highest reading for this parameter was just 82. It appears, then, that we may have occasional but not sustained water quality issues in South Lake. However, these numbers do not come close to those observed in the Vista Grande tests.
There are two possible reasons for these differences:
1) The
SF-PUC readings are taken in the middle of the lake. The Vista
Grande readings were captured at the shoreline. Runoff from the
shore may contain relatively high levels of coliform, affecting that
water that can be sampled standing on the shore.
2) The Vista Grand readings were taken after significant storms. Again, this may increase the shoreline runoff, consequently increasing the shoreline to lake center differential.
2) The Vista Grand readings were taken after significant storms. Again, this may increase the shoreline runoff, consequently increasing the shoreline to lake center differential.
In order to confirm the after-storm runoff affect three additional samples were taken on 12/5/03. There had been some rainfall in the preceding two to three days; however, this was not regarded as a significant storm. Still, that factor may have had some influence on the readings:
Average Maximum
Total coliform – MPN 1,840.0 4,260.0
E-coliform – MPN 380.0 630.0
While these readings are significantly lower than the after-storm readings taken at the same location, they remain significantly higher than the mid-lake readings taken by the SF-PUC chemist.
The effect of adding Vista Grande water to Lake Merced
The planned Vista Grande Canal test will not add stormwater runoff from the Canal directly into Lake Merced. Rather, 300 feet of perforated pipe is being installed on the top of the bank along the shoreline between the Pacific Rod & Gun Club and the Impound Bridge. It is expected that the process of both percolation and surface runoff through the shoreline flora will have a cleansing effect on that water, hopefully removing significant amounts of coliform.
While the SF-PUC characterizes this as a small test, the volumes of water may be relatively large. The capacity of this system is 10 cubic feet per second. That translates into 281,160 gallons per hour, or 20 acre-feet per day. Since the area being inundated is just 300 ft. by 10 ft., this translates into a potential inundation of that area equivalent to approximately 300 feet of rainfall in a twenty-four hour period. That may or may not be enough to wash out the lake bank. However, it is clear that much of that water will flow directly into Lake Merced, greatly exacerbating the observed effect of post-storm runoff of water contaminated with coliforms.
The fact that existing levels of coliform near the lakeshore rise significantly after storms is troubling in itself. Adding significant volumes of water containing quantities of coliform as great as those observed in the Vista Grande Canal should be approached with great care. We encourage the Regional Water Quality Control Board and the San Francisco Department of Public Health to review this program carefully before it goes forward.
Unfortunately, we have been denied access to the plans for this test, including the monitoring program that has been proposed.
John Plummer
December 8, 2003
[1] MPN stands for “Most Probable Number.” This is an estimate of the number of colonies of coliform that exist in a sample of water of 100 milliliters, and is expressed MPN/100mL.
