The following e-mail was sent to Jim Salerno, Environment Manager for the San Francisco Public Utilities Commission. Jim's department collects and publishes the water quality data usually reported bi-monthly by the PUC. We have noted several trends in this data that seem to have statitical significance, and have asked Jim to comment on their practical significance if any. We will publish that report here when it is available.
July 26, 2005
Jim:
As you know, for several years now David Dingman has provided Friends of Lake Merced with periodic reports on water quality measurements taken at the lake. We have enough experience now to identify several apparent trends in this data. While apparent, however, I have no ability to comment on their significance with regard to the health of the lake. Perhaps you would be willing to do so.
These seem to me to be significant trends:
1) Plankton count is going down in North and East Lakes, from nearly 2 million cells per cubic liter to less than one-fourth that count.
2) At the same time, Secchi Disc readings have remained nearly constant except for a sharp increase in East Lake.
3) There seems to be a gradual decrease in dissolved oxygen at the surface, beginning about mid-year 2001 but decreasing at a nearly constant rate since that date.
4) Dissolved oxygen levels at a depth of 15 feet have remained relatively constant during that same period, with the result that the surface to deep water differential has been reduced.
5) Flouride levels have increased in all lakes, while the South Lake to North Lake differential has remained relatively constant.
6) After showing wide swings in level of Total Organic Carbon in the early period, these levels have been nearly constant since mid-2002.
7) Alkalinity shows a definite downward trend, particularly in South Lake. While the variability has been greater in North and East Lakes, the general trend is apparent there as well.
8) While pH levels seemed to be declining through the end of 2003 in all lakes, there now appears to be a rebound in level coupled with a much more pronounced seasonal fluctuation.
9) A similar trend is apparent in Hardness, although here the trend is much more pronounced in South Lake.
10) Ditto Conductivity.
Again, I am in no position to evaluate the significance of these trends in terms of potential impact on either natural or recreational uses of the lake. However, if you would be willing to provide such a commentary I would be more than happy to publish that on our web site.
Thank you in advance.
John Plummer
Friends of Lake Merced
