From: Keith Lichten [mailto:email@example.com]
Sent: Monday, January 24, 2005 5:25 PM
To: Julie Gantenbein
Cc: Dale Bowyer; Lawrence Kolb; Laurie Taul; Shin-Roei Lee
Subject: Re: DeSilva's Monitoring Reports for Leona Quarry Project
Dear Ms. Gantenbein,
Thank you for your email this morning and telephone calls of last week.
Overall, we believe that the sediment and erosion protection at the
Quarry site have been significantly improved over the conditions we
observed in October and early November.
We did receive a monitoring report from DeSilva last Tuesday afternoon.
The cover narrative included with the report states that the primary
(lower) detention basin on the site did discharge during the storms
beginning December 8 (perhaps 6?) and 31, 2004, and again during the
storm that began on January 2, 2005. The submitted results appear to
suggest that there were discharges from the lower detention pond and/or
from the lower portion of the site to the south of the existing
construction entrance, and turbidity exceedances, on December 7, 8, 27,
30, 31, and January 3. The monitoring report does not include a
discussion of measures taken to further reduce turbid discharges;
however, we understand that the storage/detention volume of the large
lower detention basin was recently increased.
The current design of DeSilva's sediment filtering system treats flows
at a rate of approximately 0.78 cubic feet per second. That is, if the
large lower detention basin was full and had no additional flows
entering it, it would be emptied by this treatment system (assuming the
system was operating 24 hrs/day) in about 2 days and 2 hours. We
understand the lower basin to have a storage capacity of about 3.3 acre
feet. This is somewhat large in an absolute sense, but smaller when
compared to the area that discharges into it. The challenge, then, is
that when multi-day storm events come in, as we recently have seen,
then the relatively low storage capacity of the lower basin is
exhausted, causing a discharge of turbid water.
We also note that, based on the submitted information, it is not clear
that flocculant tests were completed on each day that the system was
operating. If the system was operated correctly, then the likelihood
of flocculant discharge was low. However, we will be contacting
DeSilva on this question.
While we appreciate and support the efforts that DeSilva has made to
improve erosion and sediment control on its site, and believe that they
have likely improved the quality of discharge over the conditions
present in October and early November, we note that there remain
discharges of turbid water from the site. As such, we are hopeful that
further refinement of the system and the continued establishment of
some of the site's vegetated erosion control measures, will act to
reduce additional turbid discharges.
If you have any questions or comments, please contact me as below.
-Keith H. Lichten, P.E.
Acting Section Leader
San Francisco Bay Regional Water Quality Control Board
1515 Clay Street, Suite 1400
Oakland, CA 94612