Transcript The Leaky Sewer Hypothesis

The Need For A New Conceptual
Site Model
Peter Strauss
December 11, 2013
CPEO’s Moffett-MEW Community Advisory Board
Mountain View, California
TCE is not a naturally occurring
contaminant
There are a scattering of locations in the area
between Whisman and Steven’s Creek and E.
Middlefield North to Highway 101 that have
greater than detection level “hits” of TCE
There is a need to understand how these
releases occurred, and if they are merely the tip
of the iceberg
Some of areas are under active
investigation
:
 Evandale/Devonshire/ Fairchild Dr.
Leong Dr.
Moffett Gateway
Moffett Gateway Property
CONCLUSION
• TCE impacts to groundwater in the western
portion of the Site appears relatively limited in
aerial extent and is primarily found in the upper
A1 zone above 25 feet bgs proximal to the
sanitary sewer line.
• TCE impacts to groundwater in the eastern
portion of the Site were identified in the upper
and lower A1 zones, A2/B1 zone, and at one
location in the B2 zone. The highest
concentrations of TCE in groundwater were
identified in the A2/B1 zone proximal to the
sanitary sewer line.
CONCLUSION
• Off-site releases appear to be contributing to TCE
detections in groundwater in the northeast
portion of the Site.
• The direct correlation between distinct areas of
high concentration of TCE in groundwater along
the sanitary sewer line strongly suggest that
historical discharges of TCE-containing wastes
into the sanitary sewer may have occurred and
then leaked at various locations both on the Site
and offsite resulting in impacts to groundwater.
“Dry Cleaners – A Major Source of PCE in
Groundwater,” Central Valley Regional Board, 1991
This report presents several case studies
where dry cleaning fluid (PCE)
contamination of soil and groundwater,
facilitated by sewer lines, has created
plumes extending more than half a mile
distant from the dry-cleaner source.
A Comprehensive Groundwater Protection Evaluation
for South San Francisco Bay Basins Executive Summary
May 2003, RWQCB
• Solvent groundwater contamination has been
detected where there are no nearby sources,
other than sewer lines.
• Where solvents are discovered in
groundwater without an obvious source,
investigators should take note of nearby
historic and existing sewer lines.
“Study of Potential for Groundwater
Contamination from Past Dry Cleaner
Operations in Santa Clara County” SCVWD
prepared by Thomas K.G. Mohr in 2007
The SCVWD recommended that investigations
should focus on the most common source
areas which include “Sewer and Septic Lines”
Questions
• THE SILVA WELL (OLD IRRIGATION WELL) ON
SHERLAND WAS CONTAMINATED. RP’S DID
MOST OF THE REMEDY, WITHOUT TAKING
RESPONSIBILITY (HOW DID THE
CONTAMINATION MIGRATE? DID IT COME
FROM A LEAKING SEWER LINE?)
• REMEDY WAS PUMPING INTO THE SANITARY
SEWER ALONG TYRELLA. DID IT LEAK?
QUESTIONS
• DOES THE MIX OF CHEMICALS (E.G. TCA,
FREON – 113) FOUND IN SAMPLES INDICATE
LIKELY RESPONSIBILITY?
• ARE THERE OTHER POTENTIAL SOURCES IN
THE EXPANDED STUDY AREA?
• IS THERE ANECDOTAL INFORMATION ABOUT
ILLEGAL DUMPING OR PAST DISPOSAL
PRACTICES THAT COULD HAVE LED TO THE
HOTSPOTS ALONG EVANDALE AND LEONG?
CONCLUSIONS AND RECOMMENDATIONS
• WE NEED TO FULLY UNDERSTAND HORIZONTAL
CONDUITS AND DEFINE FLOW PATTERNS
• SEWER LEAKS ARE ARE THE MOST LIKELY SOURCE OF
MANY OF THE DETECTIONS WITHIN THE EXPANDED
AREA
• WE NEED TO CONDUCT ADDITIONAL SAMPLING IN
AREAS WHERE THERE ARE DETECTIONS.
• PLACE ADDITIONAL MONITORING WELLS/AND OR
GRAB SAMPLES IN AREA BETWEEN E. MIDDLEFIELD
AND SHERLAND
• WHENEVER THE SANITARY SEWER IS REPAIRED,
GROUNDWATER SAMPLES SHOULD BE TAKEN