The Need For A New Conceptual Site Model

Peter Strauss

December 11, 2013CPEO’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