I haven’t read every document about the Lane Metals site, and I haven’t talked to anyone at DTSC about it, but here’s what I’ve found:
There a document with 2015 groundwater sampling results for Lane Metals. https://www.envirostor.dtsc.ca.gov/public/deliverable_documents/4447006046/Subsurface%20Investigation%20Report%20%5BSGI%201-14-16%5D.pdf
Figure 5 (page 34 of the PDF) shows an on-site shallow monitoring well adjacent to the San Pablo Ave. sidewalk with high levels (19,000 ppb, or parts per billion) of TCE!
Groundwater flow is toward the high school, but there no recent off-site sampling has been reported. That is, no one knows the extent of the TCE groundwater plume. Normally plumes are mapped to the 5 ppb or even 1 ppb contour line.
Groundwater concentrations are only a rough predictor of soil gas concentrations and, where there are buildings, indoor air contamination. However, that 19,000 ppb predicts indoor air contamination of 19 ppbv (parts per billion by volume), or 102 micrograms per cubic meter. If that concentration or anything close to were found under the school, it would call for an urgent response - which is apparently what is happening with the school closure. See the attached EPA memo for the most common TCE vapor exposure standards.
Evidence that groundwater flows toward the west: "The direction of groundwater flow within the East Bay Plain is from east to west, and generally
follows surface topography (CRWQCB, 1999)…. Groundwater flow was inferred to be to the west (PIERS, 2004)." See https://www.envirostor.dtsc.ca.gov/public/deliverable_documents/4510314762/Former%20Lane%20Metal%20Finishers%20Site%20RI%20Report%20Vol%20I%20Report%2001.24.11.pdf
(p. 2-1; p. 19 of PDF).
While DTSC and its consultants seem to be working hard to clean up the source area on the Lane Metals property, they have been ignoring off-site migration. This is wrong but all too common. One explanation is that this is an “orphan” site, with no private entity responsible for paying for the cleanup. This project is apparently being paid for by taxpayers through DTSC’s budget, not the polluters. See the conclusion at https://www.envirostor.dtsc.ca.gov/public/deliverable_documents/2794817864/201736%20Lane%20Metal%20Finishers%20Orphan%20Memo_Final%20Signed_08.16.2017.docx.pdf