I'm looking for a place in the Heritage District of Sunnyvale, and I recently learned about the many Superfund sites in the South Bay [1]. My main finding is that it is safe to live in the Heritage District. I'm sharing this report in case anyone can point out problems, and for others with similar concerns.

Background

A superfund site is a place where the EPA has classified the area as needing to have hazardous waste cleaned up:

> Thousands of contaminated sites exist nationally due to hazardous waste being dumped, left out in the open, or otherwise improperly managed. These sites include manufacturing facilities, processing plants, landfills and mining sites. (https://www.epa.gov/superfund/what-superfund)

These sites can contain many different kinds of hazardous substances, including:

• Volatile organic compounds (VOCs), such as trichloroethylene (TCE). "Volatile" means they evaporate easily, so these are the chemicals responsible for the vapor-intrusion concern described below.

• Non-volatile contaminants such as PCBs (polychlorinated biphenyls). These do not evaporate, so they do not create a vapor-intrusion risk.

A body of groundwater contaminated with these chemicals and its movement underground is called a plume. Although South Bay drinking water is not drawn from the contaminated aquifer, the main concern with a plume is vapor intrusion. VOCs evaporate out of the groundwater, and the vapor can rise through the soil into buildings above. This pathway applies only to volatile compounds like TCE and not to non-volatile ones like PCBs. Vapor intrusion can be affected by changes in the water table (like rain) and by disturbing the soil, for example, by drilling.

In Superfund sites, the EPA addresses the contamination and any vapor intrusion with measures including:

1. Pumping the contaminated groundwater out and treating it

2. Excavating the contaminated soil and disposing of it elsewhere.

3. Capping contaminated soil, and/or installing sub-slab vapor-mitigation systems inside buildings above the plume to vent or block vapors.

4. Restricting activities (such as drilling or groundwater use) on the land above the plume.

Downtown Sunnyvale Superfund sites

The closest Superfund sites to downtown (DT) Sunnyvale are the Westinghouse / Northrop Grumman plant ("the plant") and the Triple Site.

The Triple Site is a cluster of three source sites: AMD (901/902), TRW Microwave, and Philips (formerly Signetics), plus a fourth operable unit, the Offsite Operable Unit (OOU), which is the commingled plume that migrated north off those three properties into the surrounding neighborhood.

Both the plant and the Triple Site are to the northeast of DT Sunnyvale, which is crucial for concluding safety, because the groundwater is moving north. The plant is directly adjacent to the North-East corner of the Heritage District. Its latest report is the Fifth Five-Year Review (2021): https://semspub.epa.gov/work/09/100025428.pdf (the Sixth is due around September 2026). The plant's primary contaminants are PCBs and chlorinated benzenes, which are largely non-volatile, so vapor intrusion is less of a concern here than at the Triple Site. As discussed below, the contamination is also relatively contained to a small on-site area as of 2021.

The Triple Site is about a bit further North-East compared to the Plant. The site's hub page is the EPA Triple Site profile: https://cumulis.epa.gov/supercpad/SiteProfiles/index.cfm?fuseaction=second.cleanup&id=0900265, and the latest report is the Sixth Five-Year Review (September 2024): https://semspub.epa.gov/work/09/100038437.pdf. The Triple Site is much less contained. Its main contaminant of concern is TCE (trichloroethylene), a VOC and a carcinogen. The plume has been measured extending more than a mile north of the source sites.

Method

To assess safety I look at two things:

1. the direction the groundwater moves each year, which tells whether the contamination is getting closer to DT Sunnyvale

2. the boundary of the plume, which tells whether the plume is already under DT Sunnyvale

Flow direction

At the plant, the shallow A aquifer flows northwest and the deeper B aquifer flows north-northeast [2], both directions away from DT Sunnyvale, which lies to the southwest. The plant's contamination is also actively captured by an on-site groundwater extraction system, so it isn't migrating outward regardless of gradient.

At the Triple Site, the groundwater flows north [3,4,5].

Because the flow at both sites is away from DT Sunnyvale, if the plume is not already underneath the area, it won't be in the future.

Plume boundary

The plant: see Figure 6 in https://semspub.epa.gov/work/09/100025428.pdf. The legend's "Approximate area above cleanup levels" marks where contaminant concentrations exceed safe levels. It is contained on the property within an 800-foot radius of the source area. Importantly, it does not reach the property's southwest, downtown-facing edge. The "Unit A Monitoring Wells (non-detect during previous five years)" show that measurements outside that zone are within safe levels.

The Triple Site has four parts: the three source sites and the OOU, where the plume has migrated north into residential areas. For a location/overview map, see Figure 2 of https://semspub.epa.gov/work/09/100038437.pdf. For TCE concentrations, see Figure B-24. This is the shallow "A" zone, which is the depth most relevant to vapor intrusion. (Figure B-25 maps the deeper "B1" zone; it renders more legibly in the PDF, but because it is a deeper measurement it has less bearing on vapor intrusion.)

The plume's southern extent is at the Signetics source on East Arques Avenue, and it extends north from there, consistent with the northward gradient. Arques Avenue runs east-west across the city: its eastern segment hosts borders the Signetics source from below, and its western segment is the northern boundary of the Heritage District. Since the plume lies north of the Arques line and the Heritage District lies south of it, the district is outside the plume. The safety margin is smallest at the district's northeast corner (roughly half a mile southwest of the source) and much larger toward Mathilda Avenue, which is about 1.3 miles southwest of the source.

Conclusion

In conclusion, any threat posed by the Superfund sites closest to the heritage district are negligible. This is because there is no overlap with the sites. The plant, although being really close, is well-contained, doesn’t have VOCs so it doesn’t pose a threat for vapor plumes. The triple site, although having concerning levels of TCE, is far enough away that it isn’t a concern.

Sources

[1] https://www.nbcbayarea.com/news/local/toxic-plumes-the-dark-side-of-silicon-valley/1971023/

[2] https://webapp1.dlib.indiana.edu/virtual_disk_library/index.cgi/2766887/FID3643/rods/Region09/R0900956.pdf — "The regional groundwater flow is generally northward. In the A aquifer, the gradient, which flows to the northwest, is relatively flat and is estimated to be between 0.0005 to 0.010 ft/ft. Over most of the study area, groundwater in the B aquifer flows toward the north-northeast with a shallow hydraulic gradient of approximately 0.0014 ft/ft. Velocities have been estimated at 2.6 to 522 feet per year in the A aquifer, and from .7 to 73 feet per year in the B aquifer."

[3] https://www.epa.gov/newsreleases/us-epa-reaches-settlements-study-and-mitigate-indoor-air-and-groundwater-contamination — "Contamination from the Triple site migrated through groundwater northwards past Highway 101. The Triple Site also includes the residential neighborhood located over this plume of contaminants that is referred to as the Offsite Operable Unit (OOU)."

[4] https://cumulis.epa.gov/supercpad/SiteProfiles/index.cfm?fuseaction=second.cleanup&id=0901389 — "This plume consists of VOCs, including TCE, and extends from these sites more than a mile north in Sunnyvale to past Highway 101."

[5] https://semspub.epa.gov/work/09/2400267.pdf — "The sites are owned and operated by semiconductor/microprocessor manufacturers and have contributed to a commingled plume of ground water contaminated with organic solvents that has migrated off the sites to the north."

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Edit: Some people believe this is AI generated text. Please take a look at the pictures I've put here: https://imgur.com/a/non-ai-proof-2ichPsH

The first one is Pangram labs AI detector. Pangram was used at one of the top AI conferences (https://www.pangram.com/blog/pangram-predicts-21-of-iclr-reviews-are-ai-generated) to detect AI reviewers and can accurately determine if text is human written, AI edited, or AI generated. Pangram classifies my post as 100% human written. Please feel free to reproduce for yourself by pasting in my text and also pasting in AI generated text to see the differences in the classifications made by their model.

The second picture is the version history in MS word as I was drafting this. As you can see, I've been working on the write-up for the past 5 hours today.

The third picture is the gimp editor I used to create the map, which is a combination of Google maps and 2 figures from 2 of the EPA reports linked in my write-up.

Howdy. A bit about me:

1. I'm 28
2. I'm an absolute moron
3. I'm a complete idiot

I forgot to pay my card for 3 months. How? See points 2 and 3.

It took so long to get up to 790. Are there any quick fixes other than to never forget again and wait for it to go back up?