Mapping Shallow Volatile Groundwater Plumes in Silicon Valley

Volatile organic groundwater plume are shown underlying childcare sensitive uses in Palo Alto, CA. Clicking the map image will show open a new window, showing the plume within the interactive CleanupDeck. Other plumes in Silicon Valley can be viewed.

Volatile organic groundwater plumes are shown underlying childcare sensitive uses in Palo Alto. This map can be viewed at this link to the CleanupDeck. A public view of the same data is within Terradex WhatsDown

Silicon Valley prides itself as a leader in environmental sustainability, but opportunities for improvement are ever present.  This post introduces new map-based services showing contaminated groundwater areas in Silicon Valley, helping to provide intuitive environmental data meant to inform all interested stakeholders.

The Valley has evolved from former industrial areas that held semiconductor industry that discharged carcinogens to groundwater. Over the past thirty years industry has transformed to new office parks hosting businesses like Google and Facebook.  The workforce is smart, growing and young – but also vulnerable to carcinogenic vapors from shallow contaminated groundwater plumes from legacy businesses.

Former industries, gas stations and dry cleaners spilled  chemicals in shallow groundwater plumes that persist as well as migrated  beneath newly constructed workplaces and homes.  The residual chemicals can vaporize into the soil gases and migrate to structures.  One key chemical, trichloroethene (TCE), is carcinogenic and poses an impact to a young fetus, thus any young woman is particularly vulnerable.   The circumstance might be a “perfect storm” where a young workforce  unknowingly inhales vapors from unknown underlying groundwater plumes containing potential carcinogens. Terradex is building a map service called the CleanupDeck that seeks to communicate this information in a form intuitive to a public user.

At Silicon Valley’s Mountain View Google campus, vapor intrusion from groundwater plumes was found to be migrating into the underlying structures of some of the office buildings.  As reported in the press, the halogenated vapors migrated based upon inadvertent actions by a contractor.  The outcry framed a vulnerability – the young working women at childbearing are vulnerable to these vapors.

One ingredient to assessing vulnerability is knowing where these plumes are in relation to homes and offices.  Hundreds of groundwater plumes exist across Silicon Valley, and many contain volatile chemicals that could migrate upwards to occupied structures, and then be inhaled by occupants.

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This post introduces a mapping resource that shows where groundwater contamination plumes are located as well as basic information to begin to judge their impact.   The map is presented using Terradex’s CleanupDeck – a web mapping service. We describe the challenges faced when assembling a groundwater plume maps, the maps relevance and relation to USEPA’s attention to vapor intrusion hazards in Silicon Valley, and we describe the impact of a subset of these plumes – the ten that are deemed Superfund sites.  Terradex is working to complete and build upon the mapping accessible via links further below within this post.

Since this post, Terradex has released WhatsDown that shares the same information for public consumption.

The Challenge of Knowing Where Groundwater Plumes Occur

Knowing where groundwater plumes are located is basic to assessing the hazard posed when volatile organics vaporize from these plumes. Current groundwater plume maps, however, fail to clearly reveal the occurrence of groundwater plumes to key stakeholders – local government, contractors, and the general public.  Plumes are typically studied site-by-site, and not in aggregate.  The site-specific groundwater maps are typically formed as a written deliverable from the potential responsible parties (PRPs) to the environmental regulatory agency that oversees incremental cleanup progress; a map’s audience is an environmental professional, not the public. These site-specific maps often segregate chemical constituents of concern (COCs) and impacted aquifers (e.g.,  one map for TCE in the A-level aquifer, one map for TCE in the B-level aquifer, and one map for TCE in the C-level aquifer.  The maps multiply as the pattern repeats with each constituent of concern).  The site-specific maps often do not show a plume boundary as an iso-concentration line – such as a 5 part per billion TCE – a delineation that would indicate an area likely for vapor intrusion.

Another challenge is that information on cleanup sites in Silicon Valley is distributed between four government information repositories:  Santa Clara County Department of Environmental HealthGeotrackerEnviroStor or Cleanups in My Community. In addition to the decentralized nature of this cleanup data, web maps from these government sources display only “point” icons at the facility where the spill occurred.  The websites do not show groundwater plumes in their “polygon” shape. 1

Introducing the CleanupDeck

Clicking on a plume brings information to the user including typical depth to groundwater, links to the agency web page, and overview of the method. In the near future, chemical information will also be shown highlighting those constituents with a potential to volatilize.

Clicking on a plume brings information to the user including links to the agency web page, depth to groundwater, a unique plume ID, and an overview of the method. The feature welcomes feedback to better shape a given plume. In the near future, chemical information will also be shown highlighting those constituents with a potential to volatilize.

Terradex created the CleanupDeck to provide stakeholders with a web-based mapping tool to view the approximate location of groundwater plumes.  Rather than revealing multiple complex maps, the Cleanup Deck simplifies these maps into one area of impact – a summary groundwater plume.   The map is an information starting point.

Within the CleanupDeck the user sees the extent of the plume, finds information on the depth of the plume, chemicals contained in the plume, and a link to an agency web resource on the plume. The Cleanup Deck also permits viewing associated cleanup sites, institutional controls applied to limit land use after cleanup, and some key sensitive uses such as daycare and schools. The user clicks on the plume with the “i” (information) button, and a pop-up shows further information.

USEPA Brings Focus to Plumes and Vapor Intrusion Risk

A recent USEPA letter, sent to the San Francisco Regional Water Quality Control Board, addressed vapor intrusion hazards in Silicon Valley.  The letter warns that extensive shallow groundwater plumes, containing TCE and the closely related tetrachlroethene (PCE) contaminant, exist at about 15 to 50 feet below the surface at ten Superfund sites in Silicon Valley, and the letter called for the need to further assess vapor intrusion risks due to these plumes.  But the letter did not include maps showing or estimating the underlying plumes.  The lack of such maps led to the natural question – where are the potentially affected areas?  This inspired Terradex to introduce its ongoing project to build a groundwater plume layer for Silicon Valley.

Quantifying the Impact of Silicon Valley Groundwater Plumes

Terradex’s CleanupDeck  shows where groundwater plumes occur within Silicon Valley, including plumes associated with the ten Superfund sites addressed by the USEPA letter.  The table below provides a launch pad to the CleanupDeck. There is a link to each groundwater plume map in the CleanupDeck associated with the thirteen Superfund sites (three Superfund sites are combined into the MEW Study Area).

The ten Silicon Valley Superfund site plumes, in aggregate, affect nearly six square miles, 1,100 unique property parcels, 4,300 households, and a population of approximately 10,500 residents. Based on demographic profiles, approximately 2,011 women of childbearing age (22 years to 39 years) are among the residents2.  These plumes affect sensitive uses including three schools and ten child daycare facilities.

While the USEPA letter focused on Superfund sites,  approximately 516 additional plumes occur in Silicon Valley.   The majority of these plumes are associated with releases from gas stations or dry cleaners, but many are other solvent spills where cleanup was not controlled by Superfund protocols.  The CleanupDeck shows an approximation of where these plumes occur as well.

The Health Hazards of Volatile Organics

Driving this discussion is the health impacts posed by these chemicals.  A focus in on trichloroethene, which is a halogenated organic compound formerly used for degreasing in the semiconductor industry.  Exposure is through inhalation of TCE vapors evolving from shallow groundwater into building.

A March 2013 summary study entitled “Human Health Effects of Trichloroethylene: Key Findings and Scientific Issues”  reviewed the health impacts of TCE.  The study concluded “TCE is carcinogenic to humans by all routes of exposure and poses a potential human health hazard for non cancer toxicity to the central nervous system, kidney, liver, immune system, male reproductive system, and the developing embryo/fetus.”  The  potential adverse impact on the developing embryo/fetus focuses a short-term exposure risk on young women was featured.

How to Build a Groundwater Plume Map


The groundwater plume maps in the CleanupDeck are assembled by making overlays of multiple groundwater contour maps to form an outer contour. Deeper plumes are projected to the surface so that a user might know if they overlie a plume at any depth.

To estimate the groundwater plumes displayed in the Cleanup Deck, Terradex followed the method summarized here.  Each plume map is assembled from public regulatory data associated with a given spill site.  The approach taken to apply this data has varied.  Generally the estimating effort is performed by one of two methods:  1) interpreting the contour based on review of groundwater monitoring reports; or 2) generating a polygon formed by the outer ring of the monitoring well network (i.e., convex hull).  When interpreting groundwater monitoring reports, the multiple groundwater plume maps were layered into Google Earth as overlays, allowing the outer ring to be interpreted for the plume layer.  For the plumes formed by a convex hull, the GeoTracker-provided monitoring well location was downloaded, which contains the latitude and longitude of each well.   Release sites with networks of three or more wells were used to form convex hulls, which were then inserted into the groundwater plume layer.  In either case, when shallow and deeper aquifers existed we combined all contaminated data into one simplified surface projection.  3

Introducing a How Local Government Applies the Groundwater Plume Information in Planning Decisions

Allowing information on plumes to be constructively applied by local government would help safeguard the public.  Local government oversees development and can thereby detect new occupancies that given their location and profile might require further evaluation.Terradex began efforts to work with the City of Palo Alto and City of Santa Clara to allow the groundwater plume information to be used by the city staff.  We found a simple way to integrate this information into the city’s geographic information system (GIS).   A planner using the CleanupDeck could flag proposed development projects that overlie plumes or cleanup sites where vapor intrusion hazards might require evaluation. The presentation  to City of Palo Alto can be viewed at this link.

Next Steps with The CleanupDeck in Silicon Valley

The groundwater plume maps presented here and available in the Cleanup Deck remains a work in progress.  Additional plumes have yet to be mapped. For example, groundwater contamination can exist beneath municipal landfills, and these have not yet been incorporated into the map layer.  The use of convex hull techniques allowed a rapid characterization of the outer boundaries of groundwater plumes, but contaminant plumes maps would be better formed though site-by-site review of groundwater plume maps.  Given the complexity involved with mapping groundwater plumes, we envision a mapping platform that allows for the direct input by local stakeholders – helping to refine, shape, and identify any errors in plume maps through indiscriminate ability of stakeholders to “weigh in.”   Regardless of the openness of the process, there inevitably will be disagreements about the precise boundaries of a plume or the precise areas of vapor intrusion risk.   Given that the purpose is primarily to inform safe land use it would seem that uncertainties should not get in the way of transparency  – when in doubt, make the contour larger.

Knowing the innovative culture of Silicon Valley, Terradex looks forward to building local collaboration that would further develop this information. Terradex is a technology service with core competencies that include the process assimilating data, preparing intuitive maps, and developing web-based mapping platforms.  Terradex does not itself use its data or maps to advocate for one position or the other.  Rather, it strives to provide intuitive environmental information to all stakeholders.  With information properly conveyed, interested stakeholders can start to tame the “perfect storm” and allow safe use and development across Silicon Valley in areas where groundwater may be impaired.

  1. To appreciate the difference, activate the Cleanup Sites layer in the CleanupDeck, and compare point icons to the groundwater plumes.  Polygons can inform about an area of impact synonymous with the nature of a plume.  Stakeholders might inadvertently take comfort seeing that a “point” icon placed away from their household or business, even though a plume that would be shows as a polygon might underlie their household or business.
  2. Terradex appreciates the assistance of the Association of Bay Area Governments for interpreting demographic data.
  3. Terradex based the Cleanup Deck plume maps on the most current data it could find as data made public on the Internet. Terradex has applied its best effort to assemble these maps, but reliance on them should be cautionary and the maps should be considered as estimates only.  These plume maps are provided “as is” without warranty of any kind, express or implied.   Use of these maps is at the sole risk of the user. Terradex welcomes contributions to improve the quality of these plume estimates. Terradex forbids the resale or other commercial use of the information provided in the Cleanup Deck without our written consent.


  1. Zohra Majid
    Posted August 21, 2014 at 11:54 pm | Permalink

    Thank you for the valuable information. I just have tow questions please, and I understand the answers may not be definite–just approximate. What is the general considered area of the air affected by the plumes? And do high efficiency air cleaners help get rid of the organic, volatile compounds?

  2. admin
    Posted September 16, 2014 at 12:23 am | Permalink

    The area (or air) affected by the plumes is highly variable, and the literature will show considerable variance in predictions. In general, laterally from the edge of a plume at no detection of constituents, the volatiles may occur several hundred feet. There does not appear to be a vertical limit to their migration, in other words a plume cannot be too deep to not have a volatile impact.

    The mitigation approaches generally involve ventilation, or the installation of barriers. The theory is to prevent the pathway from being complete from the source to the receptor. A parallel effort would be to clean the problem by removing the contaminants, and this occurs in consort with the protective steps to sever a complete pathway.

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