This paper presents a case study for a complex contaminated groundwater site impacted by a historical release of chlorinated solvents in Silicon Valley, California. The original conceptual site model (CSM) inferred a contaminant migration pathway based on the groundwater gradient interpreted from groundwater elevation data, which is based on the underlying assumption that the subsurface conditions are homogeneous. However, the buried channel deposits render the underlying geology highly heterogeneous, and this heterogeneity plays a significant role in the subsurface migration of contaminants. Chemical fingerprinting evidence suggested that contamination at the downgradient property boundary was related to an off-site contaminant source. But, this alone was not a compelling argument. However, Environmental Sequence Stratigraphy (ESS), a geology-based environmental forensic technique, was applied to define the permeability architecture or the “plumbing” that controls subsurface fluid flow and contaminant migration. First, the geologic and depositional setting was synthesized based on regional geologic data, and representative facies models were identified for the site. Second, the existing CSM and site lithology data were reviewed and existing lithology data were graphically presented to display vertical grain-size patterns. This analysis focused on the nexus between the depositional environment and the site-specific subsurface data resulting in correlations/interpretations between and beyond data points that are based on established stratigraphic principles. The depositional environment results in buried river channels as the primary control on subsurface fluid flow, which defines hydrostratigraphic units (or HSUs). Finally, a hydrostratigraphic CSM that includes maps and cross sections was constructed to depict the HSUs present as a framework to integrate hydro-geology and chemistry data. This study demonstrates that: 1) Highly per-meable buried river channel deposits control subsurface fluid flow and contaminant transport, and have distinct chemical constituents and concentrations (i.e., they represent distinct HSUs), 2) Mapping of such HSUs is feasible with existing boring log data, 3) In settings such as the Santa Clara Valley where groundwater flow is governed by subsurface channel deposits, a hydrostratigraphic mapping approach is superior to a depth-based aquifer zonation approach, and 4) For heterogeneous subsurface, a detailed geology-based definition of the subsurface is an integral component of an environmental forensic analyses to determine contaminant source(s) and pathways.
At a contaminated site, it is generally understood that a conceptual site model (CSM) is an indispensable road map for the remedial investigation and throughout the remediation life cycle. Among the components of the CSM, contaminant sources are of the most critical as they are the starting points of the migration pathways, which controls ultimate distribution (or extent) of contaminants (Lu, 2015 [
Conventional site investigation typically relies on the assumption of homogeneous subsurface conditions where “hot spots” (i.e., areas with distinctively higher COC concentrations exceeding cleanup levels) are identified and addressed during remediation (Lu et al., 2016 [
The Silicon Valley site presented herein is a prime example of a complex site where the subsurface heterogeneity is defined by the geology and controls fluid flow and contaminant migration, and these conditions have a significant effect on source identification. At this site, despite considerable source remediation work over the past two decades, increasing contaminant concentrations were observed in monitoring wells at the supposed “down-gradient” property boundary from the source area, and a CERCLA five-year review recommended additional source remediation. Using the ESS approach, two channel deposits underlying the site were mapped, one of which could be traced back to the on-site source area, and another which was oriented oblique to the presumed groundwater gradient and was interpreted as a contaminant pathway from an off-site source. Analysis of contaminant constituents associated with these two pathways revealed differing “chemical fingerprints” and indicated that these channel deposits are in fact separate and distinct hydrostratigraphic units (HSUs). These findings enabled the responsible party to differentiate which monitoring wells were representative of on-site-related contamination, and those impacted by off-site sources. The multiple lines of evidence provided by hydrostratigraphic mapping and groundwater chemistry fingerprints indicate off-site contaminant contributions to “downgradient” property boundary monitoring wells.
“Environmental Sequence Stratigraphy”, or “ESS” as used herein, refers to the application of both the concepts of sequence stratigraphy and facies models (discussed below) to the types of datasets collected for environmental groundwater investigations, which are typically at the outcrop or reservoir unit scale (tens to hundreds of feet vertically, hundreds to thousands of feet laterally).
ESS analyses have been applied to groundwater remediation and water resource studies since the 1990s (Ehman and Cramer, 1996 [
The science of sequence stratigraphy was initially developed in the petroleum industry based on basin-scale reflection seismic studies, and identification of termination of seismic reflectors on continental margins as related to global sea level changes for petroleum exploration purposes (e.g., Mitchum et al., 1977 [
This site is representative of many contaminated groundwater sites in the Santa Clara Valley, or “Silicon Valley” of northern California (
The heterogeneous aquifers in the Silicon Valley are composed of high-permeability sand and gravel-rich channel-fill deposits encased in low permeability clay and silt floodplain deposits and/or paleosol horizons. The sandy channel deposits result in complex groundwater flow and contaminant migration pathways that are not reliably discerned with groundwater gradient maps. This results in challenges in contaminant plume characterization (particularly with comingling plumes) and remedy design, performance, and monitoring.
The Quaternary alluvial stratigraphic section, which comprises the impacted aquifers in the Silicon Valley, was deposited in fluvial channel and floodplain environments by mildly sinuous (anastomosing or meandering-type) streams
draining the Santa Cruz Mountains and flowing into San Francisco Bay (
Groundwater flow and contaminant transport occurs primarily within the permeable channel deposits, and the variable orientation of channels deflects contaminant migration directions from the regional groundwater gradients. This can cause plumes to appear to spread laterally, and assume complex plan-view morphologies (i.e.,
The database for this project consisted of boring logs (from direct push, hollow-stem auger, and mud-rotary drilling methods), well construction diagrams, and chemical analyses from groundwater samples. Graphic grain size logs were constructed from the boring log information to highlight vertical grain size patterns captured in the boring logs. As shown in
In order to address increasing contaminant concentrations in areas downgradient of the onsite source area, cross section A-A’ (location shown on
The following principles of stratigraphic interpretation in fluvial deposits were applied to correlate the grain size patterns between boring logs, as depicted in
• Channel deposits tend to have erosive bases and relatively flat tops, and clays make superior correlation markers (likely paleosol horizons);
• Gravels define channel bases and grain size fines upward.
Channel margins are sharp and erosive, and result in strong segregation of channel-fill sands and gravels from floodplain clays.
Inspection of cross section A-A’ (
Extensive on-site contaminant source removal over time resulted in significant decrease in VOC concentrations in groundwater near the source area, but little to no decreases in VOC concentrations at the downgradient property boundary of the on-site source area. The in-situ bioremediation performed in the source area resulted in generation of vinyl chloride (VC) as a daughter product. However, monitoring well T-9B at the downgradient extent of the property showed increasing VOC concentrations, up to 390 µg/L, an order of magnitude higher than other on-site wells. In light of the observed control of channels on groundwater chemistry observed at the site (e.g.,
As mentioned, on-site monitoring wells typically contain VC, occurring as a daughter product of TCE. Freon-113 is associated with the off-site source and was not used in on-site operations. Thus, VC is unique to the on-site source and Freon-113 is unique to the off-site source. After completing the ESS assessment, groundwater contaminant chemistry data (trichloroethene [TCE], tetrachloroethene [PCE], cis-1,2-dichloroethene [cDCE], vinyl chloride [VC], and Freon 113 [freon]) were interrogated with respect to the updated stratigraphic framework (i.e., HSUs) to provide an independent line of evidence for off-site related contamination (
Cross section B-B’ (
high-concentration, deep HSU-2 channel in T-5B. Note that the wells screened only across HSU 1 (T-10B, T-8B, and T-2B) contain groundwater with TCE, cDCE, and VC, and lack Freon-113. The well that is screened only across HSU 2 (T-5B) contains groundwater with Freon-113, and lacks VC. Well T-9B is screened across both HSU 1 and HSU 2 and thus contains mixed groundwater with both indicator parameters (VC and Freon-113).
A similar trend is observed in cross section C-C’, which illustrates the continuity of the HSU 2 channel sands, which is corroborated by the chemistry fingerprint. The wells that are screened solely in HSU 2 lack the on-site source indicator VC and contain Freon-113 (T-4B has historically contained Freon-113, but not during the timeframe used to create fingerprint graphs). Well T-9B is screened in both HSU 1 and HSU 2 and contains groundwater that is a mixture of HSU 1 and HSU 2, containing all four analytes.
The chemistry fingerprint data provide an independent line of evidence, and corroborate the geologic interpretation that channel HSU 1 is a contaminant
pathway representative of the on-site contaminant source and channel HSU 2 is a contaminant pathway representative of the off-site contaminant source.
This case study exemplifies why a detailed understanding of the subsurface geology is critical for distinguishing potential source areas and hydrostratigraphic pathways for complex sites. As shown in
the multiple sources responsible for the commingled plumes. As a geology-based environmental forensic tool, this approach can be a significant asset to forensic analysis of complex contaminated groundwater sites.
Cramer, R., Lu, J., Shultz, M., Plank, C. and Levine, H. (2018) Use of Environmental Sequence Stratigraphy (ESS) as an Environmental Forensic Tool to Identify Chlorinated Solvent Sources at a Complex Site in Silicon Valley, California. Journal of Environmental Protection, 9, 554-566. https://doi.org/10.4236/jep.2018.95035