Box 3 Outcomes from the Santa Susana Field Laboratory
At the Santa Susana Field Laboratory (SSFL) in California, USA, intensive subsurface investigation of groundwater contamination in the local sedimentary bedrock has been conducted since the 1980s, and field-based research has been undertaken since the mid-1990s. The bedrock is dominated by sandstones with lesser siltstones and conglomerates interbedded with shales. This site is the perfect field laboratory to study how faults and fractures impact groundwater flow and contaminant transport given multiple high-resolution data sets. Indeed, numerous cored boreholes and an extensive groundwater monitoring network consisting of conventional wells and depth-discrete multilevel systems, along with geophysical tools and structural geology analysis, have been used to understand the hydrogeologic conditions governing the migration of contaminants, with trichloroethylene being of primary interest (Sterling et al., 2005; Cherry et al., 2009; Meyer et al., 2014).
At this location, groundwater flow occurs almost entirely in the fractures because the rock matrix has low permeability (Parker et al., 2012). Well data suggest that hydraulically active fractures are present nearly everywhere at SSFL and, together with larger faults, create an interconnected network of fractures of variable length and aperture that determine the rate and direction of flow and influence the nature of the contaminant plume (Cherry et al., 2009; Guiheneuf et al., 2020). Fault zones at SSFL have contrasting hydraulic behavior: some are barriers to cross-flow, whereas others are conduits (Figure 42 of this book). Cilona and others (2015) focused on the mechanisms responsible for the barrier behavior and its impact on contaminant transport. Guiheneuf and others (2020) showed the variability of hydraulic response behavior in multiple observation wells in the area shown in Figure 42a; this behavior depended on the depth interval of the monitoring intervals and on the intersection with the larger-aperture and longer fractures associated with the Shear Zone Fault, given the location of the pumping well within the orthogonal IEL fault (shown in Figure 42a). As shown in Figure 42c, groundwater levels differ by as much as 75 m across a major NE-SW striking Shear Zone Fault zone. The hydraulic heads on the southeast side of this fault are systematically higher than those on the northwest side. Despite this strong NW-SE (fault-perpendicular) groundwater hydraulic gradient, the migration of contaminant plumes is significantly restricted from east to west across this fault (Cherry et al., 2009), and the concentration of contaminants suggests that the plume southeast of the fault is elongated parallel to fault strike (Cherry et al., 2009). These observations are consistent with the hydraulic conductivity of the fault core, which was estimated to be more than two orders of magnitude lower than the bulk hydraulic conductivities of surrounding sandstone units, based on the geometric mean and median values of multiple measurements (Figure 43a,b).
The model that was proposed to explain the barrier effect of the Shear Zone Fault involves the presence of deformed shales in the fault core, which were incorporated into the fault zone by a mechanism known as shale smearing. In this mechanism, the platy clay minerals of shales are re-aligned parallel to the shear direction resulting in reduced hydraulic conductivity of the deformed shale along the fault zone that generates strong contrasts in hydraulic conductivity along the zone. The resultant anisotropy in hydraulic conductivity influences groundwater flow.
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