Groundwater Soil Conditions similar to Marinwood and Silveira Ranch

This groundwater model shows soil conditions very near the the Toxic Plume.
Note the speed, toxic waste spreads in the layer of the confined aquifer.  This is analogous to the Franciscan Plate of the Miller Creek Streambed where the PCE toxic waste has spread.


From the Geologica Remedial Action Plan page 9-10:

2 SURFACE AND SUBSURFACE CONDITIONS

Sections below describe topography and surface water near the Site, soil lithology, groundwater occurrence, and flow conditions.

2.1 TOPOGRAPHY AND SURFACE WATER

The subject property is located in the Gallinas Valley at an elevation of approximately 40 feet above Mean Sea Level (MSL). Surface topography slopes gently to the east. The Site is located approximately 500 feet north of Miller Creek, which flows in a generally easterly direction towards San Francisco Bay. San Francisco Bay is located approximately 2 miles east of the Site.

2.2 SOIL LITHOLOGY

The site and vicinity are underlain by Quaternary Alluvium consisting of unconsolidated deposits of silt, clay, sand and gravel. Franciscan bedrock comprised of sandstone and shale reportedly outcrops in highland areas north, west, and south of the site. Borings have been advanced to maximum depths of 52 feet (ft) below ground surface (bgs) on the property and 78 feet bgs on the Silveira Ranch property to the east across Highway 101. The Site and vicinity are underlain by up to 50 to 60 feet of silt, sand, and gravel deposited by a meandering ancestral Miller Creek over fractured bedrock of the Franciscan Complex. Borings advanced at and near the Site indicate that these stream deposits are variable in texture both laterally and vertically and generally become coarser with depth. Key lithologic boring locations are shown on Figure 2. Schematic subsurface cross sections are presented on Figures 3, 4, and 5. Soils encountered at the Site generally consist of a surficial sandy clay horizon overlying as many as three horizons of clayey sand separated by grading to clayey medium sand as depicted on Cross Section A-A’ (Figure 3A). The upper silt and clay horizon was encountered to depths of approximately 15 to 25 feet bgs. Varying thicknesses of more permeable sand horizons and less permeable silt and clay horizons were encountered below that depth. However, many of the permeable sand layers may have limited lateral continuity.

2.3 GROUNDWATER OCCURRENCE AND FLOW

Groundwater is encountered under semi-confined to unconfined conditions in the uppermost permeable soil horizon beneath the Site; groundwater present in deeper permeable strata appears to be semi-confined or confined by overlying finer-grained strata. As evidence of these conditions, groundwater is typically first encountered at depths of 16 to 20 feet bgs during drilling, and rises following boring completion. Groundwater levels in monitoring wells completed at the Site fluctuate seasonally, ranging from 6 to 12 feet bgs in late winter and several feet lower in late fall (see Figure 6). Generally easterly to southeasterly horizontal groundwater gradients have been identified at the property and off-site on the Silveira Ranch to Remedial Action Plan Page 5 Former Prosperity Cleaners San Rafael, CA the east. Figure 7 presents a groundwater elevation contour map for the most recent quarterly monitoring event, which was conducted in November 2015. Based on off-site investigations, groundwater north and east of Miller Creek was encountered below the upper silt and clay horizon under partially confined conditions. South of Miller Creek on the Silveira property, groundwater was encountered in a shallow sand horizon overlying the upper silt and clay horizon as well as in deeper confined units underlying the upper silt and clay horizon (See Schematic Cross Sections B-B’ and C-C’, Figures 4 and 5). Offsite investigation completed in August/September 2015 estimated groundwater elevation ranging between approximately 18 and 31 feet MSL, which is consistently above the water level in Miller Creek, indicating that the creek gains water from the underlying groundwater-bearing zone as it crosses the Silveira Ranch property.