wells and words

coMbininG GeoPHysical loGs and sequence stratiGraPHy and aPPlyinG to site cHaracterization

by David W. Abbott, P.G., C.Hg., Consulting Geologist

In mid-December 1986, I had the opportunity to conduct a wide-ranging groundwater contamination review and study (Phase 1) for a chemical recycling facility in San Jose, CA (discussed briefly in a previous Wells and Words [Fall 2021] article and merits further discussion here). This facility was established in 1973 to reclaim, recycle, and dispose of industrial solvents used in Silicon Valley. Since 1983, the facility was under regulatory review by the CA Regional Water Quality Control Board and the CA Department of Health Services. Later, the property would be placed temporarily on the Federal Superfund Site List (low priority) by the US Environmental Protection Agency.

The 3.2-acre property (ground surface elevation ≈70 feet [ft] above mean sea level) is located ≈7 miles south of San Francisco Bay and along the central axis of the Santa Clara Valley. The shape of the property is a quadrilateral with roughly equal side lengths (≈365 ft). Between 1983 through 1986, two consulting firms installed 85 borings which included 67 monitoring wells that were distributed between five “aquifers” in order to characterize the subsurface geology, hydrogeology, and environmental contaminants (at least 28 chemicals of concern were identified). In addition, 2 extraction trenches to ≈15 ft were installed to contain and retrieve the groundwater contamination in the shallow aquifer; later a third (onsite) and fourth (offsite) trench were installed to complete the containment system and 43 additional monitoring wells were installed to fill in data gaps, to replace “dry” monitoring wells, and to define offsite migration of contaminants. Significant amounts of historical data (soil and groundwater) and numerous reports were reviewed for completion of the Phase 1 report which had a short time-line (3.5 months).

Late in this initial investigation, I discovered data from 8 mud-rotary borings that were drilled in each corner of the site (identified by the E-W-S-N labels) at the top of Figure 1 and midway between the corners along each side of the property. The data from these borings were “buried” in an obscure Appendix and were not fully utilized by previous consultants. All of the borings were drilled to a depth of 150 ft and were logged with 3 down-hole geophysical tools (Spontaneous Potential, Short-and Long-Normal Resistivity, and Natural Gamma Radiation). These geophysical logs provided a coherent picture of the subsurface stratigraphy displayed in Figure 1. Note that the geologic descriptions provided by the field geologist are less objective than the geophysical logs. This modified fence diagram (around the perimeter of the property) is consistent with the expected depositional environments of the stratigraphy beneath the property. A representative geophysical log is shown in Figure 2. Combining the geophysical logs with Sequence Stratigraphy helped to re-define and simplify the stratigraphic units beneath the property and to place them in a hydrogeologic framework. Sequence Stratigraphy is the study of rock relationships within a chrono-stratigraphic framework of repetitive genetically related strata bounded by surfaces of erosion or non-deposition, or their correlative conformities1 A less cumbersome definition of Sequence Stratigraphy is “the study of sedimentary deposits in the context of their depositional environments and changes in relative sea-level, sediment supply, and available sediment storage areas”2.

Wells And Words

You will find all article figures and endnotes at the end of the article.

Previous consultants had divided the subsurface geology into five hydrogeologic units (A through E) that were separated by clays. Aquifer A (water table) was the shallow system (depths of ≈20 ft) where most of the industrial solvents reside. Units B and C were discontinuous lenses (it had always puzzled me that if one coarse-grained unit was identified in a boring as shown on Figure 1: What label should be assigned to it – B or C?) Aquifers D and E were clearly continuous beneath the property. The geophysical logs, facies models3,4, and Sequence Stratigraphy resolved this conceptual subsurface architecture which resulted in modifications to the groundwater and soil investigations and remediation plans.

The eight geophysical logs (see Figure 2) were consistent across the property and indicated a repetitive upwardfining sequence of sands that ended in a clay layer with the exception of Units B and C. The geographic location of the property suggested a low-lying fluvial depositional (and periodic saltwater coastal environments from changes in sea level representing Aquifers D and E) environments such as would be expected along the lower elevations along the axis of the Santa Clara Valley where meandering streams could rework the sediments and deposit them in apparent isolated lens of sand, especially for Units B and C. These sediments were derived from the upland areas of the Santa Clara Valley and deposited by coalescing alluvial fans. The sandy lenses of Units B and C were identified on some (not all) of the geophysical logs and had lower resistivities (25%) compared to Aquifers D and E, which had nearly identical resistivity peaks (Figure 2). Aquifers D and E are continuous beneath the property at an elevation below the current sea level and were probably deposited in a saltwater coastal environment when sea level was at a lower elevation.

The subsurface units were re-defined as Aquifer A (Transmissivity [T] ≈200 gpd/ft), combined Unit B/C (T ≈90 gpd/ft), and Aquifers D and E (T >200 gpd/ft). Aquifer A is a distinct, thin (2 to 5 ft thick), shallow, low permeable, unconfined aquifer that occurs from a depth of 20 to 25 ft below ground surface and consists of silt and sand of re-worked fluvial sediments. Unit B/C is a relatively thick sequence of silts and clays (≈50 ft thick) with discontinuous lenses of sandier sediments deposited by meandering streams. Aquifers D and E are a thick sequence (≈70 ft thick) of interbedded and layered sands, silts, and clays probably deposited along an ancient coastal shoreline.

Previous consultants recognized (with the installation of the first two trenches) that most of the contamination was in Aquifer A but continued to install monitoring wells in Unit B/C. Sampling of these Unit B/C monitoring wells indicated that the presence of contaminants was sporadic suggesting that borehole leakage occurred between Aquifer A and Unit B/C and was responsible for this deeper unintended contamination. Aquifers D/E showed no signs of contamination.

This project investigation continued for another 14 years with the installation of several Aquifer A monitoring wells to fill data gaps, more detailed evaluation of existing soil data, and review of quarterly monitoring reports. Among the firsts for me with this project was the application of Sequence Stratigraphy to environmental contamination, hydrogeology, and the evaluation of a mixture of organic chemicals which included Ketones (mainly acetone), Benzenes (mainly Xylene), -ethenes (mainly Tricloroethene [TCE]), -ethanes (mainly 1,1,1-Trichloroethane [1,1,1,-TCA]), and -methanes (dichloromethane). The Chemical Rubber Company (CRC) handbook5 was the main reference for the chemical properties of each contaminant, including: formula, molecular weight, boiling point, melting point, density, and the relative solubility of the chemicals to water, alcohol, ether, acetone, and benzene. These physical properties were used to establish that acetone was the solvent that promoted movement of the other organic chemicals. I was academically trained in Geology (mostly inorganic chemistry) and had never worked with organic chemistry: let alone, I had not taken an organic chemistry class!

This particular facility had several groundwater industry firsts, including: (1) a contour map of 1,4-Dioxane6 that was generated from Tentatively Identified Compounds (TIC’s) provided by the analytical laboratory; and (2) a pilot steam injection and vapor extraction (SIVE) program that was supervised by Nicholas Sitar, Ph.D., Kent S. Udell, Ph.D. (now at University of Utah), and their graduate students from UC Berkeley. The pilot steam injection program was successful and eventually went full-scale to promote and accelerate removal of contamination beneath the property, especially from fine-grained sediments (i.e., silts).

Endnotes

1 Neuendorf, Klaus, K.E., James P. Mehl, Jr., and Julia A. Jackson, 2005, Glossary of Geology (5thEdition) published by the American Geological Institute, Alexandria, VA, 779 p.

2 Shultz, Michael R., Richard S. Cramer, Colin Prank, Herb Levine, and Kenneth D. Ehman, September 2017, Best Practices for Environmental Site Management: A Practical Guide for Applying Environmental Sequence Stratigraphy to Improve Conceptual Site Models, published by the USEPA, Groundwater Issue EPA/600/R-17/293.

EPA/600/R-17/293, 62 p.

3 Walker, Roger G. (editor), 1981, Facies Models (4thPrinting), published by Geological Association of Canada – Geoscience Canada 1976-1979, Toronto, Ontario, Canada, 211 p.

4 Reading, H.G. (editor), 1978, Sedimentary Environments and Facies, Elsevier, NY, 569 p.

5 Weast Robert C. (Editor), 1970, Handbook of Chemistry and Physics (51st Edition), published by the Chemical Rubber Co., Cleveland, OH, about 2,042 p.

6 Mohr, Thomas K.G., 2010, Environmental Investigation and Remediation: 1,4-Dioxane and other Solvent Stabilizers, published by CRC Press, Taylor and Frances Group, Boca Raton, Florida, 520 p. (see pages 373-378)