Theme: 10. Characterization, Fate and Transport

Topic Area: 10d. Conceptual Site Models: Improvements in Development and Application

Formatted Title:

Conceptual Site Model Enhancement and Performance Monitoring Applications Using Multi-Level Groundwater Monitoring Systems

Background/Objectives:

Multiple Solid Waste Management Units (SWMUs) are located within a 200-acre active chemical manufacturing facility bordering the Sacramento-San Joaquin River in the Bay Area of California. The facility is underlain by over 130 feet of shallow fill and unconsolidated alluvium. Deposited over the last two million years, these sediments contain interbedded clays, silts, sands, and vegetative layers formed by meandering watercourses that have drained through the site over time.

A site wide Conceptual Site Model (CSM) had previously been developed depicting the subsurface as simplified horizontal layers of high and low permeability materials. This legacy CSM was largely based on geological groupings and did not accurately capture small scale variations in the subsurface lithology that define the hydrostratigraphy and control the distribution of constituents of concern. The focus of this work is to gain a more detailed understanding of the hydrostratigraphy in a highly localized portion of the site and to leverage those learnings to provide context for an improved interpretation of the extensive site-wide legacy dataset. The goal is to improve the CSM to inform interpretation of on-going groundwater monitoring programs, performance monitoring programs, and pre-design investigations, which will ultimately result in more effective and efficient remediation.

Approach/Activities:

A phased approach integrating a combination of assessment tools was adopted to achieve a higher resolution understanding of subsurface stratigraphy and hydrogeology. Updating the CSM included an initial data gap analysis, integrating 25-year-old boring data into a digital space alongside more recent well borings and long-term groundwater monitoring data; then cross referencing these data with previous membrane interface probe and passive soil gas studies. Using these analyses, locations for the multi-level systems were selected to address the identified data gaps.

The multi-level system selected was the Solinst CMT© (Continuous Multichannel Tubing) system, which allows for up to seven discrete sampling intervals per well. At each location, detailed core logging and wireline geophysical logging of natural gamma were conducted to identify zones of interest from which to select the discrete monitoring intervals at each location to gain the most in insight into variations in hydraulic head and chemistry with depth. The CMT data were compared with data from the legacy well network, providing context and additional insight for the refinement of the CSM.

Results/Lessons Learned:

Integrating the CMT data was key in re-framing of the localized CSM, with learnings to be leveraged site-wide. Additional sampling is planned and will be used to further differentiate between impacts from various SWMUS, to refine remedial approaches and improve remediation performance monitoring.

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Tom Wallis