Improved remediation design and operation resulted from effectively using reservoir-aquifer characterization tools to identify hydraulic flow units and connectivity of sediments in contaminant-affected shallow aquifer-aquitard systems. The objective of this approach was to utilize an improved understanding of subsurface conditions to develop more effective remediation designs and operating plans. This was accomplished by developing an increased understanding of aquifer storage and flow properties, preparing geologic models, and identifying contaminant migration pathways and permeability barriers. Characterization methods practiced in petroleum exploration and production were applied to near-surface sedimentary sequences to describe the subsurface hydrogeologic setting. Stratigraphic analysis using continuous core, detailed core descriptions and analogous sedimentary environments of deposition was employed to develop deterministic geologic models of the subsurface. Predictive mapping was used to delineate contaminant-affected soil and groundwater, and interpretations were adjusted in an iterative manner with the acquisition of data.
This paper will discuss case histories of overall project characterization and remediation of two sites located in Southern California, USA. These case studies demonstrate the effect of subsurface soil heterogeneities and variation in contaminant distribution on remediation.
The method of reservoir-aquifer characterization uses analogous sedimentary environments to describe a type of sedimentary locality (e.g., meandering stream), in which the resultant deposits are determined based upon specific lithological characteristics of texture, composition and sedimentary structures.1,2 A litho-stratigraphic soil description was developed to supplement the typical soil description to create an input data set useful for stratigraphic analysis, correlation and mapping of stratigraphic units based on sedimentary environments of deposition.
Sedimentary sequences were described in terms of overall lithology and textures. Sands and silts were further described in terms of grain size range (Wentworth scale), median size, angularity, roundness, sorting and mineralogical composition (accessory minerals and fossils, and percent quartz, feldspar and lithic fragments) and sedimentary structures (bedforms, plastic deformation, tool marks, bioturbation, plant-root casts, etc.). Secondary porosity and permeability development in clay was identified by the presence of plant-root casts, carbonaceous plant fragments and caliche nodules. Continuous core sampling was conducted from the ground surface to total depths of 30 to 60 ft-bgs (feet below ground surface). Soil cores were checked for visible and detectable contaminants.
The lithology depth-profile served as the basis for well-to-well correlation and identification of vertical sedimentary sequences and bedding. Confidence in the well-to-well correlations was gained by the close similarity of lithology profiles between neighboring wells. Mapping was an iterative process based on comparison of lithology depth-profiles with similar and applicable sedimentary environments of deposition from which to develop a site-specific geologic model. Predictive structure and isopach maps and cross sections were constructed and refined after each phase of drilling. The maps and cross sections were used in depicting the extent of contaminant-affected subsurface sediments and for selecting future drilling locations.