Formatted Title
The Evolution and Application of the Puchack Site Geologic Model: From Paper to Digital, from Lithostratigraphic to Environmental Sequence Stratigraphy
Background/Objectives
The Puchack Well Field Superfund site lithostratigraphic geologic model has evolved from paper to digital, from 2-dimensional (2D) to 3-dimentional (3D). Now environmental sequence stratigraphy (ESS) is being applied to improve the geologic model and support the Phase III Remedial Design (RD), which is focused on the Intermediate Sand hydrostratigraphic unit at the site. The site is located north of Camden, New Jersey, covers approximately 180 acres, and is underlain by the unconsolidated Potomac-Raritan-Magothy system to a depth of 300 feet. In the 1980s hexavalent chromium contamination in groundwater shut down the well field. The U.S. Environmental Protection Agency (USEPA) began a Remedial Investigation (RI) in 2000. Using lithologic and natural gamma log data from borings completed at the site during the RI, the U.S. Geological Survey (USGS), USEPA, and CDM Smith developed a lithostratigraphic geologic model of the site. The project team applies this model to ensure the correct placement of extraction, injection, and monitoring well screen, critical to the success of the RD and Remedial Action (RA). The RA involves the injection of sodium lactate (a reducing agent) in groundwater to reduce hexavalent chromium to trivalent chromium.
Approach/Activities
The lithostratigraphic model started on paper as a series of cross sections developed by the USGS. This model was used to guide well and screen placement during the Phase I RD and RA. Lithologic and natural gamma data from the RI and Phase I RD/RA were stored in gINT software which was used to prepare new boring logs and cross sections. These data were then used to develop a 3D solid lithostratigraphic geologic model in Leapfrog Works software. The resulting lithostratigraphic surfaces were used in gINT to guide the placement of well screens during the Phase II RD and RA. In 2019 AECOM performed an ESS analysis of the site at the request of USEPA using data from 30 boreholes (out of more than 600). This ESS study included an assessment of depositional facies at the site (Phase 1 ESS analysis). During the Phase II RA (2019 to 2022), CDM Smith prepared grain size logs (GSLs), Phase 2 of the ESS process, for all 158 Phase II RA borings and selected RI and Phase I RA borings. In support of the Phase III RD, CDM Smith used the assessment of depositional facies completed by AECOM and the newly prepared GSLs and natural gamma logs to define hydrostratigraphic units (Phase 3 of the ESS process) to improve the geologic model of the site with a focus in the intermediate sand, which is the target of the Phase III RA.
Results/Lessons Learned
To improve the geologic model of the intermediate sand, which is the target of the Phase III RA, six cross sections were prepared which incorporated all 20 of the planned extraction wells and 40 of the planned injection and monitoring wells. These 60 wells constitute 28% of the planned total of 215 wells. GSLs and natural gamma logs from existing wells were included on the cross sections to provide the data needed to define hydrostratigraphic units (Phase 3 ESS analysis). In the lithostratigraphic model the Intermediate Sand is represented as a sand layer sandwiched between upper and lower confining units. Experience at the site suggests the Intermediate Sand is not homogeneous. The ESS analysis showed that the intermediate sand consists of a series of fluvial channel bars separated by splay and overbank deposits and estuarine and coastal clay. The estuarine and coastal clay forms the overlying and underlying confining units. The ESS analysis indicated that one (5%) of the 20 planned extraction wells would probably be screened in a low transmissivity estuarine and coastal clay and should therefore be relocated. Overall, the analysis indicated that up to 10% (about 20) of the planned 215 wells (20 extraction, 142 injection and 53 monitoring wells) may be screened in estuarine and coastal clay or on the flank of a channel and therefore relocation should be considered if practical. In conclusion, the ESS process provides a powerful tool to optimize well placement, thereby improving the quality of data collection and remediation during the design phase and during remedial action.