Formatted Title
Site Recharacterization: Conceptual Site Model Evaluation of Fractured Bedrock Sites in the Newark Basin
Background/Objectives
The long-term management of contaminated sites can exacerbate the difficulties of developing conceptual site models (CSM) for fractured bedrock sites. Mismanagement of historical data, loss of institutional knowledge, and an overreliance on previous interpretations are regular challenges that are faced when working on legacy sites. Two bedrock sites in north and central New Jersey were recharacterized using a consistent, contemporary methodology which relies heavily on pre-existing data. The application of this high resolution site characterization process yields highly technical, data driven hydrostratigraphic cross sections in a schedule and budget conscious manner. This process was used at Site “A” to inform the development of a full-scale groundwater remedial action work plan. At Site “B”, recharacterization was implemented to remedy an inaccurate CSM. The outdated model misinformed the design of a groundwater extraction system that pumped water from the wrong discrete aquifer unit. Due to confidentiality reasons, the exact locations of these sites cannot be named.
Approach/Activities
This method for site recharacterization follows a three-step approach. In the first step, the existing CSM is evaluated through a detailed review of available historical data. This includes Geophysical (ATV, gamma, caliper, and fluid temperature), hydrogeologic (K, h), analytical, boring log and well construction data. The data are inventoried, digitized, and stored in an environmental database. Hydrostratigraphic cross sections are developed based on this readily available data, and additional data needs are identified. The second step of this process consists of a field mobilization designed to collect specific data at prescribed locations and depth intervals. Traditional aquifer testing methods utilize existing monitoring networks. Geophysical tools such as gamma and nuclear magnetic resonance logging can be performed in open bedrock wells and through pre-existing PVC cased monitoring wells. At Site “A”, a novel approach of using continuous sonic wave logging to identify structural features through PVC cased wells was performed. Where additional drilling is required, the scope of work is optimized to be budget and schedule conscious. In the final step, newly acquired data are processed and loaded into the previously developed hydrostratigraphic cross sections and the CSM is updated. These data driven visualizations are then used to inform client needs as they pertain to the selection and design of a remediation strategy for the site.
Results/Lessons Learned
An overview of the recharacterization process will be provided. Hydrostratigraphic cross sections from both Site “A” and Site “B” will be presented that contain stratigraphic interpretations based on historic and newly acquired gamma data and observations made during bedrock coring. Brittle structural features such as bedding planes and connected fractures of significant aperture will be identified through ATV, caliper, fluid temperature, and continuous sonic wave data. Low and high K zones will be identified by incorporating NMR, h, and aquifer test data. This holistic approach will allow for the identification of discrete aquifer units within the bedrock that serve as the major modes of groundwater and contaminant transportation. The Selected remediation technologies based off these recharacterization efforts will also be presented along with the estimated cost savings when compared to more traditional methods of site characterization. For each site, a comparison of the pre- and post- recharacterization CSM will be discussed.