Formatted Title
Application of Environmental Sequence Stratigraphy to Sedimentary Bedrock Aquifers with Commingled and Co-located VOC and PFAS Plumes
Background/Objectives
The Newark basin in Connecticut, New York, New Jersey, and Pennsylvania contains cyclically sequenced sedimentary bedrock with monoclinally dipping bedding that imparts first-order control on groundwater flow. Preferential flow along strike direction in discrete tabular aquifer units is described by the leaky multi-aquifer system model, which is generally accepted by state and federal regulators. Identifying, characterizing, and correlating dipping bedrock hydrostratigraphic units is key to developing a conceptual site model (CSM) that can be effectively used to determine migration pathways and identify on and off-site sources. We will present two case studies where conventional CSMs struggled for over a decade to explain head data from monitoring wells, groundwater flow directions and the sources of VOCs as well as new contaminants of concern; PFAS and 1,4-dioxane. We will present how we used environmental sequence stratigraphy to update the CSMs and map tabular aquifer units thousands of feet, including to off-site source areas.
Approach/Activities
The original CSMs used a depth-based aquifer zonation approach described as shallow, intermediate, and deep bedrock aquifer units and inferred contaminant pathways using plan view plume distribution maps. Using environmental sequence stratigraphy, a geology-based forensic method, we performed detailed hydrostratigraphic mapping utilizing borehole geophysics, drilling logs and outcrop data. Our site borehole sequencing was correlated with logs from off-site industrial and commercial sites up to 2,000 feet away along strike, which are contributing to the commingled plumes. The Newark basin sedimentary rocks are profoundly cyclical, with four overlapping depositional cycles ranging from 20 thousand years to 2 million years. Bedding expression can be very subtle and difficult to follow; however, the stacking patterns of strata expressed in gamma logs, and associated water-bearing bedding-parallel fractures, can be correlated between cores by a trained geologist. To confidently identify and correlate sequences, which repeat and look very similar, we tied our model to the borehole data from the National Science Foundation–funded Newark Basin Coring Project (NBCP), which produced 6,770 meters of continuous core and geophysical logging. The cycles in the NBCP cores have been worked out and interpreted in great detail (centimeter scale) by others and traced over more than 100 kilometers.
Results/Lessons Learned
Our improved CSMs, which included integration of head and chemical data and pumping and packer test findings, was an important part of our environmental forensic analyses that resulted in the determination of contaminant (VOCs, 1,4-dioxane and PFAS) sources, both on and off site, and an understanding of stratigraphic flux and associated contaminant transport. Our presentation will include descriptions of the methods, correlation graphics, maps, cross-sections, and 3D EVS visualizations, and describe the value of improving hydrostratigraphic CSMs using environmental sequence stratigraphy to integrate hydrogeology and contaminant data.