DEFINING THE HYDROSTRATIGRAPHY OF A CONTAMINATED, MULTI-LAYERED SEDIMENTARY ROCK AQUIFER SYSTEM: COMBINING NEW AND CONVENTIONAL DATA SETS

The hydrostratigraphy of a 150 m thick sequence of fractured flat-lying sedimentary rock, primarily sandstone, was developed to provide a framework for comprehensive assessment of contaminant distributions and behavior of a dissolved-phase plume emanating for decades from a multi-component DNAPL source. Eleven hydrostratigraphic units (HSUs) were defined based on examination of continuous rock core, borehole geophysical logs, borehole flow metering, and high-resolution temperature logging in open boreholes and boreholes sealed with FLUTe™ liners. In addition, contaminant migration pathways were identified using detailed vertical profiles of contaminant concentrations in the rock matrix obtained from analysis of closely spaced rock core samples. The complementary data sets were obtained from each of three coreholes across the 7.3 km² site. Although the lithostratigraphy based on core inspection and borehole geophysics served as a starting point, HSU definition was refined using the unique data sets obtained from temperature logging within the sealed borehole, detailed hydraulic head profiles, and contaminant concentration profiles from the rock core samples. For example, the coreholes were instrumented with multi-level monitoring devices including a 38-port Westbay® MP system. The extremely detailed hydraulic head profile from the Westbay® MP system played an important role in defining the hydrostratigraphy given that there were 4 distinct kinks in the profile indicating changes in vertical hydraulic conductivity within the system. DNAPL accumulation and the most laterally extensive dissolved-phase plume occur within a sandstone HSU that has unique characteristics demonstrated by the above-mentioned data types. The detailed, complementary data sets provide a robust framework for defining the hydrostratigraphy and a realistic representation of the 3-D flow and transport conceptual model that will be used to improve decision-making regarding protection of the two zones used locally for water supply.