Development of a Robust Geological Model to Explain Creosote Flow and Transport in a Sole-Source Carbonate Aquifer

Fractured rock aquifers pose challenges for flow system characterization due to the complex nature of fracture networks controlling the bulk hydraulic conductivity within and across hydrologic units. The distinct depositional characteristics of mud-rich carbonate facies as well as the post-depositional alteration of material via diagenesis often result in highly anisotropic flow systems. The lithostratigraphic properties and sequence stratigraphic surfaces in sedimentary bedrock are known to affect fracture development and connectivity. The objective of our study is to build a robust geologic model to inform fracture network connectivity influencing vertical and horizontal flow of groundwater and creosote DNAPL in a shallow Silurian limestone aquifer located on the island of Gotland, Sweden. 

Previous consulting studies using conventional wells showed widespread presence of dissolved phase creosote constituents and occasional presence of non-aqueous phase components near and away from the historical operations area. The current research investigation aims to use recently advanced high-resolution methods to build a robust, hydraulically-calibrated geologic framework.  It began with six ODEX air-rotary boreholes, drilled 30m below the top of rock around the perimeter of known contamination at the site.  Four additional boreholes were cored using GeoBore-S diamond bit wireline drilling within the contaminated zone to provide continuous core for lithology and fracture feature logging.  These four cores were also used to inform sample locations near and away from fractures for contaminant concentrations and rock physical properties. All 10 new boreholes were geophysically logged to inform the placement of temperature and pressure transducers in the boreholes, which were then sealed in place using flexible, impermeable fabric liners (FLUTe™) for depth-discrete dynamic hydraulic head monitoring.

Lithology and fracture-feature logs were collected and confirmed the sitewide presence of limestone-marl alternations with a high propensity for laterally extensive, bedding plane fractures dipping south-southeast. Depth-discrete rock samples for VOC and PAH contaminant analyses confirmed that: 1) matrix diffusion of PAH compounds from hydraulically active fractures was limited due to low aqueous solubilities, low porosity and high sorption, 2) downward migration of creosote through vertical fractures was likely inhibited by the lack of vertical connectivity between fine-grained limestone facies imparting strong anisotropy, the near neutral density of the creosote as a NAPL and the high sorption of low solubility solutes in the organic-rich marlstone units. Downhole geophysics and core logs also confirmed that a weathered bedrock zone at the overburden-bedrock interface exists sitewide and contained the highest contaminant concentrations.  This sediment-bedrock interface may serve as a preferential pathway for the lateral migration of the marginally dense creosote oil. In combination, these data provide a process-based conceptual site model that can be used to accurately model fluid flow and plume mobility, improving risk assessment and remediation efficacy.