Formatted Title
Field Characterization of Contaminant and Oxidant Interactions in a Fractured Dolostone Aquifer Using Push-Pull Tests
Background/Objectives
Fractured sedimentary bedrock aquifers are among the most complicated to characterize and remediate. This is largely due to a poor understanding of the interconnectedness of fractures and the interactions of contaminants in this network with the porous, but low permeability, rock matrix. Though historically assumed to be minimal, it is now known that significant contaminant mass becomes stored within the rock matrix, creating a long-term secondary contaminant source as it back-diffuses into the fractures. Lab-based methods for characterizing contaminant mass fluxes and oxidant demands provide valuable information but only represent the hydrochemical reactions under ideal conditions not representative of field conditions. Therefore, it is important to conduct field-based experiments to collect results that supplement those collected in the lab. In this study, push-pull tests (PPTs) were used as a simple field technique to provide data analogous to a “leaky” batch reactor, providing insights at the interconnected fracture network scale in a toluene-contaminated aquifer.
The project site is a former industrial facility in Guelph, ON, where activities nearly 30 years ago resulted in toluene contamination of the underlying fractured dolostone aquifer. Since 2014, this site has been used as a research site for characterizing contaminant transport as well as natural and enhanced attenuation processes. At an area of approximately 250 m2, the site is well characterized and instrumented with a high-density monitoring network of 20 state-of-the-science depth-discrete multilevel wells (comprising 128 ports total), arrays of transducers, and fiber-optic cables for flow system and contaminant plume monitoring. The main objectives of this study are to: i) characterize the mechanisms governing the interactions of injected reactive and non-reactive tracers in the dual-porosity system; ii) derive the rate constants governing advection-dominated fracture transport and diffusion-dominated matrix transport; and iii) quantify toluene back diffusion. This characterization will be used to inform the design of a sitewide remediation program involving the injection of persulphate for a dual-action oxidation and enhanced biodegradation via sulphate addition.
Approach/Activities
A pair of field-scale PPTs were conducted at the same location within the contaminated source zone. PPTs involve the injection (push) of a tracer solution into the aquifer, followed by a drift/reaction period, then an extraction (pull) of the solution from the injection well. PPTs are distinct from dipole tracer tests in that the injection and withdrawal occur from the same location. Results from PPTs are derived primarily by fitting empirical equations to the concentration breakthrough curves generated during the withdrawal phase. The first PPT (PPT1) involved the injection of a suite of non-reactive tracers including potassium bromide (KBr), pentafluorobenzoic acid (2,3,4,5,6-PFBA), and Sulphorhodamine B (SRB). PPT1 piloted the PPT method and will be used to estimate advective and diffusive transport properties, providing insight into the back-diffusion of toluene. The second PPT (PPT2) involved the injection of a reactive tracer, sodium persulphate (Na2S2O8), and a non-reactive tracer, lithium bromide (LiBr). Informed by PPT1, the aim of PPT2 was to evaluate the fate and distribution of the oxidant, persulphate, and its potential oxidation of toluene. A suite of 28 constructed dipole resistivity probes were deployed during both PPTs to track movement of tracer plumes, data not typically collected during PPTs.
Results/Lessons Learned
Complementing this study are lab microcosm experiments studying the dual-action remediation potential of persulphate and dipole tracer tests to study site fracture connectivity. Synthesizing the results of these three studies will provide a comprehensive conceptual model with which the success of future remediation activities may be evaluated. Field collected data include general chemistry and tracer breakthrough curves, conductivity, temperature, and pressure transducer data, and DRP resistivity data. Lab experiments will also be conducted to study tracer sorption and reactivity.