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ScopeChemistry and use of persulfate for in situ chemical oxidation of subsurface contaminants, including free radical and other reaction mechanisms, catalysts, subsurface transport, and contaminant treatability. Key ConceptsPersulfate reaction chemistry is complex. Persulfate can react through direct electron transfer or free radical reactions. Electron transfer reactions are relatively slow and selective. Upon activation, the free radicals generated have nonspecific reactivity that allows for the degradation of a wide range of organic contaminants. The radicals that are presently understood to play a major role in reactions are the sulfate radical and the hydroxyl radical. However, there is emerging evidence that the superoxide anion and the perhydroxyl radical also may be important. Persulfate activation may be achieved by heat, chelated or non-chelated transition metals (especially iron), hydrogen peroxide, and alkaline pH conditions. The efficiency, effectiveness, and reaction products may vary between contaminants, activation methods, and the porous media to be treated. Carbonate, bicarbonate, or chloride ions can act as free radical scavengers and reduce reaction efficiency and effectiveness. The kinetics of reaction between persulfate and target compounds is complex. To simplify, pseudo first-order kinetics are often assumed. However, these usually require laboratory estimation because extrapolating between systems is difficult. The interaction of aquifer solids and persulfate is not well understood; persulfate does react with aquifer solids resulting in oxidant consumption, but the rate and magnitude of this process are not well characterized. Persistence varies from days to months, depending on conditions. The impact of persulfate on metal mobility is not well understood. Conceivably, persulfate could impact metal concentrations in groundwater through modification of pH, oxidation of metals, injection of activation amendments, and other mechanisms.

At a low permeability clay till site contaminated with chlorinated ethenes (Gl. Kongevej, Denmark), enhanced reductive dechlorination (ERD) was applied by direct push injection of molasses and dechlorinating bacteria. The performance was investigated by long‐term groundwater monitoring, and after 4 years of remediation, the development of degradation in the clay till matrix was investigated by high‐resolution subsampling of intact cores. The formation of degradation products, the presence of specific degraders Dehalococcoides spp. with the vinyl chloride (VC) reductase gene vcrA, and the isotope fractionation of trichloroethene, cis‐dichloroethene (cis‐DCE), and VC showed that degradation of chlorinated ethenes occurred in the clay till matrix as well as in sand lenses, sand stringers, and fractures. Bioactive sections of up to 1.8 m had developed in the clay till matrix, but sections, where degradation was restricted to narrow zones around sand lenses and stringers, were also observed. After 4 years of remediation, an average mass reduction of 24% was estimated. Comparison of the results with model simulation scenarios indicate that a mass reduction of 85% can be obtained within approximately 50 years without further increase in the narrow reaction zones if no donor limitations occur at the site. Long‐term monitoring of the concentration of chlorinated ethenes in the underlying chalk aquifer revealed that the aquifer was affected by the more mobile degradation products cis‐DCE and VC generated during the remediation by ERD.

Heterogeneous glacial sediments, such as clayey till, dominate large parts of the near-surface geology of the Northern Hemisphere (Houmark-Nielsen, 2010). Sand layers and lenses control water and contaminant flow pathways in the otherwise low-permeable clay matrix. Delineation and characterization of these sand structures and bodies are necessary to determine the timing, the amount and the quality of the water percolating through these sediments (e.g., Gravesen et al., 2014).A method for mapping these sand occurrences is by using crosshole ground penetrating radar (GPR). Crosshole GPR is a fast, minimally invasive, electromagnetic (EM) method, which is based on transmission of radio frequency EM waves, traveling from a transmitter, located in one borehole, to a receiver located in a neighboring borehole. The recorded traveltime and amplitude of the wave provide information on subsurface dielectric properties, which can be linked to parameters important for flow and transport processes, such as volumetric