Boreholes drilled through contaminated zones in fractured rock create the potential for vertical movement of contaminated ground water between fractures. The usual assumption is that purging eliminates cross contamination; however, the results of a field study conducted in a trichloroethylene (TCE) plume in fractured sandstone with a mean matrix porosity of 13% demonstrates that matrix-diffusion effects can be strong and persistent. A deep borehole was drilled to 110 m below ground surface (mbgs) near a shallow bedrock well containing high TCE concentrations. The borehole was cored continuously to collect closely spaced samples of rock for analysis of TCE concentrations. Geophysical logging and flowmetering were conducted in the open borehole, and a removable multilevel monitoring system was installed to provide hydraulic-head and ground water samples from discrete fracture zones. The borehole was later reamed to complete a well screened from 89 to 100 mbgs; persistent TCE concentrations at this depth ranged from 2100 to 33,000 microg/L. Rock-core analyses, combined with the other types of borehole information, show that nearly all of this deep contamination was due to the lingering effects of the downward flow of dissolved TCE from shallower depths during the few days of open-hole conditions that existed prior to installation of the multilevel system. This study demonstrates that transfer of contaminant mass to the matrix by diffusion can cause severe cross contamination effects in sedimentary rocks, but these effects generally are not identified from information normally obtained in fractured-rock investigations, resulting in potential misinterpretation of site conditions.

<p>Acidified rivers may have increased CO<sub>2</sub> emissions because their low pH transforms inorganic carbon in the form of bicarbonate anions to CO<sub>2</sub>, which can evade to the atmosphere, thus interrupting the delivery inorganic carbon to the oceans, a key flux in the long-term carbonate silicate cycle. Enhanced weathering (EW) is a carbon dioxide removal (CDR) strategy aiming to increase drawdown of atmospheric CO<sub>2</sub> through accelerated carbonation weathering of crushed minerals with targeted carbonate sequestration in oceanic stores. To date, EW research has been focused on terrestrial application of crushed minerals, and the CDR capability of enhancing weathering via addition of crushed minerals to rivers from lime dosers is essentially unexplored. Lime dosers have been used for decades to directly deposit crushed carbonate rock to rivers as a function of river flow in Norway and Nova Scotia, Canada, yet their potential as a CDR tool has yet to be verified in the field. In this study, we adapt CO<sub>2</sub> flux sensors (eosFD) designed for soils to be deployed in rivers. We conducted field trials on the Killag River, Nova Scotia, upstream and downstream of a lime doser over a period of six weeks in the autumn of 2020. Preliminary analysis shows elevated CO<sub>2</sub> evasion rates upstream of the lime doser and decreased evasion rates downstream. Aside from flood waves, CO<sub>2</sub> evasion at the downstream (treated) site is reduced to almost zero for extended periods of time. Next steps are to identify whether the reduced CO<sub>2</sub> evasion is due to CO<sub>2</sub> drawdown via increased carbonation weathering of the crushed dolomite or through reduced CO<sub>2</sub> evasion due to increased pH, or from a combination of the two processes. The results of this study may have implications for carbon credit programs for acidification mitigation and may encourage more widespread use of enhanced weathering as a CDR tool in rivers.</p>

As cities grow, surrounding rural communities often experience development stresses such as increased demand on local aquifers. Rural aquifers can have different threats than city centres, such as nitrates from agriculture or pathogens related to septic systems that may not have historically been a dominant concern, but become so under the increased water demands. Case in point is the growing community of Greely, near Ottawa, Ontario which is surrounded by agricultural lands, homes have individual septic systems, and a shallow bedrock aquifer provides the sole water supply from a communal well. The City of Ottawa has engaged the project team to examine the deeper Nepean sandstone as an alternative water supply. An initial borehole was continuously cored through the Paleozoic bedrock sequence into the Precambrian close to the existing pumping well. The detailed Discrete Fracture Network - Matrix (DFN-M) approach was applied including rock core logging, chemical analysis, geophysical logging, hydrogeologic and hydro-geophysical testing, with cross-hole testing pending. We present the study approach, preliminary results and future plans. In addition to dealing with Greely's challenges, the multiple high-resolution data sets provide insight into this complex geologic/hydrogeologic setting and an initial assessment of the sandstone's viability as a water supply elsewhere.

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