How useful are isoscapes in fractured rock and what can they tell us about governing hydrogeologic parameters?

Groundwater isoscapes are increasingly applied to a variety of hydrologic investigationsincluding recharge, paleoclimates and catchment processes. Generating stable waterisoscapes from a single sample taken from open-hole wells, as is typically the case, posesissues for fractured rock environments that are not present in porous media. To explore thevalue of isoscapes produced in fractured rock using open boreholes, a study was undertakenacross Central and Eastern Ontario, Canada where both sedimentary and crystalline bedrockare highly prevalent as aquifers. The study also aims to reveal the hydrologic factorsdominating the stable isotopic value of shallow groundwater in fractured rock. Toexplore this, a network of nine multi-level wells (25 intervals total) across threefield sites were sampled monthly from February 2017 to November 2018 for δ2Hand δ18O. Two rounds of tritium sampling were also conducted during that timeperiod. Multi-level wells were designed based on the results of 2-m interval constanthead tests, which enabled the identification of high transmissivity zones (2–3 perwell). To contrast these results, a larger, regional groundwater geochemistry datasetproduced by the Ontario Geologic Survey (OGS) was utilized. The OGS dataset,consisting of domestic and monitoring open-hole wells, was filtered to includeonly drilled bedrock wells in the same geologic formations as the multi-level wells(n=162). Spearman correlations between δ2H and various hydrologic parameters wereanalysed for both data sets to identify if stronger correlations existed from multi-levelwells than open-hole wells. Parameters included drift thickness, bedrock geology,surficial geology, vadose zone thickness, well depth, well transmissivity, tritiumvalue and average annual precipitation among others. For the temporal multi-leveldata, δ2H was averaged seasonally to explore whether a seasonal component ofsampling drives the strength of the correlations observed. This could be used tooptimize how isoscapes are generated in fractured rock. From the results of thecorrelation analysis, parameters that highly correlated with δ2H were used in amultiple regression analysis to identify the dominant parameters governing stableisotopes in shallow fractured rock. Results of these analyses will be presented. [ABSTRACT FROM AUTHOR]