The definition of a predominant geogenic source of ions and its respective background ranges are powerful tools for detecting contaminations coming from the unconventional gas industry at earlier stages. Reference data to compare pre- vs post-drilling may enforce the planning, monitoring, and management in favor of unconventional production sustainability in the long-term. In 2022, the Brazilian Government launched a pilot project named "Transparent well" for public consultation, intending to foment its incipient unconventional hydrocarbon industry. In this context, this study presents preliminary results to compound an environmental baseline to structure reference data about water resources in a pre-drilling scenario. Then, a characterization of surface water and a geochemical background proposal of Indaiá and Borrachudo basins, two potential tight gas plays inside the São Francisco Basin, was performed to understand hydrochemical processes and susceptibilities previously the unconventional industry development. Two sampling campaigns collecting 13 samples each were made to characterize the study area in two different scenarios. Chemical analysis was performed using an Inductively Coupled Plasma - Mass Spectrometry. To settle a hydrochemical background and its respective environmental thresholds, the ±2 Median Absolute Deviation method was applied. Piper and Chadha's diagrams were used to define water-type and the major cations/anions dynamics in the environment. Piper diagram shows three distinct predominant water-type in the study area, Group 1 (Ca-Mg-HCO3-type); Group 2(Na-K-Ca-HCO3-type); and Group 3 (Ca-Na-HCO3-type). Groups 1 and 2 reflect a geological control in the hydrochemistry due to an interaction between the surface water and interlayered strata of Carbonates and Siltstones from the Bambuí Group whereas Group 3 suggest a seasonal influences in water chemistry. Background values and Upper/Lower environmental thresholds for nineteen elements in two different seasons were also proposed for the study area. Reference data to compare pre- vs post-drilling may enforce the planning, monitoring, and management in favor of unconventional production sustainability in the long-term. Environmental baselines focused on water resources may be a key to the responsible development of the incipient unconventional gas industry in Brazil.

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In the middle of the São Francisco basin, the sub-basins of Indaiá and Borrachudo river highlight the considerable potential for unconventional hydrocarbon exploration. The present study seeks to evaluate the groundwater resource's intrinsic and specific vulnerability in the Indaiá and Borrachudo basins as an environmental baseline study for further development in the shale gas industry. The overlay and index GOD method was applied to assess the aquifer's intrinsic susceptibility regarding the Groundwater confinement (G), the Overlying Strata (O), and the Depth of the groundwater table (D). The specific vulnerability assessment considered the intrinsic vulnerability and the land cover/use of the area, concerning the diversity and the toxicity of pollutants inherent in each anthropogenic activity. The results indicate that 52.69%, 25.12%, and 17.57% of the aquifer area have Medium, High, and Extreme intrinsic vulnerability, respectively. The specific vulnerability assessment suggests an overall vulnerability reduction due to the low anthropogenic stress on the basin. Three gas research wells overlay moderate intrinsic vulnerability. One well is above the high vulnerability area, which concerns how the unconventional industry could develop regarding groundwater susceptibility in this ecosystem.

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