Radionuclides baseline assessment of water resources, sediments, source rocks, and NORM flux simulation prior to shale gas development in Brazil

Hydraulic fracturing (fracking) is a controversial yet profitable technique used to stimulate ultra-low permeability reservoirs in unconventional hydrocarbon production. A significant environmental concern associated with fracking is its potential to mobilize radionuclides, leading to contamination aquifers, sediments, and surface waters. Despite this risk, the potential for radionuclide contamination in watersheds due to unconventional oil and gas operations is frequently underestimated, resulting in hazardous consequences globally, including health risk and ecosystem damage. One effective approach to mitigating such risks is establishment of environmental baselines prior to shale gas development. This study aims to establish a radionuclide baseline for surface water, groundwater, sediments, and source rocks in the São Francisco Basin (Brazil), prior to the introduction of unconventional gas production. The study also simulates radionuclide flux in produced water, a byproduct of hydraulic fracturing, and assesses annual dose of radioactivity exposure using Monte Carlo simulations. Radionuclides U-238, Th-232, K-40, Pb-210, Ra-226, and Ra-228 were analyzed using Neutron Activation Analysis (NAA) and gamma spectrometry. Environmental thresholds and geochemical background were calculated using 2mMAD (Median Absolute Deviation) whereas a multivariate statistical analysis was performed through Principal Components Analysis using Compositional Data transformations. A Monte Carlo simulation, based on 10,000 iterations, was used to compare potential NORM flux from fracking with natural background levels and those in other shale plays worldwide. Current radionuclide concentrations in water resources are low, however the NORM flux and Annual Exposure simulations indicate a potential radiological risk to water security in the event of unconventional gas production. This research provide novel insights into the NORM-related risks of fracking in the São Francisco Basin, offering a foundation for further assessments before shale gas development, which can be also used in unconventional reservoirs around the world.