Exploration of unconventional natural gas reservoirs such as impermeable shale basins through the use ofhorizontal drilling and hydraulic fracturing has changed the energy landscape in the USA providing a vastnew energy source. The accelerated production of natural gas has triggered a debate concerning thesafety and possible environmental impacts of these operations. This study investigates one of the criticalaspects of the environmental effects; the possible degradation of water quality in shallow aquifers overlyingproducing shale formations. The geochemistry of domestic groundwater wells was investigated inaquifers overlying the Fayetteville Shale in north-central Arkansas, where approximately 4000 wells havebeen drilled since 2004 to extract unconventional natural gas. Monitoring was performed on 127 drinkingwater wells and the geochemistry of major ions, trace metals, CH4 gas content and its C isotopes(d13CCH4), and select isotope tracers (d11B, 87Sr/86Sr, d2H, d18O, d13CDIC) compared to the composition offlowback-water samples directly from Fayetteville Shale gas wells. Dissolved CH4 was detected in 63%of the drinking-water wells (32 of 51 samples), but only six wells exceeded concentrations of 0.5 mgCH4/L. The d13CCH4 of dissolved CH4 ranged from _42.3‰ to _74.7‰, with the most negative values characteristicof a biogenic source also associated with the highest observed CH4 concentrations, with a possibleminor contribution of trace amounts of thermogenic CH4. The majority of these values are distinctfrom the reported thermogenic composition of the Fayetteville Shale gas (d13CCH4 = _35.4‰ to _41.9‰).Based on major element chemistry, four shallow groundwater types were identified: (1) low (<100 mg/L)total dissolved solids (TDS), (2) TDS > 100 mg/L and Ca–HCO3 dominated, (3) TDS > 100 mg/L and Na–HCO3 dominated, and (4) slightly saline groundwater with TDS > 100 mg/L and Cl > 20 mg/L with elevatedBr/Cl ratios (>0.001). The Sr (87Sr/86Sr = 0.7097–0.7166), C (d13CDIC = _21.3‰ to _4.7‰), and B(d11B = 3.9–32.9‰) isotopes clearly reflect water–rock interactions within the aquifer rocks, while thestable O and H isotopic composition mimics the local meteoric water composition. Overall, there wasa geochemical gradient from low-mineralized recharge water to more evolved Ca–HCO3, and higher-mineralizedNa–HCO3 composition generated by a combination of carbonate dissolution, silicate weathering,and reverse base-exchange reactions. The chemical and isotopic compositions of the bulk shallowgroundwater samples were distinct from the Na–Cl type Fayetteville flowback/produced waters (TDS_10,000–20,000 mg/L). Yet, the high Br/Cl variations in a small subset of saline shallow groundwater suggestthat they were derived from dilution of saline water similar to the brine in the Fayetteville Shale.Nonetheless, no spatial relationship was found between CH4 and salinity occurrences in shallow drinkingwater wells with proximity to shale-gas drilling sites. The integration of multiple geochemical and isotopicproxies shows no direct evidence of contamination in shallow drinking-water aquifers associatedwith natural gas extraction from the Fayetteville Shale.