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The rural population in the Dry Zone of Sri Lanka is largely affected by Chronic Kidney Disease of Unknown etiology (CKDu). According to the multidisciplinary research carried out so far, quality of groundwater is considered one of the possible causative factors for CKDu. Therefore, assessment of the quality of groundwater being used for drinking and its evolution mechanism is the key to identifying the linkage between CKDu and drinking water. This study aimed to perform a detailed investigation on groundwater sources using isotopic, chemical, and hydrogeological methods in the CKDu-endemic (site A) and the control area (sedimentary formation—site B) in the Malwathu Oya basin and the control areas in the Malala Oya basin (site C) selected for a systematic comparison. Our investigation shows that elevated levels of TDS, magnesium, and fluoride in the shallow groundwater affected by climatic, geochemical, and hydrogeological processes may contribute to the CKDu in the Dry Zone of Sri Lanka. All the groundwater samples analysed have exceeded the hardness threshold. Prominent Mg hardness proportion together with excess F− in the CKDu endemic area may produce nephrotoxic MgF2 complexes that may trigger renal damage. In contrast, NaF complexes in the CKDu control area leads to reduction of F− toxicity in the human body. Elevated F− and Mg2+ are found in site A, low F− and high Mg2+ in site B, and either combinations of low F− and low Mg2+, high F− and low Mg2+, or low F− with high Mg2+ in site C. TDS, hardness, Mg2+, Na+, and F− are formed with different mechanisms in the three selected areas. The primary process that regulates the evolution of groundwater types and contents in sites A and C is the weathering of silicates. Similarly, in site A, carbonate dissolution and reverse ion exchange are quite strong. Cation exchange and evaporite dissolution are more pronounced in site C. Shallow groundwaters are evapo-concentrated, hence their quality deteriorates more significantly than the deep groundwater in the CKDu endemic area. Dilution decreases the ion content in site A while evaporite dissolution increases it in site C after the rainy season. Evaporation and seawater mixing affect the quality of groundwater in site B. It is also found that a statistically significant difference exists in the F−/Na+, F−/Mg2+, and F−/Ca2+ between the endemic and control areas. Intensive rock weathering combined with desorption has added excess F− to the groundwater in site A, while cation exchange and fluorite dissolution are contributing factors in site C. [ABSTRACT FROM AUTHOR]

• A framework to determine sensitive monitoring and diagnostic indicators. • Authentic formation water originated from seawater and mixed with meteoric water. • Cl, Br, Na, Li, Ba, 87Sr/87Sr, and δ11B are the most sensitive indicators. • About 0.05% of formation water in shallow groundwater can be identified. • A groundwater contamination case has confirmed the effectiveness of the method. Potential environmental impacts on shallow groundwater from shale gas development facilitated by horizontal drilling and hydraulic fracturing is a widely debated "hot topic". In terms of potential aqueous phase contamination (including flowback fluids and produced water), there is a large gap in knowledge of the indicators for routine monitoring and contamination tracing, which should be considered critical and should be prioritized for analysis. Since formation water from shale formations is the main source of flowback fluids and produced water, and there are significant differences in some specific inorganic geochemical and isotopic compositions between shallow groundwater and formation water, this study has provided a framework to determine sensitive monitoring and diagnostic indicators for tracing potential groundwater contamination from produced water using end-member analysis. The results from a case study of the Fuling Gasfield, Sichuan Basin, SW China as the first and largest commercial shale gas development site in China, shows that produced authentic formation water with similar Br/Cl and Na/Cl ratio with seawater and low δ2H and δ18O values compared to the evaporated seawater might originate from evaporated seawater modified by water-rock interactions and be mixed with fresh meteoric water. The inorganic geochemical and isotopic indicators, such as Ba, Li, Na, Cl, Br, 87Sr/86Sr (as ε Sr sw) and δ11B are sensitive to the detection of contamination of fresh shallow groundwater by produced formation water, even in very small fractions (0.05%). Meanwhile, we present a groundwater contamination case related to shale gas development in the Fuling Gasfield. The robust conservative geochemical (Cl and Br) and isotopic (δ11B and ε Sr sw) mass balances show that 0.2–0.9% of formation water entered the shallow groundwater causing contamination. The case has also confirmed the effectiveness of those indicators. Findings from this study may help to improve groundwater monitoring and environmental regulations in countries with shale gas exploration and development. [ABSTRACT FROM AUTHOR]