Characterization of groundwater nitrate exposure using Monte Carlo and Sobol sensitivity approaches in the diverse aquifer systems of an agricultural semiarid region of Lower Ganga Basin, India.

Prevalence of nitrate in different aquifer systems is a growing environmental and public health concern. Efforts were made for the first-time to achieve a higher accuracy in health risks characterization associated with the nitrate in groundwater of the diverse aquifer systems on the residents of a semi-arid rural tract of Lower Ganga Basin using Monte Carlo Simulations and Sobol Sensitivity analyses. The nitrate levels in groundwater varied between 0 and 508.3 mg/L with a mean of 19.79 ± 32.78 mg/L and 0-435.0 mg/L with a mean of 24.44 ± 35.15 mg/L during the pre-monsoon and post-monsoon periods, respectively. About 847.12-1000.25 km area of the survey area (total area 4545 km) exhibited nitrate concentrations (C) > the pre-intervention limits (45-50 mg/L). Minor populations, especially the infants from the granite gneiss, Rajmahal traps, laterite, recent alluvial and old alluvial aquifer zones under the Central Tendency Exposure (CTE) condition and all the aquifer zones (including the Gondwana supergroup aquifer zone) under Reasonable Maximum Exposure (RME) scenarios, were characterized as being at high risks of methemoglobinemia, primarily due to ingestion of untreated nitrate contaminated groundwater. Residents of the alluvial aquifer zones of the study area were found to the most vulnerable to the groundwater nitrate toxicity through oral and dermal exposures. The study validated the prediction accuracies of different interpolation methods including the Spline, Kriging, polynomial and Inverse Distance Weighted and revealed that Kriging predicted the Spatio-seasonal variations of groundwater nitrate of the district more accurately. Sobol Sensitivity analysis revealed C and the interaction effects of C and groundwater Ingestion Rate (IR), and C and Fraction of skin area contacted with groundwater (F) as the influential parameters for oral and dermal health risks exposure models. Therefore, the study recommends to residents of the study area to consume treated groundwater to mitigate nitrate related health morbidities.