Groundwater in arid and semi-arid regions is increasingly threatened by human activi-ties. However, local baseline concentrations remain poorly defined, necessitating region-specific evaluation. This study responds to this need by quantifying the NBLs and TVs of main physicochemical variables and selected metal(oid)s, including Zn, Hg, As, Cr, and Pb, in rural and urban drinking water resources in Isfahan Province, Iran. The calculations were performed using Iterative 2 sigma-outlier test, CDF, and IGT. Before NBL estimation, dataset pre-selection was implemented using NO3 due to its stability and its sensitivity to pollutions. Various methods-such as the Q-Q plot, BW, BWI, GT, MSD, and MAD-were employed to determine the ABL of NO3 (44.65 mg/L). Considering the capability of individual techniques to approach a normal distribution, the IGT and 95th percentile were selected as the final approaches for determining NBLs. NBLs calculated using IGT showed a very strong correlation (R2 = 0.997) with those obtained from Iterative 2 sigma and CDF, but NBLs determined by IGT were lower than values obtained from other techniques and the WHO guideline levels. Establishing these lower NBLs and TVs can help protect water quality by limiting the contamination potential posed by local communities. We recommend integrating geochemical and land-use datasets to validate thresholds, improve source discrimination, and support groundwater protection strategies.