Assessing the Spatial Distribution and Quality of Groundwater Resources in the Dehradun Region of India

This study examines the spatiotemporal variability in groundwater quality at five monitoring locations in Dehradun, Uttarakhand, India, from 2021 to 2025, based on 40 site-period observations covering 33 physicochemical, ionic, trace metal and pesticide parameters. Most parameters complied with BIS IS:10500 (2012) standards, with total dissolved solids and total hardness well within limits. Sulphate peaked at 260.5 mg/L at a landfill site in 2023, and iron reached 13.2 mg/L at an industrial site, while arsenic, mercury, lead, cadmium and four organochlorine pesticides stayed below detection. Trend analysis showed significant declines in TDS, sulphate and sodium and a significant rise in pH, indicating improving water quality since 2021, although a sensitivity check restricted to 2022–2024 showed the pH and sulphate trends were driven mainly by recovery from the 2021 baseline rather than a continuing multi-year trend. A major-ion Water Quality Index classified groundwater as excellent to good at all sites, while a Heavy Metal Pollution Index, driven mainly by chromium in 2021 and nickel in post-monsoon 2022 rather than by iron, exceeded the critical value of 100 in 11 of 40 site-period observations. Piper trilinear analysis identified a predominantly Ca-HCO3 hydrochemical facies at the individual-sample level, with a minority of samples showing mixed or sulphate-leaning compositions. A human health risk assessment based on copper, zinc, nickel and chromium showed hazard indices above the safety threshold at all sites in 2021, largely due to a conservative assumption that all measured chromium was present as the more toxic Cr(VI) form, with substantially lower hazard-index values from 2023 onward under the primary zero-substitution convention for below-detection-limit metals. A sensitivity check substituting half of the laboratory’s documented detection limits for non-detect readings in these metals placed all 40 site-periods in the critical HPI band and raised the child hazard index above 1 in every 2022–2025 round outside post-monsoon 2022, so the post-2021 improvement is assumption-dependent. Conventional irrigation indices indicated low sodium and salinity hazards, although the very low electrical conductivity implies potential infiltration constraints under FAO EC-SAR guidance.