Isotope-based investigation on the groundwater flow and recharge mechanism in a hard-rock aquifer system: The case of Ranchi urban area, India Investigation sur le flux de nappe et sur le mécanisme de recharge d'un système aquifère du socle basée sur les isotopes: Cas de la zone urbaine de Ranchi, Inde

摘要

Ranchi urban area (257 km~2) depends on aquifers for 30 of its total drinking-water supply of 17 million m~3year~(-1). Local hydrostratigraphy is represented by a heterogeneous, weathered and fractured aquifer system, typical of the Precambrian suite of rocks in the Indian subcontinent. Intensive development of the fractured aquifers, up to 200 m below ground, has lowered the hydraulic head and resulted in dwindling yields from fractures during the summer. To understand the groundwater flow regime and aquifer recharge mechanism, the present study examines δ~(18)O and δD variation in aquifer-specific samples along with water levels, yield of the fractures, EC and Cl-. Three types of groundwater have been identified based on isotopic composition and d-excess values, each representing different recharge source-water and pathways. The major source of recharge for the aquifers is infiltration from rainfall. Two large reservoirs and an excavated lake within the study area contribute to the recharge process but insignificantly. Isotopic compositions and the relatively low EC and low Cl~- concentrations of high-yielding bore wells in some places indicate the presence of fast-conducting fracture zones receiving copious recharge from rainfall. Such fractures can be developed further through bore wells for drinking supply with due provision for artificial recharge.