title: Geochemical controls of high fluoride groundwater in Umarkot Sub-District, Thar Desert, Pakistan.
abstract: Groundwater samples (n=152) were collected in the Thar Desert of the Umarkot Sub-District, Pakistan to evaluate the geochemical controls on the occurrence of high fluoride (F(-)) levels within the study area. Fluoride concentrations range from 0.06 to 44.4 mg/L, with mean and median values of 5.22 and 4.09 mg/L, respectively; and roughly 84% of the samples contain fluoride concentrations that exceed the 1.5mg/L drinking water standard set by WHO. The overall groundwater quality reflects the influences of silicate mineral weathering and evaporation. Fluoride originates from the weathering of minerals derived from Type-A granite and possibly anion exchange (OH(-) for F(-)) on clays and weathered micas under high pH conditions. High fluoride levels are associated with Na-HCO3 type water produced by calcite precipitation and/or base ion exchange. Depleted calcium levels in groundwater allow higher fluoride concentrations to occur before the solubility limit for fluoride is reached.
authors.affiliation: Applied Chemistry Research Centre, PCSIR Laboratories Complex, Karachi 75280, Pakistan. Electronic address: tahirrafique92@yahoo.com.
authors.forename: Tahir
authors.initials: T
authors.lastname: Rafique
authors.affiliation: Department of Geology, University of Karachi, Karachi 75270, Pakistan.
authors.forename: Shahid
authors.initials: S
authors.lastname: Naseem
authors.affiliation: Department of Geography/Geology, University of Wisconsin-Stevens Point, USA.
authors.forename: David
authors.initials: D
authors.lastname: Ozsvath
authors.affiliation: Geophysical Division, Geological Survey of Pakistan, Karachi, Pakistan.
authors.forename: Riaz
authors.initials: R
authors.lastname: Hussain
authors.affiliation: HEJ Research Institute of Chemistry, ICCBS, University of Karachi, Karachi, Pakistan.
authors.forename: Muhammad Iqbal
authors.initials: MI
authors.lastname: Bhanger
authors.affiliation: Applied Chemistry Research Centre, PCSIR Laboratories Complex, Karachi 75280, Pakistan.
authors.forename: Tanzil Haider
authors.initials: TH
authors.lastname: Usmani
keywords: Base ion exchange; Calcite precipitation; Evaporation; Fluoride ions; Mineral weathering; Thar Desert; Umarkot