Elucidating various geochemical mechanisms drive fluoride contamination in unconfined aquifers along the major rivers in Sindh and Punjab, Pakistan.

The present study aims to investigate the spatial distribution and associated various geochemical mechanisms responsible for fluoride (F⁻) contamination in groundwater of unconfined aquifer system along major rivers in Sindh and Punjab, Pakistan. The concentration of F⁻ in groundwater samples ranged from 0.1 to 3.9 mg/L (mean = 1.0 mg/L) in Sindh and 0.1-10.3 mg/L (mean = 1.0 mg/L) in Punjab, respectively with 28.9% and 26.6% of samples exhibited F⁻ contamination beyond WHO permissible limit value (1.5 mg/L). The geochemical processes regulated F⁻ concentration in unconfined aquifer mainly in Sindh and Punjab were categorized as follows: 1) minerals weathering that observed as the key process to control groundwater chemistry in the study areas, 2) the strong correlation between F⁻ and alkaline pH, which provided favorable environmental conditions to promote F⁻ leaching through desperation or by ion exchange process, 3) the 72.6% of samples from Sindh and Punjab were dominated by Na⁺- Cl⁻ type of water, confirmed that the halite dissolution process was the major contributor for F⁻ enrichment in groundwater, 4) dolomite dissolution was main process frequently observed in Sindh, compared with Punjab, 5) the arid climatic conditions promote evaporation process or dissolution of evaporites or both were contributing to the formation of saline groundwater in the study area, 6) the positive correlation observed between elevated F⁻ and fluorite also suggested that the fluorite dissolution also played significant role for leaching of F⁻ in groundwater from sediments, and 7) calcite controlled Ca2⁺ level and enhanced the dissolution of F-bearing minerals and drive F⁻ concentration in groundwater. In a nut shell, this study revealed the worst scenarios of F⁻ contamination via various possible geochemical mechanisms in groundwater along major rivers in Sindh and Punjab, Pakistan, which need immediate attention of regulatory authorities to avoid future hazardous implications.

Ali W ，Aslam MW ，Junaid M ，Ali K ，Guo Y ，Rasool A ，Zhang H ... -  《-》

Geochemical factors controlling the occurrence of high-fluoride groundwater in the western region of the Ordos basin, northwestern China.

Hydrogeochemistry and isotope hydrology were carried out to investigate the spatial distribution of fluoride (F-) and the mechanisms responsible for its enrichment in the western region of the Ordos basin, northwestern China. Sixty-two groundwater samples from the unconfined aquifer and fifty-six from confined aquifer were collected during the pre-monsoon (June 2016). Over 77% of groundwater samples from the unconfined aquifer (F- concentration up to 13.30 mg/L) and approximately 66% from confined aquifer (with a maximum F- concentration of 3.90 mg/L) exhibit F- concentrations higher than the Chinese safe drinking limit (1.0 mg/L). High-F- groundwater presents a distinctive hydrochemical characteristic: a high pH value and HCO3- concentration with Ca-poor and Na-rich. Mineral dissolution (e.g., feldspar, calcite, dolomite, fluorite), cation exchange and evaporation in the aquifers predominate the formation of groundwater chemistry, which are also important for F- enrichment in groundwater. Mixing with unconfined groundwater is a significant mechanism resulting in the occurrence of high-F- groundwater in confined aquifer. These findings indicate that physicochemical processes play crucial roles in driving F- enrichment and that may be useful for studying F- occurrence in groundwater in arid and semi-arid areas.

Su H ，Wang J ，Liu J 《-》

High levels of fluoride contamination in groundwater of the semi-arid alluvial aquifers, Pakistan: evaluating the recharge sources and geochemical identification via stable isotopes and other major elemental data.

Younas A ，Mushtaq N ，Khattak JA ，Javed T ，Rehman HU ，Farooqi A ... -  《-》

Diverse mechanisms drive fluoride enrichment in groundwater in two neighboring sites in northern China.

Excessive amounts of fluoride in drinking groundwater are harmful to human health, but the mechanisms responsible for fluoride enrichment in groundwater are not fully understood. Samples from two neighboring areas with endemic fluorosis were collected to test the hypothesis that there are distinctly different mechanisms responsible for the enrichment of fluoride in these groundwater. Hydrochemistry, stable isotopes and geochemical simulation were conducted together to investigate the fluoride spatial distribution and the diversity of responsible mechanisms. Our results showed that the spatial distributions of fluoride are different: I) high [F] in fresh shallow groundwater (SGQJ) and II) medium [F] in fresh to brackish deep groundwater (DGQJ) in the Qiji area; and III) medium [F] in brackish shallow groundwater (SGYH) and IV) low [F] in fresh deep groundwater (DGYH) in the Yanhu area. We also found that the fluoride concentration in groundwater is primarily controlled by the dissolution equilibrium of fluorite, as suggested by the correlation between [F] and [Ca]. However, there are other significant mechanisms: 1) for SGQJ, fluoride-bearing minerals (such as fluorite) dissolution, along with moderate evaporation, cation exchange and the more alkaline conditions are the driving factors; 2) for SGYH, the contributing factors are strong evaporation, the salt effect, dissolution of evaporites, gypsum and dolomite, bicarbonate-fluoride competition and anthropogenic activity; 3) for DGQJ, cation exchange, alkaline conditions and competitive adsorption are major factors; and 4) dolomite dissolution promotes the [F] increase in DGYH. Our findings suggest that the hydrogeochemical conditions play key roles in the enrichment of fluoride and that caution should be taken in the future when evaluating fluoride occurrence in groundwater, even in nearby areas.

Li D ，Gao X ，Wang Y ，Luo W ... -  《-》

Fluoride prevalence in groundwater around a fluorite mining area in the flood plain of the River Swat, Pakistan.

This study investigated the fluoride (F-) concentrations and physicochemical parameters of the groundwater in a fluorite mining area of the flood plain region of the River Swat, with particular emphasis on the fate and distribution of F- and the hydrogeochemistry. To better understand the groundwater hydrochemical profile and F- enrichment, groundwater samples (n=53) were collected from shallow (24-40m), mid-depth (48-65m) and deep (85-120m) aquifers, and then analysed using an ion-selective electrode. The lowest F- concentration (0.7mg/L) was recorded in the deep-aquifer groundwater, while the highest (6.4mg/L) was recorded in shallow groundwater. Most groundwater samples (62.2%) exceeded the guideline (1.5mg/L) set by the World Health Organization (WHO); while for individual sources, 73% of shallow-groundwater samples (F- concentration up to 6.4mg/L), 42% of mid-depth-groundwater samples, and 17% of deep-groundwater samples had F- concentrations that exceeded this permissible limit. Assessment of the overall quality of the groundwater revealed influences of the weathering of granite and gneisses rocks, along with silicate minerals and ion exchange processes. Hydrogeochemical analysis of the groundwater showed that Na+ is the dominant cation and HCO3- the major anion. The anionic and cationic concentrations across the entire study area increased in the following order: HCO3>SO4>Cl>NO3>F>PO4 and Na>Ca>Mg>K, respectively. Relatively higher F- toxicity levels were associated with the NaHCO3 water type, and the chemical facies were found to change from the CaHCO3 to (NaHCO3) type in calcium-poor aquifers. Thermodynamic considerations of saturation indices indicated that fluorite minerals play a vital role in the prevalence of fluorosis, while under-saturation revealed that - besides fluorite minerals - other F- minerals that are also present in the region further increase the F- concentrations in the groundwater. Finally, a health risk assessment via Dean's classification method identified that the groundwater with relatively higher F- concentrations is unfit for drinking purposes.

Rashid A ，Guan DX ，Farooqi A ，Khan S ，Zahir S ，Jehan S ，Khattak SA ，Khan MS ，Khan R ... -  《-》