(639a) Occurrence of Fluoride in Alluvial Shallow Aquifers of Semiarid Region of Punjab, North-West India

Fluoride has considerable physiological significance for man and animal. Fluoride (F^-) in drinking water to 1.50 mgl^-1 results in a great reduction in the incidence of dental caries. So fluoridation is recommended where water supply contained F^- less than 1.50 mgl^-1. The manual of water supply (Ministry of Health, Government of India 1962) lists several places in India including semiarid region of Punjab where people are reported to suffer from fluorosis through use of underground waters containing relatively high concentrations F^- anion. High concentration of F^- in drinking waters abstracted from shallow aquifers in semiarid region of Punjab may be the primary cause for yellowing of teeth of local population. It also caused joint pains, severe dental and skeletal   fluorosis in some people living in those areas. The present investigation aims to determine the concentration of F^- in alluvial shallow aquifers of semiarid region of Punjab, north-west India.  This study also points to exposure of water to air and co-occurrence of iron (Fe) as the two main factors responsible for considerable lowering of F^-  in water abstracted from shallow aquifers by hand pumps and tube wells. Fifty six water samples were collected in October 2008 from shallow aquifers located in the semiarid region of Punjab, north-west India. The soils in this region can be classified on physiographic units of sand dunes, interdunal areas and alluvial terraces. This region comprises of   Mansa and Talwandi sabo and geographically, the area of sampling lays between 30⁰11^/N latitude and 75⁰00^/E longitude. Before sampling the water was drawn for half an hour to empty the hand pump and tube well pipes in order to collect the fresh water from the aquifers.  Polyethylene bottles of 50 ml volumes were used for collection of water samples. Bottles were fully filled with waters and closed to avoid any in air bubble. Field measurement of redox potential (Eh) were also made at the time of sampling. Water samples collected from various locations were brought back to laboratory in cooled ice box during transportation and stored refrigerated (at 5⁰) until analysis. The analyses were performed within 48 hour after collections of water samples. The F^- concentration in water was determined potentiometrically, using the F1052F solid-state membrane electrode in conjunction with K401 calomel electrode. Water samples were also analyzed for pH, total dissolved solids, calcium, magnesium, sodium, potassium, bicarbonate, chloride and sulfate ions.

Fluoride concentration in alluvial shallow aquifers of semiarid region varied from 0.88 to 14.04 mgl^-1 with a mean value of 8.22 mgl^-1. The median, 75^th and 90^th F^- percentiles concentration were 8.46, 9.84 and 10.95 mgl^-1, respectively. WHO (2004) has laid down a limit of 1.5 mg/l F^- in drinking water. This maximum limit protects tooth decay and enhances proper bone growth. On the basis of WHO (1.5  mgl^-1) limit, 98 percent of the water sample contained F^- well above the safe limit for human consumption. The high pH and concentration of total dissolved solids slightly enhanced the F^- content in ground water. The concentration of F^- showed an increasing trend simultaneously with Mg, Na and K and the effect was more pronounced with monovalent than divalent cation. However higher concentration of dissolved Ca depressed the concentration of F^- in ground water. The mean, median, 75^th and 90^th F^- percentiles concentration enhanced with increasing concentration of Cl^-, HCO₃^- and SO₄^2- ion concentration in water of alluvial aquifers. Relationship of F^- concentration with redox potential of water samples determined immediately at the time of sampling illustrated in figure 1.

The redox potential of alluvial aquifers varied from -26.0 to +130 mV with a mean value of +9.31 mV. The median of redox potential of these shallow aquifers was +3 mV. The redox values for 75^th and 90^th percentiles were +14.75 and +37.4 mV for shallow alluvial aquifers. The results in the present investigation showed an inverse trend between F^- content and redox potentials recorded at different location. Thus, higher positive redox potential in underground water elucidating the oxidized conditions which could also be the reason for low F^- content in it. Similarly, the low redox potential values for waters extracted from alluvial aquifers depict reduced conditions and eventually could be the cause of elevated F^- content in the latter case. Thus redox processes is mainly responsible for the F^- concentration in many ground water systems. Clay minerals, Fe and Mn are commonly associated with aquitards and have shown to be significant adsorbent of F^- anion. The extent of adsorption or desorption of F^-  is also influenced to some extent by the chemistry of  aqueous phase which include pH, total dissolved solids and the concentration of  competing anions (Cl^-, NO₃^-, SO₄^2- and PO₄^3-). In an oxidizing environment, native soluble iron and manganese oxidized to solid iron and manganese oxy-hydroxides and served as the sorbing sites and eventually reduced the concentration of F^- in natural waters. However, under reduced environment, the solid Fe(III) and Mn(IV)  oxides tend to dissolved to soluble Fe(II) and Mn(II) and thus releasing F^-  from the sorption sites as well as fixed with precipitated oxy-hydroxides of iron and manganese compounds and thereby enhanced its concentration in ground waters. It is conclusively evident from this investigation that redox status of alluvial aquifers in semiarid region of Punjab is predominantly governing the concentration of F^- in underground waters. Fluoridation of water abstracted from alluvial aquifers of semiarid region of Punjab is required to maintain 1.5 mgl^-1 F^- when the redox potential of water above +95 mV and remediation is recommended when redox potential of water is below +95mV. The elevation of redox potential technique is most reliable, cheap and simple to adopt for remediation of fluoride in drinking ground water at rural household level. It needs to collect underground water from hand pumps or tube wells in a big container, earthen pots or cemented open tanks having large surface area and expose for few days to get it aerated or to interact with atmospheric oxygen in order to achieved elevated redox potential. Higher redox potential reduces fluoride content in stored water, which can used after decantation filtration to another vessel.