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Stable isotope (18O) investigations on the processes controlling fluoride contamination of groundwater
JOURNAL OF
Contaminant Hydrology ELSEVIER
Journal of Contaminant Hydrology 24 (1996) 85-96
Stable isotope (180) investigations on the processes controlling fluoride contamination of groundwater P.S. Datta *, D.L. Deb, S.K. Tyagi Nuclear Research Laboratory, IARI, New Delhi-l lO012, India
Received 22 June 1995; accepted 7 February 1996
Abstract Groundwater is being used extensively in the Delhi area for both irrigation and raw water requirement. Fluoride contamination in groundwater is therefore a matter of concern for the planners and managers of water resources. Stable isotope (lSo) and fluoride signatures in groundwater have been discussed, in this context, to characterise the sources and controlling processes of fluoride contamination. The study indicates that almost 50% of the area is affected by fluoride contamination beyond the maximum permissible limit. The wide range (0.10-16.5 ppm) in fluoride concentration suggests contributions from both point and non-point sources. Very high fluoride levels in groundwater are mostly found in the vicinity of brick kilns. Significant quantities of evaporated (isotopically enriched) rainfall, irrigation water and surface runoff water from surrounding farmland also percolate along with fluoride salts from the soils to the groundwater system. The process of adsorption and dispersion of fluoride species in the soil as well as lateral mixing of groundwater along specific flow-paths control the groundwater fluoride and '80 composition. The groundwater system has more than two isotopically distinct non-point source origins, causing spatial and temporal variations in fluoride concentration. Issues related to harmful effects of excessive use of high-fluoride groundwater and management options have also been discussed.
1. Introduction In many countries, standard advice to agriculture is to increase crop yield and production growth rates with an attempt to reduce fluoride concentration in vegetation, heavy use of fertilizers, intensive farming and irrigation. Because, it is generally believed that plants take up very small amounts of fluoride from the soil-water
* Corresponding author. 0169-7722/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S 0 1 6 9 - 7 7 2 2 ( 9 6 ) 0 0 0 0 4 - 6
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P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
continuum, accumulation of fluoride in soils eventually results in leaching by irrigation water and rainfall and deep percolation leading to groundwater contamination. However, cereal crops like jowar, maize and pearl millet, grown on alkaline soils absorb higher amounts of fluoride and thereby also contribute to the spread of fluorosis. In India, fluorosis is endemic in many places and is associated with high concentrations of fluoride ions in drinking water in areas having alkaline soils, rocky strata and dry climate (Singh and Dass, 1993). Also, under the Revised Minimum Needs Programme of the country, excessive salinity and fluoride contents in water are some of the important criteria for defining problem villages (Ran and Datta, 1980). Whether it is a typical rural area or a semi-urbanised rural area surrounding urbanised centre like Delhi, assured safe drinking water by the end of the decade (1981-1990) was the main objective of the International Drinking Water and Sanitation Decade. Since nearly 50% of the Delhi's total population is almost totally dependent on groundwater for its raw water requirement and nearly 70% of the area under irrigation is irrigated by groundwater, fluoride contamination in groundwater is a matter of concern for the planners and managers of water resources. Common sources of fluoride in soil and groundwater are the following: (1) Natural minerals such as fluorspar, fluorapatite, amphiboles (e.g., hornblende, tremolite) and some micas (weathered products of alkali, silicates, igneous and sedimentary rocks, especially shales). (2) Wet and dry depositions of gaseous (e.g., HF, SiF4) and particulate fluorides (e.g., A1F3, NaAlF6, CaF2) emissions from steel, aluminium, glass, phosphate fertilizer, brick and tile industries. Burning of coal and fly-ash deposition (Picketing, 1985; Skjelkvale, 1994). (3) Phosphate fertilizers, fumigants, rodenticide, insecticides and herbicides containing fluoride as impurity or constituent (Ware, 1975; Poovaiah, 1988), e.g., cryolite, barium fluorosilicate, sodium silicofluoride, sulfuryl fluoride, trifluralin. No known major geological source of fluoride exists in the Delhi area (Singh and Dass, 1993) and large quantities of industrial fluorides are seldom discharged into the environment unless an industrial spillage occurs, although aluminium smelters are known to be major point sources of fluoride (Skjelkvale, 1994). Therefore, fluoride contamination to groundwater from sources (1) and (2) can be ruled out in the Delhi area. In the absence of these two major sources, high concentration of fluoride could be due to presence of fluoride-bearing minerals in rocks, brick kilns and widespread application of phosphate fertilizers (which contains ~ 1-3% F), insecticides and herbicides. However, the fragments of fluoride-bearing minerals present in most soils and rocks have very low solubilities (Picketing, 1985), hence, fluoride enhancement effects can arise mainly from accumulations of man-made fluorides. Consumption/application of phosphate fertilizers in the Delhi area during 1990-1992 is reported to be to the extent of ~ 64-78 k g / h a (Fertilizer Statistics, 1993-1994). Regular application of such agrochemicals containing fluoride creates a blanket (non-point) source of fluoride. In this background, the stable isotope (18O) signatures in groundwater in integration with fluoride concentrations have been presented and discussed in this paper to determine clearly the sources of fluoride contamination in groundwater and the processes controlling it, which otherwise becomes difficult to ascertain from ion-concentration data alone.
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
87
2. Physiography and hydrogeology of the area The Delhi area (28°24'17"-28°53'00 " N, 76°56'24"-77°20'37 " E; 1485 km z) occupies a part of the Indo-Gangetic Alluvial Plains, transected by a quartzite ridge (interbedded with micaceous schists and included pegmatites) in the southeastern part (Fig. 1). The drainage to the east of the ridge enters the Yamuna River and to the west of it, the southwestern part receives the surface runoff. The alluvium mainly consists of beds of clay (92-98%), sand and gravel with a thickness not less than 122 m (Sett, 1964). A large part of the area has alkaline and saline soils. The climate of the area is
2!
sd
45 r
35;
30:
25: lso- F contours -,I" 30 ~ ' , Delhi Quartzite i ...... a Sampling points • Groundwater flow ~
28" "20'
I
55'
77o0 , I
!
5"
I
10"
"" "L o, ,, ,, ,. :, m. 15'
a
i
28~ 20 r
20"
Fig. 1. lso-fluoride contours of groundwater (1991 ) in the Delhi area, showing two high-fluoride plumes from west migrating towards the central part.
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
88
Table I Fluoride and lSO composition in groundwater of Delhi Area Sample No.
Location
F (ppm)
O (%~ vs. SMOW)
Alipur Block (DOS 29.06.90): A-I A-2 A-3 A-4 A-6 A-7 A-8 A-9 A- 10
Barwala (TW) Kherakhurd (HP) Alipur (HP) Singhola (TW) Narela (TW) Bawana (HP) Bakoli (TW) Puthkhurd (HP) Pehladpur (HP)
0.96 0.57 2.96 8.34 0.39 0.10 1.87 0.62 1.80
-
8.5 7.2 7.1 6.5 7.5 8.5 7.6 3.2 5.7
Nangloi Block (DOS 29.06.90): N-8 N-9 N- 10 N-10a N-11 N-12 N-12a N-I 3
Dariyapur (TW) Dariyapur (OW) Punjabkhor I (TW) Punjabkhor II (TW) Chandpur (HP) Tikrikalan I (TW) Tikrikalan II (HP) Nangioi (HP)
1.28 0.41 3.92 11.33 9.39 16.53 4.34 3.27
Shahdara Block (DOS 06.07.90): SB- 1 SB-2 SB-3 SB-4 SB-5 SB-6 SB-7 SB-8 SB-9 SB- 10 SB-11 SB-12
Wazirabad (YR) Khajoori (HP) Sadatpur (HP) Sabhapur (HP) Samepur (HP) Tilla (HP) Mandoli (HP) Gokulpur (HP) Mojpura (HP) Seelarnpur (HP) Gandhinagar (YR) ITO Bridge (YR)
- 7.1 - 7.9 - 6.5 - 7.6 - 7.2 - 5.8 -7.0 - 7.0 is
1.32 0.88 1.12 1.10 0.53 0.41 0.55 0.93 0.45 0.47 0.32 0.45
- 7.9
1.08 0.98 0.76 1.30
- 7.6 - 4.5 - 5.5 -
-
5.1 7.3 6.0 7.4 7.6 8.2 7.8 6.5 8.4
Mehrauli Block (DOS 05.07.90): MB-2 MB-5 MB-6 MB-7
Mandi (TW) Ghatomi (HP) Chander Hula (HP) Raipur (TW)
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
Table 1 (continued) Sample No.
Location
F (ppm)
89
O (%ovs. SMOW)
Indian Agricultural Research Institute (DOS 10.07.90):
TW-I TW-2 TW-7 TW-8 TW-14 TW-17 TW-18 TW-Gen-2 HP-New Area
1.37 1.98 0.65 1.42 0.45 2.36 0.87 2.81 0.25
- 5.9 - 6.0 - 7.2 -6.0 -7.1 1-86.6 - 8.0 -
DOS = date of sampling; TW = tube well; OW = open well; HP = hand pump; YR = Yamuna River. semi-arid, with average annual rainfall (1931-1991) of 71 cm and mean annual potential evaporation of ~ 254 cm. The groundwater in the area is under regional water table conditions (Seth and Khanna, 1969). 1SO studies in the area indicate that the natural movement of the groundwater is generally slow and sluggish and the aquifer does not constitute a homogeneous system in its lateral extent (Datta et al., 1994, 1996). The phreatic aquifer recharge is significantly controlled by lateral flow through highly permeable zones in the north and southwest, irrigation return flow and surface water collected in low-lying areas (Datta et al., 1994). The detailed features of the area, based on isotopic and geophysical investigations, have already been reported (Sett, 1964; Datta et al., 1994, 1996; Datta and Tyagi, 1996) describing the groundwater occurrence, recharge conditions, hydrodynamic zones, flow-pathways of mixing and flow regimes.
3. Methods and instrumentation Total 84 groundwater samples (from Alipur, Nangloi, Najafgarh, Mehrauli, Shahdra Blocks and IARI Farm) and three river water samples were collected during July 1990, April-October 1991 and January 1992, and analysed for fluoride and lSo contents. Wherever possible, two samples were collected to observe the variation in ions and isotopic composition within a short distance. The map showing the location of the sampling points in different blocks is given in Fig. 1. Fluoride ion concentration was measured by anion chromatography (Dionex System) and 180 content was measured by VG Isogas Mass-Spectrometer 602D by equilibrating a tank CO z gas with 2 mi water at 25°C using standard procedures. Tables 1-3 show the fluoride and 180 composition of groundwater in different blocks of the investigated area. 61SO (%o) in the tables is expressed as:
8= [(Rsamp,o/Rs,d ) - 1] × 10 3
P.S. Datta et a L / Journal of Contaminant Hydrology 24 (1996) 85-96
90 Table 2 Fluoride and Sample No.
180 composition in groundwater of Delhi
Area
Location
F (ppm)
O (%~ vs. SMOW)
Alipur Block (DOS 05.04.91): A-1 A-2 A-3 A-4 A-5 A-6 A-7
Barwala (TW) Khera Khurd (HP) Alipur (HP) Singhola (TW) Singhola (HP) Narela (HP) Bawana (HP)
2.87 0.53 3.02 2.95 2.52 2.25 0.24
-
8.5 7.2 5.23 4.92 6.39 5.9 8.66
4.80 1.58 3.07 2.39 9.44 4.07
-
6.37 4.48 3.79 5.97 4.88 6.18
Nangloi Block (DOS 05.04.91): N-8 N-9 N-10 N-11 N- 12 N- 13
Dariyapur (TW) Dariyapur (OW) Punjabkhor (TW) Chandpur (HP) Tikrikalan (TW) Nangloi (HP)
Najafgarh Block (DOS 04.07.91): Naj. 1 Naj.2 Naj.3 Naj.4 Naj.5 Naj.6 Naj.7
Mitraon (TW) Dhansa (TW) Gumman Hera (TW) Najafgarh (TW) Chhawla (TW) Bajheda (TW) Kapashera (HP)
2.78 5.02 0.33 2.97 11.60 3.90 2.3 !
Mehrauli Block (DOS 06.04.91): MB-! MB-2 MB-3 MB-4 MB-5
Bhatti (TW) Mandi (TW) Bheem Basti (HP) Manglapuri (HP) Ghatomi (TW)
0.34 1.23 2.06 0.51 0.33
Indian Agricultural Research Institute (DOS 23.10.91): TW- 1 TW-2 TW-8 TW-14 "IaN-17 TW-18 TW-S.B. TW-Mid
1.69 1.84 1.15 0.24 1.91 0.83 1.31 0.58
DOS = date of sampling; TW = tube well; OW = open well; HP = hand pump.
- 6.27 - 4.19 - 4.73 18- 5.21 - 6.34
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
91
Table 3 Fluoride and ISo compositionin groundwaterof the Delhi area Sample No.
Location
F (ppm)
O (%, vs. SMOW)
0.44 2.96
- 8.66 - 5.90
1.28 1.81 3.14 3.22
-
3.34 2.90 3.36
- 4.73 - 6.34 - 3.58
Alipur Block (DOS 21.1.92):
AB-1 AB-6
Bawana (HP) Narela (HP)
Nangloi Block (DOS 21.1.92):
N-8 N-9 N-I 1 N- 13
Dariya pnr (TW) Dariya pur (OW) Chandpur (HP) Nangloi (HP)
6.37 4.48 5.97 6.18
Najafgarh Block (DOS 22.1.92):
Naj.4 Naj.7 Naj.8
Najafgarh (TW) Kapashera (HP) TilangpurKotla (OW)
18
Mehrauli Block (DOS 20.1.92):
MB- 1 MB-2 MB-4
Bhatti (TW) Mandi (TW) Mangla puri (HP)
0.25 1.26 0.40
- 4.68 - 5.57 - 5.47
DOS = date of sampling;TW = tube well; OW = open well; HP = hand pump. where R is 1 8 0 / 1 6 0 and std. is Standard Mean Ocean Water (SMOW). The natural abundance of 180 being small, the 180 content of water samples are expressed in terms of per mil deviation from a reference standard. The ~80 data are treated as physical tracer to study water movement and mixing. The isotopic composition of water is a conservative property unless affected by mixing of water of different isotopic composition. The 180 isotope maintains a " m e m o r y " of their recharge conditions once reaching the groundwater and reflects evaporative effects. The isotopic data were correlated with fluoride ion content data in order to signify the processes controlling fluoride contamination in groundwater.
4. Results and d i s c u s s i o n The fluoride concentration and 180 content in the groundwater of Delhi area vary widely from location to location as well as within the small parts of the region, showing fluoride ranges of 0.10-16.5, 0 . 3 3 - 1 1 . 6 and 0 . 2 5 - 3 . 4 0 ppm in 1990, 1991 and 1992, respectively. Wide variation in the stable isotope contents of groundwater is due to significant variation in the 3180 contents ( - 19.2%o to - 2 . 6 0 % ° ) of rainfall with space and time, as well as intensity and distribution of rainfall (Datta et al., 1991, 1994).
92
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
Superimposed on these variations are different groundwater recharge contributions from evaporated irrigation water and surface-runoff water as well as intermixing of groundwater (Datta et al., 1994). In 1990, 1991 and 1992 respectively ~ 50%, ~ 73% and ~ 75% of the samples collected in respective years showed fluoride concentrations exceeding the general acceptability limit of 1.0 ppm (WHO, 1984). Variation in percentage is due to different number of samples taken in different years. In 1990, fluoride content of the Najafgarh Block could not be measured, although this part had high levels in other years. About 31%, 61% and 59% of the samples collected in 1990, 1991 and 1992, respectively, had fluoride levels more than the maximum permissible limit of 1.5 ppm (WHO, 1984). Very high-fluoride (3-16 ppm) groundwaters occurred mostly in the northern, western and southwestern parts of the area in the Alipur Block, Nangloi Block and Najafgarh Block. Thus, it is evident that a quite significant part of the Delhi area is affected by fluoride contamination in the groundwater beyond the maximum permissible limit. Very skewed distribution of fluoride concentration in the groundwater suggests two different sources (point and non-point) of pollution. Groundwaters with very high fluoride levels (3-16 ppm) clearly suggest the possibility of point-source contamination. Most of these wells are located adjacent to brick industries which commonly use fluoride salts. Non-uniform distribution of fluoride (0.25-2.8 and 0.24-1.9 ppm in 1990 and 1991, respectively) in the groundwater of the IARI farm (having homogeneous soil) suggests that the practice of furrow irrigation together with agronomic practices might have had an effect on the distribution of this anion. However, contributions from agrochemicals can be ruled out, because the farm area received large amounts of agrochemicals during the last 4 - 5 decades and even then F content in groundwater is relatively low. The fluoride concentration of groundwater in any location is controlled by water recharging through the unsaturated zone and by lateral groundwater flow from the surrounding area. High fluoride contents (1.5-13.0 ppm) have been reported (Singh and Dass, 1993) to be present in the groundwaters of areas surrounding Delhi (e.g., Sonepat and Karnal to the north, Rohtak to the west and Gurgaon and Faridabad to the south). Interpolative contouring (taking average value of IARI samples) of the iso-fluoride (Fig. 1) and 6180 (Datta et al., 1994) clearly shows that plume-like features from the western, southwestern and northern parts have a tendency to migrate towards the urbanised central part. Therefore, lateral inflow from these areas may also contribute to some extent to the fluoride concentration in the groundwater of the Delhi area. It is interesting to note from Fig. 2 that, taking all the data points as single grouping and barring the groundwater samples having < 1 ppm F, high-fluoride groundwaters are associated with high-180 (isotopically enriched) water. The samples collected from different blocks are shown by different legends, although the area has homogeneous characteristics otherwise. The reasons for non-linearity in the relationship will be discussed below. This clearly indicates that significant quantities of evaporated (isotopically enriched) irrigation water and surface-runoff water from surrounding farmland infiltrate (Datta et al., 1994) along with the fluoride salts from soils to the groundwater system. Direct contribution of rainfall to fluoride contamination is small because the average fluoride content in rain is normally of the order of 0.01-0.02 ppm. Since groundwater is used for irrigation, the initial 180 and F contents of irrigation water
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 8 5 - 9 6
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P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
remains the same as that of groundwater. The 180 content of surface-runoff water is the same as that of rainfall which causes runoff. Although the original F content of the surface-runoff water has not been measured, however, a comparison of the 180 contents of different types of water sources with that of groundwater suggests that groundwater recharge from rainfall is small (Datta et al., 1994, 1996). The groundwater recharge mostly takes place from lateral flow, irrigation return flow and surface-runoff water collected in low-lying areas (Datta et al., 1994). Low levels of fluoride ( < 1 ppm) in groundwater also reflect the possibilities of redistribution of infiltrating water and soluble fluorides in the unsaturated soil zone. Multiple regression analysis (Picketing, 1985) showed that fluoride ion is strongly retained in the soil with high clay content (r = 0.92). However, at any equilibrium fluoride concentration, adsorption of fluoride in soil was found to decrease with increase in pH. Yet, anion exchange in soil can be both a sink or source of fluoride (Picketing, 1985). Therefore, in the Delhi area, in spite of the surface soil being clayey, because of its alkaline nature and underlying sandy loam deposits, extensive migration of fluoride into the groundwater system may occur, as significant deposition of water-soluble fluorides (e.g., HF and alkaline fluorides) is reported to be associated with clay fraction in soils (Shanwal et al., 1989). However, the source of such deposition could not be identified. The amount of fluoride retained in soil is also reported to be concentration dependent and limited by the availability of sorption sites and hence, could be described by non-linear Langmuir-type adsorption isotherm equations (Pickering, 1985), referring to processes with slow sorption kinetics. Therefore, two opposite mechanisms: adsorption and dispersion, will affect the movement of the adsorbing fluoride species. Consequently, more than one solution of the isotherm equations may be expected, e.g. low dispersion-low adsorption and high dispersion-high adsorption (Grattoni et al., 1993). The effect of these processes is likely to be manifested in the groundwater fluoride and 180 composition, because, longer retention time of fluoride containing water in the top soil and on the surface of the soil will have more evaporation leading to 180 enrichment and increase in fluoride concentration. Thus, considering the data points as a single grouping, various trends in ~ 180-F relationship can be visualised (Fig. 2). This also reflects that the groundwater system has two or more isotopically distinct, non-point source origins (irrigation water and surface-runoff water), with spatial and temporal variations in 180 and F concentrations. As described earlier, there is also a wide spatial and temporal variation in 180 contents of rainfall (Datta et al., 1991, 1994), influenced by intensity and distribution of rainfall and evaporation during the course of its fall. Evaporation of irrigation water in the unsaturated zone also causes enrichment in groundwater resulting in large isotopic differences over small lateral distances (Datta et al., 1994). Depending on degree of evaporation/recharge (Datta et al., 1996) and amount of fluoride salts on the soil, the points in Fig. 2 will deviate from the main trend (curve I). The non-linearity in the trend is due to combined effects of evaporation, superimposed with mixing, leaching and desorption/adsorption. Large variation in fluoride content with relatively little change in isotopic composition in the upper parts (> 3 ppm F) of the trends (I and II) also indicates leaching of fluoride species, and the lower parts ( < 3 ppm F) suggest effects of evaporation and mixing. Trends I (1990) and II (1991, 1992) suggest moderate evaporation, high leaching. Trend I (1991, 1992) reflects high evaporation, low leaching. Trend
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
95
II (1990) indicates low evaporation, high leaching, and the trend III (1991, 1992) suggests very high evaporation and low leaching.
5. Concluding remarks The study clearly indicates that even by the end of the decade (1981-1990) almost 50% of the Delhi area is affected by fluoride contamination in groundwater, exceeding the maximum permissible limit in drinking water. Highly skewed distribution of fluoride concentration in the groundwater suggests contribution from both point as well as non-point sources. Although there is no firm evidence, yet, brick industries appear to be a major point source for very high levels of fluoride in groundwater. The non-point sources are irrigation water and surface runoff water dissolving fluoride form surrounding soils. 180 in integration with fluoride can clearly characterise leaching of fluoride from soil and lateral mixing of groundwater, Whatever be the source and mode of contamination, the fluoride ion inhibits many enzymes (containing calcium, magnesium and iron) involved in the energy production in cells (Ware, 1975), causing alterations in metabolism of plants and net decrease in growth, productivity and crop yield (Maclean and Schneider, 1981; Pushnik and Miller, 1985). One 10-cm application of irrigation water containing 10 m g / l F - ion in a 1-ha field can supply 10 kg of fluoride to the soil. This, together with man-made disposals (e.g., from brick industries and agrochemical inputs), are likely to exceed the " s i n k " capacity of the watershed soils over a period of time, increasing return flow of high-fluoride irrigation water to the groundwater. Hence, to protect the groundwater from further deterioration, it is desirable to put a control over the use of the deleterious man-made inputs and indiscriminate disposal of industrial wastes. Further studies need to be carried out to determine the generality of this approach.
Acknowledgements Technical assistance of Mr. K. Sarkar, Division of Environmental Sciences, IARI, is thankfully acknowledged.
References Datta, P.S. and Tyagi, S.K, 1996. Major ion chemistry of groundwater in Delhi area: chemical weathering processes and groundwater flow regime. J. Geol. Soc. India, 47: 179-188. Datta, P.S., Tyagi, S.K. and Chandrasekharan, H., 1991. Factors controlling stable isotopic composition of rainfall in New Delhi, India. J. Hydrol., 128: 223-236. Datta, P.S., Bhattacharya, S.K. and Tyagi, S.K., 1994. Assessment of groundwater flow conditions and hydrodynamiczones in phreatic aquifer of Delhi area using oxygen-18. Proc. Int. Workshopon Groundwater Monitoring and Recharge in Semi-Arid Areas, Hyderabad, IAH/UNESCO Publ., S IV: 12-24. Datta, P.S., Bhattacharya, S.K. and Tyagi, S.K., 1996. 180 studies on recharge of phreatic aquifers and groundwater flow-paths of mixing in Delhi area. J. Hydrol., 176: 25-36.
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Fertilizer Statistics, 1993-1994. Fertilizer Association of India, New Delhi, pp. 1-98-1-113. Grattoni, C.A., Dawe, R.A. and Binder, M.S., 1993. On the simultaneous determination of dispersion and nonlinear adsorption parameters from displacement tests by using numerical models and optimisation techniques. Adv. Water Resour., 16: 127-135. Maclean, D.C. and Schneider, R.E., 1981. Effects of gaseous hydrogen fluoride on yield of field grown wheat. Environ. Pollut. (Ser. A), 24: 39-44. Picketing, W.F., 1985. The mobility of soluble fluoride in soils. Environ. Pollut. (Ser. B), 9: 281-308. Poovaiah, B.W., 1988. Senescence and ageing of plants (ed. L. Nooden and A.C. Leopold). Academic Press, New York, NY. Pushnik, J.C. and Miller, G.W., 1985. The effects of fluoride on membrane properties of oxidative phosphorylation in plant mitochondria. In: Fluoride Toxicity. Publ. Int. Soc. Fluoride Res. (ISFR), pp. 47-59. Ran, B.B. and Datta, M.M., 1980. Norms for rural water supply. World Water, II(3): 19-31. Seth, N.N. and Khanna, S.P., 1969. A note on groundwater condition in Delhi area. Geol. Surv. India, Misc. Publ., 14(Part-llI): 101-110. Sett, D.N., 1964. Groundwater geology of Delhi region. Bull. Geol. Surv. India, Ser. B, 16: 1-35. Shanwal, A.V., Dahiya, I.S. and Dahiya, S.O. 1989. Soil fluoride as an indicator of profile development in the Yamuna alluvial plain, India. Fluoride, 22(3): 119-127. Singh, B. and Dass, J., 1993. Occurrence of high fluoride in groundwater of Haryana. Bhujal News, 8(1): 28-31. Skjelkvale, B.L., 1994. Factors influencing fluoride concentration in Norwegian lakes. Water, Air Soil Poilut., 77: 151-167. Ware, G.W., 1975. Pesticides. W.H. Freeman, San Francisco, CA, 70 pp. WHO (World Health Organization), 1984. Guidelines for drinking water quality. World Health Org., Geneva, 1: 53-73.
Contaminant Hydrology ELSEVIER
Journal of Contaminant Hydrology 24 (1996) 85-96
Stable isotope (180) investigations on the processes controlling fluoride contamination of groundwater P.S. Datta *, D.L. Deb, S.K. Tyagi Nuclear Research Laboratory, IARI, New Delhi-l lO012, India
Received 22 June 1995; accepted 7 February 1996
Abstract Groundwater is being used extensively in the Delhi area for both irrigation and raw water requirement. Fluoride contamination in groundwater is therefore a matter of concern for the planners and managers of water resources. Stable isotope (lSo) and fluoride signatures in groundwater have been discussed, in this context, to characterise the sources and controlling processes of fluoride contamination. The study indicates that almost 50% of the area is affected by fluoride contamination beyond the maximum permissible limit. The wide range (0.10-16.5 ppm) in fluoride concentration suggests contributions from both point and non-point sources. Very high fluoride levels in groundwater are mostly found in the vicinity of brick kilns. Significant quantities of evaporated (isotopically enriched) rainfall, irrigation water and surface runoff water from surrounding farmland also percolate along with fluoride salts from the soils to the groundwater system. The process of adsorption and dispersion of fluoride species in the soil as well as lateral mixing of groundwater along specific flow-paths control the groundwater fluoride and '80 composition. The groundwater system has more than two isotopically distinct non-point source origins, causing spatial and temporal variations in fluoride concentration. Issues related to harmful effects of excessive use of high-fluoride groundwater and management options have also been discussed.
1. Introduction In many countries, standard advice to agriculture is to increase crop yield and production growth rates with an attempt to reduce fluoride concentration in vegetation, heavy use of fertilizers, intensive farming and irrigation. Because, it is generally believed that plants take up very small amounts of fluoride from the soil-water
* Corresponding author. 0169-7722/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S 0 1 6 9 - 7 7 2 2 ( 9 6 ) 0 0 0 0 4 - 6
86
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
continuum, accumulation of fluoride in soils eventually results in leaching by irrigation water and rainfall and deep percolation leading to groundwater contamination. However, cereal crops like jowar, maize and pearl millet, grown on alkaline soils absorb higher amounts of fluoride and thereby also contribute to the spread of fluorosis. In India, fluorosis is endemic in many places and is associated with high concentrations of fluoride ions in drinking water in areas having alkaline soils, rocky strata and dry climate (Singh and Dass, 1993). Also, under the Revised Minimum Needs Programme of the country, excessive salinity and fluoride contents in water are some of the important criteria for defining problem villages (Ran and Datta, 1980). Whether it is a typical rural area or a semi-urbanised rural area surrounding urbanised centre like Delhi, assured safe drinking water by the end of the decade (1981-1990) was the main objective of the International Drinking Water and Sanitation Decade. Since nearly 50% of the Delhi's total population is almost totally dependent on groundwater for its raw water requirement and nearly 70% of the area under irrigation is irrigated by groundwater, fluoride contamination in groundwater is a matter of concern for the planners and managers of water resources. Common sources of fluoride in soil and groundwater are the following: (1) Natural minerals such as fluorspar, fluorapatite, amphiboles (e.g., hornblende, tremolite) and some micas (weathered products of alkali, silicates, igneous and sedimentary rocks, especially shales). (2) Wet and dry depositions of gaseous (e.g., HF, SiF4) and particulate fluorides (e.g., A1F3, NaAlF6, CaF2) emissions from steel, aluminium, glass, phosphate fertilizer, brick and tile industries. Burning of coal and fly-ash deposition (Picketing, 1985; Skjelkvale, 1994). (3) Phosphate fertilizers, fumigants, rodenticide, insecticides and herbicides containing fluoride as impurity or constituent (Ware, 1975; Poovaiah, 1988), e.g., cryolite, barium fluorosilicate, sodium silicofluoride, sulfuryl fluoride, trifluralin. No known major geological source of fluoride exists in the Delhi area (Singh and Dass, 1993) and large quantities of industrial fluorides are seldom discharged into the environment unless an industrial spillage occurs, although aluminium smelters are known to be major point sources of fluoride (Skjelkvale, 1994). Therefore, fluoride contamination to groundwater from sources (1) and (2) can be ruled out in the Delhi area. In the absence of these two major sources, high concentration of fluoride could be due to presence of fluoride-bearing minerals in rocks, brick kilns and widespread application of phosphate fertilizers (which contains ~ 1-3% F), insecticides and herbicides. However, the fragments of fluoride-bearing minerals present in most soils and rocks have very low solubilities (Picketing, 1985), hence, fluoride enhancement effects can arise mainly from accumulations of man-made fluorides. Consumption/application of phosphate fertilizers in the Delhi area during 1990-1992 is reported to be to the extent of ~ 64-78 k g / h a (Fertilizer Statistics, 1993-1994). Regular application of such agrochemicals containing fluoride creates a blanket (non-point) source of fluoride. In this background, the stable isotope (18O) signatures in groundwater in integration with fluoride concentrations have been presented and discussed in this paper to determine clearly the sources of fluoride contamination in groundwater and the processes controlling it, which otherwise becomes difficult to ascertain from ion-concentration data alone.
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
87
2. Physiography and hydrogeology of the area The Delhi area (28°24'17"-28°53'00 " N, 76°56'24"-77°20'37 " E; 1485 km z) occupies a part of the Indo-Gangetic Alluvial Plains, transected by a quartzite ridge (interbedded with micaceous schists and included pegmatites) in the southeastern part (Fig. 1). The drainage to the east of the ridge enters the Yamuna River and to the west of it, the southwestern part receives the surface runoff. The alluvium mainly consists of beds of clay (92-98%), sand and gravel with a thickness not less than 122 m (Sett, 1964). A large part of the area has alkaline and saline soils. The climate of the area is
2!
sd
45 r
35;
30:
25: lso- F contours -,I" 30 ~ ' , Delhi Quartzite i ...... a Sampling points • Groundwater flow ~
28" "20'
I
55'
77o0 , I
!
5"
I
10"
"" "L o, ,, ,, ,. :, m. 15'
a
i
28~ 20 r
20"
Fig. 1. lso-fluoride contours of groundwater (1991 ) in the Delhi area, showing two high-fluoride plumes from west migrating towards the central part.
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
88
Table I Fluoride and lSO composition in groundwater of Delhi Area Sample No.
Location
F (ppm)
O (%~ vs. SMOW)
Alipur Block (DOS 29.06.90): A-I A-2 A-3 A-4 A-6 A-7 A-8 A-9 A- 10
Barwala (TW) Kherakhurd (HP) Alipur (HP) Singhola (TW) Narela (TW) Bawana (HP) Bakoli (TW) Puthkhurd (HP) Pehladpur (HP)
0.96 0.57 2.96 8.34 0.39 0.10 1.87 0.62 1.80
-
8.5 7.2 7.1 6.5 7.5 8.5 7.6 3.2 5.7
Nangloi Block (DOS 29.06.90): N-8 N-9 N- 10 N-10a N-11 N-12 N-12a N-I 3
Dariyapur (TW) Dariyapur (OW) Punjabkhor I (TW) Punjabkhor II (TW) Chandpur (HP) Tikrikalan I (TW) Tikrikalan II (HP) Nangioi (HP)
1.28 0.41 3.92 11.33 9.39 16.53 4.34 3.27
Shahdara Block (DOS 06.07.90): SB- 1 SB-2 SB-3 SB-4 SB-5 SB-6 SB-7 SB-8 SB-9 SB- 10 SB-11 SB-12
Wazirabad (YR) Khajoori (HP) Sadatpur (HP) Sabhapur (HP) Samepur (HP) Tilla (HP) Mandoli (HP) Gokulpur (HP) Mojpura (HP) Seelarnpur (HP) Gandhinagar (YR) ITO Bridge (YR)
- 7.1 - 7.9 - 6.5 - 7.6 - 7.2 - 5.8 -7.0 - 7.0 is
1.32 0.88 1.12 1.10 0.53 0.41 0.55 0.93 0.45 0.47 0.32 0.45
- 7.9
1.08 0.98 0.76 1.30
- 7.6 - 4.5 - 5.5 -
-
5.1 7.3 6.0 7.4 7.6 8.2 7.8 6.5 8.4
Mehrauli Block (DOS 05.07.90): MB-2 MB-5 MB-6 MB-7
Mandi (TW) Ghatomi (HP) Chander Hula (HP) Raipur (TW)
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
Table 1 (continued) Sample No.
Location
F (ppm)
89
O (%ovs. SMOW)
Indian Agricultural Research Institute (DOS 10.07.90):
TW-I TW-2 TW-7 TW-8 TW-14 TW-17 TW-18 TW-Gen-2 HP-New Area
1.37 1.98 0.65 1.42 0.45 2.36 0.87 2.81 0.25
- 5.9 - 6.0 - 7.2 -6.0 -7.1 1-86.6 - 8.0 -
DOS = date of sampling; TW = tube well; OW = open well; HP = hand pump; YR = Yamuna River. semi-arid, with average annual rainfall (1931-1991) of 71 cm and mean annual potential evaporation of ~ 254 cm. The groundwater in the area is under regional water table conditions (Seth and Khanna, 1969). 1SO studies in the area indicate that the natural movement of the groundwater is generally slow and sluggish and the aquifer does not constitute a homogeneous system in its lateral extent (Datta et al., 1994, 1996). The phreatic aquifer recharge is significantly controlled by lateral flow through highly permeable zones in the north and southwest, irrigation return flow and surface water collected in low-lying areas (Datta et al., 1994). The detailed features of the area, based on isotopic and geophysical investigations, have already been reported (Sett, 1964; Datta et al., 1994, 1996; Datta and Tyagi, 1996) describing the groundwater occurrence, recharge conditions, hydrodynamic zones, flow-pathways of mixing and flow regimes.
3. Methods and instrumentation Total 84 groundwater samples (from Alipur, Nangloi, Najafgarh, Mehrauli, Shahdra Blocks and IARI Farm) and three river water samples were collected during July 1990, April-October 1991 and January 1992, and analysed for fluoride and lSo contents. Wherever possible, two samples were collected to observe the variation in ions and isotopic composition within a short distance. The map showing the location of the sampling points in different blocks is given in Fig. 1. Fluoride ion concentration was measured by anion chromatography (Dionex System) and 180 content was measured by VG Isogas Mass-Spectrometer 602D by equilibrating a tank CO z gas with 2 mi water at 25°C using standard procedures. Tables 1-3 show the fluoride and 180 composition of groundwater in different blocks of the investigated area. 61SO (%o) in the tables is expressed as:
8= [(Rsamp,o/Rs,d ) - 1] × 10 3
P.S. Datta et a L / Journal of Contaminant Hydrology 24 (1996) 85-96
90 Table 2 Fluoride and Sample No.
180 composition in groundwater of Delhi
Area
Location
F (ppm)
O (%~ vs. SMOW)
Alipur Block (DOS 05.04.91): A-1 A-2 A-3 A-4 A-5 A-6 A-7
Barwala (TW) Khera Khurd (HP) Alipur (HP) Singhola (TW) Singhola (HP) Narela (HP) Bawana (HP)
2.87 0.53 3.02 2.95 2.52 2.25 0.24
-
8.5 7.2 5.23 4.92 6.39 5.9 8.66
4.80 1.58 3.07 2.39 9.44 4.07
-
6.37 4.48 3.79 5.97 4.88 6.18
Nangloi Block (DOS 05.04.91): N-8 N-9 N-10 N-11 N- 12 N- 13
Dariyapur (TW) Dariyapur (OW) Punjabkhor (TW) Chandpur (HP) Tikrikalan (TW) Nangloi (HP)
Najafgarh Block (DOS 04.07.91): Naj. 1 Naj.2 Naj.3 Naj.4 Naj.5 Naj.6 Naj.7
Mitraon (TW) Dhansa (TW) Gumman Hera (TW) Najafgarh (TW) Chhawla (TW) Bajheda (TW) Kapashera (HP)
2.78 5.02 0.33 2.97 11.60 3.90 2.3 !
Mehrauli Block (DOS 06.04.91): MB-! MB-2 MB-3 MB-4 MB-5
Bhatti (TW) Mandi (TW) Bheem Basti (HP) Manglapuri (HP) Ghatomi (TW)
0.34 1.23 2.06 0.51 0.33
Indian Agricultural Research Institute (DOS 23.10.91): TW- 1 TW-2 TW-8 TW-14 "IaN-17 TW-18 TW-S.B. TW-Mid
1.69 1.84 1.15 0.24 1.91 0.83 1.31 0.58
DOS = date of sampling; TW = tube well; OW = open well; HP = hand pump.
- 6.27 - 4.19 - 4.73 18- 5.21 - 6.34
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
91
Table 3 Fluoride and ISo compositionin groundwaterof the Delhi area Sample No.
Location
F (ppm)
O (%, vs. SMOW)
0.44 2.96
- 8.66 - 5.90
1.28 1.81 3.14 3.22
-
3.34 2.90 3.36
- 4.73 - 6.34 - 3.58
Alipur Block (DOS 21.1.92):
AB-1 AB-6
Bawana (HP) Narela (HP)
Nangloi Block (DOS 21.1.92):
N-8 N-9 N-I 1 N- 13
Dariya pnr (TW) Dariya pur (OW) Chandpur (HP) Nangloi (HP)
6.37 4.48 5.97 6.18
Najafgarh Block (DOS 22.1.92):
Naj.4 Naj.7 Naj.8
Najafgarh (TW) Kapashera (HP) TilangpurKotla (OW)
18
Mehrauli Block (DOS 20.1.92):
MB- 1 MB-2 MB-4
Bhatti (TW) Mandi (TW) Mangla puri (HP)
0.25 1.26 0.40
- 4.68 - 5.57 - 5.47
DOS = date of sampling;TW = tube well; OW = open well; HP = hand pump. where R is 1 8 0 / 1 6 0 and std. is Standard Mean Ocean Water (SMOW). The natural abundance of 180 being small, the 180 content of water samples are expressed in terms of per mil deviation from a reference standard. The ~80 data are treated as physical tracer to study water movement and mixing. The isotopic composition of water is a conservative property unless affected by mixing of water of different isotopic composition. The 180 isotope maintains a " m e m o r y " of their recharge conditions once reaching the groundwater and reflects evaporative effects. The isotopic data were correlated with fluoride ion content data in order to signify the processes controlling fluoride contamination in groundwater.
4. Results and d i s c u s s i o n The fluoride concentration and 180 content in the groundwater of Delhi area vary widely from location to location as well as within the small parts of the region, showing fluoride ranges of 0.10-16.5, 0 . 3 3 - 1 1 . 6 and 0 . 2 5 - 3 . 4 0 ppm in 1990, 1991 and 1992, respectively. Wide variation in the stable isotope contents of groundwater is due to significant variation in the 3180 contents ( - 19.2%o to - 2 . 6 0 % ° ) of rainfall with space and time, as well as intensity and distribution of rainfall (Datta et al., 1991, 1994).
92
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
Superimposed on these variations are different groundwater recharge contributions from evaporated irrigation water and surface-runoff water as well as intermixing of groundwater (Datta et al., 1994). In 1990, 1991 and 1992 respectively ~ 50%, ~ 73% and ~ 75% of the samples collected in respective years showed fluoride concentrations exceeding the general acceptability limit of 1.0 ppm (WHO, 1984). Variation in percentage is due to different number of samples taken in different years. In 1990, fluoride content of the Najafgarh Block could not be measured, although this part had high levels in other years. About 31%, 61% and 59% of the samples collected in 1990, 1991 and 1992, respectively, had fluoride levels more than the maximum permissible limit of 1.5 ppm (WHO, 1984). Very high-fluoride (3-16 ppm) groundwaters occurred mostly in the northern, western and southwestern parts of the area in the Alipur Block, Nangloi Block and Najafgarh Block. Thus, it is evident that a quite significant part of the Delhi area is affected by fluoride contamination in the groundwater beyond the maximum permissible limit. Very skewed distribution of fluoride concentration in the groundwater suggests two different sources (point and non-point) of pollution. Groundwaters with very high fluoride levels (3-16 ppm) clearly suggest the possibility of point-source contamination. Most of these wells are located adjacent to brick industries which commonly use fluoride salts. Non-uniform distribution of fluoride (0.25-2.8 and 0.24-1.9 ppm in 1990 and 1991, respectively) in the groundwater of the IARI farm (having homogeneous soil) suggests that the practice of furrow irrigation together with agronomic practices might have had an effect on the distribution of this anion. However, contributions from agrochemicals can be ruled out, because the farm area received large amounts of agrochemicals during the last 4 - 5 decades and even then F content in groundwater is relatively low. The fluoride concentration of groundwater in any location is controlled by water recharging through the unsaturated zone and by lateral groundwater flow from the surrounding area. High fluoride contents (1.5-13.0 ppm) have been reported (Singh and Dass, 1993) to be present in the groundwaters of areas surrounding Delhi (e.g., Sonepat and Karnal to the north, Rohtak to the west and Gurgaon and Faridabad to the south). Interpolative contouring (taking average value of IARI samples) of the iso-fluoride (Fig. 1) and 6180 (Datta et al., 1994) clearly shows that plume-like features from the western, southwestern and northern parts have a tendency to migrate towards the urbanised central part. Therefore, lateral inflow from these areas may also contribute to some extent to the fluoride concentration in the groundwater of the Delhi area. It is interesting to note from Fig. 2 that, taking all the data points as single grouping and barring the groundwater samples having < 1 ppm F, high-fluoride groundwaters are associated with high-180 (isotopically enriched) water. The samples collected from different blocks are shown by different legends, although the area has homogeneous characteristics otherwise. The reasons for non-linearity in the relationship will be discussed below. This clearly indicates that significant quantities of evaporated (isotopically enriched) irrigation water and surface-runoff water from surrounding farmland infiltrate (Datta et al., 1994) along with the fluoride salts from soils to the groundwater system. Direct contribution of rainfall to fluoride contamination is small because the average fluoride content in rain is normally of the order of 0.01-0.02 ppm. Since groundwater is used for irrigation, the initial 180 and F contents of irrigation water
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 8 5 - 9 6
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t~
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o
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--
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P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
remains the same as that of groundwater. The 180 content of surface-runoff water is the same as that of rainfall which causes runoff. Although the original F content of the surface-runoff water has not been measured, however, a comparison of the 180 contents of different types of water sources with that of groundwater suggests that groundwater recharge from rainfall is small (Datta et al., 1994, 1996). The groundwater recharge mostly takes place from lateral flow, irrigation return flow and surface-runoff water collected in low-lying areas (Datta et al., 1994). Low levels of fluoride ( < 1 ppm) in groundwater also reflect the possibilities of redistribution of infiltrating water and soluble fluorides in the unsaturated soil zone. Multiple regression analysis (Picketing, 1985) showed that fluoride ion is strongly retained in the soil with high clay content (r = 0.92). However, at any equilibrium fluoride concentration, adsorption of fluoride in soil was found to decrease with increase in pH. Yet, anion exchange in soil can be both a sink or source of fluoride (Picketing, 1985). Therefore, in the Delhi area, in spite of the surface soil being clayey, because of its alkaline nature and underlying sandy loam deposits, extensive migration of fluoride into the groundwater system may occur, as significant deposition of water-soluble fluorides (e.g., HF and alkaline fluorides) is reported to be associated with clay fraction in soils (Shanwal et al., 1989). However, the source of such deposition could not be identified. The amount of fluoride retained in soil is also reported to be concentration dependent and limited by the availability of sorption sites and hence, could be described by non-linear Langmuir-type adsorption isotherm equations (Pickering, 1985), referring to processes with slow sorption kinetics. Therefore, two opposite mechanisms: adsorption and dispersion, will affect the movement of the adsorbing fluoride species. Consequently, more than one solution of the isotherm equations may be expected, e.g. low dispersion-low adsorption and high dispersion-high adsorption (Grattoni et al., 1993). The effect of these processes is likely to be manifested in the groundwater fluoride and 180 composition, because, longer retention time of fluoride containing water in the top soil and on the surface of the soil will have more evaporation leading to 180 enrichment and increase in fluoride concentration. Thus, considering the data points as a single grouping, various trends in ~ 180-F relationship can be visualised (Fig. 2). This also reflects that the groundwater system has two or more isotopically distinct, non-point source origins (irrigation water and surface-runoff water), with spatial and temporal variations in 180 and F concentrations. As described earlier, there is also a wide spatial and temporal variation in 180 contents of rainfall (Datta et al., 1991, 1994), influenced by intensity and distribution of rainfall and evaporation during the course of its fall. Evaporation of irrigation water in the unsaturated zone also causes enrichment in groundwater resulting in large isotopic differences over small lateral distances (Datta et al., 1994). Depending on degree of evaporation/recharge (Datta et al., 1996) and amount of fluoride salts on the soil, the points in Fig. 2 will deviate from the main trend (curve I). The non-linearity in the trend is due to combined effects of evaporation, superimposed with mixing, leaching and desorption/adsorption. Large variation in fluoride content with relatively little change in isotopic composition in the upper parts (> 3 ppm F) of the trends (I and II) also indicates leaching of fluoride species, and the lower parts ( < 3 ppm F) suggest effects of evaporation and mixing. Trends I (1990) and II (1991, 1992) suggest moderate evaporation, high leaching. Trend I (1991, 1992) reflects high evaporation, low leaching. Trend
P.S. Datta et al. / Journal of Contaminant Hydrology 24 (1996) 85-96
95
II (1990) indicates low evaporation, high leaching, and the trend III (1991, 1992) suggests very high evaporation and low leaching.
5. Concluding remarks The study clearly indicates that even by the end of the decade (1981-1990) almost 50% of the Delhi area is affected by fluoride contamination in groundwater, exceeding the maximum permissible limit in drinking water. Highly skewed distribution of fluoride concentration in the groundwater suggests contribution from both point as well as non-point sources. Although there is no firm evidence, yet, brick industries appear to be a major point source for very high levels of fluoride in groundwater. The non-point sources are irrigation water and surface runoff water dissolving fluoride form surrounding soils. 180 in integration with fluoride can clearly characterise leaching of fluoride from soil and lateral mixing of groundwater, Whatever be the source and mode of contamination, the fluoride ion inhibits many enzymes (containing calcium, magnesium and iron) involved in the energy production in cells (Ware, 1975), causing alterations in metabolism of plants and net decrease in growth, productivity and crop yield (Maclean and Schneider, 1981; Pushnik and Miller, 1985). One 10-cm application of irrigation water containing 10 m g / l F - ion in a 1-ha field can supply 10 kg of fluoride to the soil. This, together with man-made disposals (e.g., from brick industries and agrochemical inputs), are likely to exceed the " s i n k " capacity of the watershed soils over a period of time, increasing return flow of high-fluoride irrigation water to the groundwater. Hence, to protect the groundwater from further deterioration, it is desirable to put a control over the use of the deleterious man-made inputs and indiscriminate disposal of industrial wastes. Further studies need to be carried out to determine the generality of this approach.
Acknowledgements Technical assistance of Mr. K. Sarkar, Division of Environmental Sciences, IARI, is thankfully acknowledged.
References Datta, P.S. and Tyagi, S.K, 1996. Major ion chemistry of groundwater in Delhi area: chemical weathering processes and groundwater flow regime. J. Geol. Soc. India, 47: 179-188. Datta, P.S., Tyagi, S.K. and Chandrasekharan, H., 1991. Factors controlling stable isotopic composition of rainfall in New Delhi, India. J. Hydrol., 128: 223-236. Datta, P.S., Bhattacharya, S.K. and Tyagi, S.K., 1994. Assessment of groundwater flow conditions and hydrodynamiczones in phreatic aquifer of Delhi area using oxygen-18. Proc. Int. Workshopon Groundwater Monitoring and Recharge in Semi-Arid Areas, Hyderabad, IAH/UNESCO Publ., S IV: 12-24. Datta, P.S., Bhattacharya, S.K. and Tyagi, S.K., 1996. 180 studies on recharge of phreatic aquifers and groundwater flow-paths of mixing in Delhi area. J. Hydrol., 176: 25-36.
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Fertilizer Statistics, 1993-1994. Fertilizer Association of India, New Delhi, pp. 1-98-1-113. Grattoni, C.A., Dawe, R.A. and Binder, M.S., 1993. On the simultaneous determination of dispersion and nonlinear adsorption parameters from displacement tests by using numerical models and optimisation techniques. Adv. Water Resour., 16: 127-135. Maclean, D.C. and Schneider, R.E., 1981. Effects of gaseous hydrogen fluoride on yield of field grown wheat. Environ. Pollut. (Ser. A), 24: 39-44. Picketing, W.F., 1985. The mobility of soluble fluoride in soils. Environ. Pollut. (Ser. B), 9: 281-308. Poovaiah, B.W., 1988. Senescence and ageing of plants (ed. L. Nooden and A.C. Leopold). Academic Press, New York, NY. Pushnik, J.C. and Miller, G.W., 1985. The effects of fluoride on membrane properties of oxidative phosphorylation in plant mitochondria. In: Fluoride Toxicity. Publ. Int. Soc. Fluoride Res. (ISFR), pp. 47-59. Ran, B.B. and Datta, M.M., 1980. Norms for rural water supply. World Water, II(3): 19-31. Seth, N.N. and Khanna, S.P., 1969. A note on groundwater condition in Delhi area. Geol. Surv. India, Misc. Publ., 14(Part-llI): 101-110. Sett, D.N., 1964. Groundwater geology of Delhi region. Bull. Geol. Surv. India, Ser. B, 16: 1-35. Shanwal, A.V., Dahiya, I.S. and Dahiya, S.O. 1989. Soil fluoride as an indicator of profile development in the Yamuna alluvial plain, India. Fluoride, 22(3): 119-127. Singh, B. and Dass, J., 1993. Occurrence of high fluoride in groundwater of Haryana. Bhujal News, 8(1): 28-31. Skjelkvale, B.L., 1994. Factors influencing fluoride concentration in Norwegian lakes. Water, Air Soil Poilut., 77: 151-167. Ware, G.W., 1975. Pesticides. W.H. Freeman, San Francisco, CA, 70 pp. WHO (World Health Organization), 1984. Guidelines for drinking water quality. World Health Org., Geneva, 1: 53-73.