Environment Pollution and Climate Change
Open Access

Contamination; Precipitation; Evaporation; Hydrochemical

Introduction

India, the world’s second most populous country, extracts more than a third of worldwide groundwater resources, more than 90% of which is used for irrigation [1]. Intense abstraction has led to severe groundwater table declines in many parts of the country, especially in the northwestern Indian states of Punjab, Haryana, and Rajasthan [2-5]. In 2013, the Indian Central Groundwater Board estimated that groundwater in the majority (66−70%) of blocks (Indian administrative division above village) in these three states was either critically exploited or overexploited.5 At the same time, parts of northwestern India that import surface water through canals are dealing with water prevalence of uranium in water and chronic kidney disease (CKD) and demonstrated that exposure to uranium through logging issues, even in arid, previously groundwater-deficient areas [2,4-6]. Overexploitation of groundwater and the use of imported surface water, combined with reported changes in precipitation patterns induced by climate change, have raised concerns about future water sustainability in India [2-4]. Yet water quality issues are perhaps even more pressing. High concentrations of salinity, fluoride, and nitrate are widespread in groundwater resources throughout the country [6-8]. Groundwater arsenic problems have been reported in the delta aquifers of West Bengal and Bangladesh, as well as along the Indo- Gangetic Basin aquifer in Pakistan [2,9]. There are also reports of high levels of uranium in groundwater, particularly in northwestern India, which is the focus of this study.

Uranium’s threat to human health comes from its chemical rather than its radiological properties. Epidemiological and toXicological studies have examined the link between the overexploited.5 At the same time, parts of northwestern India that import surface water through canals are dealing with water prevalence of uranium in water and chronic kidney disease (CKD) and demonstrated that exposure to uranium through drinking water is associated with nephrotoxic effects [10-12] (Figure 1).

Figure 1: Figure 1: (A) Distribution of major geological formations in India that compose local aquifers, combined with identified districts in India where uranium in groundwater has been reported to exceed (red zone) or not to exceed (blue zone) the World Health Organization provisional drinking water guideline value of 30 μg/L. (B) Distribution of uranium concentrations in groundwater collected in this study, together with the major geological formations and identified districts in Rajasthan and Gujarat, where uranium content in groundwater has been reported to exceed (red zone) or not to exceed (blue zone) 30 μg/L.

Consequently, the World Health Organization (WHO) has set a provisional guideline value of 30 μg/L for uranium concentrations in drinking water [13], which is consistent with the U.S. Environmental Protection Agency [14] drinking water standards. Despite this, uranium is not included in the Bureau of Indian Standards’ Drinking Water Specification, though the Atomic Energy Regulatory Board has set a radiologically based limit of 60 μg/L for uranium in drinking water [15]. Uranium also bioaccumulates in crops irrigated with water containing uranium, though the magnitude of enrichment depends on multiple factors, such as soil and crop type [16]. Elevated concentrations of uranium have been observed in groundwater worldwide from both anthropogenic (e.g., uranium mining and nuclear disposal sites) and geogenic sources (e.g., high-uranium source rocks, like granites) [17-20]. In addition to the uranium sources, other factors such as oxidation state, water−rock interactions, and the formation of soluble aqueous complexes are important factors controlling uranium occurrence, speciation, and mobility in groundwater. Though uranium has two primary oxidation states, +4 and +6, it is most soluble as uranyl ion (UO₂²⁺) in the oxidized +6 form. Uranium may be mobilized from aquifer rocks through uranium-bearing mineral dissolution and adsorption/desorption processes with clay minerals, iron oxides, and organic matter [20-24]. Water chemistry also plays an important role, as uranium forms stable complexes with several different ligands [25]. Uranyl carbonate complexes, including ternary species, are particularly important as these highly stable and mobile complexes promote desorption of uranium from the solid phases [20-27]. While previous studies of uranium in Indian groundwater have focused on relatively small geographic areas and regions, here we present, for the first time, evidence that uranium contamination in groundwater is widespread in India and occurs at different magnitudes in many of India’s aquifers. Our analysis is based on our sampling and chemical analysis of well water in Rajasthan (n = 226), combined with compiled data from previous studies to show the wide extent of uranium in India’s groundwater (Figure 1 and Table 1). The new dataset represents a variety of hydrological settings, aquifer types, and land uses. With these tools, we show the known and likely uranium distribution in Indian aquifers and suggest mechanisms that may control the occurrence of uranium in these groundwater settings.

Table 1:Standard values for the various physicochemical characteristics of soil (Provided by the CEG Test House, Jaipur).

Methodology: sampling and analytical techniques

Thirty-three samples were collected from dug wells, tube wells and hand pumps of different locations in the study area (Figure 1). The depths of wells were in the range of 30-170 m below ground level (bgl). To get representative sample from the aquifer, the wells were flushed until a constant temperature was obtained and then sample was collected. The physicochemical parameters i.e. electrical conductivity (EC), pH, temperature etc. were measured in situ using make portable hand-held water analyser meter, which were calibrated with the standard buffer solutions. Samples for total uranium measurement were collected in acid leached bottles (20 ml Tarson make) after thoroughly rising with the sample water (Figure 2).

Figure 2: Water Analyzer Kit and fluorimeter (UA1, Quantalase).

Measurement of total dissolved uranium was carried out by fluorimeter (UA1, Quantalase), in which fluorescence due to uranyl complex is measured. The phosphate uranyl complex is formed by addition of sodium pyrophosphate. This complex is preferred as it is stable and has enhanced fluorescence [23]. The typical detection limit is 0.2 μg/l. A suitable dilution was done in the case of sample containing high uranium concentration. The detailed quality control of the U measurements is described in Rishi et al [5]. Samples for environmental isotopes (δ²H and δ¹⁸O) were collected in 60 ml airtight bottles (Tarson make). In order to avoid errors in measurement due to fractionation the sample bottles were filled up to the brim so that no air gap was left. Environmental isotopes were measured using isotope ratio mass spectrometer (IRMS: IsoPrime 100). Determination of deuterium (δ²H) isotope was done using pyrolysis mode of elemental analyser, in which 10 μl of water sample is injected into the combustion chamber and the gases formed are fed into IRMS system with a carrier gas. For δ¹⁸O, 200 μl of water sample is equilibrated with CO₂ gas at 50ºC for 8 h and the equilibrated gas is introduced into the IRMS system. The analytical precisions (2 σ) of δ₂H and δ¹⁸O are ± 1.5‰ and ± 0.2‰ respectively. The results are reported in δ-notation and expressed in units of parts per thousand (denoted as ‰). The δ-values are calculated using Eq. (1). Where R denotes the ratio of heavy to light isotope (e.g. ²H/¹H or ¹⁸O/¹⁶O) and R_Sample and R_Standard are the ratios in the sample and standard respectively. Vienna Standard Mean Ocean Water (VSMOW) was used as the standard for the isotope measurements for δ²H and δ¹⁸O.

Observations

The Physicochemical analysis of the standard range for the different parameters (as provided by the CEG Test House, Jaipur) have been also considered (Table 1) for the comparative study. The five ground water samples collected from the two spots in Sanganer were analyzed for the various physicochemical parameters and the data was compared with the standard limits for industrial ground waters, to understand the type of pollution load in the ground water (Tables 2-3).

Table 2: Physico-Chemical characteristics of ground water samples collected from two spots located in Sanganer.

Table 3: Tolerance limits for industrial ground waters discharged into inland surface waters [As per IS: 10500 (Part – A: Ground waters) – 1992 Reaffirmed 1993].

Discussion

Physico-Chemical characteristics of ground water samples: the Physico-chemical analysis of the textile ground water samples gives an idea of the extent type and possible source of pollution and can be used as an argument to emphasize on the treatment of textile ground water prior to its discharge on the open land or local water bodies. The pH values for samples of spot 1st were very slightly alkaline (8.2 and 8.0) and well within the permissible limit. In case of samples from spot 2nd the values were very close to neutral (7.5-7.9). This difference at the two spots can be attributed to the higher ratio of alkaline chemicals being used during the various processing steps, in the printing/dyeing unit. Whereas in case of spot 2nd which was an open drain receiving ground water from numerous textile industries, the alkaline ground waters might be neutralized by the acidic ground waters depending upon the type of process carried out and the chemicals used in the different printing/ dyeing units. As aquatic life is highly susceptible to change in the pH of water body. Therefore, continuous addition of ground water with non-neutral pH tends to change the pH of the receiving body to the point which may drastically after the habitats. Degree of potential impact depends on the degree of alteration in that habitat. In other words, altered pH leads to dominance of some species while cause extinction of others. EC is a salinity rating of water is an indicator of dissolved solids and suspended solids. The EC values were quite higher for the sampling points of spot 2nd (3.5 and 3.6), in comparison to that of spot 1st (1.95 and 1.68). According to Goel, the higher values of EC have been assigned to higher concentration of dyes in the ground waters. So the higher values of EC at spot 2nd might be due to the higher concentration of various salt ions and dyes contributed by the ground waters from the numerous printing/dyeing houses. Chemical Oxygen Demand (COD) value is a measure of O₂ demand of water, against total chemicals. It was observed that the COD values were very low and well within the permissible limit for spot 1st, but the sampling points of spot 2nd were showing very high values (894- 912 mg/L) much higher than the permissible limit (250mg/L). This indicates towards the oxygen deficiency in the drain due to discharge of textile ground waters (from the numerous units), rich in organic compounds. High values of this parameter indicate potential depletion of dissolved O₂ in the water body. Deficiency of O₂ in receiving water could cause adverse effects on biological activity in water environment. In worst case, this can result in total depletion of O₂ in receiving water, causing an anaerobic environment, thus changing the habitat from aerobic to anaerobic life. The total solids were present at much higher concentration at the sampling points of Spot 2nd (1286-1664 mg/L), in comparison to their concentration at spot 1st (958 and 973 mg/L). This higher Total solid content at spot 2nd can also be attributed to the contribution of numerous units in comparison to the contribution of a single unit at spot 1st. Total solid concentration in textile ground water may not be very high; however, total load of this parameter may still be significant, as the use of large amount of water in textile industry offsets the effect of low concentrations. Many dissolved solids are undesirable for health of the receiving water body. Dissolved minerals and organic constituents may produce aesthetically displeasing color and odour. The solids will increase the turbidity of water, induce septic conditions in the water body by retarding the photosynthetic activity affecting the symbiotic process, and also interface with O₂ transfer mechanism of air- water interface. As it can be seen that the concentration of all the cations (Ca, Mg and Na) except K, are present at slightly higher concentrations at the spot 2nd when compared to their presence at spot 1st. In case of K, its concentration was similar at both the spots. The presence of these cations at both the spots indicates the use of chemicals based on these metals in the textile industry. The spot 1st was showing lower concentrations of all the anions tested (Cl, HCOO₃ and CO₃), in comparison to sampling points of spot 2nd. These anions are component of the various chemicals utilized during bleaching, printing/ dyeing and fastening/washing steps in a printing/dyeing unit. All the heavy metals (Cu, Zn, Cd, Pb, Cr, Mn and Fe) were present at lower concentrations at the sampling points of spot 1st, in comparison to the spot 2nd. The Cu content was above the permissible limit (3.0 mg/L) at the spot 1st but was within the limit at spot 1st. Zn and Cd content was within the standard limits (5mg/L and 2.0 mg/L respectively) at both the spots. Although Pb contents was within limit at spot 1st but much above the limit (0.1mg/L) at spot 2nd. The presence of Cr and Mn was within limit (2.0 mg/L each) at spot 1st but not at spot 2nd. Although Fe was detected at all the 5 sampling points, but within the permissible limit (3.0 mg/L). The detection of these heavy metals at all the spots might be due to frequent use of metal-based dyes in the textile industry located in the sampling area. These heavy metals can be toxic to plant and animal life. Usually heavy metals have cumulative effect, thus their concentration tend to increase in the food chain.

Summary

This study was attempted to demarcate the uranium contaminated sites in Jaipur and districts of Rajasthan and to identify the recharge source to groundwater. Results indicate that the dissolved uranium concentration ranges from 5 to 145 μg/l with an average concentration of 49 μg/l. Dissolved uranium was correlated with other physicochemical parameters and stable isotopes in order to understand the processes governing U release into groundwater. From depth profile of EC, two water sources with salinity 3000 μS/ cm and 6000 μS/cm were identified which are contribut- ing salinity to groundwater. The decrease in EC of ground- water with depth can be attributed to flushing of the local saline groundwater by regional fresh groundwater flows. Isotope systematics infer three major sources of groundwa- ter recharge; (i) evaporative surface water or contribution from irrigational return flow, (ii) evaporated rainwater and (iii) direct precipitation without evaporation. No correlation was observed between dissolved U and the corresponding δ18O composition, while a positive correlation was observed between U and EC values for contamination up to 75 μg/l. Hydrochemical and isotopic results infer that leaching of minerals present in the subsurface is the main cause for elevated levels of U in the study area.

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