Hydrogeochemistry of intermittent alluvial aquifers controlling arsenic and 1 fluoride contamination for corresponding influencing health risk 2 assessment

22 Identification and management of safe groundwater supply in a high As and F – contaminated 23 region is a major concern facing most of the developing countries worldwide. This study 24 provides a comprehensive hydrogeochemistry result for a complex hydro-stratigraphy soil 25 aquifer, which is bound between the Shillong plateau and Himalayan ranges in the North- 26 Eastern region of India. In this study, distinct contaminated regions of As and F – were identified 27 in rainy (n=94) and winter (n=50) seasons groundwater samples. The maximum dissolved 28 concentration of As was measured as 71 µg L –1 in rainy and 211 µg L –1 in winter season 29 groundwater samples, whereas maximum F – concentration was measured as 7 mg L –1 in rainy 30 and 6 mg L –1 in winter season groundwater samples. Identified minerals saturation condition, 31 weathering, and dissolution results of hydrogeochemistry were well corroborated with the 32 Bivariate plot, Gibbs and Pourbaix diagrams. Results of health risk assessment showed that 33 population of age below 18 years old are prone to carcinogenic diseases and symptoms of non- 34 carcinogenic health risk due to daily consumption of As contaminated groundwater. Children 35 of age below 18 years old, in total 39% (i.e. 6834) and 64% (i.e. 13937) were found more 36 susceptible to arsenic ingestion effect. The male population was found prone to As cancer risk 37 than the female population. Overall, this study provides a critical result about the cause of high 38 As and F – concentration in groundwater and health risk assessment, it provides a prime concern 39 matter for the young generation which is at higher risk of As cancer.

Bharalu River is one of the tributaries of Brahmaputra river, it originates in the Khasi Hills     The minerals saturation Indices (SI) for predominant minerals were calculated using 202 Visual MINTEQ 3.1 software using groundwater hydrogeochemistry results in following Eq.

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(1) Gustafsson, 2014):    where 'C O ' is the nugget effect; 'C' is the sill; and 'a' is the range. The spatial data required 249 for semi-variogram models provided by theoretical semi-variogram model used in these 250 analyses.

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To classify the groundwater quality WQI method was followed. This a common method For each parameter, using following equation a quality rating scale i.e. 'Qi' was calculated Where, estimated concentration is abbreviated as 'Vi' for 'i th ' parameter, is abbreviated as The unit weight 'Wi' for each sample was calculated by using following equation: Where, proportionality constant is abbreviated as 'K' and calculated using following Obtained WQI values were categorized into five grades (i.e. A to E) and rated for each 268 groundwater samples. The WQI values laid in between 0 to 25 was categorized in "Grade -A" 269 and rated as "Excellent water quality" which suggest that it is safe for drinking purposes. The

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WQI value between 26 to 50 is rated as "Good water quality" and graded in the "B" category.

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The third category is "Grade-C" and rated as "Poor water quality" for WQI values laid between 272 51 to 75. The "Grade-D" category water rated as "Very poor water quality" for WQI values 273 laid between 76 to 100. The WQI values above 100 is graded as "E" and rated as "Unsuitable 274 for drinking purpose".

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Carcinogenic health risk assessment posed by drinking groundwater with high dissolved 277 concentration of As on present population was assessed by following USEPA 2004 guideline.

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Mainly ingestion of As and risk assessment for dermal was evaluated by using the following 279 empirical equations (11) & (12): Where, heavy metals average concentration in groundwater samples is ' W C ' (µg L -1 ); 'IR',  (Table 2a and   such as clay present in groundwater bearing aquifer. Therefore, the risk of groundwater is high at strong acidic (pH < 6) as well as high alkaline (pH > 6.5) groundwater condition (Wenzel 372 and Blum 1992).   (Figure 6b, 6d, 6f). 487 Table 3 shows   High Fconcentration in groundwater and stable mineral were observed near Meghalaya state 501 foothill, which were further carried down by the surface water bodies (Table 3).  (Table 4). It indicates 508 groundwater bearing aquifers are highly susceptible for surface water infiltration and dilute the 509 dissolved As and Fgroundwater concentration.

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Based on groundwater hydrogeochemistry results a health risk assessment analysis was 511 carried out. The causes of primary symptoms and chronic diseases such as arsenical skin lesions 512 and diarrhoeal disease risk were assessed by the arithmetical health risk assessment method.

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Results were presented in Table 5a, which depicts that children of age below 18 years old are 514 highly susceptible (viz. 39% in rainy season and 64% in winter season) for As ingestion 515 diseases. In Table 5b, suggests that people of age above 18 years old (i.e. female and male) 516 were found highly susceptible to hyper pigmentation and keratosis. In a total highest affected 517 population was female, in the rainy season total of 7112 (i.e. 33%) and winter season 5952 (i.e. 518 52%) female population were found susceptible to dermal effect (Table 5b). The overall results 519 of health risk assessment suggest that in a total 29,616 population of Guwahati city is at a risk of As contaminated groundwater pollution. The fluorosis effect on the present population was 521 not found at significant risk in this health risk assessment method.  Influence of tributary river (i.e. Bharalu river) and Brahmaputra river water infiltration 540 was observed in both seasons aquifers depths (Figure 7a,b & 9a,b). Comparatively opposite 541 trends were observed between Mn and Fe concentration gradient in all aquifer depths for rainy 542 groundwater samples (Figure 7c,d, 8c,d, 9c,d & 10c,d). Distribution of Fin the Gaussian 543 model kriging indicated that the eastern region of Guwahati city is a predominantly geogenic 544 source for the study area (Figure 6c,d, 7c,d, 8c,d and 9c,d). Concentration distribution within all aquifers was observed to be similar, which suggested that vertical distribution of Fe, As, Mn   The overall hydrogeochemistry of rainy and winter season data concludes that the shallow 564 depth aquifers are influenced by surface water infiltration. The identified hydrogeochemical 565 facies such as Na + -Cland SO 4 -2 -Ca +2 suggest that the groundwater has long residential time, 566 which allows to the Fe, As, Fand Ca +2 bearing minerals to precipitate and interact in the 567 aquifer. Gibbs diagram revealed that the groundwater of the alluvium aquifer is susceptible to 568 dissolution mechanism of rock-water interaction and precipitation of minerals. The high concentrations of Na + , Ca +2 and HCO3support ion exchange process to be prevailing in the 570 groundwater aquifer. Results of this study agree with the mineralization and dissolution 571 mechanism observed in minerals saturation indices found for dominant As, F -, Fe and Mn 572 bearing minerals. The groundwater quality index results identified that majority of winter 573 season groundwater samples fall in excellent to good water category, which suggest that winter 574 groundwater is likely to be more suitable than rainy groundwater samples for drinking water 575 purposes, whereas 52% of rainy season groundwater samples were classified as unsuitable for 576 drinking water purposes. The main cause behind is depletion of groundwater table and 577 simultaneously, evaporation and adsorption of dissolved As and Fon soil matrix. Whereas, in 578 rainy season groundwater table rises up and inducing reducing condition at shallow depth 579 aquifer, which increases dissolved concentration of As and Fin groundwater.

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Arithmetical health risk assessment from rainy and winter season groundwater samples 581 has predicted that children population of around 39 % and 64 % may suffer by As ingestion 582 risk. Furthermore, rainy and winter season groundwater samples have reviled that 4% and 2% 583 respectively, children population may suffer by an arsenical skin lesion (i.e. dermal effect). The 584 health risk assessment results have suggested that considerably high percentage of dermal 585 effect may be found in the women population than the male population in the study area.

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Ordinary kriging method results for different depths aquifers have suggested that the 587 closely spaced contours indicates groundwater extraction rate is high from these regions. From 588 this study it can be concluded that the aquifer of depth 250 ft. to 500 ft. can be delineated as 589 safe aquifer and can be used for installation of deep tube wells for drinking water purposes.

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Ground water quality monitoring result for this study area from year 2019 to 2020, will help 591 the public authorities to provide a pertinent and technologically viable solution to the study 592 area peoples.