Anthropogenic nitrate pollution in groundwater and its health risks in the 1 view of background concentration in a semi-arid area of Rajasthan 2

11 An increased nitrate (NO 3- ) concentration in groundwater has been a rising issue on a global 12 scale in recent years. Through different consumption mechanisms, it clearly illustrates the 13 adverse effects on human health. The goal of this present study is to assess the natural and 14 anthropogenic NO 3- concentration in groundwater and its related risks to human health in the 15 different groups of ages such as children, males, and females. Groundwater samples (n=101) 16 were obtained and analysed for their physicochemical components, along with the nitrate 17 concentration in a semi-arid area of Rajasthan. The results show that most of the samples 18 were influenced by anthropogenic activities. The background and anthropogenic levels had 19 been estimated and marked as 7.2 mg/L and 13.3 mg/L for the background and anthropogenic 20 concentrations, respectively. About 83% of nitrate samples were exceeded the background 21 limit, while 28% of the samples were beyond the permissible limit of 45 mg/L as stated by 22 the Bureau of Indian Standards (BIS). Nitrate health risks were also measured by oral intake 23 and dermal contact sources for the residents in this area. The oral exposure of nitrate was 24 very high as compare to dermal contact. With regards to the non-carcinogenic health risk, the 25 total Heath Index (HI Total ) values of groundwater nitrate in the study area varied from 0.045 26 to 3.153 with an average of 0.964 for males, 0.053 to 3.726 with an average of 1.139 for females, and 0.061 to 4.278 with an average of 1.308 for children. The nitrate health risk 28 assessment shows that about 38%, 46%, and 49% of groundwater samples constitute the non- 29 carcinogenic health risk to males, females, and children, respectively.

as atmospheric deposition, hereinafter AD) and releases of both domestic and industrial 52 sewage systems and modification in nitrogen-fixing crops in natural vegetation [11][12][13] . In most 53 natural waters, nitrate forms a critical portion of the ionic charge. Because of the harmful 54 effects on humans at high doses, NO3ions are used in international regulations and 55 guidelines 14 . However, the long-term intake of elevated nitrate concentrations can cause 56 serious health hazards in children, such as methemoglobinemia, which is also known as a 57 blue baby syndrome, and stomach cancer in adults as well 15,16 . In view of this, the World  Usually, background levels are estimated either temporally (concentrations before 67 anthropogenic activity) or spatially (concentrations in the areas not influenced by 68 anthropogenic activity) 28 . It may be difficult to establish background concentrations for 69 certain pollutants, mainly those that have several and non-point sources or are 70 environmentally reactive. Nitrate (NO3 -) is an example of an ion for which it is difficult to 71 establish a threshold concentration due to its different geogenic and anthropogenic origins 72 and its reaction 28,29 . The issues with establishing a nitrate (NO3 -) threshold value, is that 73 natural or geogenic processes will differ greatly in time and space that influences NO3-N 74 concentrations. It is generally found that NO3-N concentrations in aquifers decrease with 75 down gradation or with depth 28,30 . If the groundwater reaches at redoxcline depth (redox 76 boundary), where nitrate and oxygen suddenly vanish due to denitrification, this will occur 77 within a relatively slight difference in depth 31,32 . These redox boundaries can shift with time, 78 and there may be a limit to how much NO3can be reduced in some aquifers, mainly those 79 with low concentration organic carbon and ferric iron 33 . Thus an important aspect influencing 80 the determination of background concentrations is the well depth from which groundwater 81 samples are taken. The NO3-N concentrations can also be influenced by the water mixing of 82 different ages and/or from different origin. Mixing can takes place naturally (e.g., due to 83 quick recharge) or anthropogenically (e.g., due to construction of the well, wells are 84 frequently uncased over greater depth). Other significant variables include lithology, soil 85 composition, the thickness of unsaturated zone, and bioactivities 28 . 86 The investigated area falls under the arid and semi-arid climate state Rajasthan where the 87 only source of fresh water is groundwater. According to the CGWB report 34 , most of the 88 areas of the state have been categorised as over-exploitation zones. Along with the over-89 exploitation of groundwater, the quality of this precious resource is also highly degraded. 90 Thus, the key objectives of this research are: to (1) analyse groundwater chemistry, (2) 91 evaluate regional natural background concentration of nitrate, (3) investigate nitrate toxicity 92 for understanding the potential source and contamination mechanism of nitrate, and (4) 93 quantify the potential non-carcinogenic health risk induced by groundwater exposure to 94 nitrate using recommended model of the US Environmental Protection Agency 35,36 . To our 95 knowledge, this is the first study of health risk assessment considering the natural background 96 level of NO3in groundwater. The findings of this analysis will provide an important 97 scientific and logical understanding of the human health risk of NO3in groundwater and they 98 will also lead to improve the levels of water quality.

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Three significant river basins are present in the study area; Banganga basin (covering about 103 63% of the study area in the northern part), Morel basin (covering about 34% of area in the 104 southern part), and a small part by Ghambhir basin (around 3% of the area in the south of 105 Mahwa) 37 . Agriculture production is scattered over both kharif, and rabi cultivation in the 106 area; kharif cultivation is based on precipitation (rainfed) and rabi cultivation is especially 107 based on groundwater source. Because of significant daily temperature variations and low, moderate rainfall, the climate of 114 the area can be categorized as semi-arid. It is markedly seasonal and is characterised by a dry 115 and steadily increasing hot season between March and May, a dry and cold winter between 116 October and February, but from June to September a monsoon season. In the study region, 117 the minimum and maximum temperatures reported are below 10 °C in January, and 48 °C in 118 June. The gross annual evapotranspiration potential is 1744.7 mm 34 .

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Geology and Hydrogeology. In the study area, the Alwar group of rocks consists of 120 quartzite and schist, alluvium and wind-blown sand occupied the north-eastern and south-121 western parts (Fig.1b) (Fig.1c). The occurrence of groundwater is marked under unconfined to confined aquifer 131 conditions with the primary porosity of alluvium 38 .

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The data used for this study. For assessing nitrate contamination and background criteria,    of human health risk 35,36 . In this study, the risk assessment was carried out in three groups of 179 the exposed population, including children, females, and males.

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The non-carcinogenic health risk from oral intake was calculated as follows 5,17,18,51 : where, in equation (2), CDI is referred as chronic daily intake (in mg/kg/day); "C" is the 184 concentration of groundwater nitrate (in mg/L); IR is denoted for daily ingestion rate of 185 groundwater (in L/day) for both males and females ingestion rate is 2.
where, in equation (4)     original) have been shown in Fig.2b.

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The Shapiro-Wilk test was used (in Table S1) to test the normality of different populations 265 (NBL and APL). If the Shapiro-Wilk test significance value is greater than 0.05, the data is 266 normal. If it is smaller than 0.05, the data will deviate greatly from the normal distribution. In also cause a nitrification reaction that normally converts ammonia (NH3) to nitrate (NO3 -), 299 and hence also raises nitrate concentration in the groundwater 5,24,65,66 . The following equation 300 can be understood as the whole nitrification mechanism 67 . 301 4 + 2 2 → 3 − + 2 + 2 + (7)

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The box plots in Fig. 3 fertilizers, manure, and sewage), which cause anthropogenic pollution above the background 310 concentration. A good correlation between NO3and Clwas noted in the study area (Fig. 4b).

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It also indicated that groundwater nitrate was may be due to the sources of animal and human 312 waste from these ions. A researcher 71 claimed that an increase in chloride concentration with 313 an increase in nitrate was mainly due to the septic tank.      The non-carcinogenic hazard index (HI) map (as shown in Fig. 5) clearly indicates that the 384 northern, central, and southern parts of the study area have higher health risk zones for males, 385 females, and children. Especially it can be noticed that the area of health risk due to 386 groundwater nitrate is high for children as compared to the adults (Fig. 5c). As is visible from 387 Fig.5a,b,c, there is no detrimental impact of a non-carcinogenic risk on the visible green 388 colour in the spatial distribution of hazard index in the study area, although the yellow and 389 red regions show that population in these areas pose potential health threats in the study area.

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It indicates that the areas of yellow and red zones were in danger due to higher HI values; 391 these areas were not recommended for direct intake of drinking water. The spatial distribution 392 of the background values of nitrate (Fig.5d)