Geological and Geochemical Characteristics of Low-Arsenic Groundwater in The Area Between Two High Arsenic Areas of ,

Qiao Li (  [email protected] ) Xinjiang Agricultural University https://orcid.org/0000-0002-1514-8572 Hongfei Tao Xinjiang Agricultural University Mahemujiang Aihemaiti Xinjiang Agricultural University Youwei Jiang Xinjiang Agricultural University Wenxin Yang Xinjiang Agricultural University Jun Jiang Xinjiang Agricultural University

Research Article

Keywords: Low arsenic groundwater area, Tectonic and sedimentary evolution, Groundwater geochemistry, Xinjiang, China

Posted Date: May 10th, 2021

DOI: https://doi.org/10.21203/rs.3.rs-498060/v1

License:   This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License

Page 1/15 Abstract

The groundwater of several regions in Xinjiang, China, including the Kuitun and the Manas River Basins in the , is heavily polluted with arsenic. However, the arsenic content of the groundwater of the Karamay area located within the Junggar Basin is relatively low and below the recommended drinking water limit. In our study, we analyze the factors that result in this anomaly. The geological and geochemical characteristics of the water-bearing system in this area were investigated by analyzing water samples, carrying out hydrogeological surveys, and statistical techniques. Since the Carboniferous, the geological development and subsequent structural evolution resulted in a lower arsenic concentration in groundwater of the Karamay region than that of the Kuitun River Basin and the Manasi River Basin. The missing high-energy sedimentary environment in the Middle-Upper Permian and the composition of sediments controlled the characteristics of the multi-layer aquifer in this area. We fnd that the lack of arsenic sources, neutral and slightly alkaline environment, water injection to extract oil, and the River Diversion to Urumqi Project, result in better groundwater quality and lower arsenic pollution in this area.

1. Introduction

Arsenic (As) is a toxic element that is widely distributed in the environment (Guo et al. 2013). The formation of arsenic in groundwater is related to the geology and geomorphology (Nordstrom. 2012). Prolonged exposure to high levels of arsenic can cause cancer, cardiovascular diseases, nerve damage, and other diseases (He et al. 2020). Polluted drinking water (in countries such as Argentina, China, India, Bangladesh, the United States, Mexico, and Chile) is one of the primary routes through which people get exposed to arsenic (Tweed et al. 2020). Therefore, it is crucial to understand the geological and geomorphological factors that regulate the mobilization, chemical form, and fate of arsenic in groundwater.

In the 1970s, people residing in the Kuitun river basin started drinking groundwater polluted with arsenic instead of clean river water. The noticeable decline in the health of the residents led to the discovery of China’s frst large-scale endemic arsenic poisoning in the 1980s (Hong. 1983; Yu et al. 2006; Wang et al. 1983). Later studies indicated that the arsenic content in groundwater of the adjacent Manas River Basin ranged from 0.003–0.491 mg/L, with an average of 0.011mg/L (Zhou et al. 2017), which exceeded the limit of arsenic in drinking water stipulated by China (Ministry of Health, PRC. 2006)and the World Health Organization: 0.01 mg/L (World Health Organization. 2011). The high arsenic content in the Kuitun River Basin is mainly caused by abundant metal reserves (the gold, copper, and pyrite-bearing sand deposits on the southern slope of the Mount Zhayier in the north and the northern slope of Yilianhabierga Mountain in the south) and geological conditions favorable for enrichment (most of the high-arsenic groundwater is distributed in the Chepaizi Depression, an artesian basin with very gentle strata as well as topographic altitude) (Li et al. 2017). On the other hand, high levels of arsenic in the groundwater of the Manas River Basin can be attributed to arsenic-rich coal seams such as the Honggou Coal Mine and the Xiaogou Coal Mine in the southern mountainous regions of the study area (Luo et al. 2017; Luo et al. 2006; Guo et al. 2013). Besides, the weakly permeable aquifer and fne-grained sediments in the high-arsenic groundwater areas lead to the further enrichment of arsenic in groundwater (Zeng et al. 2018).

Zhou et al. investigated the spatial variations in the distribution of arsenic in the groundwater in Xinjiang, China (Zhou et al. 2017). They found that high- arsenic groundwater is mainly distributed in the Barkol-Yiwu Basin, Chepaizi area, , and Yanqi Basin, excluding the Karamay area. The arsenic levels in the groundwater of the Karamay region is lower than the limit for arsenic in drinking water stipulated by China and the World Health Organization despite being located between the heavily polluted Kuitun and the Manas River Basins. The environmental (such as alluvial plains, sand dunes), hydrogeological and geochemical factors that result in this anomaly are poorly understood. More investigation is needed to understand the geological and geochemical characteristics that result in low-arsenic levels in this area (Giacobone et al. 2018). This paper takes the Karamay area between the Kuitun River and the Manas River basin as the study area. The water quality of the Dushanzi Reservoir, one of the water sources in the study area that uses groundwater as its water source, is evaluated as Class I water (Zhu et al. 2017). The largest urban area corresponds to the Karamay City, which is located in the northwest of the study area, with a population of more than 400,000 (Sixth National Population Census of the People’s Republic of China, 2010) (National Bureau of Statistics, PRC. 2011). The biggest threat to groundwater quality in the study area originates in human pollution and excessive exploitation of groundwater resources. Anthropogenic pollution in some areas has caused total dissolved solids in groundwater to reach 2.8 g/L (Li et al. 2011).

The objective of this study is to analyze the possible tectonic, depositional, and geochemical processes that lead to low arsenic concentration in groundwater of the Karamay study area under the background of two high-arsenic watersheds. We collect groundwater samples and measure the existing aquifer composition. Further, we analyze the sediment origin, geological structure evolution, aquifer hydraulics, groundwater geochemical characteristics. We also characterize the spatial distribution of groundwater arsenic concentration in the Karamay area using a map. The growing global population has resulted in a simultaneous increase in the demand for clean water resources. Our study highlights the importance of better understanding the natural processes that create conditions conducive for good-quality groundwater, which is especially relevant for arid areas like Xinjiang.

2. Materials And Methods 2.1 Study area

Location formation of the Karamay area

The Karamay area is located on the northwestern margin of the Junggar Basin, between the Kuitun and Manas River Basins (Fig. 1). It is a well-known oil exploration site in China. The Zhayier Mountain lie to the northwest of the study, while the Depression lie to the northeast. The Zhongguai salient, Hongche Fault Zone and Dabasong salient lie to the south of the study area.

The map in this fgure was modifed after Zhou Y et al.(2017) with permissions of

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Geology and stratigraphy of the Karamay area

With the development and evolution of the PaleoAsian Ocean in the Neoproterozoic, the junggar region and its adjacent areas fnally formed the basement in the late Paleozoic, then deposit carboniferous and quaternary cap. The basement of the early Paleozoic in the study area is composed of volcanic arc rock, turbidite and ophiolite. (Buckman and Aitchison, 2004; Choulet et al., 2012). The carboniferous system in the sedimentary cover is mainly distributed on both sides of The Wuxia fault. The lower Carboniferous is relatively intact, and the lithology is mainly basalt, andesite, pyroclastic rock, siliceous rock, mudstone. The upper Carboniferous is mainly distributed around The Hararat Mountains, and the lithology is mainly volcanics, intermediate basic volcanic rocks, mudstones, sandstones and limestone lens bodies.The Permian is only developed in the lower Permian, which is distributed in a small amount near the Wuxia fault in a fne band. The lithology is basal conglomerate and sand conglomerate. The Triassic is developed in the upper Triassic, distributed in fne strips in the north of Karamay and unconformably in contact with Jurassic. The lithology is mainly sandy mudstone, conglomerate and sandstone. Jurassic outcrop is relatively complete, distributed around Karamay, lithology is mainly conglomerate, sandstone, mudstone and coal seam. The lower Cretaceous is exposed in the east of Karamay, unconformably in contact with Jurassic, and its lithology is conglomerate, sandstone and mudstone. The study area lacks upper Cretaceous, Paleogene and Miocene. The upper Holocene - quaternary series are distributed sporadically, mainly conglomerate, sandstone and sandy mudstone.(Fig. 2).

According to the hydrogeological Survey Report on Ecological Agriculture Project of Xinjiang Karamay in 2000, the strata exposed by 100m boreholes in the study area showed that they were mainly composed of Mesozoic Cretaceous, Neogene and quaternary strata.

Cretaceous: mainly distributed in Zhayier mountain area outside the study area, the lithology is greyish green, light brown fne sandstone and mudstone interbedded with relatively hard rock and well-developed fssures. The buried depth of the roof of this layer is generally between 20 and 40m, and it is in contact with the quaternary system in a way of pseudosconformity, and it is the relative water-proof foor of the pore water of the upper quaternary system.

Upper Neogene - Changjihe Formation: mainly distributed in the southwest corner of the study area, the exposed area is not large, and the lithology is grayish yellow conglomerate, brownish yellow conglomerate, mudstone and argillaceous sandstone. It is in pseudo-conformable contact with the underlying Cretaceous, with a thickness of 15-26m.

al Quaternary middle Pleistocene alluvial layer (Q2 ): distributed in the south of the study area, near the distribution area of the old riverbed, lithologically sandy gravel.

l Quaternary late Pleistocene lacustrine (Q3 ) : the silty soil, clay and silty clay with fne sand are widely distributed in the middle of the study area.

eol−al Quaternary Late Pleistocene - Holocene alluvial deposits and alluvial deposits (Q3 − 4 ) : mainly distributed in the southeast of the region, lithology is sandy gravel and silty clay, with local fne sand.

eol−l Quaternary Late Pleistocene -- Holocene eolian deposits and lacustrine deposits (Q3 − 4 ) : they are widely distributed in the middle part of the area. The lithology of the strata is mainly silty clay, silty soil and clay, and some of them contain fne sand lens.

eol Quaternary Holocene sedimentary deposits (Q4 ) : mainly distributed in the east and southeast of the area, the lithology is grayish yellow fne sand, medium fne sand, the general layer is not thick, exists in the state of dune and sandbag, and there are large sand ridges distributed sporadically.

Stratigraphic division in the Karamay area

Since the study area is severely cut by faults, the chemical changes in the groundwater are complicated, and its water chemistry type varies considerably. According to a previous study (Chen et al. 2000), the northwestern edge of the basin can be divided into an overlying pinch-out zone, fault-step zone, and slope zone; the groundwater in different zones has different chemical characteristics.

The map in this fgure was modifed after Zhou Y et al.(2017) with permissions of

Elsevier (https://www.sciencedirect.com/science/article/abs/pii/S088329271630302X through QGIS—software (version 3.14.16); https://eur03.safelinks.protection.outlook.com/? url=https%3A%2F%2Fwww.qgis.org%2Fen%2Fsite%2F&data=04%7C01%7C%7Ca2eff60fe9454196008a08d8b0b20556%7C5b406aaba1f14f13a7aadd573da3 license Creative Commons Attribution-ShareAlike 3.0 licence (CC BY-SA). 2.2. Sample collection and analysis

The main aquifer in the study area is a porous quaternary sediment with a flling thickness of 80–350 meters, which constitutes a unconfned aquifer at less than 60 m and a semi-confned aquifer below 60 m. These quaternary sediments include sand, sand and clay sand. In this study, we collected two surface water and ffty-one groundwater samples from the Karamay area and analyzed their chemical characteristics. Figure 3 shows the location of sampling points, while Table 1, 2, and 3 provide the measured physical and chemical properties. The temperature, pH, electroconductivity, alkalinity of the water samples were measured on-site using multi-parameter portable water quality analyzers DDB-303A, PHB-4, etc. We also collected recorded the well type, well depth, water

Page 3/15 table depth. Before each sample collection, the sampling bottle was rinsed with the stabilized groundwater stock solution. The sample was fltered through a 0.45 µm flter membrane, and then put into a polyethylene sampling bottle and sealed. It was stored at a temperature below 4℃ before being analyzed. The collection, storage, and delivery of water samples were strictly in accordance with the Technical specifcations for environmental monitoring of groundwater (HL/T164-2004) (State Environmental Protection Administration of China. 2004). The analysis of the samples was performed by the laboratory of the Second + + 2+ 2+ − 2− − Hydrogeological Brigade of the Xinjiang Bureau of Geology and Mineral Resources. The major ions measured were K +Na , Ca , Mg , HCO3 , CO3 , Cl , 2− SO4 , and As. The calculation using the anion and cation balance method shows that the absolute error of the charge balance of all water sample data was less than 5%, meaning the data are reliable.

3. Results

The surface water and groundwater samples collected in this study were mainly distributed in the Karamay area, located in the northwestern part of the Junggar Basin (Fig. 3). It is evident that the concentration of arsenic in the surface water samples and most of the groundwater samples in the Karamay area was below the detection limit (< 0.00005 mg/L) (Table 1). In samples where arsenic was detected (sample numbers: KA08 ~ 10; KA13 ~ 15), the concentration of arsenic ranged from 0.0016 to 0.0045 mg/L, which is lower than the drinking water limit stipulated by the World Health Organization: 0.01 mg/L (Table 1, Fig. 3).

The map in this fgure was modifed after Zhou Y et al.(2017) with permissions of

Elsevier (https://www.sciencedirect.com/science/article/abs/pii/S088329271630302X through QGIS—software (version 3.14.16); https://eur03.safelinks.protection.outlook.com/? url=https%3A%2F%2Fwww.qgis.org%2Fen%2Fsite%2F&data=04%7C01%7C%7Ca2eff60fe9454196008a08d8b0b20556%7C5b406aaba1f14f13a7aadd573da3 license Creative Commons Attribution-ShareAlike 3.0 licence (CC BY-SA).

In terms of major ions’ geochemical characteristics, most of the water samples belonged to the HCO3·SO4-Ca type, which evolved normally to the

HCO3CO3·SO4-Na type (Table 2, Fig. 4). In one sample from the northern part of the study area, SO4-Ca was the primary ion (sample KA39), which could be due to the dissolution of secondary gypsum in the sediments of the Qigu Formation (Yuan. 2015). Sample KA21 also had high SO4 concentration, which can be attributed to its higher degree of evolution as it is located in the central region of the study area. The average and the maximum value of the pH value of groundwater in the study area is 7.86 and 8.80, respectively, which means that the environment is neutral and slightly alkaline (Table 1). The average value of electroconductivity is 782 µs/cm, and the maximum value is 3940 µs/cm (KA53) (Table 1). The average pH of surface water is 7.7, and its average electroconductivity is 156 µs/cm, with both values lower than those of groundwater.

The groundwater depth in the study area ranged from 1.5 to 138 m, and the deepest well had a depth of 298 m (Tables 1 and 3). The lithology of the water- producing layer of the wells was mostly gravel and sand (Table 3). The hydraulic conductivity of the well was in the range of 0.3 ~ 372.3 m2/day (Table 3).

Page 4/15 Table 1 List of water samples in this study. Location, pH, conductivity, and As concentrations. n Type of sample Date Longitude Latitude Static level of well(mbgs) pH Conductivity(µs/cm) As(mg/L)

KA01 Surface water 2019 84°46′35.7″ 44°30′31.6″ —— 7.80 123 < 0.00005

KA02 well 2019 84°48′21.7″ 44°51′26.5″ 13.5 8.80 450 < 0.00005

KA03 well 2019 85°03′08.8″ 45°09′53.0″ 16 8.17 397 < 0.00005

KA04 well 2019 85°40′56.8″ 46°05′53.3″ 15 8.20 875 < 0.00005

KA05 well 2019 85°26′33.5″ 45°58′24.7″ 25.3 8.33 325 < 0.00005

KA06 well 2019 85°13′29.6″ 44°53′47.2″ 12.5 8.05 705 < 0.00005

KA07 well 2019 85°14′45.7″ 44°51′27.6″ 16.5 7.40 669 < 0.00005

KA08 well 2019 85°19′42.3″ 45°18′35.7″ 12.9 7.94 331 0.0016

KA09 well 2019 85°00′38.3″ 45°12′21.0″ 21 7.99 198 0.0023

KA10 well 2019 84°53′47.0″ 45°09′32.8″ 12.9 8.25 523 0.0035

KA11 well 2019 84°51′35.6″ 44°53′55.5″ 14.5 8.45 298 < 0.00005

KA12 well 2019 84°59′50.2″ 45°10′32.7″ 23.2 7.99 1311 < 0.00005

KA13 well 2019 84°50′35.8″ 45°15′42.2″ 23.1 8.32 421 0.0024

KA14 well 2019 84°55′37.9″ 45°08′33.6″ 22.3 8.56 455 0.0036

KA15 well 2019 85°18′29.8″ 45°12′49.6″ 12.5 8.32 887 0.0045

KA16 well 2019 85°10′23.7″ 45°07′11.0″ 14.6 7.67 399 < 0.00005

KA17 well 2019 84°58′50.2″ 45°06′12.3″ 13.5 7.45 363 < 0.00005

KA18 well 2019 85°18′46.9″ 45°00′56.7″ 30.5 7.23 984 < 0.00005

KA19 well 2019 85°11′42.5 44°32′43.6″ 35 7.35 423 < 0.00005

KA20 well 2019 85°14′12.1″ 44°43′32.8″ 41 7.64 687 < 0.00005

KA21 well 2019 85°19′16.3″ 44°43′55.5″ 37 7.35 776 < 0.00005

KA22 well 2019 85°00′45.5″ 44°41′12.7″ 23 7.35 335 < 0.00005

KA23 well 2019 84°53′11.4″ 44°38′23.6″ 41 7.64 257 < 0.00005

KA24 well 2019 84°50′55.8″ 44°37′45.3″ 45 7.67 387 < 0.00005

KA25 well 2019 84°55′23.8″ 44°40′39.2″ 23 7.81 398 < 0.00005

KA26 well 2014 84°48′04.0″ 44°47′58.0″ —— 7.94 646 < 0.0005

KA27 well 2014 84°41′56.0″ 44°56′34.0″ —— 7.81 2930 < 0.0005

KA28 well 2014 85°05′15.1″ 44°30′15.5″ —— 8.21 472 0.0007

KA29 well 2014 85°02′38.0″ 44°33′13.0″ —— 7.55 1218 < 0.0005

KA30 well 2014 85°01′48.6″ 44°51′55.6″ —— 7.45 1120 < 0.0005

KA31 well 2014 85°03′35.0″ 44°56′35.0″ —— 8.19 769 < 0.0005

KA32 well 2014 85°00′20.7″ 44°40′36.9″ —— 7.27 1431 < 0.0005

KA33 Surface water 2017 84°50′22.0″ 44°45′48.4″ —— 7.60 189 < 0.00005

KA34 well 2017 84°51′31.6″ 45°08′24.2″ —— 7.80 3090 < 0.00005

KA35 well 2017 84°55′47.3″ 44°54′18.7″ 15 7.40 3770 < 0.00005

KA36 well 2017 84°55′47.2″ 44°48′31.6″ 40 8.30 160 < 0.00005

KA37 well 2017 84°54′28.7″ 44°39′27.4″ 19 8.30 133 < 0.00005

KA38 well 2017 85°20′30.8″ 44°58′51.4″ —— 8.20 165 < 0.00005

KA39 well 2017 85°03′29.8″ 44°56′42.6″ —— 8.20 365 < 0.00005

KA40 well 2017 84°59′43.5″ 44°48′30.5″ 38 7.70 195 < 0.00005 mbgs = meters below ground surface.

Page 5/15 n Type of sample Date Longitude Latitude Static level of well(mbgs) pH Conductivity(µs/cm) As(mg/L)

KA41 well 2017 84°55′04.8″ 44°58′20.7″ 15 8.00 183 < 0.00005

KA42 well 2017 85°10′20.5″ 44°56′34.6″ 42 7.50 864 < 0.00005

KA43 well 2017 85°06′01.9″ 44°47′24.4″ 35 7.60 1380 < 0.00005

KA44 well 2017 84°30′27.2″ 44°38′45.3″ 45 7.40 1440 < 0.00005

KA45 well 2017 84°36′16.9″ 44°51′42.2″ 60 7.80 336 < 0.00005

KA46 well 2017 84°36′45.7″ 44°53′38.7″ 1.5 7.90 430 < 0.00005

KA47 well 2017 84°44′09.5″ 45°35′25.7″ 45 7.40 110 < 0.00005

KA48 well 2017 84°47′44.6″ 45°33′24.4″ 7 7.70 440 < 0.00005

KA49 well 2017 84°49′54.4″ 45°34′27.8″ 45 7.40 804 < 0.00005

KA50 well 2017 84°45′38.4″ 45°20′51.6″ 105 8.30 200 < 0.00005

KA51 well 2017 84°37′39.8″ 45°23′14.7″ 138 8.00 263 < 0.00005

KA52 well 2017 84°43′09.6″ 45°24′47.7″ 120 8.00 165 < 0.00005

KA53 well 2017 84°37′52.1″ 45°43′05.6″ —— 7.70 3940 < 0.00005 mbgs = meters below ground surface.

Page 6/15 Table 2 Composition of major ions in groundwater and surface water. n Type of sample K + Na(mg/L) Ca(mg/L) Mg(mg/L) Cl(mg/L) HCO3(mg/L) SO4(mg/L) NO3(mg/L)

KA01 Surface water 9.976 35.13 7.68 7.92 99.53 57.98 4.09

KA02 well 75.5 4.01 0.49 22.69 97.63 48.99 ——

KA03 well 1892.27 176.51 122.75 2652.16 112.08 861.98 ——

KA04 well 52.5 36.07 23.09 38.99 170.86 91.26 ——

KA05 well 75.18 49.67 16.43 50.89 199.25 102.63 ——

KA06 well 106.05 104.21 55.91 77.99 457.65 209.41 ——

KA07 well 65.59 110.83 14.94 37.27 330 122.31 5.13

KA08 well 24.8 106.6 11.7 92.2 85.4 153.7 2.37195

KA09 well 321.28 86.62 124.24 152.68 643.82 599.69 0.04

KA10 well 18.98 82.1 8.47 50.18 115.81 107.55 ——

KA11 well 0.3 40.23 4.98 17.2 99.62 14.94 ——

KA12 well 21.79 52.54 9.71 16.49 155.66 61.31

KA13 well 95.98 98.11 25.65 28.67 286.42 241.41 4.2

KA14 well 61.71 54.19 15.69 28.67 229.13 80.86 2.59

KA15 well 154.2 66.91 31.87 46.59 260.27 283.78 8.09

KA16 well 64.6 78.82 13.94 33.69 206.72 159.75 ——

KA17 well 58.93 115.35 26.14 48.03 209.21 263.24 ——

KA18 well 164.8 77.99 33.61 129.74 176.83 321.29 0.03

KA19 well 48.33 5.34 3.73 9.32 84.68 27.67 < 0.01

KA20 well 19.37 29.56 1.99 7.17 107.1 17.83 1.16

KA21 well 256.5 267.65 76.69 257.33 405.97 769.1 ——

KA22 well 72.64 122.74 13.94 50.18 273.97 205.2 ——

KA23 well 44.25 103.86 14.94 28.67 216.51 119.35 8.32

KA24 well 119.71 137.11 46.81 43.01 323.78 407.93 6.07

KA25 well 26.28 24.63 4.98 14.34 99.62 27.67 0.73

KA26 well 34.32 85.65 15.19 60.24 111.21 150.45 15.52

KA27 well 408.89 197.42 65.87 335.04 390.69 872.5 62.85

KA28 well 20.72 61.11 12.27 25.33 125.84 95.13 13.24

KA29 well 40.55 180.62 37.66 102.34 239.98 329.07 11.04

KA30 well 42.95 166.27 37 34.26 304.36 343.19 7.37

KA31 well 43.82 78.67 27.43 51.21 242.9 98.47 41.09

KA32 well 79.57 197.28 48.22 77.76 359.96 409.59 25.58

KA33 Surface water 8.878 32.22 7.33 7.82 91.62 53.85 3.05

KA34 well 688.236 374.53 56.18 853.35 111.17 1389.29 21.26

KA35 well 598.014 199.75 217.89 649.26 283.42 1547.76 8.58

KA36 well 2.553 28.19 4.89 7.11 95.29 34.55 2.43

KA37 well 2.868 21.34 4.4 7.11 95.29 15.24 1.37

KA38 well 2.868 26.18 4.15 12.8 95.29 27.79 2.37

KA39 well 3.11 56.38 7.82 41.96 83.07 82.86 7.93

KA40 well 2.75 31.41 6.11 12.8 106.28 39.37 1.37

KA41 well 4.291 25.37 7.33 14.22 107.5 39.37 2.04

Page 7/15 n Type of sample K + Na(mg/L) Ca(mg/L) Mg(mg/L) Cl(mg/L) HCO3(mg/L) SO4(mg/L) NO3(mg/L)

KA42 well 83.327 98.67 21.01 147.91 97.73 234.19 24.26

KA43 well 111.49 178.41 40.3 253.16 94.06 379.59 40.25

KA44 well 140.77 169.14 43.97 207.65 245.55 364.63 22.24

KA45 well 12.41 44.7 11.48 28.45 108.72 60.15 4.27

KA46 well 27.11 37.86 11.24 28.45 124.61 89.61 0.32

KA47 well 93.39 140.15 44.95 106.67 339.61 331.00 0.53

KA48 well 36.67 49.13 20.27 35.56 250.43 72.24 0.96

KA49 well 72.96 73.7 26.14 192 98.95 113.05 0.96

KA50 well 2.63 32.62 7.08 28.45 100.17 27.79 2.99

KA51 well 3.94 32.22 7.82 28.45 97.73 28.75 2.86

KA52 well 3.11 26.18 6.6 8.53 108.72 27.79 1.47

KA53 well 496.1 251.7 300.45 625.81 486.2 1574.08 0.63

Table 3 Depth of the drilled wells, lithology of the productive layers (in screen sites), and transmissivity of the wells. n Type of sample Depth of well(m) lithology of the productive layers Transmissivity of the wells (m2/day)

KA10 well 180 —— 38.2

KA11 well 190 conglomerate 97.2

KA12 well 200 Conglomerates, sand, coarse sand 141.8

KA16 well 120 boulders, coarse gravel, coarse sand 3.5

KA18 well 120 coarse-medium sand, interbed gravels 0.3

KA19 well 130 medium-coarse gravel 37.4

KA21 well 180 coarse gravel, medium-fne gravel 65.1

KA22 well 120 boulders, medium sand, coarse gravel 1.8

KA23 well 150 coarse-medium sand, fne gravel 17.3

KA24 well 90 coarse gravel, medium-fne gravel, fne sand 107.5

KA25 well 80 fne-medium gravel, coarse sand 201.3

KA31 well 80 fne gravel with medium-coarse sand 69.2

KA35 well 80 fne-medium gravel, sand gravel 0.7

KA36 well 120 coarse gravel 372.3

KA38 well 160 fne-medium gravel, coarse sand 32.1

KA40 well 298 fne-medium gravel, coarse sand 115.8

KA41 well 280 coarse-medium sand, fne gravel 109.2

KA42 well 180 —— 52.7

KA43 well 180 Gravel with sandy matrix 82.3

KA44 well 200 gravel, sand 81.5

KA45 well 160 coarse gravel, medium-fne sand 25.3

KA46 well 58 fne gravel with medium-coarse sand 3.2

KA47 well 120 medium-coarse sand 127.2

KA49 well 230 boulders, coarse sand, medium sand 32.1

KA50 well 190 coarse gravel, medium sand with fne gravel 7.6

KA51 well 260 coarse gravel, fne gravel, boulders 181.4

KA52 well 260 —— 89.8

KA53 well 69 coarse-medium sand, fne gravel 0.6

Page 8/15 4. Discussion

Due to the low arsenic content of groundwater in the study area, we analyzed the following possibilities:

Compared with the Kuitun River Basin and the Manas River Basin, the study area lacks the infuence of geological sources of arsenic or geological processes, aquifer characteristics, and groundwater geochemistry on the enrichment of arsenic in groundwater. 4.1. Comparison of the Karamay area with the surrounding areas of high arsenic content 4.1.1 The Kuitun River Basin

There are more than 20 gold and copper deposits on the southern slope of the Zhayier Mountain in the northern part of the study area. The northern slope of the Yilianhabierga Mountain in the south contains deposits such as gold, copper, nickel, lead, zinc, pyrite, and manganese. Among the above-mentioned deposits, arsenopyrite (whose main chemical component is FeAsS) is also a common mineral component and is especially abundant in the vein gold mines[20]. The arsenic content in these deposits can reach as high as 39%, which is generally around 10%. For example, in some polymetallic deposits to the west of the study area, the arsenic content in the surface oxidation zone can already reach 2.45% − 4.5%, which serves as an important source of arsenic in the Kuitun River Basin. In addition, arsenic sources in rocks, such as mud, shale, and granite, cannot be ignored. As per studies (Hong. 1983), the arsenic content in mud and shale can reach more than 10 ppm, and that in granite is 1.5–2 ppm. Although the content of arsenic in the deposit is low, the quantity of the deposit is quite large because of the wide distribution and the large area. The arsenic in these mountains is transported to adjacent plains through various forms of runoff, which is the primary source of arsenic. According to literature data, about 10.5 t of arsenic was carried by multiple runoffs from mountainous areas into plains (4.5 t carried by surface runoff and 6 t carried by underground runoff).

This area was a subsidence depression during the Quaternary Period. Although the sedimentary center moved many times in various stages of the Quaternary Period, the Chepaizi area of the Kuitun River Basin was always located in the center of the sedimentary area, forming lacustrine deposits dominated by clayey, argillaceous, and siltstones sediments with a thickness of more than 50 meters. The site also experienced many swamping stages, so its strata are rich in humus and other organic matter deposits. This depositional environment and the formed sediments are conducive to the occurrence and enrichment of arsenic (Hong. 1983; Yuan et al. 2014). 4.1.2 The Manas River Basin

The average elevation of the plain area in the Manas River Basin is 450.8 m, and its terrain generally slopes from south to north. To the south of the Wuyi Highway in front of the mountain, there is a single phreatic aquifer composed of huge thick pebble with good hydraulic permeability. The north of the plain reservoir is located to the north of the , and the City gradually transitioned into a multi-layer confned aquifer. The upper part of the aquifer lithology is a pebble layer, and the lower part is a gravel, gravel-sand, or sand layer. The lithological particles are fner northwards and downwards, with water abundance and permeability becoming moderate to poor (Li et al. 2015).

The evolution of the plain area of the Manas River Basin is mainly infuenced by the combined effect of basement structure, piedmont fold hills, and a series of rivers such as the Manas River. Since the Quaternary, under the infuence of the neotectonic movement, the Tianshan Mountains and their two slopes have been continuously uplifted. The northern foothills of the Tianshan Mountains gradually shifted to the Junggar Basin about 40 km, infuenced by thrust and napped of the North Tianshan Mountains, which form three rows of folds. Since the Quaternary, with the expansion of the piedmont zone, the piedmont settlement center has continued to advance northward by about 30–40 km. When the subsidence center advances northward, the spatial position of the plain zone will inevitably move northward with it, and the closed environment of the deposition center makes it easy to develop high arsenic groundwater (Fig. 1). The arsenic in this area comes from arsenic-rich coal seams such as the Honggou Coal Mine and the Xiaogou Coal Mine on the northern slope of Tianshan Mountain in the southern part of the study area (Luo et al. 2006). 4.1.3 The Karamay area

The Karamay area, located in the northern part of the Zhayier Mountains, lacks arsenic deposits, thus lacking arsenic sources. As mentioned above, after the study area experienced tectonic movements, a large deep fault developed extending from the Paleozoic strata to the Middle and Upper Jurassic strata. The large thrust fault-controlled the depression and deposition center of the basin. Large thrust faults also controlled the subsidence and sedimentary center of the basin. The groundwater near the Hala’alat Mountain belonged to the HCO3-Na type. However, a closed high-salinity primary Cl-Ca type water was formed at a deeper level. Near the pinch-out zone of the formation, the groundwater transformed into the SO4-Na type due to the intrusion of surface water (Fig. 2). As this area is close to the arsenic source in the southern mountainous area, arsenic (KA8-10, KA13-15) was detected in some water samples. During the middle and late Permian Period, a unifed sedimentary basin was formed. The Middle and Upper Permian strata were missing only in the Chepaizi Uplift, the southern margin, and parts of the eastern uplift regions, where the groundwater was mainly HCO3-Na type water. In the vicinity of the step-fault zone, due to the infuence of syngenetic faults, the upper water, and the lower water were mixed in the Permian pinch-out zone, producing SO4-Na and Cl-Mg water, and no As was detected in groundwater samples. During the Triassic, the lake basin of the basin further expanded, and its groundwater was dominated by the Cl-Ca type water. However, the HCO3-Na type water was still maintained in the continuous depositional areas of the Upper Permian and Triassic Baikouquan Formation, due to which As was detected in some of the groundwater samples. 4.2. Infuence of aquifer composition and granulometry, and groundwater geochemistry

An important reason for the low arsenic content in groundwater in the Karamay area is the composition of the aquifer sediments. The sediments originated from the end of the Cretaceous to the Quaternary when the sedimentary base of the basin was in uneven and undulating metamorphic rocks. The sandstone and mudstone within the formation lacked the source of arsenic derived from geogony. This example of similar low-arsenic conditions can be observed in Page 9/15 certain specifc environments in the Kuitun River Basin. These conditions are formed by the interbedded river sedimentary layers in aeolian loess deposits (Jiao. 2001). River sediments do not contain arsenic, which helps improve porosity, permeability, and groundwater circulation. The groundwaters are formed by high-velocity fow and circulate in coarse river sediments composed of inert mineralogy. The interaction time is short, resulting in low As concentration (1– 10 lg/L).

In the Karamay area, even if there is a small quantity of deposits containing arsenic minerals, the geochemical environment does not promote its dissolution.

Moreover, if arsenic is present in groundwater at a neutral and slightly alkaline pH, it will be adsorbed onto the surface of Al- Fe- and Mn-oxides and hydroxides

(such as hematite, goethite, Fe(OH)3, magnetite, and gibbsite), and will precipitate in the aquifer (Guo et al. 2013). The pH values of the basin are likely the largest concern for arsenic release to the aqueous phase. Next we will take mechanism study on weakly alkaline groundwater (high pH values) with low arsnic.

There are no redox potential data in the samples in the study area, which is crucial for understanding the mobility of As. Under oxidizing conditions, the migration of arsenic is most likely to occur in a coarse aquifer because the adsorption process of secondary aluminum iron, manganese oxide, and hydroxide minerals will enhance this process. However, the adsorption surface area of the particles in the coarse aquifer is smaller, so the possible mobility of arsenic is lower (Erickson. 2005). Under reducing conditions, As gets released into groundwater and may be more prone to dissolution and desorption through reducibility of metal hydroxides. Ni (Ni et al. 2016)and Gan (Gan et al. 2014) observed this process in the Hetao basin and the Jianghan Plain in China. All these reports indicate that the oxidation-reduction potential is an essential factor for understanding the migration and should be analyzed in more detail in the Karamay area.

Finally, the dilution of groundwater caused by water injection and exploitation of oil in the Karamay area and the Irtysh River Diversion to the Urumqi Project also made led to low levels of arsenic in the groundwater. The Karamay area is a typical resource-based area. According to estimates, oil can be exploited for at least a hundred years. Nevertheless, oil resources are limited will be exhausted one day.

In order to realize the sustainable development of Karamay, to make its economic structure more reasonable, and avoid the market risk caused by the development of a single petroleum industry, the research has begun to develop modern large-scale agriculture in recent years, which has been identifed as a new economic growth point and an alternative industry to establish after the exhaustion of oil resources (Lu et al. 2011). Irrigation water in the area is drawn from the Irtysh River. With the deepening of the development of the large agricultural area, the groundwater level continues to rise (Chen. 2016). Besides, nearly 87% of China’s oilfelds adopt the development method of water injection (Luo et al. 2016). The water injection source of the Karamay Oilfeld is clean water or mixed produced/freshwater. Irrigation water source and oilfeld water injection both use surface water with arsenic content not exceeding the standard, which will also lead to low arsenic content in groundwater in this area (Liu et al. 2017).

5. Conclusion

The arsenic content in groundwater in the Karamay area is lower than the safe drinking water limit (10 µg/L) recommended by the World Health Organization and by China. The arsenic levels were also signifcantly lower than the surrounding Kuitun River Basin and Manas River Basin. Our study yielded the following conclusions:

1. The low arsenic concentration in groundwater within the study area can be attributed to the combined reasons: 1) natural processes and water

geochemistry: groundwater is mainly of HCO3·SO4-Ca type with near-neutral pH, 2) lack of a typical arsenic source of geogony, the sediment composition of aquifers: the interaction time between groundwater and the coarse inert sediment layer is short, 3) the result of water injection during oil exploitation in the Karamay area and the dilution of the groundwater caused by the Irtysh River Diversion to Urumqi Project. 2. The lack of the Middle and Upper Permian strata is a favorable condition for the characteristics of the high-energy sedimentary environment in this area.

However, a better investigation of the correlation of redox conditions, mineralogy, and geochemistry of the Permian-Triassic strata is necessary to fully understand the reasons behind low As levels in the Karamay area. As the population increases and the demand for water resources grows, building reservoirs for storing high quality groundwater can ensure uninterrupted water supply in areas like Karamay.

Declarations

Acknowledgement

This study is fnancially supported by the National Natural Science Foundation of China (41762018) and “Tianshan Youth Program” of Xinjiang Science and Technology Department (2020Q079). Special thanks are given to the anonymous reviewers for their constructive comments.

Ethical Approval

Our work has nothing to do with ethical issues.

Consent to Participate and publish

All authors have read and agreed to the published version of the manuscript.

Authors Contributions

Page 10/15 Conceptualization, Q. L. ; methodology, Y. J.; software, A.M.; validation, Q.L. and H.T.; formal analysis, Q.L. and Y.J.; investigation, Q.L. and J. J.; resources, Q.L.; data curation, W.Y.; writing—original draft preparation, Q.L.; writing—review and editing, Q.L.; supervision,Q.L. ; project administration, Q.L.; funding acquisition, Q.L.

Competing Interests

The work described here has not been submitted elsewhere for publication, in whole or in part, and all the authors listed have approved the manuscript that is enclosed.The authors declare no confict of interest.

Availability of data and materials

All the data and materials we used in this paper were obtained through feld surveys and tests in qualifed laboratories.

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Figures

Figure 1

Areas with high arsenic content in Kuitun river Basin and Manas River Basin and the location of the Karamay area. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Page 12/15 Figure 2

Geological map（a） and stratigraphic columnwith a detail (b) of the study area. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Page 13/15 Figure 3

Arsenic concentrations in surface and groundwaters of the study area Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Page 14/15 Figure 4

Piper diagram for surface and groundwaters of the Karamay area.

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