Identification of arsenic spatial distribution by hydrogeochemical processes represented by different ion ratios in the Hohhot Basin, China

The Hohhot Basin, a typical inland basin of the Yellow River Basin in China, has high concentrations of arsenic (As) in its shallow groundwater, while the factors dominating the distribution of high arsenic levels remain to be further identified. An analysis of the ratio of hydrogeochemical compositions can help to reveal the spatial characteristics of the shallow groundwater environmental conditions and the distribution of high-arsenic water (As >10 μg/L). In this study, a total of 170 samples of shallow groundwater in the Hohhot Basin were collected and water samples with As >10 μg/L accounted for 29.4% of the total. Based on the slope changes of the cumulative frequency curves of (HCO3− + CO32−)/SO42−, Ca2+/(HCO3− + CO32−), Ca2+/Mg2+, and Na+/Ca2+, the groundwater in the study area can be categorized into six different zones according to the environmental characteristics including redox condition, water recharge intensity, and cation exchange level. The result shows that the groundwater in the front of the piedmont alluvial plain and platform is in a weak reducing condition with high lateral recharge intensity, fast runoff, and weak cation exchange. In the Dahei River alluvial plain, which serves as the groundwater discharge zone, the groundwater runoff is sluggish with poor lateral recharge, sufficient exchange between cations in the groundwater and the aquifer matrix, and enhanced reducibility. The degree of oxidation increased in the groundwater near the Hasuhai Lake and the drainage canal, which adverse to the arsenic enrichment. High-arsenic groundwater is mainly distributed in aquifers of (HCO3− + CO32−)/SO42 > 10, Na+/Ca2+ > 13, and Ca2+/(HCO3− + CO32−) < 0.1, which represent the strong reducing condition, low surface water recharge intensity, and strong cation exchange condition. Reductive dissolution of iron oxide, strong evaporation and concentration process, and competition from phosphate in aquifers jointly lead to the release of arsenic into groundwater.


Introduction
Arsenic, a common nonmetallic element in groundwater, can cause neurological and skin diseases if ingested at high concentrations. Long-term exposing to the high arsenic can lead to increased cancer risks to the liver, bladder, and other organs (Chen and Ahsan 2004;Sheng et al. 2021;Silvera and Rohan 2007). Hence both the World Health Organization (WHO) and China set the standard that the guideline value of arsenic in drinking water is no more than 10 μg/L. High-arsenic groundwater were widely reported in more than 70 countries such as Bangladesh, India, the United States, and China, affecting approximately 100-200 million of global population (Fendorf et al. 2010;Mandal and Suzuki 2002;Wang et al. 2019).
The formation of high-arsenic groundwater is mainly caused by geological origin and human activities  (Nordstrom 2002). High-arsenic groundwater from geological origin primarily occurs in rivers and lake basins with arid or semi-arid climate (Li et al. 2022b) or river deltas, which can be enriched by means of leaching enrichment, burial dissolution, compaction release, and evaporation concentration (He et al. 2020;Wang et al. 2020). The distribution of high-arsenic groundwater in Bangladesh and China indicated that it mainly occurred in the anaerobic reductive environment of Holocene strata (Guo et al. 2011;Zheng et al. 2004). The higharsenic groundwater generally existed in a weak alkaline environment with high Fe and Mn concentrations. In the reducing environment, arsenic could be released into the groundwater through the reduction dissolution of iron and manganese oxides (Guo et al. 2008;He et al. 2020). However, the chemical characteristics of groundwater differ between inland basins and delta areas. For example, HCO 3 − , Na + , and TDS contents are high in Datong Basin and Hetao Basin, while the distribution of cations is dispersive in high-arsenic river deltas (Guo et al. 2014a;He et al. 2020;Xie et al. 2009).
China is severely affected by high-arsenic groundwater, especially in the Yellow River basin, including Yinchuan Basin (Guo et al. 2014b), Hetao Basin (Guo et al. 2014a), and Xinxiang area which locates in the lower reaches of the Yellow River (Ren et al. 2021). The Hohhot Basin is located in the upper and middle reaches of the Yellow River basin and its shallow groundwater has been widely used for agriculture but with a high arsenic level up to 300 μg/L (Guo et al. 2014a;L. et al. 2003). The geomorphic characteristic results in high-arsenic groundwater of the Hohhot Basin mainly exist in the river flat.
Considering the great spatial variability of the arsenic occurrence, scholars have adopted different approaches to identify the characteristics of arsenic spatial distribution. In the Hetao Basin, indicators including the soil-sand ratio and the number of clay layers are positively correlated with the concentration of arsenic, making it convenient to identify high-arsenic areas (Cao et al. 2017). The concentrations of stable isotopes of carbon, hydrogen, and oxygen are likewise positively correlated with the arsenic content in groundwater (Zhou et al. 2018); Fe and S isotopes also gather in high-arsenic groundwater (Guo et al. 2013a;Wang et al. 2014). The ratio of different ions can indicate the hydrogeochemical formation of the groundwater under certain circumstances (Jia et al. 2017) in that Ca 2+ / (HCO 3 − + CO 3 2− ) could show the intensity of lateral recharge while SO 4 2− /Cl − and HCO 3 − /SO 4 2− reflect the degree of groundwater reducibility in arid and semi-arid areas (Cao et al. 2017;Jia et al. 2014). Because of the great stability of major hydrogeochemical compositions of the groundwater, it is relatively accurate to conclude groundwater environmental characteristics from analysis of specific ion ratios. The cumulative frequency distribution curves of various ion ratios demonstrate the accumulative process of different groundwater ratios from 0 to 100%. The obvious changes of the slope in the cumulative frequency curves mean that the distribution range of the ratio has changed greatly, revealing a significant change of the groundwater environment (McMahon 2004;Reimann et al. 2005). Therefore, changes in the slope of cumulative frequency curves can help divide the study area for research purposes.
In this paper, the shallow groundwater in the Hohhot Basin is divided into several zones based on the arsenic spatial distribution characteristics and the slope changes in cumulative frequency curves of four ionic ratio types, which serves to (1) analyze the different types of hydrogeochemical processes in the shallow groundwater of the study area; (2) provide a new method for identifying the spatial distribution of high arsenic content by cumulative frequency curves of ion ratios; and (3) investigate the mechanism of arsenic enrichment in various zones divided by different ion ratios.

Geographic feature
The Hohhot Basin is located in Inner Mongolia of China, where the terrain slopes from northeast to southwest in general and from north to south in the west (Fig. 1). The plain is narrow in the west and wide in the east, and the Yellow River flows east along the southern edge of the plain. The eastern plain is covered by alluvial deposits of the Dahei River and thick lake deposits gather in the lower part. On the eastern part of the plain lies the Manhan Mountain, southern part the Horinger platform, and northern part the Daqing Mountain. The flat terrain slightly inclines to the west and the surface lithology is mainly Holocene sandy loam in the Dahei River alluvial plain.

Hydrology and meteorology
The Hohhot Basin is in an arid and semi-arid climate with obvious seasonal variations, meaning long winter time and short summer time. This climate is characterized by low precipitation, high evaporation, and high diurnal temperature. The Dahei River is a tributary of the Yellow River in the study area, consisting of three parts: the mainstream of Dahei River in the east, the tributaries in the west, and Hasuhai Lake and drainage canal.

Hydrogeological condition
There are two kinds of aquifer types in the Hohhot Basin. The monolayer aquifer is distributed in front of Daqing Mountain, Manhan Mountain, and the northern edge of the Horinger platform. They consist of coarse grains and have good hydraulic connections due to devoid of stable and continuous waterproof layers. The double-layer aquifer is distributed in front of the piedmont inclined plain and alluvial plain of the basin center.
In the monolayer aquifer, the shallow groundwater is mainly recharged by the lateral runoff from the groundwater in the mountainous area and discharged by flowing to the double-layer aquifer and artificial exploitation. The shallow groundwater in the double-layer aquifer is primarily recharged by lateral runoff from the monolayer aquifer, and secondarily recharged by other sources including precipitation, canal system infiltration, irrigation infiltration, and leaking recharge from confined groundwater.
The shallow groundwater in the piedmont plain mainly flows from northeast to southwest, east to west, or southeast to northwest. In the piedmont alluvial plain, the aquifer matrix consists of coarse particles, resulting in a large permeability coefficient and hydraulic gradient. Meanwhile, the runoff rate is fast and the shallow water is mainly discharged by runoff. When entering the Dahei River alluvial plain, the groundwater mainly flows from northeast to southwest. Due to the gentle topography and fine particles of the aquifer in the alluvial plain, the hydraulic gradient of groundwater decreases and the groundwater runoff rate becomes very slow.

Field sampling
The study area is an important agriculture district of the Inner Mongolia, China. In this study, a total of 170 groundwater samples were collected in 2016. All samples were from shallow water, with well depths less than 100 m. The shallow groundwater mainly has been used for the irrigation in this area.
The water table values were determined before sampling in the beginning, followed by extracting the groundwater which volume was more than 3 times of the volume of water stored in the wellholes in order to ensure the accuracy of the test, except those used with large daily water volume that were directly pumped for sampling. The sampling depth was 0.5 m below the groundwater surface. During the sampling process, the water sample containers were cleaned 3 times with well water at the beginning and then filled with water samples, ensuring that no gaps were left at the top after the protective agent was added. Two-milliliter HCl per 500 mL sample was added to ensure the pH <2, which was used for determining the trace elements. The water sample containers were tightly capped immediately, sealed with tapes, and labeled. All water samples were stored in the environment of 4 °C, and analyzed and tested in the laboratory within 7 days.

Site test
The water temperature, pH conductivity (EC), and total dissolved solids (TDS) were tested by sensION+MM150(MM156) portable multi-parameter water quality analyzer produced by HACH Company on the sampling day, and the turbidity was tested by WGZ-200B portable turbidimeter (meter) produced by Shanghai Xingrui Company.

Laboratory test
K + , Na + , Ca 2+ , Mg 2+ , Fe, Mn, and other cations were measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and inductively coupled plasma-mass spectroscopy (ICP-MS) in the laboratory of the Institute of Hydrogeology and Environmental Geology (IHEG), Chinese Academy of Geological Sciences (CAGS). The arsenic content in groundwater was measured by inductively coupled plasma-mass spectroscopy (ICP-MS) (7500C, Agilent). The error range of cation and anion concentration for all samples was less than 5%, ensuring the accuracy of all data.

Division method of the cumulative frequency curve
The cumulative frequency curve can show the accumulative distribution in the ratios of different hydrogeochemical compositions. As the ratio grows and the number of samples increases, the cumulative frequency increases gradually from 0 to 100%. All data would form several straight lines with different slopes based on the inflection point (threshold) at which the ratio exhibits a significant increase. The significant variations of the line slopes indicate a change of the groundwater environment and hydrogeochemical processes (María et al. 2007). In this study, the cumulative frequency curves of ion ratios were plotted, and the curves were divided into several ratio ranges according to different slopes, which were displayed on the map. Finally, according to the ranges and spatial distributions of ratios, the differences of hydrogeochemical components in groundwater were determined, revealing the specific chemical processes in groundwater of different areas.

Hydrogeochemical distribution of groundwater
The concentration ranges of ions and TDS in 170 samples differed from 2 to 3 orders of magnitude (Table 1), indicating strong spatial variability of shallow groundwater in the study area. The pH of shallow groundwater in the Hohhot Basin was slightly alkaline, ranging from 7.25 to 9.05. The types of anions and cations with the highest concentration were HCO 3 − and Na + , with a median value of 378 and 98.3 mg/L, respectively. TDS ranged from 262 to 6.11 × 10 3 mg/L, with a median value of 579 mg/L, and saline water (TDS >1 g/L) accounted for 28.8% of the total.
The Ca 2+ concentration was 5.70-260 mg/L, and the groundwater with a high Ca 2+ content was mainly distributed in the piedmont plain of Daqing Mountain and Manhan Mountain with strong runoff. The Na + concentration ranged from 9.93 to 2.10 × 10 3 mg/L, and the groundwater with high Na + concentration was distributed in the alluvial plain of Dahei River, which is the main discharge region in the study area. Meanwhile, Na + and TDS showed a linear positive correlation (R 2 = 0.898), indicating that high TDS in groundwater was affected by the increase of Na + concentration (Fig. S1). The piper diagram showed that water chemical types were correlated with arsenic concentration.

Spatial distribution of arsenic
The overall arsenic concentrations in the groundwater of the Hohhot Basin ranged from 0.05 to 357.5 μg/L, with an average value of 26.9 μg/L and a median value of 3.15 μg/L (Table 1). High-arsenic groundwater (As >10 μg/L) accounted for 29.4% of the total. To further identify the distribution characteristics of arsenic in shallow groundwater of different geomorphic units, the 170 water samples were divided into three groups according to different arsenic concentrations, namely, 0-10, 10-50, and >50 μg/L. An analysis of the concentration and spatial distribution of arsenic in the study area revealed that the distribution of arsenic was affected by the sedimentary environment. Most of the arsenic groundwater was distributed in the alluvial plain of Dahei river, and a small part was distributed along the Yellow River and near Hasuhai Lake (Fig. 3). Specifically, water with arsenic levels above 50 μg/L were concentrated in the alluvial plain of Dahei River, while the arsenic levels in other areas were generally below 10 μg/L. The low-arsenic groundwater was mainly distributed in wells with depths of 0-70 m but the high-arsenic groundwater is concentrated in wells of 0-40-m depth, indicating that the shallower aquifers are more favorable for arsenic enrichment in groundwater.

Cumulative frequency curves and distribution characteristics of ion ratios
In the reducing environment of arid and semi-arid areas, sulfates are converted into S 2− under the reactions of microorganisms, and the concentration of HCO 3 − increases through the oxidative transformation of organic compounds (Guo et al. 2008;L. et al. 2003). In this climate, (HCO 3 − + CO 3 2− )/SO 4 2− increases with the enhancement of groundwater reducibility, meaning that it can be used as an indicator of the groundwater redox status. Most groundwater samples in Group 1 were distributed around Hasuhai Lake and the drainage canal and a small part were distributed in the southeast of the study area. Samples in Group 2 and 3 were mainly distributed in the piedmont areas of Daqing Mountain in the north, Horinger platform in the south, and Manhan Mountain in the east. Samples in Group 4 were mainly distributed in the groundwater discharge zone of the Daqing Mountain Fig. 2 Piper diagram of shallow groundwater in the study area piedmont alluvial plain front and the Dahei River alluvial plain (Fig. 4).
The Ca 2+ content in groundwater can indicate the degree of recharge by surface water (Cao et al. 2017). Meanwhile, increasing of Ca 2+ /(HCO 3 − + CO 3 2− ) could attribute to the dissolution of silicate minerals (Gao et al. 2021a). The piedmont lateral recharge could also cause augment of Ca 2+ /(HCO 3 − + CO 3 2− ) in the Hohhot Basin since Manhan Mountain is mostly composed of silicate minerals such as quartz and mica. Overall the Ca 2+ / (HCO 3 − + CO 3 2− ) content increases with the enhancement of surface water recharge intensity or lateral recharge intensity of piedmont area. According to the slope change of the Ca 2+ /(HCO 3 − + CO 3 2− ) cumulative frequency curve, the study area can be divided into four groups (Fig. S2). Group 1 samples were distributed in the west of the Dahei River plain, while Group 2 samples were located in the east of the Dahei River, the Horinger platform front, and the front of the Daqing Mountain piedmont alluvial plain. Group 3 samples were mainly distributed in the Daqing Mountain piedmont alluvial plain and east of the Horinger platform. Group 4 samples were mainly distributed in the Manhan Mountain piedmont alluvial plain. Along the flow direction of the groundwater from the piedmont plain to the river plain, the Ca 2+ /(HCO 3 − + CO 3 2− ) index gradually (2) + ∕ − + − has been decreased, indicating that the groundwater in the discharge area was less affected by surface water or lateral recharge.
The dissolution and leaching in various lithologic aquifers result in the difference of Ca 2+ /Mg 2+ ratio in water. It indicates that the groundwater is dissolved by carbonate such as calcite and dolomite when the ratio is close to 1~2 (McMahon et al. 2018), while the further increase of the ratio reflect that the groundwater is also affected by the leaching of silicate minerals (Asare et al. 2021;Xiao et al. 2016). Consequently the Ca 2+ /Mg 2+ ratio also reflects the intensity of groundwater lateral recharge, in that the increase of the former can manifest the enhancement of the latter. Based on the slope changes of the Ca 2+ /Mg 2+ cumulative frequency curve (Fig. S3), the study area was divided into four groups: 1 (0-0.4), 2 (0.4-1.4), 3 (1.4-2.2), and 4 (>2.2). Group 1 water samples were mainly distributed in the Dahei River alluvial plain and near the Hasuhai Lake. Group 2 groundwater samples were mainly located in the Daqing Mountain alluvial plain, the eastern Dahei River alluvial plain, and near the Horinger platform. Groups 3 and 4 groundwater samples were mainly distributed in the piedmont plain of the Manhan Mountain. Cation exchange between groundwater and minerals of aquifers occurs when local runoff conditions are poor. The Na + /Ca 2+ ratio can indicate the cation exchange degree in groundwater . With the enhancement of cation exchange, both the Na + content and the ratio of Na + / Ca 2+ gradually increase. According to the slope change of , distribution of ratio groups, and hydrochemistry types the Na + /Ca 2+ cumulative probability curve, the study area can be divided into four groups: 1 (0-1.1), 2 (1.1-2.4), 3 (2.4-13), and 4 (>13) (Fig. S4). Group 1 groundwater samples were mainly distributed in the vicinity of the Hasuhai Lake, the drainage canal, and the alluvial plain of the Manhan Mountain. Group 2 groundwater samples were mainly located near the Daqing Mountain alluvial plain and the southern Horinger platform. Group 3 groundwater samples were widely distributed in the margin of the Dahei River alluvial plain, while Group 4 groundwater samples were concentrated in the discharge center of the Dahei River alluvial plain.

Recognition of arsenic spatial distribution characteristics by different ion ratios
Since the occurrence of arsenic is significantly affected by hydrogeochemical processes in the groundwater environment, the spatial distribution characteristics of different ion ratios can not only help to identify hydrogeochemical processes in groundwater, but also effectively facilitate identifying the distribution mechanism of arsenic in the study area. In the light of the concentration characteristics and spatial distribution of different ion ratios in the foregoing sections, the groundwater in the study area was divided into six zones according to the final boundary that was set according to boundaries of overlapping areas of the four ion ratios (Fig. 5e). The Manhan Mountain piedmont alluvial plain zone (Zone I) was characterized by Na + /Ca 2+ < 1.1, Ca 2+ / (HCO 3 − + CO 3 2− ) > 0.7, and Ca 2+ /Mg 2+ > 1.4, located in the front edge of Manhan Mountain (Fig. 5a). The median value of Ca 2+ concentration in this zone was 69.7 mg/L, which was the highest among the six zones (Table S1). The high Ca 2+ concentration in Zone I was mainly caused by strong piedmont lateral recharge considering the geomorphological characteristics of this area. The median Na + content was 33.7 mg/L, which was the lowest among six zones, indicating a weak cation exchange and a good groundwater runoff condition in this area. The mean and median values of arsenic concentration in this region were 1.08 and 0.785 μg/L. The arsenic concentrations of all samples were less than 10 μg/L in Zone I, which is the lowest arsenic distribution area among the six regions.
The Horinger platform zone (Zone II) was characterized by groundwater with ion ratio 0.4 < Ca 2+ /Mg 2+ < 1.4, 2 < (HCO 3 − + CO 3 2− )/SO 4 2− < 5, and 0.1 < Ca 2+ / (HCO 3 − + CO 3 2− ) < 0.5 in the northern front of Horinger platform (Fig. 5b). The median values of Na + , Ca 2+ , and Mg 2+ concentration in this zone were 165, 40.7, and 45.2 mg/L, respectively (Table S1). The values of Ca 2+ /Mg 2+ and Ca 2+ /(HCO 3 − + CO 3 2− ) indicated that the Ca 2+ content in this area was relatively lower than that in the vicinity of the Hasuhai Lake and the Manhan Mountain, but higher than that in the Dahei River alluvial plain, representing that the groundwater was weakly recharged from the platform region. The value of (HCO 3 − + CO 3 2− )/SO 4 2− indicated that the redox condition of groundwater is in a weak reducing state. The average arsenic concentration of groundwater in this area was 3.33 μg/L, and only one water sample with As >10 μg/L was distributed in Zone II.
The Daqing Mountain piedmont alluvial plain zone (Zone III) was located in the alluvial plain in front of Daqing Mountain (Fig. 5b) with ion ratios of 2 < (HCO 3 − + CO 3 2− )/SO 4 2− < 5 and 0.5 < Ca 2+ /(HCO 3 − + CO 3 2− ) < 0.7. The ion ratio indicated that the groundwater in this place was under a weak reducing and medium runoff intensity condition. The groundwater was affected by the lateral recharge in front of Daqing Mountain, which was stronger than the lateral recharge intensity in the Horinger platform. The mean value of arsenic concentration in this area was 17.8 μg/L, and the groundwater had a low arsenic level near the mountain front but a relative high arsenic level near the Dahei River alluvial plain.
The transition zone (Zone IV) was represented by ion ratios of 0.4 < Ca 2+ /Mg 2+ < 1.4, 5 < (HCO 3 − + CO 3 2− )/ SO 4 2− < 10 and 0.1 < Ca 2+ /(HCO 3 − + CO 3 2− ) < 0.5. The transition zone was located between the piedmont alluvial plain and the Dahei River alluvial plain (Fig. 5c). The ion ratios demonstrated that the groundwater in Zone IV was affected by a medium lateral recharge and medium reducing environment that was stronger than Zone II and Zone III. The median value of arsenic concentration was 17.9 μg/L and 47.4% of water samples had arsenic levels higher than 10 μg/L. The Dahei River alluvial plain zone (Zone V) was mainly situated in the groundwater discharge area of Dahei River alluvial plain (Wen et al. 2013) in view of the distribution range of (HCO 3 − + CO 3 2− )/SO 4 2 > 10, Na + /Ca 2+ > 13, and Ca 2+ /(HCO 3 − + CO 3 2− ) < 0.1 (Fig. 5d). The ratio of groundwater with (HCO 3 − + CO 3 2− )/SO 4 2− > 10 was the highest in the study area, indicating the strongest reducibility of groundwater in the Hohhot Basin. The median values of Ca 2+ and Na + concentrations were 28.2 and 215 mg/L, which, respectively, were the minimum and maximum values among all zones (Table S1). The ratios of Ca 2+ /(HCO 3 − + CO 3 2− ), Na + /Ca 2+ , and ion content jointly indicated that the surface water recharge and lateral recharge effect of Zone IV were both the weakest in the whole study area, and neither the Hasuhai Lake nor the piedmont groundwater can recharge to the groundwater of this area. At the same time, the cation exchange level between Ca 2+ in the aquifer and Na + in the groundwater was sufficient. The average arsenic concentration in the groundwater of Zone V reached 91.6 μg/L, which was significantly higher than that in other zones.
The drainage canal zone (Zone VI) consisted of groundwater samples with Na + /Ca 2+ < 1.1 and (HCO 3 − + CO 3 2− )/ SO 4 2− < 2 that was mainly distributed in the east of the Hasuhai Lake and the drainage canal (Fig. 5c). The mean value of arsenic concentration in the groundwater of this area is 10.0 μg/L. The ratio of (HCO 3 − + CO 3 2− )/SO 4 2− in Fig. 5 Ion ratio distribution and regional recognition in shallow groundwater Zone VI is the smallest in the study area, indicating the strongest oxidation degree of groundwater. Studies have shown that the groundwater could be affected by the water quality of its nearby lakes and canals (Guo et al. 2011;Jessen et al. 2012). Groundwater would mix with surface water of higher oxygen content, resulting in increased oxidation of the groundwater. Besides, the Na + /Ca 2+ ratio of Zone VI is the smallest in the study area, indicating an extremely weak cation exchange between Ca 2+ and Na + in the aquifer and a good runoff condition. It is worth noting that although the Na + /Ca 2+ value in Zone VI is the smallest in the whole study area, the median and mean value of Na + concentrations were 203 and 298 mg/L, respectively, which were only lower than that in Zone V among the six zones. Meanwhile, TDS, Mg 2+ , and SO 4 2− concentrations were the highest in the study area, and the median and average values of Ca 2+ reached 64.3 and 88.9 mg/L, only lower than that of Zone I (Table S1).
Compared with other ions and ratios of high-arsenic areas, the ions of the study area were less than that in the Hetao Basin but intensities of cation exchange and evaporation were much more than other areas like Datong Basin and Yinchuan Basin, which attribute to the same environment of slow groundwater flow and arid climate (Guo et al. 2013b). It can be concluded that the distribution of arsenic and ion ratios in groundwater had significant consistent characteristics. The median values of arsenic concentration in the six zones were 0.785, 2.76, 2.87, 4.30, 54.3, and 2.48 μg/L, respectively (Fig. S5a). It can be found that the total arsenic content of Zone V was significantly higher than that of other zones, while the total arsenic content in Zone I was the lowest. The arsenic concentrations of Zones II, III, IV, and VI were roughly similar, between which in Zones V and IV. The result of ion ratios revealed that Zone V has been affected by the weakest lateral recharge but the strongest reduction and cation exchange effect. In contrast, Zone I was influenced by the strongest piedmont lateral recharge effect and the best runoff condition among the six zones.
Therefore, the occurrence characteristics of arsenic can be identified by the value distribution of the four kinds of ion ratios (Table 2). High-arsenic groundwater was distributed in the groundwater discharge zone of the Dahei River alluvial plain. This zone was in a strong reducing environment with high (HCO 3 − + CO 3 2− )/SO 4 2− , low Ca 2+ /(HCO 3 − + CO 3 2− ), and low Ca 2+ /Mg 2+ , indicating that it was not affected by surface water or lateral recharge. Meanwhile, under the poor runoff conditions of the groundwater, the increase of Na + / Ca 2+ ratio represented a strong cation exchange. The lowarsenic area, which was affected by recharge from piedmont and the Hasuhai Lake, had good groundwater runoff which was not conducive to arsenic release.

Hydrogeochemical processes in different zones
Zone V consisted of the highest arsenic concentration groundwater, with the shallow depth to the water table ranging between 1.33 and 7.83 m and a median value of 2.68 m ( Figure S5c). The median TDS in this zone reached 901.3 mg/L, which was only lower than that in Zone VI. The median depth to the water table and TDS in Zone VI were 2.7 m and 1416 mg/L, respectively (Table S1). Zones V and VI showed the shallowest water level in all zones (Fig. S5a, b). There was a negative relationship between arsenic content and water level overall except the low arsenic content and water level in Zone VI (Fig. 6a). In other words, the shallower groundwater could generally provide the environment which was conducive to the enrichment of arsenic level. Therefore, it suggested that strong evaporation and concentration mechanism occurred in Zones V and VI (Liu et al. 2017;Smedley and Kinniburgh 2002), and arsenic was further enriched under evaporation concentration (Wen et al. 2013). This also explained that although cation exchange interaction intensity was high in Zone IV, the Na + content was still significantly higher than that in the piedmont plain and platform. On the contrary, the median value of the groundwater table and TDS reached 14.09 m and 477.1 mg/L, respectively, in Zone I, meaning that the groundwater was not affected by evaporation and TDS also showed desalination characteristics ( Fig. S5c and Table S1). Arsenic and iron concentrations in groundwater were positively correlated (R 2 = 0.0554) (Fig. 6b), indicating that the release of arsenic was related to the increase of iron content. It showed that the groundwater in Zone V had the strongest reducing condition and also the highest concentration of iron in the basin, with a median concentration of 0.48 mg/L (Table S1). Under the reducing condition, iron oxide gained electrons and dissolved as ferrous ions into groundwater through reductive dissolution. Consequently, arsenic adsorbed on iron oxide was released into the groundwater (L. et al. 2003;Li et al. 2022a;Luo et al. 2012). The low-arsenic areas were affected by the recharge of oxygenbearing water and had a low iron content.
Ammonia-nitrogen and nitrate-nitrogen concentrations were greatly affected by redox conditions, meaning that they could be considered as sensitive indicators of redox state. The transformation of nitrogen in the groundwater was also significantly different. Arsenic concentration was negatively correlated with nitrate concentration but positively correlated with ammonium concentration (Fig. 6c,  d). In other words, nitrate concentration was significantly lower in high-arsenic groundwater while ammonium concentration was higher in low-arsenic groundwater (Guo  (Fig. S5a,e, and f). The NO 3 − concentration was found to be the highest in Zones I and VI, where oxidation was the strongest, with the median concentration reaching 28.8 and 42.2 mg/L, respectively. Meanwhile, the median ammonium concentration in these two zones (0.02 mg/L and 0.04 mg/L) was significantly lower than that of other zones. The groundwater in Zones VI and I were affected by oxygenated water recharge from the Hasuhai Lake and the drainage canal as well as lateral recharge from the Manhan Mountain, and therefore, the oxidation of Zones I and VI were significantly higher than that in other zones (Guo et al. 2010).
Ammonia-nitrogen in oxidized groundwater would be converted to nitrate nitrogen by nitrification (Rotiroti et al. 2017). Low nitrate-nitrogen (median 3.08 mg/L) and high ammonium nitrogen (median 0.16 mg/L) were found in Zone V. Compared with the groundwater quality of Manhan Mountain piedmont plain, the concentration of ammonia nitrogen in Zone V significantly increased while the concentration of total inorganic nitrogen decreased (Fig. S6). Groundwater was in an anaerobic state under the strong reducing condition of Zone V. In the presence of organic compounds, nitrate-nitrogen was not only released N 2 through denitrification but also transformed into ammonium nitrogen through dissimilatory nitrate reduction to ammonium (DNRA) reaction (Gao et al. 2021b;T. et al. 2011).
Meanwhile, the median concentration of HPO 4 2− in Zone V reached 1.2 mg/L, which was significantly higher than that in other zones ( Fig. S5d and Table S1). The release of phosphorus (P) under anoxic conditions was mainly controlled by the reductive dissolution of iron oxide (Li et al. 2022c). The high phosphate concentration promoted the release of arsenic in groundwater by competing for adsorption sites (Smedley and Kinniburgh 2002). There was no evident correlation between F and As in the study area (Fig. 6e). Highly fluoridated groundwater was mainly found in Zones II and V, where the Na + level was high. Because Ca 2+ and F − in groundwater were able to form fluorite precipitation, high Ca 2+ was not conducive to the enrichment of fluoride. Arsenic enrichment was enhanced by weak evaporation and concentration due to the shallow water level, but the weak runoff and oxidation environment inhibited the release of arsenic into groundwater in Zones II and III. The position and ion ratios of Zone IV manifested that the environment of the transition zone slightly promoted the release of arsenic. Thus, as the aquifer environment altered along the direction of Fig. 7 Hydrogeochemical model of shallow groundwater in the study area groundwater flow, the arsenic release mechanism and the hydrogeochemical reaction model changed accordingly (Fig. 7).

Conclusion
In this study, the slope change thresholds of the cumulative frequency curves of (HCO 3 − + CO 3 2− )/SO 4 2− , Ca 2+ / (HCO 3 − + CO 3 2− ), Ca 2+ /Mg 2+ , and Na + /Ca 2+ were confirmed to identify the spatial ranges of different types of physical and chemical processes in the shallow groundwater of the Hohhot Basin. The groundwater in the Basin was divided into six zones accordingly. The arsenic concentration in the groundwater of the Manhan Mountain piedmont alluvial plain zone was 0.785 μg/L (median value, the same below), and all its samples had an arsenic level of less than 10 μg/L. The groundwater in the Horinger platform zone was receiving platform recharge, with an arsenic concentration of 2.76 μg/L. The groundwater in the Daqing Mountain piedmont alluvial plain zone had an arsenic concentration of 2.87 μg/L. Samples collected in The transition zone had an arsenic concentration of 4.30 μg/L. The groundwater in the Dahei River alluvial plain zone gathered in the discharge zone of the Dahei River alluvial plain, possessing the highest arsenic distribution in the basin. The median value of its arsenic concentration was 54.3 μg/L and the arsenic concentration of 79.5% of samples collected in this zone exceeded 10 μg/L. The drainage canal zone was located near the Hasuhai Lake and the drainage canal, with an arsenic concentration of 2.48 μg/L.
In the Dahei River alluvial plain, the groundwater runoff was slow and the lateral recharge was weak. Arsenic was released into the groundwater through the reductive dissolution of iron oxide in this strong reducing environment. At the same time, the strong evaporation and concentration effect due to the shallow water level would further lead to arsenic enrichment. High phosphate concentration also contributed to the release of arsenic by competing with arsenic for adsorption sites. In the piedmont-alluvial plain and platform front area, the groundwater was affected by strong lateral recharge and weak cation exchange. The weak reducing state is not favorable for the dissolution of iron oxide in groundwater. Moreover, the significant increase in water level depth led to weaker evaporation. The evaporation of groundwater near the Hasuhai Lake and the drainage canal was intensive, but the oxidation of groundwater was significantly enhanced by the recharge of surface water with high oxygen content, resulting in difficult enrichment of arsenic.