High-Arsenic Groundwater from Huaihe River Basin, China(cid:0)Geochemistry and Hydrogeochemical Processes

Arsenic (As) poses a danger to environmental health, and drinking arsenic-rich groundwater is a key exposure risk for humans. The distribution, migration, and enrichment of As in groundwater is an important worldwide environmental and public health problem that requires research. Huaihe River Basin has been newly identied as a region of high-arsenic groundwater in China. This study aims to analyze the hydrogeochemical data of high-arsenic groundwater, trace its formation and evolution, and evaluate its potential pollution risks. The results showed that As and F were the main inorganic chemical substances affecting the water quality in the study area, with concentrations of 5.75±5.42 μg L -1 and 1.29±0.40 mg L -1 , respectively, exceeding the recommended drinking water standards of the World Health Organization by 23% and 31%, respectively. The proportion of groundwater with a high As content presents a high exposure risk. According to the hydrochemical diagram and the calculation of mineral saturation indices, the groundwater in the study area underwent evaporation, halite dissolution, and water-rock interaction. The total alkalinity of high-arsenic groundwater ranges mainly between 400–700 mg L -1 , and the chemical type is mainly of HCO 3 -Na. High-arsenic groundwater is largely affected by evaporation and cation exchange. In an alkaline environment, As in high-arsenic groundwater derives from the dissolution and release of arsenic sulde in aquifer sediments and poses a potential threat to human health through food and drinking water.


Introduction
Arsenic (As) is ubiquitous in nature and listed as a Class-I speci c carcinogen by the International Agency for Research on Cancer (IARC) (Shahid et al., 2018; WHO, 2011). The most sensitive toxicity threshold of As concentration in drinking water has not been determined. The recommended limit of As concentration in drinking water is 10 μ L -1 according to the guidelines for drinking-water quality by the World Health Organization (WHO, 2011). According to the United States Environmental Protection Agency (EPA) and the National Research Council (NRC), the long-term consumption of water with As concentrations as low as 5μg/L, or even 3μg/ L, may cause adverse chronic health effects on humans, especially cancer (Taheri et al., 2017). Drinking groundwater rich in As is the main route for human exposure to this element.
Globally, more than 100 million people are exposed to high-arsenic groundwater, including 19 Wang et al., 1998;Zhang et al., 2017). Huaihe River Basin is an area in China where higharsenic groundwater has been newly discovered. Ever since arsenic in drinking-water was discovered in this basin in the 1980s, the enrichment of arsenic in groundwater in this area has received extensive attention, and preliminary hydrogeological environmental surveys and scienti c research have been conducted (Chen et al., 2013;Li et al., 2017;Wen et al., 2013;Zhang et al., 2010). A statistical prediction based on groundwater data from Huaihe River Basin was conducted in 2010. The analysis found that the probability of arsenic exposure risk in Huaihe River Basin was predicted to be greater than 0.40 and the proportion of As concentration exceeding 10 μ L -1 in the monitored wells of villages in the statistical survey area was 17%, with the highest detection value being 620 μ L -1 . Previous research has mainly focused on the hydrogeochemical distribution of high-arsenic groundwater and the geographical distribution of endemic diseases as a consequence water arsenic poisoning through drinking water. Such research has lacked in-depth analyses of the formation processes, evolution mechanisms, and in uencing factors that result in high-arsenic groundwater.
There are many aquifers in Huaihe River Basin. Due to the long-term and large-scale exploitation of groundwater, the environment of the groundwater ow system has changed, potentially leading to the reduction or oxidative dissolution of arsenic minerals in the aquifer, and posing a potential threat to human health through, for example, food and drinking water. Given the extensive harmful effects of arsenic on the natural environment and public health, conducting geochemical studies on arsenic pollution in groundwater of Huaihe River Basin is necessary. This study selected the representative smallscale ow eld of high-arsenic groundwater in the Huaihe River Plain (Taihe County, Anhui Province) to analyze the sources and occurrence of arsenic contaminants and identify the hydrogeochemical processes. This may provide a scienti c basis and engineering reference for the treatment and public health risk control of arsenic contamination.

Study Area
Huaihe River Basin is located in Eastern China. It originates in Tongbai and Funiu Mountains in the west, faces the Yellow Sea in the east. The geographical coordinates are 111°55′E to 121°25′E and 30°55′N to 36°36′N, with an area of 270,000 km². Huaihe River Basin is located in the north and south climatic transition zone in China and belongs to the warm temperate subhumid monsoon climate zone, with an annual average temperature of 11-16℃. Huaihe River Basin is geologically located at the junction of three tectonic units that are the North China Block, the Yangtze Block and the Qinling Orogenic Belt (Zhang et al., 2015). The terrain tilts slightly from northwest to southeast, with alluvial-proluvial plain as the main landform. The terrain is at, with sea-level elevation generally ranging from 15 to 50 meters ( Fig.   1).
Since the Neogene period (23 Ma), loose Neogene and Quaternary sediments of immense thickness have formed in Huaihe River Basin, providing good hydrogeological conditions for the formation and distribution of groundwater in the region. The groundwater system of Huaihe River Basin is divided into shallow, middle and deep aquifer systems, from top to bottom, and groundwater ow runs from northwest to Southeast generally. The shallow groundwater occurs in the Holocene and late Pleistocene strata of 50 m and is in uenced by meteoric precipitation and surface water. The groundwater depth is generally 2-4 m within the limit of evaporation depth, and evaporation is the main drainage route of shallow groundwater. The middle groundwater occurs in the middle and lower Pleistocene strata of 50-150 m, while the deep groundwater mainly occurs in the Neogene strata of 150-500 m (Fig. 1). Due to the deep burial of the middle and deep groundwater (buried depth of more than 50 m), the aquifers that are separated by cohesive soil layers cannot directly receive the recharge from atmospheric precipitation and the runoff is slow. Exploitation is the main method through which deep groundwater gets discharged.

Methods
According to the principle of thermodynamics, the dissolution and precipitation of minerals in water-rock reactions are determined by the saturation index (SI) of various minerals in groundwater. The SI can be used to identify the water quality and hydrochemical evolution process ( The mathematical expression of SI is: where IAP is the ionic active product and Ks is the equilibrium constant of the mineral. SI 0, SI = 0 and SI 0 are the thermodynamic criteria for the dissolution, equilibrium, and precipitation of minerals, respectively, and 0.5 >SI >-0.5 is generally considered as near saturation. According to the analysis of a hydrogeological survey recently conducted in Huaihe River Basin, Taihe County in Anhui Province is a typical high-arsenic groundwater area in the Huaihe River Plain (Fig. 1). For this study, we selected a small-scale zone of high-arsenic groundwater in Maji Town as a natural experimental eld to collect and test groundwater samples. The groundwater sampling points were selected via the grid method, with an accuracy of 1 km × 0.5 km or 1 km × 1 km, and 62 water samples were collected. All the groundwater samples were from the shallow aquifer, which formed during the Holocene to Late Pleistocene, with 4-50 m deep water tables. The aquifers consist of quaternary sandstone, ne sandstone, and siltstone. An instantaneous sampling method was implemented for groundwater sample collection. Before sampling, sample bottles and stopcocks were washed three to ve times with the water to be collected; then, the samples were acidi ed with nitric acid (pH < 2) for the analysis of cations. The pH, temperature and total dissolved solids (TDS) were measured in the eld using portable meters (HANNA, HI8424 THERMO scienti c, ORION) and calibrated using standard solution.
All samples collected and analyzed in this study were from shallow pore groundwater and the porous water system formed during the Holocene to Late Pleistocene. The hydrochemical concentrations of ions such as As, K + , Na + , Ca 2+ , Mg 2+ , Cl -, SO 4 2-, HCO 3 -, F -, and Br -, total alkalinity, and total acidity were determined at the Laboratory of the China Geological Survey Nanjing Center. The As concentration in the groundwater was determined by atomic uorescence spectrometry. The detection limit of As by uorescence spectrometer (AFS-820, China) was 0.05μ L -1 , with < 1.0% precision. Cations (Na + , K + , Ca 2+ , Mg 2+ ) were determined by inductively coupled plasma-atomic emission spectrometry (ICP-OES), while anions (SO 4 2− , Cl − , F -,Br -) were determined via ionhromatography. Total alkalinity and total acidity, and HCO 3 were determined by acid titration.There were 42 in-situ sediment samples collected for X-ray diffraction, determined by X-ray diffractometer (D/MAX 2500, Japan) at the Laboratory of the China Geological Survey Nanjing Center.
For the statistical analysis, SPSS19.0 was used as a platform for the descriptive statistical analysis, correlation analysis,and regression analysis. showing clear spatial variability. The proportion of high-arsenic groundwater samples above 1 μg L -1 reached 74%, nearly 1/4 of which exceeded the threshold of > 10 μg L -1 recommended by the WHO (Fig. 1). The Fconcentration of groundwater was 1.29±0.40 mg L -1 , and 31% of the considered samples exceeded the WHO-recommended limit of 1.50 mg L -1 .
The dominant ions determine the groundwater types. According to the Piper diagram (Fig. 2), the groundwater types in the study area are dominated by HCO 3 Na, followed by HCO 3 Na•Mg, HCO 3 Na•Ca, and HCO 3 Na•Ca•Mg. The high-arsenic groundwater types are dominated by HCO 3 Na (Fig. 2). . The Gibbs diagram is an analytical method that uses the relationship between the ratios of Na/(Na+ Ca) and Cl/(Cl+HCO 3 ), and TDS to re ect the controlling factors of main ions in the water. According to the Gibbs diagram (Fig. 3), TDS in the study area was 722±296 mg L -1 , Cl/(Cl+HCO 3 ) ranged from 0.01 to 0.03, and Na/(Na+Ca) ranged from 0.23 to 0.94. Most of the analytical samples were located in the areas of water-rock interaction and evaporation crystallization (Fig. 3), con rming that the water-rock interaction and evaporation processes have an impact on the formation and evolution of groundwater in the study area.  and Na + above this ratio may have other sources. In this study, the Na/Cl ratios of groundwater samples collected in the entire region were 9.63±57.4 and those of most samples were substantially larger than 1:1, showing signi cant spatial variability. The Na/Cl ratio decreased with the increase of Cl concentration. The Na/Cl ratios of contaminated groundwater (As≥10 μg L -1 ) were 15.7±16.0, above the dissolution line of halite (Fig. 5). It can therefore be inferred that the Na + of groundwater in the study area is not derived only from the dissolution of halite. The groundwater may experience overall strong cation exchange and the ion exchange of high-arsenic groundwater is more signi cant.

Source of arsenic and its mobilization
Under the pH and Eh conditions of the natural environment, arsenic exists mainly as As ( ) in an inorganic oxidation state or As ( ) in a reduction state. Arsenic minerals in sediments usually exist in mineral phases such as arsenate, arsenite, and sul de. There are many possible hydrogeochemical factors that trigger the release of arsenic from the solid phase into the groundwater. Changes in groundwater regime, redox potential (Eh), acidity, and alkalinity (pH) exert an in uence on arsenic in sediments, through the adsorption and resolution process and then affect the concentration of As in water (Chen et  Alkalinity is a chemical measurement of a water's ability to neutralize acids. Alkalinity is also a measure of a water's buffering capacity or its ability to resist changes in pH upon the addition of acids or bases The SO 4 2-in the groundwater could be derived from both gypsum dissolution and sul de oxidation and there was a positive correlation between As and SO 4 2contents in the test samples (correlation coe cient R=0.58) (Fig. 6). The mean concentrations of SO 4 2in the groundwater with As < 3, 3 ≤ As < 5, 5 ≤ As < According to the phase analysis by X-ray diffraction, the main mineral components of the sediments in Due to the long-term exploitation of groundwater in large quantities, the environment of the groundwater ow system has changed, breaking the equilibrium of dynamic exchange between the solid and liquid phases of the aquifers, and triggering the release of arsenic from the solid phase into the groundwater. The dissolution of carbonate minerals usually increases the alkalinity (pH). Under high pH conditions, the oxidation of arsenic-containing sul de leads to the release of arsenic and sulfur into the groundwater,

Conclusion
The formation of high-arsenic groundwater requires the combined action of multiple factors in the process of water-rock interaction, such as the accumulation of arsenic-bearing minerals, the dissolution and precipitation of solid arsenic, and the hydrogeological conditions of arsenic enrichment. Huaihe River Basin is a typical area of high-arsenic groundwater in China. In this study, we selected a typical smallscale region of high-arsenic groundwater in the basin for the natural eld experiment to analyze the formation and evolution of high-arsenic groundwater and trace the source of arsenic and its release mechanism. The concentration of As in groundwater in the study area was 5.75±5.42 μg L -1 , showing clear spatial variability. The proportion of groundwater with a high As content ( 1 μg L -1 ) reached 74%, showing a high exposure risk. According to the analysis of hydrochemical composition, the groundwater in the study area underwent the effects of evaporation, halite dissolution, and water-rock interaction. Cl -, Fand SO 4 2-in the groundwater are partly derived from the dissolution and release of halite, uorite, gypsum, and anhydrite minerals. The chemical type of high-arsenic groundwater is mainly HCO 3 -Na.
High-arsenic groundwater is affected signi cantly by evaporation and cation exchange. The high-arsenic groundwater is of in-situ origin, and arsenic probably derived from the dissolution and release of the primary arsenic in the aquifer sediments. In an alkaline environment, the oxidative dissolution of arseniccontaining sul de is the main process leading to the formation of high-arsenic groundwater.