The provenance of deep groundwater and its relation to arsenic distribution in the northwestern Hetao Basin, Inner Mongolia

High-arsenic (As) groundwater has been widely found throughout the world. The source of groundwater would determine spatial distribution of groundwater As. In order to trace the source of high-As deep groundwater (DGW, depths > 50 m), groundwater, sediments, and local bedrock samples were taken to investigate chemical and isotopic compositions in the Hetao Basin, China. Results showed that 87Sr/86Sr in DGW gradually decreased with the increase in As concentrations along the approximate flow path. In recharge-oxic zone (Zone I), DGW was mainly recharged by fissure water, influenced mostly by weathering of phyllite bedrock and meta-basalt. In groundwater flow-moderate reducing zone (Zone II), DGW was mainly related to incongruent dissolution of feldspar. However, in groundwater flow-reducing zone (Zone III), DGW was partly recharged from shallow groundwater (SGW) with depths < 50 m. The mixing contributions of SGW to DGW in Zone III mostly exceeded 80% during groundwater irrigation season. In Zone I, DGW As concentrations were mostly lower than 50 μg/L due to oxic conditions. In Zone II, the weakly alkaline pH and the decreasing Ca/Na resulting from incongruent dissolution of feldspar caused As desorption, which was the major contribution to As mobilization (As mostly > 200 μg/L). In Zone III, the recharge of SGW introduced labile organic matter to support reduction of Fe(III) oxyhydroxides/oxides and predominantly led to As release into groundwater (As > 300 μg/L). This study has provided insights into the source of high-As DGW and the effect of SGW mixing on As mobilization.


Introduction
In China, high arsenic (As) groundwater has mainly been found in inland basins like the Yinchuan basin, the Hetao basin and Songnen basin [1]. Especially in the Hetao basin, high As groundwater has been observed in both shallow and deep aquifers [1,2]. Since deep groundwater is mainly used as water source for irrigation, groundwater As would enter the food chain, and thus threaten the health of local residents [3]. However, most studies are concentrated on aqueous geochemistry and its roles in As mobilization. With regard to source of high arsenic groundwater, it remains an open question. Strontium isotopes have been widely used to trace the source of contaminants in groundwater due to its isotopic stability [4,5]. Based on evaluating strontium isotope characteristics, this study aims at: (1) characterizing the spatial distribution of 87 Sr/ 86 Sr and the sources of high arsenic in DGW, and (2) revealing the relationship between sources of high arsenic DGW and arsenic mobilization.

Study area
The Hetao basin is located in the northwest of Inner Mongolia, to the south of Langshan mountains and to the north of Yellow River. It is a typical semiarid to arid inland basin. There are clay layers with several meters in thickness at depths around 40 m below land surface. Shallow groundwater (SGW) is hosted overlying clay layers, while deep groundwater (DGW) occurs underlying the clay layers [2].

Sampling and analysis
Sixty-two water samples were collected in the study area with an area of around 90 km 2 during July to August of 2016, including 57 DGW samples and 5 SGW samples (Fig.1). Groundwater was sampled after parameters like temperature, pH, Eh, and EC became stable. Laboratory analyses were carried out for determination of major ions, trace elements and DOC. 87 Sr/ 86 Sr analyses were conducted on Isoprobe T TIMS after sample purification through Sr-spec resin. In addition, selected six sediment samples were dried and milled for XRD analysis.

Distribution of 87 Sr/ 86 Sr and As
The 87 Sr/ 86 Sr in DGW decreased from the alluvial fan to the flat plain with the range from 0.7152 to 0.7117 (Fig. 1b). However, arsenic concentration showed an increasing trend with the range from 0.53 to 447 μg/L, which is in line with previous studies [6]. There was a negative correlation between As concentrations and 87 Sr/ 86 Sr. Around 77.6% of samples exceeded the 10 μg/L (the maximum concentration level recommended by the WHO). Arsenic concentrations below 10 μg/L mainly occurred in alluvial fans, especially near the front of mountain area.

Source of high arsenic groundwater and its relation to arsenic distribution
Since Ca 2+ and Sr 2+ have equal charge and similar size, they have similar geochemical behavior. We identified three sources (S1, S2 and S3) of high arsenic groundwater based on 87 Sr/ 86 Sr and Sr/Ca. S1 is the fissure water recharging, mainly located near the front of mountain area in alluvial fan. Groundwater exhibited higher 87 Sr/ 86 Sr than that in rainfall ( 87 Sr/ 86 Sr = 0.7134), and had the narrow range of Sr/Ca from 0.005 to 0.01. The fissure water was considered to be under-saturated or just over-saturated with respect to calcite. Therefore, these indices indicated that the bedrock fissure water derived from rainfall recharged the deep groundwater. However, As in DGW was less than 50 μg/L, and more than 61% of samples had As content less than 10 μg/L. DGW with 87 Sr/ 86 Sr >0.7134 largely originated from fissure water, usually had low As concentration due to the direct recharge from bedrock fissure groundwater under oxic conditions. S2 is related to incongruent dissolution of plagioclase. It is located in the transition zone between the alluvial fans and the flat plain. Groundwater 87 Sr/ 86 Sr in this zone decreased further from 0.7134 to 0.7117, but Sr/Ca increased from 0.008 to 0.027. Plagioclase commonly exits in sediments of the study area based on XRD analyses, not just in the transition zone [6]. Around 69% DGW samples had arsenic concentrations greater than 50 μg/L, and more than 55% had arsenic concentrations greater than 150 μg/L. On the one hand, the incongruent dissolution of plagioclase can generate OHand make pH maintain weakly alkaline (average 7.7, compared with 7.59 in S1). On the other hand, this kind of incongruent dissolution can decrease Ca/Na in groundwater since calcite was over-saturated but Na-smectite was under-saturated. The appropriate pH and the decreasing Ca/Na all favored desorption of As from Fe/Mn oxide/oxyhydroxide [7][8][9]. Both of these processes may be the reason that arsenic increased further in this area.
S3 is associated with the vertical mixing from SGW. It is located in the flat plain. Groundwater 87 Sr/ 86 Sr were approximately 0.7117 and Sr/Ca continually increased from 0.017 to 0.037. The 87 Sr/ 86 Sr of SGW was 0.7117, and Sr/Ca ranged from 0.025 to 0.032. During the irrigation season (April -July), the decrease in DGW level could result in the recharge of SGW into DGW, especially in the flat plain [2]. SGW usually had the evapotranspiration signal due to the recharge from southern groundwater and/or surface water influenced by the diverted Yellow River with constant 87 Sr/ 86 Sr (0.7115) [10]. Arsenic concentration reached the highest level. Nine water samples had As concentrations exceeding 150 μg/L, and five were above 300 μg/L. The vertical mixing brought abundant fresh organics from nearsurface to support bacteria utilization. Therefore, degradation of the fresh organics was coupled with the reductive dissolution of Fe(Ⅲ) Based on the variation of 87 Sr/ 86 Sr and Sr/Ca in groundwater from the alluvial fan to the flat plain, the possible sources of high As DGW in the Hetao basin of Inner Mongolia were obtained, including fissure water recharging, incongruent dissolution of plagioclase and vertical mixing from SGW. In the alluvial where DGW comes from fissure water, As mobilization is limited with concentrations generally less than 50 μg/L. In the transition area where DGW is related to incongruent dissolution of plagioclase, the appropriate pH and the decreasing Ca/Na favored elevating groundwater As to more than 150 μg/L. In the flat plain where DGW is associated with the vertical mixing with SGW, the fresh organics carried by SGW could be used by As/Fe reducing bacteria, and thus promote As release via As/Fe oxides reduction, leading to high As DGW with As >300 μg/L.