Forms and Sources of Arsenic in the Groundwater of the Northeastern Tectonic Active Zone of the Qinghai-Tibet Plateau

doi:10.21203/rs.3.rs-3862111/v1

High-altitude tectonic zones are active areas for geothermal groundwater, and the elevated arsenic content within them has emerged as a significant resource and environmental concern. This study selected the Gonghe-Guide Basin in the northeastern Qinghai-Tibet Plateau as the subject of investigation, employing field tests of geothermal water samples, hydrochemistry, thermodynamic simulations, and statistical methods to explore the forms, distribution, and sources of arsenic in geothermal groundwater. The research data indicates that the geothermal groundwater in the area displays weak alkalinity, medium to high mineralization, with the primary hydrochemical types being SO₄-Cl·Na and Cl·Na. Arsenic concentration shows a significant negative correlation with Cl⁻ and a positive correlation with water temperature and DO. Thermodynamic simulations suggest that the predominant form of arsenic is As⁵⁺. Rock weathering, evaporative concentration, and ion-exchange adsorption collectively shape the hydrochemical characteristics of the study area, laying the environmental groundwork for the formation and migration of arsenic. Notably, the ion-exchange between sodium ions and calcium and magnesium ions significantly impacts the arsenic concentration. This study provides insights into the behavior, forms, and origins of arsenic in geothermal groundwater and offers a reference for similar research in other geothermally active regions.

arsenic

geothermal

hydrochemical features

causative analysis

Gonghe Basin

Geothermal hot springs, as unique natural thermal energy repositories, offer invaluable therapeutic, energy, and research benefits to human society^[1]. Globally, the utilization of geothermal hot springs has increasingly exhibited a trend towards scaling and industrialization^[2–3]. According to the latest data, their application now spans over fifty countries, contributing billions of dollars to the global economy. However, as their exploitation deepens, associated environmental concerns have progressively come to the fore. Among these, the pervasive presence of arsenic emerges as an issue that cannot be overlooked: numerous studies have shown that arsenic concentrations in geothermal hot springs far exceed safety standards, and prolonged exposure can pose a severe threat to human health^[4–5]. More critically, the migration and accumulation of arsenic have led to groundwater contamination in certain areas, compromising drinking water safety for local inhabitants and irreversibly impacting the local ecology^[6–11]. Thus, delving into the geochemical behavior and environmental effects of arsenic in geothermal hot springs and groundwater not only helps address current environmental issues but is also crucial for the sustainable and safe use of geothermal resources.

The concentration of arsenic in geothermal systems results from the interplay of multiple factors. On one hand, temperatures ranging from 150–250°C facilitate the mobilization of arsenic from rocks and minerals to geothermal fluids, whereas excessively high temperatures might release toxic arsenic^{[5, 12–13]}. Moreover, arsenic concentrations are typically higher in deep neutral chloride waters compared to shallow acidic sulfate waters. The forms in which arsenic exists relate to the subterranean redox conditions, encompassing various biological and abiological processes such as oxidation, reduction, and methylation^[14–15]. Within this context, microbial activity plays a pivotal role, especially in the transformation of arsenic forms. The migration of arsenic within geothermal systems is intricately tied to its sources, geochemical conditions, and microbial activities. In deep geothermal reservoirs, arsenopyrite and chalcopyrite are the primary sources of arsenic^[13]. In some regions, the arsenic concentration in geothermal water is associated with rock leaching, and alkaline geothermal waters typically have elevated arsenic levels. Notably, neutral hot springs, representative of deep circulating geothermal fluids, exhibit more pronounced arsenic concentrations compared to acidic springs.

The Qinghai-Tibet Plateau boasts abundant geothermal resources, and concomitantly, there is a notable release of high concentrations of arsenic in its geothermal waters^[16]. It is postulated that these heightened arsenic concentrations might be associated with magma chambers contaminated by crustal materials. Additionally, rock leaching, fluid-rock interactions, and specific chemical components of magmatic fluids also influence the release of arsenic. Renowned as the "Roof of the World," the Qinghai-Tibet Plateau holds a globally significant position in terms of groundwater and geothermal resource scale. Current regional studies and data indicate that the objective conditions of arsenic in the geothermal and groundwater of the Qinghai-Tibet Plateau are relatively severe. Compared to other similar regions or neighboring areas, the arsenic content is conspicuously elevated^[17]. This arsenic contamination hinders further development and utilization of geothermal and groundwater resources. Research also demonstrates the efficacy of arsenic removal technologies in practice and potential risks associated with arsenic-contaminated water sources. Therefore, a profound study of the geochemical characteristics of arsenic in the groundwater and geothermal of the Qinghai-Tibet Plateau, aiming to elucidate its sources, migration mechanisms, and influencing factors, not only aids in formulating effective remediation strategies but also ensures the safe utilization of these vital resources.

2.1. Overview of the Study Area

The Gonghe-Guide Basin is situated on the northeastern edge of the Qinghai-Tibet Plateau, primarily influenced by the uplift of the Qinghai-Tibet Plateau and the northward collision of the Indian Plate. The geological structure is intricate, predominantly encompassing three major faults: the northeast-trending Gonghe Fault, the north-south trending Gandu Fault, and the southeast-trending Suli Fault, in addition to several secondary faults and strike-slip faults. Under the influence of tectonic movements, numerous structural uplifts and depressions have formed within the basin^[18].

The groundwater in the study area mainly originates from precipitation in the mountains and rivers flowing through the basin. The geothermal gradient is pronounced around the mountainous regions surrounding the basin. Consequently, mountain precipitation, upon infiltrating underground, gets heated, leading to the formation of underground thermal water. Loose rock layers with high porosity and permeability, such as sand and gravel rocks, conglomerates, and sandstones, constitute significant groundwater reservoir and migration layers, facilitating the accumulation and flow of underground thermal water^[19]. Moreover, hydrogeological structures like joints and fissures also exert certain impacts on the storage and flow of underground thermal water. Regarding the genesis of underground thermal water, the thermal water in the Gonghe-Guide Basin primarily arises due to the interplay between regional tectonics and the Earth's internal heat. This interaction causes underground rocks to experience thermal expansion and pressure-induced deformation, creating relatively stable thermal water storage spaces. Simultaneously, the process of groundwater movement introduces a certain amount of dissolved ions, enriching the underground thermal water with abundant mineral elements. The basin's heat sources mainly stem from deep crustal geothermal energy, the Earth's internal geothermal energy, and volcanic activities.

In the study area, the subterranean thermal waters are predominantly contained within the Lower Pleistocene and Neogene low-temperature geothermal reservoirs ^[20–21]. The caprock of the Lower Pleistocene geothermal reservoir maintains stability between depths of 100 to 200 meters, characterized by subclay and subsandy soils with abundant water resources, exhibiting temperatures ranging from 18℃ to 42℃. The geothermal input for these waters is attributed to the Quaternary Lower Pleistocene strata, composed of a mix of fine silty sand, medium to coarse sand, and gravel, forming a thermal reservoir of medium to coarse granularity with a thickness exceeding 100 meters. The Neogene geothermal reservoir, with water temperatures potentially reaching 40℃ to 85℃, consists of Neogene-era siltstone, fine sandstone, medium sandstone, and pebbly medium-coarse sandstone, buried at depths extending from 800 to 1,150 meters. The principal heat source for this reservoir is the geothermal convective system associated with the tectono-magmatic belt of the Ela and Waligong mountains^[22]. Accordingly, this study delineates the two geothermal reservoirs using 40℃ as the threshold and categorizes the thermal environments into high-temperature hot springs (HT) and low-temperature hot springs (LT). Temperatures of the LT hot springs vary from 14℃ to 37.3℃, while the HT hot springs display temperatures ranging from 40℃ to 83℃.

2.2. Sample collection and chemical analysis

In April 2022, a collection of 48 groundwater samples was methodically undertaken from hot springs, thermal wells, and geothermal boreholes situated within the Lower Pleistocene and Neogene thermal reservoirs of the Gonghe Basin to conduct comprehensive hydrochemical analyses (Fig. 1). Sampling commenced after ensuring that the discharge from the hot spring or wellhead had stabilized, a process which involved persistent observation of the outflow. Subsequently, samples were meticulously gathered directly from above the submersible pumps or at the point of emergence of the hot springs. The temperature of the samples at the time of collection ranged from 14°C to 83°C, encapsulating the thermal variance across the sampling sites. For the purpose of chemical analysis, samples were stored in two 500 mL polyethylene bottles. Field procedures dictated that all water samples were promptly filtered through a 0.45 µm membrane filter subsequent to cooling, to exclude particulate matter. Furthermore, for the cation analysis, the pH of the samples was carefully adjusted below 2 using reagent grade HNO₃, with the aim to inhibit oxidation.

The temperature of the water was measured on site with a digital thermometer to the accuracy of 0.3°C. Total dissolved solids (TDS), pH, electrical conductivity (Ec), redox potential (ORP), and dissolved oxygen (DO) were determined by the Hach HQ40d at the field site (Beijing Hach Instruments Co., Ltd.) with the precision of ± 2% full scale for TDS, ± 0.01 for pH, ± 0.5% full scale for Ec, ± 0.1 mV for ORP and ± 0.1mg/L for DO. The Ca²⁺, Mg²⁺, Na⁺, K⁺ and As concentrations were analyzed by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Thermo Electron Corporation, USA) within 2 weeks after sampling. The CO₃²⁻ and HCO₃⁻ concentrations were measured by potentiometric titrations. The SO₄²⁻ and Cl⁻ concentrations were determined on an unacidified sample according to Ion Chromatography (Dionex-900). Qinghai Geological and Mineral Testing Application Center performed all of the chemical analyses. At the end of the hydrochemical run, we monitored and verified the accuracy of the experimental data based on chemical equilibrium theory (charge balance, precipitation balance, etc.) .

2.3 Data Processing

The obtained data was analyzed for spatial distribution characteristics using the Mapgis software. Data compilation, organization, and statistical analyses were performed using Excel 2007 and SPSS version 25.0. The PHREEQC software, utilizing the wateq4f.dat database, was employed for the calculation of arsenic valence states and the mineral saturation index (SI). Water chemistry characteristics and causal relationships were analyzed and studied using Piper trilinear diagrams, Gibbs diagrams, chloro-alkaline indices, and Gaillardet plots.

3.1 Hydrochemical Characteristics

Statistical analysis revealed that the geothermal water temperatures within the sampling region ranged from 14°C to 83°C, with an average of 35.52°C (Table 1). The pH values indicated a mildly alkaline milieu, spanning from 7.22 to 9.01, with a mean of 8.18. Measurements of Total Dissolved Solids (TDS) reflected varying degrees of mineralization, peaking at 2239 mg/L, descending to a minimum of 297.3 mg/L, and averaging 1256.90 mg/L, thereby denoting the extensive water-rock interactions occurring within the geothermal systems. Ion analysis accentuated sodium (Na⁺) as the predominant cation with an average concentration of 409.37 mg/L. The average concentrations of calcium (Ca²⁺) and magnesium (Mg²⁺), at 35.95 mg/L and 10.38 mg/L respectively, are indicative of prolonged circulation of groundwater within the thermal reservoir and its interaction with carbonate minerals. Regarding anions, chloride (Cl⁻) and sulfate (SO₄²⁻) averaged 380.38 mg/L and 257.11 mg/L, respectively, while bicarbonate (HCO₃⁻) averaged 235.53 mg/L. Consequently, the hydrochemical facies of the groundwater in the study area are predominantly characterized as sulfate-chloride-sodium (SO₄-Cl·Na) and chloride-sodium (Cl·Na) types (Fig. 2).

With regard to temperature fluctuations, the pH values of the high-temperature (HT) and low-temperature (LT) groups displayed minimal variability. The oxidation-reduction potential (ORP) averaged − 206.14 mV in the HT group, as opposed to -42.34 mV in the LT group, potentially reflecting the differences in the redox environments under varying thermal conditions. The average Total Dissolved Solids (TDS) in the HT group was 1608.86 mg/L, indicative of higher mineralization, while the LT group averaged 1111.97 mg/L, suggesting a comparatively lower degree of mineralization and implying more intensive water-rock interactions in the HT group samples. Chemically, the HT group exhibited an average sodium (Na⁺) concentration of 531.24 mg/L, in contrast to 359.19 mg/L in the LT group, which may be associated with the former's increased interaction with sodium-bearing minerals such as feldspars due to higher temperatures. The average concentration of potassium (K⁺) differed between the HT and LT groups, recorded at 11.55 mg/L and 2.92 mg/L respectively, suggesting that elevated temperatures may enhance the dissolution of potassium minerals. In terms of anions, the HT group had higher average concentrations of chloride (Cl⁻) and sulfate (SO₄²⁻) at 440.74 mg/L and 340.39 mg/L respectively, compared to 355.53 mg/L and 222.82 mg/L in the LT group. This could reflect a greater degree of evaporation concentration processes or dissolution of sulfate and chloride minerals in the HT group. The disparity in bicarbonate (HCO₃⁻) concentrations, with averages of 294.54 mg/L in the HT group and 211.23 mg/L in the LT group, may indicate higher CO₂ pressures at elevated temperatures, facilitating the dissolution of more carbonate minerals.

The observed differences between the HT and LT groups in terms of water temperature, mineralization, major cation, and anion contents reveal distinct characteristics of water-rock interactions within two separate geothermal environments. The HT group's high-temperature setting promotes the dissolution of a greater number of minerals, leading to increased mineralization and affecting the hydrochemical composition, while the LT group's cooler conditions result in relatively lower mineralization and distinct chemical signatures.

Table 1

Concentrations of in-situ physicochemical parameters, major and trace chemical constituents in hot spring samples
		T (℃)	pH	ORP (mv)	DO (mg/L)	TDS (mg/L)	TAlk (mg/L)	TAcid (mg/L)	COD_Mn (mg/L)	F⁻ (mg/L)	As (mg/L)
HT	Max	83	8.94	-39.5	4.16	2164	358	2.5	1.19	8.84	0.531
	Min	40	7.42	-364.1	0	467	31.7	0	0.24	0.38	0.02046
	Avg	57.9	8.19	-206.14	1.91	1608.86	103.76	0.5	0.776	4.35	0.2068
LT	Max	37.3	9.01	122.3	3.71	2239	268	2.5	1.9	4.2	0.1032
	Min	14	7.22	-248.8	0.92	297.3	112	0	0.48	0.22	0.001
	Avg	24.71	8.18	-42.34	2.13	1111.97	193	1.79	1.04	0.83	0.0344
		K⁺ (mg/L)	Na⁺ (mg/L)	Ca²⁺ (mg/L)	Mg²⁺ (mg/L)	Cl⁻ (mg/L)	SO₄²⁻ (mg/L)	HCO₃⁻ (mg/L)	NO₃⁻ (mg/L)	NO₂⁻ (mg/L)	NH₄⁺ (mg/L)
HT	Max	51.1	727.1	64.2	16.3	831.6	554	851.5	10.8	0.024	0.72
	Min	2	113.9	9.1	0.14	96.2	77.9	0.04	0	0	0.17
	Avg	11.55	531.24	30.52	3.36	440.74	340.39	294.54	2.47	0.0072	0.404
LT	Max	5.49	730.9	76.9	38	814.1	573.6	415.5	12.1	1.23	0
	Min	1.5	30.7	10.6	2.13	27.1	33.2	134.8	0	0	0
	Avg	2.92	359.19	38.19	13.28	355.53	222.82	211.23	2.64	0.45	0
Note: T: Temperature; ORP: Oxidation-Reduction Potential; DO: dissolved oxygen.; TDS: total dissolved solids

3.2 Causes of Hydrochemical Characteristics

The Gonghe Basin is situated on the fault zone of the eastern edge of the Tibetan Plateau. The active fault structures within this region might lead to the contact of groundwater with substances such as sulfides released by deep geological structures, thereby influencing its hydrochemical characteristics. Due to the influence of geological structures, the types of rocks and mineral compositions through which the water flows are diverse. As a result, the ion concentration in the water is affected by factors such as the mineral composition of underground rocks, solubility, and ion exchange^[23]. This leads to regional differences in the hydrochemical characteristics of the groundwater in the study area. The formation of alkaline water bodies might be associated with the release of sodium, calcium, magnesium ions, and bicarbonate anions due to the acidification of silicate minerals like feldspar and mica in rocks. Additionally, tectonic activities might accelerate the groundwater circulation, further intensifying the interaction between groundwater and rocks, thereby increasing ion concentration and mineralization in the groundwater^[24].

In terms of causative differences, higher temperatures possibly enhance the dissolution of rocks, which explains the higher sodium and chloride ion concentrations in the HT group. In the LT group, owing to lower temperatures, the dissolution of certain minerals like feldspar and mica might be slower, leading to calcium and sulfate being the dominant ions. Moreover, the geological tectonic activity within the Gonghe Basin might allow the groundwater in the HT group to have more contact opportunities with deep rocks, thus being influenced by substances such as sulfides released from deep geological structures. In contrast, the LT group might be more influenced by rocks and soils closer to the surface.

3.3 Arsenic Speciation and Origin

3.3.1. Arsenic Speciation

The valence state of arsenic in groundwater holds profound geological implications, as it provides in-depth insights into the geochemical conditions and processes within aquifers^[25]. Different valence states of arsenic, particularly As(III) and As(V), distinctly indicate the redox conditions of the groundwater system and the presence of associated electron acceptors and donors^[26]. These valence states not only determine the mobility, solubility, and reactivity of arsenic within the aquifer but also offer crucial information regarding arsenic sources and pathways. This, in turn, aids in a deeper understanding of its geological conditions in the aquifer.

From the data, it's evident that the concentrations of both arsenic species are influenced by the groundwater temperature. Under HT conditions, the concentration of As³⁺ is noticeably lower, whereas As⁵⁺ concentration relatively increases. Such concentration variations might be related to specific chemical reactions promoted by high temperatures. In the groundwater of the study area, As⁵⁺ predominates, especially under oxidizing conditions. Under reducing conditions, the microbial decomposition of organic matter can facilitate the reduction of As⁵⁺ to As³⁺. However, in the samples from the study area, even in the ORP-lower LT settings, the concentration of As³⁺ remains low. This might imply the presence of a mechanism that limits the conversion of As⁵⁺ to As³⁺, or As³⁺ gets rapidly oxidized back to As⁵⁺ upon formation. Specifically, in the HT environment, there's a slight increase in As⁵⁺ concentration, potentially related to the chemical reaction equilibrium influenced by high temperatures. The high TDS values exhibited in both settings further indicate strong interactions between groundwater and the surrounding rocks and minerals, which might influence arsenic concentration and speciation.

Table 2

Composition of arsenic species across different temperature ranges
		As³⁺	As⁵⁺
LT	max	3.394E-17	1.380E-06
	min	1.547E-31	1.421E-07
	avg	5.018E-18	7.943E-07
HT	max	1.405E-21	3.309E-06
	min	1.876E-29	2.735E-07
	avg	2.810E-22	1.479E-06

3.3.2. Source of Arsenic

The concentration of arsenic in groundwater is correlated with several ions and parameters. Specifically, the ion Cl⁻ has a significant negative correlation with arsenic concentration, while water temperature and DO exhibit a positive correlation with arsenic levels. This aligns with prior research, suggesting that sodium, calcium, magnesium, sulfate, and chloride ions in groundwater might be associated with the presence of arsenic. The relationship between these ions and arsenic might be linked to the dissolution from arsenic-bearing stratified rocks upon contact. Moreover, although previous studies indicated a more acidic pH value for high arsenic groundwater, in our data, the correlation between pH and arsenic concentration isn't significant. This might imply that the presence of arsenic doesn't always linearly correlate with pH value and could be related to other geological and chemical factors.

Table 3

Correlation between hydrogeochemical indicators in groundwater and arsenic content
Parameter	Spearman Correlation Coefficient	p-value	Significance
Ec	0.7741	0.0031	**
ORP	-0.6084	0.0358	*
Water Temperature	0.3427	0.2756
DO	-0.1997	0.5339
pH	-0.1972	0.5390
TDS	0.6224	0.0307	*
Total Alkalinity	0.4904	0.1055
Total Acidity	0.2414	0.4497
Ca²⁺	-0.6783	0.0153	*
Cl⁻	0.7972	0.0019	**
K⁺	0.3497	0.2652
Mg²⁺	-0.3077	0.3306
Na⁺	0.7203	0.0082	**
SO₄²⁻	0.0979	0.7621
CO₃²⁻	-0.1391	0.6664
HCO₃⁻	0.4895	0.1063
F⁻	0.2937	0.3541
NO₂⁻	0.0214	0.9475
NH₄⁺	0.2729	0.3907
NO₃⁻	0.2325	0.4672

3.4 Arsenic Formation Environment

3.4.1 Evaporative Concentration Mechanism

To decipher the formation, migration, and distribution mechanisms of arsenic in groundwater, we further employed the Gibbs diagram to qualitatively study the hydrochemical composition and its origins in groundwater^[27–28]. The Gibbs method can categorize the factors affecting natural water chemistry into three types: “Atmospheric Precipitation Control Type,” “Rock Weathering Type,” and “Evaporation-Concentration Type.” The vertical axis of this method represents the total amount of TDS in groundwater on a logarithmic scale, while the horizontal axis presents the ratio of Na⁺/(Na⁺ + Ca²⁺) or Cl⁻/(Cl⁻ + HCO₃⁻) (Fig. 3).

Observing our data, samples primarily lie around the areas denoting evaporative concentration and rock weathering^[29]. This observation further substantiates that arsenic formation and migration in Guide Basin groundwater are predominantly influenced by rock weathering processes. Given the annual precipitation in the Guide Basin amounts to merely 200 mm, its impact on the hydrochemical composition of groundwater within the basin is relatively limited. Factors likely playing a pivotal role in arsenic formation, migration, and enrichment include groundwater runoff pathways and circulation speed, as evidenced by the low HCO₃⁻/Cl⁻ ratio in the samples.

Building upon previous research, high arsenic groundwater may be influenced by precipitation recharge in specific areas, such as the southern mountainous regions. The runoff pathways and water circulation durations in these areas are prolonged, possibly leading to the migration and enrichment of arsenic. Additionally, sedimentary arsenic content enriches with increasing depth. Iron-manganese oxides and organic matter may constitute the primary enrichment mechanisms of arsenic in groundwater. These findings offer crucial references for future research and groundwater resource management.

3.4.2. Ion Exchange Adsorption

Ion exchange adsorption is a central mechanism in the groundwater chemistry process, determining the ion concentrations and equilibrium in the aqueous system^[30]. According to Gibbs' theory, ion exchange can be described as the mutual substitution between cations and anions on solid-phase surfaces or vice versa. This process plays a pivotal role in determining concentrations of specific ions, such as As, in groundwater.

Based on the data in this study, ICAI-1 primarily reflects the ion exchange adsorption between chloride ions and sodium and potassium ions^[31]. The data indicates a certain correlation between the ICAI-1 values and As concentration. As the strength of ion exchange adsorption between chloride and sodium/potassium intensifies, the concentration of As also increases (Fig. 4). This might be attributed to the conditions of specific ion activity wherein certain ions are adsorbed from the aqueous phase onto the solid phase, leading to the release of As from the solid phase into the water phase. Meanwhile, the design of ICAI-2 is more comprehensive, considering a broader array of ions such as bicarbonates, sulfates, carbonates, and nitrates in ion exchange adsorption with chloride ions. From the data, the relationship between ICAI-2 and As concentration appears to be more complex, suggesting that the influence of ion exchange adsorption of various ions on As concentration might be multifaceted. We posit that under specific ion activity conditions, ion exchange adsorption significantly affects As concentrations. However, due to the ion exchange involving multiple ions, the concentration of As is not solely influenced by a single ion but is a result of the cumulative effects of multiple ion exchange adsorptions.

3.4.3. Water-Rock Interaction

To further deepen our understanding of ion exchange adsorption, we compared γ(Na⁺)−γ(Cl⁻) and γ(Ca²⁺)+γ(Mg²⁺)−[γ(HCO₃⁻)+γ(SO₄²⁻)] to further reflect the interaction between cations and anions. Most data points were observed to cluster around a reference line with a slope of -1 (Fig. 5). This distinctly indicates cation exchange adsorption between sodium ions and calcium and magnesium ions. This observation is consistent with the theoretical prediction of ion exchange between sodium and calcium and magnesium ions. However, some sample points significantly deviate from this reference line, which might be attributed to interference from other geochemical processes like ion precipitation, redox reactions, etc. Furthermore, the data distribution's non-uniformity could hint at varying groundwater flow paths, signifying interaction complexities and other potential influencing factors. For instance, high ion concentrations in certain areas might arise from mineral weathering or contributions from other sources.

To further ascertain the sources of the water's chemical composition, we employed the Gaillardet diagram as our primary analysis tool. According to Gaillardet et al. (1999), this diagram, by comparing the molar ratios of Ca²⁺/Na⁺ and HCO₃⁻/Na⁺, offers an effective means to determine the primary sources of water's chemical constituents^[32]. Evidently, from our data, most sample points cluster around the end-member of evaporite dissolution (Fig. 6). This suggests that the chemical characteristics of groundwater in the study area are majorly influenced by the dissolution of evaporites. This discovery aligns with the geological history and features of the underground basin. The abundant organic matter in the confined aquifers and prolonged contact with surrounding rock allow groundwater ample time to react with the surrounding rock, leading to a significant amount of soluble salts dissolving into the groundwater. Additionally, redox reactions within the aquifers might also impact the water's chemical composition, especially the reduction reactions involving iron oxides, potentially leading to increased HCO₃⁻ concentrations^[33]. The presence of HCO₃⁻ relates to the form and mobility of arsenic. Under alkaline conditions, HCO₃⁻ might form complexes with As(III), influencing its mobility and solubility. The combined effects of evaporite dissolution processes and redox reactions shape arsenic's forms and concentrations, which align with our prior observations regarding groundwater's chemical properties and arsenic forms.

The geothermal waters of the Gonghe Basin exhibit slightly alkaline characteristics, with their average conductivity indicating medium to high mineralization. The dominant hydrochemical types are SO₄-Cl·Na and Cl·Na. The chemical properties of the water are closely related to the basin's geographical location, geological structure, and the types of rocks it flows through. The HT group of groundwater shows stronger alkaline features and higher sodium and chloride ion concentrations, while the LT group is primarily characterized by calcium and sulfate ions.

Arsenic in groundwater predominantly exists in the As⁵⁺ form, which relates to the redox conditions of the groundwater. The concentration of arsenic shows significant correlations with other ions and parameters, such as Cl⁻, water temperature, and DO, suggesting that arsenic's presence might be linked to contact dissolution with stratum rocks. Regarding the formation mechanisms, rock weathering, evaporative concentration, and ion exchange adsorption are key factors affecting arsenic's formation and migration in groundwater. Notably, ion exchange adsorption, especially between sodium and calcium/magnesium ions, has a significant impact on arsenic concentration. Moreover, the chemical characteristics of groundwater in the Guide Basin are primarily influenced by evaporite dissolution. Prolonged contact with surrounding rocks and redox reactions might further affect the concentration and forms of arsenic.

This study offers insights into the behavior, forms, and sources of arsenic in geothermal groundwater, providing scientific support for managing geothermal water resources in the Gonghe Basin. It also serves as a reference for related studies in other geothermally active areas. In future research, integrating techniques like stable isotope technology and high-throughput microbial sequencing will help further unveil the sources and migration processes of arsenic.

Author Contribution

Conceptualization, Q.F. and H.Z.; methodology, H.Z.; software, S.S.; validation, G.Z. and Z.N.; formal analysis, G.Z.; investigation, Z.N.; resources, J.Z.; data curation, H.Z.; writing—original draft preparation, H.Z.; visualization, T.Z.; supervision, W.Z.; project administration, W.Z.; funding acquisition, Q.F., S.Z. and H.Z.. All authors have read and agreed to the published version of the manuscript.

Acknowledgements.

This work is funded by Scientific research project of State Grid Qinghai Electric Power Company (B7280722E072) and Key Laboratory of Ionic Rare Earth Resources and Environment, Ministry of Natural Resources of the People’s Republic of China (No.2022IRERE403)

Data Availability Statement

The data generated in this study are part of an ongoing research initiative and are subject to intellectual property considerations. In accordance with the terms and conditions of our funding agreements with the State Grid Qinghai Electric Power Company Research Institute and the Key Laboratory of Ionic Rare Earth Resources and Environment, Ministry of Natural Resources of the People's Republic of China, the dissemination of this data is governed by confidentiality clauses. Consequently, the public availability of these data is restricted until the completion of the project and the resolution of any related patent processes. We commit to facilitating access to the data as far as is permissible under the prevailing intellectual property rights and funding agreement stipulations in the future.

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No competing interests reported.

Forms and Sources of Arsenic in the Groundwater of the Northeastern Tectonic Active Zone of the Qinghai-Tibet Plateau

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and Methods