地下水是淡水的重要来源,约占地球上可用淡水资源总量的95%。它不仅用于日常用水需求,还用于农业灌溉、工业用途、生态补给和发电。
Groundwater constitutes a vital source of freshwater, accounting for roughly 95% of the total available freshwater resources on Earth [
1]. It is utilized not only for daily water needs but also for agricultural irrigation, industrial purposes, ecological recharge, and power generation [
2]. Therefore, groundwater holds significant value as a resource and plays a critical role in the environment. The degradation of groundwater quality represents a significant issue within the context of global environmental and climate change today. Since the Industrial Revolution, there has been widespread concern over the deterioration of groundwater quality [
3]. Among the various groundwater quality issues, the release of high concentrations of heavy metals has had a significant impact on groundwater quality, and serious consideration must be given to its potential risks and hazards to human health. In particular, As is considered by the United States Agency for Toxic Substances and Disease Registry (ATSDR) to be the pollutant that poses the highest potential risk to human health due to the release from natural sources and the resulting high geogenic concentrations in groundwater [
4]. The sources of As in groundwater primarily include natural origins such as geological formations, volcanic activity, and hydrothermal processes, as well as anthropogenic activities including mining, coal combustion, and petroleum extraction [
5]. The majority of global health issues caused by As are linked to the consumption of water with high As concentrations. Due to the wide range of negative effects of high As concentrations on human health, the World Health Organization (WHO), the United States, and the European Union (EU) have lowered the Maximum Contaminant Level (MCL) of As in drinking water from 50 μg/L to 10 μg/L as a safe limit for As concentration in drinking water [
6,
7]. High-As groundwater is defined as groundwater with As concentrations above the WHO drinking water standard. The enrichment of high-As groundwater is primarily influenced by a combination of natural sources and hydrogeochemical conditions, with the majority of natural high-As groundwater primarily being a result of geological arsenic contamination [
5]. Despite the established risks, many countries, such as Bangladesh, Nepal, Pakistan, Mexico, and Argentina, continue to adopt the 50 µg/L standard for arsenic concentration in their national drinking water guidelines, due to a lack of professional expertise, economic considerations, and the low-level arsenic detection technology [
8].
Human exposure to As occurs through direct and indirect pathways. Direct exposure involves drinking water with a high As concentration, contact with skin, and inhalation of gasses with a high As concentration. Indirect exposure mainly occurs through the food chain; this includes eating crops, vegetables, and fruits cultivated in As-contaminated soil or irrigated with As-rich groundwater, as well as consuming meat products from animals raised in such environments. Prolonged exposure to As, regardless of the route, can result in serious health disorders affecting the skin, blood vessels, and nervous system. Extended periods of high As exposure also notably increase the risk of developing cancers in organs like the lungs, liver, kidneys, and skin [
9,
10] (
Figure 1).
Figure 1. Different pathways of arsenic exposure in groundwater and effects on humans (MMA-Monomethylarsenite; DMA-Dimethylarsenite).
Environmental As exists in groundwater in both organic and inorganic forms, with varying levels of toxicity associated with different forms. The three primary forms of inorganic arsenic are as follows: pentavalent arsenate [As(V)], trivalent arsenite [As(III)], and metallic arsenic. Arsenic in organic form often occurs as various organic arsenic compounds such as Monomethylarsenite (MMA) and Dimethylarsenite (DMA) [
11]. Among these, inorganic arsenic is more toxic to humans, and the toxicity significantly differs between the oxidation states of As(III) and As(V). The toxicity of As(III) is more than 60 times higher than that of As(V) and 70 times higher than that of methylated arsenic [
12]. The heightened toxicity of As(III) is partially because of its reactivity towards biologically relevant molecules [
13]. The methylated arsenic forms, including MMA and DMA, exhibit moderate toxicity, while other organic forms, such as arsenobetaine (AsB) and arsenocholine (AsC), are generally considered non-toxic [
12]. In aqueous solutions, As(III) and As(V) primarily exist as oxyanions due to the high charge and small ion radius of As
3+ and As
5+. The presence and dispersion of distinct arsenic compounds within hydrological systems are markedly influenced by both the redox potential and pH levels prevailing in aquatic environments [
14]. Under circumstances characterized by moderate-to-high redox potentials, As tends to stabilize into the As(V) form (H
3AsO
4, H
2AsO
4−, HAsO
42−, or AsO
43−). Conversely, in environments featuring predominantly acidic or weakly alkaline reducing conditions, and lower redox potentials, As(III) tends to be prevalent as the uncharged H
3AsO
3 molecule [
15].
2. Global Distribution of Geogenic High-Arsenic Groundwater
High-As groundwater is widespread worldwide. According to statistics, 107 countries are affected by high-As groundwater, with the highest number in Asia (32) and Europe (31), followed by Africa (20), North America (11), South America (9), and Australia (4) [
16]. The most affected countries are Bangladesh, India, Pakistan, China, Nepal [
7], Laos [
17], Cambodia [
18], Myanmar [
19], Vietnam [
20], and the United States. The world map (
Figure 2) displays the global distribution of geogenic high-As groundwater, predominantly found in inland basins and river deltas in South Asian, East Asian, and South American countries. Major countries are shown in
Table 1. Generally, the river-marine sedimentary shallow (Holocene) aquifers in the river deltas are the main areas where high-As groundwater occurs naturally, and it occurs mainly under reducing aquifer conditions [
21].
Figure 2. Main countries worldwide affected by geogenic high-arsenic groundwater (≥10 μg/L) (taking the maximum arsenic concentration), see Table 1 for specific data.
Globally, the problem of geogenic high-As groundwater is particularly prominent in South and Southeast Asia, especially in Bangladesh and India [
22]. In Bangladesh, 61 areas have been identified as having high-As groundwater. The potential population at risk is approximately 20 million people [
23]. According to the National Drinking Water Survey of Bangladesh, around 8% of the water samples had As levels exceeding the Bangladesh standard of 50 μg/L, while around 18% of the samples were above the WHO guideline of 10 μg/L [
24]. The concentration of As in groundwater is higher in Bangladesh compared to other countries, and some tube wells even contain As concentrations as high as 4730 μg/L [
25,
26]. In India, high-As groundwater has already affected twenty states and four union territories, and about 100 million people are under threat from the toxicity of high-As groundwater [
16,
27,
28,
29]. The impact of high-As groundwater in India is concentrated on the Ganges–Yarlung Tsangpo Plain, seen on the neo-alluvial (Holocene) floodplains of the rivers in the Himalayas [
30,
31]. Approximately 50–60 million individuals in Pakistan consume high-As groundwater (>50 μg/L) in vulnerable areas [
32]. A meta-analysis of groundwater affected by As in Pakistan showed that 73% of these groundwater samples contained arsenic above 10 µg/L [
33]. China is also one of the world’s most representative areas of high-As groundwater, with more than 20 provinces/autonomous regions having high-As groundwater problems. These high-As groundwater provinces are mainly located in the fluvial/alluvial-lacustrine plains and basins (Yinchuan Plain, Hetao Plain, Guide Basin, Hohhot Basin, Junggar Basin, Datong Basin, etc.) located in arid/semi-arid regions and alluvial plains/basins and river deltas in humid/semi-humid regions (Yangtze River, Yellow River Delta, Pearl River Delta, Delta, Huaihe River, Alluvial Plain, Yellow River, Yuncheng Basin, Taiyuan Basin, Songnen Plain, etc.) [
34,
35,
36,
37]. The population affected by high-As groundwater contamination in China was estimated to be about 19.6 million according to a statistical risk assessment model developed by Rodríguez-Lado et al. [
38].
Table 1. The occurrence of high-As groundwater reported by major countries in the world.
在欧洲,地下水的污染归因于地热和热液系统,以基岩和火山沉积物为主[
67]。潘诺尼亚盆地(罗马尼亚,塞尔维亚和匈
[2]牙利)的情况尤其值得注意,因为超过600,000名居民可能暴露于高砷地下水[
57]。此外,芬兰基岩地下水中砷的最大浓度为 1040 μg/L。意大利南部伊斯基亚岛地区记录到的最高砷浓度为1479微克/升,是MCL的148倍。热液活动和热控制似乎是导致砷从矿物中释放的主要因素[
68]。
美国和加拿大也经历了广泛的地质成因高砷地下水污染,但其浓度低于亚洲国家[
69]。在拉丁美洲,地下水中的砷化合物主要来源于地热流体和火山活动[
43]。在墨西哥31个州中,有13个州的饮用水含量过高[
70]。特别是,在La Laguna地区的孔隙弱渗透层中发现了5000 μg/L的浓度[
71]。地下水:在瓜纳华托州的Juventino Rosas和墨西哥州的Ixtapan de la Sal和Tonatico已经确定了地下水作为地热来源[
72]。阿根廷地下水中受As影响最大的地区是Chaco-Pampean平原,在收集的86个地下水样本中,约有88%超过了WHO的指导值,阿根廷的高危人群约为400万人[
54]。
在非洲,只有少数几个地区(主要是西部和南部地区)发现了高砷地下水,这更多是由于研究不足,而不是缺乏问题[
67]。非洲有20个国家记录了地下水中砷的高浓度,包括博茨瓦纳、布基纳法索、埃塞俄比亚和加纳[
67]。布基纳法索地下水中As的最大浓度为1630 μg/L,而加纳地下水中的As最高浓度为1760 μg/L[
56,73]。
This entry is adapted from the peer-reviewed paper 10.3390/w16030478