Scholarly article on topic 'Hydrogeochemical Zonation of Deep Groundwaters Near Langshan Mountain of the Hetao Basin, Inner Mongolia'

Hydrogeochemical Zonation of Deep Groundwaters Near Langshan Mountain of the Hetao Basin, Inner Mongolia Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Yongfeng Jia, Huaming Guo

Abstract Drinking water in the Hetao basin relies significantly on deep groundwater. Little is known about the occurrence of As and its distribution in deep groundwaters of the Hetao Basin. This study observed that this deep groundwater is affected by the presence of As in a nearby mountainous area. Arsenic concentrations gradually increase from the alluvial fans to the flat plain of the basin. Four hydrogeochemical zones were distinguished based on hydrogeological conditions, together with the distribution of redox-sensitive elements and their relationship with As concentrations. Although As mobilization is fundamentally controlled by the reductive dissolution of Fe-bearing minerals, reduction of SO4 2-also affects groundwater As concentrations. If the supply of SO4 2- is sufficient, further reduction of SO4 2-may lead to a decrease of As levels through co-precipitation with pyrite. This information is helpful for choosing localities for safe drinking water wells in mountain-edge alluvial fans.

Academic research paper on topic "Hydrogeochemical Zonation of Deep Groundwaters Near Langshan Mountain of the Hetao Basin, Inner Mongolia"

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Procedia Earth and Planetary Science 7 (2013) 393 - 396

Water Rock Interaction [WRI 14]

Hydrogeochemical zonation of deep groundwaters near Langshan Mountain of the Hetao Basin, Inner Mongolia

Yongfeng Jia a' b, Huaming Guo a' b' *

a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, P.R. China b School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, P.R. China

Abstract

Drinking water in the Hetao basin relies significantly on deep groundwater. Little is known about the occurrence of As and its distribution in deep groundwaters of the Hetao Basin. This study observed that this deep groundwater is affected by the presence of As in a nearby mountainous area. Arsenic concentrations gradually increase from the alluvial fans to the flat plain of the basin. Four hydrogeochemical zones were distinguished based on hydrogeological conditions, together with the distribution of redox-sensitive elements and their relationship with As concentrations. Although As mobilization is fundamentally controlled by the reductive dissolution of Fe-bearing minerals, reduction of SO42- also affects groundwater As concentrations. If the supply of SO42- is sufficient, further reduction of SO42-may lead to a decrease of As levels through co-precipitation with pyrite. This information is helpful for choosing localities for safe drinking water wells in mountain-edge alluvial fans.

© 2013TheAuthors. Publishedby ElsevierB.V.

Selectionand/orpeer-review under responsibilityof theOrganizing and ScientificCommitteeof WRI14- 2013

Keywords: arsenic; groundwater; hydrogeochemistry; reduction.

1. Introduction

Elevated As groundwaters are present in many parts of the world, including the Indian subcontinent region, Southeast Asia, North America, and Europe [1]. In general, As concentrations in deep groundwaters at depths greater than 150 m are usually lower than in shallow groundwater (mostly < 10 ^g/L, the World Health Organization's (WHO) drinking water guideline value) [1-3]. In the Hetao basin of Inner Mongolia, high As groundwaters are typically found in shallow aquifers, which poses a significant health risk to more than one million residents [4]. Limited data indicates that As

* Corresponding author. Tel.: +86-10-82321366; fax: +86-10-82321081. E-mail address: hmguo@cugb.edu.cn.

1878-5220 © 2013 The Authors. Published by Elsevier B.V.

Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 -doi:10.1016/j.proeps.2013.03.094

concentrations tend to increase with the sampling depths at depths less than 20 m [5]. However, few deep groundwaters were sampled in previous studies, so that their aqueous geochemistry is still largely unknown. More than 3000 deep wells used for irrigation have been drilled at mountain fronts along the margin of the Hetao basin. Some water supply wells are also present. Arsenic concentration in these well waters would significantly affect As cycling in the human food chain and is thought to also affect health of local resident. Therefore, this investigation of the groundwater geochemistry of these deep wells was undertaken to (i) characterize the As distribution and related groundwater chemistry in the near-mountain area, (ii) evaluate the important hydrogeochemical processes controlling As enrichment in deep aquifers, and (iii) identify target aquifers with safe water for human consumption.

2. Methods

One hundred and three groundwater samples were collected in August from wells at depths above 50m in the first semi-confined aquifer. Groundwaters were sampled after pumping sufficiently (usually 20 min) until the flowing water reached a stable temperature, pH, DO (dissolved oxygen), and Eh. Parameters, including water temperature, conductivity, pH, Eh, H2S, Fe(II), NH4-N, and alkalinity were measured at the time of groundwater sampling and samples also were collected for subsequent laboratory analysis. Arsenic was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, iCAP 6300, Thermo), with detection limit 0.98 ^g/L and relative standard deviation less than 2%.

3. Results and Discussion

3.1. Distributions of As and As species

Around 31% of the Hetao basin groundwaters sampled meet the WHO drinking water guideline value of 10 ^g/L. Arsenic is predominantly present as As(III), with As(In/As up to 70%. There are 13 samples in alluvial fans, of which 12 samples had As concentration below 10 ^g/L. Groundwater As content displays a gradually rise from < 1.0 ^g/L in front of alluvial fans, reaching a value of 390 ^g/L in the basin plain. This shows that groundwater in alluvial fans is a relatively safe drinking resource.

3.2. Hydrogeochemical zonation

The hydrogeochemical zonation of the Hetao basin helps to explain the chemical characteristics of groundwater and delineate the geochemical processes occurring in aquifers, and therefore to interpret the processes regulating As mobilization. Previous studies [4] showed that groundwater As is mainly controlled by subsurface redox conditions. Therefore, redox-sensitive elements can be used to delineate the pattern of hydrogeochemical zonation (Fig.1A). Since evaporation of water in the semi-arid climate of this region of Mongolia increases SO42- concentration of groundwater, SO42- values were normalized to the conservative tracer Cl- to eliminate this effect [6]. Arsenic concentrations, SO42"/Cl", and major redox-sensitive elements, permit recognition of the following hydrogeochemical zones with distinct characteristics:

Zone I is the Groundwater Oxic Zone, mainly located in the alluvial fans (Fig.1B). Groundwater here primarily occurs in an oxic environment, as indicated by high oxidation-reduction potentials (average = 19.8 mV) and high concentrations of NO3- (average = 33mg/L). Ammonia is not detected in 4 of 14 samples, while others have average NH4-N concentration of 0.23 mg/L. Iron(II) was only present in 7 of 14 samples. Together, this information suggests that groundwaters in this zone are oxic waters that are beginning to be affected by NO3-/O2 reduction. Dissolved As concentrations are relatively low (average =

5.7 pg/L) in this zone, with 6 samples having As values less than 1.0 pg/L. It is inferred that adsorption of As on the aquifer sediments is occurring in this zone.

.« •• t

Fe'-(mg/L|

Fig. 1. Plot ofSO/"/CI" ratio vs. ferrous iron concentration (A) and hydrogeochemical zonation in study area (B).

Zone II is the Transition Zone, between the oxic conditions of Zone I and the reducing conditions of Zone III. It is located at the basin margin at the toe of alluvial fans (Fig.1B). Ammonia is present in all 30 samples. Lower NO3- concentration (average = 18.4 mg/L) and higher NH4-N concentrations (average = 0.3 mg/L), compared with Zone I, suggest a relatively more reducing groundwater environment. SO42-/Cl-values here are the highest among all zones, suggesting SO42- is less affected by reduction. Iron(n) contents range from 0.22 to 0.96 mg/L, with the median value of 0.47 mg/L being much higher than in Zone I. Therefore, in contrast to the stage of NO3-/O2 reduction present in Zone I, the stage of NO3-reduction predominates in Zone II during which Fe minerals begin to be reduced. Arsenic(III) is the major As species in most of samples (As(InyAs>80%). Arsenic concentrations (median = 8.5 pg/L) are higher than in Zone I, indicating that As release to the groundwater accompanies the reduction of Fe(III). A sequential reductive process is envisioned in which reductive dissolution of Fe minerals first occurs, releasing absorbed As(V) to the water, which then is reduced to As(III). However, four samples were dominated by As(V) (As(V)/As>50%), which is indicative that portions of Zone II have less reducing conditions in which released As(V) is not reduced to As(III).

Zone III is the Moderate Flow Groundwater Reducing Zone, located distant from the alluvial fan margins in the groundwater runoff area (Fig.1B). Here the average NH4-N concentration is 1.1 mg/L, with a median value of 1.07 mg/L and average Fe(n) value is 0.7 mg/L, with median value of 0.65 mg/L. The reduction of Fe(III) minerals predominates throughout this zone. Besides, SO42- concentrations are lower than in the other zones, indicating the occurrence of sulfate reduction. The presence of detectable hydrogen sulfide is the evidence of the reduction process. Much higher concentrations of As (average 114 pg/L, median 108 pg/L) were observed in this zone, with As(In) as the major As species (As(In)/As>70%).

Zone IV is the Low Flow Groundwater Reducing Zone, generally located towards the basin of Zone III (Fig.1B). Concentrations of NH4+ reach a maximum here, with an average concentration of 3.2 mg/L and median value of 3.2 mg/L. This documents the increasingly trend of reducing groundwater conditions toward the center of the basin. Average Fe(II) concentrations reach 0.98 mg/L, with a median value of 0.98 mg/L. SO42- concentrations are lower than in the other zones, suggesting a greater extent of sulfate reduction. Similarly, As concentrations are highest in Zone IV, with an average of 267 pg/L and median value of 308 pg/L. Arsenic(In) accounts for 75% of total As. Iron-bearing mineral reduction continues throughout Zone IV concomitant with SO42- reduction. This, in turn, releases more As to groundwater, leading to the highest observed As concentrations in this zone. The field study of Postma and Jakobsen [7] also observed that bacterial Fe reduction occurred simultaneously with SO42- reduction.

However, the continued sulfide production due to SO42- reduction may subsequently lead to a decline in dissolved As concentration due to the co-precipitation with pyrite, should this point in the geochemical evolution of the system be reached. Previous studies have assessed S-As geochemistry at various field sites [8-9], showing that the enhanced reducing condition would consume more SO42- and increase the possibility of S-As precipitation.

4. Conclusion

Arsenic in groundwaters in the mountainous Hetao basin of Inner Mongolia exhibits significant zonation as a consequence of redox process zonation. Arsenic release is considered to be mainly linked to the reductive dissolution of Fe-bearing minerals and this study documented that progressive sulfate reduction outward from the mountain front into the basin plays an important role in regulating As mobilization. When reducing condition reach the stage of SO42- reduction, Fe-bearing minerals are dissolved, releasing As to the groundwater, whereas continued reduction of SO42- may lead to the co-precipitation of pyrite and As scavenging from groundwater. Groundwater in alluvial fans is largely free of As, and therefore is a potential safe drinking water resource. Since both dissolution and precipitation of Fe minerals can influence groundwater compositions, low As groundwater may occur in some areas of the central basin plain. However, delineation of these areas needs further investigation.

Acknowledgements

The study has been financially supported by the National Natural Science Foundation of China (Nos. 41222020 and 41172224), the Program for New Century Excellent Talents in University (No. NCET-07-0770), and the Chinese Universities Scientific Fund (No. 2010ZD04).

References

[1] BGS/DPHE (British Geological Survey, Dept. of Public Health Engineering). Arsenic Contamination of Groundwater in Bangladesh, Final Report. Keyworth, UK: British Geological Survey; 2001.

[2] Fendorf S, Michael HA, van Geen A. Spatial and temporal variations of groundwater arsenic in south and southeast Asia.

Science 2010; 328:1123-1127.

[3] Radloff KA, Zheng Y, Michael HA, Stute M, Bostick BC, Mihajlov I. et al. Arsenic migration to deep groundwater in

Bangladesh influenced by adsorption and water demand. Nat Geosci 2011; 4: 793-798.

[4] Guo HM, Yang SZ, Tang XH, Li Y, Shen ZL. Groundwater geochemistry and its implications for arsenic mobilization in

shallow aquifers of the Hetao Basin, Inner Mongolia. Sci Total Environ 2008; 393: 131-144.

[5] Guo HM, Zhang Y, Xing LN, Jia YF. Spatial variation in arsenic and fluoride concentrations of shallow groundwater from the

Shahai town of the Hetao basin, Inner Mongolia. Applied Geochemistry 2012, doi:10.1016/j.apgeochem.2012.01.016.

[6] Buschmann J, Berg M. Impact of sulfate reduction on the scale of arsenic contamination in groundwater of the Mekong, Bengal

and Red River deltas. Applied Geochemistry 2009; 24: 1278-1286.

[7] Postma D, Jakobsen R. Redox zonation: Equilibrium constraints on the Fe(III)/SO4-reduction interface. Geochim Cosmochim Acta 1996; 60: 3169-3175.

[8] O'Day PA, Vlassopoulos D, Root R, Rivera N. The influence of sulfur and iron on dissolved arsenic concentrations in the

shallow subsurface under changing redox conditions. Proc Nat Acad Sci USA 2004; 101: 13703-13708.

[9] Polizzotto M, Harvey C F, Sutton S R, Fendorf S. Processes conducive to the release and transport of arsenic into aquifers of

Bangladesh. Proc Natl Acad Sci USA 2005; 102: 18819-18823.