hydrogeological and geochemical comparison of high arsenic groundwaters in inland basins p r china

Three inlands basins the Hetao basin HT, the Yinchuan basin YC, and the Songnen basin SN have been investigated to characterize chemistry and geochemistry of groundwaters and sediments a

Procedia Earth and Planetary Science 17 ( 2017 ) 416 – 419

1878-5220 © 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of WRI-15

doi: 10.1016/j.proeps.2016.12.105

ScienceDirect

15th Water-Rock Interaction International Symposium, WRI-15 Hydrogeological and geochemical comparison of high arsenic

groundwaters in inland basins, P.R China Huaming Guoa,b,1, Di Zhanga,b, Ping Nia,b, Yongsheng Caoa,b, Fulan Lib

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

High As groundwater has been widely found in inland basins of P.R China, which has posed a serious heath effect on local residents Although these inland basins experience arid-semiarid climate, and are filled with Quaternary sediments, As concentrations show big variations Three inlands basins (the Hetao basin (HT), the Yinchuan basin (YC), and the Songnen basin (SN)) have been investigated to characterize chemistry and geochemistry of groundwaters and sediments and to evaluate their controls on As concentrations Arsenic concentrations ranged between <0.1 and 105 μg/L (average 27.8 μg/L), between <0.1 and

338 μg/L (average 94.0 μg/L), and between 0.33 and 857 μg/L (average 130 μg/L) in YC, SN, and HT, respectively In those basins, HCO 3- is the major anions, and Na+ the major cation Both Cl- and SO 42- concentrations are much lower in SN than those

in HT and YC In YC, although Fe and Mn concentrations are the highest, groundwaters have the lowest As concentration Contents of ionically bound As (S1) and strongly adsorbed As (S2) are the highest in HT and the lowest in YC Results show that groundwater As is predominantly regulated by active As forms in sediments (S1 and S2) At the scale of multi-basin, groundwater flushing evidently regulates groundwater As Due to the similar groundwater flow rate at the scale of the site, redox conditions are the key factor controlling groundwater As, with high groundwater As under reducing conditions

© 2017 The Authors Published by Elsevier B.V

Peer-review under responsibility of the organizing committee of WRI-15

Keywords: Redox; Multi-basin; Sequential extraction; Partition; Reductive dissolution

1 Introduction

Groundwater As has posed a serious threat to resident health, with millions of people suffering from chronic arsenic poisoning1 The World Health Organization, the European Union, the United States, and China have set a guideline value of 10 μg/L for drinking water As2

* Corresponding author Tel.: +86-10-82321366; fax: +86-10-82321081

E-mail address: hmguo@cugb.edu.cn

© 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of WRI-15

High As groundwater has been found worldwide under both oxic conditions and reducing conditions1,3 More groundwater with high As concentrations occurs under reducing conditions than under oxic conditions, especially in Asia, which has been observed in river deltas experiencing humid tropical climates4,5 and in inland basins experiencing arid/semiarid climates2,6,7 In China, inland basins have been more frequently reported to host high As groundwater relative to river deltas, including the Yinchuan basin, the Hetao basin, the Huhhot basin, the Datong basin, the Yuncheng basin, the Songnen basin, the Guide basin and the Dzungaria basin2 Hydrogeological and geochemical comparison of groundwater in inland basins would help in better understanding As mobilization and behaviors in aquifers This study aims at delineating hydrogeological and geochemical comparison of high As groundwater in the Hetao basin, the Yinchuan basin, and the Songnen basin

2 The study areas

In this study, the Hetao basin (HT), the Yinchuan basin (YC), and the southwestern Songnen basin (SN) were selected The Yinchuan basin and the Hetao basin lies along the Yellow River from the west to the east, in the northwestern China, while the Songnen basin is located in the west of Jilin province, in the northeastern China (Fig.1) These basins lie in arid-semiarid areas, with much higher average annual potential evaporation than average annual precipitation Detailed hydrogeologic settings of these basins can be found in8-12

3 Materials and methods

3.1 Groundwater sampling and analysis

Two hundred and twenty-three groundwater samples (61 in YC, 87 in SN, and 75 in HT) were taken Both field measurements (water temperature, EC, pH, Eh, S2-, alkalinity, Fe(II) and NH4-N) and laboratory analyses (major cations, trace elements, As species, DOC, δ2H and δ18

O) were carried out, which can be found in11,13

3.2 Sampling and analysis of sediments

Two hundred and thirty sediment samples were taken, (66 inYC, 61 in SN, and 103 in HT) Elemental compositions were determined by ICP-MS after total digestion Sequential extraction were carried out accordingly

to the method by14 The grain-size distribution of sediments was measured in triplicate using a laser-diffraction particle-size analyzer (Mastersizer 2000, Malvern)

4 Results and discussion

4.1 Hydrogeochemistry

Groundwater data in YC, SN and HT can be found in11,15,13, respectively In all these basins, HCO3

was the major anion, and Na+ the major cation Both Cl- and SO42- concentrations were much lower in SN than those in HT and YC Redox sensitive elements showed big variations in these inland basins Most groundwater had relatively low Eh values, showing suboxic to anoxic conditions Generally, HT groundwater had the lowest Eh values, followed by YC groundwater and SN groundwater NH4-N showed opposite trends to Eh values, which was the highest in the HT groundwater and the lowest in the SN groundwater

Groundwater As ranged between 0.33 and 857 μg/L (average 130 μg/L) in HT, between <0.01 and 338 μg/L (average 94.0 μg/L) in SN, and between <0.01 and 105 μg/L (average 27.8 μg/L) in YC Although the highest Fe and Mn concentrations were observed in the YC groundwater, these waters had the lowest As concentrations

4.2 Sediment geochemistry and grain-size distribution

The SN sediments had the lowest Fe2O3 and Mn contents among the three basins, which would be associated with the lowest As, P and S contents Although Fe2O3 contents of HT sediments were identical to those of YC sediments, both As and P contents were higher in comparison with YC sediments It indicated that As was relatively enriched in the HT sediments The relative enrichment of As in HT sediments led to the highest contents of ionically bound-As (S1) and strongly bound-As (S2), which ranged from <0.05 and 30.4 mg/kg, and from 0.06 to 4.62 mg/kg,

respectively The YC sediments had the lowest contents of S1 and S2, with ranges between <0.05 and 0.33 mg/kg and between <0.05 and 5.31 mg/kg, respectively

d10 and d60, representing 10% and 60% grain fraction, respectively, were used to characterize size distribution of sediments Although sediment d10 was identical in these three basins, d60 was the highest for YC sediments and the lowest for HT sediments The ratio of d60 to d10 is the coefficient of uniformity16, which was the highest in the SN sediments The YC sediments and the HT sediments had identical d60/d10 ratios

4.3 Linkage of groundwater As on sediment As

In these basins, groundwater As was sourced from sediments, instead of anthropogenic activities8-11,15 Although high As contents were observed in several sediment samples in these basins (up to 104 mg/kg), average As contents were 11.5, 7.6 and 16.1 mg/kg in sediments of YC, SN and HT, respectively, which is comparable to other aquifer sediments hosting high As groundwater1,5

No good correlation between groundwater As and sediment As (Fig.1a), was observed, thus indicating that total

As in groundwater was not directly related to sediments This has been documented in other aquifer sediments hosting high As groundwater17 This bias would be dependent on As forms in the sediments At the scale of multi-basin, groundwater As was closely related to ionically bound-As and strongly bound-As of sediments (Fig.1b) Due

to the nature of strong binding and low P concentrations in groundwater, S2 would have relatively low contribution

to groundwater As However, this form would be desorbed by coexisting anions via competitive desorption18

Fig.1 Average groundwater As versus average sediment As (a) and averages of ionically bound-As and strongly bound-As (b) from three basins

4.4 Role of groundwater flushing

At the scale of multi-basin, negative correlations were observed between hydraulic conductivities and S1 and between hydraulic conductivities and S2 The relatively low ionically bound-As and strongly bound-As in sediments with high hydraulic conductivities indicated that groundwater flushing played a role in decreasing these active As forms With high groundwater flow rates, dissolved As was readily flushed out of aquifers19, which decreased contents of active As forms in aquifer sediments In West Bengal and Bangladesh, low As groundwater was found in shallow aquifers underlying sandy soils with high hydraulic conductivity20 In contrast, both dissolved As and active

As forms were kept in groundwater systems under stagnant hydraulic conditions with low hydraulic conductivities12 However, no negative correlation was found between dissolved As and hydraulic conductivity at the scale of contaminated site in the Hetao basin It indicated that, locally, groundwater flushing may not be the predominant factor controlling groundwater As Other factors, including redox conditions, would likely play more important role

in regulating groundwater As in the Hetao basin

4.5 Role of redox conditions

A negative correlation was observed between As and Eh values in groundwater from YC, SN and HT basins (data not shown) It indicated that reducing conditions enhancemobilization from aquifer sediments via reductive dissolution of Fe/Mn oxides and reductive desorption of As(III), as suggested by3 Organic carbon in sediments and groundwater would be the triggers for development of reducing conditions15 Both SOM and DOC were high in

these inland basins Microbial oxidation of organic matters coupled with reduction of Fe/Mn oxides and sulfate would be major biogeochemical processes for As release from sediments into groundwater4,21

Acknowledgements

The study has been financially supported by National Natural Science Foundation of China (Nos 41222020 and 41172224), the program of China Geology Survey (No 12120113103700), the Fundamental Research Funds for the Central Universities (No 2652013028), and the Fok Ying-Tung Education Foundation, China (Grant No 131017)

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