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Arsenic speciation in aquifer sediment under varying groundwater regime and redox conditions at Jianghan Plain of Central China
Science of the Total Environment 607–608 (2017) 992–1000
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Arsenic speciation in aquifer sediment under varying groundwater regime and redox conditions at Jianghan Plain of Central China Yanhua Duan a, Yiqun Gan a,b, Yanxin Wang a,b,⁎, Chongxuan Liu c, Kai Yu a, Yamin Deng d, Ke Zhao a, Chuangju Dong d a
School of Environmental Studies, China University of Geosciences, 430074 Wuhan, China State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 430074 Wuhan, China School of Environmental Science and Engineering, Southern University of Science and Technology, 518055 Shenzhen, Guangdong, China d Geological Survey, China University of Geosciences, 430074 Wuhan, China b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Arsenic speciation was determined by linear combination fits of As K-edge XANES. • Sediment As speciation mainly controlled by redox conditions in the groundwater system. • Groundwater regime and redox conditions greatly affected As content and speciation in the sediments. • Sediment As speciation and reactivity controlled groundwater As concentration.
a r t i c l e
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Article history: Received 24 February 2017 Received in revised form 2 July 2017 Accepted 2 July 2017 Available online xxxx Editor: F.M. Tack Keywords: Arsenic pollution Redox controls Speciation Mobility Central Yangtze basins
a b s t r a c t At Jianghan Plain of central Yangtze basins where the health of N73, 000 people has been affected by long term intake of high arsenic groundwater, over 100 sediment samples from four boreholes at the field monitoring sites were collected and analyzed to delineate the distribution and speciation of As in the shallow aquifer sediment. Results showed that sediment As concentration is generally dependent on the lithological conditions, with the higher As concentration present in fine particle sediment, especially in the silty sand layers underlying clay or silty clay layers. High As concentration in the sediment mainly occurred in three different depth ranges: b 5 m, 15–35 m, and N35 m. Both the groundwater regime and redox conditions played important roles in controlling sediment As speciation. Arsenate (86%) was the dominated As species in the near surface sediment. As the redox turned to be reducing, arsenite (64%) became the dominant species in the underlying clay and silty clay layers. But in the silty sand aquifer near the boundary of unconfined aquifer and confined aquifer, arsenate (85%) became the dominant species again as results of redox potential elevation. In the deep medium to coarse sand aquifers (N 35 m deep), As-sulfides (49%–63%) were the main species of As. The speciation and reactivity of sediment As strongly controlled the spatial distribution of groundwater As concentration, while seasonal variation in groundwater As concentration and speciation affected the content and speciation of sediment As. © 2017 Elsevier B.V. All rights reserved.
⁎ Corresponding author at: No. 388, Lumo Road, Wuhan 430074, China. E-mail address: [email protected] (Y. Wang).
http://dx.doi.org/10.1016/j.scitotenv.2017.07.011 0048-9697/© 2017 Elsevier B.V. All rights reserved.
Y. Duan et al. / Science of the Total Environment 607–608 (2017) 992–1000
1. Introduction Geogenic arsenic (As) pollution of drinking water has been reported in N70 countries, posing a serious health hazard to an estimated 150 million people world-wide (Brammer and Ravenscroft, 2009), and 19.6 million people in China (Rodriguez-Lado et al., 2013). Long time exposure of As may cause various diseases such as melanosis, keratosis, non-petting oedema, gangrene, leucomelanosis, respiration system problems and an increased cancer risk for skin, bladder, and lungs (Chen and Ahsan, 2004; Chowdhury et al., 2000; Kapaj et al., 2006). As also causes cardiovascular disease and inhibits children mental development (Chen et al., 1996; Wasserman et al., 2004). Sediment As content and speciation are the two critical factors controlling As partitioning and transport. In oxidized aquifer sediment, As(V) adsorbs more strongly than As(III), and As(III) could be oxidized to As(V), leading to strong As retention (Stollenwerk et al., 2007). Under reducing conditions, As can be released from colloids or Fe/Mn oxide surfaces through desorption, reductive dissolution of Ascontaining Fe (III) minerals, and As(V) reduction to more labile As(III), which subsequently enhances As mobility and toxicity in the aquifers (Bhattacharya et al., 1997, 2002; Donselaar et al., 2017; Fendorf et al., 2010; Guo et al., 2008; Nickson et al., 2005; Paul et al., 2015; von Brömssen et al., 2007; Zheng et al., 2004). However, As can be removed from the aqueous phase via precipitation or incorporation of As-Fe sulfides under reducing conditions (Bostick et al., 2004; Lowers et al., 2007; O'Day et al., 2004). Sorption of both As(V) and As(III) is strong on Fe(III) oxides under circumneutral pH conditions (Dixit and Hering, 2003), small goethite particles can enhance As sorption and/or coprecipitation during As fixation (Xie et al., 2016). Incorporation of As in carbonates can effectively mobilize As from sediment into groundwater (Anawar et al., 2004; Kim et al., 2000; Saunders et al., 2005; Sengupta et al., 2004). Additionally, As(V) can substitute for SiO44 − within silicate minerals (Seddique et al., 2008), and As(V) is retained more appreciably than As(III) on silicate minerals, such as chlorite and halloysite (Lin and Puls, 2000). These previous results indicate that sediment As speciation directly controls As concentration, mobility and toxicity in groundwater. Characterizing the distribution and speciation of As in the sediment is therefore critical for understanding the speciation and spatial variation of groundwater As. Since the first reports of waterborne As poisoning in Shahu Village within the Jianghan Plain in 2005, studies by Deng et al., 2014, Y. Gan et al., 2014; Y.Q. Gan et al., 2014, Duan et al., 2015, Li et al., 2015, and Schaefer et al., 2016 have made efforts to understand the processes and factors leading to high concentrations of As in groundwater. The elevated As concentration in groundwater has affected the health of N 73,000 people, including 20,000 children (Li et al., 2010; Wang and Zhao, 2007). Our previous work was mostly focused on the effects of groundwater hydrogeochemistry on As distribution (Y. Gan et al., 2014; Duan et al., 2014) and dynamic changes in groundwater As concentration (Deng et al., 2014; Duan et al., 2015; Y.Q. Gan et al., 2014; Schaefer et al., 2016). Although a great deal of effort has been made in most high arsenic groundwater regions to investigate the mineralogy and geochemistry of the aquifers sediment, and sediment-associated As that are expected to have great impacts on As enrichment in groundwater (Bhattacharya et al., 2001; Hasan et al., 2009; Hossain et al., 2014; Mukherjee et al., 2009; Neidhardt et al., 2013; von Brömssen et al., 2008), litter work has been done in the Jianghan Plain. The results of previous work indicated that the sediment samples from Shahu Village contained As concentration ranging from 1 to 107 mg/kg (Duan et al., 2014; Li et al., 2015), with a median of 12.8 mg/kg (Li et al., 2015). Higher As concentration was observed in the silt and clay samples, with the highest concentration near 20 m depth. All sediment samples had high concentrations of Fe and Mn, with Fe2O3 contents ranging from 4% to 9% (Y. Gan et al., 2014). Li et al. (2015) also reported that 10%–70% (with a median value of 38%) As in the sediment could be extracted by ammonium oxalate, implying
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that a large amount As in the sediment was bound with amorphous Fe oxides. The main mineralogical compositions in the sediment were clay minerals (montmorillonite, chlorite, illinite and kaolinite) and quartz (Y. Gan et al., 2014). And the main source of As in the sediment at Jianghan Plain was thought to be the As-bearing minerals (As-bearing Fe oxides, and As-bearing sulfides) deposited during fluvial-lacustrine deposition (Li et al., 2015). The objectives of this research are: (1) to understand the hydrostratigraphy and the relationship with the sediment types; (2) to determine As concentration and speciation in sediment as a function of depth in the aquifer; and (3) to correlate sediment-associated As with groundwater As to provide insights into the factors controlling arsenic concentration in groundwater at Jianghan Plain. 2. Materials and methods 2.1. Site descripting Jianghan Plain is an alluvial plain formed by the Yangtze and Han rivers located in the Middle Reaches of the Yangtze River that includes the central and southern regions of Hubei Province (Fig. 1). It has a subtropical monsoonal climate, the average annual precipitation increases from 800 mm in the northwest to 1500 mm or more in the southeast, 30–50% of which occurs in summer. Jianghan Plain is a semi-closed Quaternary basin with a higher elevation in the north and a lower elevation in the south. The field monitoring site for this study was constructed in Shahu Village, the interior of the Jianghan Plain (Fig. 1), The site is surrounded by four rivers (Tongshun River, Dongjing River, Kuige River, and Lüfeng River), and covered by other abundant surface water bodies such as ponds, irrigation channel, and wetlands. Surface water levels at the field site fluctuate 5.6–7.6 m annually and are closely tied to monsoon rains. Well water levels follow a trend similar to surface water, but the magnitude of the fluctuations is approximately 0.7–2.4 m and the timing of falling and rising well water levels lags surface water changes (Duan et al., 2015; Schaefer et al., 2016). Strong surface watergroundwater interactions were observed here. During the wet summer monsoon when surface water levels are higher than groundwater levels, a groundwater recharge flow gradient develops; during drier, winter months, surface water levels drop below groundwater levels, and groundwater flow reverses and moves toward surface water. Further details of the field area are provided in previous publications (Duan et al., 2015; Deng et al., 2015; Schaefer et al., 2016). 2.2. Sampling Groundwater samples, 729 in total, were collected in Xiantao, Honghu, Qianjiang, and Jianli in the Jianghan Plain from 2011 to 2014. Temperature, conductivity, pH and oxidation-reduction potential (ORP) were measured in the field using a HACH HQ40D multimeter. ORP was converted to Eh depending on the water temperature. Samples were filtered and acidified with concentrated HCl for analysis of total dissolved As concentration. Sediment samples were collected during well installation in the Shahu field site from boreholes SY15 (N 30°09′23.73″, E 113°40′ 42.59″), SY03 (N 30°09′23.51″, E 113°40′39.21″), SY14 (N 30°09′ 18.62″, E 113°40′21.03″), and SY07 (N 30°09′19.66″, E 113°39′58.76″) (Fig. 1). Sediment samples from SY03 and SY07 were collected in November 2011, samples from SY14 and SY15 were collected in November 2014. A Geoprobe DT21 rig was used to get continuous in-situ sediment samples from depths b 20 m. The sediment (18 cm long) in sampling tube were cut and sealed for lab analysis. A rotary drill rig and splittube sampler with an internal stainless steel tube (17.5 cm length, 3.8 cm diameter) was used to obtain sediment samples N 20 m deep. After sample collection, both ends of the sampling tubes were covered with polytetrafluoroethylene caps and sealed with paraffin wax,
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Fig. 1. Location of the monitoring wells and boreholes (SY07, SY14, SY03, and SY15) in the Shahu field site at Jianghan Plain. Sediment samples were collected during well installation.
vacuum sealed in a vacuum bag and placed in a gas-tight box containing anoxic pouches, and placed on ice in the field and stored at 4 °C in the refrigerator prior to shipment to Stanford University for further analysis.
2.3. Samples analysis In the laboratory, total dissolved As concentration in groundwater was determined using a hydride generation-atomic fluorescence spectrometer (HG-AFS, 930, Titan, China). The detection limit of the instrument for As was 0.05 μg/L and the relative standard deviation was b1.0%. All samples were diluted several times to adjust for the operating range and then analyzed. Sediment samples were transferred to a glove box (95% N2, 5% H2), and then removed from the sample tube and allowed to dry under anoxic conditions. Samples' mixing, grinding, sieving, and preparation were also conducted in the glove box. Sediment pH was measured in 0.01 M CaCl2·2H2O (Dittmar et al., 2007). Total C (TC) content was measured using Carlo-Erba NA 1500 Elemental Analyzer. Sediment were acid fumigated to remove carbonates prior to the measurements of total organic carbon (TOC) (Harris et al., 2001). Total elemental composition was determined by X-ray fluorescence spectrometry (XRF). Mineralogical compositions of the sediment collected at SY03 were measured using an automated powder X-Ray Diffractometer (Cu-Kα radiation and a graphite monochromator) (X'Pert PRO DY2198, PANalytical) with a detection limit of 2%. Synchrotron powder X-ray diffraction for the samples at SY14 was performed at beam line 11-3 at the Stanford Synchrotron Radiation Laboratory (SSRL). Peak identification was performed using HighScore Plus.
Similar to the analyses described in Stuckey et al. (2015a), bulk As Xray absorption (XAS) (8 samples at SY14) and bulk Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy (only two samples were analyzed due to time limit for users) were performed at beam line 11-2 at SSRL. For As K-edge XANES spectroscopy, subsamples were packed into 2 mm thick polycarbonate slot mounts in Kapton tape (25 μm thickness) in glove box, and stored in gas-tight box containing anoxic pouches before the measurement. The energy selection was maintained by a Si (220) monochromator detuned 30–40% for harmonic rejection, and energy calibration was achieved by assigning a K-edge position of 11,874.0 eV to a Na3AsO4 standard. Fluorescent X-rays were measured with a multi-element Ge detector. For Fe EXAFS spectroscopy, sediment samples were deposited on a 0.2 μm cellulose nitrate filter paper and packed in Kapton tape (25 mm thickness). Energy selection was accomplished using a Si (220) monochromator detuned 40–50% for harmonic rejection, and energy calibration was achieved by assigning a K-edge position of 7111.0 to a Fe foil. Fluorescent X-rays were measured with a Lytle detector fitted with 3 μm thick Mn filter. Linear combination fitting of XANES and EXAFS data was used to quantitatively determine As and Fe species in sediment samples with standard compounds. 3. Results 3.1. Lithology and mineralogy of the sediment Sediment lithology at the Shahu site ranged from clay to coarse sand, with brownish-yellow to dark gray color (Figs. 2–6). The boundary of the phreatic aquifer and confined aquifer was located around 17.5 m
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Fig. 2. Hydrogeological profile from SY07 to SY15 of the field monitoring site.
below the land surface (bls). Above 17.5 m were interlayers of brownish-yellow to gray silty clay with brownish-gray clays. Thin brownish-gray to gray silty sand layers occurred discontinuously between the clay and silty clay layers. From 17.5 to 50 m bls, the main lithology of the sediment was sand, with particle size increasing with depth. Gray silty sand spanned 17.5–21 m, and gray to livid fine sand 21–40 m, with brownish-livid medium-coarse sand at depth between 40 and 50 m. Below the medium-coarse sand layers, livid fine sand layers occurred between 50 and 60 m. Several gray clay and silty clay lens were “sandwiched” by the sand layers. Gravels and spiral shells occurred in the sand layers from 34 to 55 m (Figs. 2–6). The main sediment mineralogical compositions were clay minerals and quartz. The content of montmorillonite, chlorite, illite and kaolinite in sediment samples from different depths ranged from 0–15 wt%, 8– 16 wt%, 12–25 wt%, and 0–10 wt%, respectively, while that of quartz from 31 to 54 wt%. The sandy sediment also contained a high content of feldspar (15–24 wt%). Synchrotron powder X-ray diffraction results also showed that the main mineralogical compositions for the sediment were quartz (account for 20%–78%) and silicate minerals (ranging from 55% to 77%). Higher contents of calcite (up to 20%), and albite (up to
21%) were also detected in the sand sediment N 40 m deep. For some samples, iron sulfide (FeS, 2.6 and 3.8 m), pyrrhotite (Fe7S8, 14.0 m), and westerveldite (AsFe, 8.0 and 21.5 m) were also detected. 3.2. Sediment chemical compositions For the sediment collected at SY07, higher As concentration were observed in the silty clay layer located at b 4.0 m depth (mean 13.4 μg/g) and in the clay (mean 17 μg/g) and silty sand (mean 20.7 μg/g) layers located from 15 to 17.5 m (Fig. 3). The sediment from SY03 and SY14 both showed higher As in the shallow clay and silty clay layers (b 5 m deep), and the highest As concentration in the silty sand layers, with 39.8 μg/g at 18.5 m for the sediment at SY14 and 107.5 μg/g at 20.1 m for the sediment at SY03 (Figs. 4, 5). For the sediment collected at SY15, only the shallow clay and silty clay (b 5 m) had higher As concentration (Fig. 6). Additionally, higher As concentrations also occurred in deep clay and silty clay lens “sandwiched” by the sand layers (Fig. 7). On the whole, the concentration of sediment associated As ranged from 0.1 to 107.5 μg/g, with a median of 7.0 μg/g, and a mean of 10.0 μg/g. Sediment As concentration is generally dependent on lithological
Fig. 3. Geochemical profiles of the sediment samples collected from SY07. Only the top 18 m sediment samples were collected due to the damage of the auger.
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Fig. 4. Geochemical profiles of the sediment samples collected from SY14.
conditions in our study area, which is similar to the findings in high As groundwater regions at Bangladesh (Hossain et al., 2014; von Brömssen et al., 2007). Fine particle sediment contained a higher As concentration at each borehole, especially in the silty sand layers underneath clay or silty clay layers. Based on sediment As concentration, the vertical As distribution can be divided into 3 sections: b 5 m deep, between 15 and 35 m deep, and N35 m deep (Fig. 7). Arsenic concentration in the shallow clay and silty clay sediment (b 5 m) ranged from 9 to 21 μg/g, with a mean value of 14.6 μg/g. For the sediment below 35 m, As concentration were higher in the intermittent clay and silty clay lens than in the sand layers. Arsenic concentration of the sediment between the depth of 15 m and 35 m bls ranged from 0.1 to 107.5 μg/g, with a mean value of 13.1 μg/g. Extremely high As concentration occurred in the silty sand layers around 20 m deep (Figs. 5, 7). All of our samples had high concentrations of Fe and Mn (Figs. 3–6). Total Fe concentration ranged from 10 to 70 mg/g, with a median
value of 43 mg/g, and a mean value of 41 mg/g. Iron concentration in the shallow clay and silty clay (mean 47 mg/g) were much higher than in the deep sand (mean 34 mg/g). The highest Fe concentration was observed at the depth of 52 m in the medium-coarse sand layer at SY14. Sediment samples with As concentration b 20 μg/g showed positive relationship between As and Fe concentrations (Fig. 8).Total Mn concentration ranged from 220 to 4400 μg/g, with a median value of 630 μg/g, and an average value of 750 μg/g. Like the Fe concentration in the sediment, the highest Mn concentration (4400 μg/g) also occurred at the depth of 52 m in the medium-coarse sand layer at SY14. Sediment TC and TOC contents were low, with a mean value of 1.0% and 0.5%, respectively. For the shallow clay and silty clay layers, the TOC contents were almost equal to TC contents, while in the deep sand layers, TOC were much lower than TC contents (Figs. 3–6), with an average TOC content of 0.74% in the shallow clay and silty clay and 0.38% in the deep sand.
Fig. 5. Geochemical profiles of the sediment samples collected from SY03.
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Fig. 6. Geochemical profiles of the sediment samples collected from SY15.
3.3. Arsenic and iron speciation in the sediment Arsenic speciation in sediment profiles at SY14 was measured through XANES analysis. Results showed that sediment As speciation varied with depth and lithology. Arsenic species changed from arsenate (HxAsOx4 − 3) to arsenite (H3AsO3) with depth: arsenate decreased from 86% to 28% from 1.5 m to 4.9 m, while arsenite increased from 14% to 64%. Arsenate increased again to 85% from 4.9 m to 18.5 m, then decreased to 15% at 36.3 m and 35% at 51.7 m. On the contrary, arsenite decreased to 15% at 18.5 m, 22% at 36.3 m and 17% at 51.7 m. Arsenian pyrite [Fe(S, As)2] was the dominant speciation in the deep medium-coarse sand at 36.3 m (63%) and 51.7 m (49%). High contents of As-sulfides (realgar and arsenopyrite) were also observed in the shallower clay sediment at 3.8 m (16%) and 8.0 m (35%) (Fig. 9). Bulk Fe K-edge EXAFS for the two samples from SY14 showed that 43% sediment Fe was associated with silicates, 25% with ferrihydrite, and 32% with ferrous smectite for the sample at 3.8 m, while for the
sample at 8.0 m, 94% sediment Fe contained in silicates and 6% in hematite.
3.4. Groundwater chemistry Arsenic concentrations of most samples were between 10 and 400 μg/L, including 43% samples in 10–100 μg/L, 9% in 100–200 μg/L, 4% in 200–400 μg/L. Only 14 samples (2%) showed As concentrations N 400 μg/L, with the highest As concentration being 2620 μg/L. High arsenic groundwater occurs mostly in areas along the Dongjin and Tongshun rivers, and in areas where the watercourse of the Yangtze River changes (Y. Gan et al., 2014; Y.Q. Gan et al., 2014). The distribution of arsenic in groundwater was highly heterogeneous, as indicated by the arsenic concentration of adjacent sampling points varying greatly (Y. Gan et al., 2014). Vertically, high arsenic groundwater primarily occurred at a depth between 15 and 35 m (Fig. 7, region b), which is the depth of most residential wells in the region.
Fig. 7. Relationship between depths and groundwater arsenic concentrations (left), and sediment As concentration (right) in the Jianghan Plain.
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Fig. 10. Relationship between groundwater Eh and sampling depths. Fig. 8. Relationship between sediment As and Fe concentrations.
Groundwater Eh changed with depths, decreasing from 2 to 11 m, increasing from 12 to 20 m, and decreasing again from 21 m to 26 m (Fig. 10). For the groundwater samples below 26 m, changes of average Eh were indistinctive, ranging between 85 and 110 mV (Fig. 10). 4. Discussion 4.1. Effects of groundwater regime and redox conditions on As concentration and speciation in the sediment In the field monitoring site, groundwater water tables fluctuated seasonally, with higher water levels during the rainy season and lower
water levels during the dry season. Dissolved arsenic concentration at most monitored wells showed temporal variations, which were positively correlated with groundwater levels changes, with a lower concentration corresponding to a lower water level during dry season and a higher concentration corresponding to a higher water level during rainy season (Duan et al., 2015; Schaefer et al., 2016). Higher As concentrations with a high percentage of As (III) were observed in the monsoon season (July–August) when groundwater table was high, and a decrease in the post-monsoon season (November–December) with lower groundwater level. The variability of total As and As speciation was more significant at the depth of 25 m (Deng et al., 2015). Sediment samples from SY14 were collected in November when percentage of As(V) in groundwater increased, more As, mainly As(V), would be scavenged onto fresh iron oxyhydroxides or silicate minerals, resulting in
Fig. 9. a. Liner combination fitting (LCF) of As XANES spectra at various depths. The XANES spectra of the As standards used in the fit are shown. b. Arsenic speciation determined by linear combination fits of As K-edge XANES as a function of sediment collection depth. Arsenic sulfides are the sum of the relative area of arsenopyrite, realgar, and arsenian-pyrite, which were used as standards in linear combination fitting of XANES spectra. Sodium arsenate and sodium arsenite were used as arsenate and arsenite standards.
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increased As concentration in the sediment. Besides, relatively high groundwater Eh around 20 m deep (Fig. 10) indicated higher sediment Eh, which may further promote As oxidation. These explained why arsenate was the main speciation in the silty sand sediment around 20 m deep. Groundwater table at SY14 ranged from 1.78 m (in the wet season) to 3.94 m bls (in the dry season) (data not shown), corresponding the observed transition from arsenate to arsenite below 4.0 m with annually saturated conditions (Fig. 9). Thus, groundwater regime played a key role in controlling As concentration and speciation in the shallow sediment. The potential release mechanism of dissimilatory As (V) reduction may also occur in the unsaturated zone when it becomes saturated in the wet season. Arsenic present predominantly as arsenate in the near surface, where more dissolved oxygen was existed as a result of diffusion from air in the unsaturated zone (Williams and Oostrom, 2000). In the deeper saturated zones, arsenite became the dominant species in the underlying clay and silty clay layers where anaerobic conditions prevailed. More reduced As speciation (As-sulfides) was observed in the medium to coarse sand aquifers N 35 m deep as the conditions became more reducing (Fig. 9). Thus, the redox condition directly controlled the speciation of As in the sediment. Meanwhile, low permeability layers, such as clay and silty clay layers, could maintain reducing conditions in the aquifers, which may increase the contents of reduced As speciation (arsenite and/or As-sulfides) in the sediment. Additionally, sediment As concentration in the silty sand layers underlying clay or silty clay layers were elevated. The possible reason was that arsenic released under reducing conditions in the clay layers over a long period of time could be transported down into the silty sand, resulting in the higher As concentration in this layer. 4.2. Effects of sediment As speciation and reactivity on groundwater As concentration High dissolved As concentrations typically occurred at the depth between 15 and 35 m, with the highest value around 20 m deep (Fig. 7). Correspondingly, sediment at the same range of depth (15–35 m) also contained higher concentration, with the highest concentration around 20 m deep (Fig. 7, zone b). Ammonium oxalate extraction results showed that 13 to 68% (mean 39%) As in the sediment at 15–35 m deep was bound with amorphous Fe oxides (Li et al., 2015). Thus, Fe oxides did host appreciable As within this high-As zones, and reductive dissolution was a major mechanism of As release. Additionally, main sediment As species at the same range of depth were As(V) and As(III), with 45% and 85% As(V) respectively at 16.3 m and 18.5 m. Arsenic could be released into groundwater by dissimilatory reduction of As(V) to less sorptive As(III) and/or dissimilatory reduction of Fe oxides that reduced As sorption sites (Ahmed et al., 2004; Berg et al., 2007; Islam et al., 2004; Paul et al., 2015; Stuckey et al., 2016; von Brömssen et al., 2008). Although higher As concentration also occurred in the near surface sediment (Fig. 7, zone a) and in the deep medium-coarse sand (Fig. 7, zone c), groundwater As concentration around the same depth were relatively low. For the near surface sediment above the groundwater table, the dominant As speciation was arsenate, which could be strongly sorbed onto iron oxides under oxidizing conditions. Many studies have demonstrated that ferrihydrite, a short-range order material common in soils and sediment, has a strong sorption capacity for arsenate and arsenite (Dixit and Hering, 2003; Kocar et al., 2006; Masue et al., 2007; Raven et al., 1998). Sediment sample at 3.8 m contained 25% ferrihydrite of the total Fe, indicating that As could be absorbed or coprecipitated with Fe oxides. The scavenged As could be released into groundwater by dissimilatory As(V) reduction and/or dissimilatory Fe oxides reductive dissolution when the conditions in the aquifers became reduced (Berg et al., 2007; Paul et al., 2015; Stuckey et al., 2016). While in the deep medium-coarse sand, the dominant As speciation in the sediment was As-sulfides (arsenian pyrite), which were stable minerals under
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reducing conditions. Under present geochemical conditions, arsenian pyrite represents a stable host of As in the reducing aquifer environment, and would be a source of As to groundwater only if the arsenian pyrite were oxidized (Stuckey et al., 2015b; Appelo and Postma, 1996; Chowdhury et al., 1999). 5. Conclusions Sediment As concentration is generally dependent on the lithological attributes, with the higher As concentration present in fine particle sediment, especially in the silty sand layers underneath clay or silty clay layers. Arsenate, arsenite, and As-sulfides were detected in the sediment samples at different depths, both the groundwater regime and redox conditions played important roles in controlling sediment As speciation. Sediment-associated As and groundwater As were highly correlated, the speciation and reactivity of sediment As strongly controlled the spatial distribution of groundwater As concentration, while seasonal variation in groundwater As concentration and speciation affected the content and speciation of sediment As. Arsenic was released into groundwater by dissimilatory As(V) reduction and/or dissimilatory Fe oxides reductive dissolution under reducing conditions. In the near surface, As (mainly as arsenate) was scavenged onto iron oxides under the oxidizing conditions. While in the deep medium-coarse sand, the dominant As speciation in the sediment was As-sulfides (arsenian pyrite), which were stable minerals under reducing conditions. More detailed work is still needed for source apportionment of arsenic in groundwater systems at Jianghan Plain, and in the Yangtze River basins at large. In respect of the significant seasonal variation of groundwater As concentration, sediment samples from other seasons need to be collected to reveal temporal variations of sediment As concentration. Acknowledgement The research work was financially supported by National Natural Science Foundation of China (No. 41521001, No. 4142217, and No. 41572226), Ministry of Science and Technology of China (No. 2014DFA20720), and the US National Science Foundation (grant number EAR-0952019), with additional support from South University of Science and Technology (GG01296001). We would like to thank Prof. Scott Fendorf, Dr. Guangchao Li, Doug Turner, Dr. Kristin Boye, Dr. Juan Salvador Lezama Pacheco and many graduate students at Stanford University for their help with laboratory measurements and data analysis. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. Constructive comments provided by anonymous reviewers helped to clarify and improve this manuscript. References Ahmed, K., Bhattacharya, P., Hasan, M., Akhter, S., Alam, S., Bhuyian, M., Imam, M., Khan, A., Sracek, O., 2004. Arsenic enrichment in groundwater of the alluvial aquifers in Bangladesh: an overview. Appl. Geochem. 19:181–200. http://dx.doi.org/10.1016/j. apgeochem.2003.09.006. Anawar, H.M., Akai, J., Sakugawa, H., 2004. Mobilization of arsenic from subsurface sediments by effect of bicarbonate ions in groundwater. Chemosphere 54:753–762. http://dx.doi.org/10.1016/j.chemosphere.2003.08.030. Appelo, C.A., Postma, D., 1996. Geochemistry, Groundwater and Pollution. Balkema Publishers, Leiden. Berg, M., Stengel, C., Trang, T K]–>P.T.K., Viet, P.H., Sampson, M.L., Leng, M., Samreth, S., Fredericks, D., 2007. Magnitude of arsenic pollution in the Mekong and Red River Deltas-Cambodia and Vietnam. Sci. Total Environ. 372:413–425. http://dx.doi.org/ 10.1016/j.scitotenv.2006.09.010. Bhattacharya, P., Chatterjee, D., Jacks, G., 1997. Occurrence of arsenic-contaminated groundwater in alluvial aquifers from delta plains, eastern India: options for safe drinking water supply. Int. J. Water Resour. Dev. 13:79–92. http://dx.doi.org/10. 1080/07900629749944. Bhattacharya, P., Jacks, G., Jana, J., Sracek, A., Gustafsson, J.P., Chatterjee, D., 2001. Geochemistry of the Holocene alluvial sediments of Bengal Delta Plain from West Bengal, India: implications on arsenic contamination in groundwater. In: Jacks, G., Bhattacharya, P., Khan, A.A. (Eds.), Groundwater Arsenic Contamination in the Bengal
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Arsenic speciation in aquifer sediment under varying groundwater regime and redox conditions at Jianghan Plain of Central China Yanhua Duan a, Yiqun Gan a,b, Yanxin Wang a,b,⁎, Chongxuan Liu c, Kai Yu a, Yamin Deng d, Ke Zhao a, Chuangju Dong d a
School of Environmental Studies, China University of Geosciences, 430074 Wuhan, China State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 430074 Wuhan, China School of Environmental Science and Engineering, Southern University of Science and Technology, 518055 Shenzhen, Guangdong, China d Geological Survey, China University of Geosciences, 430074 Wuhan, China b c
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Arsenic speciation was determined by linear combination fits of As K-edge XANES. • Sediment As speciation mainly controlled by redox conditions in the groundwater system. • Groundwater regime and redox conditions greatly affected As content and speciation in the sediments. • Sediment As speciation and reactivity controlled groundwater As concentration.
a r t i c l e
i n f o
Article history: Received 24 February 2017 Received in revised form 2 July 2017 Accepted 2 July 2017 Available online xxxx Editor: F.M. Tack Keywords: Arsenic pollution Redox controls Speciation Mobility Central Yangtze basins
a b s t r a c t At Jianghan Plain of central Yangtze basins where the health of N73, 000 people has been affected by long term intake of high arsenic groundwater, over 100 sediment samples from four boreholes at the field monitoring sites were collected and analyzed to delineate the distribution and speciation of As in the shallow aquifer sediment. Results showed that sediment As concentration is generally dependent on the lithological conditions, with the higher As concentration present in fine particle sediment, especially in the silty sand layers underlying clay or silty clay layers. High As concentration in the sediment mainly occurred in three different depth ranges: b 5 m, 15–35 m, and N35 m. Both the groundwater regime and redox conditions played important roles in controlling sediment As speciation. Arsenate (86%) was the dominated As species in the near surface sediment. As the redox turned to be reducing, arsenite (64%) became the dominant species in the underlying clay and silty clay layers. But in the silty sand aquifer near the boundary of unconfined aquifer and confined aquifer, arsenate (85%) became the dominant species again as results of redox potential elevation. In the deep medium to coarse sand aquifers (N 35 m deep), As-sulfides (49%–63%) were the main species of As. The speciation and reactivity of sediment As strongly controlled the spatial distribution of groundwater As concentration, while seasonal variation in groundwater As concentration and speciation affected the content and speciation of sediment As. © 2017 Elsevier B.V. All rights reserved.
⁎ Corresponding author at: No. 388, Lumo Road, Wuhan 430074, China. E-mail address: [email protected] (Y. Wang).
http://dx.doi.org/10.1016/j.scitotenv.2017.07.011 0048-9697/© 2017 Elsevier B.V. All rights reserved.
Y. Duan et al. / Science of the Total Environment 607–608 (2017) 992–1000
1. Introduction Geogenic arsenic (As) pollution of drinking water has been reported in N70 countries, posing a serious health hazard to an estimated 150 million people world-wide (Brammer and Ravenscroft, 2009), and 19.6 million people in China (Rodriguez-Lado et al., 2013). Long time exposure of As may cause various diseases such as melanosis, keratosis, non-petting oedema, gangrene, leucomelanosis, respiration system problems and an increased cancer risk for skin, bladder, and lungs (Chen and Ahsan, 2004; Chowdhury et al., 2000; Kapaj et al., 2006). As also causes cardiovascular disease and inhibits children mental development (Chen et al., 1996; Wasserman et al., 2004). Sediment As content and speciation are the two critical factors controlling As partitioning and transport. In oxidized aquifer sediment, As(V) adsorbs more strongly than As(III), and As(III) could be oxidized to As(V), leading to strong As retention (Stollenwerk et al., 2007). Under reducing conditions, As can be released from colloids or Fe/Mn oxide surfaces through desorption, reductive dissolution of Ascontaining Fe (III) minerals, and As(V) reduction to more labile As(III), which subsequently enhances As mobility and toxicity in the aquifers (Bhattacharya et al., 1997, 2002; Donselaar et al., 2017; Fendorf et al., 2010; Guo et al., 2008; Nickson et al., 2005; Paul et al., 2015; von Brömssen et al., 2007; Zheng et al., 2004). However, As can be removed from the aqueous phase via precipitation or incorporation of As-Fe sulfides under reducing conditions (Bostick et al., 2004; Lowers et al., 2007; O'Day et al., 2004). Sorption of both As(V) and As(III) is strong on Fe(III) oxides under circumneutral pH conditions (Dixit and Hering, 2003), small goethite particles can enhance As sorption and/or coprecipitation during As fixation (Xie et al., 2016). Incorporation of As in carbonates can effectively mobilize As from sediment into groundwater (Anawar et al., 2004; Kim et al., 2000; Saunders et al., 2005; Sengupta et al., 2004). Additionally, As(V) can substitute for SiO44 − within silicate minerals (Seddique et al., 2008), and As(V) is retained more appreciably than As(III) on silicate minerals, such as chlorite and halloysite (Lin and Puls, 2000). These previous results indicate that sediment As speciation directly controls As concentration, mobility and toxicity in groundwater. Characterizing the distribution and speciation of As in the sediment is therefore critical for understanding the speciation and spatial variation of groundwater As. Since the first reports of waterborne As poisoning in Shahu Village within the Jianghan Plain in 2005, studies by Deng et al., 2014, Y. Gan et al., 2014; Y.Q. Gan et al., 2014, Duan et al., 2015, Li et al., 2015, and Schaefer et al., 2016 have made efforts to understand the processes and factors leading to high concentrations of As in groundwater. The elevated As concentration in groundwater has affected the health of N 73,000 people, including 20,000 children (Li et al., 2010; Wang and Zhao, 2007). Our previous work was mostly focused on the effects of groundwater hydrogeochemistry on As distribution (Y. Gan et al., 2014; Duan et al., 2014) and dynamic changes in groundwater As concentration (Deng et al., 2014; Duan et al., 2015; Y.Q. Gan et al., 2014; Schaefer et al., 2016). Although a great deal of effort has been made in most high arsenic groundwater regions to investigate the mineralogy and geochemistry of the aquifers sediment, and sediment-associated As that are expected to have great impacts on As enrichment in groundwater (Bhattacharya et al., 2001; Hasan et al., 2009; Hossain et al., 2014; Mukherjee et al., 2009; Neidhardt et al., 2013; von Brömssen et al., 2008), litter work has been done in the Jianghan Plain. The results of previous work indicated that the sediment samples from Shahu Village contained As concentration ranging from 1 to 107 mg/kg (Duan et al., 2014; Li et al., 2015), with a median of 12.8 mg/kg (Li et al., 2015). Higher As concentration was observed in the silt and clay samples, with the highest concentration near 20 m depth. All sediment samples had high concentrations of Fe and Mn, with Fe2O3 contents ranging from 4% to 9% (Y. Gan et al., 2014). Li et al. (2015) also reported that 10%–70% (with a median value of 38%) As in the sediment could be extracted by ammonium oxalate, implying
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that a large amount As in the sediment was bound with amorphous Fe oxides. The main mineralogical compositions in the sediment were clay minerals (montmorillonite, chlorite, illinite and kaolinite) and quartz (Y. Gan et al., 2014). And the main source of As in the sediment at Jianghan Plain was thought to be the As-bearing minerals (As-bearing Fe oxides, and As-bearing sulfides) deposited during fluvial-lacustrine deposition (Li et al., 2015). The objectives of this research are: (1) to understand the hydrostratigraphy and the relationship with the sediment types; (2) to determine As concentration and speciation in sediment as a function of depth in the aquifer; and (3) to correlate sediment-associated As with groundwater As to provide insights into the factors controlling arsenic concentration in groundwater at Jianghan Plain. 2. Materials and methods 2.1. Site descripting Jianghan Plain is an alluvial plain formed by the Yangtze and Han rivers located in the Middle Reaches of the Yangtze River that includes the central and southern regions of Hubei Province (Fig. 1). It has a subtropical monsoonal climate, the average annual precipitation increases from 800 mm in the northwest to 1500 mm or more in the southeast, 30–50% of which occurs in summer. Jianghan Plain is a semi-closed Quaternary basin with a higher elevation in the north and a lower elevation in the south. The field monitoring site for this study was constructed in Shahu Village, the interior of the Jianghan Plain (Fig. 1), The site is surrounded by four rivers (Tongshun River, Dongjing River, Kuige River, and Lüfeng River), and covered by other abundant surface water bodies such as ponds, irrigation channel, and wetlands. Surface water levels at the field site fluctuate 5.6–7.6 m annually and are closely tied to monsoon rains. Well water levels follow a trend similar to surface water, but the magnitude of the fluctuations is approximately 0.7–2.4 m and the timing of falling and rising well water levels lags surface water changes (Duan et al., 2015; Schaefer et al., 2016). Strong surface watergroundwater interactions were observed here. During the wet summer monsoon when surface water levels are higher than groundwater levels, a groundwater recharge flow gradient develops; during drier, winter months, surface water levels drop below groundwater levels, and groundwater flow reverses and moves toward surface water. Further details of the field area are provided in previous publications (Duan et al., 2015; Deng et al., 2015; Schaefer et al., 2016). 2.2. Sampling Groundwater samples, 729 in total, were collected in Xiantao, Honghu, Qianjiang, and Jianli in the Jianghan Plain from 2011 to 2014. Temperature, conductivity, pH and oxidation-reduction potential (ORP) were measured in the field using a HACH HQ40D multimeter. ORP was converted to Eh depending on the water temperature. Samples were filtered and acidified with concentrated HCl for analysis of total dissolved As concentration. Sediment samples were collected during well installation in the Shahu field site from boreholes SY15 (N 30°09′23.73″, E 113°40′ 42.59″), SY03 (N 30°09′23.51″, E 113°40′39.21″), SY14 (N 30°09′ 18.62″, E 113°40′21.03″), and SY07 (N 30°09′19.66″, E 113°39′58.76″) (Fig. 1). Sediment samples from SY03 and SY07 were collected in November 2011, samples from SY14 and SY15 were collected in November 2014. A Geoprobe DT21 rig was used to get continuous in-situ sediment samples from depths b 20 m. The sediment (18 cm long) in sampling tube were cut and sealed for lab analysis. A rotary drill rig and splittube sampler with an internal stainless steel tube (17.5 cm length, 3.8 cm diameter) was used to obtain sediment samples N 20 m deep. After sample collection, both ends of the sampling tubes were covered with polytetrafluoroethylene caps and sealed with paraffin wax,
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Fig. 1. Location of the monitoring wells and boreholes (SY07, SY14, SY03, and SY15) in the Shahu field site at Jianghan Plain. Sediment samples were collected during well installation.
vacuum sealed in a vacuum bag and placed in a gas-tight box containing anoxic pouches, and placed on ice in the field and stored at 4 °C in the refrigerator prior to shipment to Stanford University for further analysis.
2.3. Samples analysis In the laboratory, total dissolved As concentration in groundwater was determined using a hydride generation-atomic fluorescence spectrometer (HG-AFS, 930, Titan, China). The detection limit of the instrument for As was 0.05 μg/L and the relative standard deviation was b1.0%. All samples were diluted several times to adjust for the operating range and then analyzed. Sediment samples were transferred to a glove box (95% N2, 5% H2), and then removed from the sample tube and allowed to dry under anoxic conditions. Samples' mixing, grinding, sieving, and preparation were also conducted in the glove box. Sediment pH was measured in 0.01 M CaCl2·2H2O (Dittmar et al., 2007). Total C (TC) content was measured using Carlo-Erba NA 1500 Elemental Analyzer. Sediment were acid fumigated to remove carbonates prior to the measurements of total organic carbon (TOC) (Harris et al., 2001). Total elemental composition was determined by X-ray fluorescence spectrometry (XRF). Mineralogical compositions of the sediment collected at SY03 were measured using an automated powder X-Ray Diffractometer (Cu-Kα radiation and a graphite monochromator) (X'Pert PRO DY2198, PANalytical) with a detection limit of 2%. Synchrotron powder X-ray diffraction for the samples at SY14 was performed at beam line 11-3 at the Stanford Synchrotron Radiation Laboratory (SSRL). Peak identification was performed using HighScore Plus.
Similar to the analyses described in Stuckey et al. (2015a), bulk As Xray absorption (XAS) (8 samples at SY14) and bulk Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy (only two samples were analyzed due to time limit for users) were performed at beam line 11-2 at SSRL. For As K-edge XANES spectroscopy, subsamples were packed into 2 mm thick polycarbonate slot mounts in Kapton tape (25 μm thickness) in glove box, and stored in gas-tight box containing anoxic pouches before the measurement. The energy selection was maintained by a Si (220) monochromator detuned 30–40% for harmonic rejection, and energy calibration was achieved by assigning a K-edge position of 11,874.0 eV to a Na3AsO4 standard. Fluorescent X-rays were measured with a multi-element Ge detector. For Fe EXAFS spectroscopy, sediment samples were deposited on a 0.2 μm cellulose nitrate filter paper and packed in Kapton tape (25 mm thickness). Energy selection was accomplished using a Si (220) monochromator detuned 40–50% for harmonic rejection, and energy calibration was achieved by assigning a K-edge position of 7111.0 to a Fe foil. Fluorescent X-rays were measured with a Lytle detector fitted with 3 μm thick Mn filter. Linear combination fitting of XANES and EXAFS data was used to quantitatively determine As and Fe species in sediment samples with standard compounds. 3. Results 3.1. Lithology and mineralogy of the sediment Sediment lithology at the Shahu site ranged from clay to coarse sand, with brownish-yellow to dark gray color (Figs. 2–6). The boundary of the phreatic aquifer and confined aquifer was located around 17.5 m
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Fig. 2. Hydrogeological profile from SY07 to SY15 of the field monitoring site.
below the land surface (bls). Above 17.5 m were interlayers of brownish-yellow to gray silty clay with brownish-gray clays. Thin brownish-gray to gray silty sand layers occurred discontinuously between the clay and silty clay layers. From 17.5 to 50 m bls, the main lithology of the sediment was sand, with particle size increasing with depth. Gray silty sand spanned 17.5–21 m, and gray to livid fine sand 21–40 m, with brownish-livid medium-coarse sand at depth between 40 and 50 m. Below the medium-coarse sand layers, livid fine sand layers occurred between 50 and 60 m. Several gray clay and silty clay lens were “sandwiched” by the sand layers. Gravels and spiral shells occurred in the sand layers from 34 to 55 m (Figs. 2–6). The main sediment mineralogical compositions were clay minerals and quartz. The content of montmorillonite, chlorite, illite and kaolinite in sediment samples from different depths ranged from 0–15 wt%, 8– 16 wt%, 12–25 wt%, and 0–10 wt%, respectively, while that of quartz from 31 to 54 wt%. The sandy sediment also contained a high content of feldspar (15–24 wt%). Synchrotron powder X-ray diffraction results also showed that the main mineralogical compositions for the sediment were quartz (account for 20%–78%) and silicate minerals (ranging from 55% to 77%). Higher contents of calcite (up to 20%), and albite (up to
21%) were also detected in the sand sediment N 40 m deep. For some samples, iron sulfide (FeS, 2.6 and 3.8 m), pyrrhotite (Fe7S8, 14.0 m), and westerveldite (AsFe, 8.0 and 21.5 m) were also detected. 3.2. Sediment chemical compositions For the sediment collected at SY07, higher As concentration were observed in the silty clay layer located at b 4.0 m depth (mean 13.4 μg/g) and in the clay (mean 17 μg/g) and silty sand (mean 20.7 μg/g) layers located from 15 to 17.5 m (Fig. 3). The sediment from SY03 and SY14 both showed higher As in the shallow clay and silty clay layers (b 5 m deep), and the highest As concentration in the silty sand layers, with 39.8 μg/g at 18.5 m for the sediment at SY14 and 107.5 μg/g at 20.1 m for the sediment at SY03 (Figs. 4, 5). For the sediment collected at SY15, only the shallow clay and silty clay (b 5 m) had higher As concentration (Fig. 6). Additionally, higher As concentrations also occurred in deep clay and silty clay lens “sandwiched” by the sand layers (Fig. 7). On the whole, the concentration of sediment associated As ranged from 0.1 to 107.5 μg/g, with a median of 7.0 μg/g, and a mean of 10.0 μg/g. Sediment As concentration is generally dependent on lithological
Fig. 3. Geochemical profiles of the sediment samples collected from SY07. Only the top 18 m sediment samples were collected due to the damage of the auger.
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Fig. 4. Geochemical profiles of the sediment samples collected from SY14.
conditions in our study area, which is similar to the findings in high As groundwater regions at Bangladesh (Hossain et al., 2014; von Brömssen et al., 2007). Fine particle sediment contained a higher As concentration at each borehole, especially in the silty sand layers underneath clay or silty clay layers. Based on sediment As concentration, the vertical As distribution can be divided into 3 sections: b 5 m deep, between 15 and 35 m deep, and N35 m deep (Fig. 7). Arsenic concentration in the shallow clay and silty clay sediment (b 5 m) ranged from 9 to 21 μg/g, with a mean value of 14.6 μg/g. For the sediment below 35 m, As concentration were higher in the intermittent clay and silty clay lens than in the sand layers. Arsenic concentration of the sediment between the depth of 15 m and 35 m bls ranged from 0.1 to 107.5 μg/g, with a mean value of 13.1 μg/g. Extremely high As concentration occurred in the silty sand layers around 20 m deep (Figs. 5, 7). All of our samples had high concentrations of Fe and Mn (Figs. 3–6). Total Fe concentration ranged from 10 to 70 mg/g, with a median
value of 43 mg/g, and a mean value of 41 mg/g. Iron concentration in the shallow clay and silty clay (mean 47 mg/g) were much higher than in the deep sand (mean 34 mg/g). The highest Fe concentration was observed at the depth of 52 m in the medium-coarse sand layer at SY14. Sediment samples with As concentration b 20 μg/g showed positive relationship between As and Fe concentrations (Fig. 8).Total Mn concentration ranged from 220 to 4400 μg/g, with a median value of 630 μg/g, and an average value of 750 μg/g. Like the Fe concentration in the sediment, the highest Mn concentration (4400 μg/g) also occurred at the depth of 52 m in the medium-coarse sand layer at SY14. Sediment TC and TOC contents were low, with a mean value of 1.0% and 0.5%, respectively. For the shallow clay and silty clay layers, the TOC contents were almost equal to TC contents, while in the deep sand layers, TOC were much lower than TC contents (Figs. 3–6), with an average TOC content of 0.74% in the shallow clay and silty clay and 0.38% in the deep sand.
Fig. 5. Geochemical profiles of the sediment samples collected from SY03.
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Fig. 6. Geochemical profiles of the sediment samples collected from SY15.
3.3. Arsenic and iron speciation in the sediment Arsenic speciation in sediment profiles at SY14 was measured through XANES analysis. Results showed that sediment As speciation varied with depth and lithology. Arsenic species changed from arsenate (HxAsOx4 − 3) to arsenite (H3AsO3) with depth: arsenate decreased from 86% to 28% from 1.5 m to 4.9 m, while arsenite increased from 14% to 64%. Arsenate increased again to 85% from 4.9 m to 18.5 m, then decreased to 15% at 36.3 m and 35% at 51.7 m. On the contrary, arsenite decreased to 15% at 18.5 m, 22% at 36.3 m and 17% at 51.7 m. Arsenian pyrite [Fe(S, As)2] was the dominant speciation in the deep medium-coarse sand at 36.3 m (63%) and 51.7 m (49%). High contents of As-sulfides (realgar and arsenopyrite) were also observed in the shallower clay sediment at 3.8 m (16%) and 8.0 m (35%) (Fig. 9). Bulk Fe K-edge EXAFS for the two samples from SY14 showed that 43% sediment Fe was associated with silicates, 25% with ferrihydrite, and 32% with ferrous smectite for the sample at 3.8 m, while for the
sample at 8.0 m, 94% sediment Fe contained in silicates and 6% in hematite.
3.4. Groundwater chemistry Arsenic concentrations of most samples were between 10 and 400 μg/L, including 43% samples in 10–100 μg/L, 9% in 100–200 μg/L, 4% in 200–400 μg/L. Only 14 samples (2%) showed As concentrations N 400 μg/L, with the highest As concentration being 2620 μg/L. High arsenic groundwater occurs mostly in areas along the Dongjin and Tongshun rivers, and in areas where the watercourse of the Yangtze River changes (Y. Gan et al., 2014; Y.Q. Gan et al., 2014). The distribution of arsenic in groundwater was highly heterogeneous, as indicated by the arsenic concentration of adjacent sampling points varying greatly (Y. Gan et al., 2014). Vertically, high arsenic groundwater primarily occurred at a depth between 15 and 35 m (Fig. 7, region b), which is the depth of most residential wells in the region.
Fig. 7. Relationship between depths and groundwater arsenic concentrations (left), and sediment As concentration (right) in the Jianghan Plain.
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Fig. 10. Relationship between groundwater Eh and sampling depths. Fig. 8. Relationship between sediment As and Fe concentrations.
Groundwater Eh changed with depths, decreasing from 2 to 11 m, increasing from 12 to 20 m, and decreasing again from 21 m to 26 m (Fig. 10). For the groundwater samples below 26 m, changes of average Eh were indistinctive, ranging between 85 and 110 mV (Fig. 10). 4. Discussion 4.1. Effects of groundwater regime and redox conditions on As concentration and speciation in the sediment In the field monitoring site, groundwater water tables fluctuated seasonally, with higher water levels during the rainy season and lower
water levels during the dry season. Dissolved arsenic concentration at most monitored wells showed temporal variations, which were positively correlated with groundwater levels changes, with a lower concentration corresponding to a lower water level during dry season and a higher concentration corresponding to a higher water level during rainy season (Duan et al., 2015; Schaefer et al., 2016). Higher As concentrations with a high percentage of As (III) were observed in the monsoon season (July–August) when groundwater table was high, and a decrease in the post-monsoon season (November–December) with lower groundwater level. The variability of total As and As speciation was more significant at the depth of 25 m (Deng et al., 2015). Sediment samples from SY14 were collected in November when percentage of As(V) in groundwater increased, more As, mainly As(V), would be scavenged onto fresh iron oxyhydroxides or silicate minerals, resulting in
Fig. 9. a. Liner combination fitting (LCF) of As XANES spectra at various depths. The XANES spectra of the As standards used in the fit are shown. b. Arsenic speciation determined by linear combination fits of As K-edge XANES as a function of sediment collection depth. Arsenic sulfides are the sum of the relative area of arsenopyrite, realgar, and arsenian-pyrite, which were used as standards in linear combination fitting of XANES spectra. Sodium arsenate and sodium arsenite were used as arsenate and arsenite standards.
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increased As concentration in the sediment. Besides, relatively high groundwater Eh around 20 m deep (Fig. 10) indicated higher sediment Eh, which may further promote As oxidation. These explained why arsenate was the main speciation in the silty sand sediment around 20 m deep. Groundwater table at SY14 ranged from 1.78 m (in the wet season) to 3.94 m bls (in the dry season) (data not shown), corresponding the observed transition from arsenate to arsenite below 4.0 m with annually saturated conditions (Fig. 9). Thus, groundwater regime played a key role in controlling As concentration and speciation in the shallow sediment. The potential release mechanism of dissimilatory As (V) reduction may also occur in the unsaturated zone when it becomes saturated in the wet season. Arsenic present predominantly as arsenate in the near surface, where more dissolved oxygen was existed as a result of diffusion from air in the unsaturated zone (Williams and Oostrom, 2000). In the deeper saturated zones, arsenite became the dominant species in the underlying clay and silty clay layers where anaerobic conditions prevailed. More reduced As speciation (As-sulfides) was observed in the medium to coarse sand aquifers N 35 m deep as the conditions became more reducing (Fig. 9). Thus, the redox condition directly controlled the speciation of As in the sediment. Meanwhile, low permeability layers, such as clay and silty clay layers, could maintain reducing conditions in the aquifers, which may increase the contents of reduced As speciation (arsenite and/or As-sulfides) in the sediment. Additionally, sediment As concentration in the silty sand layers underlying clay or silty clay layers were elevated. The possible reason was that arsenic released under reducing conditions in the clay layers over a long period of time could be transported down into the silty sand, resulting in the higher As concentration in this layer. 4.2. Effects of sediment As speciation and reactivity on groundwater As concentration High dissolved As concentrations typically occurred at the depth between 15 and 35 m, with the highest value around 20 m deep (Fig. 7). Correspondingly, sediment at the same range of depth (15–35 m) also contained higher concentration, with the highest concentration around 20 m deep (Fig. 7, zone b). Ammonium oxalate extraction results showed that 13 to 68% (mean 39%) As in the sediment at 15–35 m deep was bound with amorphous Fe oxides (Li et al., 2015). Thus, Fe oxides did host appreciable As within this high-As zones, and reductive dissolution was a major mechanism of As release. Additionally, main sediment As species at the same range of depth were As(V) and As(III), with 45% and 85% As(V) respectively at 16.3 m and 18.5 m. Arsenic could be released into groundwater by dissimilatory reduction of As(V) to less sorptive As(III) and/or dissimilatory reduction of Fe oxides that reduced As sorption sites (Ahmed et al., 2004; Berg et al., 2007; Islam et al., 2004; Paul et al., 2015; Stuckey et al., 2016; von Brömssen et al., 2008). Although higher As concentration also occurred in the near surface sediment (Fig. 7, zone a) and in the deep medium-coarse sand (Fig. 7, zone c), groundwater As concentration around the same depth were relatively low. For the near surface sediment above the groundwater table, the dominant As speciation was arsenate, which could be strongly sorbed onto iron oxides under oxidizing conditions. Many studies have demonstrated that ferrihydrite, a short-range order material common in soils and sediment, has a strong sorption capacity for arsenate and arsenite (Dixit and Hering, 2003; Kocar et al., 2006; Masue et al., 2007; Raven et al., 1998). Sediment sample at 3.8 m contained 25% ferrihydrite of the total Fe, indicating that As could be absorbed or coprecipitated with Fe oxides. The scavenged As could be released into groundwater by dissimilatory As(V) reduction and/or dissimilatory Fe oxides reductive dissolution when the conditions in the aquifers became reduced (Berg et al., 2007; Paul et al., 2015; Stuckey et al., 2016). While in the deep medium-coarse sand, the dominant As speciation in the sediment was As-sulfides (arsenian pyrite), which were stable minerals under
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reducing conditions. Under present geochemical conditions, arsenian pyrite represents a stable host of As in the reducing aquifer environment, and would be a source of As to groundwater only if the arsenian pyrite were oxidized (Stuckey et al., 2015b; Appelo and Postma, 1996; Chowdhury et al., 1999). 5. Conclusions Sediment As concentration is generally dependent on the lithological attributes, with the higher As concentration present in fine particle sediment, especially in the silty sand layers underneath clay or silty clay layers. Arsenate, arsenite, and As-sulfides were detected in the sediment samples at different depths, both the groundwater regime and redox conditions played important roles in controlling sediment As speciation. Sediment-associated As and groundwater As were highly correlated, the speciation and reactivity of sediment As strongly controlled the spatial distribution of groundwater As concentration, while seasonal variation in groundwater As concentration and speciation affected the content and speciation of sediment As. Arsenic was released into groundwater by dissimilatory As(V) reduction and/or dissimilatory Fe oxides reductive dissolution under reducing conditions. In the near surface, As (mainly as arsenate) was scavenged onto iron oxides under the oxidizing conditions. While in the deep medium-coarse sand, the dominant As speciation in the sediment was As-sulfides (arsenian pyrite), which were stable minerals under reducing conditions. More detailed work is still needed for source apportionment of arsenic in groundwater systems at Jianghan Plain, and in the Yangtze River basins at large. In respect of the significant seasonal variation of groundwater As concentration, sediment samples from other seasons need to be collected to reveal temporal variations of sediment As concentration. Acknowledgement The research work was financially supported by National Natural Science Foundation of China (No. 41521001, No. 4142217, and No. 41572226), Ministry of Science and Technology of China (No. 2014DFA20720), and the US National Science Foundation (grant number EAR-0952019), with additional support from South University of Science and Technology (GG01296001). We would like to thank Prof. Scott Fendorf, Dr. Guangchao Li, Doug Turner, Dr. Kristin Boye, Dr. Juan Salvador Lezama Pacheco and many graduate students at Stanford University for their help with laboratory measurements and data analysis. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. Constructive comments provided by anonymous reviewers helped to clarify and improve this manuscript. References Ahmed, K., Bhattacharya, P., Hasan, M., Akhter, S., Alam, S., Bhuyian, M., Imam, M., Khan, A., Sracek, O., 2004. Arsenic enrichment in groundwater of the alluvial aquifers in Bangladesh: an overview. Appl. Geochem. 19:181–200. http://dx.doi.org/10.1016/j. apgeochem.2003.09.006. Anawar, H.M., Akai, J., Sakugawa, H., 2004. Mobilization of arsenic from subsurface sediments by effect of bicarbonate ions in groundwater. Chemosphere 54:753–762. http://dx.doi.org/10.1016/j.chemosphere.2003.08.030. Appelo, C.A., Postma, D., 1996. Geochemistry, Groundwater and Pollution. Balkema Publishers, Leiden. Berg, M., Stengel, C., Trang, T K]–>P.T.K., Viet, P.H., Sampson, M.L., Leng, M., Samreth, S., Fredericks, D., 2007. Magnitude of arsenic pollution in the Mekong and Red River Deltas-Cambodia and Vietnam. Sci. Total Environ. 372:413–425. http://dx.doi.org/ 10.1016/j.scitotenv.2006.09.010. Bhattacharya, P., Chatterjee, D., Jacks, G., 1997. Occurrence of arsenic-contaminated groundwater in alluvial aquifers from delta plains, eastern India: options for safe drinking water supply. Int. J. Water Resour. Dev. 13:79–92. http://dx.doi.org/10. 1080/07900629749944. Bhattacharya, P., Jacks, G., Jana, J., Sracek, A., Gustafsson, J.P., Chatterjee, D., 2001. Geochemistry of the Holocene alluvial sediments of Bengal Delta Plain from West Bengal, India: implications on arsenic contamination in groundwater. In: Jacks, G., Bhattacharya, P., Khan, A.A. (Eds.), Groundwater Arsenic Contamination in the Bengal
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