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Science of the Total Environment 427-428 (2012) 286–297
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Science of the Total Environment
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Occurrence and geochemical behavior of arsenic in a coastal aquifer–aquitard systemof the Pearl River Delta, China
Ya Wang a, Jiu Jimmy Jiao a,⁎, John A. Cherry b
a Department of Earth Sciences, The University of Hong Kong, Hong Kong, PR Chinab School of Engineering, University of Guelph, Guelph, ON, Canada N1G 2W1
⁎ Corresponding author. Tel.: (852) 2857 8246; fax: (E-mail address: [email protected] (J.J. Jiao).
0048-9697/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.scitotenv.2012.04.006
a b s t r a c t
a r t i c l e i n f oArticle history:Received 30 December 2011Received in revised form 31 March 2012Accepted 1 April 2012Available online 2 May 2012
Keywords:ArsenicOccurrenceGeochemical behaviorCoastalAquifer–aquitard systemThe Pearl River Delta
Elevated concentrations of arsenic, up to 161 μg/L, have been identified in groundwater samples from theconfined basal aquifer underlying the aquitard of the Pearl River Delta (PRD). Both aquatic arsenic in porewater and solid arsenic in the sediments in the basal aquifer and aquitard were identified. Arsenic speciationof groundwater in the basal aquifer was elucidated on a pH-Eh diagram. In the PRD, arsenic is enriched ingroundwater having both low and high salinity, and arsenic enriched groundwater is devoid of dissolved ox-ygen, has negative Eh values, is slightly alkaline, and has abnormally high concentrations of ammonium anddissolved organic carbon, but low concentrations of nitrate and nitrite. Results of geochemical and hydroche-mical analyses and sequential extraction analysis suggest that reductive dissolution of iron oxyhydroxidecould be one of the important processes that mobilized solid arsenic. We speculate that mineralization of sed-imentary organic matter could also contribute to aquatic arsenic. Scanning electron microscope analysis con-firms that abundant authigenic pyrite is present in the sediments. Sulphate derived from paleo-seawaterserved as the important sulfur source for authigenic pyrite formation. Co-precipitation of arsenic with authi-genic pyrite significantly controlled concentrations of aquatic arsenic in the coastal aquifer–aquitard system.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Quaternary aquifers are important sources of groundwater re-sources, and are also important links between terrestrial and marinebiogeochemical cycles in coastal zones (Moore, 1999; Charette andSholkovitz, 2002; Slomp and Van Cappellen, 2004). Groundwaterflowing from coastal aquifers to the sea can bring large amounts ofeutrophication-causing nutrients andmetals into the marine environ-ment (Shaw et al., 1998; Moore, 1999; Montlucon and Sanudo-Wilhelmy, 2001; Burnett et al., 2003). Because of its importance,arsenic contamination of groundwater in Quaternary aquifers hasbeen studied extensively in many parts of the world (Nickson et al.,1998; Ahmed et al., 2004; McArthur et al., 2004; Agusa et al., 2006;Berg et al., 2007). The enrichment and geochemical behavior ofarsenic in Quaternary groundwater systems in coastal regions is ofboth scientific and environmental importance.
The Pearl River Delta (PRD), located in the coastal area of SouthChina, was selected for this study. The PRD has a large coastal aquifer–aquitard system, and some aspects of its hydrogeochemical conditionsand geochemistry have already been well documented (Jiao et al.,2010; Wang and Jiao, 2012). Elevated concentrations of arsenic ingroundwater, exceeding both international and national standards(10 μg/L and 50 μg/L, respectively) have been reported in several
852) 2517 6912.
rights reserved.
localities in North China, mainly in Hetao Basin, Shanxi, Jilin Provincesand Xinjiang (Guo et al., 2008; Xie et al., 2009; Tang et al., 2010;Currell et al., 2011; Guo et al., 2011; Wang et al., 2011). Yu et al.(2007) estimated that approximately 580,000 people in North Chinaare exposed to drinking water containing >50 μg/L arsenic. However,very few studies of groundwater arsenic have been carried out inSouth China. Hydrogeological surveys in 2007 and 2008 showed forthe first time that the basal sand and gravel aquifer of the PRD is richin dissolved arsenic, mostly higher than the current World HealthOrganization limit for drinking water.
Arsenic mobilization processes can vary with geochemical condi-tions (Das et al., 1996; Mandal et al., 1998; Nickson et al., 1998;McArthur et al., 2001), thus studies of specific geochemical conditionsof areas with arsenic-rich groundwater are important for understand-ing the mechanisms involved. Most previous studies have focused onthe geochemical conditions of arsenic in aquifers (Foust et al., 2004;Ravenscroft et al., 2005; Postma et al., 2007; Nguyen et al., 2009;Kumar et al., 2010), and the hydrogeological and geochemical charac-teristics of aquitards above and below the aquifers were largelyignored. In this study, we treated the aquifer–aquitard system as ahydrogeological continuum, and focused on the hydraulic relation-ship between them in order to understand the mechanisms of arsenicmobilization and the behavior of arsenic in this specific system. To thebest of our knowledge, very few studies of groundwater arsenic havebeen carried out in a coastal aquifer–aquitard system, which is stillinfluenced by paleo-seawater intrusion.
287Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
2. Geological settings of the study area
The central and southern parts of the PRD are mostly covered byQuaternary sediments, although bedrock crops out both around andwithin the delta plain (Fig. 1A). The basement rocks include shale,sandstone, limestone, dolomite, granite, and gneiss ranging in agefrom Cambrian to Tertiary (GHT, 1981; Long, 1997). Within thedelta plain the elevation ranges from 6–9 m above sea level in thenorthwest to 1–2 m near the southeast coast (GHT, 1981). A rivernetwork is extensively developed in the delta plain (Fig. 1), and thegeneral direction of regional groundwater flow in the sand and gravelaquifer follows roughly the major river flow, which is from northwestto southeast.
Late Quaternary stratigraphic units in the PRD basin are generallydivided into four sequences: two terrestrial units (T1 and T2) and twomarine units (M1 and M2) (Zong et al., 2009a) (Fig. 1B). The oldterrestrial unit (T2), mainly composed of sand and gravel, is widelydistributed in paleo-valleys formed prior to the last transgression in
Macao
GuangzhouN
30 km
MinzhoMZ4
HJ1
PK25ZK83
DL1
PK27D22
GK2
-55
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
5
A
B
Alt
itu
de(
m)
Soil Fine sand
Silt and clay Sand and gravel
Weathered clay Bedrock
PK25 ZK8
0 (km)15
HJ1
T1
M2
T 2
M1
8.8-8.4
V I E 0
105
Fig. 1. A. Map of the study area. Yellow areas are the Quaternary plain, and green areas are bare marked in blue. The site (MZ4) at Minzhong Town with multi-level piezometers was mCalibrated carbon-14 ages were shown on the cross-section for reference.
the late Pleistocene. The old marine unit (M2), mainly composed ofsilt and clay, formed during the interglacial period starting at about130 ka before present (Yim, 1994; Yim et al., 2002; Zong et al.,2009a). In most parts of the plain, the upper part of M2 contains alayer of weathered clay, formed during low sea level caused by thelast glacial event. After this, a younger terrestrial unit of alluvial sed-iment, mainly composed of sand, was laid down along paleo-riverchannels. The youngest terrestrial unit (T1) consists of both weath-ered clay and alluvial sediments. After radiocarbon age of 8.2 calibrat-ed ka before present (cal. ka BP), rapid rise of sea level during thepostglacial period resulted in a large-scale Holocene transgression,and led to the formation of younger marine sediments (M1), 5 to20 m thick (Zong et al., 2009a). The calibrated radiocarbon ages mea-sured on sediments are available from various studies of the PRD(Huang et al., 1982; Li et al., 1991; Zong et al., 2009a) and some ofthem are shown in Fig. 1B. The fine-grained silts and clays of M1and M2 comprise the aquitards, and the terrestrial unit T2 of sandand gravel is the basal aquifer of this coastal groundwater system.
Hong Kong
Shenzhen
Pearl River Deltaplain area
South China Sea
ng
3 DL1 PK27 D22 GK2
Sequence boundary
21-20.3
10.7-
9.9
7.7-7.4
7.0-6.3
5.5-
5.3
South China Sea
Pearl River delta and estuary
C H I N A
T N A M200 km
°E 110°E 115°E
20°N
25°N
edrock outcrops. Boreholes are marked in red and the boreholes along the cross sectionarked with a red triangle. B. Simplified cross-section of the PRD Quaternary sediments.
288 Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
3. Sampling and analytical methods
3.1. Field sampling and measurements
A total of 18 boreholes (Fig. 1A) were drilled using rotary drillingduring the dry season from December 2007 to March 2008. Sedimentsamples were collected from the central parts of the drill cores toavoid contamination and then sealed in plastic sampling bags. Afterdrilling, temporary wells were made using steel pipes with a diame-ter of 110 mm; open intervals were fixed in the basal aquifer andscreened. After each well was adequately developed, groundwaterwas sampled when the pumped water from the well became clean.Groundwater samples were filtered by passing through 0.45 μmmembrane filters and were used to fill two 125 ml high density poly-ethylene sampling bottles. One bottle, which was used for major cat-ion and trace element analyses, was acidified using double distillednitric acid until the pH was lower than 2; the second bottle, with nomodification, was used for major anion analysis. The same procedureswere used to sample groundwater from piezometers at MinzhongTown (MZ4 site, close to borehole MZ4). Water and sediment sam-ples were preserved with frozen blue ice in a cooler in the field andanalyzed in the laboratory as soon as possible.
Temperature, pH, total dissolved solids (TDS), dissolved oxygen(DO) and redox potential (Eh) of the groundwater were measuredin the aquifer by placing YSI 6-Series Multiparameter Water QualitySondes (6920V2) (YSI, 2006) directly into the well at a depth equalto the middle of the aquifer. Readings were taken after stability wasestablished between the sondes and the groundwater. The sondeswere checked with standard solutions before every use and calibratedif necessary. NH4
+ was analyzed in the field with a HANNA HI93733Ion Specific Meter, and HCO3
− was measured by using a HACH DigitalTitrator (Mode 16900). The method of standard addition was used forquality control, and recovery rates were within 95–104%.
3.2. Groundwater sample analysis
Major ions chloride and sulphate were analyzed by ion chroma-tography and dissolved organic carbon (DOC) was analyzed on anIL550 TOC-TN Analysis System. Ca2+, Mg2+, Na+ and K+ were ana-lyzed by Inductively Coupled Plasma Atomic Emission Spectrometry(ICP-AES). Concentrations of groundwater trace elements (As, Mn,Se, Sr, V) were analyzed by Inductively Coupled Plasma Mass Spec-troscopy (ICP-MS). Standard reference material (SRM) 1640 wasused for quality control, and relative errors for the analyzed elementswere within 8%. The Eh–pH diagram was drawn using Geochemist'sWorkbench software (Bethke, 1996).
3.3. Sediment sample analysis
Analysis of total organic carbon (TOC) in sediments wasperformed on a ThermoQuest Italia S.P.A. EA 1110. Prior to analysis,sediment samples (about 20 g for each) were oven dried at a temper-ature of 55 °C for 72 h, treated with 1M HCl to remove inorganiccarbon, and then ground into powder with a particle size less than75 μm using an agate pestle and mortar. The contents of majorelements (K, Ca, Na, Mg, Al, Fe, Mn and Ti oxides) in the sedimentswere determined by wavelength-dispersive X-ray fluorescencespectroscopy (WD-XRF) (Philip). Glass disks were prepared usingsediment samples and lithium borate (1:7 ratio). Relative errors ofmajor element determination were within 5%. Total arsenic contentsin the sediment samples were measured by WD-XRF (Philip) onpressed powder pellets using standard curves based on Chinese,American and Japanese rock standards. SRM (aqueous sedimentGBW07311 (GSD-11) and rock powder GBW07104 (GSR-2)) wereused for quality control.
The bulk mineralogy of the sediments was determined by X-raypowder diffraction (XRD) using a D8 Advance Diffractometer (BrukerAXS) equipped with a Cu X-ray tube operated at 40 kV and 40 mA.The system was calibrated by SRM 660a (lanthanum hexaboride,LaB6), obtained from the U.S. National Institute of Standard and Tech-nology, for the baseline position. Scans were collected from 5 to 80°2-theta, with a step width of 0.02 and a sampling time of 2 s perstep. Approximate relative abundance ratios of major minerals wereestimated from the relative intensities. Sediment samples from aqui-tards were used to exam mineralogical details using a scanning elec-tron microscope (SEM), combined with an energy dispersive X-rayspectrometer.
Sequential extraction was organized for freeze-dried sedimentsamples from cores of SD1 and MZ4 to separate arsenic contents indifferent phases by the following widely used extraction procedure(Keon et al., 2001; Wenzel et al., 2001; Anawar et al., 2003; Rezaet al., 2010):
Step 1, acid extractable: Each sediment sample of 0.5 g was placedfor 5 h at 25 °C in 8 ml 0.1M acetic acid, and the mixed solutionwas adjusted to pH 5.0. These samples were agitated continuously.Step 2, reducible: The sediment residue from step 1 was placed for30 min in 20 ml 0.1M NH2OH.HCl in 0.01M nitric acid to extractAs from Mn oxide and carbonate phases. Then the sedimentresidue was treated for 30 min at 50 °C with 0.1M NH2OH.HCl in0.25M HCl to extract As from poorly crystalline Fe oxides. Thesesamples were agitated occasionally.Step 3, organic matter: The sediment residue from step 2 wastreated with 0.1M sodium pyrophosphate at pH 10.0 for 12 h.Mixtures of sodium pyrophosphate and sediment were heatedon a hotplate and dissolved into 1M ammonium acetate to makea final volume of 20 ml.Step 4, residual: Any remaining residue from step 3 was dissolvedusing a mixture of concentrated HCl and potassium chlorate.
The above extractions were conducted in 50 ml centrifuge tubes.Distilled and deionized water was used to rinse the sediment samplesafter each extraction and the rinsed solution was added to theextracted solution from each step of the procedure. Sediment and so-lution mixtures were separated by centrifuging at 3000 rpm for20 min. Supernatants were filtered by passing through 0.45 μm Milli-pore filters. Arsenic was analyzed by ICP-MS (Perkin-Elmer SCIEXELAN 9000, Wellesley, MA, USA) (USEPA Method 6020A), with cali-bration standards covering a range from 1 μg/kg to 50 μg/kg usinghigh-purity standards (Charleston, SC, USA).
4. Results and interpretation
4.1. Hydraulic relationship between the aquitard and aquifer
The hydraulic profile of the aquitard studied by Jiao et al. (2010) atMZ4 site (Fig. 1A) indicated downward flow through the aquitard intothe aquifer. The total vertical head difference between the top and thebottom of the aquitard was about 1.5 m, most of which was dissipatedbetween 17 and 30 m, suggesting that the deeper aquitard zonehad vertical hydraulic conductivity significantly lower than the zoneabove it. The vertical hydraulic conductivities at different depths inthe aquitard, estimated from slug tests in the piezometers, rangedfrom 2.57×10−5 to 1.16×10−4 m/day, and vertical groundwater ve-locities were estimated to range from 0.39×10−3 to 1.84×10−3 m/yr(Wang, 2011).
The regional groundwater flow in the PRD basal aquifer is alsostagnant and paleo-seawater derived from Holocene transgression isstill trapped in this unit (Wang and Jiao, 2012). Previous studiesalso demonstrated that ammonium concentrations in the basal
10km
N
0 160160 80 80
As: µ g/L
MZ4
Fig. 2. Concentrations of arsenic in the PRD basal aquifer. The size of the dots indicates the values of arsenic concentration. The scale of the concentration is shown in the dot at SD1.
289Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
aquifer are largely controlled by ammonium concentrations in theoverlying aquitard (Jiao et al., 2010). In the current study, the aquiferand aquitard were treated as an integrated system to investigate theoccurrence and geochemical behavior of arsenic.
4.2. Concentration and speciation of arsenic in the basal aquifer and aquaticproperties of the aquifer-aquitard system enhancing arsenic mobilization
Groundwater arsenic concentrations show variable spatial distri-butions in the basal aquifer (Fig. 2), with concentrations rangingfrom 3.3 to 161 μg/L (Table 1). Physical and chemical parametersof the groundwater samples are provided in Table 1. A study of thevertical distribution of As in the groundwater showed all of theenriched concentrations (>10 μg/L) occur in the basal aquifer buried6.6 to 42 m below ground surface (Table 1). In contrast, groundwatersamples taken from the basal aquifer with burial depths less than6.6 m are basically arsenic free (Table 1).
Table 1 shows that pH values of water from the basal aquiferrange from 5.7 to 7.9, with most above 7, indicating a slightly alkalineenvironment. This apparently is a common characteristic of arsenic-richgroundwater in strongly reducing aquifers (Smedley and Kinniburgh,2002). Values of Eh are all below zero, ranging from −63 to −337 mV.A plot of groundwater Eh and pH values in a Pourbaix diagram (Fig. 3)shows that As(III) is the dominant species in the basal aquifer.Dissolved oxygen (DO) concentrations are below limit of detection insome of the investigated boreholes (Table 1). Elevated ammonium(NH4
+>10 mg/L) originating from mineralization of sedimentaryorganic matter existed in the Pleistocene basal sand aquifer, which hasa total area >1600 km2 (Jiao et al., 2010). Nitrate and nitrite nitrogenconcentrations are less than 5 mg/L and 0.09 mg/L (unpublished data),respectively, in this confined, ammonium-rich aquifer. Negative Ehvalues, high ammonium concentrations, low nitrate nitrogen and nitritenitrogen and a lack of DO in the groundwater of this aquifer indicate amoderately to strongly reducing environment. Reducing conditionscan enhance solid arsenic mobilization and lead to enriched arsenic con-centrations in groundwater (Kim et al., 2000; McCreadie and Blowes,2000; McArthur et al., 2001; Ahmed et al., 2004; Reza et al., 2010).
Arsenic concentrations are enriched (>50 μg/L) in groundwaterwith both high (SD1, SD17 and SD18) and low salinity (SD3 and
SD6) in the basal aquifer (Table 1). Groundwater types are mainlyNaCl-type, Ca(HCO3)2-type and NaHCO3-type (Wang and Jiao,2012). Salinity in the confined Quaternary aquifer is mainly con-trolled by seawater fractions derived from the large-scale Holocenetransgression (Wang and Jiao, 2012). A lack of significant statisticalcorrelations between As and TDS or Cl- (figures not shown) indicatethat arsenic enrichment in the groundwater of the basal aquifer is not as-sociated with a marine source. Elevated concentrations of SO4
2− (up to490 mg/L) are expected in aquifers influenced by seawater, but SO4
2−
concentrations in this aquifer are lower than the concentrations basedon theoretical mixing between fresh groundwater and seawater.The deficiency of SO4
2− is attributed to SO42− reduction in this reducing
environment (Wang and Jiao, 2012).Concentrations of major ions and trace elements determined for the
aquitard are shown in Table 2, and their vertical variations are shown inFig. 4. Arsenic concentrations range from 1.7 to 28.7 μg/L in the slightlyalkaline environment (pH between 7.8 and 9.5). Abnormally highconcentrations of ammonium, ranging from 32.1 to 268.8 mg/L in theaquitard porewater, indicate a reducing environment. The major ionchemistry of piezometer porewater demonstrated that there is a signif-icant correlations between chloride and TDS (r=0.99, at the signifi-cance level of 0.01) and between chloride and sodium (r=0.99, at thesignificance level of 0.01), indicating a single source of chloride, sodiumand TDS. Furthermore, plots of chloride versus TDS and chloride versussodium show that the points are on, or close to, the theoretical mixinglines between fresh groundwater and seawater (figures not shown).Therefore, we conclude that the brackish water originated from paleo-seawater, which existed in the overlying aquitard, similar to that in theconfined basal aquifer. In this aquitard, which was significantly influ-enced by paleo-seawater, SO4
2− ranges from 1 to 28.8 mg/L (Table 2),suggesting significant removal of SO4
2− from porewater.
4.3. Solid arsenic distribution, physical and chemical properties of thebulk sediments
On the basis of the XRF analysis, solid-phase arsenic contents in thesediments vary from 5.0 to 39.6 mg/kg, with an average value of17.9 mg/kg (Table 3 and Fig. 5). These values are similar to, or slightlyhigher than, those in previous studies in other coastal aquifers. For
0 10 20 30
0
5
10
15
20
25
30
35
40
0 2500 5000 7500 10000
As concentration (µg/L)
Dep
th (
m)
Fe
Mn
Se
Sr
As
A) Fe, Mn, Se and Sr concentrations (µg/L)
0 10 20 30
0
5
10
15
20
25
30
35
40
0 100 200 300
TDS (g/L), Cl-(g/L),SO 42-(mg/L) and As
concentrations (µg/L)
Dep
th (m
)
NH4+
As
TDS
SO42-
Cl-
B) NH4+(mg/L) in piezometer water
290 Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
example, arsenic contents of sediments in a coastal plain, New Jersey,USA range from b1 to 49.6 mg/kg (Barringer et al., 2011); those fromthe head of Waquoit Bay, along the south shore of Cape Cod, MA, USArange from 3.6 to 7.5 mg/kg (Bone et al., 2006); those of a sand aquiferof eastern Australia range from 3.99 to 9.33 mg/kg (O'Shea et al.,2007); and those of an Indian Sundarban mangrove ecosystem rangefrom 0.6 to 1.53 mg/kg (Mandal et al., 2009). The enrichment of solidAs may reflect a detrital source from the widely distributed bedrockoutcrops in the Pearl River drainage basin (Long, 1997), concentrationduring sediment transport and deposition, and/or post-depositionalgeochemical and biologic processes.
A vertical lithofacies sequence of the sediments from four bore-holes (SD1, SD3, SD15 and MZ4) is shown in Fig. 5. Two of the profiles(SD1 and MZ4) include particle size variations of the sediments.Based on their textures, the sediments are classified as fine-grained(clay, silt and silty clay) and coarse-grained (silty sand, fine tomedium sand, sandy gravel and gravel). The distribution of solid arse-nic is clearly related to the grain size of the sediments, being signifi-cantly higher in the fine-grained samples (11.1–39.6 mg/kg, average20 mg/kg) than in the coarse-grained material (5.0–32.0 mg/kg, aver-age 14.5 mg/kg) (Table 4). This close correlation between arsenicconcentration and sediment grain size is also demonstrated in Fig. 5.Sediments in cores from borehole SD15, which range from siltysand to medium-grained sand and gravel, also have low solid As(5–17.4 mg/kg), with an average of 10.4 mg/kg. The enrichment ofsolid arsenic in fine-grained sediments is thought to be due to themuch greater surface of such material for adsorption (Padmalal etal., 1997; Singh et al., 2005). Such adsorbed arsenic may be releasedinto groundwater as a result of the pH-dependent desorption behav-ior of arsenate (Smedley and Kinniburgh, 2002). Elevated solidarsenic at the marine silts and clays of the PRD also suggests a marineinfluence on solid arsenic enrichment.
Sand and gravel layers devoid of sedimentary organic matter aregenerally gray, yellowish-grey or grayish-white in color. The sand andgravel basal aquifer in the PRD is dominantly composed of quartz, feld-spar, mica and carbonates and locally enriched in oxyhydroxides,sulfide minerals and silicate minerals (Huang et al., 1982; Lan, 1991).The clays and silts of the aquitard are dark gray, indicating the presenceof abundant organic matter. Organic carbon is relatively enriched in the
H3AsO
4
H2AsO4
-
HAsO42-
AsO43-
As(OH)3
As(OH)4-
AsO2OH2-
Orpiment
Realgar
1.2
.8
.4
0
-.4
-.80 2 4 6 8 10 12 14
pH
Eh(
V)
SD17 SD1
SD10
SD20
SD3
SD6
SD7
SD15
SD18
SD17
SD1
SD10
SD20
SD3SD6SD7
SD15
SD18
Legend
Fig. 3. Eh–pH diagram for As at average groundwater temperature of 24 °C and fixed Asactivity of 10−6 M. Dashed lines show stability limits of water at 1 bar pressure. The Ehand pH values of the tested groundwater samples are shown as points with boreholecode.
Fig. 4.Variations of porewater As, Fe,Mn, Se and Sr concentrations (A) and EC, Cl-, SO42− and
NH4+ concentrations (B) with depth from 7 piezometers installed in the aquitard at MZ4 site.
fine-grained sediments (ranging from 0.89 to 2.51 wt.%, with a meanvalue of 1.40 wt.%), compared with the coarse-grained sediments(ranging from 0.01 to 2.01 wt.%, with a mean value of 0.72 wt.%)(Table 3 and Fig. 5). Previous studies have shown that organic-rich sed-iments usually host As-rich groundwater under reducing conditions(e.g., Reza et al., 2010). A significant correlation between sedimentaryorganic carbon and solid As in the PRD (Fig. 6C, correlation coefficientr=0.63 at the significance level of 0.01) suggests that significantamounts of arsenic are contained in the organic matter. Many studiesindicate that sedimentary organic matter can serve as a redox driverfor arsenic mobilization, and some of the studies also suggest thatmicrobial metabolism of organic matter can enhance arsenic release(Dowling et al., 2002; Anawar et al., 2003; Postma et al., 2007; Jessenet al., 2008).
4.4. Mineralogical and geochemical composition of the PRD sediments
In general, detrital sediment compositions can be related to the na-ture of the source terrain, processes that occur during transport and de-position and post-depositional modification (Norman and DeDeckker,1990). Previous studies have shown that arsenic distribution in sedi-ments and groundwater is controlled by factors such as lithology,buried minerals, sedimentary organic matter, redox conditions and
Table1
Prop
erties
andmajor
ions
ofgrou
ndwater
from
theba
sala
quife
rin
thePR
D.
Hole
code
Sampling
date
Boreho
lelocation
Dep
thof
aquifer(m
)Te
mp.
TDS
Na+
K+
Ca2+
Mg2
+NH4+
Cl-
SO42−
HCO
3−DO
DOC
pHEh
As
TotalF
eMn
Sr
Long
itud
eLatitude
Top
Bottom
°Cg/L
g/L
mg/L
mg/L
mg/L
mg/L
g/L
mg/L
mg/L
mg/L
mg/L
mV
μg/L
mg/L
mg/L
mg/L
SD1
28-Jan
-08
E113
°12
′36.6″
N22
°55
′12.4″
36.4
39.9
24.0
16.0
3.72
136
498
486
310
8.27
bLO
D14
00Nd
35.8
6.70
−12
616
121
.20.50
8.30
SD2
13-Jan
-08
E113
°09
′19″
N22
°52
′13″
20.9
26.3
24.0
1.50
0.22
321
.943
.335
.559
.00.35
2.40
618
Nd
12.7
7.30
Nd
24.4
0.50
0.70
0.50
SD3
5-Jan-
08E1
13°05
′40″
N22
°46
′51″
11.7
18.0
24.0
0.80
0.03
914
.069
.222
.558
.90.07
0.60
418
Nd
19.0
7.00
−19
212
60.90
0.20
0.30
SD4
16-Jan
-08
E113
°07
′56″
N22
°48
′52″
18.6
24.2
24.0
2.60
0.68
519
.798
.929
.014
.71.35
19.9
34.6
Nd
Nd
6.40
Nd
10.3
21.9
1.20
0.50
SD5
10-Jan
-08
E113
°15
′24″
N22
°49
′09″
8.10
14.0
24.0
1.00
0.10
511
.012
930
.23.30
1.79
41.8
466
Nd
4.10
7.10
Nd
3.30
1.70
0.40
0.60
SD6
8-Jan-
08E1
13°13
′35″
N22
°47
′16″
6.60
16.8
24.2
0.50
0.02
48.40
77.7
16.2
0.80
0.00
64.20
353
Nd
1.70
7.20
−17
361
.51.80
0.20
0.30
SD7
16-D
ec-07
E113
°20
′17″
N22
°49
′15″
10.2
15.0
23.7
7.70
1.48
53.4
352
216
8.00
3.48
347
334
bLO
D1.60
7.10
−18
718
.5bLO
D1.10
6.50
SD10
31-D
ec-07
E113
°15
′10″
N22
°42
′7.4″
22.2
24.9
23.5
7.20
1.31
31.9
686
129
17.6
3.95
182
12.7
Nd
2.00
6.60
−63
.018
.737
.03.30
2.90
SD11
30-D
ec-07
E113
°13
′3.9″
N22
°41
′27.4″
3.40
9.90
24.0
0.60
0.06
97.10
67.0
15.3
11.1
0.08
69.30
280
Nd
5.30
7.30
Nd
4.60
bLO
D1.10
0.20
SD13
17-Jan
-08
E113
°21
′42″
N22
°45
′10″
28.3
40.0
24.0
5.80
1.17
56.5
89.5
163
57.2
2.26
9.00
411
Nd
1.20
7.50
Nd
48.0
8.90
0.40
1.00
SD15
29-D
ec-07
E113
°17
′13.9″
N22
°40
′15.4″
30.2
36.0
24.2
17.0
3.91
93.8
319
456
117
7.97
50.4
390
bLO
D7.80
7.60
−29
325
.7bLO
D3.80
3.50
SD17
17-D
ec-07
E113
°29
′3.3″
N22
°43
′54.4″
14.5
20.0
24.2
15.0
2.49
90.4
1396
605
15.4
8.39
490
367
bLO
D1.70
6.40
−12
060
.1bLO
D72
.06.80
SD18
23-D
ec-07
E113
°26
′44″
N22
°41
′44″
42.0
46.6
24.4
26.0
6.38
239
463
711
102
13.1
bLO
D10
26Nd
21.4
7.90
−33
757
.3bLO
D0.70
10.0
SD19
19-D
ec-07
E113
°24
′24″
N22
°36
′4.8″
25.2
28.3
24.0
8.50
1.23
38.4
618
234
48.7
4.75
88.8
29.1
Nd
2.90
5.70
Nd
20.4
452
28.0
5.90
SD20
25-D
ec-07
E113
°22
′2″
N22
°39
′06″
25.0
29.8
24.2
7.90
1.25
38.1
564
214
65.8
4.12
15.2
3.70
Nd
4.70
6.70
−10
935
.910
79.00
3.10
MZ4
23-Feb
-09
E113
°31
′47″
N22
°36′02
″34
.451
.024
.027
.08.55
253
153
594
347
14.1
bLO
D11
21Nd
Nd
8.10
−14
338
.20.70
2.50
4.20
bLO
Dmea
nslower
than
limitof
determ
ination;
Ndmea
nsno
tde
tected
.
291Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
water-rock interactions (Anawar et al., 2003; Hasan et al., 2007; Rezaet al., 2010). Thus, we explored all of these factors in the PRD.
XRD analysis (figures not shown) of 5 samples at different depthsfrom 3 boreholes (SD1: 5.5 m and 39.5 m; MZ4: 7.3 m and 36 m;SD15: 9.4 m) showed that the sediments are dominantly composedof quartz, illite, kaolinite, feldspar and mica. Quartz is abundant inall of the samples, whereas clay minerals (illite and kaolinite) aremostly concentrated in the fine-grained sediments. Heavy mineral(specific gravity >4.5) assemblages are generally similar in the fine-grained and coarse-grained deposits but are more abundant andmore diverse in the fine-grained samples. Previous studies of heavyminerals in the PRD sediments indicate that they make up 2.8–16 wt.% of the analyzed samples and consist chiefly of oxides, silicatesand hydroxides (Huang et al., 1982; Lan, 1991; Long, 1997).
Bulk contents of Fe, Mn, Al, Mg, Ti and P in core samples analyzedby XRF (Table 3) are low in SD15, whereas those in SD1, SD3 and MZ4are relatively high with uniform values. This probably reflects differ-ences in sediment character at those locations; sediments from SD1,SD3 and MZ4 are composed mainly of silt and clay, whereas thosefrom SD15 are mainly sand. The elements investigated were mainlyderived from terrestrial sources (Lan, 1991). XRF analysis of thecore sediments (Table 3) show that the MnO contents of sedimentsfrom the four boreholes are generally less than 0.15 wt.% (Table 3and Fig. 5). Generally low contents of P2O5 (0.03–0.19 wt.%, mean0.12 wt.%) and a significant correlation between P2O5 and Fe2O3
(r=0.90 at the significance level of 0.01) suggest that P is fixed iniron-bearing minerals, such as iron oxyhydroxides and vivianite.Titania contents (Table 3) in the silt and clays range from 0.72 to1.54 wt.%, with an average of 0.99 wt.%, which are higher than thosein the sand and gravel (0.13–1.11 wt.%, mean 0.57 wt.%). The abun-dance of TiO2 is consistent with the presence of ilmenite as a heavymineral in the PRD sediments (Huang et al., 1982; Wu and Zhao,1982; Lan, 1991). The Fe2O3 contents are generally enriched in thefine-grained sediments (3.96–8.9 wt.%, mean 6.0 wt.%) compared tothe coarser-grained sands (0.99–5.38 wt.%, mean 3.21 wt.%). A signif-icant correlation between Fe and Al (r=0.83 at the significance levelof 0.01) indicates that the iron-bearing heavymetals are concentratedin silt- and clay-rich sediments, consistent with the findings of Lan(1991). MgO contents are also significantly enriched in the fine sedi-ments (1.06–2.06 wt.%, mean 1.43 wt.%) vs. the coarser-grained varie-ties (0.10–1.47 wt.%, mean 0.72 wt.%). SiO2 is the most abundantoxide in the sediment samples (56.46–91.56 wt.%, mean 68.25 wt.%),but is negatively correlated with the other major elements (e.g. Fe2O3,MgO, K2O and P2O5, figures not shown) implying that the silica-richare mainly concentrated in the coarse-grained material. Significantcorrelations of solid arsenic with Fe2O3, MnO, Al2O3 (Fig. 6A, B and D)indicate that terrestrial detrital deposits from the PRD basin are themajor sources of solid arsenic in the sediments.
Arsenic and V are typically associated with Fe and Mn oxides(Smedley et al., 2005). The similar distribution of trace and majorelements in the sediments suggests that both are controlled by thesame processes (Abraham, 1998; Singh et al., 2005). Copper and Bashow similar variation trends in the profiles of the four boreholes. El-evated Ba (>400 mg/kg) is one of the important indicators of marinedeposits in the PRD (Lan et al., 1987; Lan, 1991). A significant correla-tion between Ba and As (r=0.64 at the significance level of 0.01)verifies a marine influence on the solid arsenic enrichment in thePRD deposits. Likewise, a very significant correlation between V andNi (r=0.92 at the significance level of 0.01) suggests that they mostlikely originated from the same source. Both Cu and Ni have a strongaffinity to sedimentary organic matter, Fe oxides and clay minerals(Tessier et al., 1994; Singh et al., 2005; Reza et al., 2010) and havesignificant correlations with solid arsenic in the PRD (r=0.74 at thesignificance level of 0.01). However, the good correlations betweenAs and the other trace elements do not prove a genetic link and theinherent relations between these elements remain uncertain.
Table 2Properties and major ions of pore water from 7 piezometers installed in the aquitard at MZ4.
Depth TDS K+ Ca2+ Na+ Mg2+ NH4+ Cl- SO4
2− HCO3− pH As TFe Mn Se Sr
m g/L mg/L mg/L mg/L mg/L mg/L g/L mg/L mg/L μg/L μg/L μg/L μg/L μg/L
P4 2.93 6.20 26.6 146 460.3 72.6 32.1 1.02 4.80 538 7.80 28.7 8257 859 23.9 899P2 7.16 15.5 225 47.0 4043 292 103 7.50 3.30 235 8.90 2.00 487 464 115 1815P8 11.6 22.5 384 69.8 5714 666 148 10.5 2.40 3590 8.60 1.80 317 532 172 3225P5 22.0 28.8 544 42.2 7920 698 269 15.1 1.00 1552 8.80 1.90 315 454 256 3517P6 27.3 23.6 473 85.1 7660 681 133 14.9 7.30 1978 8.80 2.60 3847 456 213 4327P3 29.2 25.1 451 56.7 7166 323 206 13.5 5.40 421 9.20 7.10 563 442 203 2907P7 35.1 12.4 178 45.2 3216 231.2 106 5.87 28.8 216 9.50 1.70 173 420 103 526
NH4+, pH and EC of pore water in piezometers (P2, P3, P4, P5, P6, P8) and piezometer (P7) were tested on 22 June 2009 and 16 December 2010, respectively.
292 Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
Clay minerals, noncrystalline aluminosilicate phases, amorphousFe, Mn, and Al hydroxides, and sedimentary organic matter arethe main phases that contain solid arsenic in the PRD sediments.Significant correlations were identified between arsenic and Fe, andbetween Mn and Al (Fig. 6), suggesting that the As is especially asso-ciated with Fe and Mn oxides and noncrystalline aluminosilicatephases. Correlation of arsenic with Al can also be caused by iron oxy-hydroxide coatings on altered silicate grains. A correlation betweenarsenic and iron in sediments has been documented by many studiesand interpreted as the result of adsorption of As onto iron hydroxides,followed by its release when the hydroxides undergo reductive disso-lution (Widerlund and Ingri, 1995; Mirlean et al., 2003; Zheng et al.,2004).
In the PRD, TOC also correlates well with arsenic in the sediments(r=0.63 at 0.01 significance level) (Fig. 6). Sequential extractionanalysis revealed that the dominant leachable materials containingsolid arsenic in the sediments are reducible phases, organic matterand residual phases, and that their contents are in the range of 0.84
Table 3XRF results of sediments from 4 boreholes (SD1, SD3, MZ4 and SD15) at different depth.
Holecode
Lithofacies Depth SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2
m wt.% wt.% wt.% wt.% wt.% wt.% wt.% wt.
SD1 Silt 4.50 56.5 1.30 19.4 7.80 0.10 1.40 0.70 0.3SD1 Silt 5.50 60.1 1.10 17.4 7.20 0.11 1.20 0.90 0.3SD1 Silty clay 9.50 62.7 0.90 13.9 6.20 0.09 1.60 3.40 0.5SD1 Silty clay 14.5 71.0 0.70 11.0 4.20 0.08 1.10 2.30 0.5SD1 Silty clay 18.5 71.1 0.80 10.9 4.00 0.08 1.10 2.70 0.5SD1 Silty clay 23.5 62.5 0.90 14.6 6.00 0.13 1.40 2.40 0.6SD1 Silty clay 25.5 61.2 0.90 14.5 7.30 0.12 1.50 2.20 0.7SD1 Silty clay 28.5 64.5 1.00 15.9 4.10 0.07 1.30 1.50 0.7SD1 Silty clay 31.5 63.6 0.90 15.4 5.10 0.10 1.30 1.70 0.6SD1 Fine sand 36.5 64.2 0.70 14.1 5.10 0.14 1.20 2.20 0.6SD1 Medium sand 39.5 83.7 0.20 6.80 2.10 0.05 0.40 1.20 0.3SD3 Silty clay 5.50 59.5 1.10 14.2 6.80 0.13 1.40 3.90 0.3SD3 Silty clay 9.10 60.2 1.10 13.8 6.10 0.12 1.20 4.70 0.4SD3 Medium sand 11.5 84.2 0.30 3.80 2.90 0.05 0.30 3.80 0.3SD3 Silty sand 15.8 74.3 0.60 9.80 3.70 0.06 0.70 2.50 0.2SD3 Medium sand 19.8 69.9 0.80 12.3 4.80 0.06 1.10 2.20 0.3MZ4 Silt 3.00 56.3 1.50 17.8 8.90 0.15 1.60 1.10 0.5MZ4 Silt 7.30 59.4 1.00 16.8 6.80 0.12 1.70 1.90 0.7MZ4 Silt 11.4 58.9 0.90 16.1 6.80 0.11 2.10 2.70 1.1MZ4 Silt 17.4 67.6 0.70 12.2 5.50 0.10 1.50 2.90 0.8MZ4 Silt 22.4 63.9 0.90 13.9 5.800 0.10 1.50 2.60 1.0MZ4 Silt 27.4 61.1 0.90 16.7 5.30 0.10 1.70 1.90 1.1MZ4 Silt 29.1 63.7 0.90 14.2 5.90 0.12 1.50 2.10 1.0MZ4 Coarse sand 35.0 59.4 1.00 17.0 5.40 0.15 1.50 1.90 1.0MZ4 Coarse sand 36.0 64.3 1.10 18.0 3.10 0.03 1.20 0.40 0.8SD15 Clay 3.50 66.6 1.00 11.5 4.90 0.07 1.20 4.40 0.5SD15 Silty sand 6.40 69.1 1.00 9.90 4.30 0.07 0.90 4.70 0.3SD15 Silty sand 9.40 84.7 0.40 5.60 2.50 0.05 0.50 1.20 0.2SD15 Silty clay 17.4 63.3 1.00 13.9 5.30 0.08 1.50 4.20 0.6SD15 Silty sand 24.4 77.4 0.70 8.20 3.60 0.06 0.90 2.60 0.4SD15 Fine sand 27.4 84.5 0.40 4.90 2.10 0.04 0.50 2.40 0.3SD15 Medium sand 34.4 91.5 0.10 3.10 1.10 0.02 0.10 0.50 0.2SD15 Gravel and sand 39.4 91.6 0.20 3.40 1.00 0.02 0.10 0.60 0.2
to 4.61 mg/kg, 1.6 to 5.42 mg/kg and 4.2 to 11.04 mg/kg, respectively.The dominant contribution of As is from residual sulfide and silicatephases (Fig. 7).
4.5. Mobilization of arsenic
In previous studies elsewhere, arsenic-rich pyrite has been gener-ally considered to be the dominant sources of dissolved arsenic ingroundwater, and oxidation of the pyrite is considered the principalmechanism of arsenic mobilization into groundwater (Das et al.,1996; Armienta et al., 1997; Schreiber et al., 2000). However, eventhough pyrite is present in the PRD sediments (Huang et al., 1982;Lan, 1991), oxidation cannot be the mobilization mechanisms inthe basal aquifer because this is a moderately to strongly reducingenvironment as indicated by deficient DO concentrations, negativeEh, high concentrations of ammonium, and very low concentrationsof nitrate and nitrite. Large amounts of methane gas have also beenidentified in the basal aquifer by previous surveys (Zhao, 1974). In
O K2O P2O5 TOC As V Cu Ni Cr Ba As/Fe
% wt.% wt.% wt.% mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg μg/mg
0 2.70 0.17 1.47 39.6 241 53.1 65.0 95.9 423 0.70 2.60 0.15 1.31 31.4 213 45.8 53.8 82.7 453 0.60 2.30 0.14 2.51 19.8 159 37.3 45.8 75.0 376 0.50 2.10 0.11 1.01 13.8 124 29.8 29.7 87.5 363 0.50 2.10 0.10 0.89 11.1 117 37.4 31.3 65.8 332 0.40 2.60 0.16 1.34 13.9 145 31.6 40.3 74.6 388 0.30 2.60 0.19 1.41 19.2 165 39.4 45.2 78.4 443 0.40 2.70 0.12 1.78 19.1 149 44.0 49.8 92.4 420 0.70 2.60 0.14 1.78 20.3 148 33.4 43.0 83.7 415 0.60 2.70 0.17 1.18 26.0 138 40.3 39.4 90.9 418 0.70 2.50 0.05 0.39 14.8 38.6 23.1 12.3 61.2 268 1.00 2.30 0.19 1.48 21.8 198 52.1 49.6 86.7 446 0.50 2.20 0.17 1.27 19.8 181 45.9 47.0 94.6 462 0.50 1.20 0.08 0.06 8.8 37.2 12.3 13.6 77.2 186 0.40 2.10 0.10 0.96 10.2 94.2 21.8 28.0 68.6 363 0.40 2.30 0.10 1.26 32.0 133 28.8 39.8 87.8 433 0.90 2.50 0.19 1.78 29.6 276 62.1 73.9 93.0 350 0.50 2.80 0.18 1.02 25.1 177 44.1 47.2 84.3 387 0.50 2.80 0.11 1.25 16.8 169 35.6 53.3 41.0 290 0.40 2.30 0.09 0.98 16.3 142 23.2 39.6 49.4 290 0.40 2.40 0.11 1.46 15.0 149 22.5 38.3 27.3 263 0.40 2.70 0.13 1.51 22.7 153 29.1 47.3 52.4 328 0.60 2.50 0.12 0.99 14.3 155 26.2 41.2 38.8 310 0.30 3.00 0.15 2.01 20.2 176 41.3 53.5 36.5 357 0.50 3.10 0.09 1.65 23.2 193 37.8 89.4 45.9 339 1.10 2.10 0.11 1.06 12.6 162 43.0 43.9 84.9 376 0.40 1.80 0.12 0.76 14.1 136 32.8 33.0 55.2 387 0.50 1.70 0.06 0.32 11.6 63.7 15.0 16.3 74.0 336 0.70 2.40 0.13 1.68 17.4 150 33.6 41.6 99.4 362 0.50 1.70 0.07 0.58 9.80 125 25.1 27.1 63.8 340 0.40 1.50 0.06 0.10 5.20 60.6 16.8 16.4 111.1 267 0.30 1.50 0.03 0.01 7.50 26.5 11.3 7.8 87.9 235 1.00 1.60 0.03 0.05 5.00 37.0 14.6 8.7 98.9 264 0.7
-25
-20
-15
-10
-5
0
0 10 20 30 40
0 5 10 15 20
Soil
Clay
Silt clay
Sand
Sand and grave
Bed rock
-45
-30
-15
0
MediumSilt
FineSilt
Very fineSilt
0 10 20 30 40
4 5 6 7 8 0 5 10 15 20Coarse
SiltFineSilt
-45
-30
-15
0
Dep
th (
m)
Dep
th (
m)
Dep
th (
m)
Dep
th (
m)
Medium Fine Very FineSilt Silt Silt
0 10 20 30 40 50
5 6 7 80 5 10 15 20
-60
-45
-30
-15
0
0 5 10 15 20
0 5 10 15
A SD1 Particle size TOC, Al2O
3, Fe
2O
3and MnO(wt%)
Phi
C MZ4 Particle sizePhi
B SD3
As(gk/gm)
As(gk/gm) As(gk/gm)
As(gk/gm)
D SD15
Al2O3
Fe2O
3MnOTOCAs
TOC, Al2O
3, Fe
2O
3and MnO(wt%)
TOC, Al2O
3, Fe
2O
3and MnO(wt%)TOC, Al
2O
3, Fe
2O
3and MnO(wt%)
CoarseSilt
FineSilt
Fig. 5. XRF and TOC analyses of the core sediments from SD1 (A); SD3 (B); MZ4 (C) and SD15 (D). Particle size distribution of SD1 and MZ4 was shown in (A) and (C), lithofaciessequences of SD3 and SD15 were shown in (B) and (D).
293Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
such a moderately to strongly reducing environment, the absence of acorrelation between dissolved arsenic and iron (Fig. 8B), as well as apH >7 (Table 1) in water of the basal PRD aquifer provide strong ev-idence that sulfide oxidation is not the major mechanism of As mobi-lization. A few of the of the water samples have pH values below 7.0,which are attributed to protons of ionized carbonic acid and increasedpCO2, released from decomposition of organic matter. Hence, the con-tribution of pyrite to arsenic concentrations in groundwater in thebasal aquifer is presumed to be negligible. Instead this aquifer isvery similar to others, such as those in the Bengal Basin, the HetaoBasin, and the Red River delta, where reducing processes have beenidentified as the main mechanism of arsenic mobilization (Ahmedet al., 2004; Eiche et al., 2008; Guo et al., 2008).
Iron oxyhydroxide is a ubiquitous secondary mineral in deltaicand marine sediments and is abundant in the PRD. It has a veryhigh specific surface area (around 600 m2/g) so it has a very high ad-sorption capacity for As (Davis and Kent, 1990; van der Zee et al.,2003). Arsenic is adsorbed more readily to amorphous rather thancrystallized iron oxyhydroxides, probably due to a larger surfacearea (Mohapatra et al., 2005). Iron oxyhydroxide reduction common-ly exists in natural anoxic environments and it is widely used to ex-plain the presence of dissolved arsenic in anoxic groundwater
Table 4Comparison of trace element contents in the fine and coarse grained sediments.
V Ni Cu Zn Sr Ba Pb As
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
Fine-grainedsediments
Average 169 46.4 38.5 153 94.2 374 53.7 20.0Highest 276 73.9 62.1 205 146 462 185 39.6lowest 117 29.7 22.5 94.4 69.2 263 24.1 11.1
Coarse-grainedsediments
Average 96.9 29.6 24.7 114 56.7 323 33.0 14.5Highest 138 89.4 41.3 453 106 433 138 32.0lowest 26.5 7.80 11.3 30.0 19.1 186 10.3 5.00
(Matisoff et al., 1982; Korte, 1991; Korte and Fernando, 1991;Nickson et al., 1998, 2000; McArthur et al., 2001; Reza et al., 2010).Thus, iron oxyhydroxide reduction is considered one of the most im-portant processes for arsenic mobilization in the PRD. Reduction ofiron oxyhydroxide is driven by microbial metabolism of sedimentaryorganic matter, which is indicated by the presence of CH2O(McArthur et al., 2004; Nickson et al., 2000). Sedimentary organicmatter in the PRD ranges from 0.013 to 3.95 wt.% (Wang, 2011) andDOC concentrations are enriched in the basal aquifer (Table 1). Sedi-mentary As/Fe (μg/mg) ratios vary from 0.3 and 1.1, with an averageof 0.55, similar to values of sediments where amorphous iron hydrox-ides have been identified to be efficient scavengers of dissolved arse-nic, and the distribution of iron hydroxides exerts a major influenceon the biogeochemcial cycling of arsenic in sediments (Bone et al.,2006). When arsenic-rich iron oxyhydroxides undergo microbiallymediated reductive dissolution, arsenic is mobilized, along withsoluble iron and bicarbonate (Shimada, 1996; Nickson et al., 1998;Anawar et al., 2003). A significant correlation between As andHCO3
− (Fig. 8A) also suggests reduction of iron oxyhydroxide usingsolid organic matter as an electron donor. Our data show no signifi-cant correlation between As and Fe, consistent with observations ofmany other studies of reducing aquifers (Nickson et al., 2000;McArthur et al., 2001; Anawar et al., 2003; Reza et al., 2010), thatshow removal of iron from the groundwater by the formation of sec-ondary iron-bearing minerals, such as pyrite, siderite and vivianite.
McArthur et al. (2001) suggested that the extent of arsenic pollu-tion in groundwater is controlled by the distribution of sedimentaryorganic matter, which acts as a redox driver for the reduction ofiron oxyhydroxide. However, microbial metabolism of sedimentaryorganic matter could be an alternative explanation for arsenic mobili-zation. Our sequential extraction results (Fig. 7) show that a signifi-cant part of the solid arsenic is in sedimentary organic matter. Thepresence of methane gas in the PRD basal aquifer (Zhao, 1974) re-flects fermentation of sedimentary organic matter, production of
r= 0.52
0.00
0.04
0.08
0.12
0.16
Mn
O(w
t%)
As (mg/kg)
r = 0.71
0
2
4
6
8
10
Fe 2O
3(w
t%)
As (mg/kg)
r=0.63
0
1
2
3
0 15 30 45
0 15 30 45 0 15 30 45
0 15 30 45
TO
C (
wt%
)
As (mg/kg)
r = 0.79
0
10
20
30
Al2
O3(
wt%
)
As (mg/kg)
BA
DC
Fig. 6. Relationship of solid phase As with Fe2O3 (r=0.71 at the significance level of 0.01) (A); MnO (r=0.52 at the significance level of 0.05) (B); TOC (r=0.63 at the significancelevel of 0.01) (C); Al2O3 (r=0.79 at the significance level of 0.01) (D).
0
5
10
15
20
25
30
35
40
45
0 20 40 60 80 100
Dep
th(m
)D
epth
(m)
15
20
25
30
35
40
0 10 20 30 40 50
As in reducible,organic matter and residualphases in SD1(%)
As in reducible,organic matter and residualphases in MZ4(%)
Reducible phase Organic matter Residual phase
A
B
Fig. 7. Vertical distribution and proportions of As in different phase of sediments from(A) core SD1 and (B) core MZ4.
294 Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
ammonium and reduction of iron oxyhydroxides. DOC concentrationsin the aquifer, ranging between 1.2 and 35.8 mg/L (Table 1), are muchgreater than DOC concentrations (b1 mg/L) commonly found ingroundwater elsewhere (Aiken et al., 1985). Abnormally high ammo-nium concentrations (up to 390 mg/L) occur widely in the basal aqui-fer (Jiao et al., 2010), and there is a strong positive correlationbetween arsenic and ammonium concentrations (Fig. 8D). There isalso a significant positive correlation between aquatic arsenic andDOC (Fig. 8C). Therefore, we speculate that microbial mediated min-eralization of sedimentary organic matter in a strong reducing envi-ronment can lead to degradation of the organic material andgenerate elevated concentrations of ammonium and DOC, as well asrelease contained arsenic. Significant correlations between aquatic ar-senic and concentrations of NH4
+ and DOC in groundwater have alsobeen identified in aquifers of other regions (Dowling et al., 2002;Anawar et al., 2003; Postma et al., 2007; Jessen et al., 2008). There-fore, release of arsenic by degradation of organic matter may be acommon and important mechanism for arsenic mobilization. Theabove discussion did not include the arsenic concentration in thebasal aquifer at MZ4 and the reasons are given in Section 4.6.
4.6. Control of arsenic concentrations in groundwater
SEM analysis of the PRD sediments shows that they contain abun-dant authigenic minerals. The distinction between authigenic and de-trital sources is based on grain morphology (Lowers et al., 2007).Authigenic pyrite, both framboidal and massive (Fig. 9), is especiallyabundant in the fine-grained silts and clays, mainly in the marinelayers. Pyrite is the dominant form of sulfur in the PRD (Huang etal., 1982; Lan, 1991; Long, 1997), and plays an important role in de-termining the arsenic content of sediments because it can act eitheras an arsenic source (Das et al., 1996; Mandal et al., 1998) or sink(Lowers et al., 2007). As discussed above, the aquifer–aquitard systemis under reducing conditions, which favor authigenic pyrite forma-tion, but not pyrite oxidation. The formation of authigenic mineralscan remove arsenic from the groundwater by co-precipitation withpyrite and siderite in strongly reducing environments (Haynes et al.,1987; Welch and Lico, 1998; Nickson et al., 2000; Lowers et al.,2005; Lowers et al., 2007; Reza et al., 2010). Therefore, authigenicpyrite is a sink for, not source of, aquatic arsenic in the PRD, and effec-tively controls the arsenic concentration in groundwater.
295Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
In this study, a significant amount of arsenic incorporated in pyritewas confirmed by the finding of large amount of arsenic accumulatedin sulfide phase in the sediments (Fig. 7). In a coastal aquifer–aquitardsystem like the PRD, where groundwater is significantly influenced byformer seawater intrusions, sulphate that originated from a marinesource can provide the sulfur needed to form pyrite and thus can con-trol arsenic concentrations in the groundwater. For example, in theaquitard porewater where SO4
2− reduction is significant, and will con-tinue to be significant, aquatic arsenic concentrations are very low(1.7–28.7 μg/L). The highest concentration (28.7 μg/L) was found inthe shallowest piezometer (2.93 m), which was likely under oxidiz-ing, rather than reducing conditions. On the other hand, arsenic con-centrations can be enriched where the sulfur source is exhausted. Forexample, the highest concentration of arsenic in the basal aquifer atSD1 (161 μg/L) (Table 1), occurs where SO4
2− concentrations arebelow the limit of detection. Although SO4
2− in groundwater sampleMZ4 is also below the limit of detection (Table 1), SO4
2− is presentin the aquitard porewater (Table 2 and Fig. 4). Because the paleo-shoreline of the PRD migrated from the northwest to the southeast(Zong et al., 2009b), the influence of SO4
2− reduction on groundwatersamples should also decrease in the same direction because SO4
2− re-duction would have persisted much longer in the northwest than inthe southeast. Because the arsenic concentration in groundwatersample MZ4 is dominantly influenced by the co-precipitation process,it was not included in the discussion of arsenic mobility processes inSection 4.5. It should be noted that pyrite formation is the majorprocess controlling groundwater arsenic concentration in the PRD.Although formation of other secondary minerals, such as siderite(FeCO3), in the PRD sediments (Huang et al., 1982), could also causearsenic co-precipitation, pyrite is thought to play the dominant rolein this process because of its abundance (Huang et al., 1982).
Pyrite formation can significantly control arsenic concentrations ingroundwater. The solid arsenic contents of the detrital sediments in thePRD are relatively enriched compared with those of the Narayanganjand Chapai-Nawabganj Districts of Bangladesh where solid arsenic con-tents in core sediments range from 0.05 to 12.77 mg/kg (with a meanvalue of 4.55 mg/kg) (Anawar et al., 2003) and from 1.4 to 55 mg/kg(with a mean value of 10.3 mg/kg (Reza et al., 2010), respectively.
r=0.66
0
400
800
1200
1600
0 50 100 150 200
0 50 100 150 200
HC
O3-
(mg
/L)
A
r=0.78
0
10
20
30
40
DO
C(m
g/L
)
As (µg/L)
C
Fig. 8. Correlation of arsenic to (A) HCO3−, (B) Total Fe, (C) DOC and (D) NH4
+. Correlatio
However, the arsenic concentrations of groundwater in the PRD aquiferand aquitard porewater are much lower than those in Bangladesh,where the arsenic concentrations are up to 750 μg/L and 462 μg/L(Anawar et al., 2003; Reza et al., 2010), respectively. This probably re-flects the very little pyrite in those areas, due the absence of a significantsource of sulfur (Ahmed et al., 2004).
5. Conclusions
Our study demonstrates that groundwater arsenic concentrationsin the basal PRD aquifer range from 3.3 to 161 μg/L. Enriched arsenicconcentrations occur in groundwater with a range of salinity. The ar-senic enriched groundwaters are characterized by a lack of DO, nega-tive Eh values, slightly alkaline environments, and abnormally highammonium and DOC concentrations, but low nitrate and nitrite con-centrations. There are significant spatial variations of arsenic concen-tration in the basal aquifer, reflecting the local geochemistry andredox processes. A correlation between arsenic concentration andgrain size in the vertical profiles suggest that sediment grain size isthe key factors controlling solid arsenic distribution. The sedimentsin the PRD are mainly dark gray, fine-grained, marine silts and clayswith abundant sedimentary organic matter. Heavy minerals, sedi-mentary organic matter and most major elements (Fe, Mn, Ti, Al, P)and trace elements (As, V, Cu, Ni, Cr, Ba) are concentrated in thefine-grained layers. Significant correlations of arsenic with Fe2O3,MnO and Al2O3 indicate that terrestrial detrital deposits were one ofthe major sources of solid arsenic in the delta sediments, whereas acorrelation between solid arsenic and Ba contents probably reflectsa marine influence on solid arsenic enrichment. Positive correlationsbetween arsenic and Al, TOC, Fe and Mn, and similar depth variationsbetween As and Fe2O3, Al2O3 and TOC, as well as the results of se-quential extraction experiments, suggest that reductive dissolutionof iron oxyhydroxide is one of the major processes that mobilizessolid arsenic. The above relationships, combined with sequential ex-traction results and positive correlations between arsenic and ammoni-um and between arsenic and DOC, suggest that mineralization ofsedimentary organic matter can release arsenic as well. The aquaticarsenic concentrations are significantly controlled by co-precipitation
0 50 100 150 200
0 50 100 150 2000
100
200
300
400
500
TF
e(m
g/L
)
B
r=0.72
0
100
200
300
400
NH
4+ (m
g/L
)
As (µg/L)
D
n coefficients (r) for (A), (C) and (D) are provided and the significance level is 0.01.
10µmB
A
Fig. 9. SEM images of pyrite forms in the PRD marine sediments. (A) Open framboidtexture. (B) Individual grains with diameter over 10 μm. Sediment samples werefrom core SD1 at the depth of 25.5 m and core MZ at the depth of 11.4 m.
296 Y. Wang et al. / Science of the Total Environment 427-428 (2012) 286–297
of arsenic and authigenic pyrite in the sediments, causing arsenicconcentrations in groundwater closer to the sea to be less elevatedthan those located farther from the coast.
Acknowledgments
This studywas financially supported by the General Research Fund ofthe Research Grants Council, the Hong Kong Special AdministrativeRegion, China (HKU 702707P, HKU 703109P, and HKU 703010P), ChinaGeological Survey, Guangdong Geological Survey and “SustainableWater Environment” Strategic Research Sub-Theme in HKU. We thank
Kouping Chen, Vicky Fung, and Yali Sun for assistance with well drilling,sampling, testing in thefield andmajor ion analysis in the laboratory.Wealso would like to thank the anonymous reviewers for their valuablecomments and suggestions to improve the quality of the paper.
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