Geochemical, mineralogical and statistical characteristics of arsenic in groundwater of the Lanyang Plain, Taiwan

Journal Pre-Proof Research papers Geochemical, Mineralogical and Statistical Characteristics of Arsenic in Groundwater of the Lanyang Plain, Taiwan Chen-Wuing Liu, Ming-Zhe Wu PII: DOI: Reference:

S0022-1694(19)30695-X https://doi.org/10.1016/j.jhydrol.2019.123975 HYDROL 123975

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Journal of Hydrology

Received Date: Revised Date: Accepted Date:

11 April 2019 16 July 2019 20 July 2019

Please cite this article as: Liu, C-W., Wu, M-Z., Geochemical, Mineralogical and Statistical Characteristics of Arsenic in Groundwater of the Lanyang Plain, Taiwan, Journal of Hydrology (2019), doi: https://doi.org/10.1016/ j.jhydrol.2019.123975

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Submit to Journal of Hydrology 2nd revised on July 16, 2019

Title：Geochemical, Mineralogical and Statistical Characteristics

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of Arsenic in Groundwater of the Lanyang Plain, Taiwan

Authors：Chen-Wuing Liu*, Ming-Zhe Wu

*Corresponding

Author:

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University, Taipei, 10617, ROC

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Affiliation：Department of Bioenvironmental Systems Engineering, National Taiwan

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Phone: +886-2-2362-6480 Fax: +886-2-2363-9557

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E-mail address: [email protected]

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SUMMARY High arsenic concentrations (average 0.1 mg/L) of groundwater were found in Lanyang plain of Taiwan. In this study, 39 groundwater samples from 23 wells were collected and 14 hydrogeochemical parameters were analyzed. Factor analysis was applied to determine major influence factors of the arsenic enriched groundwater quality, and PHREEQC was used to

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calculate the distribution of aqueous species and saturation index of which affected the

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hydrogeochemistry of groundwater. 393 geological core samples from 9 drilling wells were

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collected and analyzed the contents of total arsenic and iron. Moreover, core samples associated with high arsenic concentration groundwater were selected, mineralogical phases

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were analyzed using x-ray fluorescence (XRF), high resolution x-ray photoelectron (XPS) and scanning electron microscope and energy dispersive spectrometer (SEM-EDS). Results of the arsenic enrichment factor determined by factor analysis indicated that infiltration of the

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organic and nitrogen pollutants from anthropogenic activities to shallow groundwater, and the reductive dissolution from iron oxyhydroxides in the deep aquifer were the main processes of

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arsenic release to groundwater from the sediment. Total arsenic and iron contents of the core samples were well correlated in marine sequences. The presence of clay layer within the subsurface may increase in the As contamination in groundwater aquifer. However the time

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for As release from clay layer to lower aquifer may require tens or hundreds years to complete under natural environment condition. Surface analyses of core sample performed by XPS showed that arsenic was adsorbed or co-precipitated with non-crystalline iron oxyhdroxides

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and sulfides. After a long term burial of sediment, microbial metabolism of organic matter creates a more reducing environment, arsenic may then be gradually released from iron

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oxyhydroxides by reductive dissolution or desorption to aqueous phase. The framboidaldiagenetic type phase was identified by XPS and the groundwater is supersaturated with

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respect to pyrite and orpiment determined by PHREEQC are suggesting sulfide minerals coprecipitate As. Arsenic in sediments is released into groundwater primarily by the reductive dissolution of As-bearing Fe-oxyhydroxides in reducing environment in the Lanyang plain. Keywords： Arsenic, Factor analysis, Mineralogical analysis, Geochemical modelling, Lanyang plain

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1. Introduction In the southwestern coast of Taiwan, blackfoot disease (BFD) is known as an endemic peripheral vascular disease. Arsenic has been identified as a major risk factor for BFD (Tseng,

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1977). Ingestion of arsenic compounds in well water has been associated with age-adjusted mortality from diabetes (Lai et al., 1994), hypertension and cerebrovascular disease (Brown

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and Chen, 1995; Chiou et al., 1997), and cancer of lung, liver, bladder, kidney, prostate and

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nasal cavity (Chen and Wang, 1990; Chiou et al., 2001).

In the northeastern Taiwan, the Lanyang plain is also an arsenic-affected region. Arsenic

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concentrations in groundwater largely exceed the Taiwan EPA drinking water limit of 10μg/L. Some well waters are up to 600μg/L or higher (Chiou et al., 1997). Residents of the Lanyang plain have used shallow wells water (depths <40m) to obtain drinking since the 1940s (Chiou

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et al., 2001). Although nowadays 90% of households have tap water supply, groundwater is still commonly used as a source of drinking water and aquacultural water in the rural village

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and coastal fishpond of Lanyang plain (Liang et al., 2013; 2018). Significant dose-dependent relationships between the arsenic concentration in well water and an increased risks of cerebrovascular disease, urinary cancer and other cancers, and adverse pregnancy outcomes

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has also been found (Chiou et al., 2001; Lee et al., 2007). The use of high As groundwater for aquacultural needs may bio-accumulate As in farmed fish causing a potential cancer risk for

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consumption (Liang et al., 2013).

The arsenic is of natural origin and is believed to be released to groundwater as a result

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of a number of mechanisms (Mukherjee et al., 2019; Coomar et al., 2019; Maity et al., 2017). Geochemical processes involving redox reactions, mineral dissolution, precipitation,

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adsorption, and desorption substantially influence As mobilization (Jia et al., 2014). This release appears to be associated with the burial of fresh sediment and the generation of anaerobic groundwater conditions (Guo et al., 2008). The arsenic is then desorbed and dissolved from iron oxides which had earlier scavenged the arsenic from river water during their transport as part of the normal river sediment load. Natural variation in the amount of iron oxide at the time of sediment burial may be a key factor in controlling the distribution of high arsenic groundwater (Yang et al., 2016). Release of As in groundwater is generally correlated with geological and sediment environment. Redox conditions and their reactive 2

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processes determine the variation of As species concentrations in groundwater (Zheng et al., 2004; Polizzotto et al., 2005; Hsu et al., 2010). The As contents of core sample at three monitoring wells locate at the mid- and distal-fan of Lanyang plain were analyzed (Chen, 2001). The average As contents of the core samples at three wells were 7.6 mg/kg, 2.4 mg/kg and 3.4 mg/kg, respectively. The highest As content of 51.9 mg/kg, 26.1 mg/kg and 15.9

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mg/kg were found at 37 m, 106 m, 136 m, respectively, below the ground (Chen, 2001).

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Release from natural source is the dominant cause of elevated As concentrations in groundwater (Smedly and Kinniburgh, 2002; Kao et al., 2013; Maity et al., 2007; Mukherjee

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et al., 2019).

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The study investigated the distribution of As species and the redox geochemistry in groundwater in the Lanyang plain. 39 groundwater samples from 22 wells were collected. The concentrations of 14 water quality parameters were measured, and relations between As and

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other chemical parameters were statistically assessed using correlation and factor analysis. Arsenic and iron contents and the mineralogical characteristics of core samples from 9 drilling

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wells were evaluated. Aqueous chemical speciation and mineral saturation calculations were performed using PHREEQC (Parkhurt, 1995). The objectives of the study are to 1) characterize the sources, distribution, geochemical and mineralogical properties of As in

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groundwater and sediments in the Lanyang plain; 2) assess the main hydrogeochemical factors controlling As mobilization.

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2. Methods and materials

2.1. Study site

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The Lanyang plain located in Yilan County in northeastern Taiwan, it is an alluvial fan

flooded by the Lanyang river (Fig. 1.). The region is triangular, bordered by the Pacific Ocean

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to the east, the Snow mountain to the northwest, and the Central mountains to the southwest. The Lanyang river flows through the middle in the same direction of groundwater from west to east. The triangular plain area is approximately 400 km2 with 30 km in each side. The groundwater region of Lanyang plain is divided by the Lanyang river of the north and south banks according to recharge sources. The north bank is recharged by the Snow mountain whereas the south bank is recharged by the Central mountain (Peng, 1995). According to the subsurface stratigraphic analysis of the Lanyang plain, the sequence boundary of the basement is formed with weathered and un-integrated surfaces at about 18 Ka BP. Marine sequence is 3

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deposited with alluvial sediment an transgressive lags in the range 6 - 18 Ka BP, and reaches to maximum flooding surface with a stable seawater table at 6 Ka BP (Chen, 2000). The basin filled up with increasing quantities of alluvial sediments through aggradations during 3 – 6 Ka BP. Progradation occurred toward the sea at 3 Ka BP (Chen, 2000) is non-marine sequence. The depth of alluvial plain is in the range several hundred meters. The surface layer is covered

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by sediments from the Quaternary period, including silt, sand and clay, and the plain is

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partitioned into proximal-, mid-, and distal-fan areas (Chen, 2000). The hydrogeological profile of Lanyang plain (Fig. 3) is constructed according to the cross section of stratigraphic

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survey denoted in Fig. 2. Additionally, The 23 hydrogeological stations with depths from 13

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to 227 m were setup by the Taiwan Water Resources Agency (WRA) based on the hydrogeological property for monitoring groundwater levels and quality. Groundwater aquifers are divided into two-depth intervals according to marine and non-marine sequence of

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aquifer 1 (13-70 m) and aquifer 2 (70-250 m).

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2.2. Groundwater sample analysis

Thirty-nine groundwater samples were collected from 23 hydrogeological stations, including 23 shallow groundwater samples (Aquifer 1) and 16 deep groundwater samples

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(Aquifer 2). pH, dissolved oxygen (DO), redox potential (Eh) and electrical conductivity (EC) were measured in situ. The remaining 14 physicochemical parameters of groundwater

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including Na+, K+, Ca2+, Mg2+, HCO3-, Cl-, SO42-, NH4+, NO3-, HS-, As, TOC, Fe and Mn were analyzed in the laboratory.

The field sampling method followed the NIEA (National

Institute of Environmental Analysis, NIEA) code W103.52B established by the Taiwan

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Environmental Protection Administration. At least three wellbore volumes of groundwater were purged before taking samples. Water samples were collected only after pH and EC

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stabilized and when the fluctuations of pH and relative EC were less than 0.1% and 5%, respectively. Samples were then kept in ice boxes and delivered to the laboratory within 24 h. Alkalinity was analyzed using the Gran titration method. Sulfide was determined through spectrophotometry by using turbidimetric methods. Un-acidified parameters (NO3-, Cl-, and SO42-) and acidified parameters (NH4+, Ca2+, Mg2+, Na+, and K+) were measured using ion chromatography (DIONEX ICS-900). The acidified parameters of Fe and Mn were measured using inductively coupled plasma and atomic emission spectrometry (Varian, VISTA-MPX). The TOC was measured using the high-temperature combustion method by Shimadzu TOC4

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5000A (Lenore et al., 1998). The lower detection limit was 0.05 mg/L; variances of duplicate measurements were less than 3%; recoveries of check and spike samples were between 90% and 110%. The analytical method for As(III) and As(V) was followed closely from our previous study (Huang et al., 2003). After pumping, all groundwater samples were filtered through a 0.2 μm pore membrane syringe filter to prevent microbial activity and remove

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suspended particles. In this study, we used 1 mL of 8.7 M acetic acid for the preservation of

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As species per 100 mL of groundwater sample. All samples were stored at 4°C and kept in the dark before analysis within 2 weeks. Concentrations of As species were analyzed using a high

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performance liquid chromatograph (Hitachi 7110) connected to a hydride generation (FIAS

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400 Perkin–Elmer) and atomic absorption spectroscopy (Perkin–Elmer AA100). The detection limits of As(III) and As(V) were 0.4 and 0.3 μg/L, respectively. Samples were spiked with As species to determine the recovery rate in the laboratory procedure, which

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yielded As(III) and As(V) recovery rates of 100.7 ± 3.8% and 97.2 ± 4.0%, respectively. The coefficient of variation was used to test the reliability and was less than 5% in all experiments.

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For sulfides, the lower detection limit was 0.03 mg/L; variances of duplicate measurements were less than 10%; recoveries of check and spike samples were between 85% and 115%. For all parameters (except As and sulfides), a total of 10 samples comprising blank, spiked,

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duplicate, and check samples (standard solution from Merck) were sequentially measured (Lenore et al., 1998). The variance of duplicate measurements was less than 3%; the

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recoveries of the check and spiked samples were between 90% and 110%.

2.3. Factor analysis

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Factor analysis (FA) is a multivariate statistical method, and yields a general relationship

between measured chemical variables by elucidating the multivariate patterns that may help to

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classify the original data. It can be used to determine the geographical distribution of the resulting factors. The geochemical interpretation of factor analysis provides insight into the main processes that may govern the distribution of hydrochemical variables. It is frequently employed to characterize the quality of groundwater (Liu et al., 2003). Sixteen hydrochemical parameters of the Lanyang plain (Table 1) were examined to evaluate the characteristics of groundwater using FA with SPSS software (SPSS Inc., 1998).

2.4. Geological core samples analysis 5

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Geological core samples were collected from 9 drilling stations of Water Resources Agency—w03, w04, w05, w07, w11, w13, w16, w17 and w23 (not water monitoring well) at 5-m intervals to the bottom of the well. As and Fe contents of a total of 393 geological core samples were analyzed. A 30% H2O2 and 9.6 M HCl solution were added to geological core sample to remove organic matter, and to destroy structure of soil grain. After filtering, all

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samples were examined using an electro-thermal atomic absorption spectrometer (AA100;

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PerkinElmer, Wellesley, MA) and a hydride generation system (FIAS100; PerkinElmer); 0.5% NaBH4 in 0.1% NaOH and 1 M HCl solution was added to reduce arsenic to arsine. The

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total As concentration was determined. Additionally, a concentrated HNO3, 30% H2O2 and

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concentrated HCl solution was added to geological core sample to digest. After filtering, all samples were examined using a flame atomic absorption spectrometer (FLAA400; PerkinElmer, Wellesley, MA), and the total Fe concentration was determined. Moreover, the

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amounts of various elements (eg. Si, Al, Fe, Ca, K and Mg) in the core samples were determine by X-ray fluorescence (XRF) (Spectro XE-POS).

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Feature of main As-bearing phase (such as probable ferrihydrites with various Fe/As molar ratio) exits in which As is present as surface-sorbed and/or incorporated species was explored. Chemical characterization of well-screen core sample of three drilling stations –w03,

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w07 and w11 – locate at mid- and distal-fan area, respectively, are identified by high resolution x-ray photoelectron spectroscopy (HR-XPS; ULVAC-PHI Quantera SXM) and scanning electron microscope (SEM; Hitachi S-3000N). Both XPS and SEM are surface

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chemical analysis techniques that can be used to analyze the surface chemistry of a material. XPS provides information on the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within the surface or reaction layers of minerals

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and grains (Frau et al., 2005). SEM is designed for direct studying of surface. SEM images

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have great depth of field yielding a characteristic three-dimensional appearance useful for understanding surface structure of a mineral (Hou et al., 2006).

2.5. Geochemical speciation and saturation index The geochemical program PHREEQC based on the database WATEQ4F.dat was applied to calculate the distribution of aqueous species (Parkhurst and Appelo, 1999). The program adopted the ion-associated theory of aqueous solutions to perform various aqueous geochemical computations. PHREEQC can also determine which solids might precipitate or 6

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dissolve by employing the saturation index (SI). The SI is defined as the logarithm of the ratio of the ion activity product (IAP) of component ions for a solid in solution to the solubility product (Ksp) of the solid, SI=log(IAP/Ksp) (Nordstrom and Alpers, 1999). In this study, 16 hydrogeochemical parameters of groundwater samples including, pH, DO, Eh, Na+, K+, Ca2+, Mg2+, HCO3-, Cl-, SO42-, NH4+, NO3-, HS-, As, Fe and Mn concentrations were used to

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calculate equilibrium speciation and SI.

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3. Results and discussion 3.1. Groundwater compositions

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The hydrochemical data of 39 analyzed groundwater samples are listed in Table 1. Fig. 4 shows the plot of a Piper diagram of the groundwater quality of Lanyang plain. According to

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the classifications of the Piper diagram, Type I represented the carbonated temporary hardness, Type II represented the alkali carbonate, Type III represented the non-carbonate/permanent

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hardness, and Type IV represented saline (Piper, 1953). The principal water type in the Lanyang plain is Type I (n=20), followed by Type II (n=15). Only three groundwater samples in the coastal area of north bank were Type IV which were caused by the sea water intrusion (Kao et al., 2015). The high prevalence of carbonate water in the Lanyang plain is attributed

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to the high hydraulic conductivity of the subsurface aquifer. The pH is ranged 7-8, Ehs are < 0 mv except in the recharge zone of the mountain proximal fan.

The average arsenic

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concentration is 0.1 mg/L with highest arsenic concentration of 1.01mg/L at w06. 84% groundwater As exceed the drinking water standard of 0.01 mg/L. Fig. 5(a)-(e) plots the As concentration distribution versus depth, Eh, DO, NO3- and

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SO42- in the Lanyang plain. The monitor wells located in the north and south banks of the Lanyang river were denoted by different legends. High As concentrations mostly appear in

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low Eh, DO, NO3- and SO42- of the south bank aquifers. Because the reductive dissolution of As-rich Fe oxyhydroxide is the most probable mechanism of the As release to groundwater in sedimentary aquifers (Wang et al., 2007; Nath et al., 2008), Fig. 6 shows the regressional analyses of (a) As vs HCO3-, (b) As vs TOC, (c) Fe vs HCO3-, (d) Fe vs TOC and (e) As vs Fe. As vs HCO3- and TOC yield higher correlations in the south bank of Lanyang plain with R2 of 0.45 and 0.13 respectively. Similarly, Fe vs HCO3- and TOC also present higher correlations in the south bank of Lanyang plain with R2 of 0.25 and 0.46. However As vs Fe show low correlation. The decoupling of As and Fe in reducing aquifers is commonly found around 7

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world including the Chianan Plain of Taiwan (Nath et al., 2008; Sracek et al., 2018). The redox state (redox potential, Eh) of groundwater is an important parameter affecting As mobility and transformation. The redox conditions of groundwater vary widely from approximately + 300 mV to -300 mV in the Lanyang aquifers. We herein delineated four redox boundaries using four redox couples at standard state (Mn4+/Mn2+, As5+/As3+, Fe3+/Fe2+

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and SO42-/HS-) and plot in Fig. 7(a) Fe concentration vs Eh, Fig. 7(b) As concentration vs Eh

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(Signes-Pastor et al., 2007). The As and Fe concentrations were low in redox state of Eh > 80 mV, and high in the redox state of -200 mV
3.2. Distribution of arsenic species

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minerals and decrease the aqueous concentrations.

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sulfate and As may be reduced to precipitate as pyrite, arsenopyrite realgar or orpiment

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Table 2 shows the As species concentrations in aquifer 1 and 2 of Lanyang plain. The major arsenic species in all groundwater wells was As(Ⅲ). The ratio of As(III)/As(total) 65%

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and 78% in aquifer 1 and 2, respectively, implying that the groundwater systems were in reduced condition. Other redox parameters including negative redox potential with average of -105 mV, and low DO value of 0.74 mg/L all support this reductive environment finding.

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Previous studies have reported the predominant As(Ⅲ) species in groundwater of Taiwan and US (Chen et al., 1994; Hering and Chiu, 2000). However, others have found that As(Ⅴ) was

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the dominant species in groundwater at several regions (Chen et al., 1999; Smedley et al., 1996), or As (V) occurred in roughly equimolar proportion with As(Ⅲ). While As(Ⅲ) may be metastable to varying degrees in aerobic environments, As(Ⅲ ) exists more stable under

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anaerobic conditions. Several different bacteria strains were capable of reducing As(Ⅴ ) to

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As(III) (Liao et al., 2011) and act as a detoxification mechanism (Ji and Silver, 1995; Rosen et al., 1995). Reduction of As(Ⅴ ) to As(Ⅲ ) was observed in the nature gradient tracer experiment (Kent et al., 2001) where As(Ⅴ ) was injected into an anoxic groundwater zone containing high concentration of dissolved Fe(Ⅱ) and reduced directly to As(III) by Fe(Ⅱ) with the aid of microorganisms.

3.3. Factor analysis Table 3 presents the eigenvalues of first four factors, their percentage of variance and 8

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cumulative percentage of variance. It reveals that the eigenvalues of the four factors, which exceed one, explain 86.49% of the total variance of the data set. Table 4 presents the loading of the varimax rotation factor matrix for the four factors model. The term “strong”, “moderate”, and “weak” as applied to factor loadings, refer to absolute loading values of > 0.75. 0.75-0.5 and 0.5-0.3, respectively (Liu et al., 2003). This study selected absolute factor

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loading of over 0.6 to elucidate the relationships between the factors and hydrochemical data.

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A factor score associated with each monitoring well was determined. They were plotted to illustrate the spatial characteristics of the quality of groundwater in the Lanyang plain.

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Factor 1, which has strong positive loading on Cl-, K+, EC, Mg2+, Na+, Mn and Ca2+,

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explained 48.81% of the total variance. The strong positive terms are the major solutes in seawater. Factor 1 is thus called the salinization factor. Over pumping and salty water infiltration from coastal fishponds are the two main causes. Fig. 8(a1) and (a2) show the

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spatial score distribution of factor 1 of aquifer 1 and 2 where only shallow aquifer 1 in the north coast has salinization, the deep aquifer 2 is free from brackish water. Peng (1995) used

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the isotope analysis to survey the groundwater age. He confirmed that the northern coast groundwater was relative young suggesting the seawater intrusion and the infiltration of salty water responsible for the salinization.

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Factor 2 characterizes by strong positive loading of Fe and TOC which accounts 19.29% of the total variance. Fig. 8(b1) and (b2) show the high score distribution of factor 2 in the north mid-fan and in the south coast of aquifer 1 and 2, respectively. The source of TOC in

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aquifer 1 was infiltrated from the sanitary landfill leachate whereas the TOC source in aquifer 2 was dissolved from the organic-rich sediment. High TOC in groundwater creates a more reducing environment causing Fe(II) release to groundwater via reductive dissolution of Fe-

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oxides in sediment (Chen, 2001; Lawati et al., 2012). Fe oxides acts as an important electron

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acceptor to reductive dissolution and release of Fe2+ into groundwater. Factor 3 explains 11.4% of the total variance which associates with strong loading of As

and HCO3- and moderate loading of NH4+. Fig. 8(c1) and (c2) display that the high scores of Factor 3 distribute along the middle sea coast and downstream area of the aquifer 2 in south bank of Lanyang plain. The mineralization of sedimentary organic matters results in bicarbonate production and subsequent dissolution of calcite, rising the alkalinity. The dissolved carbonate may influence the adsorption of trace metals by competing for sites on mineral surface, forming carbonate complex that can either enhance or suppress adsorption, 9

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and forming precipitates (Villalobos and Leckie, 2001). The mobilization of adsorbed As with dissolved bicarbonate has proposed to be one of the major causes of high levels of As in groundwater (Appelo et al., 2002; Wang et al., 2007). Denitrification reduced NO3- to NH4+ resulting in depletion of NO3- and increase in NH4+ leading to the release of As from Asbearing Fe oxyhydroxides into groundwater (Weng et al., 2017). Factor 3 is thus called the As

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enrichment factor.

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Factor 4, which has strong positive and negative loadings on Eh and pH, respectively, explains 7.25% of the total variance. It represents the geochemical characteristics of the study

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region, suggesting a tendency of redox potential decrease from the proximal fan to the distal

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fan. Spatial distribution of Factor 4 scores from positive to negative along the direction from west to east (Fig. 8(d1) and (d2)). The inverse relationship between pH and Eh controls the solubility of minerals. Eh-pH diagrams are commonly applied to represent the dominate

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chemical species and mobility of metals in groundwater (Appelo and Postma, 1993). Factor 4

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is called the redox factor.

3.4. Sediment characteristics and distribution of elements As and Fe contents in 393 core samples of nine drilling stations were analyzed. Fig. 9

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illustrates the vertical distribution of As and Fe contents in north and south banks of Lanyang plain. Low As and Fe contents generally found in the proximal fan in both north (W 16, 17) and south (W11) banks of Lanyang plain. Fig. 10 and 11 show that As and Fe contents in

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marine sequence were positively correlated, but were poorly correlated in non-marine sequence. Marine sequence consists of clay/silt whereas non-marine sequence comprises by

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sand/gravel. The small grain size of sediment materials provides a large surface area, enabling high adsorption capacity of arsenic and resulting increased As enrichment in clay and silt.

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Moreover, the marine formation contains fine clay with sea organisms that may have high arsenic content resulting from bioaccumulation and bioconcentration (Franceconi et al., 1998; Liu et al., 2006). The moderate to low correlation between As and Fe in marine sediment may be due to the fraction of silicate bound As and As retained by Fe oxhydroxide (Norrs et al., 2005). Nine core samples from various intervals in the well screens of 3 drilling stations in the south bank of Lanyang plain were selected from bulk analysis of elemental distribution by XRF (Table 5). The main chemical constituents of sediment incudes SiO2, Al2O3, Fe2O3, K2O, 10

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CaO2, MgO and TiO2 accounting for the majority of the samples weight. Manganese, sulfur and arsenic were minor compositional elements, ranging from 2.94 to 67.4 mg/kg, 348.3 to 4404 mg/kg, and 6.7 to 22.4 mg/kg, respectively. Among these samples As content was weekly correlated with Mn (R2=0.34, p<0.05), and Fe (R2=0.24, p<0.05), indicating Asbearing Fe, Mn phases were the predominant As-host minerals in the sediment.

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Nine core samples from three drilling stations in the south bank of Lanyang plain were

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selected for XPS analyses. Each XPS result for Fe (Fig.12) comprises to curves (1) the experimental curve after smoothing and (2) curve of the fitted components. The results of the

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Fe2p3/2 lines can be fitted by multiple components. Six types of Fe minerals were observed,

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including a mixing-valence type (Fe3O4), two reducing form types (FeO, FeS), and three oxidizing form types (Fe2O3, FeOOH and FeF3). In the non-marine sequence, FeO and FeOOH are dominate minerals. In the marine sequence the Fe minerals distributions are

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complex. The detection limit is such that the spectra of Mn2p3/2 and Asd5/2 yielded no information regarding specific mineral compounds of Mn and As.

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Samples w11-2 and w07-2 of marine sequence were chosen for further investigation by SEM EDS. Figure 13 shows images of w11-2 and w07-2 by SEM (Fig. 13a and b); and the spectra show Fe, S, Si, O are main peaks and As is small peaks (Fig. 13c and d). The presence

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of colloid-like fabrics shows amorphous or poorly crystalline Fe-As phase as coating on silicate grain due to co-precipitation. Pyrite was found in both samples and was similar to that found in the southern Choushui River alluvial fan, Taiwan (Lu et al., 2010) where it was

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identified as the framboidal-diagentic type (Liu et al., 2013; Akai et al., 2004).

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3.5. Aqueous speciation and saturation indices Table 6 presents the ratios of the particular chemical species concentration to the total

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chemical element concentration calculated by PHREEQC. The major species of As, C and S were HAsO3, HCO3- and SO42-, respectively. However, the major Fe species were Fe2+, FeHCO3+, and FeCO3 because of the high alkalinity in the groundwater. Table 7 shows the mineral saturation index of 13 groundwater samples computed using PHREEQC. The groundwater was the supersaturated with respect to orpiment in 9 well samples. Moreover, groundwater was oversaturated with respect to siderite and was close to saturate with respect to calcite. It suggests that the elevated HCO3- levels are not only controlled by the dissolution of 11

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the carbonate minerals in the Lanyang aquifer. High HCO3- concentrations correlate with the levels of dissolved organic carbon (TOC up to 24.1 mg/L) in groundwater. TOC levels showed distinct trends of variation with As and Fe concentrations. NO3- levels in groundwater are generally low with no correlation to As but NH4+ correlated well with As. CO2 is the primary degraded product of organic matters causing carbonate minerals dissolved including

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the reduction of NO3- as well as the reductive dissolution of Fe oxyhdroxides in solid phase.

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This explains that the Fe and TOC formed as the iron reduction factor, and As associated with HCO3- and NH4+ as As enrichment factor in the factor analysis. Groundwater was under-

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saturated with respect to anhydrite, epsomite and magnesite.

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The SI values for most of iron minerals including goethite, hematite, magnetite, siderite and pyrite, were greater than zero, indicating that these minerals may be precipitated. The XPS and SEM-EDS results confirmed these findings. The positive SIs of pyrite (>10)

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indicated the high potential for precipitation. Fe2+ may be co-precipitated with HS-, resulted in low HS-. The results of SEM also showed that amorphous or poorly crystalline Fe-As phase in

3.6. Discussion

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samples 11-2 and 7-2 were present.

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Factor analysis shows that the iron reduction factor and As enrichment factor both include NH4+ with moderate loading. These two factors consist of Fe, TOC, NH4+, As and HCO3- are generally responsible for moderately reducing environment, and arsenic release to

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groundwater might be caused by mineralization or organic in reducing conditions (Liu et al., 2003; Lawati et al., 2012). Scores of factor 2 are distributed close to the landfill (W31) and

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theme park (W08) in the aquifer 1. The site was for traditional dumping which was operated from 1990 to 1999 and buried municipal waste with no impermeable liner. The theme park is

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a popular scenic area, and attracts more than one million tourists annually. The anthropogenic organic pollutants infiltrate to groundwater likely causing the adsorbed As ions release from Fe-oxyhdroxide surface to the aquifer 1. However, arsenic concentrations in aquifer 1 is less than those of in aquifer 2 due to the high As was accumulated in the marine formation. Generally, in the suboxic to anoxic conditions, the presence of dissolved Fe corresponds to the dissolved As, NH4+-N and HCO3- which are consistent with mechanism of reductive dissolution of Fe-oxyhydroxides via respiration of organic matter (Nickson et al., 2000; McArthur et al., 2004). HCO3- presents the local baseline that results from weathering, 12

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oxygen consumption and nitrate reduction, which is the major dissolved inorganic carbon (DIC) (Nickson et al., 2000; Anawar et al., 2004). Respiration of dissolved organic carbon (DOC) produces ammonia and CO2, and latter may induce calcite dissolution (Harvey et al., 2002). Carbon isotopes (14C) of DIC (bicarbonate) in groundwater of Lanyang plain was analyzed by Peng (1995), indicating the groundwater is the mixture of invading fresh water

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with older resident water at mid-fan and distal-fan areas of Lanyang plain, and recharge from

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the western mountains are the primary source of groundwater in the aquifers. Young groundwater and surface water contain less bicarbonate, while it is preponderantly high in old

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and reducing groundwater (McArthur et al., 2001). The older DIC can be oxidative product of

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DOC that mobilized from sediment, and invades to groundwater. Appelo et al. (2002) indicates that sediment-groundwater interface contains high dissolved bicarbonate content, and As may be mobilized by displacement from sediment surface.

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A poor correlation exists between Fe and As in groundwater which may be caused by adsorption of Fe(II) onto solid surface or precipitation as Fe(II) or mixed valence solid

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(Nickson et al., 2000). This study identifies various iron compounds by XPS analysis. Anawar et al. (2004) demonstrates bicarbonate ion exchange/substitution may be another important process to As mobilization, and may explain the poor correlation between aqueous As and Fe.

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PHREEQC simulation indicates Fe2+ and FeHCO3+ are primary species for iron, and molar ratio of FeHCO3+ increases with increasing depth based on geochemical modeling; and further form siderite (Welch and Lico, 1998). Groundwaters are supersaturated with respect to

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other iron minerals including hematite (Fe2O3), goethite (FeOOH) and magnetite (Fe3O4). High concentrations of HCO3- are partially due to the moderately dissolving tendency of calcite and dolomite based on the geochemical modelling (Table 7). Reductive dissolution of

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Fe-oxide minerals and the subsequent release of adsorbed and/or co-precipitated As are main

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causes of high As concentrations in groundwater under reducing conditions (Ahmed et al., 2004; Jia et al., 2014). However, gradual reductive dissolution of Fe-oxides could also allow re-sorption of As released by this process onto the residual solid until the sorption capacity of the solid reached and/or the solid is lost by dissolution (McArther et al., 2004; Sracek et al., 2018). The de-coupling of As and Fe in reducing aquifers is common and has been observed in many sites around the world where As is released as a consequence of redox processes including Bangladesh, West Bengal, Assam in India, Cambodia and Taiwan (Srack et al., 2018). 13

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As and Fe contents of core samples in marine sequence were positively correlated. Marine sequence comprised clay, silt and organic matter. The As contents in marine sequence were generally higher than that of in non-marine sequence (Liu et al., 2006). Moreover, organic matter can strongly influence the solubility and mobility of As through redox reactions, competitive adsorption, desorption and complexation reactions suggesting that the organic

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matter is a redox driver and also one of the sources of As in groundwater (Anawar et al.,

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2013). The presence of clay layer within the subsurface may thus increase in the As contamination in groundwater aquifer. However the time for As release from clay layer to

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lower aquifer may require tens or hundreds years to complete under natural environment

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condition. Recently, Erban et al. (2013) reported that As released from clayey aquitards caused by pumping-induced land subsidence may take a decade or more to affect the As concentration in the lower aquifer in Mekong Delta, Vietnam. Their study provides a clue to

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investigate the time lag process of As released from clay layer to deep aquifers.

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The nonlinear relation between sorbed and dissolved As is important factor for the mobility and transport of As in aquifer (Swartz et al., 2004).

The extent of As adsorbed onto

Fe-oxyhydroxides is strongly influenced by As oxidation state and presence of inorganic and

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organic solutes (Hering et al., 1997). Arsenite and bicarbonate ions species become primary species for As and carbon, respectively, in groundwater at mid-fan and distal-fan area, and their molar ratios rapidly increase with increasing depth based on PHREEQC modelling. In

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aquifer 1, mineralization of organic matter in sediment cause onset of reducing conditions, and Fe-oxyhydroxides are the important acceptors of electrons, providing for the mineralization of organic matter (Stumm and Morgan, 1996). This leads to dissolution of As-

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rich FeOOH coating on soil particles, or reduction of some of the adsorbed arsenate to

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arsenite, and its release to groundwater. However, some arsenate may re-adsorb onto FeOOH. In aquifer 2, mineralization of organic matter progressively drives reductive dissolution of FeOOH, and converts arsenate to arsenite (McArthur et al., 2001; Lee et al., 2007). Additionally, high levels of bicarbonate may contribute to limit adsorption sites of Feoxyhydroxides to sorb As (Harvey et al., 2002), resulting in higher level of As. Many shallow alluvial aquifers in various parts of the world such as Bangladesh, West Bengal (India), China and Taiwan have the As contamination problems (Smedley and Kinniburgh, 2002). Arsenic is generally derived from weathering of As-rich minerals and then 14

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enter to the groundwater by reductive dissolution of Fe oxhydroxides under anaerobic conditions in the alluvial systems (Nath et al., 2008; Wang et al., 2007). Reza et al. (2011) conducted a comparative study on As in alluvial aquifers of Bengal delta plain, Chianan plain and Lanyang plain. The As concentrations are generally higher in the Chianan plain groundwater than those in the Lanyang plain and Bengal delta plain groundwater. The mean

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As concentrations in Bengal delta plain, Chianan plain, and Lanyang plain are 50.65 μg/L

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(2.8-170.8 μg/L, n =20), 393 μg/L (9-704μg/L, n=5), and 104.5μg/L (2.51-543 μg/L, n=6), respectively (Reza et al. 2011). The reductive dissolution of As-adsorbed Mn oxyhydroxides However, in the Chianan plain and Lanyang plain, microbially mediated reductive

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2007).

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is the most probable mechanism for mobilization of As in the Bengal delta plain (Hasan et al., dissolution of As- adsorbed amorphous/crystalline Fe oxyhydroxides in organic-rich sediments is the primary mechanism for releasing As to groundwater (Liu et al., 2013; Lee et Arsenic, organic matter and humic substance in the Lanyang plain groundwater

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al., 2007).

are comparatively lower than in the Chianan plain groundwater and the BFD has not been High levels of As and humic substances may possibly play a

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reported in the Lanyang plain.

critical role in causing the unique BFD in the Chianan plain of southwestern Taiwan (Reza et al., 2011).

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4. Conclusion

Thirty-nine groundwater quality samples and 393 geological core samples with 5m

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interval from 9 drilling stations of the Lanyang plain, Taiwan, were collected analyzed. Factor analysis (FA) was applied to wells with 16 hydrogeochemical parameters of 39 groundwater

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samples. High resolution X-ray photoelectron spectroscopy (HR-XPS) and scan electron microscope (SEM) were used to determine the mineralogy of core samples. Fe and As

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contents of core sample were also analyzed. data for groundwater aquifers 1 and 2. arsenic enrichment.

Factor analysis extracted four factors from the

These factors are mostly related to salinization and

Analytical results of FA suggest that the distribution of the salinization

is in aquifer 1, and the main causes of saline groundwater are infiltration from fishpond salty water and sea water intrusion. The leachate from the landfill site infiltrated to the aquifer 1, and caused reducing condition with high As concentrations in the local aquifer. Analytical results of core samples indicate that As and Fe contents was correlated with location of clay layer in marine sequence, and As adsorbed or co-precipitated with Fe-oxyhydroxides. The 15

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major minerals identified by XPS and SEM-EDS were goethite, hematite, magnetite, pyrite, and siderite, agreeing with the SI values calculated by PHREEQC. The arsenic-enrichment in the reduced geological environment and the adsorption/co-precipitation of As on the Feoxyhydroxides may be used to interpret the possible processes of arsenic release to groundwater in the aquifers. Arsenic in sediments is released into groundwater primarily by

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the reductive dissolution of As-bearing Fe-oxyhydroxides in reducing environment in the

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Lanyang plain.

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Acknowledgements

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The authors would like to thank the Ministry of Sciences and Technology of the Republic of China for financially supporting this research under Contracts Nos. 104-2313-B-

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002-026-MY3 and 107-2313-B-002-019-MY3.

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Table 1 Physic-chemical parameters and major ionic constituents in groundwater of the Lanyang plain.

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depth(m)

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Table 2 Measured arsenic species in groundwater aquifers 1 and 2 of Lanyang plain. Aquifer 1 (below 13-70m)

Aquifer 2 (under 70m)

As(Ⅲ) As(Ⅴ) (μg/L)

Station

depth(-m) (μg/L)

39

62.56

N.D*

w02-2

77

9.46

N.D*

w03-2

69

N.D*

54

w03-3

109

326.26

25.68

w06-1

50

N.D*

N.D*

w03-4

1.92

54.09

w07-1

35

8.44

0.57

w06-2

148

N.D*

16.29

w09-1

34

2.39

N.D*

w07-2

73

N.D*

31.96

w14-1

63

N.D*

5.54

w07-3

164

672.72 120.08

w15-1

27

41.42

N.D*

w09-2

111

0.62

N.D*

w17-1

61

8.63

N.D*

w11-2

115

14.99

N.D*

w18-1

37

2.22

12.07

w11-3

186

0.32

N.D*

w11-4

227

5

N.D*

w14-3

168

N.D*

46.99

w15-2

121

47.29

20.19

w17-2

147

N.D*

35.99

F

w02-1

(μg/L)

O

depth(-m) (μg/L)

E-

Station

As(Ⅲ) As(Ⅴ)

AL

PR

PR

O

144

R N

* Not Detected Table 3

U

Eigenvalues, percentage of variance and cumulative percentage of variance of the first

JO

four factors for eigenvalues exceed one in the factor analysis. Factor

Eigenvalue

1 2 3 4

6.83 2.70 1.56 1.01

Percentage of variance(%) 48.81 19.29 11.14 7.25

25

Cumulative percentage of variance(%) 48.81 68.10 79.24 86.49

JOURNAL PRE-PROOF

Table 4 Loadings for varimax-rotated factor matrix in four-factor model. Factor 4

1 0.99*

2 0.02

3 -0.02

0.00

0.99*

0.02

0.02

0.00

EC

0.99*

0.06

0.03

Mg2+

0.99*

0.04

-0.04

0.02

Na+

0.99*

0.06

0.06

-0.01

Mn

0.95* 0.81*

0.10 0.06

-0.10 -0.11

0.07 0.22

O

O

0.00

PR

Ca2+

F

Variable ClK+

Fe

0.20 -0.11 -0.07 -0.09 -0.09 -0.82* 0.81*

AL

PR

E-

0.40 0.81* 0.05 TOC -0.08 0.77* 0.11 As -0.02 -0.14 0.90* HCO3-0.04 0.43 0.76* NH4+ -0.03 0.57 0.65* pH -0.22 -0.41 0.10 Eh -0.08 -0.38 -0.11 * Absolute factor loading of over 0.6 are shown in bold typeface

R N

Table 5 XRF analysis of major element (weight %), Mn, S and As(mg/kg) contents of nine core

W11

W11

W11

W07

W07

W03

W03

W03

W03

depth(m)

115

185

225

75

165

25

70

100

145

Si

56.74

46.68

62.17

60.07

50.32

52.52

48.52

60.35

60.53

Al

25.74

15.95

24.65

25.17

17.22

13.89

17.33

25.57

26.12

Fe

7.188

6.024

4.340

5.339

5.560

5.060

5.596

4.995

4.923

K

7.109

5.285

6.450

6.682

5.777

4.393

5.506

6.329

6.073

Ca

0.803

2.086

0.418

1.387

1.529

0.763

1.090

0.421

0.390

Mg

1.912

3.112

3.508

2.861

2.952

2.779

2.702

4.030

3.288

Ti

1.054

0.796

0.799

0.857

0.868

0.679

0.918

0.884

0.856

Mn

67.4

6.74

3.20

9.46

15.17

2.94

3.70

3.70

3.70

S

1505

4404

1961

1044

2437

1516

1252

348.3

4002

As

22.4

11.8

14.7

17.1

17.8

15.3

18.9

6.7

17.6

JO

Well

U

samples.

26

JOURNAL PRE-PROOF

Table 6 Ratio (%) of different chemical species concentration to the total chemical element concentration of 13 groundwater samples in the Lanyang plain computed using PHREEQC. W17-1 40

W17-2 75

W21 32

W15-1 25

W13 30

W08 35

W06-1 50

W11-1 30

W05-1 25

W06-2 165

W11-3 115

W02-1 40

W05-1 25

CO2 HCO3CO32-

8 90 0

1 97 0

0 95 2

4 94 0

3 94 0

12 85 0

24 76 0

15 83 0

2 97 1

10 88 0

14 86 0

5 94 0

2 97 1

H3AsO3 H2AsO3HAsO42H2AsO4-

98 2 0 0

86 7 7 0

0 0 99 0

97 3 0 0

95 5 0 0

0 0 77 23

0 0 60 40

99 1 0 0

89 7 4 0

0 0 81 19

99 1 0 0

97 3 0 0

89 7 4 0

Fe2+ FeCO3 Fe(OH)2 FeHCO3 + Fe(OH)3 Fe(OH)2

69 6 0 24 0 0

70 16 0 11 0 0

35 47 0 14 1 0

74 16 0 7 0 0

78 7 0 10 0 0

54 0 0 2 15 9

SO42CaSO4 MgSO4 HS-

70 5 4 11

91 0 0 0

88 4 4 0

89 6 5 0

83 6 11 0

H2S

0

0

0

0

O

O

E-

PR 43 0 0 6 22 27

77 3 0 19 0 0

76 12 0 9 0 0

74 2 0 15 4 1

71 0 0 26 0 0

77 6 0 14 0 0

76 12 0 9 0 0

78 13 9 0

88 6 6 0

84 6 5 2

84 5 4 7

89 6 5 0

62 5 4 16

88 6 3 1

84 5 4 7

0

0

1

0

0

10

0

0

PR

AL 0

N U R

+

F

Well Depth(m)

27

JOURNAL PRE-PROOF

Table 7 Saturation index of 13 groundwater samples of nine drilling stations in Lanyang plain computed using PHREEQC (SI>0 precipitate；SI<0 dissolve). W17-1 40

W17-2 75

W21 32

W15-1 25

W13 30

W08 35

W06-1 50

W11-1 30

W05-1 25

W06-2 165

W11-3 115

W02-1 40

W05-1 25

Arsenolite (As4O6)

-31.1

-31.4

-38.0

-20.1

-31.2

-56.8

-60.8

-21.9

-26.5

-59.5

-21.9

-20.7

-26.5

Claudetite (As4O6)

-30.9

-31.1

-37.7

-19.8

-30.9

-56.5

-60.5

-21.7

-59.3

-21.7

-20.5

-26.2

Realgar (AsS)

-5.7

-7.9

-12.0

--*

-6.9

-19.3

--*

-7.3

-3.1

-3.5

-7.3

Orpiment (As2S3)

5.1

2.2

-2.0

--*

3.5

-8.5

Calite (CaCO3)

-0.1

0.1

1.0

-0.1

0.2

0.0

Anhydrite (CaSO4)

-3.8

-3.0

-2.8

-3.7

-2.2

Dolomite (CaMg(CO3)2)

-0.2

0.1

2.0

-0.2

Epsomite (MgSO4．7H2O)

-6.2

-5.5

-5.1

-6.1

Magnesite (MgCO3)

-0.7

-0.6

0.4

N

O

O

--*

--*

10.4

5.5

--*

10.3

9.4

5.5

-1.1

-0.6

-0.1

-0.4

-0.3

-0.2

-0.1

-2.3

-3.1

-3.6

-2.4

-4.0

-2.9

-3.6

PR

-3.6

E-

PR

-1.7 -0.1

-2.2

-1.2

-0.3

-0.8

-0.6

-0.7

-0.3

-4.2

-4.2

-4.7

-5.5

-6.0

-4.8

-6.5

-5.5

-6.0

-0.7

-1.7

-1.2

-0.8

-1.0

-5.3

-1.1

-0.8

-0.1

--* smaller than sulfide detected limit (< 3.25 μg / L)

U R

-26.2

0.6

AL -0.7

F

Well Depth(m)

28

JOURNAL PRE-PROOF

Table 7

Magnetite (Fe3O4) Siderite (FeCO3) Pyrite (FeS2) Scorodite (FeAsO4．H2O) Thenardite (Na2SO4)

W13 30

W08 35

W06-1 50

W11-1 30

W05-1 25

W06-2 165

W11-3 115

W02-1 40

W05-1 25

2.7

4.0

6.0

2.7

2.6

6.9

7.0

2.0

4.4

6.4

1.5

2.6

4.4

7.4

10.0

14.0

7.4

7.2

15.8

16.0

6.0

F

(Fe2O3)

W15-1 25

10.8

5.1

7.1

10.8

10.4

13.1

16.9

10.1

9.5

16.0

15.5

7.9

15.3

7.0

9.8

13.4

1.6

0.5

0.4

0.8

0.0

-1.0

-1.8

0.6

-0.1

-0.9

0.8

0.3

-0.1

11.3

11.8

15.4

--*

10.8

21.9

--*

11.4

13.8

--*

10.4

10.6

13.8

-18.0

-14.5

-9.8

-14.9

-16.8

-10.3

-9.7

-8.0

-10.6

-6.9

N U R

13.4

-6.5

-5.6

-15.6

-11.0

-9.2

-17.1

-15.4

-11.0

-9.6

-9.7

-10.1

-11.0

-9.3

-10.7

-9.9

-11.0

AL

--* smaller than sulfide detected limit (< 3.25 μg / L)

14.8

O

Hematite

W21 32

PR

(FeOOH)

W17-2 75

E-

Geothite

W17-1 40

PR

Well Depth(m)

O

(continued)

29

F

JOURNAL PRE-PROOF

groundwater direction groundwater level (m)

O

O

Pacific Ocean

Snow mountain

AL

PR

E-

PR

Ilan river

R N

Fig. 1. Lanyang plain map showing location of study areas, groundwater level and

JO

U

flow direction.

30

JOURNAL PRE-PROOF

0

10000 (m)

5000

Taiwan

W16

Pacific Ocean

W18

PR

Snow Mountain mountain Snow

W20

O

drilling stations W17

W23

O

W21 W19

monitoring wells

F

W22

W15

W14

W12 W10

PR

W09

W11

W06

AL

W08

E-

W13

W07 W05

W04

Central mountain Central Mountain

W02 W03 W01

JO

U

R N

Fig. 2. Locations of 23 groundwater monitoring wells and 9 drilling stations.

31

JOURNAL PRE-PROOF

Mountain 50

w08

Proximal-fan Recharge

Distal-fan

Mid-fan w07

w11

w06

0

w05 Uppermost

aquifer1 1 Lower

-50

F

Elevation (m)

5 km

0

100

bedrock

-100 -150

O

Lower 22 aquifer

W

-200

O

N E

PR

S -250

Sand

Silt

Clay

Flow direction

Fig. 3.

PR

E-

Gravel

Stratigraphic profile in the south bank of Lanyang river according to the red

JO

U

R N

AL

broken line denotes in Fig.2.

32

AL

PR

E-

PR

O

O

F

JOURNAL PRE-PROOF

JO

U

R N

Fig. 4. Piper diagram of groundwater properties in the Lanyang plain.

33

O

O

F

depth

JOURNAL PRE-PROOF

(b)

PR

E-

PR

(a)

(d)

JO

U

R N

AL

(c)

(e)

Fig. 5. Groundwater As concentration distribution versus (a) aquifer depth, (b) Eh, (c) DO, (d) NO3- and (e) SO42- in the Lanyang plain (▇ denotes wells in the north bank, ◆ in the south bank of Lanyang plain).

34

O

F

JOURNAL PRE-PROOF

(b)

PR

E-

PR

O

(a)

(c)

JO

U

R N

AL

(d)

(e)

Fig. 6. Correlations of groundwater As concentrations vs (a) HCO3-, (b) TOC; groundwater Fe concentrations vs (c) HCO3-, (d) TOC; and As vs Fe in the Lanyang plain (▇ denotes wells in the north bank, ◆ in the south bank of Lanyang plain).

35

JOURNAL PRE-PROOF

PR

O

O

F

(a)

R N

AL

PR

E-

(b)

Fig. 7. (a) Iron, (b) As concentration distribution in the corresponding four redox

JO

U

states of groundwater in the Lanyang plain.

36

JOURNAL PRE-PROOF

(b1) TOC, Fe

w16

w09

w11

w09

M ou nt a

w33

0

-0.5

w28

w10

w12

w07 w03 w06

w23 w15 w22

w10

M ou nt ai n SN O W

w11

w09 w08

w01

-1.5

Central Mountain

Central Mountain

(d2) Eh, pH

0.5 0

w30

w28

w33

w30

-0.5

w28

-1

w18 w12

w07 w03 w06

-1.5

w22 w15 w10

w18 w12

w03 w06 Central Mountain

AL

Central Mountain

PR

w12

w23 w15 w22

w18

w20 w13

w05 w02 w04

1

-1

-2

w17

w16

1.5

w33

E-

w30

w28

w25

2

in

0.5

-1

w18

O

PR

(c2) As, Alk, NH4

SN O W

w30

0 -1

Central Mountain

in

in M ou nt a SN O W

1

0

w19

w09

1

1

w26

w01

2

w33

w29

w05 w02 w04

(b2) TOC, Fe

4 3

w11

w01 Central Mountain

(a2) Cl, K, EC, Mg, Na, Ca, Mn

2

w16

w20 w13

w05 w02 w04

Central Mountain

w34

w31

w24

-1.5

w17

w08

w01

-0.5

w25

w24

w08 w05 w02 w04

w26

0

w37

w32

-1

w19

w20 w13

w08

w29

w38 w36

in

w11

w34

w31

M ou nt a

w16

w17

0.5

w32

-1 -1.5

w19

w20 w13

w25

w24

-2

w17

w26

1

w37

M ou nt a

w24 w19

-0.5

w29

-1

w25

w34

1.5

F

w26

0

SN O W

w29

w32 w31

w38 w36

0.5

w37

O

w34

w31

0 -1

0

SN O W

1

in

w32

SN O W

2

1

w37

w38 w36

M ou nt a

M ou nt a

in

3

2

1

2

w38 w36

w39

w39

M ou nt ai n

4

(d1) Eh, pH

(c1) As, Alk, NH4

w39

SN O W

w39

SN O W

(a1) Cl, K, EC, Mg, Na, Ca, Mn

w22 w15 w10 w03 w06

Central Mountain

U R

N

Fig. 8. Spatial distribution of the score of (a) factor 1 (salinization) (b) factor 2 (iron reduction) (c) factor 3 (As enrichment) (d) factor 4 (redox) (a1 and a2 denote factor 1 of aquifer 1 and 2, respectively; and so on). 37

JOURNAL PRE-PROOF

W16 10 20 30

W17 0

10 20 30

W13

W23 0

10 20 30

0

0

-100

-100

-150

-150

-200

-200

20

40

0

10 20 30

0

W04

10 20 30

0

10 20 30

W03 0

20

40

O PR E-

PR -250

AL

-250

O

F

-50

0

W05

W07

W11

10 20 30

-50

N

As (mg/kg) ● non-marine core Fe (mg/kg x5000) North bank South bank Fig. 9. Vertical distribution of As and Fe contents in core samples at nine direling stations with Lanyang plain.△ marine core As (mg/kg) ▲ marine core Fe (mg/kg x5000)

U R

depth(m)

0

0

○ non-marine core

38

O

O

F

JOURNAL PRE-PROOF

(b) w17

AL

PR

E-

PR

(a) w16

(d) w13

(c) w23

R N

Fig. 10. Correlation of analytical As and Fe contents of core samples in non-marine and marine sequences in the north bank of Lanyang river. (a)w16, (b)w17, (c)w23, (d)w13. (◆

JO

U

represents marine；■ represents non-marine).

39

O

O

F

JOURNAL PRE-PROOF

(b) w07

AL

PR

E-

PR

(a) w11

(c) w05

JO

U

R N

(d) w04

(e) w03 Fig. 11. Correlation of analytical As and Fe contents of core samples in non-marine and marine sequences in the south bank of Lanyang river (a)w11, (b)w07, (c)w05, (d)w04, (e)w03. (◆ represents marine；■ represents non-marine). 40

(c) w11-4 non-marine

(e) w07-2 marine

(f) w07-3 marine

(h) w03-2 marine

(i) w03-4 marine

JO

U

R N

AL

(d) w07-1 non-marine

PR

E-

PR

(b) w11-3 marine

O

(a) w11-2 marine

O

F

JOURNAL PRE-PROOF

(g)w03-1 non-marine

Fig. 12. Fe solid phase compounds of w11, w07 and w03 at various depths determined by curve fitting of Fe2p3/2 narrow scan spectrum.

41

O

F

JOURNAL PRE-PROOF

O

(a) (b)

PR

E-

PR

(c)

JO

U

R N

AL

(d)

Fig. 13. (a) and (c) are SEM-EDS image and spectrum of core sample for w11-2 at 115 m depth; (b) and (d) are image and spectrum of core sample for w07-2 at 75 m depth.

42

JOURNAL PRE-PROOF

Research Highlights  We analyzed 39 groundwater and 393 core samples of arsenic–affected aquifer in Yilan, Taiwan.  Factor analysis shows aqueous As closely associated with HCO3- and NH4+.

O

F

 High As and Fe contents were correlated in marine formation.

JO

U

R N

AL

PR

E-

PR

O

 As was adsorbed or co-precipitated with non-crystalline Fe oxyhydroxides and sulfides.

43