Hydrogeochemistry of Groundwater and Arsenic Adsorption Characteristics of Subsurface Sediments in an Alluvial Plain, SW Taiwan

Abstract

Many studies were conducted to investigate arsenic mobilization in different alluvial plains worldwide. However, due to the unique endemic disease associated with arsenic (As) contamination in Taiwan, a recent research was re-initiated to understand the transport behavior of arsenic in a localized alluvial plain. A comprehensive approach towards arsenic mobility, binding, and chemical speciation was applied to correlate groundwater hydrogeochemistry with parameters of the sediments that affected the As fate and transport. The groundwater belongs to a Na-Ca-HCO₃ type with moderate reducing to oxidizing conditions (redox potential = −192 to 8 mV). Groundwater As concentration in the region ranged from 8.89 to 1131 μg/L with a mean of 343 ± 297 μg/L, while the As content in the core sediments varied from 0.80 to 22.8 mg/kg with a mean of 9.9 ± 6.2 mg/kg. A significant correlation was found between As and Fe, Mn, or organic matter, as well as other elements such as Ni, Cu, Zn, and Co in the core sediments. Sequential extraction analysis indicated that the organic matter and Fe/Mn oxyhydroxides were the major binding pools of As. Batch adsorption experiments showed that the sediments had slightly higher affinity for As(III) than for As(V) under near neutral pH conditions and the As adsorption capacity increased as the contents of Fe oxyhydroxides as well as the organic matter increased.

1. Introduction

High arsenic (As) concentrations of natural origin are often found in groundwater of alluvial aquifers in different parts of the world. Such As-enriched aquifers also exist in the coastal Chianan Plain of southwestern Taiwan. This region is unique for its endemic cases of a peripheral vascular disease (i.e., gangrene), known as Blackfoot Disease (BFD), since 1960s [1]. The occurrence of BFD was strongly correlated with the direct ingestion of groundwater containing, on average, 671 ± 149 µg/L total dissolved As [2]. Several works addressed their heterogeneous distribution in the alluvial aquifers of the Chianan Plain and a number of geogenic sources and release mechanisms of As were reported in this region [3,4,5]. Among them, reductive dissolution of As-bound Fe/Mn oxyhydroxides in organic-rich sediments was considered as one of the major processes [4,5,6]. Other processes, like pyrite oxidation and siderite dissolution were also responsible for high As in Taiwan [7]. Chemical weathering of As-rich silicate minerals such as biotite in the Western Foothill Belt of the Central Mountain Range of Taiwan might also be one of the sources of As contamination in the Chianan Plain [8,9].

Spatial variation of As distribution in Chianan plain was controlled by local hydrogeological factors [9]. However, vertical distribution of As did not reflect any significant correlation with the depth [4,5,6]; arsenic in different solid phases and its associated release processes were mainly responsible for such As enrichment. However, in-depth cross sectional studies on subsurface sediments are limited, which may make it difficult to elucidate the characteristics of solid phase As uptake and its release mechanism. Although the arsenic-laden groundwater is no longer used for drink water purpose, it is still used in fishery of the region. Thus, developing a cost-effective method to remove arsenic from water remains as a challenge.

Sequential extraction may help in explaining partitioning of As in different solid phases in subsurface sediments as well as identifying the chemical speciation of As. Arsenic in solid phases was generally the result of chemisorption of As(V) or As(III) on soil colloid surfaces, mainly Fe oxide/hydroxides [10], in addition to be retained by minerals like biotite, goethite, and ferrihydrite [11]. In calcareous sediments, As could be adsorbed on carbonate minerals [10]. Different affinities of As(III) and As(V) can influence the extent of their adsorption on sediment constituents, which in turn affects the As concentrations in natural water [12]. Iron oxides have positive surface charges possessing high capability to adsorb negative-charged arsenate (e.g., H₂AsO₄⁻, and AsO₄³⁻) in water, whereas reduced As in the form of uncharged complex (e.g., H₃AsO₃) is less adsorbed by oxides and other minerals. Thus, As(III) is more mobile than other As species. For these reasons, chemical speciation analysis of sediment samples is essential to identify the forms of As present along the soil profile, which helps understand its transport, fate, and availability in natural systems.

The major objectives of this study are to: (i) understand the distribution of As in groundwater and core sediments; (ii) study the distribution of As among various hosting solids, metal-sediment interactions (mainly Mn and Fe), and As speciation in sediments; (iii) elucidate the adsorption characteristics of arsenic speciation in aquifers; and (iv) understand the binding, mobility, and fate of As in the As-enriched aquifer.

2. Materials and Methods

2.1. Geologic Setting

The alluvial Chianan Plain is located in the southwestern part of Taiwan (Figure 1) with an area of 2400 km². It is surrounded by the Peikang River in the north, the Ernjen River in the south, the Western Foothill Belt of the Central Mountain Range in the east, and the Taiwan Strait in the west. Subsurface sediments of this coastal plain were deposited during the Quaternary period in a deltaic environment where the groundwater was reducing in nature [4,5,13]. The plain is covered by alluvial deposits originating from the Western Foothill Belt of the Central Mountain Range through erosion and fluvial transport [6,9]. The sediments deposited into the plain are fine-grained clastic, while the subsurface water is highly saline, especially near the coastal areas. The sediments, deposited in shallow to deep sea environments, locally known as the Gutingkeng Formation at a depth of 100 to 280 m, often contain high amounts of organic ooze and humic acids [14]. Detailed local geological and hydrogeological setting was reported elsewhere [9].

2.2. Drilling and Sediment Collection

One borehole was drilled to a depth of 200 m in the town of Yichu in southwestern Taiwan. The site is located in the transition zone between the BFD-affected area and the non-affected area (Figure 1). The core sediments were sampled using a split-spoon sampler with rotary drill rigs at various depths and were immediately sealed in zipper packets and preserved in an anaerobic chamber for speciation analysis.

2.3. Core Sediments Characterization

The particle size distribution was analyzed using the pipette method [15]. The specific surface area (SSA) and cation exchange capacity (CEC) were determined by the methylene blue method. The total organic carbon (TOC) was measured using a LECO carbon analyzer after the sediments were first treated with 2 mol/L H₂SO₄ to remove free inorganic carbonates. The trace elements of core sediment were analyzed using ICP-MS version (Agilent 7500 cs, Santa Clara, CA, USA).

2.4. Groundwater Sampling and Analysis

Twenty-nine samples of groundwater were collected from the surrounding areas near the drilling site. Temperature, pH, electrical conductivity, salinity, total dissolved solids (TDS), and uncorrected oxidation-reduction potential (ORP) were measured on site using portable instruments. Groundwater samples were acidified with HNO₃ or HCl to pH 2 for major cations and As speciation analyses and un-acidified for major anion analyses. The major cations of Ca²⁺, Mg²⁺, K⁺, Na⁺, and NH₄⁺ and major anions of Cl⁻, NO₃⁻, NO₂⁻, SO₄²⁻, and F⁻ were analyzed with an ion chromatograph (Dionex, Suunyvale, CA, USA) using external calibration (accuracy: ±6%; precision: ±5%). Alkalinity and phosphate were analyzed spectrophotometrically. Dissolved organic carbon (DOC) was analyzed using a carbon analyzer (Analytikjena Multi N/C 2100, Überlingen, Germany). Elemental composition was measured using ICP-MS (Agilent 7500 cs). Arsenic speciation was performed with the HG-AFS technique using the Millennium Excalibur system PSA 10.055 (PSA Analytical Ltd., Kent, UK) with a detection limit of 1 µg/L.

2.5. Sequential Extraction Procedures

Sequential extraction of As was intended to determine the content of As bounded on different solid phases: (1) water soluble; (2) clay and carbonate; (3) Mn oxyhydroxides; (4) Fe oxyhydroxides; and (5) organic matter and sulfides in As-enriched sediments [16]. For all extraction 2 g of sediment and 16 mL of extraction solution was used. The sediments were mixed with 1 mol/L MgCl₂ for 1 h in extraction (1). For extraction (2), 1 mol/L sodium acetate/acetic acid buffered at pH 4.5 was mixed with the residues from extraction (1) for 15 h. For extraction (3), residues from extraction (2) was extracted with 40 mL of 0.04 mol/L hydroxylamine hydrochloride in 25% acetic acid at 95 °C for 5 h. For extraction (4), 100 mL of 0.2 mol/L oxylate + 0.2 mol/L oxalic acid was used for extractions of residues from extraction (3) and the extraction lasted for 4 h in the dark. For extraction (5), 6 mL of 0.02 N HNO₃ and 10 mL of 30% H₂O₂ were mixed with the residues from extraction (4) and heated to 85 °C for 2 h. The amounts of the residual fraction of As were computed as the difference between bulk concentration (determined by Aqua Regia total digestion) and the sum of leachable fractions.

2.6. Arsenic Speciation

The As speciation analyses were determined using the method by Georiadis [17]. A mild extraction media of phosphate solution [18] and sodium diethyldithiocarbamate trihydrate (NaDDC) were used as extractants. The supernatant solution was filtered through a 0.45 μm PVDF Acrodisc^® syringe (Pall Corp., Port Washington, NY, USA) filter into a 2 mL HPLC vial and was immediately analyzed using the HG-AFS technique with the Millennium Excalibur system.

2.7. Batch Study for As Adsorption

Samples of fine particles with relatively high As contents were selected for further adsorption experiments to determine the As adsorption capacity. To each 50 mL centrifuge tube, 1 g of sediments and 10 mL of As(V) or As(III) solution made from Na₂HAsO₄·7H₂O or NaAsO₂ at concentrations of 0.1, 0.4, 0.6, 0.8, 1.2, 1.6, and 2.0 mM were mixed for 24 h on a shaker table. No background electrolyte was added. The solution pH was between 7.6 and 8.8, under which the As(V) was in HAsO₄²⁻ form while As(III) was in H₃AsO₃ form, and the pH was not maintained. After centrifugation, the As concentrations in the supernatants were measured and the amount of As adsorbed were determined by the differences between the initial and the equilibrium As concentrations. The adsorption data were fitted to the Langmuir adsorption isotherm and the As adsorption capacity was then calculated.

3. Results and Discussion

3.1. Chemical Characterization of Sediments

A total of 38 core sediment samples at various depths down to 200 m were analyzed for their physico-chemical characteristics, particle size distribution and bulk geochemical (major ions and trace elements) composition. The average clay percentage was predominantly higher than that of sand and silt throughout the entire core, with the highest value (74%) at 85 m (Figure 2a). Overall, the percentage of sand was slightly decreased with depth, whereas the percentage of silt was about the same throughout the entire depth of the core, exhibiting a slight increasing trend with depth for clays. The sediments at alluvial Chianan Plain at various depths were similar to those of the Bengal Delta Plain [5,6].

The Yichu core sediments were reddish brown to gray in color with alteration of light and dark sediments along the entire depth. The gray color of the subsurface sediments suggested reduced environment in these aquifers, and the adsorbed As could be released to the groundwater via reductive dissolution of Fe/Mn oxyhydroxides [19]. This suggested potential release of the adsorbed As if the geochemical parameters such as pH and ORP changed.

The elemental composition of the Yichu core sediments is presented in

Table 1. Elemental composition of core sediment samples of Yichu, Chianan Plain.

Sample Depth (m)	As (mg/kg)	Fe (%)	Mn (mg/kg)	Ni (mg/kg)	Co (mg/kg)	Pb (mg/kg)	Zn (mg/kg)	Cu (mg/kg)	Ca (%)	Al (%)	P (%)	Mg (%)	K (%)	NH4 (mg/kg)	Cl− (mg/kg)	NO3− (mg/kg)	HCO3− (mg/kg)
5	6	1.83	234	18.8	8.4	9.1	49	8	NA	NA	NA	NA	NA	2.29	0.41	105.4	57.42
10	13.5	3.19	500	29.6	13.2	18.1	75	15.5	0.41	1.09	0.036	0.43	0.18	2.04	2.56	6.98	115.0
15	23	3.77	742	35.1	15.7	22.3	89	18.9	0.93	1.56	0.045	0.8	0.2	3.47	0.81	10.65	63.82
20	$10.9$	2.07	337	18.7	8	12	53	9.5	1.05	1.71	0.049	0.92	0.2	2.58	0.62	16.92	64.49
25	$6.5$	1.9	294	17.7	7.9	8.3	52	5.3	0.74	0.95	0.032	0.48	0.12	2.51	0.35	6.07	67.97
30	$8.4$	2.35	334	23.8	9.9	11.1	60	8.1	0.75	0.98	0.031	0.52	0.12	2.99	1.27	0.776	81.91
35	$6.8$	2.26	351	20.4	9.3	11	54	7.6	0.82	1.16	0.038	0.62	0.13	3.36	1.11	0.745	77.33
40	$18$	3.81	672	38.1	16	28.2	102	28.8	0.9	1.08	0.042	0.62	0.13	3.81	11.29	0.454	134.2
45	$6.6$	2.03	351	21.6	9.5	12.2	51	7.7	1.23	1.8	0.049	1.05	0.17	BDL	9.04	0.217	92.42
50	$14$	3.92	808	38.9	18	21.9	86	19.5	1.01	0.99	0.069	0.62	0.13	5.66	7.54	0.195	50.54
55	$12$	2.1	431	22.1	10.2	11.5	55	9.2	1.2	1.83	0.052	1.07	0.21	7.90	3.57	31.67	71.16
60	$2.7$	1.96	354	19.4	8.6	10.4	51	9.5	1.09	1	0.035	0.63	0.13	7.90	6.26	1.857	83.47
65	$2.7$	1.92	262	20.2	8.2	10.3	67	7.9	0.93	1.04	0.043	0.55	0.17	3.39	1.84	41.11	65.74
70	$3.6$	1.86	99	19.6	8.1	8.6	54	9.3	0.8	0.98	0.036	0.52	0.12	3.65	0.34	4.29	81.33
75	$6.9$	2.36	136	34	17.4	18.4	78	16.6	0.12	1.05	0.036	0.37	0.14	10.73	1.15	189.3	81.33
80	$6.4$	3.69	815	31.6	14	18.8	82	17.7	0.18	1.44	0.043	0.51	0.13	2.78	0.71	82.06	86.42
85	$13$	4.21	531	41.1	16.6	21.8	97	22	0.57	1.68	0.055	0.71	0.14	5.61	0.76	74.12	36.6
90	$22.8$	2.46	504	21.9	10.4	12.1	57	10	0.67	2.03	0.046	0.96	0.18	2.72	0.33	73.09	34.31
95	$0.8$	1.04	69	14.2	7.7	8.4	41	5.1	0.1	1.25	0.038	0.36	0.15	5.27	0.64	5.45	31.47
100	$21.5$	2.35	411	21.1	9.8	12	52	8.5	0.07	0.58	0.013	0.19	0.1	3.09	0.58	0.184	76.45
105	$3.4$	2.55	622	17.4	8.9	9.9	43	6.6	0.14	1.22	0.051	0.33	0.14	3.21	0.39	3.113	75.08
110	$13.4$	1.98	448	18.7	8.4	10.3	45	7.7	0.17	0.88	0.026	0.29	0.12	2.99	0.23	59.77	87.39
115	$4$	1.42	181	15.4	7.3	8.1	42	5.8	0.45	0.85	0.025	0.38	0.11	6.45	0.32	77.08	129.62
120	$7.9$	2.17	129	23.4	9.2	11.3	53	7.3	0.36	0.78	0.022	0.33	0.12	4.6	0.18	60.81	17.08
125	$8.4$	2.48	391	25.5	11.1	12.3	62	22.4	0.11	0.99	0.032	0.42	0.13	1.59	0.32	76.34	34.56
130	$5.9$	2.72	164	25.5	11.2	13.4	71	14.2	0.82	1.22	0.045	0.56	0.14	1.58	0.31	32.26	130.1
135	$1.7$	2.43	157	26.9	10	16.1	85	13.3	0.08	1.54	0.038	0.47	0.17	2.22	0.23	18.02	45.75
140	$7.6$	1.91	203	19.9	9.2	12	49	7.4	0.12	1.45	0.026	0.55	0.15	2.56	0.32	34.06	41.37
145	$17.3$	3.47	805	32.9	15.3	19.1	83	16	0.12	0.95	0.034	0.37	0.12	4.89	0.34	10.53	128.7
150	$4$	1.39	112	14.6	7.1	8	37	4.4	1.12	1.59	0.048	0.83	0.14	2.41	0.16	6.89	82.59
155	$17.5$	5.6	1511	38.8	16.8	24.5	101	22.7	0.08	0.69	0.027	0.27	0.1	3.24	0.24	3.71	30.84
160	$10.3$	4.78	1248	36.2	15.3	26.3	92	20.4	1.73	1.9	0.125	0.91	0.17	7.38	0.07	5.384	BDL
165	$5.2$	1.24	82	12.9	6.5	6.5	37	5	0.98	1.81	0.102	0.88	0.17	1.99	0.15	4.148	97.6
170	$3.9$	1.58	146	14.4	6.5	7.4	45	5.3	0.05	0.69	0.013	0.25	0.1	63.46	3.53	3.415	106.5
175	$16.3$	3.27	478	28.5	13	20.1	78	18.7	0.22	0.83	0.02	0.34	0.1	81.35	8.05	6.05	76.68
180	$18.8$	3.08	422	30.8	14	18	79	15.7	0.95	1.5	0.044	0.8	0.14	64.42	4.11	3.191	57.51
185	$4.7$	2.57	338	23	8.3	11.6	65	7	0.91	1.44	0.042	0.8	0.13	53.68	4.29	11.79	75.08
190	$18.4$	4.05	694	34.4	15.6	33.5	102	21.2	0.75	1.33	0.038	0.7	0.15	2.29	0.40	105.4	57.42
195	$13.2$	2.95	400	27.4	10.9	17.4	74	11	1.07	1.97	0.05	1.08	0.19	2.04	2.56	6.98	115.0
200	$9.3$	2.7	398	25.8	10.3	15.7	73	10	0.87	1.46	0.047	0.77	0.17	3.47	0.81	10.65	63.82
Min	0.80	1.04	69	12.9	6.5	6.5	37.0	4.4	0.05	0.58	0.013	0.19	0.10	0	0.07	0.184	0
Max	22.8	5.60	1511	41.1	18.0	33.5	102	28.8	1.73	2.03	0.125	1.08	0.21	81.4	11.3	189.3	134.2
Mean	9.93	2.64	429.1	25.0	11.0	14.7	65.5	12.2	0.63	1.26	0.042	0.60	0.14	9.94	1.96	29.78	72.6
STD	6.20	1.00	306.6	7.82	3.4	6.36	19.3	6.29	0.44	0.39	0.020	0.25	0.03	19.2	2.79	40.99	31.8
Median	8.15	2.40	372.5	23.2	9.95	12.0	61.0	9.50	0.75	1.22	0.038	0.55	0.14	3.30	0.63	8.76	75.1

. The As, Fe, and Mn contents showed a large variation with depth (Figure 3), ranging from 0.80 to 22.8 mg/kg (mean 9.9 ± 6.2 mg/kg), 1.0% to 5.6% (mean 2.6% ± 1.0%), and 69 to 1511 mg/kg (mean 429 ± 306 mg/kg), respectively. Highest levels of As were found at depths of 15, 40, 90, 100, 180, and 190 m. Regression analysis between As and other elements and TOC (Figure 4) showed positive correlations. For Fe, Mn, and TOC, the correlations were R² = 0.72, 0.81, and 0.55, respectively, with p < 0.05, suggesting their strong association. Positive correlations between As and Fe (R² = 0.65, p < 0.05), or Mn (R² = 0.42, p < 0.05), were also observed in a previous study of sediments collected from an adjacent Budai area (Figure 1), which indicates that Fe and Mn in the sediments could be in the form of adsorbed phases on fine-grain (clay size) mineral surfaces [20]. Positive correlations were also found between As and Ni (R² = 0.55, p < 0.05) or Co (R² = 0.65, p < 0.05) for sideraphile, and between As and Cu (R² = 0.50, p < 0.05), or Zn (R² = 0.54, p < 0.05), for chalcophile, indicating that sulfide solids may serve as local sinks for As under sulfate-reducing conditions.

3.2. Hydrogeochemistry and Occurrences of As in Groundwater

The physico-chemical characteristics of the groundwater collected from the alluvial Chianan Plain are presented in

Table 2. Physico-chemical characteristics of groundwater samples (n = 29) collected from Yichu, Chianan plain, southwestern Taiwan.

Sample Location	Latitude/Longitude	Depth (m)	Temp (°C)	EC (mS/cm)	TDS (mg/L)	Salinity (‰)	pH	ORP (mV)	DOC (mg/L)
Beimen 2B	N 23°17′23.4″/E 120°8′57.8″	60	24.8	67.9	OFL	46.2	7.1	−128	23
Yenshui 2	N 23°18′4″/E 120°15′12″	23	20.9	1.2	575	0.4	7.83	−158	26.4
Yichu (house)	N 23°19′52.3″/E 120°13′27.2″	20	24.7	3.08	1481	1.5	7.23	−108	17.
Lucao1B (rice field)	N 23°24′59.9″/E 120°17′50.6″	13	18.1	1.52	727	0.6	7.54	−54	11.3
Lucao1A (house)	N 23°24′59″/E 120°17′51″	30	24.3	1.79	857	0.7	7.54	−118	19.6
Liujiao2	N 23°30′27.3″/E 120°16′19.1″	67	24.9	1.044	501	0.3	7.31	−148	13.7
Budai-Shinwen	N 23°20′22″/E 120°7′57.9″	313	24.7	1.15	554	0.4	7.63	−140	158
Budai-3	N 23°20′29.7″/E 120°9′37″	233	32.3	0.97	463	0.3	8.25	−96	129
Beimen CN 9	N 23°18′33.3″/E 120°8′45.7″	100	31.7	1.67	799	0.7	8.1	-72	157
Beimen-Jinhu	N 23°18′26″/E 120°9′8.2″	300	28.5	1.42	682	0.5	7.44	−139	159
Beimen 2A	N 23°30′27.6″/E 120°16′19″	277	24.7	1.86	893	0.8	7.33	−144	163
Yenshui 3	N 23°18′6.7″/E 120°15′11.1″	23	24.7	1.56	751	0.6	7.92	−158	119
Yenshui 1	N 23°18′2.4″/E 120°14′57.2″	23	24.7	1.37	657	0.5	8.12	−133	124
Beimen 1	N 23°17′23.3″/E 120°5′41.2″	200	26.4	1.43	684	0.5	7.68	−149	205
Budai 5	N 23°22′56.8″/E 120°9′49.6″	200	24.5	0.86	410	0.2	8.34	8	97.7
Budai 4	N 23° 19′37.8″/E 120° 9′3.2″	300	23.9	3.55	1704	1.8	7.76	−100	141
Hsuechia 2	N 23°13′46.1″/E 120°10′4.3″	3	21.4	5.47	2626	2.9	7.38	−96	105
Siaying 3	N 23°14′14.7″/E120°14′21.7″	233	27.5	0.85	408	0.2	8.13	−17	111
Siaying 1	N 23°14′7.9″/E 120°14′40″	150	25	3.37	1619	1.7	7.05	−27	103
Yichu 5	N 23°19′59.7″/E 120°13′11.6″	20	26.1	1.57	751	0.6	7.24	−119	125
Yichu 6	N 23°19′29.7″/E 120°12′48.1″	20	25.1	2.32	1116	1	7.5	−102	142
Yichu 7	N 23°18′55.5″/E 120°10′5.8″	72	29.1	1.13	545	0.4	8.1	-192	136
Yichu 8	N 23°18′57.1″/E 120°10′25.6″	143	25.2	5.44	2750	2.9	7.9	−10	138
Lucao 3	N 23°22′40.4″/E 120°16′51.8″	4.5	27.8	1.10	531	0.3	7.3	−91	87
Lucao 4	N 23°23′15.1″/E 120°17′39.2″	83	26.5	1.49	716	0.6	7.84	−148	101
Lucao 5	N 23°23′25.3″/E 120°17′20.4″	300	26.3	2.08	998	0.9	7.63	−153	98.3
Hsuechia 3	N 23°17′7.5″/E 120°11′6.7″	67	27.8	0.96	462	0.2	7.65	−72	122
Hsuechia 4	N 23°16′22.2″/E 120°9′36.2″	5	24.1	5.05	2610	2.7	7.34	−113	155
Hsuechia 5	N 23°14′39.6″/E 120°9′14.7″	7	27.1	2.24	1076	1	7.59	−118	115
	Min	3	18.1	0.85	408	0.2	7.05	−192	11.3
	Max	313	32.3	67.9	2750	46.2	8.34	8	204
	Mean	113	25.6	4.32	998	2.46	7.64	−107	107
	STD	110	2.86	12.3	678	8.44	0.35	49.2	52.2
	Median	67	25	1.56	739	0.6	7.63	−118	119

and

Table 3. Hydrochemical characteristics of groundwater samples (n = 29) collected from Yichu, Chianan plain, southwestern Taiwan.

Sample Location	Alkalinity (mg/L)	F (mg/L)	Cl (mg/L)	NO2 (mg/L)	NO3 (mg/L)	SO4 (mg/L)	Na (mg/L)	NH4 (mg/L)	K (mg/L)	Mg (mg/L)	Ca (mg/L)	PO4 (mg/L)	As(III) (μg/L)	As(V) (μg/L)	Fe (μg/L)	Mn (μg/L)
Beimen 2B	184	2.89	332	0.00	8.17	10.4	582	0.00	26.2	51.4	66	3.00	102	224	18,809	32.3
Yenshui 2	182	2.85	57.0	0.00	7.70	29.1	138	2.3	17.9	63	64.7	3.17	560	184	3725	91.0
Yichu (house)	190	3.16	269	0.00	7.76	336	277	0.00	20.1	88.1	236	0.44	102	10.5	8338	160
Lucao1B (rice field)	188	2.88	89.7	0.00	8.28	90.7	1.41	0.00	3.34	71.2	144	1.20	4.95	3.94	4879	75.9
Lucao 1A (house)	185	2.86	108	0.00	0.00	111	1.71	0.00	3.4	70.7	176	2.60	41.1	5.23	28	1.19
Liujiao 2	175	2.97	24.9	0.00	7.66	72.3	75.9	0.00	3.63	37.7	121	1.09	89.8	5.03	12,129	237
Budai- Shinwen	185	0.00	171	0.00	3.50	0.00	422	0.00	4.56	24.8	34.8	2.18	604	100	1271	30.3
Budai-3	177	0.00	9.85	0.00	0.00	3.33	223	10.1	8.25	12.8	16.2	3.92	654	64.4	533	40.7
Beimen CN 9	173	0.00	104	0.00	3.35	11.4	391	0.00	16.9	23	11.7	6.08	212	40.6	365	58.9
Beimen-Jinhu	180	0.00	137	0.00	3.36	3.11	362	28.3	6.88	31.4	42.3	1.86	356	184	4033	32.9
Beimen 2A	172	0.00	116	0.00	3.39	5.88	332	24.3	5.71	35.5	74.5	3.00	158	220	3537	28.0
Yenshui 3	177	0.00	76.8	0.00	0.00	86.8	194	4.45	20	77.2	102	1.82	954	177	2994	27.6
Yenshui 1	176	0.00	70.2	0.00	2.91	8.85	245	0.00	32.7	45.8	26.4	2.22	274	41.6	466	25.5
Beimen 1	340	0.00	12.4	1.29	3.79	3.16	333	0.00	3.66	12.6	33.8	4.16	372	161	2326	28.9
Budai 5	157	0.00	17.6	1.13	4.63	4.71	170	0.00	6.56	7.37	33.4	1.42	1.96	319	381	17.6
Budai 4	166	0.00	212	473	3.89	11.4	622	0.00	13.3	51.5	98.7	1.72	24.5	285	5832	28.7
Hsuechia 2	175	0.00	0.00	859	3.72	234	698	0.00	14.7	106	531	0.00	10.3	3.22	10,260	2609
Siaying 3	154	0.00	5.94	0.00	3.65	3.55	191	5.84	10.7	13.6	26.8	1.38	57.9	10.2	519	70.1
Siaying 1	171	0.00	0.00	207	385	0.95	531	0.00	8.55	32	112	2.43	190	9.15	12,093	263
Yichu 5	183	0.00	73.2	0.00	3.71	92.7	151	2.66	16.1	70.4	161	0.21	94.2	12.9	9385	107
Yichu 6	192	0.00	96.3	0.00	3.6	262	300	0.00	32.2	86.7	163	1.31	49.8	6.60	7022	1047
Yichu 7	182	0.00	23.4	0.00	3.49	3.43	259	11.8	12.8	16.2	17.9	2.89	305	104	1062	85.8
Yichu 8	189	0.00	258	539	4.46	39.8	816	0.00	29.9	77.1	59.9	1.91	0.62	96.1	956	17.5
Lucao 3	163	0.00	14.6	0.00	0.00	128	34	0.00	6.02	66.2	203	0.00	22.9	1.36	14,508	1957
Lucao 4	177	0.00	112	0.00	3.67	12.9	295	0.00	18.8	27.9	37.7	2.92	188	51.9	3428	87.9
Lucao 5	177	0.00	187	0.00	3.87	5.45	273	12.4	19.4	58.9	68.6	1.04	57.8	18.2	5389	192
Hsuechia 3	176	0.00	19.7	0.00	3.64	3.14	194	8.46	7.81	20.2	42	1.04	510	112	5281	161
Hsuechia 4	181	0.00	226	965	4.66	51.9	872	0.00	59.9	103	130	0.14	34.1	3.68	12,104	359
Hsuechia 5	179	0.00	155	0.00	3.83	126	316	0.00	18.9	66.2	157	0.34	34.8	0.23	10,965	339
Min	154	0.00	0.00	0.00	0.00	0.00	1.41	0.00	3.34	7.37	11.7	0.00	0.62	0.23	28.1	1.19
Max	340	3.16	332	965	385	335	871	28.2	59.8	105	531	6.08	954	318	18,809	2609
Mean	183	0.61	103	105	17.1	60.4	320	3.81	15.4	49.9	103	1.91	209	84.6	5607	283
SD	31.5	1.21	91.4	261	70.8	86.6	224	7.32	12.2	28.6	102	1.40	242	94.9	5046	595
Median	177	0	89.8	0	3.71	11.4	276	0	13.3	51.4	68.5	1.82	102	41.5	4033	75.9

(n = 29). The hydrochemical properties of the groundwater in the neighboring areas of the drilling site exhibited considerable variations in a range of well depth from 3 m to 313 m, i.e., water temperature (18.1 to 32.3 °C; mean 25.6 ± 2.86 °C), pH (7.05 to 8.34; mean 7.64 ± 0.35), EC (0.85 to 67.9 mS/cm; mean 4.32 ± 12.3 mS/cm), salinity (0.2‰ to 46.2‰; mean 2.46‰ ± 8.44 ‰), TDS (408 to 2750 mg/L; mean 998 ± 678 mg/L), DOC (11.3 to 204 mg/L, mean= 107 ± 52.2 mg/L), and ORP (−192 to 8 mV; mean −107 ± 49.2 mV). The alkalinity ranged from 154 to 340 mg/L (mean 183 ± 31.5 mg/L).

Among the major anions, chloride ranged from <0.1 to 332 mg/L (mean 103 ± 91.4 mg/L), and nitrate varied from 3.50 to 7.80 mg/L (mean 4.60 ± 1.80 mg/L). Elevated levels of sulfate were observed up to 385 mg/L (mean 17.1 ± 70.8 mg/L). Fluoride was mostly absent with the exception of four locations showing the maximum at 3.16 mg/L. Similarly, nitrite was only found in six sites (mean 105 ± 261 mg/L). Phosphate showed a meager variation between <0.1 to 6.08 mg/L (mean 1.91 ± 1.40 mg/L). Among the major cations, Na⁺ was found to vary within 1.41 to 871 mg/L (mean 320 ± 224 mg/L). NH₄⁺ was observed at five sites (mean 3.81 ± 7.32 mg/L). Other cations K⁺ (3.34 to 59.8 mg/L; mean 15.4 ± 12.2 mg/L), Mg²⁺ (7.37 to 105 mg/L; mean 49.9 ± 28.6 mg/L) and Ca²⁺ (11.7 to 531 mg/L; mean 103 ± 102 mg/L) also varied considerably. The general water type was found to be an Na-Cl type as observed from the Piper diagram (Figure 2b). The major cation Ca²⁺ may derive from carbonate minerals such as calcite and dolomite, while Mg²⁺ may originate from biotite, chlorite, and carbonate minerals, and K⁺ may derive from orthoclase and clay minerals [21].

The As(III) and As(V) concentrations ranged from 0.62 to 954 μg/L (mean 209 ± 242 μg/L) and from 0.23 to 318 μg/L (mean 84.6 ± 94.9 μg/L), respectively (Table 3), among the 29 groundwater wells. Only a few samples contained arsenic concentrations within the WHO standard of <10 µg/L, and most of the groundwater samples contained concentrations higher than 100 µg/L.

3.3. Solid Phase Partitioning of As and Speciation

Figure 5 shows the fraction of extractable and non-extractable solid phase As at different depths. The relative proportion of different As pools did not show much depth variation. The percentage of leachable As did not vary much until at a depth of 110 m. Only a small fraction (0.03% to 2.17%) of As was present in the water soluble/leachable form and clay/carbonate bound fraction (0.01% to 1.43%). Similarly, the exchangeable of As accounted for 4% in the sediments of Zhalong wetland in Northeastern China ([22]. Among the various extractable pools, the organic matter (0.90%–51.4%) and Fe/Mn-oxyhydroxides were found to be the major binding phases (1.08%–51.1% bound to Mn-oxyhydroxide, and 0.58%–25.5% to Fe-oxyhydroxides). These results confirmed that the As in the sediments interacted with the Fe/Mn phases via adsorption. The amounts of As in the sodium pyrophosphate and 0.1 M hydroxylamine extracts were found to be higher among various extractable pools, suggesting that organic matter and Mn-oxyhydroxides are the dominant hosts of solid phase As. A general trend of increasing As bound to organic matter was observed below a depth of 130 m, whereas As bound to Mn-oxyhydroxide was uniform until a depth of 150 m, and thereafter it increased markedly. The majority of the As fractions is bound to the recalcitrant fraction. The percentage of non-extractable As was uniformly high and ranged widely between 41.4% and 96.8%, suggesting that most As was associated with residual phases of crystalline sulfides, except at a depth of 135 m where the non-leachable fraction was only 1.60%. In a similar study, 88%–91% of the total As concentration was found associated with residual fraction in mariculture sediments from Dongshan Bay in China across from Taiwan Strait [23].

The predominant species in the Yichu core sediments was As(III) (Figure 6). The As(III) ranged from 0.9 ± 0.2 to 14 ± 0.78 μg/kg, whereas As(V) ranged from 0.13 ± 0.06 to 6.48 ± 0.63 μg/kg. It is well known that As(V) is reduced to As(III) under reducing conditions. Due to the lack of negative charged for arsenite under near neutral pH condition, its affinity to iron and manganese oxides is much weaker than As(V), and can be released into water [24]. In addition, due to the weak binding, 92%–99% of As(III) could be extracted under subcritical conditions [25].

3.4. Adsorption Characteristics of Arsenic in Sediment and Its Implication on Mobilization

A characteristic adsorption isotherm was observed when the adsorption of As(III) and As(V) was plotted against the equilibrium As concentration at an equilibrium pH of 7.5 (Figure 7). Overall, a slightly high As(III) adsorption on these sediments were observed. This finding is similar to the trend observed in the Budai core sediments of the Chianan Plain [20] and agrees well with the As speciation analyses of the core samples (Figure 6). Adsorption of As(III) on ferrihydrite was considerably faster than that of As(V) at pH 7 to 8 [26]. Multiple linear regression analyses showed the following results:

As(V) = 0.03 Clay − 0.01 Mn + 0.48 Fe + 3.52 TOC + 7.01 SSA − 54.51 CEC, adjusted R² = 0.99

As(III) = −0.04 Clay + 0.002 Mn + 1.17 Fe + 18.5 TOC + 6.19 SSA − 48.08 CEC, adjusted R² = 0.99

They suggest positive correlation with Fe, TOC, SSA, and negative correlation with CEC, confirming the observations for correlations between As contents and elements concentrations in these core sediment samples (n = 40) (Figure 4).

Due to the strong adsorption of As(V) and As(III) onto organic matters, they are likely to remain in soils for a long time, especially in fine-textured soils with high Fe content [27]. Similar results were observed for estuarine sediments in SW Taiwan [28] and in the sediments of silty clay and silty sand of the Yun-Lin Plain, Taiwan [29].

The groundwater contained low levels of sulfate, indicating that oxidation of pyrite may not be the likely source of As. The element image analyses showed no correlation between pyrite and As contents [30]. Thus, reductive dissolution of As-contained ferric iron under anaerobic conditions is suggested to be a major mobilizing force for As release to the groundwater of the Chianan Plain. Although no such correlation has been observed between As and bicarbonate concentration, high As groundwater usually contains relatively elevated concentrations of bicarbonate in the Chianan Plain aquifer. This may result in desorption of As oxyanions under the competitive sorption effect of elevated concentrations of bicarbonate [31].

4. Conclusions

The hydrogeochemical characteristics of groundwater and subsurface sediments and the adsorption characteristics of arsenic speciation in aquifers were investigated in the As-enriched areas of the Chianan Plain in southwestern Taiwan. The concentration of aqueous As was strongly depth-dependent, with the maximum at 1131 µg/L. Redox-sensitive parameters were closely related to bicarbonate and electrical conductivity. Dissolved As was significantly correlated with Fe contents, suggesting that the Fe-bearing phases is likely to be a sink of As. The aquifer sediments were characterized with higher clay content and alkaline pH and were enriched with organic matter, which bears a positive correlation with As content at different depths under consideration. In core samples, As, Fe, and Mn exhibited strong depth dependence, suggesting reductive dissolution to be a major mobilizing force for As under moderately reducing conditions. Sequential separation showed that the organic matter and Fe/Mn-oxyhydroxides are the major binding phases of As. The speciation analyses showed dominance of As(III) over As(V) in the sediment samples under consideration. Arsenic adsorption on the sediments followed the Langmuir adsorption isotherm with a higher adsorption capacity for As(III) than for As(V), confirming the major adsorbed species was As(III) in the sediments.