Groundwater is a major water resource in Southwestern Taiwan; hence, long-term monitoring of water quality is essential. The study aims to assess the hydrochemical characteristics of water in the arsenic-contaminated aquifers of Choushui River alluvial fan and Chianan Plain, Taiwan using multivariate statistical methods, namely, factor analysis (FA), cluster analysis (CA), and discriminant analysis (DA). Factor analysis is applied to reveal the processes controlling the hydrochemistry of groundwater. Cluster analysis is applied to spatially categorize the collected water samples based on the water quality. Discriminant analysis is then applied to elucidate key parameters associated with the occurrence of elevated As concentration (>10 μg L(-1)) in groundwater. Major water types are characterized as Na-Ca-Cl and Na-Mg-Cl in the Choushui River alluvial fan and Chianan Plain, respectively. Inorganic species of arsenic (As), particularly As(III), prevail in these two groundwater catchments, and their levels are higher in the Chianan Plain than in the Choushui River alluvial fan. Through FA, three factors, namely, the degree of salination, As reduction, and iron (Fe) reduction, are determined and denoted irrespective of some differences between the factorial compositions. Spatial distribution patterns of factors As reduction and Fe reduction imply that the redox zonation is delineated by As- and Fe-dominance zones separately. The results of CA demonstrate that three main groups can be properly explained by the factors extracted via FA. Three- (Fe(2+), Fe(3+), and NH (4) (+) ) and four-parameters (Fe(2+), Fe(3+), NH (4) (+) , and Ca(2+)) derived from discriminant analysis for Choushui River alluvial fan and Chianan Plain are elucidated as key parameters affecting the distribution of As-contained groundwater. The analytical results indicate that the reductive dissolution of Fe minerals is prerequisite for the mobilization of As, whereas the shift of redox condition from Fe- to As-reducing leads to the accumulation of dissolved As in this area.
This work determined scopes of arsenic(As)-contaminated groundwater using risk-based indicator classification approaches in blackfoot disease hyperendemic areas of southern Taiwan. Indicator kriging was first used to establish a conditional cumulative distribution function at each cell. Three approaches--the p-quantile estimate, the E-type estimate and the minimization of the expected loss--were then adopted to delimit contaminated regions for a regulated standard of As concentrations in groundwater. According to a risk assessment model established in our previous research, the standard was set to 250 microg/l for aquacultural use, corresponding to the 77.1th percentile of observed concentrations. Misclassification risks and uncertainty were examined for the classification approaches. The analyzed results reveal that contaminated areas are the largest using the 0.771-quantile estimate, whereas they are the smallest using the minimization of the expected loss. Proportions of credible polluted areas with low risks to false positives maintain a constant, 12.9-13.2%, for the classification approaches. To reduce a great impact on human health, As-polluted groundwater should be strictly prohibited to cultivate fish in credible polluted zones and monitored persistently in polluted zones with high risks to false positives.
High arsenic (As) concentration in groundwater potentially poses a serious threat to the health of local residents in southwestern Taiwan. Although the As release to groundwater is responsible for the reducing bacteria-mediated reductive dissolution of As-rich Fe hydroxides, the influences of FeRB and different organic substrates on As and Fe mobility and transformation were rarely discussed. An experiment that involved As-adsorbed synthetic amorphous Fe(III) hydroxide (HFO) and the inoculation of in situ Fe-reducing bacteria (FeRB) was performed to evaluate the contribution of FeRB to the As mobility and transformation. The batched experiment of As-free HFO showed that the reducing bacteria rapidly induced the reduction of amorphous Fe oxyhydroxide to Fe(II) by reductive dissolution of HFO and formation of Fe-citrate complexation. For aqueous As(V) reduction experiment, arsenate was effectively reduced to As(III) by the facultative anaerobic bacterium in the cultured FeRB. In the experiment of As-containing HFO reduction, the aqueous As(V) acts as an electron acceptor and reduced to As(III) after the reductive dissolution of Fe(III) on HFO. However, the increase in the As(III) concentrations with time for various organic substrates in the As-adsorbed HFO-reducing experiment differ from the rates of As(V) reduction with various organic substrates in the As(V)-reducing experiment. The decrease in sorption sites by coupled reductive dissolution of HFO and the competitive desorption of small molecular organic carbon is apparently the important factor of As mobility. For large molecular organic carbon (i.e., citrate), the significant contribution of citrate on As mobility is the complexation of iron citrate. A working hypothesis model of As biogeochemical cycling is proposed to illustrate the relevant processes in the groundwater aquitard of southwestern Taiwan.
The distribution of inorganic arsenic (As) species in a groundwater sample, including As(III) and As(V), requires adequate preservation to prevent the adsorption caused by precipitates of Fe oxyhydroxides. Twenty-two groundwater samples with varied As and Fe concentrations were collected from three catchments in Taiwan. After filtration of samples in the field, inorganic As species were immediately preserved by adding EDTA/acetate acid. Analytical results showed that arsenite was the dominant species of inorganic As in most of samples. However, the distribution of Fe(II) varied widely and was not consistent with the equilibrium calculation by the geochemical program, PHREEQC, as a result of which the Fe(II) should be the primary Fe species under reducing conditions. The positive saturated index (SI) of Fe oxyhydroxides indicated that the Fe precipitation may occur. Indeed, the oxidation of Fe(II) rapidly forms Fe oxyhydroxides in controlled samples, causing the adsorption and/or co-precipitation of inorganic As and the decrease of aqueous As concentrations. EDTA/acetate acid can effectively slow the rate of As(III) oxidation and eliminates the precipitation of Fe and As, especially for high-Fe (>6.45 mg/L) groundwater samples. However, for low-Fe groundwater, other preservation methods should be considered, such as the phosphoric acid and hydrochloric acid.
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