BIOGEOCHEMICAL PROCESSES CONTROLLING ARSENIC SPECIATION IN GROUNDWATER AQUIFERS AND AQUACULTURE PONDS OF THE CHIANAN PLAIN, SOUTHWEST TAIWAN

The Chianan Plain region of western Chiayi and Tainan Counties, southwest Taiwan, has historically been the primary endemic area for Blackfoot Disease (BFD), a peripheral vascular disease that has been linked to the long-term ingestion of arsenic-contaminated groundwater from deep wells (>100 m) drilled in the region’s alluvial aquifers during the 1940’s – 1960’s. Some workers have suggested that the co-occurrence of high concentrations of dissolved, highly fluorescent humic substances in association with high As concentrations may be a primary causative factor for BFD. Although the high-As groundwater is no longer directly ingested by the region’s inhabitants, these aquifers are still heavily produced for irrigation and aquaculture purposes. The inorganic As concentration of farmed fish from this area has been reported to co-vary with the total-As concentration of pond water (Huang et al., 2003). We measured the concentration and speciation of As associated with sediments and groundwater from wells drilled in or near the BFD endemic area, as well as from aquacultural fish-ponds that are supplied by the regional groundwater. Total phosphate-exchangeable As concentrations up to 1.4 mg/kg were measured in aquifer sediment below 100 m depth, with the majority of adsorbed As present as As(V) (>87%). Groundwater from 100 m depth also contained the highest dissolved As concentration (175.4 µg/L), with As present primarily as As(III) and organic-As forms. Arsenic concentrations in fish-pond water from three different locations on the Chianan Plain were highest in Beimen Township (142.9 µg/L), followed by Budai Township (78.9 µg/L) and Yichu Township (20.3 µg/L). Each of the ponds contained detectable amounts of dissolved organic and inorganic forms of As, with As(V) representing 70 – 90% of dissolved As in the Beimen and Yichu ponds. By contrast, the majority of dissolved As in the Budai pond (93 %) was present as an unidentified organo-As form. We also conducted sediment microcosm bioassays to investigate microbiological pathways for As bio-transformation that may affect the mobility and bioavailability of As in both the aquifer and fish-pond environments. A population of As(V) reducing bacteria in the aquifers and pond sediments readily reduced As(V) to As(III) using endogenous organic electron donors.