Distribution of Arsenic Sulfides in Van Phuc, Vietnam, and Their Relationship to Aquifer Arsenic Concentrations

Groundwater arsenic concentrations are highly heterogeneous, in part due to spatial variability in sediment geochemical and hydrological properties on fine scales. The concentration of As in groundwater depends on the balance between sources of dissolved As, and the sinks that can return it to the solid phase. While much effort has focused on Fe(III) reduction as a source of groundwater arsenic, much less research has been devoted the variation in the sinks of arsenic in exhibiting sharp contrasts between As-impacted and As-free waters. It is critical to understand these processes at such boundaries to properly evaluate the long-term viability of low-As aquifers as alternative water sources. Here, we examine the Fe, As, and S speciation in sediments collected while installing a transect of wells across a sharp gradient in dissolved As levels in Van Phuc village, Vietnam. Marked variations in solid phase As speciation as As(V), As(III) and As2S3 were observed as a function of core location and depth. Arsenic sulfides were not found in shallow, weakly reducing and oxic sediments associated with groundwater low in As, but were extensive in some gray sediments from reduced aquifers. Many reduced aquifers also exhibited low dissolved sulfate concentrations. These observations indicate that the formation of these sulfides is tied to sulfate reduction within anoxic aquifers, but the prevalence of arsenic sulfide is not correlated directly to aqueous As levels. Iron speciation indicates that many, but not all, sediments from low-As aquifers materials contain appreciable Fe(III). In such low-As aquifers, As concentrations may be buffered by adsorption to iron oxides. In low-As aquifers within reduced sediments, however, Fe(III) minerals are rare and sulfide minerals may play a dominant role in regulating the fate of arsenic. In either case, the quantity of Fe(III) and/sulfate as terminal electron acceptors limits the sediment’s capacity to scavenge As. Thus, the boundaries between high- and low-As groundwaters may not be stable over time. This is a concern in Van Phuc because high-As groundwater elevated appears to be passing through the low-As portion of the aquifer. Extensive groundwater pumping of low-As groundwater may also facilitate this process.