Defining Landforms and Rate-Limiting Biogeochemical Factors Controlling the Release of Arsenic to Groundwater Within the Mekong Delta.

Arsenic-contaminated groundwater is pervasive in the deltaic regions of S/SE Asia, posing a massive human health risk. Under the flooded conditions of these deltaic sediments, microbially driven oxidation of organic carbon coupled to the dissimilatory reductive dissolution of arsenic-bearing iron (oxyhydr)oxides causes the transfer of arsenic from the solid to the aqueous phase. Substrate reactivity dictates the arsenic release rate, and a field-representative characterization of the relative reactivity of carbon and arsenic is lacking. Here the reactivity of organic carbon and As-bearing iron (oxyhydr)oxides was examined along three representative sediment profiles: 1) a perennial lake receiving high carbon input, 2) a wetland experiencing intermediate flood duration (~ 6 mo.) and carbon input, and 3) a wetland of short flood duration (~ 3 mo.) and low carbon input. Incubations of fresh sediment with sterile, anoxic groundwater medium (pH 7.1) were initiated in the field immediately upon sediment collection under nitrogen atmosphere. The following treatments were prepared: control (no amendment), 2 mM glucose, 3.5% Fe³⁺ (as As-loaded goethite), and 2 mM glucose + As-loaded goethite (3.5% Fe³⁺).  Arsenic and iron release in the control treatment was observed at 0.3 m and 0.4 m in the lake sediments, but not at 0.7 m; no arsenic or iron release occurred in the control treatment in either wetland profile down to ~30 m. Arsenic-loaded goethite did not promote arsenic or iron release. However, 2 mM glucose and As-loaded goethite together caused the greatest arsenic release to occur. Glucose addition stimulated arsenic and iron release in all three profiles for all depths measured, indicating organic carbon reactivity was the rate-limiting factor controlling arsenic release.  Arsenic reactivity decreased with increasing depth in all three depositional environments. Thus, in the Mekong Delta, active biogeochemical arsenic release is evidently constrained to continuously flooded, near-surface sediments, and is rate-limited by organic carbon reactivity.