The Role of Gases in an Arsenic Contaminated Aquifer

Arsenic (As) contamination of groundwater remains a problem for many of the river deltaic areas in South-East Asia; where concentrations regularly exceed the 10μ/L currently recommended by the Word Health Organization. The focus of this study, is to determine noble and reactive gases in groundwaters at a location where As mobilisation is active, to constrain the sites hydrology in such a highly reducing environment. The small village of Van Phuc, Vietnam, presents an ideal opportunity for such research as is it well studied and accessible, however As dynamics here are still not well understood.

Gas concentrations in 21 wells at varying depths and locations were analysed in Van Phuc with the miniRUEDI, a portable mass spectrometer capable of measuring noble gases: He, Ar, Kr, and reactive gases: CO₂, CH₄, N₂ and O₂. Water samples were additionally taken in copper tubes for later analysis, in an effort to date the groundwater using the ³He ingrowth method. Dating such samples is particularly difficult in environments such as Van Phuc, where Methane tends to oversaturate and foster in-situ degassing of the groundwater.

First results show a progressive depletion of the atmospheric gases (Ar, Kr and N₂) with increasing CH₄ concentrations. He, shows the opposite behaviour such that it increases in concentration as CH₄ approaches in-situ saturation within the groundwater. The conceptual picture these results indicate, is that the production of Methane bubbles reduces the hydraulic conductivity in the aquifer; allowing enough time for He to accumulate, whilst simultaneously depleting Ar, Kr and N₂ in the groundwater as a result of their partitioning into the free CH₄ gas phase, which is subsequently degassed.