One of the reasons the processes resulting in As release to groundwater in southern Asia remain
poorly understood is the high degree of spatial variability of physical and chemical properties in shallow
aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as
a slurry from precisely the same interval was developed in Bangladesh. Recently published results from
Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of
grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered
groundwater ranging in depth from 3 to 36 m that were analyzed for As (1-1000 ?g/L), Fe (0.01-40 mg/L),
Mn (0.2-4 mg/L) and S (0.04-14 mg/L) are compared. The P-extractable (0.01-36 mg/kg) and HClextractable
As (0.04-36 mg/kg) content of the particulate phase was determined in the same suite of samples,
in addition to Fe(II)/Fe ratios (0.2-1.0) in the acid-leachable fraction of the particulate phase. Needlesampler
data from Bangladesh indicated a relationship between dissolved As in groundwater and Pextractable
As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under
sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution
coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As
concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to
Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe
>0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be
particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of
increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the
flushing of shallow aquifers of their initial As content are explored. ?? 2008 Elsevier Ltd. All rights
reserved.
