Extreme spatial heterogeneity has emerged as a salient characteristic of groundwater arsenic in many complex fl uviodeltaic environments. Here we examine patterns of arsenic heterogeneity in the shallow (<23 m) groundwaters of a well-studied fl oodplain setting in Araihazar, Bangladesh. Ninetyfi ve augers and 200 shallow wells sampled at spacings of 10 1 -10 3 m in the 25 km 2 area indicate that the concentration of arsenic in shallow groundwater largely varies with the grain size, thickness, and distribution of fi ne-grained (<63 µm) sediments that overlie buried aquifer sands. The overall pattern shows that lower arsenic concentrations are typically found where aquifer sands outcrop at or near the surface, whereas higher arsenic levels typically underlie, or are adjacent to thicker, fi ne-grained deposits. Furthermore, chronostratigraphic reconstructions of aquifer sediments indicate that sediment distribution, and consequently the patterning of dissolved arsenic, is readily explained in the context of local river history and fl oodplain development within the past 1000 yr. An important implication is that complex patterns of groundwater arsenic in affl icted fl uviodeltaic settings can be better understood through reconstructions of local aquifer history. This fi nding is especially relevant because the village and tube-well locations are closely linked with surface landforms such as former levees and bars. An additional and worrisome fi nding is that the artifi cial fi lling of villages to protect from fl ooding can mimic the natural fi ne-grained stratigraphy commonly associated with high concentrations of arsenic.
Measurement of low temperature (90ºC-120ºC) Thermoluminescence (TL) sensitivity of natural quartz samples subjected to pre-heating and optical stimulation indicate that significant sensitivity changes can occur during measurement of the natural Optically Stimulated Luminescence (OSL). During the measurement of natural signal, the luminescence sensitivity of samples can change by 40%. The sensitivity changes both during the initial preheat and the measurement of natural OSL. The currently used version of Single Aliquot Regeneration (SAR) protocol measures and corrects for the sensitivity changes after preheat and readout of natural OSL. However, it does not take into account the changes in sensitivity during the readout of the natural signal. We therefore developed a correction procedure so that both the natural and the regenerated OSL intensities can be measured and plotted with the same sensitivity and suggest that in the absence of such a correction, a considerable fraction of the SAR based ages could have systematic errors. The correction for the sensitivity is based on the use of sensitivity of 110ºC TL quartz peak, which is correlated to OSL signal (Murray and Roberts, 1998). The use of 110ºC peak provides a reasonable measure of the changes in OSL sensitivity of quartz. A modified Natural Sensitivity Corrected-SAR (NSC-SAR) procedure, that comprises the measurement of, 1) the TL intensity of 110ºC peak for a test dose on sample as received (i.e. natural sample) and, 2) the sensitivity of the 110ºC peak of the same sample after the preheat and read out of the natural OSL, is proposed. This ratio, termed as Natural Correction Factor (NCF), then provides a way to correct for sensitivity changes. Results on samples from diverse depositional environments indicated that the NSC-SAR consistently (without exception) provided improved distribution in paleodoses i.e. a lower scatter compared to the standard SAR protocol. In addition, the use of this protocol also resolved anomalous cases where the intensity of natural OSL was significantly above the saturation intensity of the regenerated OSL. Implicitly, this study implies a caution on the use of palaeodoses obtained from single grains as such a correction is not possible in the currently used automated single grain OSL measurement systems. The only way now on will be to analyze aliquots with only a grain on them.
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