VARIABILITY OF ARSENIC CONCENTRATIONS IN TUBE WELLS FROM ONE VILLAGE IN ARAIHAZAR UPAZILA, NARAYANGANJ DISTRICT OF BANGLADESH

In the Ganges-Brahmuputra Delta, concentrations of arsenic in tube wells have been known to vary spatially on both the basin (tens of km) and the local (tens of meter) scale, but to what extent the spatially variable arsenic concentration will change seasonally is not clear. In partnership with Bangladeshi scientists, water samples from 5000 contiguous tube wells in Araihazar Upazila, ~20 km east of Dhaka, were collected between March and May of 2000, and analyzed by graphite-furnace atomic absorption spectrometry subsequently. The geo-referenced data indicate an extremely heterogeneous spatial distribution of arsenic.

An important objective of our study is to understand the geological, hydrological and geochemical factors that contributed to the mobilization of arsenic. In January 2001, we began our multi-disciplinary investigation in village Dari Satyabhadi, where arsenic concentrations in 157 wells ranged from <5 mg/L to 800 mg/L (123 ¢´ 154 mg/L), with 51% of wells containing > 50 mg/L. Water from the lower Pleistocene Aquifer (> 100 ft) is characterized by low levels of arsenic (< 5 mg/L), positive oxidation-reduction potential (ORP). Radiocarbon analyses of monitoring wells (100 ft, 140 ft and 300 ft) found 28 percent modern carbon in dissolved inorganic carbon, indicating that the water is thousands of years old. In contrast, the upper Holocene Aquifer (25 ft-50 ft) displayed a wide range of arsenic concentrations corresponding to ORP values from -200 mV to slightly positive, with bomb-radiocarbon found in monitoring wells at 20 ft and 40 ft. The two aquifers are separated by a clay formation (50 ft-98 ft), likely the Madhupur clay, because a thin peat layer within the clay was AMS dated to be radiocarbon-dead. About 20 existing tube wells from Dari Satyabhadi were re-sampled in January 2001 and subsequently, ~ 50 wells were re-sampled in June 2001. Results will be used to constrain to what extent the spatially variable arsenic concentrations vary with season.