Limited Temporal Variability of Arsenic Concentrations in 20 Wells Monitored for 3 Years in Araihazar, Bangladesh

Millions of people in Bangladesh have probably switched
their water consumption to wells that meet the local
standard for As in drinking water of 50 μg/L as a result of
blanket field testing throughout the country. It is therefore
important to know if As concentrations in those wells could
change over time. To address this issue, we report here
precise groundwater As analyses for time-series samples
collected from a suite of 20 tube wells containing
≤50 μg/L As and ranging from 8 to 142 m in depth. For 17
out of 20 wells, the standard deviation of groundwater
As concentrations was <10 μg/L over the 3-year monitoring
period (n = 24−44 per well). Six of the 17 wells are
community wells, each of which serves the needs of several
hundred people in particularly affected villages. Of the
three wells showing larger fluctuations in chemical
composition including As, two are very shallow (8 and
10 m). Variations in As concentrations for one of these wells
(50 ± 32 μg/L, n = 36), as well as another shallow well
showing smaller variations (48 ± 5 μg/L, n = 36), appear
to be coupled to seasonal precipitation and recharge
linked to the monsoon. The other shallow well showing
larger variations in composition indicates a worrisome and
steady increase in As concentrations from 50 to 70 μg/L
(n = 36) over 3 years. The time series of As (30 ±
11 μg/L, n = 24) and other constituents in one deep
community well (59 m) show large fluctuations that suggest
entrainment of shallow groundwater through a broken
PVC pipe. Even though the majority of wells that were initially
safe remained so for 3 years, our results indicate that
tube wells should be tested periodically.