Clay layers may be contaminating groundwater with arsenic in parts of Bangladesh, according to a recent study.
The study, published in Nature Communications, suggests that clay layers do not always protect against arsenic and could potentially contaminate some wells.
"Our findings challenge a widely held view, namely that impermeable clay layers necessarily protect an aquifer from perturbation," said Alexander van Geen, a research professor at Columbia University's Lamont-Doherty Earth Observatory. "In this context, we show from several different angles - failed attempts to lower local exposure, high-resolution drilling, monitoring, and groundwater dating - that this is actually not the case for groundwater arsenic, because distant municipal pumping can trigger remotely the release of arsenic below such a clay layer."
The study was set in motion after two manually pumped community wells drilled to intermediate depths in the vicinity of Dhaka, Bangladesh, suddenly failed. The outcome was water with elevated concentrations of arsenic, according to Columbia University.
The researchers found that the recent changes in arsenic near Dhaka were the result of pumping from deeper aquifers. Water levels under Dhaka are 100 meters below what they would naturally be, since the aquifer is not filling quickly enough, according to the study.
"In Dhaka, the pumping probably accelerated the release of arsenic and allowed us to document the changes within a decade," van Geen added. "We would not have figured this out without having been there monitoring wells for at least 10 years. Monitoring is not very exciting, but because of the monitoring we discovered something fascinating."
According to the research team, this discovery is especially worrisome for households on the outskirts of Dhaka that have been privately reinstalling wells, ultimately accessing the shallow aquifers beneath the impermeable clay layer, reported Columbia University.
The long-term diffusion of dissolved organic carbon from clay layers may also explain why private wells screened just below a clay layer in other sedimentary aquifers are more likely to be contaminated with arsenic than deeper wells would be, according to the findings.
While the geochemical conditions surrounding every aquifer are different, the problem of arsenic and other contaminants leaking into deep aquifer groundwater is not unique to Dhaka. "It's a warning and it means that in some areas you need to probably test wells more frequently than others," said van Geen.