Retardation of arsenic transport through a Pleistocene aquifer

Abstract

Holocene aquifers are the source of much arsenic poisoning in south and southeast Asia, whereas Pleistocene aquifers are mostly safe; here the delayed arsenic contamination of a Pleistocene aquifer is described and modelled. Millions of people across southeast Asia are exposed to arsenic-contaminated drinking water drawn from Holocene aquifers, layers of sand deposited less than 5,000 years ago. By contrast, Pleistocene aquifers, deposited about 12,000 years ago, have lower levels of contamination and are increasingly being exploited as safe sources of drinking water. This study reports the gradual penetration of arsenic into a low-arsenic Pleistocene aquifer south of Hanoi, Vietnam. Changes in groundwater flow and the redox state of the aquifer sands induced by pumping are introducing contamination from the high-arsenic Holocene aquifer. Contamination so far is limited owing to the absorption of arsenic onto aquifer sands, which delays arsenic movement by decades. Groundwater drawn daily from shallow alluvial sands by millions of wells over large areas of south and southeast Asia exposes an estimated population of over a hundred million people to toxic levels of arsenic1. Holocene aquifers are the source of widespread arsenic poisoning across the region2,3. In contrast, Pleistocene sands deposited in this region more than 12,000 years ago mostly do not host groundwater with high levels of arsenic. Pleistocene aquifers are increasingly used as a safe source of drinking water4 and it is therefore important to understand under what conditions low levels of arsenic can be maintained. Here we reconstruct the initial phase of contamination of a Pleistocene aquifer near Hanoi, Vietnam. We demonstrate that changes in groundwater flow conditions and the redox state of the aquifer sands induced by groundwater pumping caused the lateral intrusion of arsenic contamination more than 120 metres from a Holocene aquifer into a previously uncontaminated Pleistocene aquifer. We also find that arsenic adsorbs onto the aquifer sands and that there is a 16–20-fold retardation in the extent of the contamination relative to the reconstructed lateral movement of groundwater over the same period. Our findings suggest that arsenic contamination of Pleistocene aquifers in south and southeast Asia as a consequence of increasing levels of groundwater pumping may have been delayed by the retardation of arsenic transport.