Arsenic contamination of Bangladesh aquifers exacerbated by clay layers

Ivan Mihajlov,M Rajib H Mozumder,Peter Schlosser,Alexander Van Geen,Peter S K Knappett,Brian J Mailloux,Kazi Matin Ahmed,Martin Stute,Imtiaz Choudhury,Benjamín C Bostick

doi:10.1038/s41467-020-16104-z

Abstract

Confining clay layers typically protect groundwater aquifers against downward intrusion of contaminants. In the context of groundwater arsenic in Bangladesh, we challenge this notion here by showing that organic carbon drawn from a clay layer into a low-arsenic pre-Holocene (>12 kyr-old) aquifer promotes the reductive dissolution of iron oxides and the release of arsenic. The finding explains a steady rise in arsenic concentrations in a pre-Holocene aquifer below such a clay layer and the repeated failure of a structurally sound community well. Tritium measurements indicate that groundwater from the affected depth interval (40–50 m) was recharged >60 years ago. Deeper (55–65 m) groundwater in the same pre-Holocene aquifer was recharged only 10–50 years ago but is still low in arsenic. Proximity to a confining clay layer that expels organic carbon as an indirect response to groundwater pumping, rather than directly accelerated recharge, caused arsenic contamination of this pre-Holocene aquifer.

Highlights

Confining clay layers typically protect groundwater aquifers against downward intrusion of contaminants
The second installation of a community well screened within a few meters of the initial well confirmed that the orange sands are capped by a 10-m thick layer of clay at the site but this well failed after producing low-As water for several months
In the case of advection, using a vertical difference of 1 m in hydraulic head across the 10-m thick clay layer (Fig. 3a) and a plausible range of vertical hydraulic conductivities for the clay of 10−9–10−7 m/sec[39], the Darcy velocity of clay pore water into the pre-Holocene aquifer is on the order of 0.3–30 cm per year, i.e., 3–300 L/m2 per year (Supplementary Note 5)

Summary

Introduction

Confining clay layers typically protect groundwater aquifers against downward intrusion of contaminants. In the context of groundwater arsenic in Bangladesh, we challenge this notion here by showing that organic carbon drawn from a clay layer into a low-arsenic pre-Holocene (>12 kyr-old) aquifer promotes the reductive dissolution of iron oxides and the release of arsenic. Proximity to a confining clay layer that expels organic carbon as an indirect response to groundwater pumping, rather than directly accelerated recharge, caused arsenic contamination of this pre-Holocene aquifer. A similar process can occur in pre-Holocene sands where it is made apparent by the conversion of orange Fe(III) to gray Fe(II) oxides in response to a supply of organic carbon[24,25,26,27] The sources of this organic carbon could be immobile plant matter co-deposited with aquifer materials or mobile reactive dissolved organic carbon (DOC) advected by groundwater flow. The new results offer the most direct evidence yet of a new mechanism for groundwater contamination with As triggered by pumping, which was inferred from observations elsewhere in Bangladesh, the Mekong delta of Vietnam, and the Central Valley of California[15,16,17]

Methods

Results

Conclusion