T54 Arsenic: Fate and Transport in Natural Waters and Aquifers from Basin to Pore-Space Scale II

Sunday, 4 November 2012
1:30 PM - 5:30 PM
213D

ARSENIC CONTAMINATION POTENTIAL FOR A PLEISTOCENE AQUIFER IN CAMBODIA

Paper number: T54-8
Program: Technical Sessions
Time: 3:15 PM – 3:30 PM

Authors

Abstract

Arsenic contamination of groundwater from Holocene aquifers has led to serious health concerns throughout South and Southeast Asia, where as many as 100 million people routinely consume well water with concentrations exceeding the World Health Organization recommended guideline of 10 ug/L. To date, there are no widely implemented mitigation strategies for avoiding high As concentrations. In some locations, costly new wells are being installed that access deeper, As‑free water found in older Pleistocene sediments, but this effort is currently proceeding with minimal understanding of aquifer geochemistry. Without a comprehensive evaluation of aquifer materials, wells are at risk to unknown water quality problems. In fact, recent studies indicate such aquifers may be susceptible to future As contamination over the timescales of human groundwater extraction. In some cases these aquifers are also contaminated with high salinity or Mn concentrations.

In an effort to better characterize the potential for future As contamination, we have investigated groundwater and sediment chemistry from a Pleistocene aquifer in Cambodia. The aquifer at our site outcrops near the surface, providing 1) access to a high density of domestic wells from which to examine water quality, 2) the ability to use local drilling techniques to install multi-depth monitoring wells and obtain redox-preserved intact sediment cores, and 3) a location with similar recharge sources as contaminated Holocene aquifers nearby. There is great spatial variability in groundwater chemistry, as well as the physical and chemical properties of aquifer sediments. Overall, dissolved oxygen and Fe concentrations are low in the majority of domestic wells. Redox measurements and Mn concentrations suggest the aquifer is poised in a state of manganese reduction. Well water is devoid of As, but based on mild acid and phosphate extractions, As may be liberated from sediments in significant quantities. Accordingly, the As-free wells at our Pleistocene field site remain vulnerable to future As contamination following land-use changes that influence recharge chemistry and groundwater flow. Work is needed to quantify the timescales and degree of such impacts in order to guide decisions about future groundwater development.