Flow and sorption controls of groundwater arsenic in individual boreholes from bedrock aquifers in central Maine, USA

Science of the Total Environment
Columbia University - Lamont-Doherty Earth Observatory; Maine Geological Survey
By: , and 

Links

Abstract

To understand the hydrogeochemical processes regulating well water arsenic (As) evolution in fractured bedrock aquifers, three domestic wells with [As] up to 478 μg/L are investigated in central Maine. Geophysical logging reveals that fractures near the borehole bottom contribute 70-100% of flow. Borehole and fracture water samples from various depths show significant proportions of As (up to 69%) and Fe (93-99%) in particulates (>0.45 μm). These particulates and those settled after a 16-day batch experiment contain 560-13,000 g/kg of As and 14-35% weight/weight of Fe. As/Fe ratios (2.5-20 mmol/mol) and As partitioning ratios (adsorbed/dissolved [As], 20,000-100,000 L/kg) suggest that As is sorbed onto amorphous hydrous ferric oxides. Newly drilled cores also show enrichment of As (up to 1300 mg/kg) sorbed onto secondary iron minerals on the fracture surfaces. Pumping at high flow rates induces large decreases in particulate As and Fe, a moderate increase in dissolved [As] and As(III)/As ratio, while little change in major ion chemistry. The δD and δ18O are similar for the borehole and fracture waters, suggesting a same source of recharge from atmospheric precipitation. Results support a conceptual model invoking flow and sorption controls on groundwater [As] in fractured bedrock aquifers whereby oxygen infiltration promotes the oxidation of As-bearing sulfides at shallower depths in the oxic portion of the flow path releasing As and Fe; followed by Fe oxidation to form Fe oxyhydroxide particulates, which are transported in fractures and sorb As along the flow path until intercepted by boreholes. In the anoxic portions of the flow path, reductive dissolution of As-sorbed iron particulates could re-mobilize As. For exposure assessment, we recommend sampling of groundwater without filtration to obtain total As concentration in groundwater.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Flow and sorption controls of groundwater arsenic in individual boreholes from bedrock aquifers in central Maine, USA
Series title Science of the Total Environment
DOI 10.1016/j.scitotenv.2014.04.089
Volume 505
Year Published 2014
Language English
Publisher Elsevier Pub. Co.
Publisher location Amsterdam
Contributing office(s) Maine Water Science Center
Description 17 p.
First page 1291
Last page 1307
Country United States
State Maine
Online Only (Y/N) N
Additional Online Files (Y/N) N