Arsenic fractionation and mineralogical characterization of sediments in the Cold Lake area of Alberta, Canada.

Elevated arsenic (As >0.01 mg L(-1)) in some domestic well water in the Cold Lake area of Alberta, Canada is of great concern for public health. To determine possible sources of groundwater As, sediments (n=135) collected from five different locations (up to ~300 m depth) in the area were characterized. Total As concentrations in the sediments varied from ~1 to 35 mg kg(-1). Sediments derived from shale contained high As (~13 mg kg(-1); n=14), particularly the shale of Lea Park formation where maximum average As was ~32 mg kg(-1) (n=2). Unoxidized sediments of Grand Centre (24.9 ± 4.2 mg kg(-1) As) and Bonnyville (19.9 ± 1.8 mg kg(-1) As) formations also contained high As. Sequential extraction procedure (SEP) revealed the dominance of exchangeable and specifically adsorbed As (6 to 46% of total As) in the sediments of varying As concentrations (0.8 to 35.4 mg kg(-1) As). The high As sediments (>7 mg kg(-1) As) also contained significant amount of sulfide bound As (11 to 34% of total As), while low As (<7 mg kg(-1) As) sediments had crystalline oxide minerals bound As (25 to 75% of total As) as major phases. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses confirmed the presence of pyrite, and μ-XRD analysis signaled the presence of arsenopyrite in sediments containing ~20 mg kg(-1) As. X-ray absorption near edge structure (XANES) spectroscopy analysis suggested dominance of arsenite (AsIII; ~60 to 90% of total As) in all the sediments. These findings may help to devise strategies to investigate mechanisms of As release into the groundwater.