Redox-cycling of arsenic along the water pathways in sulfidic metasediment areas in northern Sweden

Arsenic has emerged as a problem element in groundwater. The most common mechanism of mobilising As from the solid phase into water is through the reductive dissolution of ferric oxyhydroxides. This investigation was made in northern Sweden where metasediments containing pyrite, pyrrhotite and arsenopyrite underlay about 4000 km2. The overlying till contains as much as up to 100 mg kg−1 As. Speciation of the dissolved As shows that arsenite dominates largely over arsenate. The Fe oxyhydroxides formed may contain up to 0.5% As. Sandy sediments may contain 100–500 mg kg−1. Arsenic and Fe are closely correlated. The cycling of As in water, flora and fauna in wetlands has been studied. Ferric reduction occurs in wetlands and groundwater rich in Fe, and As is found to be discharging into ditches, brooks and streams. Wetland trees and plants show a moderately elevated content of As with a few species showing above 2 mg kg−1, the permissible level in fodder for domestic animals. The only plants having a high content of As are Equisetum species showing up to 26 mg kg−1. These plants make up a small fraction of the food of wild, grazing and browsing animals, for example moose and reindeer, and does not seem to constitute an environmental risk. However, the animals could be exposed to considerable amounts of As by drinking water from springs in wetlands. In the fauna, an elevated As content has so far been found in a limited number of benthic macroinvertebrate samples (1.23–42.1 mg kg−1 dry weight), in which inorganic As species (arsenate) predominate in the extractable fraction (62–82%) with lower amounts of arsenite, mono- and dimethylarsenic acid. Some samples also contained arsenobetaine, trimethylarsine oxide and tetramethylarsonium ion. To the authors’ knowledge, this is the first report on As speciation in benthic macroinvertebrates. Fish species from polluted streams (pike and brown trout) had normal As levels (0.57–1.84 mg kg−1 dry weight), mainly present in a form of arsenobetaine (brown trout) or arsenobetaine and dimethylarsinic acid (pike). As both fish species also contained minor amounts of arsenite and arsenate, it is estimated that there is no serious health risk to potential consumers.