In recent years, elevated arsenic concentrations have been found in
waters and soils of many, countries, often resulting in a health threat
for the local population. Switzerland is not an exception and this paper
deals with the release and subsequent fate of arsenic in a 200-km(2)
mountainous watershed, characterized by crystalline silicate rocks
(gneisses, schists, amphibolites) that contain abundant As-bearing
sulfide ore deposits, some of which have been mined for iron and gold in
the past. Using analytical methods common for mineralogical, ground
water and soil studies (XRD, XRF, XAS-XANES and -EXAFS, electron
microprobe, extraction, ICP, AAS with hydride generator, ion
chromatography), seven different field situations and related dispersion
processes of natural arsenic have been studied: (1) release by rock
weathering, (2) transport and deposition by water and ice; (3) release
of As to the ground and surface water due to increasing pH; (4)
accumulation in humic soil horizons; (5) remobilization by reduction in
water-saturated soils and stagnant ground waters; (6) remobilization by
using P-rich fertilizers or dung and (7) oxidation, precipitation and
dilution in surface waters. Comparison of the results with experimental
adsorption studies and speciation diagrams from the literature allows us
to reconstruct and identify the typical behavior of arsenic in a natural
environment under temperate climatic conditions. The main parameters
identified are: (a) once liberated from the primary minerals, sorption
processes on Fe-oxy-hydroxides dominate over Al-phases, such as
Al-hydroxides or clay minerals and limit the As concentrations in the
spring and well waters between 20 and 300 mug/l. (b) Precipitation as
secondary minerals is limited to the weathering domain, where the As
concentrations are still high and not yet too diluted by rain and soils
waters. (c) Although neutral and alkaline pH conditions clearly increase
the mobility of As, the main factor to mobilize As is a low redox
potential (Eh close or below 0 mV), which favors the dissolution of the
Fe-oxy-hydroxides on which the As is sorbed. (d) X-ray absorption
spectroscopy (XAS) of As in water-logged humic forest soils indicates
that the reduction to As III only occurs at the solid-water interface
and that the solid contains As as As V (e) A and Bh horizons of humic
cambisols can effectively capture As when As-rich waters flow through
them. Complex spatial and temporal variation of the various parameters
in a watershed results in repeated mobilization and immobilization of
As, which continuously transports As from the upper to the lower part of
a watershed and ultimately to the ocean. (C) 2004 Elsevier B.V. All
rights reserved.