The geochemistry of pyritic shale weathering within an active landslide.
Intense chemical weathering accompanies physical instability at
the site of a repeatedly active landslide in North Derbyshire. In order
to describe and quantify the chemical weathering, a programme of water
sampling and analysis was devised. A sequence of reactions are proposed,
based on theresults of this work, to account for the observed concentrations
of chemical species in the drainage waters.
It is thought that pyrite oxidation, accelerated by the presence
of catalytic bacteria, is responsible for the considerable acidity of these
waters. Additional reactions involving carbonates and silicates occur
at strictly comparable rates and consume over 99% of the acidity prior
to the water's emergence in a number of ochre-precipitating springs.
Analysis of the solid reactants and products confirms the suggested
sequence of events and suggests a number of ways in which chemical weathering
might be related to slope stability. Clay minerals appear to be little
affected by weathering, and the growth of precipitate minerals such as
gypsum in joints and on bedding planes might be a more important mechanism
in shale breakdown. After this initial rapid physical disintegration,
chemical weathering, at the surface, proceeds relatively slowly.
Chemical processes build up stresses within the rocks and possibly
help to maintain any inherent planes of weakness. Ultimately landslide
movement is triggered by increases in porewater pressures brought about
by fluctuations in local ground water levels.