Solid organic matter in Uk aquifers : its role in sorption of organic contaminants
A major control on the attenuation of organic contaminants dissolved in groundwater is
their sorption onto solid organic matter (OM) in the rock. Sorption modelling is currently
based on the amount of solid organic carbon in the rock. However, there are very few
available data on the total organic carbon (TOe) content in common UK geological
materials. A significant new set of Toe measurements on a range of important formations
is presented. Methods to measure TOe in geological, especially carbonate-rich, samples
are reviewed and evaluated.
The impact of the type of OM on its sorption capacity is not understood. Non-linear
isotherms measured on seven samples (of Lincolnshire Limestone, Glacial Till from
Norfolk and unconsolidated deposits) had a wide range of organic carbon normalised
distribution coefficients (Koc from 7.85 llg to 767 lIg at 0.01 gil trichloroethene). This
indicates the impact of characteristics of the OM.
Bulk geochemistry and micro-morphology characteristics of the OM that could be
important predictors for sorption capacity are investigated and identified. Organic matter
has been isolated, and the isolation technique evaluated. The major element composition
of the isolated OM has been determined; elemental results were found to correlate with
results of a pyrolysis analysis applied to whole rock samples. Morphological analysis of
microscope slides of isolated organic matter provided information on the physical types
(visually) and size (by image analysis) of the organic particles. Measured Koc values
correlated with the elemental HlO content of the sample's OM and with the pyrolysis
measurements, but not with morphological results.
The effect of selected empirical isotherms on solute transport was simulated: use of an
accurate distribution coefficient is essential, but non-linearity was found not to make a
major impact on arrival results. Recommendations are made for the improvement of
sorption modelling, and for appropriate analysis methods to supply relevant data on the
type of OM in this context.