Page 106 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 83

Groundwater transport may indeed disperse large quantities of elements but in
stratified geological materials it tends to do so laterally. This is particularly true with
respect to stratified glacial overburden, including glaciolacustrine varved clays, for
which horizontal hydraulic conductivities are typically orders of magnitude higher than
those that occur in the vertical direction (Freeze and Cherry, 1979). This results in
horizontal groundwater flow being favoured over vertical flow by a similar factor.
Selective leach anomalies that have been attributed to bedrock features are most
commonly reported as either apical (single peak) or rabbit-ear (twin peak) anomalies
lying almost directly over the feature. If groundwater transport was a significant
contributor to the mobility of the elements involved, one would expect anomalies
occurring down a long dispersal plume in the down-gradient direction of groundwater
flow. This has not been reported in surface soils. Furthermore, selective leach anomalies
are often noted in surface soils far above the water table. Since groundwater obviously
could not play a significant role in the formation of many of these anomalies, it is
reasonable to conclude that it plays a relatively minor role in the formation of selective
leach anomalies as a whole.

Gaseous transport

The measurement of soil gases has been used to identify the presence of
mineralisation by a number of workers (e.g., Lovell et al., 1983; McCarthy et al., 1986).
Gaseous transport of metals and/or other species has been suggested as a possible
mechanism in the development of geochemical soil anomalies (Klusman, 1993; Clark,
1997; Smee, 1998). Case studies in support of gaseous transport are largely cited from
arid or semi-arid areas where the water table exists 10s to 100s of metres below ground
surface. Large scale and rapid gas transport in these environments is plausible because of
the very thick vadose (unsaturated) zone. Gases in the phreatic zone (i.e., below the
water table) in the zone of meteoric groundwater (envisioned here as the zone where
lithostatic load adds nothing to the fluid pressure) typically exist in dissolved form and
therefore their transport occurs largely by diffusion or groundwater advection.
Some authors have suggested a gaseous transport mechanism below the water table
in which gaseous carriers move metals in a separate gas phase (Clark, 1997). This is
extremely unlikely. In the vadose zone, gases can exist in the gaseous phase at partial
pressures that are considerably below atmospheric pressure. This is not the case below
the water table where the sum of partial pressures of all dissolved gases must exceed the
fluid pressure for a separate gas phase to exist. By definition the "water table" in a
saturated geological medium is the point at which the fluid pressure is exactly equal to
atmospheric pressure (Freeze and Cherry, 1979). Below the water table, the fluid
pressure exceeds the vapour pressure and, above it, the vapour pressure exceeds the fluid
pressure. In the zone of meteoric groundwater, gases typically exist only as a vapour
phase below the water table in the following circumstances.