Page 118 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 95

Sources of spontaneous potential

All moist Earth materials contain redox-active species, such as O2(aq), Fe 2+, HS, OH-
and H +, which impart a bulk redox capacity, or Eh if measured against the H+-H2 half-
cell, to the soil or groundwater. The variable chemical composition of overburden,
groundwater and rock therefore results in variable redox capacity which, in turn, results
in spontaneous potential voltages and currents between different points in the Earth. The
multitude of potential processes that affect the composition of Earth materials and might
thereby affect redox processes can be divided into primary lithological processes and
surficial processes.
Primary lithological processes are defined as all those that contribute to the variable
composition of rock. They result in specific lithologies and mineral accumulations in
various places in the upper crust. The mineral assemblages in these materials can impart
a redox potential to the groundwater/rock matrix with which they are in contact and can
result in a characteristic redox signature for various rock types and mineral
accumulations. For example, the presence of large amounts of gypsum in carbonate rock
can fix the equilibrium Eh of the groundwater/rock environment to no lower than
approximately -275 mV, which is the Eh of the SO42"-HS - half-cell (at pH 8 and using
certain other assumptions; Garrels and Christ, 1965, p. 215). In this environment, species
that can impart a lower Eh are unlikely to be present because most of the reducing agents
capable of reducing 8042 to HS-would already have been consumed. However, the Eh
can be greater than -275 mV because SO42- is the only geologically-important sulphur
species in oxidised environments (Krauskopf, 1979), where its redox behaviour is
relatively inert (Bartlett and James, 1995), and therefore it adds little to the redox-
buffering of oxidised systems.
Processes such as these can occur due to mineral assemblages or dissolved species in
many other rock types. In unweathered pyritic rocks, Eh is typically maintained below
the sulphide oxidation half-cell. Ferrous olivines and clinopyroxenes in ultramafic rocks
undergoing weathering can produce very reducing conditions that approach the limits of
water stability (around -400 mV at pH 11; Barnes et al., 1978). The presence of
dissolved oxygen in oxygenated terrestrial waters typically maintains Eh at a fairly high
empirically-observed level of over 200 mV. The empirical limit is used because there is
no perfect correlation between dissolved oxygen concentration and Eh, probably due to
the myriad biological and inorganic processes that involve oxygen. There is, however, a
general relationship between the presence of dissolved oxygen and high Eh to the extent
that most oxygenated terrestrial waters are found to have an Eh of between 200 and 400
mV. The upper limit of geologically observed Eh is around 800 mV and the theoretical
electrical potential of oxygen is higher still at 1000 mV at pH 4.0 (Fig. 3-4).
The presence of water itself restricts the Eh of aqueous systems to a well-defined
stability field. Figure 3-4 shows the theoretical and empirical stability fields for water in
natural environments. As shown in Table 3-I, reducing agents that are more reducing
than H2(g) rarely exist in the shallow terrestrial environment because their oxidation by