Page 41 - Geochemical Remote Sensing of The Sub-Surface

18 O.F. Putikov and B. Wen
occur as ions, complex ions and compounds. Their absolute concentrations are of the
order of 1-20 • 10 .2 ~tg/l and their relative concentrations as compared with their total
contents in the rocks usually ranges from 0 to 0.2%, and is rarely greater than 2%
(Antropova, 1975). In capillary moisture the absolute concentrations of Pb and Cu are
about 10-2-102 ~tg/1 and the relative concentrations of these metals are about 0.1-1%
(Antropova, 1975). Absolute concentrations of sorbed metals are 10-1-102 ~tg/l for Pb
and 6 • 10 .3 to 4.7 • 102 ~tg/1 for Mo, and their relative concentrations are about 1% and
for Pb and 0.01-0.1% for Mo. Antropova (1975) showed that after extraction of heavy
metals soluble in groundwater, capillary moisture and sorbed forms, a significant
fraction of metals is still held by chemical bonds of different strengths in oxides and
hydroxides of Fe and Mn. The relative content of this form of metals is usually 1-10%
for Cu and Ni, up to 90% (frequently 16.5-86%) for Pb and up to 93% for Mo. A metal
may also form compounds with natural organic acid, making metallo-organic
compounds (MOC). Antropova (1975) found that fulvic and humic acids are special
concentrators of heavy metals and the metals they concentrate exist not as cations but as
constituents of humate and fulvate complexes. In these complexes the concentrations of
metals are usually higher than the a~erage metal concentrations in the total organic
fraction. Several researchers (Krat, 1983; Malmqvist and Kristiansson, 1984;
Kristiansson and Malmqvist, 1986; Krchmar, 1988; Dukhanin, 1990; Putikov and
Dukhanin; 1994) have pointed out that, in the upper crust and in the near-surface
atmosphere, heavy metals exist in the gas phase. But the concentration of elements in
this phase is very low, of the order of 10 .4 pg/1 (Dukhanin, 1990; Ozerova and
Mashianov, 1989).
In physico-chemical terms, metals in these different forms are held by chemical
bonds of different strengths. According to the strength of the chemical bonds, the forms
of metals may be divided into four groups:

9 strongly confined (strong chemical bonds within minerals);
9 moderately confined (MOCs and Fe-Mn hydroxides);
9 weakly confined (metals in capillary moisture, sorbed on surfaces);
9 mobile (soluble in groundwater, quasi-gaseous and gaseous).

The capacity of metals to disperse in space is clearly related to this classification:
weakly-confined metals are able to disperse for greater distances than strongly-confined
metals, and so on. In particular, mobile forms are able to migrate far from their
sourcesprovided that some natural or artificial physical field is exerted to effect
migration. However, all forms are in a state of dynamic equilibrium and can transform
into other forms. One consequence is that, with increasing concentration of metals in the
more mobile forms, there is a simultaneous increase in concentration of metals in the
forms with stronger chemical bonds. For example, dissolved Pb, Mo and Cu will interact
with oxides and hydroxides of Fe and Mn under certain physico-chemical conditions of
pH and Eh. First, due to adsorption of these metals from solution, the metals are