Environmental geochemical mapping and assessment of anthropogenic chemical changes in the Irkutsk-Shelekhov region, southern Siberia, Russia

A geochemical survey of the Irkutsk-Shelekhov agricultural-industrial area was carried out using drainage sediments and waters in an attempt to evaluate the effectiveness of this approach in assessing man-made pollution. The area sampled is 3500 km(2) and the average sample density is one site per 16 km(2). Sample media included: (1) stream sediments (<0.25 mm, + 0.25 - 1 mm, and + 1-3 mm fractions); (2) overbank material ( < 0.25 mm and +0.25 -1 mm fractions); (3) surface water; and (4) plant material (birch, willow, moss). The results indicate that small- to medium-scale geochemical mapping of the drainage system, based on inexpensive semi-quantitative analyses by optical emission spectroscopy, may be used to assess pollution and identify areas for more detailed studies. The fine fraction of the overbank sediments and stream water were found to be the most appropriate media for routine mapping of this type. Anthropogenic accumulations of heavy metals (Hg, Cd, Ph, Ag, Au) up to 2 to 3 times background were observed in the upper part (0-10 cm to 20-30 cm) of the overbank samples. Similarly high concentrations of these elements persist in some places to depths greater than 70 cm suggesting that overbank sediments may be contaminated by groundwater. Anthropogenic effects in natural environments may be recognized using: (1) specific chemical element associations uncommon in natural environments; (2) a comparative study of conjugate media, their facies and fractions; (3) a direct analysis of the correlation between the geochemistry of the environment and land use; and (4) a study of the mode of occurrence of elements to distinguish natural from anthropogenic ones.