J.K. Novak, L. Minarik, L. H. Peza , K. Melka, M. Burian

Institute of Geology, Academy of Sciences CR, Rozvojova 135, 16502 Praha 6 <inst@gli.cas.cz>

Submitted Nov 25 / Accepted Dec 13, 2000

OBSERVATIONAL CONSTRAINTS TO RISK OF METAL TOXINS FROM MEDITERRANEAN KARST BAUXITES IN MINING AREAS

Pyrite-bearing karst bauxites originated in anoxic conditions. Bauxites composed of boehmite, gibbsite and kaolinite as well as associated vitriol clays are sensitive to present-day weathering due to oxidizable sulfur compounds (pyrite, and origin of hydrated Fe-Al sulfates). A massive accumulation of these karst bauxites of Mediterranean type is opened at the lowermost level of the Dajti Bauxite Mine (Central Albania). Both spoil tips on the surface and mine water in the subsurface periodically cause increased acidity of water, especially at strong rainfall runoff. Influx of acid water due to percolation through the spoil tips usually influences the quality of the karst aquifers and represents a specific natural hazard. Even though the acid-neutralizing capacity of the limestones in the basement and surroundings is the highest of all possible rock complexes, the soluble hydroxy-alumina filtrates with released metal toxins (Cd, Be, Ni, Mn, Co) may disturb an interim quasi-equilibrium in karst aquifers, inducing damage to aquatic fauna and flora as well as drinking water quality, mainly during the pluvial periods of the year. The terrigenous components (boehmite, gibbsite, kaolinite) and disseminated pyrite and/or hydrated Fe-Al sulfates (halotrichite, melanterite) are original carriers of the trace metals including the light REEs. They can survive in natural state of metastable physical-chemical equilibrium for long times, but the exposure to weathering by mining operations and breakdown of pyrite directly cause the acidification of water and consequent mobilization of ecotoxic metals. By means of the laboratory acid leaching using suprapure 0.01 M HNO3 solution at ambient temperature and atmospheric pressure, a high efficiency of the desorption for metal ions such as Cd, Zn, and Be and far less for transition metal ions (Ni » Co ~ Fe ~ Mn) was documented in the analyzed suspensions. The water-soluble amount of Cr, Cu, As and Pb is negligible (partly due to primary depletion in source rocks). Solubility experiments on powdered rock samples provided a useful information on relative mobility of the tested bioavailable trace elements and should be used more frequently, especially for comparison with the observed change in geochemistry of the pollution agents.

The study originated within the framework of the IGCP Project 405 "Anthropogenic impacts on weathering processes".

Discussion:

by Harald Strauss, Muenster:

1. The observation that the neutralizing capacity of a limestone might not be sufficient to compensate for intense acidification is certainly a valuable observation for any region suffering from acid mine drainage (and the situation which is described resembles this scenario). That this limestone has the highest capacity in the vicinity is - as one would guess - just a relative statement. Capacity might still be limited.
2. The combination of field-based observations and laboratory experiments is certainly an excellent approach to this problem. As people are becoming more and more aware of environmental hazards, they will become equally aware of the necessity to perform basic science experiments in order to understand the natural systems. Unfortunately, this awareness is greatly reduced when it comes to the national funding agencies.
3. There is a statement that the pyrite and the hydrated Fe-Al sulfate are practically the original carriers of the trace metals and the REE. I have been involved in research related to brown coal mining operations and their environmental influence. From some meeting I recall a talk, in which it was shown that a good portion of the trace elements was not bound to the pyrite but to other minerals and/or organic matter. Now obviously, the latter part might not be relevant in the situation described (karstic bauxite deposits). But the authors should investigate in all directions, even though the common feeling would attribute the whole load of environmentally hazardous metals to the pyrite.

by Josef Zeman, Brno:

1. This insufficient neutralizing capacity of limestone for AMD (acid mine drainage) would parallel this situation with our experiments in Zlate Hory Ore Mines in 1980s. The AMD by pH 2 (high concentration of Fe, Cu and Zn; n * 0.1 L/s; site Mir Adit) was channelized through large vessels, which contained chippings of limestone. The acidity remained high, although reactive surface was large and movement of filtered AMD was slow (even by precipitation of substantial amount of metals). Conversely, the experiments with strong AMC flows (n * 1 L/s) through a series of artificial ponds were much more successful than any previous filtering in carbonate debris. During 1 day, the pH values increased to 8 and the above mentioned metals almost completely precipitated.
2. Analyzing these experiments from 1980s, we can assume that sufficient contact of water with atmosphere (mainly atmospheric carbon dioxide) would control this effective precipitation of metals by rapid decrease in acidity of water. But these are only ideas for setting of other experiments …

Location of investigated belt of bauxite deposits in Albania

EDX maps of bauxite

Mean concentrations of metals (points) and EPA Contaminant Level (blue line)