Three compost cover layer models were examined that might be beneficial in the
suppression of tailings oxidation and acidic drainage. Laboratory studies were done to
assess the potential compost cover layer benefits, and to facilitate the design of field
tests. Two grades of compost were compared: immature (4 week old) and mature (14
week old) municipal solid waste (MSW) compost. Analyses of the leachate quality from
compost cover layers and tailings were done since MSW compost may release trace
metals, organic chemicals and pathogens into the watershed outside the tailings area.
A laboratory simulation experiment to compare the three cover layer models and two
grades of compost was conducted in plexiglass columns over 290 days.
The compost and the sand cover layer models appeared to be substantially better than
the ploughed model as cover layer designs for suppressing tailings acidic drainage.
Oxygen concentrations were greatly depressed below atmospheric concentration in the
compost and tailings layers in all three cover models. Substantial compaction pressure
on the compost cover layer appeared to be of little benefit. Medium to strong reducing
conditions developed, especially under an immature compost cover layer, and this
enhanced the dissolution of oxides and increased the mobilization of iron, sulphate,
and several trace metals. The formation of black iron sulphide precipitate and methane
at the compost-tailings interface showed that acid mine drainage (AMD) processes and
acidity were being reversed under the immature compost cover. However, reductive
dissolution of tailings oxides was still proceeding in the tailings layer at the end of the
nine month simulation. This may be a transient condition until iron and sulphur become
immobilized as less soluble precipitates such as sulphides.
Compost quality tests showed that leachates from mature and fresh MSW compost
present a low environmental risk for use on mine lands. Furthermore, very low
hydraulic conductivity of the fresh compost cover layer will inhibit vertical water flow
through the compost layer, and thus reduce the leaching of acids, metals and nitrogen
compounds from the compost-tailings system.
From this preliminary study it is concluded that the compost model, using immature
MSW compost, appears to be the simplest and most cost-effective tailings cover layer
solution. Long term field tests or large lysimeter pilot experiments are needed to
determine whether the beneficial characteristics of the compost cover layer can be
maintained. Long term compaction and decomposition of fresh compost must be
examined to see if its favourable hydraulic characteristics and anaerobic conditions will
be maintained under natural conditions. In particular, the relative importance of the
physical oxygen barrier and biological oxygen consumption benefits of different grades
of MSW compost needs to be determined.
