| Summary: | This study investigates coal discard cover soil fertility and its potential for
degradation, particularly in terms of its salinisation and acidification potential. Seven
rehabilitated coal discard dumps in the Witbank, Ermelo and Newcastle regions were
used as study areas. All areas were rehabilitated with a cover soil layer, revegetated
and annually fertilised with nitrate fertilisers, super phosphate, kraal manure and
lime. Performance guideline for pH of 5.5-(6.5 i0.5)-7.5 and electrical conductivity
guideline of preferably less than 200 mS.rn-' but not higher than 400 mS.m-' were set
based on literature information. Soil chemical data from a three-year fertilisation
programme were used to assess the fertility of the cover soil surface (0-150mm).
Data collected over a three year period as well as additional electrical conductivity
and pH measurements from the cover soil surface, subsoil, cover soil/coal contact
zone and underlying coal itself were used to assess the occurrence of salinisation
and acidification of the cover soil. The soil fertility varied significantly among dumps
as well as over the three years. Results indicated an increase in ammonium acetate
extractable macro elements (calcium, magnesium and potassium). With the
exception of manganese, no micro-element toxicities were recorded. Iron
concentrations were slightly elevated in some of the sandy cover soil layers. No
increase in soluble nitrogen (nitrate and ammonium) was found and most soluble
nitrogen was in the form of nitrates. In general the Bray extractable phosphate
increased during the study period. It can be predicted that with the following fertiliser
programme increases of exchangeable macro-elements as well as available
phosphorus can be expected. The study could not indicate an increase in adsorbed
or available nitrogen. Organic carbon was initially not analysed therefore no
comments can be made whether organic matter increased. Four of the seven dumps
surveyed had comparably similar organic carbon levels to the background samples.
Overall the fertiliser programme increased the electrical conductivity and decreased
the acidity of the cover soil surface. Acidity and salinity was in general not a problem
at the surface of the cover soil and pH was even slightly higher in cover soil samples.
The acidity and especially salinity increased at the subsoil and so did the sulphate
concentrations. Calcium and magnesium sulphate were predominantly responsible
for higher electrical conductivity measurements. The percentage exchangeable
sodium was also predominantly less than 2% indicating that sodicity is not currently a
problem in cover soil. Soil fertility was satisfactory for vegetation growth and macroelement
concentrations were in the correct ratio although calcium was slightly high.
An elevated sulphate concentration, in comparison to the natural grassland soils, as
well as a high salinity and high acidity in the subsoil layers indicate that salinisation
and acidification could deteriorate without proper management. A slightly acidic
cover soil can also be attributed partially to its natural acidic pH due to the wellweathered
and leach property of burrow pit. Higher than recommended salinity
levels were found in subsoil samples but the occurrence of acidification of the subsoil
was more dump specific. In relation to acidity and salinity guidelines only the cover
soil of one dump was concerning and the larger dumps subsoil acidity and salinity
were elevated.
The following management strategies are proposed:
a) The acidification potential, and therefore the pyrite content of the coal discard
must be considered during decisions making on the rehabilitation method
(clay barriers), topsoil depth, maintenance and mine closure potential.
b) The occasional monitoring of the subsoil's and coal contact acidity is
recommended, although not much can be done to stop acidification after
cover-soil placement.
c) To ensure a more sustained from of nitrogen supplementation over the long
term the use of selected legumes should be investigated. Research in
Europe and Australia suggested that nitrogen fixation could contribute
substantially to the nitrogen for plant uptake.
d) The physical properties of the topsoil (bulk density 8 soil compaction) are also
being neglected and needs to be assessed occasionally and interpreted
together with chemical analyses. Observations in other studies indicate that
this could be the most fundamental problem for vegetation growth and not
necessarily soil fertility, since soil physical properties could have a major
impact on root development.
Key words: Coal discard, mine rehabilitation, soil fertility, topsoil degradation,
salinisation, and acidification === Thesis (M. Environmental Management)--North-West University, Potchefstroom Campus, 2004.
|
|---|
