Potential for using trees to limit the ingress of water into mine workings : a comparison of total evaporation and soil water relations for eucalyptus and grassland .
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Date
2003
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Abstract
Current mining methods used to extract coal from underground mine workings disturb
the natural environment and the existing stable geological structures. As a result, the
ingress of water into the mines increases and the quality of the water passing through
the mine workings deteriorates, irrespective of the operational status of the mines.
Water ingress is generated by regional aquifers, local aquifers, recharge from the
surface through rainfall, natural drainage paths on the surface, and surface water bodies.
The quality of water in the mines deteriorates as a result of contact with the remaining
coal in the mine workings. Mining can therefore cause an increased influx of water into
a mine and the degradation of this water. The solution to reducing the impact of mines
on the environment is to prevent, or at least reduce, the amount of water entering the
mines, and to manage this water to prevent further degradation in water quality.
This study focused on afforestation with Eucalyptus viminalis trees to manage or
inhibit ingress of water into underground mine workings. The hypothesis of this study
was that a change in vegetation, from grassland to fast-growing and potentially high
water-using trees like Eucalyptus. could possibly reduce the drainage of water below the
root-zone and into the mine workings. The hypothesis was tested by estimating the
components of the soil water balance for a grassland site and a Eucalyptus tree site. The
research site was situated in Mpumalanga, (260 36' Sand 290 08' E, 1650 m a.m.s.l.),
one of South Africa's major coal bearing areas. Although the Secunda area is a treeless
environment and conditions are not optimal for forestry, some Eucalyptus species are
suited for conditions (frost and periodic droughts) encountered in this area.
The soil water balance of grassland and E. viminalis trees were studied through a
field experiment and a long-term (30 years) modelling exercise. Total evaporation of
the grassland site was estimated using the Bowen ratio energy balance technique. The
transpiration of six representative E. viminalis trees were estimated using the heat pulse
velocity technique. The soil water storage changes at both sites were determined from
the soil water content, estimated using water content reflectometers. Measurements
were performed in a smectic clay soil which resulted in measurements difficulties.
Vertical cracks were formed under soil drying. To establish the importance of climate
and plant growth on the drainage beyond the root-zone, the soil water balance of a
grassland and an E. viminalis site were simulated over a 30-year period with the Soil
Water Atmosphere Plant (SWAP) model.
It was concluded from the comparative field experiment and modelling, that a change
in vegetation from grassland to E. viminalis will reduce the drainage of water below the
root-zone, especially under above-average rainfall conditions. The reduction in
drainage beyond the root-zone at the E. viminalis sites, compared to the grassland site, was demonstrated in the modelling exercise and can be deduced from the total
evaporation and soil water storage estimated at both sites. The results from the field
experiment confirmed the modelling results and showed that usually there were higher
transpiration rates for the E. viminalis tree site, compared to the grassland site. The
higher transpiration rates for E. viminalis trees resulted in lower relative saturation of
soil layers and lower profile soil water contents at the E. viminalis site, and higher daily
soil water storage changes at the E. viminalis site compared to the grassland site. These
differences were more pronounced during winter when the grassland was dormant.
The results from the modelling exercise showed that an E. viminalis tree stand, with a
closed canopy, reduced drainage below the root-zone compared to a grassland. The
drainage at the grassland site contributed to up to 54 % of the rainfall, compared to the
43 % at the E. viminalis site. However, under below-average rainfall conditions the
annual drainage at both sites, were similar. Further, the absolute magnitude of the
drainage was similar to the total evaporation at the grassland site under certain
conditions. The results not only suggest that a change in vegetation, from grassland to
E. viminalis trees, would reduce the drainage beyond the root-zone, but that it may
delay the onset of drainage. Under above-average rainfall conditions, the modelled
drainage at the E. viminalis site only exceeded 20 mm, a month later than at the
grassland site. The simulation results also showed that under conditions of aboveaverage
rainfall, drainage occurs whenever the rainfall exceeds the long-term average
rainfall, irrespective of the existing vegetation. However, when the rainfall is belowaverage
drainage at both sites are limited to large rainfall events. This simulation
showed that over a period of eight years, E. viminalis trees could potentially reduce the
drainage by 1235 mm more than grassland, which is equivalent to 1540 m3 ha- I a-I, or
1.54 Me ha- I a-I. The annual average reduction in drainage below the root-zone caused
by E. viminalis trees (1.79 Mf ha-1 a-\ is a small reduction when compared to the
influx of water into mineworkings. E.g. the influx of water into a bord-and-pillar mine
range between 0.5 and 4 Mt d-I per area mined and up to 17000 Mt d-I per area mined
under high extraction mining (Hodgson and Krantz, 1998; Hodgson et aI., 2001).
This work gave a comprehensive account of the differences in the soil water relations
of grassland and E. viminalis trees overlying coal mine working. Few other studies in
South Africa compared the total evaporation and soil water relations of grassland and
E. viminalis trees in so much detail. State of the art monitoring techniques were used
and produced valuable comparison of their use in expansive clay profiles. The work
should contribute to management decisions focussed on limiting ingress of water into
mine workings.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2003.
Keywords
Mine water., Mine water--Management., Mine drainage., Coal mines and mining., Eucalyptus., Theses--Agrometeorology.