Metropolitan Coal Project Independent ... - Peabody Energy

METROPOLITAN COAL PROJECT INDEPENDENT ENVIRONMENTAL AUDIT
allow for 'minimal' gas release downstream of Pool P. There is no limit to gas release upstream of
Pool P. In accordance with the Long-walls 20-22 Water Management Plan, the following actions were
undertaken once the gas release was identified:
- monitoring conducted weekly to determine the extent of the gas releases,
- gas concentration monitoring (using OdaLog meter); and
- identification of any observable environmental effects (e.g. impacts to riparian
vegetation or fish).
Gas releases were observed periodically until May 2011, however there were no observable
environmental effects of these releases. There were no further indications of gas release from this
time until the end of the review period.
Another gas release site was noted downstream of Flat Rock Crossing and upstream of LW20 by the
mine. The location of the release will be recorded and reported in the next annual report.
4.14.6 Subsidence Assessment
Performance Indicators (PI) have been presented in the Water Management Plan for connective
cracking assessment. The first two indictors are indirect measures of subsurface cracking height (i.e.
underground visual inspections of strata water make above the goaf from incumbent strata and 20-
day average water make determined from estimated mine water use). The third PI compares the
measured piezometric head profiles down through the overburden with the predicted low and highinflow
model outcomes after the extraction of LW20. An exceedence would be assessed if the
predicted piezometric heads were lower than or plotted to the left of the high-inflow model profile.
The outcomes of the review indicate that no abnormal groundwater inflows have been observed into
the goaf and mine water makes are well within the 20-day average or 2ML/day. The multi-piezometer
readings in two deep boreholes above LW22 and 600m west of the extraction limits of LW21 indicate
that the measured profiles are within the predicted model profiles that assume low and high water
flows.
It is noted that depressurization of confined aquifers in the overburden have occurred up to 100 m
above the Bulli Seam to the top of the Scarborough Sandstone or base of the Stanwell Park
Claystone Units due to connective sub-surface fracturing above the workings.
Discontinuous fracturing appears to have developed in the Bulgo Sandstone with groundwater levels
dropping by approximately 75 m. It is noted that some piezometer readings have not yet reached
equilibrium due to the low permeability of the rock mass unit in which they are situated
It is noted that a calibrated ground water model and extensive multi-piezometer monitoring program
has been developed to improve the mines ability to assess the impact of mining on the groundwater
regime within the project area.
Whilst the it is considered that the measured v. prediction model profiles is very good, it is unclear
how much the strata has been impacted by subsidence as pre-mining or base line piezometric profiles
have not been provided on the plot. Time dependent results are provided (elsewhere in the AEMR)
that clearly demonstrate how the caving process has impacted on hydraulic conductivity and
piezometric head levels above the extraction panel. The use of multi-wired extensometers above the
extracted long-walls would also complement the piezometer readings and provide invaluable data to
enable the establishment of the link between ground piezometeric behaviour and strata dilation
magnitudes.
There has been no impact to the Woronora Reservoir Storage capacity groundwater levels upstream
of the reservoir. There has however, been a temporary exceedence in the PI for the quality of water
flowing into the reservoir with a high dissolved iron reading noted. (The quality of the reservoir itself
has not changed significantly, although the mine is still trying to obtain further quality data from the
SCA (as previously mentioned) to assess base-line variation properly).
35 trevor brown & associates
applied environmental management consultants
aemc