Review of groundwater assessment model for Stratford Mining Complex

Heritage Computing (2012) developed a groundwater model using Groundwater Vistas (a graphical user interface for MODFLOW and other models; Environmental Simulations Inc, 2011) in conjunction with solver MODFLOW-SURFACT (a groundwater modelling software package; HydroGeologic Inc, 2015) for the Stratford Mining Complex. The Stratford Mining Complex encompasses both the Stratford Coal Mine and Bowens Road North Open Cut. The objectives of the groundwater modelling were to assess the potential impacts of Stratford Coal Mine and Bowens Road North Open Cut mining (active and post) development on the groundwater as well as to assess cumulative impacts with the other proposed/approved surrounding mine (Rocky Hill Coal Project) and AGL CSG operations.

The model extent is 15 km × 17 km with an approximate depth of 0.6 km, of which about 70% is active in the simulation. The cells are uniform 50 m by 50 m comprising 340 rows by 306 columns, and about 930,000 active cells. The geometry of the model has four types of units divided into 13 layers. The eastern and western boundaries were chosen to coincide with topographic ridge lines and outcrops of the Alum Mountain Volcanics, and were considered as no-flow boundaries.

The northern and southern boundaries are chosen between 5 and 6 km from future mining areas, where groundwater contours suggest lateral flows are primarily in the east−west direction. These boundaries are simulated as no-flow boundaries accordingly. The model relies on ‘river’ cells in the top two layers to receive discharge near these two boundaries. The Avon River is represented by the MODFLOW river (RIV) package, with stage heights set to 0.5 m below ground surface, and bottom elevation varying from 0.5 m, at the river head, to 2.0 m, in lowland reaches, below ground surface. Dog Trap Creek and Avondale Creek both have river stage set to 2.0 m, and bottom elevation at 2.5 m, below ground level. Minor streams are represented as drains by the RIV package and assigned stage and bottom elevations of 0.1 m below the surface; all minor streams are considered solely as discharge features. Mining operations in the coal seam layers are represented by the MODFLOW drainage (DRN) package, with invert levels (i.e. floor level) set to 0.1 m above the base of the relevant layer. CSG activity in this model is implemented as a complete dewatering of the AGL zone 1 area in all coal seams of the geological model.

The hydraulic conductivity of materials was discretised into 17 zones in the horizontal plane and cut through the 13 layers. Zone 1 is the alluvium, zones 2 to 7 are for the weathered overburden and interburden units, and zones 8 to 17 are for coal seam layers. Calibrated horizontal hydraulic conductivity for alluvia varied from 0.2 to 10 m/day. Horizontal hydraulic conductivities in the coal seams and overburden are specified to decrease with depth according to:

K h subscript overburden end subscript equals 0.0057 e x p left parenthesis negative 0.025 cross times depth right parenthesis


K h subscript coal space seams end subscript equals 0.4211 e x p left parenthesis negative 0.014 cross times depth right parenthesis


Vertical conductivity was initially set to one-tenth of horizontal conductivity, although calibration resulted in vertical conductivities of almost an order of magnitude higher than horizontal conductivity in some coal seams. Specific yield in the alluvium varied between 0.01 and 0.2, while this value is set to 5 x 10-3 for the other units. The storage coefficient for overburden and weathered rock is set to 1 x 10-4, while for the coal seams this value is 1 x 10-3 for the 0 to 100 m range which decreases to 1 x 10-4 for the deeper coal seams. Riverbed conductances per grid cell varied from 25 to 100 m2/day, but with no diagrams or indication of where the variations occur spatially. The drain conductance per grid cell was set to 1000 m2/day to minimise any resistance to inflow.

Recharge from rainfall was imposed as a fixed percentage of rainfall in five distinct zones and varied from 0.25% (hills) to 8% (alluvium). Specific recharge rates were determined solely by calibration. Evaporation was applied uniformly across the model domain, with an extinction depth of 2.0 m below ground surface, and a rate of 0.4 mm/day equivalent to 146 mm/year. There is no discussion of the choice of either the evaporation depth or rate, although at the end of the calibration period it accounts for 7.6 ML/day, or 35% of total model discharge. Stock and domestic water bores were not represented in the model, as the amount of produced water was considered to be too small in volume and too irregularly taken. Large-scale pumping associated with future CSG was represented by ‘drain’ cells in the model set to an invert level appropriate for depressurisation of the target layer.

Steady-state calibration used 39 point targets, averaged at each site over the full monitoring record (from 1994 to 2010), near past and future mining developments. Automatic calibration using the Model-Independent Parameter Estimation and Uncertainty Analysis (PEST, Doherty, 2005) software was done iteratively during steady-state and transient calibration on hydraulic conductivity, water storage properties, and recharge rates as a percentage of rainfall. Transient calibration proceeded from the heads estimated by the steady-state model, using 90 monthly stress periods from January 2003 to July 2010. Sensitivity analysis was conducted for lateral and vertical hydraulic conductivities of coal seams and weathered rock interburden as well as recharge fraction in the hill zone.

The model predicted a complex pattern of stream loss and gain due to mining operations, but overall it is confined to the minor streams of Dog Trap and Avondale creeks. The patterns of loss and gain varied as pits begin mining or are decommissioned. Cumulative impacts were assessed for the Rocky Hill Coal Project and AGL’s Gloucester Gas Project. Pits were represented by ‘drain’ cells 0.1 m above the bottom of the relevant layer, and CSG by drains with a groundwater level equal to a depressurisation target for gas production. Sensitivity was primarily assessed by pit inflow and CSG-produced water. These were most sensitive to lateral conductivity, but previous variations showed that increasing this led to a poorer fit to observed groundwater levels.

The main hydrological changes predicted by this modelling are:

  • The impact on the water level in each privately owned bore is expected to be negligible.
  • Drawdown of 1 m was predicted at end-of mining (December 2024) out to less than 1 km around all pits, except south of Roseville West Pit where it may extend to 1.6 km.
  • The final voids from the mining pits would remain as permanent groundwater sinks, with increased recharge through the mine waste rock. Total inflow to three pits in the long term is modelled to be 0.9 ML/day.
  • As mining progresses, it was anticipated there would be more leakage from the alluvium (in the near vicinity of the pits) to the weathered rocks. The direct water loss from alluvium is estimated as 0.08 ML/day, assuming 2 m of saturated thickness and 10% porosity. Water loss from the weathered rock (in the near vicinity of the pits) to pits is estimated as 1.1 ML/day over the life of mining, and may drop to 0.6 ML/day post mining.
  • Cumulative effects are expected to be substantially greater than would be produced by the proposed mining operations. CSG activity would cause pronounced drawdown in the watertable between the Stratford Mining Complex and Stratford.
Last updated:
5 November 2018
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