This is the second post on the Draft Surat Underground Water Impact Report now out for review. The Queensland Water Commission took its findings to Roma yesterday and Chinchilla today. See yesterday’s post on the background to the report and the geological formations involved.
This post looks at how the model was derived and examines QWC’s monitoring regime. There were three key steps in designing the flow model: conceptualisation, construction and calibration. In the conceptualisation phase, designers looked at geological data and formation contacts in databases held by the Geological Survey of Queensland and the Department of Natural Resources Groundwater Database.
They also took into account the distribution and depth of the geological layers of the Walloon Coal Measures based on previous studies and developed hydraulic parameter estimates based on pump and drill tests, existing models and reported results. They devised 19 model layers based the formations from the shallowest (Condamine Alluvium) to the deepest (Permian Sediments) and mapped out the groundwater flow between the layers. The layers are recharged by rain in the outcrop areas on the edges of the Basin. The model is complicated by the Walloon Coal Measures (the main CSG-bearing formation in the Surat Basin) which contains sediment layers of varying permeability. The model allocates three layers to the Walloon for simplicity.
The model covers an area 550km x 660km divided into 1.5sq km cells stacked into 19 layers. Once constructed with hydraulic parameters, the model was calibrated to replicate pre-CSG conditions in 1995 using groundwater levels from 1500 bores in the groundwater database. The model was designed to make predictions from the 1995 data including and excluding petroleum impacts. They added uncertainty analysis to provide 200 predictions of drawdown for each model cell at different time periods. The upper and lower five percent of the 200 were discarded as outliers and the maximum value of the remaining predictions was used in the report.
The report estimates the CSG industry will draw an average of 95,000 megalitres of water a year over the life of the industry. It will be 125,000 ML a year in the first three years as the industry ramps up. This is why getting the water monitoring strategy right is so important.
The water monitoring strategy involves monitoring of water levels in coal seams and surrounding aquifers. It will not monitor water quality (eg for contamination from fracking) or the volume of water extracted from wells. QWC will not conduct the monitoring – that is left to the gas companies. Someone said to me today that was like leaving Ned Kelly in charge of the bank vault but the QWC says the companies have legal responsibilities and anomalies will quickly be exposed.
The monitoring has six broad objectives. 1. Establish background trends not attributable to CSG. 2.Identify changes in aquifer conditions in petroleum development areas. 3. Identify changes in aquifer conditions near critical groundwater use (eg towns relying on groundwater), 4. Identify changes in aquifer conditions near springs. 5. Improve future groundwater flow monitoring 6. Improve understanding of connectivity between aquifers.
There will be a regional monitoring network with 142 monitoring sites (27 already exist) which will have 498 monitoring points (104 already exist). These sites will target different strata of the Surat and Bowen Basin including the Condamine Alluvium, Main Range Volcanics, Mooga Sandstone, Orallo Formation, Gubberamunda Sandstone, Westbourne Formation, Springbok Sandstone, Walloon Coal Measures, Hutton Sandstone, Evergreen Formation, Precipice Sandstone, Clematis Sandstone and Bandanna Formation.
At each site, water data is collected at least once a fortnight. Queensland’s regulatory requirements require a UWIR update every three years but there will also be an annual report.