Probabilistic Groundwater Flow, Particle Tracking and Uncertainty Analysis for Environmental Receptor Vulnerability Assessment of a Coal Seam Gas Project

Dennis Gonzalez,Daniel W. Gladish,Sreekanth Janardhanan,Daniel E. Pagendam

doi:10.3390/w12113177

Abstract

The production of coalbed methane, or coal seam gas (CSG) in Australia increased 250-fold since the 1990s to around 1502 petajoules in 2019 and continues to expand. Groundwater flow in the aquifers intersected by gas wells could potentially facilitate a transport pathway for migration of contaminants or poorer quality water from deeper formations. While regulatory and mitigation mechanisms are put in place to minimize the risks, quantitative environmental impact assessments are also undertaken. When many gas wells are drilled in a wide area where many potential receptors are also spatially distributed, potential source-receptor combinations are too numerous to undertake detailed contamination risk assessment using contaminant transport modelling. However, valuable information can be gleaned from the analysis of groundwater flow directions and velocities to inform and prioritise contamination risk assessment and can precede computationally challenging stochastic contaminant transport modelling. A probabilistic particle tracking approach was developed as a computationally efficient screening analysis of contamination pathways for a planned CSG development near Narrabri in northern New South Wales, Australia. Particle tracking was run iteratively with a numerical groundwater flow model across a range of plausible parameter sets to generate an ensemble of estimated flow paths through the main Great Artesian Basin aquifer in the area. Spatial patterns of path lines and spatial relationships with potential receptors including neighbouring groundwater extraction wells and hydrologically connected ecological systems were analysed. Particle velocities ranged from 0.5 to 11 m/year and trajectories indicated dedicated contaminant transport modeling would be ideally focused at the local scale where wells are near potential receptors. The results of this type of analysis can inform the design of monitoring strategies and direct new data collection to reduce uncertainty and improve the effectiveness of adaptive management strategies and early detection of impacts.

Highlights

The commercial production of coalbed methane or coal seam gas (CSG) as it is known began in Australia in the 1990s and has grown from around 2 petajoules (PJ) per year in 1997–1998 to 1502 PJ/year as of December 2019 with a further 31,803 PJ of remaining CSG reserves in Queensland [1]
Multiple plausible ensembles of hydraulic properties including conductivity and porosity sampled from prior distributions were used to run iterative particle tracking simulations
At a regional scale this method was appropriate as a screening assessment to identify and prioritise potential source-receptor combinations prior to employing dedicated local contaminant transport models for contamination risk assessment

Summary

Introduction

The commercial production of coalbed methane or coal seam gas (CSG) as it is known began in Australia in the 1990s and has grown from around 2 petajoules (PJ) per year in 1997–1998 to 1502 PJ/year as of December 2019 with a further 31,803 PJ of remaining CSG reserves in Queensland [1]. Gas wells provide a potential pathway for introduced and/or geogenic contaminants to enter intersected aquifers [2,3]. Prediction of contamination pathways is challenging in data sparse areas such as deep coal-bearing sedimentary basins where pre-development monitoring is rare. To account for uncertainty a probabilistic approach is useful for producing a range of plausible outcomes but is time consuming and costly for contaminant transport modelling. For efficient regional scale screening level assessment of receptor vulnerability and identification and prioritisation of source-receptor combinations that may need detailed analyses, a probabilistic particle tracking method was developed and applied for a CSG project near Narrabri, in northern New South Wales (NSW), Australia, approximately 500 km north-east of Sydney

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