Oral Presentation NCGRT/IAH Australasian Groundwater Conference 2019

Groundwater level trend analysis to inform likelihood of cumulative coal seam gas impacts in the Surat CMA (358)

Timothy R. Ezzy 1 , Sanjeev Pandey 1 , Keith Phillipson 1 , Gerhard Schoning 1 , Anna Bui Xuan Hy 1 , Daan Herckenrath 1 , Dhananjay Singh 1 , Douglass Beck 1
  1. Office of Groundwater Impact Assessment, Brisbane, QLD, Australia

The Queensland Office of Groundwater Impact Assessment (OGIA) is responsible for assessing cumulative groundwater impacts arising from coal seam gas (CSG) extraction in the Surat Cumulative Management Area (CMA), and preparing an Underground Water Impact Report (UWIR) every three years.

The main objectives of this groundwater level trend analysis project were:

  • To analyse regional groundwater level trends in the main aquifers of interest directly above (Springbok Sandstone and Condamine Alluvium) and below (Hutton Sandstone) the coal seam gas reservoir (Walloon Coal Measures) in the Surat Basin.
  • To analyse the potential causes of these groundwater level trends, with a primary aim of identifying CSG versus non-CSG water extraction impacts.

This project adopted a multiple lines of evidence approach using both qualitative and quantitative techniques. Specific statistical methods used include Mann-Kendall analysis of groundwater level trends, and Spearman correlation analysis of groundwater levels with major water balance factors such as rainfall, and local CSG and non-CSG water extraction. In addition, traditional conceptual hydrogeological analyses were used to assess if connectivity pathways are present between the reservoir and the individual aquifers. Hypothesis testing was undertaken using a simple sub-regional groundwater flow model. Finally, a new classification criteria was introduced and employed to assess the likelihood of CSG impacts at individual monitoring points within OGIA’s groundwater monitoring network.

CSG associated water extraction is a more recent stress and presents as an additional influence on an already impacted aquifer system. Long-term declining groundwater level trends are observed in all three aquifers of interest. These declining trends tend to be highly correlated with local-scale, non-CSG water use estimates and rainfall, and occur both before and after the commencement of CSG extraction, and inside and outside of CSG production areas. The key challenge of this study was to separate these more recent CSG influences on groundwater level trends, from long-term non-CSG water use impacts and climate influences.