Discrete interval groundwater samples from uncased boreholes in the Hawkesbury Sandstone, NSW, Australia, reveal mixed results.

Peterson, MA; Cendón, DI; Hankin, SI; Chisari, R

Discrete interval groundwater samples from uncased boreholes in the Hawkesbury Sandstone, NSW, Australia, reveal mixed results.

Date

2013-09-19

Authors

Publisher

International Association of Hydrogeologists

Abstract

The effects of subsidence and related fracturing from longwall coal mining have raised community concern around the Southern Coalfields of NSW. Shallow fracturing of bedrock streambeds has led to draining of perennial pools and localised diversion within some streams. Consequently, the risk of diverting surface water and shallow groundwaters towards underground mining operations has also been debated in the community. A Zone-of-Interest Groundwater Sampler (ZoIGS) with straddle packers and multiple pressure sensors was used to collect samples from discrete 5 m intervals within uncased 100 mm boreholes. Up to 6 intervals were successfully sampled in each borehole, to a maximum depth of 137 m below ground level. Hydrogeochemical information analysed comprised major ions, water stable isotopes, 3H and 14CDIC . The results were compared to geophysical data and pressure recovery after purges for each of the intervals. Each borehole displayed a unique vertical arrangement of apparent ages, hydraulic conductivities, mixing and pressure gradients. Hydrogeochemical results showed that some of the more evolved groundwater at depth, with corrected 14C ages up to a few thousand years, was also displaying a 3H signature of mixing with much younger waters. In some cases age profiles were overturned with depth, with younger water occurring below older water. Geophysical and pressure recovery data showed that each borehole intersected a number of aquifers, separated by shale aquitards up to five metres thick. The shale beds are sometimes closely associated with high horizontal hydraulic conductivities and outflow zones. 3H indicative of younger water did not always coincide with the higher hydraulic conductivity zones. The results describe a complex heterogeneous system within the Hawkesbury Sandstone, with multiple flow paths contributing to mixing of old and young groundwater at depth. The ability to sample and investigate numerous discrete intervals within uncased boreholes gave an opportunity for better understanding the system, with minimal infrastructure.

Keywords

Ground water, New South Wales, Australia, Sandstones, Longwall mining, Coal mining, Carbon 14, Surface waters, Stable isotopes

Citation

Peterson, M. A., Cendón, D., Hankin, S., & Chisari, R. (2013) Discrete interval groundwater samples from uncased boreholes in the Hawkesbury Sandstone, NSW, Australia, reveal mixed results. Paper presented to the IAH 2013, Perth, Australia : "Solving the groundwater challenges of the 21st century", International Association of Hydrogeologists 40th International Conference, Perth, Western Australia, 15-20 September 2013, (pp. 166).