Collaborating Organisations: Flinders University, Department of Environment and Natural Resources, Northern Territory Government, CloudGMS
Chief Investigator: Peter Cook , John Wischusen, Anthony Knapton
The Rocky Hill area, located to the southeast of Alice Springs airport, has been identified as a potential future bore field for Alice Springs town water supply. In 1996 a parcel of land was set aside for this bore field, and the area has been the focus of renewed hydrogeological investigations since that time. The NCGRT has assisted in the recent investigations to elucidate hydrostratigraphy, improve the hydraulic characterisation of the aquifer system and confirm the mapping of the groundwater quality distribution - both spatially and with depth. The knowledge gained from the project updates the current estimate of sustainable yield of the resource. The project also provided a basis for a more quantitative risk and impact assessment to the resource and Alice Springs’ potential future water supply.
The current bore field that supplies Alice Springs is located west of the airport, and water is pumped principally from the Mereenie Sandstone aquifer. This aquifer extends east to Rocky Hill, where most of the potable groundwater resource is hosted within the Mereenie Sandstone and the overlying Hermannsburg Sandstone. Groundwater salinity within the Rocky Hill area is low, with total dissolved solids (TDS) less than 600 mg/L over an area of more than 140 km2. Available evidence suggests that good quality water (TDS < 600 mg/L) occurs to more than 300 m depth. The extractable volume of good quality groundwater to this depth is estimated to be more than 5 000 GL. This increases to more than 10 000 GL if groundwater with TDS up to 1000 mg/L is considered and represents a much greater resource than previously identified. Recharge to the system is most likely to the result of the rare flow events in the Todd River that flood out into the Rocky Hill region.
One of the potential risks to the groundwater supply is from agricultural development located immediately northeast of the proposed bore field. Fodder crops have been intermittently grown here under irrigation since the 1970s, with intensification of irrigation since 2002 when grape vines were planted. The development of irrigated agriculture will lead to increased rates of groundwater recharge, and there is potential for this recharge to impact groundwater quality. Because concentrations of chloride, nitrate and other salts are naturally high in arid zone soils of central Australia, irrigation drainage will leach these salts into the underlying groundwater. Based on comparison of soil moisture profiles beneath the vineyards and adjacent native vegetation, we estimate that salts will begin to be leached into the aquifer within the next 5 - 10 years, with salt fluxes of approximately 13 tonnes/year per hectare of irrigated land. If irrigation continues, this leaching of salt will continue for a period of 50 – 80 years. Leaching of nitrate is not considered to be a significant problem.
A solute transport groundwater model (Knapton, 2017) was developed to enable simulation of future production bore field pumping to be undertaken from within the land parcel to the east. This work indicated that groundwater beneath the agricultural block is likely to be within the capture zone of such a bore field. However, groundwater which is derived from beneath the agricultural block will be diluted by groundwater from the local surrounding area. Therefore, it is possible that salt fluxes beneath the irrigation areas may have little effect on concentrations of groundwater pumped by the bore field. The model was used to calculate groundwater travel times to the bore field and the extent to which groundwater from beneath irrigation areas are diluted by groundwater from the local area surrounding the bore field. The model will also be used to assess the risk to a potential future bore field from other activities overlying the aquifer.
Cook PG, Knapton A and White N, 2017, The Potential Impact of Irrigated Agriculture on Groundwater Quality in the Rocky Hill Region, Northern Territory. National Centre for Groundwater Research and Training, Australia.
Cook PG, Wischusen J. and Knapton A, 2020, The Hydrogeology of the Rocky Hill Region, Northern Territory. National Centre for Groundwater Research and Training, Australia, and Department of Environment and Natural Resources, Water Resources Division. WRD Report 13/2020.