This report presents key results of the Ti Tree Basin study completed as part of Exploring for the Future (EFTF)—an eight year, $225 million Australian Government funded geoscience data and information acquisition program focused on better understanding the potential mineral, energy and groundwater resources across Australia.

As part of EFTF, Geoscience Australia undertook an assessment of available and new hydrochemical data collected in the Ti Tree Basin, Northern Territory. The basin is one of the four water control districts within the Southern Stuart Corridor Project area. Communities, irrigation farms and pastoral stations in the basin rely on groundwater, and extensive groundwater sampling and hydrochemical investigations have been undertaken over the past 50 years. An opportunity was recognised to collate and interpret the existing data, supplemented by new EFTF data, not only to add value to the understanding of groundwater processes in the basin itself but also to provide a useful knowledge base for other groundwater resources in the region that are poorly understood. This study largely relied on the available groundwater analysis data from the Northern Territory Department of Environment and Natural Resources database, supplemented by publicly available analyses from other sampling campaigns, including the EFTF, totaling 1913 groundwater samples across the district.

The key findings of the study are:

• The hydrochemistry data, particularly on salinity (total dissolved solids (TDS)), ion ratios (e.g. HCO3/Cl, Cl/(Cl+HCO3), Cl/(Cl+HCO3+SO4), Na/Cl) and radiocarbon (14C) could be used to map the three major recharge areas for the basin—the floodout of the Woodforde River to the west, the floodout of Allungra Creek in the basin centre, and the eastern basin margin. This is consistent with the current accepted interpretation that recharge is dominated by episodic run-on and infiltration in drainage floodout areas, driven by intense rainfall events that generate runoff in upland basement headwaters and ephemeral flows in basin creeks. There are no hydrochemical indicators of recharge in the vicinity of the channelised reaches of the basin creeks (i.e. both Woodforde River and Allungra Creek), located upstream of the floodouts.

• From a groundwater resource perspective, the Allungra Creek floodout has broadly the best combination of low-salinity groundwater (median TDS = 740 mg/L) and bore yield statistics (median = 10 L/s). The Woodforde River floodout also has areas with high-yielding bores (>10 L/s) of fresh groundwater (<1000 mg/L), with the borehole distribution suggesting that the fresh groundwater resource is significantly more extensive to the west of the river than that previously mapped. The eastern basin margin generally has low-salinity groundwater (median TDS = 775 mg/L) but lower bore yields (median = 4.4 L/s).

• There are differences in the recharge characteristics of the three floodout areas, due to differences in drainage catchments and floodout hydrogeology. The Woodforde River floodout has the most depleted stable isotopes, interpreted to be due to a higher rainfall/runoff threshold for recharge (>150 mm/month). It also has the largest isotopic range and the best δ18O-δ2H linear regression, suggesting the most influence of evaporation, such as a longer period of surface water ponding. In comparison, the stable isotope signature for Allungra Creek groundwaters suggests a lower rainfall/runoff threshold for recharge (>100 mm/month) and low evaporative influence, hence relatively rapid infiltration. This is also inferred to be the case for the low-salinity eastern basin margin groundwaters. For both Woodforde River and Allungra Creek, modern recharge is indicated by groundwaters with high radiocarbon activity (14C percent modern carbon (pMC) >70). For the eastern basin margin, radiocarbon activity is low to moderate (14C pMC 20–50). This is interpreted to reflect a longer travel time in the unsaturated zone.

• In the floodout areas, the dominant hydrogeochemical process relating to the fresh groundwater is water–rock interactions. Groundwater tends to be the least evolved Ca(Mg)-HCO3 or transitional Na(K)-HCO3 water type, according to Chadha plots. Zones of prevalence of carbonate-gypsum dissolution or Na-silicate weathering could be mapped using indicators such as cation chloride ratio. Ion exchange is also a likely process in these fresh groundwaters, as inferred from chloro-alkaline indices.

• Groundwater salinity is higher away from the floodout areas. This increased salinity is due to evapotranspirative concentration in addition to water–rock interactions, as inferred from ion ratios, including Cl/Br. Stable isotopes indicate that transpiration of groundwater by vegetation accessing the watertable, rather than direct evaporation, is the dominant process in these areas. This process is particularly evident in the Wilora Palaeochannel, the northern extension of the basin, which generally has the highest groundwater salinities (median TDS = 1575 mg/L), the lowest bore yields (median = 1.9 L/s) and the greatest prevalence of shallow watertables (<15 m). With higher salinities, groundwaters tend to be the evolved Ca(Mg)-Cl(SO4) and Na(K)-Cl(SO4) water types and potentially influenced by reverse ion exchange processes.

• Mountain-front recharge has previously been proposed as an additional recharge mechanism, notably near the southern basin margin. Although sampling is limited in this area, hydrochemical indicators such as low HCO3/Cl, high Na/Cl and evolved Na(K)-Cl(SO4) water type suggests that active recharge is not significant. The watertable is deep along the southern basin margin (>50 m), so groundwater chemistry can be strongly influenced by processes during downward infiltration through a thick unsaturated zone.

• Limited sampling of deeper bores (>80 m), potentially in the Hale Formation, generally have the characteristics of being more saline and lower yielding compared to bores in the shallow groundwater resource (particularly from 40 m to 80 m). However, there are deep bores with good yields of fresh groundwater; of 57 bores in the basin with interval depths exceeding 80 m, eight (14%) have the combination of yield >5 L/s and salinity <1000 mg/L. The deeper groundwaters are typically Ca(Mg)-Cl(SO4) and Na(K)-Cl(SO4) water types, with the latter, more evolved, water type dominating at depths >120 m. There are very few stable isotope analyses for the deeper groundwaters, but these are within the isotopic range for the shallow groundwaters in the same area, suggesting similarity in recharge processes and a degree of aquifer connectivity. Likewise, there are very few radiocarbon analyses for deeper groundwaters (depth >60 m), but these consistently show low 14C activity (pMC <40). The higher salinities, evolved water types and low 14C activity reflect longer residence times in the deeper groundwater system.

The study highlighted that floodout recharge, involving episodic flow of basin creeks from headwater catchments, is the most dominant mechanism, rather than direct infiltration from large rainfall events. The study also identified that recharge characteristics, particularly the rainfall threshold for effective recharge and the role of evaporation, are not consistent across the floodout zones in the basin. This likely reflects differences in upland catchment size and geology, as well as floodout landform and hydrogeology. The study also highlighted the importance of groundwater-dependent vegetation in the basin, with dominance of transpiration of groundwater rather than direct evaporation. The groundwater hydrochemistry datasets and interpretation maps can support informed water management decisions within the basin. For example, improved understanding of the spatial and temporal distribution of recharge is not only needed for defining groundwater extraction limits but also used in strategies such as managed aquifer recharge. The EFTF work adds to the knowledge base and datasets that have developed over decades for the Ti Tree Basin, which are also valuable assets for broader understanding of groundwater resources in central Australia.