The existence of submarine fresh groundwater has been recorded at continental shelves worldwide. The dynamic preservation and lifetime of fresh groundwater in the offshore environment remains an open hydrogeological problem. The mechanisms and time scales of fresh groundwater preservation are examined using numerical simulations based on a geologically representative model of the New Jersey shelf, USA. Utilizing two-dimensional depth-migrated seismic and well data, a detailed hydrogeological model is built, with a vertical resolution of 10 m. The model captures the highly heterogeneous shelf environment and accounts for porosity compaction trends derived from core data. The results show transient coupled simulations of groundwater flow and heat and salt transport from the late Pleistocene until present day and projected 18,000 years into the future. They reveal freshwater preservation patterns and yield simulated borehole salinity profiles broadly consistent with field observations. The simulations show that freshwater intervals of a thickness of 200–300 m and lateral extent of tens of kilometers may have been preserved from the Last Glacial Maximum until today. It was found that approximately 30–45% of the initial freshwater volume remains preserved after 12,000 years, depending on the recharge boundary condition. The preserved volume ranges between 15 and 30% after 30,000 years. These results improve the understanding of submarine preservation of fresh groundwater through an interdisciplinary approach which integrates seismic imaging, hydrogeological modeling and high-performance numerical simulation.

The interaction between surface-water streams and groundwater in the Maules Creek catchment of northern New South Wales, Australia has been investigated using a wide range of techniques. Zones of groundwater discharge were mapped by measuring the temperature and fluid electrical-conductivity distribution in bores and surface water. Zones where surface water appears to be recharging the aquifer were investigated by measuring the vertical head gradient between the stream and adjacent bores and by estimates of the decreasing surface flow. Geological heterogeneity appears to be the most significant factor in controlling exchange. Lithological information was assembled using geophysical logging of existing bores, supplemented by the results of electrical resistivity imaging. A preliminary water balance was assembled from the available State records of groundwater abstraction for irrigation, rainfall, evapotranspiration and flow gauging in Maules Creek and the adjacent Namoi River. The analysis has demonstrated the complexity of these coupled systems and gives an indication of the most efficient techniques to be deployed in the field to investigate these complex but important systems.

Vertical leakage (discharge to upper aquifers) is an important but poorly constrained component of water balance in the Great Artesian Basin (GAB), Australia. It ranges from negligible discharge where the GAB is overlain by aquitards, to high discharge where artesian water feeds the shallow unconfined aquifer (thereby raising the water table) causing elevated surface soil moisture and extensive surface salinisation. Adequately representing the temporal and spatial variability of vertical leakage is difficult due to the large scale over which the discharge occurs. An innovative method is presented that integrates a supervised classification of high-discharge zones using time-series Landsat data with landform mapping information to improve classification results. ‘Wetness persistence’ and ‘salt persistence’ classes, determined from the time series data, are related to groundwater discharge processes through a discharge framework that allows scaling up of field-based discharge estimates. The results show that using multi-image classification integrated with landform data will significantly reduce uncertainty by reducing false positives. No significant temporal trends were found in a time series assessment, with results featuring high variability, most likely due to image normalisation issues. The lack of a clear temporal signal suggests that an assumption of steady-state discharge is valid for estimating annual fluxes of vertical leakage. Supervised classification and landform outputs provide updated knowledge on GAB vertical leakage rates by providing useful lower and upper bounds of discharge rates respectively. Additionally, groundwater-dependent ecosystem classification, covering the full extent of the basin margins, is a new source of information resulting from the work.