Karstic groundwater basins are characterized by both point and diffuse recharge. This paper describes the hydrologic characteristics of point recharge and their influence on recharge estimation for four groundwater basins. Point recharge is highly transient and may occur in relatively short-time periods, yet is capable of recharging a large volume of water, even from a single extreme rainfall event. Preferential groundwater flows are observed in karst aquifers with local fresher water pockets of low salinity that develop around point recharge sources. Measurable fresh water plumes develop only when a large quantity of surface water enters the aquifer as a point recharge. In fresh water plumes, the difference in chloride concentrations in diffuse and point recharge zones decreases as the plumes become enriched through mixing. The relative contributions to total recharge from point sources using the measured gap between groundwater and rainwater chloride in the chloride vs. δ 18 O plot is not necessarily indicative of sinkholes not directly recharging the aquifer. In karst aquifers, recharge estimation methods based on groundwater age distribution; average annual rainfall and basin average chloride in the conventional chloride mass balance (CMB) method are questionable due to theoretical limitations and key assumptions of these methods not being met. In point recharge dominant groundwater basins, application of: watertable fluctuation, numerical groundwater modelling, Darcy flow calculation or water budget methods are more suitable for recharge estimation as they are independent of the particular mode of recharge. The duality of the recharge mechanism in karst aquifers suggests that modification to the CMB method may be required to include both point and diffuse recharge components.

A groundwater risk assessment was carried out for 30 potable water supply systems under a framework of protecting drinking water quality across South Australia. A semi-quantitative Groundwater Risk Assessment Model (GRAM) was developed based on a "multi-barrier" approach using likelihood of release, contaminant pathway and consequence equation. Groundwater vulnerability and well integrity have been incorporated to the pathway component of the risk equation. The land use of the study basins varies from protected water reserves to heavily stocked grazing lands. Based on the risk assessment, 15 systems were considered as low risk, four as medium and 11 systems as at high risk. The GRAM risk levels were comparable with indicator bacteria-total coliform-detection. Most high risk systems were the result of poor well construction and casing corrosion rather than the land use. We carried out risk management actions, including changes to well designs and well operational practices, design to increase time of residence and setting the production zone below identified low permeable zones to provide additional barriers to contaminants. The highlight of the risk management element is the well integrity testing using down hole geophysical methods and camera views of the casing condition.

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An integrated approach combining lithological logs, downhole geophysics, electromagnetic survey and the distribution of radiocarbon ( 14 C) and the stable isotopes of water molecules ( 18 O) were used to identify the conduit flow paths of a small freshwater lens. Lost circulation zones, where drilling fluid flows into geological formation instead returning up the annulus recorded during water well drilling, were considered as the major fracture zones. The presence of high porosity zones within boreholes were identified using caliper, gamma and neutron logs. These methods were used to identify the depth intervals at which cavities and the existence of conduit porosity within the boreholes. Transient electromagnetic (TEM) method was used to investigate resistivity anomalies in the profiles along nine pre-determined lines across the freshwater lens. Resistivity anomalies were related to borehole information and other surface features such as sinkholes. Low resistivity zones of the TEM tomography sections had excellent correlation to fracture zones identified during well drilling, and downhole geophysical logs. Similarly, high resistivity zones in the profiles correlate well with the zones of O signatures of the groundwater confirm the presence of conduits and potential pathways of preferential flows. This investigation illustrates the effectiveness using an integrated approach to trace the conduit flow paths in karst aquifers. The information gained from the study is currently being used for the management of the freshwater lens.

Estimation of natural recharge and potential for seawater intrusion are critical considerations for management of coastal freshwater aquifers. We show hydrochemical signatures of groundwater to identify the influence of geological control on chemical processes in a coastal groundwater system. We used dominant hydrochemical facies, salinity and magnesium ions to determine two main groundwater flow paths with different origins and ages. Mixing of groundwater with different origins and ages results in unreliable recharge estimates using chlorofluorocarbon (CFC) and chloride mass balance (CMB) methods, thus limiting available methods for recharge assessment. Interpretation of hydrochemical data suggests that calcium carbonate dissolution, ion exchange processes and mixing with sea aerosol in coastal zones are the main influencing factors on groundwater chemistry. Restricted groundwater flows due to occurrence of a basement high at the southern side of the basin boundary influence the distance to the toe of the saline wedge. Thus, knowledge of geological control over groundwater systems forms an important part of characterising basins and contributes toward effective management of groundwater resources.

Blue Lake, a volcanic crater provides municipal water supply to the city of Mount Gambier, population of 26,000. Current average annual pumping from the lake is 3.6 × 10 6 m 3. The lake is fed by karstic unconfined Gambier Limestone aquifer. Storm water of the city discharges to the aquifer via about 400 drainage wells and three large sinkholes. Average annual storm water discharge is estimated at approximately 6.6 × 10 6 m 3 through drainage wells and sinkholes within 16.8 km 2 of the central part of the city. Chemical mass balance for calcium was used to estimate groundwater inflow to the lake at 6.3 × 10 6 m 3 , almost equal to the volume of storm water discharge and slightly higher than the previous estimates using environmental isotopes (4.8-6.0 × 10 6 m 3). Considering the lake outflow volume of 2.7 × 10 6 m 3 , the net inflow to the lake equates to the current annual pumping and therefore it is considered that the current pumping rate is at the upper limit. For meeting the short-term future demand, confined aquifer water may be used and in the longerterm, an additional well field is required outside the Blue Lake capture zone, preferably to the northeast of the city. For water supply security, inflow to the lake along with water quality has to be maintained within the city. Current annual private abstraction within the capture zone is about 4.4 × 10 6 m 3 and in order to maintain aquifer water levels, no additional allocation should be allowed.

Coliform source tracking was undertaken on 48 water sources of which 42 are potable in 26 water supply systems spread across South Australia. The water sources in the study vary from unprotected springs in creek beds to deep confined aquifers. The frequency analysis of historical coliform detections indicate that aquifer types, depth to water and casing depth are important considerations; whilst maintaining well integrity and the presence of low permeable clay layers above the production zone are the dominant parameters for minimizing coliform contamination of water supply wells. However, in karst and fractured rock aquifers, pathways for coliform transport exist, as evidenced in the >200 MPN/100 mL level of coliform detection. Data indicate that there is no compelling evidence to support the contention that the wells identified as low risk are contaminated through geological strata and clay barriers. However, data strongly supports the suggestion that coliform detection from sample taps and wellheads stem from the surrounding groundwater and soil-plant sources as a result of failed well integrity, or potentially from coliform bacteria that can persist within biofilms formed on well casings, screens, pump columns and pumps. Coliform sub-typing results show that most coliform bacteria detected in town water supply wells are associated with the soil-water-plant system and are ubiquitous in the environment: Citrobacter spp. (65%)