Water Resource Management Using 3D Catchment Geology Visual Models

9th Murray-Darling Basin Groundwater Workshop 2004

Mintern, C.
Former Department of Primary Industries Research, Victoria.

Abstract: The over-allocation of water resources in Australia has, on occasions, resulted in irreparable damage of groundwater-dependant ecosystems and resulted in the compaction and contamination of aquifers. The size (volume) of water resources, key processes responsible for water quality contamination and the requirements of groundwater-dependant ecosystems were poorly understood. As a result simplistic methods were used to estimate water allocations, for example, some groundwater allocations are based on the assumption that 2% of annual rainfall is equal to recharge. Such simplistic methods to approximate water allocations are difficult to validate and then use, has lead to severe problems in places such as the Namoi Catchment where water resources have been over allocated by 247%.

New and innovative methods are now available that can improve the accuracy of water resource estimates and enhance the management of water resources in Victoria. For example 3D catchment geology models have been used in combination with other techniques to investigate complex catchment hydrological processes that link surface and groundwater, as well as aquifers. 3D geological models can incorporate a variety of data in different formats and at different scales including tacit knowledge that has been built up within the catchment by experts over time. In addition 3D geology models can simultaneously analyse of multiple data sets. This enables efficient and effective synthesis of information, compared to current methods where data sets are analysed separately. 3D model approach provide a mechanism by which water resource estimates can be verified and continually improved as more data becomes available, producing water resource estimates to a level of accuracy previously not achieved. Using this method to estimate water resources will dramatically improve the accuracy of water resource estimates in Victoria, preventing adverse impacts that may result from over-allocation.

Key Words: water resource management, 3D geology models, conjunctive water resource management.


Introduction

Developing water resources in a sustainable way has been a major challenge worldwide. Problems associated with water resource development have included;

• lowering of water tables in areas of surface water and/or groundwater extraction;
• rising groundwater tables (leading to land salinisation) in areas of irrigation,
• salt-water intrusion into freshwater aquifers;
• land subsidence; and
• lowered baseflow in streams with corresponding ecological damage.

Rapidly growing populations in water-stressed areas and global climate change are increasing the pressure on water resources, and increasing the need to manage allocations between competing users (including the ecosystem) and optimise management of supplies for optimal social, economic and environmental benefits.

For many aquifer systems, including the Murray Group Limestone Aquifer (Barnett 2002), Great Artesian Basin (Hillier and Foster 2002), the Milk River Aquifer (Phillips, Bentley et al. 1986), and the High Plains Aquifer (Sophocleous 1998; Sophocleous 2002a; Sophocleous 2002b), water is extracted at a greater rate than it is being recharged, so the aquifer is effectively being mined. In the Murray Group Limestone aquifer, the water has been estimated to be 20,000 years old (Barnett 2002) and is being mined at a rate of 5 cm/yr (Barnett 2002), however there is limited knowledge of the long-term damage caused by the mining the water resource. For most aquifer systems within Victoria, little is known about the extent of the water resource, how long current extraction rates can continue, and what the long-term repercussions of current water extraction regimes are. There is an urgent need to more accurately define water resources to prevent adverse impacts that may result from over-allocation.

In Victoria water resources are often estimated from simplistic methods that cannot be independently validated. Specifically these limitations include;

• the interaction between surface water and groundwater not being incorporated into methods used to estimate water resources;
• the significant uncertainty associated with methods traditionally used to estimate recharge; and
• analytical models that require steady state conditions to aquifer systems where groundwater is hundreds of thousands of years old violate the steady state assumption.

It is now well understood that water allocated from streams has a draw down effect on groundwater and vice versa (National Research Council 2000; MDBC 2001; Evans and Cook 2002; Sophocleous 2002). This interaction between surface and groundwater is often not reflected in methods used to estimate surface water and groundwater allocations because the allocations are made independently. Management of surface water and groundwater as one interconnected system has been recognised to be a fundamental requirement to achieve sustainable water resource management (Government of Victoria, 2000), which has been highlighted as the most challenging issue facing water resource management in Australia (MDBC, 2001).

The nature of aquifer recharge is one of the most critical variables in estimating available water resources (Evans, Coram et al. 1998; Evans and Cook 2002; Scanlon, Healy et al. 2002). The method currently used to calculate water allocations in Victoria assume that 2% of rainfall equals recharge (Government of Victoria 2000). This method assumes that recharge is a linear function of rainfall, which is satisfactory in the wet tropics and temperate regions, where annual recharge is approximately evenly distributed (Evans, Coram et al. 1998; Evans and Cook 2002). However this assumption is less satisfactory in semi- arid and arid regions, where the defining characteristic of recharge is not the average rainfall, but the length of time required to attain a threshold of rainfall intensity that allows recharge to occur (Evans, Coram et al. 1998; Evans and Cook 2002).

Other methods traditionally used to estimate recharge involves using field infiltration measurements and hydrograph interpretation, and extrapolating to areas of similar geology. Although this method is cheap and easy, it is not well suited for making allocations because estimates are limited by the accuracy with which specific yield can be determined, and the validity of the assumptions inherent in the method (Healy and Cook 2002). In addition, the method assumes that fluctuations in the water levels are solely due to recharge, when they may also be a result of changes to atmospheric pressure or other phenomena such as groundwater pumping (Healy and Cook 2002; Scanlon, Healy et al. 2002). Further this method does not take into account the dynamics of the catchment groundwater flow system, so, in areas where episodic recharge occurs recharge is significantly over-estimated, leading to over-allocation of water resources (Evans, Coram et al. 1998; Evans and Cook 2002; Scanlon, Healy et al. 2002; Sophocleous 2002).

Analytical models are another tool used to estimate water resources. A frequent misuse of analytical models involves incorrectly applying models to aquifer systems that assume the aquifer systems to be in a steady state (equilibrium) where recharge equals discharge (Evans and Cook 2002). This is certainly not the case for an aquifer system where groundwater is hundreds of thousands of years old, recharged in a previous climate, and where more water may be discharging from the aquifer than what is recharging.

The errors associated with methods traditionally used to estimate water resources in Victoria underscore the need to investigate different approaches that increase the confidence in estimates. In this paper I will investigate how innovative methods, such as 3D catchment geology models used in conjunction with multiple independent approaches, can help identify complex hydrological processes and provide estimates of water resources with a level of accuracy previously not achieved.

Approaches that could be used to improve the accuracy of water resource estimates

In the past decade, there have been numerous advances in techniques used to estimate water resources. Some of these include using multiple isotope tracers, integrating analytical models, calculating estimates at the catchment scale, and using 3D geology models.

Use of geochemical methods

Geochemical methods often use major ions and/or radioactive isotopes to investigate contaminant transport pathways and the residence time of groundwater flow systems that are unique to each catchment. Information generated from these methods cannot be derived from the physical hydrogeologic methods traditionally used to estimate recharge. The value of geochemical methods was recently highlighted where hydrogeologic investigations interpreted a geologic formation between saline and freshwater aquifers to be virtually impermeable (aquitard). This meant that no mixing occurred between these aquifers (Lawrence 1975). The use of geochemical analysis showed that there can be considerable mixing between saline and freshwater aquifers (Weaver, Swane et al. 2002) and water was transported from surface hypersaline lakes down to the freshwater aquifer, to a depth of approximately 200 m (Weaver, Swane et al. 2002).

Estimating the age of groundwater using multiple isotope tracers is another application of geochemical techniques. Groundwater recharge and discharge rates and groundwater flow velocities can be determined more accurately than traditional hydrologic methods (Cook, Bohlke et al. 2002). As there is a level of error associated with each tracer technique, multiple tracer techniques are often used to check the accuracy of results from different techniques (De Vries and Simmers 2002; Scanlon, Healy et al. 2002). Multiple isotope tracing is a powerful tool for quantifying the speed of catchment hydrologic processes and helping anticipate consequences of changed land management.

Integrating multiple analytical models

Analytical models are frequently used to estimate water resources and their sources, and the transport mechanisms of contaminants. However, an innovative approach to using analytical models has been developed where different models at different scales have been combined to increase reliability of results (Sophocleous and Perkins 2000). This integration of models provides a framework that can be used to check continuity and better constrain model parameters by calibrating them against multiple targets. More reliable results are obtained than when contaminant transport models are used separately. A key advantage of linking these models is that they require appreciably less input data and readily available data can be used (compared to fully distributed, physically based models) (Sophocleous and Perkins 2000). This approach also enhances model calibration and thus the reliability of model results (Sophocleous and Perkins 2000).

Calculating estimates at the catchment scale

In Victoria water allocations are estimated within a groundwater management area (GMA), which is a restricted area where recharge estimates cannot be validated because outside influences are not taken into account. In contrast, if water allocations were calculated at the catchment scale that is the natural boundary for surface water and groundwater flow, this would enable estimates to be validated.

3D catchment geology models

An innovative method that has been developed to more accurately estimate water resources is the use of a 3D catchment geology model in combination with a catchment water balance, geochemical techniques and analytical modelling. These methods have been successfully used in Alberta, Canada to identify and investigate complex catchment hydrologic processes that link surface water and groundwater, as well as aquifers.

3D geology maps provide a powerful tool for analysing complex aquifer systems. Maps are constructed by incorporating geology maps, cross sections, geology logs and geophysical surveys. Interpolated surfaces are created from these data and then stacked to develop a 3D map of the aquifer geometry. Benefits of using these models include;

• explicitly define the geometry of aquifer systems;
• providing valuable information to a wide variety of government and private organisations for the purpose of extension, regulation, policy development etc;
• being able to incorporate a variety of data in different formats and at different scales including tacit knowledge that has been built up within the catchment by experts over time, thus enabling water resource estimates to be calibrated against multiple independent approaches;
• providing valuable information at both local and regional studies;
• allowing resolution to be repeatedly improved as more information becomes available;
• an ability to identify and target priority areas for more detailed investigation, to highlight knowledge gaps and to communicate complex conceptual models in a sophisticated manner; and most importantly
• allowing simultaneous analysis of multiple data sets, which enables efficient and effective synthesis of information, compared to current methods where data sets are analysed separately.

A disadvantage of the use of this method is that it requires specialist knowledge to develop the 3D geology model.
The use of 3D visual models for water resource management purposes is only in its infancy, with the United States and Canada leading the way. Trials have been conducted in the United States and Canada using a combination of software packages. Types of software available include;

• Rockware (Rockworks) Goldern Colorado USA;
• EarthVision geologic modelling software;
• ESRI Arc view 3D analyst;
• Gocad; and
• View Log

A 'Three-dimensional mapping for groundwater applications' workshop (Thorleifson and Berg 2002) was held in Denver Colorado, October 2002. This workshop drew together water managers, geologists and groundwater modellers from across the United States, Canada and Scandinavia to present different methods used to build 3D geologic models, and to discuss the intricacies of data acquisition processing, model construction and model application. The Canadian Federal Government is currently deciding on the standard 3D geology model software that will be used nationally for this purpose¹¹ Harvey Thorleifson, Geological Survey of Canada, Ottawa (3/2/2003) pers. comm.


Discussion

For most aquifer systems within Victoria, little is known about the extent of the water resource, how long current extraction rates can continue, and what are the long-term repercussions of current water extraction regimes. There is an urgent need to more accurately define water resources.

No single method is capable of resolving dynamic hydrologic processes unique to a catchment. The uncertainties of each method underscore the need to apply multiple methods to increase the confidence in estimates. Estimating water resources and defining the contaminant transport is an iterative process that includes refining estimates as additional data are gathered.

The use of 3D catchment geology models in combination with a catchment water balance, geochemical techniques and analytical modelling can help in identifying and investigating complex catchment hydrologic processes that link surface water and groundwater, as well as between aquifers. This approach to documenting water resources provides a mechanism by which water resource estimates can be verified and continually improved, as more data becomes available. It provides an innovative solution to resolve conflicting pressures on water resource management issues in Victoria and by improving the accuracy of water resource estimates, preventing adverse impacts that may result from over allocation.


Acknowledgements

Thanks are due to the Science Quality Unit, Department of Primary Industries and Primary Industries Research Victoria, Bendigo for their financial support of the study tour of North America.

The author gratefully acknowledges funding support from the Wimmera Catchment Management Authority, Horsham, Victoria for the development of this paper.

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