Framework

Hydrogeological Mapping

Knowledge of the hydrogeological setting is critical in understanding groundwater-surface water interactions. This involves mapping the configuration and characteristics of groundwater flow systems within the catchment, covering aspects such as aquifer geometry, geological and stratigraphic configurations, hydraulic properties such as transmissivity and storativity, and recharge and discharge mechanisms. A hierarchy of groundwater flow systems of different size and depth can develop in a catchment depending on the combination of surface topography, geology and climate (Figure 1). There can be a combination of groundwater flow systems that are local, intermediate or regional in scale (Toth, 1963). Local flow systems are the shallowest and most dynamic, involving short flow paths (mostly < 5km) with groundwater discharging to the nearest lowland feature. In contrast, regional flow systems have the deepest and longest flow paths (typically exceeding 50km), with intermediate systems operating between these two end-members. Local flow systems tend to be dominant in areas of high topographic relief, while intermediate-regional systems are more evident in flat-lying areas. Groundwater exchange with surface water features are primarily governed by their location with respect to groundwater flow systems, the geological characteristics of their beds and climatic factors (Winter, 1999). River reaches can receive contributions of groundwater from flow systems of different scales and provenance (Figure 2).

Figure 1: Different scale groundwater flow systems within a catchment (Winter et all, 1998, after Toth, 1963)

Figure 2: Groundwater flow systems operating within an alluvial riverine valley (Winter et al, 1998)

Groundwater flow system mapping is available at a national level as well as at the catchment level. The National Land and Water Resources Audit provides access to a national coverage of groundwater flow system mapping based on recharge and flow behaviour and using a combination of geology, geological and topographical criteria (Coram et al, 2000). Local, intermediate and regional groundwater flow systems are identified across a range of geological and geomorphological terranes. The focus of the mapping was to predict how groundwater systems respond to changing recharge and for defining appropriate dryland salinity management options. Such mapping is useful in defining the provenance and time-scale of groundwater movement, but is not specific in terms of groundwater-surface water interaction.

Traditional hydrogeological maps are available for certain groundwater management regions across Australia and are published at 1:250,000 scale or more detailed, refer Hydrogeology Data. These maps depict various combinations of hydrogeological parameters such as:

1. groundwater availability, in terms of bore yield;
2. groundwater quality, typically salinity;
3. potentials, in terms of depth to watertable, elevation of groundwater surfaces, groundwater flow paths, freshwater head difference between aquifers;
4. aquifer hydraulic properties like estimates transmissivity and storativity; and
5. aquifer structure, typically aquifer boundaries, structural contours of the aquifer top and base, isopachs of aquifer thickness, and specific features such as faults.

Such information provides a useful context when evaluating the extent and direction of groundwater-surface water exchange. However, this perspective of the extent of hydrogeological mapping across Australia is by no means complete. The published hydrogeological map is only a small subset of a vast repository of mapping that has been undertaken, with mapping embedded in journals, reports, unpublished consultancies, research theses and management plans.

In the absence of specific hydrogeological mapping, more generic geological information is useful at the stream-reach level as well as the catchment level. A geological understanding of the catchment is a precursor for developing a conceptual understanding of groundwater processes. Identification of geological structures such as faults or basement highs that control the geometry and hydraulic properties of aquifers is particularly important. Stratigraphic information such as the distribution of low-permeability clay layers or paleochannels that act as preferential pathways is used to map variability of stream-aquifer connectivity. For example, connectivity in the Cudgegong Valley (near Mudgee, NSW) is controlled by geological features (Hamilton, 2004). Bedrock constrictions or faulting restricts groundwater throughflow in the alluvial aquifer resulting in shallower watertables and gaining conditions in the river. Away from these geological features, the regulated Cudgegong River is largely a losing stream.

Mapping of geomorphological features have also been used to characterise connectivity. Analysis of the geomorphology of some North American alluvial aquifers inferred a relationship between dominant groundwater direction and parameters such as channel slope, sinuosity, incision and channel width-to-depth ratio (Larkin & Sharp, 1992). Groundwater was dominantly lateral (underflow conditions) in alluvial systems with large channel gradients, small sinuosities, large width-to-depth ratios and low river incisions, with the alluvial systems with opposing attributes dominated by vertical groundwater flow (gaining or losing conditions).

Advantages

1. an understanding of the hydrogeological setting is critical in defining connectivity between streams and aquifers;
2. hydrogeological mapping is an important part of developing a conceptual model showing the broader perspective of the nature and configuration of groundwater systems, the scale and direction of groundwater flow, geomorphological features and the hydraulic properties of aquifers.

Disadvantages

1. compiling and interpreting hydrogeological data can be time consuming and complex. It can involve interpolation of limited data from a sparse network of bores, so is subject to misinterpretation;
2. a working knowledge of hydrogeological principles is required.

Data Availability

Refer Hydrogeology Data

Case Studies

Alstonville Plateau, NSW

References

Coram JE, Dyson PR, Houlder PA and Evans WR, 2000. Australian Groundwater Flow Systems contributing to dryland salinity. Report by Bureau of Rural Sciences for National Land and Water Resources Audit, Canberra.

Hamilton S, 2004. River-groundwater interactions in the Cudgegong Valley, Mudgee NSW. In: Conference Proceedings, 9th Murray Darling Basin Groundwater Workshop Bendigo, Victoria 17-19 February 2004.

Larkin RG and Sharp JM Jr, 1992. On the relationship between river-basin geomorphology, aquifer hydraulics and groundwater flow direction in alluvial aquifers. Geological Society of America Bulletin 104(12): 1608-1620.

Toth J, 1963. A theoretical analysis of groundwater flow in small drainage basins. Journal of Geophysical Research 68, 4785-4812.

Winter TC, 1999. Relation of streams, lakes and wetlands to groundwater flow systems, Hydrogeology Journal 7, 28-45.

Winter, TC, Judson, WH, Franke, OL and Alley WM. 1998. Groundwater and surface water a single resource. Circular 1139, U.S. Geological Survey, Denver.