Background

Groundwater has always been a valuable resource in the Strathbogie Ranges and with the seasons and rainfall now becoming much less predictable, its importance is only increasing.

Here in the Strathbogies groundwater sits on top of the undulating, fractured granitic rocks that comprise most of the ranges. This groundwater is held in aquifers, geological rock structures that enclose adequate saturated and permeable materials capable of delivering considerable water quantities to natural springs and wells constructed to access the groundwater. The material that forms most of the land surface in our ranges is weathered granite (regolith), with some organic matter in the top few centimeters (soil). This granitic regolith is gritty and friable, as sand (or quartzite) is a major component of granite and is very resistant to weathering. Most of the other minerals that comprise granite weather more quickly than quartzite and turn into the silt and clay that are the finer parts of this regolith. Because there is no impermeable clay layer above the groundwater, the aquifers in the Strathbogie Ranges are described as unconfined fractured rock aquifers.

Groundwater recharge

The granitic rocks and boulders we see at the surface are heavily weathered and it’s this weathered material that makes up the regolith; the finer material (eg weathered mica, feldspar) has been washed down slope, or even into nearby streams and wetlands.

Granite tors on Mt Tel show how wind, rain and climate extremes weather the granitic rock.

Water gets in to these fractured rock aquifers where the rock is exposed or close to the surface – mostly elevated, rocky sites. These are areas of groundwater recharge. Where the regolith is more than a few meters deep, most rainwater is soaked up by the soil and held as soil moisture (water that evaporates and transpires from the soil and vegetation, into the atmosphere). Once part of the groundwater, water can move considerable distances through the regolith and deep rock fractures, from hundreds of meters to many kilometers.

For example, along an east-west transect from Strathbogie township to Bald Hill (Kelly’s Lookout), there are many areas of exposed rocks, all of which are sites of groundwater recharge. Around and in between the recharge areas lie the groundwater aquifers.

A schematic depiction of recharge zones and groundwater aquifers on the Strathbogie Tableland

Groundwater flow

Most of the flow of water in the fractured rock aquifers of the Strathbogie Ranges occurs along the fractures and faults in the bedrock and through the weathered surface of the bedrock that underlies the regolith. The amount and speed of groundwater flow depends on the size of the faults/fractures and how well connected they are.

The groundwater in the Strathbogies can be thought of as a series of underground reservoirs of different size and with different water levels. All the reservoirs, or aquifers, in a particular area are, to a greater or lesser extent, connected because water flows laterally, or sideways, between the aquifers. The rate of sideways flow is usually fastest in the deep fracture system and least in the tightly packed regolith above the granite. In fact, groundwater in the ranges is always flowing (vertically and laterally), thanks to gravity and differences in hydraulic pressure.

Fractured Rock Aquifer Schematic
This diagram shows the general relationships between geology and groundwater in a granitic fractured rock aquifer.

Groundwater discharge

Because these fractured rock aquifers are unconfined, groundwater doesn’t always remain underground; in places it leaks out sideways into streams and wetlands. It’s this flow out of the ground – groundwater discharge – that creates the features so characteristic of the Strathbogie Ranges – spring-soaks and wetlands. These groundwater discharge sites were once also the favoured places for construction of farm dams, not just because there’s often a plentiful supply of water, but it’s where the clay minerals (from the weathering of the granite) accumulated – the dams held water!

[For more information on how geology creates and influences wetlands in the Strathbogie Ranges, see: The hydrology of wetlands in the Strathbogie Ranges – report]

Groundwater discharge, not rainfall run-off, is what keeps the spring-fed dams and wetlands full and the streams running – all through the dry, hot summer. At least, that’s what we’d become used to. But in the last 20 years the number of low-flow and no-flow events, even in our most reliable streams, the Hughes Ck, Seven Creeks, Honeysuckle Ck etc, have been increasing. This causes problems, not just for humans wanting to pump from streams, but for the health of the stream itself and all the animals and plants that depend on that flow.

ALL of the flow in the Seven Creeks at Polly McQuinn’s in Dec. 2007, the height of the Millenium Drought (above), came from groundwater!

Understanding groundwater

At a time when demands on groundwater are growing (the combination of stock, domestic and irrigation) and there is potentially less recharge occurring (due to reduced rainfall), the impacts on humans, industry and the natural environment are likely to increase. More than ever, we need to improve our understanding of groundwater in the Strathbogie Ranges.

One of the more common concerns for landholders is whether groundwater pumping in one area will impact on the groundwater nearby? Or, to rephrase the question, are Strathbogie Ranges aquifers egg cartons or bathtubs? These are legitimate concerns and another good reason to improve our understanding of groundwater in these ranges.

This post is part of the Bogies and Beyond Groundwater Monitoring Project, funded by the Victorian Government and managed by the Goulburn Broken Catchment Management Authority.

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