Groundwater concepts PDF (41kb)

What is groundwater?

Rainfall that enters the soil and drains down past the plant root zone becomes part of the groundwater system. The nature and movement of underground water is usually determined by drilling boreholes and measuring the static water levels.

Groundwater movement

Contrary to popular belief, groundwater does not generally flow as underground streams except in the case of cavernous limestone aquifers (e.g. as found on the Nullarbor plain).

More commonly, groundwater seeps very slowly through fine fractures and joints in rocks or through voids and pores in sandy and clayey sediments. If a particular layer is very permeable it is termed an aquifer and yields useable supplies of water to pumped bores.

The upper surface of the groundwater system is called the watertable. It usually follows the ground surface, at some depth below it, being deeper under hills and shallower under valleys. Groundwater flows from recharge areas located higher up in the landscape toward discharge areas at lower topographic levels.

Groundwater flow rates can vary from less than one metre per year (e.g. in clays) to a couple of hundred metres per year (e.g. in porous limestone).

Groundwater systems

Permeable layers that conduct groundwater beneath the surface are called aquifers. Various types of aquifer can develop depending upon geology, topography and climate, and include:

Fractured rock aquifers develop when groundwater travels along joints and fractures within hard rock. These aquifers are commonly found in uplands regions. The permeability of the aquifer is dependant upon the extent of fracture systems and degree of weathering in the rock.

Sedimentary aquifers develop in permeable layers within deposits of gravels, sands, silts and clays that are often found beneath riverine floodplains and within sedimentary basins.

Unconfined aquifers behave like a bucket or sponge being filled with water. Recharge occurs generally across the landscape and discharge occurs when the watertable comes close to the land surface.

Confined aquifers are bounded between upper and lower layers of impermeable clay or rock. They have a well defined recharge area where aquifer materials outcrop in the highlands. These aquifers are usually under pressure and discharge may occur as artesian flow from springs (e.g. “seepage eyes” and mound springs).

Perched watertables consist of a local watertable (often seasonal in nature) separated from the deeper groundwater by an unsaturated layer of low permeability.

Aquifers form into groundwater flow systems (called a GFS) and these systems are defined according to the scale at which they operate and include:

Local flow systems occur in hilly areas where groundwater recharges on the hills and discharges into the adjacent valleys. Recharge and discharge areas are separated by a relatively short distance ranging up to a few kilometers.

Regional flow systems occur on extensive plains that are underlain by deep sedimentary basins (e.g. Great Artesian Basin and the Murray Darling Basin) where groundwater can travel over large distances (e.g. 50 km) and at great depths.

Intermediate flow systems lie somewhere between local and regional systems and flow paths can range from 5-50 km.

Recharge and discharge

Recharge is unused rainwater which moves down through the soil past the root zone to the watertable, causing it to rise. Discharge is the process whereby groundwater escapes to the surface by evaporation, transpiration from vegetation, or from springs or seepage zones where dryland salinity may then result.

While the actual physical movement of water between recharge and discharge areas is very slow, it is the hydraulic pressure that drives the system. Adding more water to recharge areas increases the hydraulic head which pushes more water out at the lower end (as saline discharge). It is therefore important to manage recharge since this is the primary cause of dryland salinity.

The recharge area

Recharge can occur over the whole landscape, although some areas may drain more readily to the watertable than other areas. These are known as high recharge areas and are priority areas for management in saline catchments. Creeks, rivers, lakes and wetlands can also act as recharge areas to groundwater.

Detecting high recharge areas

Areas of higher recharge can be identified by using soil, landscape and vegetative indicators such as:

• ground surfaces that allow water to soak in rapidly (e.g. sandy, gravelly, rocky surfaces),
• freely draining subsoils (e.g. deep sands or well structured clays with old root channels),
• areas where water concentrates and drains rapidly (e.g. depressions, gravelly stream beds),
• unarable and degraded landscapes with low vegetative cover (rocky ridges, eroded sandhills).

Last update: June 2007

Author: Chris Henschke, Salinity Program Consultant, Rural Solutions SA

Agdex: 552

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