Processes and activities that threaten groundwater
include water resource development, agricultural land use, acid
sulphate soils, urban and commercial development, mining, and
plantation forestry.
Overuse of groundwater can dehydrate many ecosystems by lowering
the water table so that many plants can't reach it. Overuse also
reduces the quantity of water that seeps into rivers, and destroys
habitats in caves and aquifers.
Diversion or impoundment (i.e. dams) of surface water can raise
the water table and cause similar problems. Some groundwater-dependent
species may be advantaged, some may die from water logging. Groundwater
may contain salt and, if it rises, the salt may come to the surface,
causing soil salinity that can kill plants, and cause the loss
of native fauna that depend on those plants.
Intensive agriculture often has similar implications for the
level and quality of groundwater. The groundwater level rises,
causing irrigation salinity, changes in vegetation cover, and
a disruption to the recharge-discharge relationship in the catchment.
This can have serious consequences for birds, bats, mammals and
reptiles that rely on small pockets of native vegetation in rural
areas. A further threat is agricultural chemicals that can contaminate
groundwater.
Excavation during construction and lowering of the water table
can activate acid sulphate soils and severely degrade aquatic
ecosystems. Acid sulphate soils are wetland soils and unconsolidated
sediments that contain iron sulphide. When covered permanently
with water, the iron sulphide is stable and the soils are weakly
alkaline. However, when the water table is lowered, the iron sulphide
is exposed to oxygen, oxidises and, in the presence of water,
forms sulphuric acid (Powell and Ahern 1997). This process has
a disastrous effect on aquatic ecosystems, poisoning them with
heavy metals, acidified water, and iron.
New urban or commercial developments can lower and raise groundwater
by increased domestic watering, and recreational and industrial
uses. In turn, native vegetation and wetlands are threatened,
and salinity increases. Urban development can also affect the
quality of groundwater by effluent from septic tanks seeping into
it, leakage from underground fuel tanks, and the use of chemicals
and fertilisers.
Mining and residential development often pump large quantities
of water from aquifers. This can lower the level and reduce the
flow rate of underground water, as well as reducing aquifer pressure.
Where underground water has different water quality (pH, salinity,
chemical composition) at different depths, mining may also alter
water quality and change cave and aquifer ecosystems.
Plantation forestry reduces the rate of runoff, stream-flow and
groundwater recharge, and the pressure of underground water. All
of these may affect groundwater-dependent ecosystems in similar
ways to those mentioned above. However, the lowering of groundwater
levels may be beneficial to ecosystems where the water table has
been unnaturally elevated by irrigation.
