Dealing with salinity in Wheatbelt Valleys - Department of Water

Pannell
Proposals to construct major regional engineering
systems, fed by pumping from agricultural land,
have been made (Belford 2001; Thomas &
Williamson 2001). Particularly in view of the very
great resources involved, these proposals would
need very careful investigation before funding
could responsibly be provided.
Infrastructure
The impacts of salinity on built infrastructure have
received increasing attention. According to the
National Land and Water Resources Audit (2001),
assets across the country at high risk from shallow
saline watertables by 2050 include 67,000 km of
road, 5,100 km of rail and 220 towns.
Rural towns
Low water “transmissivity” of soils was described
earlier in relation to agriculture. It also has
important implications for rural towns on valley
floors. It is estimated that it would take
3000 years for groundwater to move from the top
of the Merredin catchment to Merredin town
(Matta 1999). Clearly, the only land that has
contributed groundwater directly to Merredin
town site in the 100 years since the region was
developed is land in or very close to the town site.
In particular, areas such as roads are an important
source of recharge waters. Hydrologists
recommend that the most important and effective
treatment for preventing salinity damage within
town sites is reducing recharge within the town
site, and/or enhancing discharge in and around the
town by engineering treatments, such as pumping
(Matta 1999; Dames & Moore – NRM 2001). It is
believed that, in most cases, benefits from
revegetation of surrounding farm land will be
insufficient and/or too slow to prevent major
damage to town infrastructure.
For towns such as Merredin, which have fresh
water piped to them for domestic use, the
problem is exacerbated by release of this imported
water into the ground from garden irrigation
systems or septic tanks. For some towns in
Western Australia (e.g. Cranbrook, Tambellup),
imported water and runoff from roofs and roads
accounts for a substantial part of the groundwater
rise within the town.
The Rural Towns Program is concerned with
42 WA towns facing salinity impacts. A number of
these towns have been subjected to hydrological
studies to identify systems of intervention which
– 6 –
would be needed to reduce the impacts of salinity,
and for six of them, detailed economic analyses of
these interventions have been conducted. These
are very important studies and they have major
implications for the management of salinity in the
towns. Some of the common findings from the six
towns are listed below, drawn from the report by
Dames & Moore – NRM (2001).
In low-lying towns like Merredin and Katanning,
groundwater beneath the towns has low rates of
lateral flow through material of low transmissivity.
The implication is that actions taken to reduce
groundwater recharge higher in the catchment will
have very little impact on groundwater behaviour
beneath the town within time frames needed to
prevent damage. The corollary is that actions to
prevent groundwater rise or to lower existing
levels need to occur within or immediately
adjacent to the townsite.
• Surface water management within the towns
is inadequate. In some cases, observed
damage is being caused by poor domestic
water management or a lack of sealed
drainage, rather than rising groundwaters.
• Roads are the biggest cost item from salinity in
the towns, amounting to about 60% of the
total.
• There is great variability in the situation in
different towns. Towns need individual
investigation and advice in determining the
most appropriate course of action. Without
specialist professional input, affected towns
are unlikely to grasp the approach to urban
water management that is needed.
Some of the actions recommended by the
consultants are cheap and could be taken up
immediately (e.g. appointment of “Water Wise”
coordinators to provide advice to householders,
businesses and builders). Nevertheless, managing
rising groundwaters effectively in most of the
towns will require expensive engineering works.
In some of the towns, the cost of the
recommended works is so high that it outweighs
the potential salinity damage costs which would be
avoided, implying that living with the salinity
damage may be more economically efficient than
attempting to prevent it. This is apparent in
Table 1, which shows a summary of the economic
analysis for each town. The costs shown are total
costs over 30 or 60 years, discounted to present
values using a 7% discount rate.