On Friday 9th February 2013, Hills’ community member and secretary of the Darling Range Branch (DRB) of the WA Naturalists’ Club, Dr Arthur Conacher (retired UWA geographer), presented a talk titled “Researching the salt problem in the Southwest of WA.”
Arthur has mentored and supervised several students researching aspects of the salinity problem in the WA wheatbelt. His illustrated talk covered aspects of the causes of the salt problems in the land, commencing with an overview of some of the problems and examples of practical work done with students on fieldwork, initially in the York-Mawson and Dalwallinu-Pithara areas during the period 1970-1973, through to the early 21st century. Accompanying photographs demonstrated some the effects of salt in WA landscapes, and some of the remedial projects that have been trialled.
Evidence of increasing soil and water salinity is widespread and is one of our most significant environmental problems. Two main types of salinity are recognized; primary and secondary — either occurring naturally or resulting from human activities, respectively. Primary salinity occurs naturally in soils and waters such as salt lakes and secondary salinity is the result of human activities, usually land development and agriculture. Secondary salinity from dryland agriculture (not irrigation) was the focus of the talk.
Within the south west of WA main rivers where water has become too salty for human consumption or irrigation include the Frankland, Warren, Blackwood, Murray, Avon and Moore. Fresh water can only be found in upland headstreams from forested areas. It was significant to the problem to know of some of the broader changes since the early days of settlement in WA; examples included the use of fresh water for orchards at New Norcia in the 19th Century and the fact that some 40 wheat belt towns are experiencing salinity problems.
Although the salt problem was noted throughout European settlement of WA, research into salinisation of land and streams and its causes only began in the 20th Century. One of the most important research reports was published by Wood in 1924, in which it was recognized that removal of native vegetation and clearing land for agriculture were the prime environmental disturbances that caused the salinity problem. Land clearing for farming reduces plant transpiration and increases the movement of water in the soil landscape, eventually leading to the formation of salt scalds (unproductive land). The salinity problem is complex and has involved diverse researches to explain how water moves to redistribute salt in the landscape, and what causes salts to concentrate in particular locations. By 1994 it was reported that 1.8 million hectares (ha) of previously productive land was salt-affected with current predictions quoted between 3 and 6 million ha.
In particular studies on fingertip streams (sites of stream origins) it was shown that from an ‘at source’ concentration of total soluble salts of 50 mg/litre, salinity can progress to concentrations of 90 thousand mg/litre downstream. Soil studies from pit sections showed the relationship of near-surface soil-water and a deeper salty zone at greater depths. One pit section, as an example showed that at a depth of about 2.5 m a clay zone provided a barrier to water infiltration, with a perched water table above it. The clay barrier prevented mixing of water between the perched water and the salty, deeper groundwater aquifer below, which is under pressure.
As had been recognised by Arthur and his students, Harry Whittington independently discovered the importance of ‘throughflow’ in relation to the salt problem. His work was reported in the “Countryman”, leading Arthur to arrange a meeting. Following advice from the US Department of Agriculture, from 1954 Harry had trialled what are referred to as “interceptor banks” on his own degraded land at Springhill, Brookton in Western Australia. His work led to the formation of WISALTS in 1976 (Whittington Interceptor Salt-Affected Land Treatment Society), an association of farmer members promoting his technology (termed the Whittington Interceptor Banks, or Harry Banks). By 1980 there were some 1000 financial members of WISALTS, representing about a third of all wheatbelt farmers with a salt problem.
Harry’s work showed how the problem of throughflow of water could be controlled by sealing fields with interceptor banks. This permitted water to be captured where it falls by controlling sub-surface throughflow, preventing waterlogging of low-lying areas and remediating salt-affected land. Seepage water from productive land then flowed into specific drainage zones or farm dams. Various follow-up researches by Arthur and his students in 1977 and 1981 tracked the efficiency of such interceptor banks, finding mixed results. It seemed that controlling throughflow alone was insufficient and that in many locations there was mixing between the relatively fresh throughflow and perched water systems, and the deeper, saline groundwater which is often under pressure. Nevertheless the principle of intercepting and using rain where it falls, before it can infiltrate to depth and cause the salt problem, remains valid. The question is: how to attain that objective.
Several other research programs were outlined to the audience. In the Darling Ranges and the Collie catchment, CSIRO, the former Public Works and Forests Departments conducted many research programs, partly in relation to land clearing for the woodchip industry and mining for bauxite. All showed the importance of more than one level of subsurface water, both fresh and saline. CSIRO and Public Works Department projects in the Collie catchment area showed that throughflow was the dominant hydrological mechanism in those landscapes and that there was mixing of deep and near-surface waters.
This mixing of near-surface and deeper groundwater and the hydrological importance of throughflow was confirmed by Richard George’s PhD research in the late 1980s (UWA geography), initially based on a detailed drilling program near Narrogin and later (1990s) extended throughout the wheatbelt in his work with the Agriculture Department.
Further work was carried out by Jessica Lothian in 2003 into the Water Corporation’s five ‘recovery catchments’, again revealing mixed results. Currently the Water Corporation is thinning vegetation in the Wungong catchment to increase runoff in spite of possible ecological effects and the consequences for salinity. This underlines some complex issues and conflict of interests in planning.
Many aspects of salinity problems were introduced in the talk and demonstrated how constructive strategies can only be developed and financed with co-operation between several Government Departments, farmers, and other landholders.
The following objectives were set out as proposals that could provide positive action and solutions to some of the interrelated problems of soil and water salinity:
- Revegetating catchments with native vegetation where feasible
- Establishing salt tolerant plants on salt land
- Pumping and deep drainage – which works only under certain conditions but causes problems with dumping salty and acidic water amongst farming neighbours and in wetlands
- Valley-side interceptors are partly successful
- Planting high transpiring crops in catchments
- Combining several of these solutions.
There is increased public demand for supplies of potable water within the State together with a situation of increasing soil and water salinity problems and a variable rainfall regime. The financial burden to agriculture becomes obvious but solutions are not straightforward.
Susan Stocklmayer and Arthur Conacher