The Chief Executive Officer of the Department of Agriculture and Food and the State of Western Australia accepts no liability whatsoever, whether negligent or otherwise, arising out of the use or release of this information or any part thereof. Don Bennett and Richard George of the Department of Agriculture and Food, Bunbury and Neil Lantzke of the Department of Agriculture and Food, South Perth provided both technical advice and references. Jane Kowald, of the Department of Agriculture and Food, Katanning undertook a technical review of this document.
Introduction
Geographic distribution of salt risk in south-west western australia
The above observations suggest that areas east of the Darling Scarp and low in the valley floor are at particular risk of salinity and, as a result, dams dependent on runoff from these areas are also at risk of salinity.
Sources of salt in dams
The salt load from surface water is dependent on the extent and severity of salt-affected soil in the dam's catchment area. If the catchment contains areas affected by salt, the surface salt can be dissolved by surface currents and deposited in the dam. The location and severity of many seepage areas on hillsides in the western part of the region's medium rainfall zone are associated with dikes and faults.
Management options for salinity in dams: site selection
Again, it is preferable to place the pond well clear of water tables and in soil that minimizes leakage to avoid this risk. Soil with a clay content of at least 35 percent (Stanton 2005) helps reduce the loss of water from the pond through leakage, while sand seams and stones increase the chance of leakage. Leakage from dams reduces the volume of storage, which in turn increases the surface area to volume ratio of the stored water, thereby increasing the proportion of water that evaporates and thus the concentration of salt in the dam.
Some spreading is beneficial as this can reduce soil porosity (Bennett et al. 2004, Heath & Raper 2005) and help seal the dam. Where site constraints require a dam to be constructed in sandy soil, clay material can be brought from off-site to serve as a liner for the dam (Coles 2003). The investment amount for site assessment should be proportional to the size of the proposed dam.
Due to the sealing of existing dams, the dam may need to be emptied; it can be expensive and it is often more practical and cheaper to build a new dam if another site is available. Similar to the paddock, scrubbing resulting from heavy livestock traffic can damage the soil structure in the dam and help seal the dam as the water level recedes in the spring, summer, and fall (Coles 2003). A source of suitable clay near a leaky dam can be used to lay a compacted clay blanket and economically seal the dam.
Bentonite swells when wet and provides an effective seal provided it is spread evenly over the wet area of the pond.
Maximising inflow of fresh water to dams
Areas in the northern and eastern wheat belt have adopted watershed over roads as standard practice due to their ability to generate runoff from low-intensity rainfall that is common in these areas. More information can be obtained from Stanton (2005), the surface water management group of the Department of Agriculture and Food, or the farm water group of the Department of Water. This runoff may be discolored by oxidizing bitumen, but this is unlikely to be a problem if the water is not intended for human drinking (Lewis 2002, Richardson et al. 2004).
Department of Agriculture and Food land and water development officials can provide design advice. While overslope channels can be used to convey runoff to a dam, Laing (1981) discusses the use of diversion banks to capture water that would otherwise bypass dams. Recent work by Rod Short and Neil Lantzke at the Department of Agriculture and Food in Western Australia has shown that some surface sealers improve the efficiency of catchments with clay lined roads.
The previous options all involved investments in the construction of works to improve the drainage of a catchment area. But there may be an opportunity to take advantage of high runoff features that already exist in a watershed, or that have been constructed to serve a different purpose. Granite rocks do not support other productive land uses, and their runoff has been used to supplement the city water supply of several wheat belt towns since the 1930s (Davis 1977).
The galvanized sheet metal commonly used for shed roofs has a run-off threshold of 1.65–2.5 mm (depending on the slope) and a very high efficiency (90–100 percent).
Discharge management: reducing salinity in surface flows
The nature of the aquifer (expected yield, water quality, disposal options) can be assessed by a qualified hydrologist. The time taken to fill the test trench with water will give an indication of the likely potential to drain the soil. Soils that are prone to spreading should be treated with gypsum to help maintain soil structure, as salt removal via drainage and refreshing the soil profile with rain infiltration can cause soil pores to collapse, effectively reducing the effectiveness of the drainage system (Bennett et al . . 2004).
The best results were achieved on lowlands on the coastal plain of the southwest (Bennett et al. 1999). Use perforated well screen and gravel pack in the bottom 6 m of the bore, and PVC pipe to line the rest of the bore (see Figure 4). If infiltration rates of the soil in the seepage areas are high enough to flush the salt through the soil profile, the salt can be removed from the soil surface.
Use a sieve with a perforated well and gravel in the bottom 6 m of the borehole, and a PVC pipe for the rest of the borehole (Figure 6). A siphon (a fully charged pipe) is inserted into the borehole and acts as a passive vacuum pump as the weight of the water under gravity in the pipe draws water out of the borehole. It is important to note that this system requires the removal of water down the dam slope in an environmentally friendly manner and that any engineering works need to be transported.
Secondary salinization of the soil surface due to capillary rise and evaporation during the dry season results in a 'first flush' of surface water (after rainfall during a summer storm or at the turn of the season) that transfers a significant proportion of the annual salt load to the row drainage. Surface flows after this first flush are often of better quality and are likely to continue to be fresh until low flows return at the end of the rainy season. This can be done by manipulating the inflow to the dam using a weir, a diversion channel, and some method of measuring the salinity of the surface water above the weir.
Management options for improving water quality in dams
This can cause problems if the dam water is rich in dissolved iron and the water must be piped or used for irrigation. The shape of the dam can have a major influence on the amount of evaporative loss (Stanton 2005). This fact also points to the importance of dam maintenance, as silt accumulation in the bottom of dams reduces the 'usable depth' while at the same time decreasing the volume to area ratio of the dam.
Once saltwater has entered a dam, it does not necessarily mean that the quality of all the water in the dam will be compromised. This provides an opportunity to rinse the salt water from the bottom of the dam while the fresh water at the. The vacuum is then used to draw water against gravity from the bottom of the dam.
It is important that the pipe outlet is not too far below the high water table in the dam, as otherwise too much water can be sucked from the dam. Sieves can be primed by filling them with a fire pump from the outlet end until the bubbles stop floating to the surface of the dam. One method (used with flexible pipes) is to tie a float to the top of the inlet and attach weights 50mm along the line to keep the inlet suspended 50mm above silt in the dam.
To assess the appropriateness of this technique, the level of stratification in the embankment and the approximate depths of the various layers must be determined.
Conclusion
Department of Natural Resources and Mines 2003, Methods for reducing evaporation from water reservoirs used for urban water supply. Engle, R, McFarlane, DJ & Street, B 1987, 'Impact of dolerite dykes on saline seeps in south-western Australia'. Farmer, D 2004, Damcat 4 Dam and improved catchment water supply design software Version 4.0, Department of Agriculture WA.
George, RJ & Frantom, PWC 1988, 'The nature, development and management of sandfield intrusions in the eastern wheat belt of Western Australia'. Airborne Geophysics Assessment for Agriculture, Forestry and Fisheries Australia and the National Dryland Salinity Program, Department of Agriculture WA. Laing, IAF, Pepper, RG & McCrea, AF 1988, Problem areas for farm water supplies in South Western Australia.
Lothian, J & Conacher, A 2005, 'Managing secondary river salinity in south-western Australia: an assessment of a catchment approach to water resource restoration'. Mayer, XM, Ruprecht, JK, Muirden, PM & Bari, MA 2004, 'A review of stream salinity in south-west Western Australia'. Schoknecht, N, Tille, PJ & Purdie, B 2004, Soil and landscape mapping in south-western Australia - a review of methodology and results.
Seymour, M 2001a, The environmental benefits of the forage bush tagasaste Farmnote 49/2001, The Meat Program, Department of Agriculture WA.
