TECHNOLOGY
SALINITY AND GROUNDWATER CONTROL SHEPPARTON REGION by W. TREWHELLA ABSTRACT The Shepparton Region is a major irrigated area with a gross annual value of regional output of more than $2.5 billion. Some 200 000 ha already have high watertables, and soil salinity is increasing. By year 2020 it is estimated that the direct annual loss to salinity will be $40 million, unless watertables are controlled. The Shepparton Salinity Plan has been developed for this purpose, and is now being implemented. Vertical drainage by groundwater pumping from the shallow Shepparton Formation aquifers is a major ¡ component of the Plan. Pumping will be carried out using both private pumps (which reuse pumped groundwater for supplementary irrigation) and public pumps installed for salinity control. The development of the groundwater pumping program is described, including the constraints imposed by the variability of the aquifers and the limited opportunities for discharge of salt from the Region.
INTRODUCTION The Shepparton Irrigation Region is an intensively irrigated area centred on Shepparton in the Goulburn Valley. It has a total area of about 500 000 ha, of which about 280 000 ha is irrigated with water diverted from the Goulburn, Campaspe and Murray Rivers. The climate is warm temperate, with average annual rainfalls ranging from 380 to 500 mm / yr. The best soils are intensively irrigated for dairying and horticulture, but large areas are irrigated less intensively for mixed farming enterprises. Removal of the native vegetation and the introduction of irrigation have increased accessions to groundwater to levels well above the carrying capacity of the underlying aquifers. This has resulted in high watertable levels regionally, with groundwater discharge and soil salinisation in some areas. Areas where the watertables are consistently within 2 m of surface are at risk to salinity, but severe problems generally only occur if the watertable is less than Im below surface. The Shepparton Salinity Management Plan has been developed to combat the problem. It combines improved irrigation management and regional drainage to reduce surface waterlogging and minimise accessions to groundwater. However, it recognises that accessions will always exceed the capacity of the aquifer systems and therefore gives high priority to subsurface drainage, particularly vertical drainage by groundwater pumping. This introduces the need for appropriate disposal of the subsurface drainage effluent, which ranges from brackish to highly saline. The Plan is designed to ensure that irrigation within the Region is sustainable, and that the irrigation and drainage works are managed in an environmentally responsible manner.
HYDROGEOLOGY The Region is located on the extensive alluvial plains deposited by the prior Murray, Goulburn and Campaspe river systems. The main geological units are the Renmark Group and the Calivil and Shepparton Formations. The Renmark Group and the Calivil Formation are the major regional aquifers, and are generally 80 to 100 m below the present land surface. They generally consist of coarse alluvial sands, gravels and pebbles which are commonly referred to as the 'deep leads'. The aquifers are effectively confined or semi-confined. Prior to development, pressure levels were believed to be some 30 m below surface, and the aquifers carried recharge from the upland areas to the south and east under the plains to discharge in the Mallee areas to the north and west. Pressures have risen in the last 50 years at a rate of 10 to 20 cm/ yr and are now generally about 10 m below surface. However, they are within 2 to 4 m below surface at several localities in the Goulburn and Campaspe valleys. In some places groundwater pumping for irrigation has stabilised the pressure levels,
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WATER December 1992
Bill Trewhella graduated B.E. (Civil) from Melbourne in 1959 and has worked in the Rural Water Corporation, (under its various names) ever since, in the field of Land anti Water Management. He is currently Regional Investigations Engineer in the Tatura office.
but the levels are still rising elsewhere. The rises are due to increased recharge in the areas upstream and downstream, as well as local recharge. Recent modelling work (RWC, 1991) suggests that recharge rates to the 'deep lead' within the Region are generally very low (less than 5 mm/ yr), but can be up to 30 mm / yr near the southern and eastern margins. The Shepparton Formation is a complex mixture of clays and sands, with aquifers scattered irregularly at all depths. The aquifers are up to 3 km wide and 5 m thick, and highly variable in texture. Transmissivities range up to 1000 m 2/ day, but are commonly 200 to 500 m 2/ day. The upper aquifers are unconfined or semiunconfined, and are recharged from local irrigation and rainfall . The deeper aquifers tend to be semi-confined, and may be recharged by leakage from both the upper and lower strata. Recharge rates to the upper aquifers can be up to"300 mm/ yr under irrigation on the very sandy soil types. However, clay-loam soils predominate and recharge rates regionally are believed to average 50 to 60 mm/ yr or less. Except for the most sandy soil types when under intensive irrigation, most recharge occurs under conditions of prolonged winter or spring rainfall when evaporation is low. As a result of irrigation the soils and subsoils are now commonly near saturation when rainfall occurs, so that conditions are ideal for recharge to occur. The watertable levels in the Shepparton Formation predevelopment probably were similar to the pressures in the underlying aquifers. Some of the earliest irrigation areas had high watertables and salinity in the 1930s, but systematic recording of watertable levels commenced in a few areas only in the 1960s. With intensification of irrigation since the 1960s the area with high watertables has spread steadily. In August 1991 220 000 ha had watertables within 2 m of surface, and 78 000 ha had watertables within 1 m (Fig.I) . It has been predicted that 274 000 ha will have watertables within 2 m of surface by 2020 (Draft Shepparton Land and Water Salinity Management Plan, 1989). The Shepparton Formation aquifers are filling up from the top, i.e. the uppermost aquifers are filled rapidly by local recharge and a small part of the recharge leaks to the lower aquifers (Fig.2). Because of the low to moderate transrnissivities and the low hydraulic gradients (generally about 1:2000) lateral dissipation is slow, and watertables rise quickly. The watertable response to development may be slow initially because the overburden soils are dry and have storage capacity. Once 'wetting up' has occurred, however, watertables can rise rapidly, and rates of up to 1 m/ yr have been recorded. Without subsurface drainage the watertable levels generally approach a quasi-equilibrium at depths of 1 to 2 m below surface (Fig.3). Capillary return from the watertable to the plant rootzone or soil surface between irrigations and rainfall events balances accessions. At the sub-regional scale the nett recharge approaches zero, and the groundwater flow system is dominated by local (vertical) inputs and outputs. Groundwater outcrops have formed