Groundwater Management: Quantity and Quality (Proceedings of the Benidorm Symposium, October 1989). IAHS Publ. no. 188,1989.

A groundwater management strategy for salinity mitigation in the Victorian riverine plain, Australia

R.S. EVANS & J. NOLAN Rural Water Commission, 590 OrrongRoad, Armadale, Victoria, 3143, Australia

Abstract The major salinity problems emerging in the Riverine Plain of Northern Victoria are described. The long term viability of agriculture in this region is now threatened by land salinization and waterlogging; a direct result of increased rates of recharge to groundwater in response to clearing of natural vegetation and intensive irrigation. Without large scale dewatering, the groundwater system is unable to dissipate the increased recharge, hence water tables will continue to rise and exacerbate the existing problem. A groundwater management strategy, based on encouraging groundwater development is described. The principle constraints on such development, the need for control of aquifer salinity and salt export, are presented in the context of an overall conjunctive use policy. Specific strategy initiatives to encourage an appropriate and sustainable level of groundwater use are presented.

Une stratégie d'exploitation des nappes aquifères pour la réduction de la salinité dans la plaine riveraine de Victoria, en Australie

Résumé Cet article présente les principaux problèmes de salinité de la plaine riveraine, au nord de Victoria, en Australie. Aujourd'hui, dans cette région, la viabilité à long terme de l'agriculture est menacée par la salinisation et la saturation des terres et des eaux. Ce fait est l'une des conséquences de l'augmentation des recharges de nappes aquifères causée par l'élimination de la végétation naturelle et par des irrigations intensives. Si cette eau en excédent n'est pas évacuée à grande échelle, le système aquifère ne peut pas dissiper les eaux excédentaires ; les nappes d'eaux souterraines continueront donc à monter et aggraveront le problème actuel. Une stratégie d'exploitation, basée sur l'incitation au développement des nappes d'eaux souterraines, est décrite dans cette étude. Les principales restrictions de tels développements, le besoin de contrôle de salinité aquifère et d'exportation de sel sont compris dans un plan d'action d'usage général et conjoint. Des initiatives de stratégies spécifiques, dont le but est d'encourager un niveau

487 R.S. Evans & J. Nolan 488

approprié et pouvant être maintenu longtemps de l'emploi des nappes d'eau, sont aussi présentées, INTRODUCTION

A groundwater management strategy for the Victorian Riverine Plain (Fig. 1) which aims at addressing the serious and rapidly expanding salinity problems of the region is presented. This strategy is only one component of the regional land and water salinity management strategy which is currently in preparation. The groundwater management component primarily applies to private groundwater development for on-farm use. The groundwater strategy centres on the development and careful management of the better quality groundwater resources in Northern Victoria. Recharge reduction, a major component of the salinity strategy, is not covered in this paper.

Fig. 1 Riverine Plain of northern Victoria.

EXISTING SITUATION

Regional perspective

The Murray Basin in southeastern Australia represents a large predominantly closed groundwater basin of approximately 320 000 km2. Tertiary and Quaternary sediments are typically 300 m thick and form an extensive sequence of alternating aquifers and aquitards. Recharge occurs 489 Groundwater management for salinity mitigation

from both point sources around the basin rim (predominantly by river recharge) and by diffuse recharge through vertical infiltration over much of the region. Discharge occurs over large areas of the basin through évapotranspiration from shallow watertables, by discharge to the River Murray and at concentrated discharge regions where groundwater outcrops to form extensive saline lake systems. The Murray Basin is divided on geomorphic and hydrogeological grounds into the riverine plain to the east, comprising predominantly good to moderate quality groundwater (500 to 3000 mg/1) and non marine sediments, and the Mallee in the west, comprising predominantly marine sediments with moderate to poor quality groundwater (10 000 to 60 000 mg/1). Increasing land and stream salinization within the riverine plain is largely attributed to land and water management changes since European settlement, some 150 years ago. The two primary changes, the widespread clearing of native vegetation, and replacement with shallow rooted pastures, and the introduction of predominantly surface water based irrigation, have led to a dramatic increase in recharge and subsequent rise in shallow watertables and deep aquifer pressures. The rising water level has mobilised salts (predominantly sodium chloride) stored in the previously unsaturated profile, and transported them towards the surface where evaporative concentration occurs. Stream salinities have increased through surface run-off of the concentrated salts and increased base flow. In general, the rising groundwater levels are continuing at the rate of 0.05 to 0.1 m/year. Large tracts of both dryland (non irrigated) and irrigated pasture are threatened or already severely affected by saline groundwater and/or waterlogging.

Hydrogeological framework

Three major stratigraphie units comprise the riverine plain. The uppermost unit, the Shepparton Formation, can be idealized as an extensive silt and clay sheet intersected by randomly distributed interconnected sand beds (prior streams) of variable salinity. The watertable level is influenced by leakage to or from the underlying Calivil Formation and Renmark Group, the "Deep Lead", which from the regional perspective may be considered to be a single highly transmissive sand and gravel aquifer of good quality water (Fig. 2). The estimated recharge over the region varies from around 10 mm/year in the highlands up to 170 mm/year beneath heavily irrigated sandy soils. On average, in the irrigated areas it is around 30 mm/year. The shallow watertable in the Shepparton Formation tends to be between 10m and 1 m below the surface. Where the watertable is below the zone of évapotranspiration (i.e. deeper than about 2 m) it is generally exhibiting a steady rise, but significantly influenced by local features. In the eastern riverine plain, the "Deep Lead" generally has a potentiometric RS. Evans & J. Nolan

|H! 200 -2000 mg/l | ] 2000 - 6000 mg/l f///| 6000 - 30000mg/t Fig. 2 Groundwater quality (a) Shepparton Formation, (b) Deep Lead. surface below that of the Shepparton Formation, hence leakage is downwards. However, the "Deep Lead" potentiometric surface is rising at a rate of up to 0.2 m/year (Macumber, 1978) and thus commonly will equal or exceed the shallow watertable within a few decades if it is not managed through controlled pumping to maintain vertical drainage of the overlying Shepparton Formation. In the western part of the riverine plain the deeper aquifer is generally artesian.

Current water allocation and usage practices

Surface water Extensive predominantly surface water based irrigation exists over much of the riverine plain. A complex channel system exists to distribute the water (typically 1.6 x 10 Ml/year) from dams located in the highlands. The irrigation system commenced in the 1880s and has undergone significant expansion up to the 1960s. Surface water entitlements to individual farms (with an average size of 70 ha) have been based on 6 Ml/ha

Groundwater All bores, other than those for domestic and stock purposes, in Victoria require a licence to extract groundwater. A maximum volume of water able to be extracted is a condition of the licence. Within the Victorian riverine plain there are approximately 950 groundwater bores licenced to extract 255 000 Ml/year for irrigation purposes (and many thousands of domestic and stock bores). The irrigation licences consist of about 750 shallow systems (less than 30 m deep multiple spear points) and 200 deep bores (typically 60 to 100 m deep). Over 70% of groundwater extraction occurs from bores installed at depths of less than 20 m. The actual irrigation groundwater usage and the authorised usage over time is shown in Fig. 3. 491 Groundwater management for salinity mitigation

The usage has increased from an estimated 4500 Ml in 1974 to 108 000 Ml in 1986. The actual groundwater usage is about 40% of the authorised volume in a normal year rising to about 80% in a drought year, as observed during 1982 and 1983. Currently groundwater usage is approximately 8% of the applied irrigation water across the region.

250-

£200 AUTHORIZED

LICENCED 150- ALLOCATION

UJ 100- < ESTIMATED ACTUAL USAGE

—i 1 1 1 82 83 84 85 1986 Fig. 3 Authorised and historical groundwater use.

Irrigation bores are primarily installed to provide additional water for irrigation, either as insurance against drought or to provide a long term supplementary source of water to the channel-supplied surface water. Groundwater bores are rarely, if ever, installed by the public primarily for salinity or water logging control. In addition to private groundwater usage, there is substantial public-scale groundwater pumping for watertable and salinity control purposes. There are 79 pumping installations in the eastern region protecting high value horticultural and pasture (18 000 ha) areas and another six pumps directed at pasture protection. Almost all the groundwater pumped is discharged to surface water drains and channels where approximately 60% is used for irrigation and 40% is discharged to the River Murray- Groundwater salinity is generally about 2600 mg/1. In the order of 10 900 Ml/year are pumped and hence 11 300 tonnes of salt enter the River Murray per year.

Land and stream salinisation

Rising groundwater levels across the Victorian riverine plains are evident beneath the irrigation regions and the adjacent dryland areas. It is estimated KS. Evans & J. Nolan 492 that salinity is costing approximately $50 million per year as lost agricultural production within the irrigated areas; about 140 000 ha have suffered from losses in agricultural productivity as a result of salinity. Ashallowwater table, less than 2 m below the surface, currently underlies 160 000 ha (32%) of the Shepparton Irrigation Region (see Fig. 4). It is estimated that if land and water management practices are not modified this will increase to 44% in the year 2000 and 55% by 2020. Areas of localized dryland salting are scattered

Fig. 4 Water table depth in August, 1987. throughout the region. In the order of 10 000 ha of dryland farming land is affected. If the shallow watertable and deeper aquifer pressure rises continue at their current rate, much of the riverine plain would become a zone of regional groundwater discharge within 100 years. Rising groundwater levels have increased the volume and salinity of discharge to streams and drains which ultimately flow to the River Murray. River Murray salinity is costing three of the states involved approximately $40 million per year. The high economic and environmental cost of salinity, predominantly on downstream water users, is a critical factor in limiting groundwater disposal through export to the River Murray. 493 Groundwater management for salinity mitigation

MANAGEMENT STRATEGY ISSUES

Agronomic and land use practices

There is little doubt that agronomic and land use practices which are inappropriate to the Murray Basin hydrological equilibrium are primarily responsible for the current salinity problem. A variety of measures are being introduced to reduce groundwater recharge e.g. revegetation of recharge areas, improved agronomic practices, using more deeply rooted crops, surface drainage, improved water use. Nonetheless, these are expected to retard rather than reverse the new hydrological process.

Surface water and groundwater pricing and allocation

Gravity surface water supplies are generally significantly cheaper than pumped groundwater. Currently at $12.10/Ml/year and rising at a rate of inflation plus 2% per year towards the full cost of $22.50 at today's value. This can be compared with fuel costs alone for pumping from the shallow aquifers of $ 15 to $20/Ml and up to $30/Ml for deep aquifer pumping. It will be many years before the surface water price equates with the groundwater costs and hence groundwater becomes economically attractive. Many irrigated farms do not have sufficient surface water allocation to meet their needs, in which case groundwater pumping is attractive. High surface water allocations in some years closely correlates with a relatively low level of groundwater usage and a corresponding increase in watertable levels and/or deep lead aquifer pressures. The development of an equitable policy to encourage groundwater usage, while not disadvantaging those water users without a viable groundwater alternative, is under consideration. Transferable Surface Water Entitlements (TWE) have recently been introduced on a temporary basis. This breaks the nexus between land and water, and enables individual farmers to sell surface water entitlements to other farmers. The approval of TWE applications will need to evaluate possible salinity consequences with care.

Scope for further groundwater development

Significant scope exists for further development of groundwater in northern Victoria. This applies primarily to shallow aquifers in the Murray Valley area and in other more localised prior stream aquifers in the Shepparton Region, and the Murray, Campaspe, Goulburn and Upper Loddon deep lead systems. Aquifers of suitable groundwater quality and yield underlie in the order of 25% of the northern Victorian irrigation regions and adjacent hinterland. In these areas it is believed that groundwater could meet in the order of 20% RS. Evans & J. Nolan 494 of the total water demand. Groundwater development is being encouraged for its resource potential and also because of its role in salinity mitigation. This encouragement takes the form of:

- Advice by government agencies on the availability and use of suitable groundwater. - A groundwater exploratory drilling scheme where shallow aquifers of a suitable quality and quantity are located on the farmers property, thus removing the exploration risk. - A two year grants scheme that provides a 50% grant for the capital costs of new groundwater bores and pumps. - A short term direct subsidy of $5/Ml for groundwater pumped from shallow aquifers and used on farms. - An Electricity Commission initiative to encourage the installation or conversion of existing groundwater pumps to electricity by offering off-peak rates over the weekend as well as at night and susidizing instal­ lation costs. - The latter three incentives have only recently been introduced.

It has been well proven that pumping from shallow aquifers can have a significant beneficial effect in lowering water tables and thus reducing salinity problems, as well as providing additional water for irrigation. Pumping from deep aquifers has a muted effect on shallow water table levels. The reduction in deep aquifer pressures (or even halting the upward trend) has a beneficial effect on long-term salinity problems by maintaining or even increasing downward leakage from the shallow aquifers. Between 1982 and 1986 deep aquifer pressures within the Campaspe Valley had stabilized as a result of groundwater pumping from deep aquifers. However, two moderately wet years since then, combined with high surface water allocations and consequent reduction of level of deep aquifer pumping, have reversed the trend, such that continued rise of deep aquifer pressures is now generally observed. Nonetheless, with generally rising deep aquifer pressures, significant deep aquifer pumping might be able to stabilise or even reverse deep aquifer pressure trends, and this must extend the timeframe within which drainage from the shallow to the deep aquifers will continue. The possibility of a substantial upstream "Deep Lead" well field with reinjection of the pumped groundwater into the surface supply system is being investigated. Also, deep lead pumping from areas outside the irrigation districts, where the groundwater recharge is lower, have a much more significant effect on shallow watertable levels than deep pumping from within the irrigation districts. In most bedrock dryland areas experiencing a salinization problem groundwaters are of unsuitable quality for re-use. Hence, a dual approach of groundwater extraction both for use, and for watertable control, is inappropriate. Subsurface drainage techniques, as are used in irrigation 495 Groundwater management for salinity mitigation areas, are almost always too costly in relation to the benefits. Opinion is strongly behind an immobilisation approach, in which changes in crop type on farms, and even returning cleared land to forest cover, would reduce recharge, and hence, eventually, discharge.

The need for salt export

Salt export generally refers to removal of saline groundwater via supply channels or drains to either another region or the River Murray, but may also refer to either immobilisation of the salt in the geological profile (be it in aquifers or in the soil profile), or alternatively to disposal in evaporation basins. It is generally recognised that groundwater use, without significant salt export, will gradually cause a decrease in groundwater quality such that over a varying timeframe (thought typically to be in the order of 20 to 50 years) the use of groundwater will be severely restricted. Also, intensive groundwater usage will inevitably cause the migration of poor quality groundwater from adjacent or deeper aquifers or aquitards. Consequently, although in the short term useful groundwater level lowering will occur, as well as additional irrigation water being available, in the long term serious groundwater quality decrease may occur. In the Tongala area significant groundwater quality decrease has been observed after only five years of intensive pumping. The salinity has increased approx. 30 mg/1/year. In this area the groundwater extraction rate of approximately 3 Ml/ha is well above the local recharge rate resulting in intrusion of poorer quality groundwater from deeper aquifers and from outside the region. To maintain groundwater quality in the long term, on-farm use must be balanced by salt disposal. A fundamental requirement for long-term sustainable irrigation is that the mass of salt removed by drainage must be at least equal to the mass of salt applied to the land surface. Currently little pumped groundwater is disposed from the region and consequently groundwater salinities are rising in some locations. It is generally recognised that significantly greater salt disposal will be required to maintain long-term aquifer quality. The salt mass required to be exported from the shallow aquifers is dependent upon the rate of drainage to the deeper aquifers. The groundwater management strategy should be aimed at maximizing the drainage to the deeper aquifers, thus reducing the shallow aquifer disposal requirements, whilst ensuring that deep aquifer water quality continues to remain at a suitable quality for irrigation purposes.

Groundwater licencing criteria

The primary legislation governing groundwater development in Victoria, the Groundwater Act (1969), was written within a context of controlling private RS. Evans & J. Nolan 496 groundwater use in groundwater deficient (or "stressed") situations. So-called "material interference" is currently the key criterion for licencing. This was designed to ensure that existing users are not disadvantaged by new groundwater development, and ensures that only minimal interference occurs between adjacent bores and between groundwater and surface waters. This restrictive criterion is not appropriate for salinity management and much greater interference between bores should be permitted to assist with the control of groundwater pressures. Salinity problems require a fundamentally different approach from that used for the typical "stressed" aquifer. Maximizing groundwater use, within salinity constraints, is the aim of most allocation policies dealing with salinity. The existing rights of owners of shallow and often inefficient bores (often those used for domestic and stock purposes) should not be the primary criterion used in assessing new applications. Many shallow bores may have

Fig. 5 Irrigation-related groundwater management principles. to be deepened, at the owners' expense, or as an integral part of a regional salinity management plan. The criterion for consideration of new licence applications should include possible groundwater quality decrease as a basis for modifying licence conditions, or even rejecting a licence application. This assumes that an allowable volume able to be extracted from a region (considering all aquifers) has been defined. This assessment would be carried out with a view to optimizing salt storage and disposal. In this case, the allowance for some migration could only be permitted in the context of a complementary disposal agreement. A conceptual arrangement is shown in Fig. 5 where strategic pumped bores located in more saline aquifers prevent the horizontal and/or 497 Groundwater management for salinity mitigation

vertical intrusion of their waters into usable quality aquifers. Specific salt export provisions are essential in this case. Careful management is required for deep lead pumping bores. Such bores must be located to minimize migration of poor quality groundwater from either bedrock or overlying aquitards. The extraction rate for a region also must be carefully assessed to achieve optimum yield and minimal water quality decrease. Finally, groundwater management costs should be borne by the whole irrigation community and not by the individual groundwater users.

SALINITY/GROUNDWATER STRATEGY PRIORITIES

Integrated water allocation and management policy

The need for a fully integrated groundwater and surface water allocation policy is essential to deal with the salinity issue. Such a policy would aim at having a consistent price for all water, where any specific subsidies to encourage groundwater use are identified and built into the total water pricing and salinity mitigation structure. Allocation criteria for new or transferred surface water and groundwater would consider the total water available to individual properties and licences would be issued accordingly. The development of such a conjunctive use policy at the irrigation region and farm scale would require declaration of groundwater control regions based upon aquifer yield and salinity, where the proposed licencing criteria, as outlined in the previous section, would apply. Surface drainage which provides benefits in reduced waterlogging, recharge control, reduced road maintenance and a salt disposal mechanism will influence the groundwater control options in regions. In regions where no surface drainage is provided or access to channels is limited, a more conservative groundwater allocation policy is called for. A long-term sustainable aquifer quality needs to be assured. This will usually lead to lower groundwater allocation. Conversely, where adequate surface drainage (or alternative disposal means) is available, higher groundwater allocation will usually be appropriate. Salt export via deep seepage or significant aquifer throughflow may override the effect of the presence or absence of surface drainage. The conjunctive use policy would also encourage deep groundwater pumping in appropriate areas on both a private and public scale.

Definition of salt export criteria

The need for long term salt export from certain regions is generally recognised, especially with increased use of poorer quality groundwater. The R.S. Evans & J. Nolan 498 primary mechanism for salt export is surface drains. The implementation of an adequate salt disposal strategy must be considered on the local, regional and interstate scales. Groundwater management strategies will be vastly different in those local areas where no surface drainage (60% of the Shepparton Region) is provided and, therefore, where export is limited to channel disposal only. Considerable technical work has been devoted to defining the minimum requirement for salt export to maintain a salt balance (Trewhella, 1984). This work indicates the need to export in the order of 80 000 tonnes of salt per year through the existing and proposed drainage and channel network from the Shepparton Irrigation Region. In the long term, any rigorous assessment of salt export criteria must be based on integrated basin management principles. The salt load originating from dryland regions (unirrigated riverine plain and bedrock regions) will increase over time and be a major factor influencing irrigation region salt export requirements. The Murray Darling Ministerial Council, with membership of the Federal Government and the four state governments within the Murray Darling Basin, have, through the Murray Darling drainage strategy, effectively specified upper limits upon the mass of salt that may be disposed of to the River Murray. This is a major constraint upon regional disposal.

Education

There is a growing awareness of the salinity benefits of groundwater pumping. Community education programmes of the advantages, and constraints, of groundwater development are being accelerated. Farm advisory services need to encourage groundwater usage. There is a need to encourage bore depths and pump setting to be determined so as not to limit possible future mitigation strategies. The problem encountered where "the shallowest bore" ties up the resource needs to be overcome. In many cases, this will mean constructing pump settings deeper than that which is currently required so as to allow for possible reducing water levels/pressures in the future. This especially applies to bores tapping the deep leads. This could only occur in appropriately defined regions. It is considered that a government grant/subsidy for deeper bores may be appropriate in this case. Early warning systems need to be encouraged. The prospect of groundwater quality decrease must be appreciated by groundwater users. Every user should be regularly taking groundwater quality measurements. In addition, observation bores, located around and beneath (in the case of shallow aquifers) the extraction bore, should be encouraged and these should be regularly monitored for both groundwater level and quality. These data can then be used to point to imminent or long-term problems. 499 Groundwater management for salinity mitigation

CONCLUSIONS

Groundwater management for salinity purposes requires a significant change of thinking compared to management for "stressed" aquifers. The recognition that groundwater quality decrease is a primary factor controlling groundwater quantity allocation represents a significant change of thinking compared to conventional management philosophies. The need for careful management of the re-used groundwater is paramount. In most hydrogeological settings within the region, long term salt export is necessary. This points to the initial importance of implementing an adequate salt disposal strategy at local, regional and interstate scales.

Acknowledgements A large number of people have developed the technical basis upon which this management philosophy is based. Considerable input by staff in the Rural Water Commission and Department of Water Resources is acknowledged.

REFERENCES

Macumber, P.G. (1978) Hydrological equilibrium in the southern Murray Basin, Victoria, Australia. In Proc. Symp. HydroL of the Riverine Plains of S.E. Australia. Griffith, 67-88. Trewhella, N.W. (1984) Projected water table levels and salt balances for the Kerang and Shepparton Regions - The non-intervention situation. In Salinity Control in Northern, Victoria. A strategic Study for the Salinity Committee of the Victorian Parliament.