Management of saltwater intrusion in coastal aquifers in Queensland, Australia 138' 12' 16' p. A 20, 1 I NTI I I 24'1 1 -, 28' I JM~Mool

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1 AGSO lournal of Australian Geology & Geophysics, 4 (2/3), Commonwealth of Australia 993 Management of saltwater intrusion in coastal aquifers in Queensland, Australia J.R. Hillier! Many Queensland rivers have extensive deltas, which have large reserves of groundwater. These deltas contain fertile land suitable for intense agricultural development with associated towns and industry, all relying on groundwater for their water supply requirements. n some areas, extensive water use, especially in dry times, has resulted in saltwater intrusion into the aquifers. Management techniques have had to be developed and imple mented to ensure the sustainability of these supplies. ndividual hydrological situations have to be managed in different ways to take full advantage of the resources available. Options for management include artificial recharge schemes where conditions are suitable and surface water is available, or the provision of supplementary supplies from surface water storages to relieve demand on the groundwater resources. Restrictions on use may have to be applied during dry period and, in some cases, deliberate overuse with subsequent intrusion of saltwater has been permitted to encourage agricultural development of certain areas, with surface water being supplied when it became available. Extensive monitoring of groundwater levels and quality is carried out to detect any movement of the freshwater/saltwater interface. Models are used to predict future movement. The experience in Queensland has been that engineering works, coupled with management linked to predictive groundwater models, can reduce saltwater intrusion and provide high sustain able yields from coastal aquifers. ntroduction Coastal Queensland is fortunate to have quite extensive groundwater resources. Many rivers have well-developed alluvial tracts and deltas with extensive sand and gravel aquifers. Large sand dune deposits occur both on shore and as nearby islands. The coastal location of these large fresh groundwater resources, though, limits their long-term yields. The sand dune areas have little fertile land and use of their groundwater resources is only commencing as an auxiliary source of supply for municipal water. The river delta systems usually contain rich soil and were an obvious target for the development of sugar cane plantations in the late 9th century. Groundwater use for irrigation commenced shortly after settlement, but it was not until the expansion of the sugar industry in the mid-20th century that irrigation was practiced extensively. With no planning for future use of the resource and with little understanding of the hydrologic system, bores were drilled as needed. Many bores were located close to the coast, often tapping freshwater overlying deeper saltwater in the same aquifer. Several dry seasons in the 960s showed the large benefits to be gained from irrigation. Numerous bores were drilled into the thick sequence of sand, gravel and clay which is saturated with fresh water. Most of this water is stored below sealevel and as development accelerated, groundwa ter levels were drawn down below sealevel and saltwater intrusion into the aquifers occurred. Management of the aquifer systems was required before excessive use of groundwater caused large-scale water quality degradation. Extent of coastal aquifer systems Not all rivers draining to the east coast of Queensland have well-developed deltas which contain fresh groundwater. Water Resources, Department of Primary ndustries, GPO Box 2454 Brisbane, Queensland Streams with low seaward gradients tend to develop wide flat alluvial deposits through which the streams meander to the sea. Although these sediments have sand and gravel sections, the upper layers are usually very clayey and this restricts direct recharge from rainfall. The long tidal reaches and the lack of vertical recharge usually results in saline groundwater in these aquifers. The rivers with steeper seaward gradients usually have deltas which contain fresh groundwater. These deltas have coarser silts and sand deposits in the upper parts of the 2' 6' 38' 20, NT 24' p. A -, OLD 44' '. 500 km ' JMMool SA r j NSW BRSBANE North Stradbroke 902 Figure. Location of main coastal groundwater systems in Queensland.

2 24 J.R. HLLER Table. Major groundwater reserves in coastal locations. Area Geology Fresh water storage Annual yield Annual use 99/92 Nth Stradbroke s(,2) Moreton sland(l) Fraser sland(l) () (3T Don R() Burdekin R(4) Herbert R (5) Mulgrave R() Tertiary seds. Alluv./delta Alluv./delta 4.0 x x x 06.4 x x 05.3 x 05.2 x x x ML Megalitre () Various unpublished data (2) Laycock (975) (3) Bedford (978) (4) Hazel and Hillier (988) (5) Cox (979) sedimentary sequence which improves recharge and very short tidal reaches. The major coastal aquifer systems are shown in Figure and are set out in Table. ncluded in this table are details of the coastal sand dune deposits, although the large resources of groundwater in these are mostly underdevel oped. Extent of saltwater intrusion Sand islands Because so little use is made of groundwater from the sand dune deposits, a strong outflow to the sea is maintained. No sign of saltwater intrusion has been noted on North Stradbroke sland, the only area to have any significant use. Several bores were drilled to bedrock on the beach near the high tide limit in an attempt to locate the freshwater/salt water interface. No saltwater was encountered in any of these bores, indicating that the interface was farther out to sea. Between 968 and 97, about 45 km 2 of coastal land was subject to water quality degradation because of landward movement of the freshwater/saltwater interface and upward vertical intrusion of deeper saline groundwater. Ground water use was calculated to be about megalitres/an num during this period, while the safe annual supply under natural conditions is about megalitres/annum (Credlin, 980). This excessive use caused the water table to be lowered more than 6 m below sealevel over large areas. Land has been developed for agricultural purposes almost to the coast and water quality degradation has occurred in some bores. The main cause of this quality degradation is that some bores are located too close to the freshwater/salt water interface. ntersection of this interface by the cone of depression of the pumping bores has drawn saline water into the bores. There does not appear to be any significant landward movement of this interface and rarely has the regional water table been lowered to sealevel even in coastal zones. Burdekin t is evident that before development much of the coastal zone was underlain by saline water at depth. Monitoring of water quality showed the freshwater/saltwater interface moved inland a small distance during the 960s. However, most of the measured quality change was a result of overuse of the limited freshwater resources adjacent to the coast which caused upward movement of deeper saline water. Engineering remedies n areas where it has been evident that groundwater could not supply the demand without continued degradation of its quality, alternative sources of supply have been provided. An integrated-use scheme has been implemented based on supplying surface water from a major storage via channels and pipelines for irrigation in areas where groundwater quality had deteriorated, or where use was greatly in excess of yield. The groundwater usage now equates roughly to the safe yield of the aquifer, but the system still needs management to achieve the maximum benefit without causing saltwater intrusion. Only now are projects being considered to supply surface water as an alternative source to the area which has been affected by saltwater intrusion, or to artificially recharge the aquifer. No commitment to works has been made at this stage. Burdekin An artificial recharge scheme, based on pumping surface water from the Burdekin River into a series of natural and artificial channels, depressions and pits was implemented in the mid-960s and has continued to expand. The volume of water diverted for artificial recharge of the aquifer is now in excess of megalitres/annum and with this addition the delta remains totally reliant on groundwater for its water supply.

3 Monitoring Monitoring of water levels and quality is carried out in all major groundwater areas, whether subject to government management or not. n most areas, monitoring to obtain information for the assessment of resources, determine yields, and assess changes in quality has been performed for about 40 years. The location of the freshwater/saltwater interface was determined initially by sampling private bores. Later, special monitoring bores were constructed to monitor movement of the interface, with screens set near the bottom of the aquifer. Nests of bores were then constructed with screens set at various depths, to allow a vertical profile of quality to be obtained. More recently, bores have been sited to intercept the freshwater/saltwater interface. These have been con structed with PVC casing which has been slotted through the full thickness of the aquifer. Conductivity probes are lowered down the bore to monitor the quality profile in these bores in situ. n most areas, the location of the interface is detected as a distinct quality change - an abrupt increase in conductivity over a to 2 m interval. Movement of this freshwater/saltwater interface is quite sensitive to water level variations and it can be followed easily. Figure 2 illustrates the typical changes in one bore in the area. Thirty similar bores have been constructed close to the coastline in this area to monitor movement of the saltwater into the aquifer. t is evident from this detailed monitoring that if production MANAGEMENT OF SALTWATER NTRUSON 25 bores are kept inland from the saltwater interface, signifi cant gradients can be induced in occasional dry seasons for short periods of time without causing much movement of the interface. When groundwater levels are below sealevel for periods of several months, saltwater intrusion will occur. However, when the cone of depression of a pumping bore intersects the interface, saltwater may be drawn directly into the bore, increasing the apparent effect of intrusion. Management Under Queensland legislation, sub-artesian groundwater resources can be declared by Order-in-Council to come under the provisions of the Water Resources Act (989). This part of the legislation is enacted only when the resource is at risk either of quality deterioration or is over-developed, and when management is required to protect the viability of supplies. Varied hydrological situations in each of the areas require different management techniques. The area was declared a sub-artesian area in 970 after it became obvious that groundwater use was far in excess of the safe yield. Extensive investigations showed that artificial recharge was not economically feasible and surface water was introduced to reduce the demand on groundwater. The area is now divided into sections, some of which remain on groundwater and others have surface water supplied. The demand on groundwater has been A B :[ '" c:.q '" a; j :[ '" 0> c: "6 2-0 Ui l, -0 3, " Conductivity micros/em Log scale (range ) -20 9/0/3 Figure 2. Saltwater interface monitoring, area. CA) Groundwater Level Fluctuation (B) Resultant borehole conductivity profiles

4 26 l.r. HLLER reduced and now this can be supplied with little risk of saltwater intrusion. A complex groundwater model has been constructed to cover the whole of the area, and this is now the key to managing the groundwater system (Hillier, 987). The model is being used to optimise the distribution of water withdrawals and this enables the maximum use to be made of groundwater without causing deterioration of quality. The total model system consists of a groundwater flow model interfaced with a soil moisture store model. The latter uses daily rainfall records and crop details to simulate the crop consumptive use, irrigation demand, groundwater recharge, and runoff. The output from this simulation forms the input to the groundwater flow model, which then calculates on a monthly time step, the resultant groundwater levels. The model has been used to analyse the effects of historic rainfall and current use distribution on the yield of the groundwater system. n the past, water users have been issued licences with a fixed water right or allocation. Now it is apparent that overall a system of variable use is much more economically attractive than a rigid system, which limits use to a pre-determined maximum, irrespective of the volume of groundwater in storage. Much more water can be with drawn in a series of wet years, but this may need to be countered by below-average use in dry seasons. Current water levels are used as input into the model at the commencement of an irrigation season and the model is run to determine the water levels which would result from various rainfall scenarios. The results of these simula tions are discussed with irrigators and decisions are made on the water allocation level for the season. The allocation usually varies from between 5 to 25% of the fixed allocation. Detailed monitoring of the location of the freshwater/salt water interface is still undertaken, and groundwater use can be restricted further if excessive movement is detected. Generally, the model predictions and the announced allocation system preclude the necessity for use restrictions and allow the landholder to commence a season with knowledge of his minimum water entitlement. The area has been subject to management of groundwater resources since it was proclaimed a sub-arte sian district in 947. Limitations on drilling have been the only control placed on groundwater use to manage saltwater intrusion. New bores have not been permitted in the area which is considered at risk. Existing use has induced some movement of the saltwater wedge in very dry seasons, though the total area affected is not great. As the demand for water is increasing consideration is being given to management options that maximise use, but at the same time control saltwater intrusion. Burdekin Although monitoring of water levels and quality is undertaken in the Burdekin, there is no active government involvement in managing the groundwater resource and the area has not been proclaimed as a sub-artesian district. The artificial recharge schemes are operated by two constituted boards whose members represent the irrigators and the local sugar mills. These boards have the power to levy fees to cover their operating costs. n nearly thirty years of operation, these boards have been able to pump sufficient water for recharge from partially regulated river flows to ensure that the demand can be satisfied without inducing further saltwater intrusion. Their operations are assisted by the input of the results of monitoring. Since the completion of the Burdekin Dam upstream of the in 987, the water requirements of the boards have been assured. Other options n part of the Burdekin rrigation area (outside the Burdekin ), saltwater intrusion has been deliberately allowed to encourage agricultural development. This area has a large reserve of freshwater, but a low natural yield because of restricted recharge. t was obvious that groundwater could support only limited irrigation develop ment in the long term and that it would be necessary to supply surface water if irrigation development was to be sustainable. Use of groundwater has been in excess of megalitres/annum, while the natural yield is about 2000 megalitres/annum. Saltwater has encroached inland under about 2 km 2. The channels bringing surface water to the area are almost completed and groundwater use will be reduced to a level which will stabilise the freshwater/ saltwater interface. Conclusion Although use of coastal aquifers is constrained by the presence of saltwater, high sustainable yields can be obtained if the aquifers can be managed in a flexible manner. Options, such as artificial recharge, alternative surface water supplies, variable annual use and if necessary restrictions on use, can be utilised to obtain maximum benefits without long-term degradation of the water quality. Monitoring is essential to allow remedial action to be taken before serious quality changes occur. Acknowledgments The permisson of the Director-General, Department of Primary ndustries, to publish this paper is acknowledged, as is the assistance given freely by various staff members of the Department. The comments made by Mr R. Abel of the Australian Geological Survey Organisation, and one anonymous person, in reviewing the manuscript were appreciated.

5 References Bedford, K.B., Report on Groundwater Resources, Pioneer Valley. Queensland rrigation and Water Supply Commission Report. Cox, R.B., Report on hydrogeology of the Herbert, ngham. Queensland Water Resources Commis sion Report. Credlin, B.L., 980: ntegration of surface and groundwa ter use in an irrigation system - rrigation Area, Queensland, Australia. n nternational Commis sion on rrigation and Drainage. Third Afro-Asian MANAGEMENT OF SALTWATER NTRUSON 27 Regional Conference, New Delhi, 980, -29. Hazel, c.p., & Hillier, J.R., Artificial recharge in Queensland, Australia. n Johnson, A.., & Finlayson, D.J. (editors), 988. Artificial Recharge of Groundwater -Proceedings of the nternational Symposium. Ameri can Society of Civil Engineers, Hillier, J.R., Techniques and strategies for managing coastal aquifers in the area. n Proceedings of the nternational Conference on Ground water Systems Under Stress. Australian Water Re sources Council, Conference Series 3,