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• Introduction
• Character of the groundwater resource
• Are groundwater levels changing?
• Further information

Introduction

The groundwater resource characteristics for the Upper Lachlan Alluvium groundwater managment unit are presented below. This includes technical detail on aquifer properties and water level change for key monitoring bores.

What is the character of Upper Lachlan Alluvium's groundwater resource?

Vital Statistics:

Aquifer Description:

Two main geological units have been differentiated within the alluvium. The shallower Cowra Formation (equivalent to the Shepparton Formation in the downstream portion of the Lachlan River - GWMA 012) is generally no more than 40 m thick and is dominated by clay and silt but has lenses of coarse sand and gravel which provide small yields. The deeper Lachlan Formation (equivalent to the Calivil Formation in the downstream portion of Lachlan River - GWMA 012), which extends to a maximum depth of about 120m in the westerly parts of the GMU, is characterised by clean quartz sand and gravels in layers which can be quite thick and which have a high level of porosity and permeability. The alluvial deposits rest on a basement of fractured crystalline rocks (granite, slate etc.) which are effectively impermeable compared with the alluvial material. Bores in these aquifers yield up to 15-20 L/s. Water in the upper aquifer is essentially unconfined, but in the deeper aquifers is semi- confined to confined. The groundwater in the upstream part of the GMU, at least in the central and deeper parts of the aquifer system, has relatively low salinity, commonly less than 500 mg/L and nearly always less than 1000 mg/L. Downstream, generally beyond the confluence of Bland Creek, the water salinity increases rapidly to the west. Between Condobolin and Lake Cargelligo (Zone 8), the salinity is too high for irrigaiton purposes and marginal for some stock use. Recharge to the aquifers is by infiltration of rainfall across the alluvial flats and infiltration of concentrated run-off from the valley sideslopes. There is also substantial recharge by infiltration from the regulated Lachlan River and its tributaries on a continuous basis, and periodically during flood events when flood recharge is distributed over the whole alluvial system.

Method used for determining sustainable yield:

One of the most basic pieces of data required for sensible management of a resource is the quantity of input to a system or recharge. The sustainable yield of an aquifer is almost always considerably less than recharge because of the provision for the environmental needs. Nevertheless, any sustainable yield study will always involve the determination of recharge as a first necessary step. The following working definition has been adopted: SUSTAINABLE YIELD IS THAT PROPORTION OF THE LONG TERM AVERAGE ANNUAL RECHARGE WHICH CAN BE EXTRACTED EACH YEAR WITHOUT CAUSING UNACCEPTABLE IMPACTS ON THE ENVIRONMENT OR OTHER GROUNDWATER USERS The actual proportion is not specified in this definition. This proportion will change according to each situation and is assigned differently to each aquifer system. An interim sustainable yield figure has been derived for NSW aquifers by applying a sustainability factor to estimated recharge. These sustainability factors are a proportion of long term annual average recharge. An arbitrary default factor of 70% (ie 30% reserved for environmental needs) has been adopted for most cases to date, but as better understanding of groundwater systems is developed, the sustainable yields can be adjusted accordingly. The initial figures are intended to be conservative while bearing in mind that it is most often easier to subsequently adjust Sustainable yield values upward rather than downward. Sustainability factors offer protection to the integrity of the groundwater system itself and ultimately all groundwater users including the environment and ensure that neither temporary nor permanent damage to the aquifer system results from overuse. Sustainable yield values can - and indeed will - change over time as our technical understanding of the dynamics of individual groundwater systems is enhanced as a result of more rigorous investigation and in response to changes in natural and socio-economic realities. In short, this is a commencement of a continuous process of periodic review and adjustment of sustainable yield estimates. It follows therefore that a set of sustainable yield figures will reflect a level of understanding that exists at a point in time. Groundwater management committees may change the sustainable yield factor to suit local conditions. High levels of accuracy in determining sustainable yield require a degree of rigorous study that would take years if not decades to achieve. As many NSW systems are either over- allocated or nearly so, it is not practical nor is it in the communitys best interests to wait those decades before adopting allocation ceilings that are technically highly accurate. In short, at this stage a very high degree of accuracy is not required to commence management consistent with the philosophy of sustainability. Nevertheless, the approach applied has generated a set of figures that have been produced as a synthesis of knowledge accumulated to the present and have been adjusted according to good hydrogeological common sense and an understanding of local issues. Additionally, the approach has been conservative in the interest of resource protection but tempered with compromise recognising the need to preserve current development and acknowledging the importance of encouraging continued development where appropriate to do so. Where rigorous numerical models have been developed and have resulted in the generation of acceptable recharge figures for an aquifer system, these values have been adopted as acceptable for use in sustainable yield determinations. In some cases systems that are similar to a modelled system have had recharge determined empirically using the modelled system as a reference. Most systems however, have not been modelled. In those cases, inputs (or recharge) to the system have generally been kept to rainfall and river components of recharge. Throughflow and underflow have in most cases been omitted from calculations in the interest of both simplicity and conservatism. Likewise, irrigation returns have not been considered even though in some situations, a certain proportion of irrigated water might be expected to access the underlying aquifer. Two equations were used to estimate recharge. Both have a limited number of terms and allow recharge values to be assigned respectively to:

1. Rainfall sourced and;
2. River sourced.

Rainfall recharge was calculated according to assessed rainfall, area and assumed proportion of rainfall accessing the aquifer. River recharge was estimated using an equation, which is a modified form of the Darcy equation that is used in the assessment of river recharge in the Modflow software package that models groundwater flow. The result is a theoretical contribution of the river to the recharge. An additional factor was applied to this result as an adjustment factor intended to reduce the theoretical river recharge and is set as a) the fraction of the year and/or b). fraction of river reach - that is considered as a loosing stream. In this way an actual river recharge component is produced: There is a strong subjective character to the results achieved by the above method which has been applied to all GMUs across NSW, but they have been made with common sense and with hydrogeological principles in mind. They are therefore valid within the needs of the present situation. Applying this procedure to the Upper Lachlan GMU, with a 30% environmental factor, led to an initial estimate of 165,000 ML/y for the sustainable yield, but this has since been modified by Regional staff to the figure shown in the database (205 000 ML/y).

Assumptions used for allocating development categories:

The classification of this GMU as Category 3 in terms of allocations is based on the latest estimate of sustainable yield and on a recent compilation of allocations by Regional staff. The classification of the GMU as Category 1 in terms of abstraction is based on a very broad estimate of total pumpage, based on the number of licensed bores and an assumed average useage of 250 ML/year. Real usage data are practically non-existent. Bish and Williams (1994) reported highest annual pumpage of 5758 ML in 1986/87, with an average 4000 ML/y from then until 1992/93. No more recent data have been collected, following the abandonment of the requirement for licencees to submit an annual return of water used. The reported allocation for 1992/93 was 34893 ML. Note that the categorisation referred to above is for the GMU as a whole. Zones 1, 2 and 3 which together comprise almost the entire body of alluvial deposits upstream of Jemalong Gap, are all overallocated with respect to their separate proportions of the total GMU sustainable yield. These three zones contain the bulk of the deeper and more highly productive aquifers.

Are groundwater levels changing?

Technical information on the key groundwater bores and monitoring stations is presented below, including hydrographs where available. Link to a discussion on groundwater levels and trends at a State level as it relates to dryland salinity.

The following groundwater management units also occur in Lachlan Province:

• Alexandra  (VIC)
• Araluen Alluvium and Weathered Granite  (NSW)
• Ascot  (VIC)
• Bell Valley Alluvium  (NSW)
• Belubula River Alluvium  (NSW)
• Billabong Creek Alluvium  (NSW)
• Bullarook  (VIC)
• Bungaree  (VIC)
• Cudgegong Valley Alluvium  (NSW)
• Glengower  (VIC)
• King Lake  (VIC)
• Lancefield  (VIC)
• Lower Macquarie Alluvium  (NSW)
• Mid Murrumbidgee Alluvium  (NSW)
• Mid and Upper Murrumbidgee Catchment Fractured  (NSW)
• Molong Limestone  (NSW)
• Moolort  (VIC)
• Mudgee Limestone  (NSW)
• Murrumbidgee  (ACT)
• Namadgi  (ACT)
• Orange Basalt  (NSW)
• Queanbeyan and Molonglo  (ACT)
• Spring Hill Groundwater Supply Protection Area  (VIC)
• Tourello  (VIC)
• Unincorporated Area - Lachlan  (VIC)
• Unincorporated Area - Lachlan Fold Belt Province  (NSW)
• Upper Macquarie Alluvium  (NSW)
• Wandin Yallock  (VIC)
• Young Granite  (NSW)

Further information

• New South Wales Water Resources Assessment 2000 Report
• New South Wales Water Resources Assessment 2000 Technical Report
• For more information about water and other natural resource issues link to www.dlwc.nsw.gov.au
• Link to data available for download on the Groundwater management units and provinces - ARC/INFO export
• Link to Map maker to make a map using this information.

Key

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Last updated: Saturday, 27-Jun-2009 09:44:04 EST