Australian Natural Resources Atlas

Groundwater Management Unit

 

Groundwater Management Unit: Molong Limestone

Introduction

The groundwater resource characteristics for the Molong Limestone 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 Molong Limestone's groundwater resource?

Vital Statistics:

Area1,771 km2
Total Water Allocated4,000 ML/yr
Total Water Used600 ML/yr
Average Salinity1,000 mg/L
Sustainable Yield7,000 ML/yr
Depth to top of aquifer25 m

Aquifer Description:

The aquifer comprises scattered lenses of limestone and calcarenite interbedded with shale, siltstone and fine sandstone near the eastern margin of a major geological structure referred to as the Molong geanticline. Folding and consequent fracturing, followed by sloution along the fracture planes, have created a substantial degree of secondary permeability in the calcareous lenses. Where they are connected to a recharge path, such zones provide a useful aquifer, but there is direct connection berween aquifers. Low salinty water with yields to around 10 L/s can be obtained from the better sites, with water occurring under essentially confined conditions.

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% of recharge is 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.

For this GMU, a preliminary estimate of 7000 ML/y for sustainable yield has been derived by assuming long term average annual recharge is 5% of rainfall in the limestone outcrop areas, which are estimated to be aabout 25% of the total GMU area.A 30% environmental factor was allowed (ie sustainable yield is 70% of recharge).

Assumptions used for allocating development categories:

The sustainable yield is a preliminary estimate, the allocation figure is an estimate, and the abstraction is based on an assumed proportion of allocation. The classification as Category 3 for allocations is probably reasonable, but is certainly subject to modification with more precise data and estimates. The classification as Category 2 for abstrasction is based on regional staff estimation and while the estimated abstraction seems very low a Category 2 classification seems reasonable.

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.

Bore IDStart recordEnd recordDepth of bore (m)Reduced level (m)
NSWGW05004501-Jan-9001-Jan-0024no data
NSWGW80030801-Jan-9401-Jan-00no datano data
NSWGW80036801-Jan-9401-Jan-00no datano data
NSWGW80036901-Jan-9401-Jan-00no datano data
NSWGW80037101-Jan-9401-Jan-00no datano data
NSWGW80037201-Jan-9401-Jan-00no datano data
NSWGW80037301-Jan-9401-Jan-00no datano data

The following groundwater management units also occur in Lachlan Province:

Further information

Key

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