2.1.3 Hydrogeology and groundwater quality


This section provides a hydrogeological assessment of the input datasets for the Namoi subregion numerical groundwater model.

There are a large amount of observed groundwater level data that have been collected in recent decades for water resources management. This data collection is focused on the alluvium where the water resources are; there are little observational data outside of the alluvium such as in the Permian formations that host the coal resource.

The focus of data collection of hydraulic properties has been the hydraulic conductivity. The hydraulic conductivities of the coal-bearing formations and interburden have a decreasing trend with increasing depth, and also a zone of enhanced hydraulic conductivity near the surface due to weathering. These relationships have been incorporated into the numerical groundwater model. There is no evidence of a depth-dependent relationship for the hydraulic properties in the Pilliga Sandstone, therefore the groundwater model will not have a depth dependence for the hydraulic properties for the Pilliga Sandstone.

The observed trends in groundwater level at nested piezometers have been used to infer the vertical connectivity in the alluvium. Zones in the alluvium that are well connected have been identified by similar trends occuring in groundwater level at multiple depths. Similarly zones that are poorly connected have been identified where the trends in water levels at multiple depths are different. This analysis of the connection of the alluvium is used in the groundwater model to inform the ratio of the vertical hydraulic conductivity to the horizontal hydraulic conductivity of the alluvium.

The recharge due to rainfall for the groundwater model domain has been determined using the chloride mass balance technique and upscaled using surface geology and mean annual rainfall as covariates.

This section provides a hydrogeological assessment for the Namoi subregion. It informs the conceptualisation that underpins the numerical groundwater model that is used for assessing the groundwater impacts of additional coal resource development in the Namoi subregion. Relevant hydrogeological datasets sourced from the NSW state agencies and groundwater models developed for the Namoi river basin in previous studies were used for this purpose. The Namoi subregion consists of two major aquifer systems: the Namoi Alluvial aquifer (Upper and Lower Namoi) and the Pilliga Sandstone aquifer. The most widely used aquifer in the Namoi subregion is the Namoi alluvium comprising the Quaternary Narrabri and Gunnedah formations. These units contain significant resources of high quality groundwater that is heavily utilised for irrigation, water supply and stock and domestic use. Pilliga Sandstone is a part of the Surat Basin and is a major regional aquifer consisting of medium- to coarse-grained sandstone and conglomerate with minor interbeds of fine-grained sediments.

Hydrogeological data, including aquifers, water levels and hydraulic properties, are required to inform groundwater modelling. In addition, estimates of the extraction of groundwater for use are needed. The hydrogeologic data used in the bioregional assessment (BA) of the Namoi subregion are detailed in Section

Last updated:
6 December 2018
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