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Assessment of Major Spring Systems in the Oolloo Dolostone, Daly River

(2011). Assessment of Major Spring Systems in the Oolloo Dolostone, Daly River<br />. Darwin, NT: Northern Territory Government, Department of Natural Resources, Environment, the Arts and Sport.

Document type: Research Report
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Title of Report Assessment of Major Spring Systems in the Oolloo Dolostone, Daly River
Editor Tickell, S. J.
Publication Date 2011
ISBN 978-1-921937-31-6   (check CDU catalogue  open catalogue search in new window)
Publisher Northern Territory Government, Department of Natural Resources, Environment, the Arts and Sport
Place of Publication Darwin, NT
Total Pages 148
Field of Research 300800 Environmental Sciences
Abstract Executive Summary

The Oolloo aquifer is an important karstic aquifer in the northern part of Australia’s Northern Territory. Its groundwater discharges into the Daly River, providing the major portion of its baseflow with flow maintained throughout the Dry season supporting the associated riverine ecosystems. This groundwater is also increasingly being extracted for agriculture. A draft Water Allocation Plan for the aquifer is to be released during early 2012 and this study comprises research into various aspects of aquifer water balance and components of the associated water cycle. It was undertaken in order to provide the most up to date scientific knowledge to support the plan.

Geological investigations including reconnaissance mapping and drilling have better defined the extent of the aquifer and has confirmed a twofold subdivision into an upper, highly permeable, fractured and karstic aquifer and a lower, less permeable, mainly fractured rock aquifer. A new geological formation, the Florina Formation was also recognised and mapped. It is important because it acts as a confining layer to the Oolloo aquifer. Drilling through the Florina Formation into the top of the Oolloo aquifer has demonstrated that the Oolloo Dolostone was exposed to the atmosphere and underwent kastification prior to the deposition of the Florina Formation during the Early Palaeozoic. This explains the widespread development of the highly permeable karstic aquifer, even at depth in the central parts of the Daly Basin. The updated knowledge about the Oolloo aquifer has been used to compile a new hydrogeological map of the area.

Groundwater discharge from the aquifer is largely through springs in the Daly and Katherine Rivers. They occur in places where confining layers are not present. Three main groups of springs were recognised based on location and discharge. They have been named the Katherine River, Stray Creek and Daly River spring zones. Spring discharge from the Oolloo aquifer varies considerably with time on a scale of decades due to medium term (decadal) rainfall changes.

Hydrochemical investigations have indicated both upward and downward leakage of groundwaters between the Oolloo aquifer and overlying Cretaceous strata, depending on the local hydraulic gradient. Localised upward leakage from the underlying Jinduckin Formation into the Oolloo aquifer was also indicated. Nitrate concentrations in groundwater are very low. Broad trends in dissolved oxygen and iron(II) concentration, coupled with the fact that measured concentrations are so low, suggests there may be some capacity for denitrification to mitigate potential future nitrogen contamination of the groundwater.

A suite of environmental tracers (222Rn, CFCs, SF6, 14C, and 4He) was used to characterise groundwater interaction between the Oolloo aquifer and Daly River. Both modern (less than about 100 years) and older (hundreds to thousands of years) waters were differentiated in the groundwater and springs. Their distribution has led to a new conceptual model of groundwater flow. At the small scale, within 1 km of the river, the Oolloo aquifer is recharged by a combination of deep drainage through surface soils and, to a lesser degree, from annual flooding and lateral flow into the aquifer. In the subsequent Dry season, discrete point source seeps provide relatively young water to the river. However, at the large scale, on the order of the Daly Basin, regional scale groundwater flow supplies water to major spring zones and most likely to submerged (concealed) seepage zones that extend downstream of Stray Creek. Along the entire section of the Stray Creek and Daly River spring zones the older regional-scale groundwater makes up approximately 35 per cent of the baseflow. In the Stray Creek spring zone it represents approximately 90 per cent of baseflow. At Oolloo Crossing, groundwater levels exhibit a hydraulic damming effect that governs interaction between the Daly River and adjacent aquifer. At this location extensive bank-storage is probably not occurring during the annual flooding.

Groundwater recharge was estimated utilising environmental tracers, analysis of time-series hydraulic head data and cross sectional numerical modelling. Vertical recharge was estimated to be 17 per cent of rainfall where the massive unit of the Oolloo aquifer is close to the ground surface and 7 per cent where overlain by Cretaceous sediments. When a dual-continua approach to modelling groundwater flow and apparent age is invoked, the unique transmission and storage properties (i.e. matrix and conduit flow) of the Oolloo aquifer can be replicated. Furthermore, qualitative comparison of apparent age appears to be useful for further constraining numerical models.

Recharge (and other water balance components) was also estimated using SVAT (soil-vapour-atmospheric transfer) models calibrated, where possible for soils, vegetation and land forms of the Daly River catchment. The SWAT model was tested over the Oolloo region, simulating surface runoff, soil profile water redistribution and evapotranspiration. Model outputs compared reasonably to observed data available. In addition, outputs from the WAVES model were extracted to generate a map of deep drainage (downward drainage beyond the root zone) and values of recharge ranged up to 150 mm/year with a mean of 67 mm/year (7 per cent of rainfall) in good agreement with geochemical based estimates. These models are limited by the paucity of soil data available to provide robust simulations, with soil hydraulics particularly import for the accurate partitioning of runoff and drainage.

An extensive set of soil hydraulic properties was obtained by field and laboratory measurements across the extent of the Oolloo aquifer to examine the degree of spatial variation and the departure from agricultural soil types for which there are some available data. Observations were made across the Oolloo region to examine variability in soil texture, bulk density, hydraulic conductivity and water release properties. Two distinct groups were identified based on water release characteristics that correspond to agricultural and non-agricultural soils. Data describing non-agricultural soils is sparse despite the area occupied across the Daly catchment, especially in headwaters landforms. A flume was established within one of these small headwater creek systems characterised by skeletal soils and runoff was three times that typically occurring from agricultural soils.

Spring dependant ecosystems will be broadly protected by the Water Allocation Plan as it sets minimum flows at various points in the river based on ecological studies. More localised impacts on spring flows caused by drawdowns from production bores located too close to the river need to be addressed. To do this buffer zones adjacent to the main spring zones are recommended. The karstic nature of the aquifer makes it difficult to set distance limits based on rigorous scientific methods. Observed groundwater levels and overall knowledge of the aquifer have been used to recommend limits of three km for the Stray Creek spring zone and 1.5km for the Daly and Katherine River spring zones.
Additional Notes Technical Report No. 22/2011D
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