Surface and Groundwater Quality
Suspended solids including clays and colloids and fine organic matter are a major impact upon the water quality in the Burdekin Dry Tropics region. The long term average sediment discharge is about 3.8 million tonnes, representing 20-40% of the total sediment being delivered to the Great Barrier Reef lagoon. Much of this occurs during cyclones or heavy monsoonal rains and is the major impact of terrestrial runoff on near shore environments of the Great Barrier Reef lagoon. Much of this is delivered during infrequent floods with intervening years having relatively low sediment discharge.
While increased suspended solids have marked effects on marine environments, there are lesser impacts on freshwater organisms which are more adapted to episodic events. However the increased nutrient levels during low flow periods has a marked effect on freshwater systems. This can be attributed to the persistence of nutrients and surface water contaminants from irrigation water and small rainfall events. These impacts are increased when baseflow of rivers and creeks ceases and waterways contract to permanent and semi permanent waterholes allowing contaminants to concentrate. This can be further impacted by cattle loitering and defecating in and around these waterholes (Roth et al 2002). During periods of elevated water flow ambient water quality in the region is the same or very similar over the length of a creek or river. When flow returns to normal creeks, waterholes and rivers begin to exhibit individual characteristics i.e. differing water quality and biotic assemblages. This leaves waterholes with variable water quality more vulnerable to disturbance whilst others are more tolerant (Roth et al 2002).
Aquatic invertebrates are a reasonably good indicator of some aspects of water quality but relatively little is known about aquatic invertebrates in the region. Fish are better indicators of water quality and water way health in some of the more heavily impacted of the region i.e. wetlands in the Lower Burdekin. Current fish communities of the floodplain are very poor when compared to their natural state, partly as a loss of water quality and degradation and loss of connectivity with estuarine areas. Fish stocks in the Upper Burdekin, Bowen and Cape river systems are known but little is known of stocks in the Belyando Suttor.
Generally ambient water quality has more to do with accumulation of localised effects with the main factor being local land use, in grazing areas it is stock and feral animal use of riparian zones and in cropping areas localised stream runoff and entry to ground water.
Major impacts on ambient water quality include: increased turbidity blocking sunlight and affecting ecological processes, increased nutrient levels contributing to eutrophication and decreased oxygen levels, low ph contributing to fish kills, accumulation of contaminants in the food chain, sublethal toxicological effects of pesticides, settlement of sediments, algal blooms and weed growth.
The Burdekin Delta is one of the largest alluvial aquifer systems in Australia. Despite its physical dimensions, the existence of significant wetlands and the risk of increased extractions leading to salt water intrusion limit further groundwater development with in the delta.
Up to 360, 000 ML is introduced to the aquifer each year through rainfall. While attempts have been made in the past to assess groundwater resources and understand the groundwater systems and its interactions with surface and irrigation water this is far from complete. A comprehensive groundwater flow and seawater intrusion models for the Burdekin coastal groundwater system are currently being developed.
At Bowen, the groundwater underlies a gross area of 15,000 ha, with the net irrigable area restricted by water allocations of 12,000 ML/a. Some farms in the lower delta have been so affected by seawater intrusion that they have ceased operation with other reported instances of deteriorating water quality.
You may also be interested in:
- Roth, C.H., Lawson, G. and Cavanagh, D. 2002 Overview of the key Natural Resource Management Issues in the Burdekin Catchment, with particular reference to Water Quality and Salinity. Burdekin Catchment Condition Study Phase 1. Report commissioned by Dept. of Natural Resources and Mines on behalf of the Burdekin Dry Tropics Board. CSIRO Land and Water Townsville.
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