Drain 31, near Millicent
2009 Aquatic Ecosystem Condition Report
- Permanently wet channel in autumn and spring.
- Moderately diverse macroinvertebrate community with no rare or sensitive species.
- Obvious signs of gross nutrient enrichment.
- Riparian vegetation limited to introduced grasses and weeds.
- Large amount of silt deposited in the channel.
About the location
Drain 31 is a small drain in the South East with a catchment area of about 116 km2. It rises at an elevation about 20 metres above sea level around Millicent, includes drainage from Drain 32, 34 and 35 and flows in a southerly direction into Drain 44, where it ultimately discharges into the northern part of Lake Bonney SE.
Drain 31 is an artificially constructed drain where the primary function is to remove surface water and draining saline groundwater to improve agricultural productivity in the region (Department for Water 2010). Given its artificial character, the drain is not expected to be in a highly rated aquatic ecosystem condition, although it does provide significant habitat for many aquatic species in the region.
The major land uses near the drain are sheep grazing and cropping, and the upper reaches of the drainage network receives stormwater from Millicent township and treated effluent from the Millicent Wastewater Treatment Plant (WWTP). The monitoring site was located downstream from the discharge from the WWTP and near the junction with Drain 34 off Lossie Road, about three kilometres south–south–west of Millicent.
The drain was given a Poor rating because the site sampled showed evidence of major changes in ecosystem structure and moderate changes to the way the ecosystem functions.There was considerable evidence of human disturbance, including nutrient enrichment, fine sediment deposition and a lack of vegetative cover in the riparian zone.
A moderately diverse community of about 32 species of macroinvertebrates was collected from the channel, which ranged in size from 6–10 metres wide and from 34 cm to over one metre deep, in autumn and spring 2009. The community was dominated by species tolerant to poor water quality such as springtails, chironomids, worms and waterbugs. However, despite the presence of a wide range of generalist species that included a mayfly (Cloeon) that tolerates quite harsh environments and an uncommonly collected waterbug from the region (Giant Water Bug Diplonychus), no sensitive or rare species were found.
The water was fresh (salinity ranged from 686–743 mg/L), generally well oxygenated (57–73% saturation) and clear, with very high concentrations of nutrients such as nitrogen (2.69–14.7 mg/L) and phosphorus (0.42–2.62 mg/L). The very high nutrient concentrations were recorded in autumn 2009, when most of the water in the drain was actually effluent discharged from the wastewater treatment plant.
The sediments were dominated by detritus, silt and clay, and included a small amount of gravel and algae; samples taken from below the surface were blackened, sulphidic and anaerobic, or lacking in oxygen. More than 10 cm of silt was deposited in the channel, presumably sourced from further upstream because levels of erosion were negligible at the site sampled.
The plants growing in the channel and on the water’s edge included a floating fern (Azolla) and duckweed (Spirodela) released from the WWTP lagoon and several emergent species (native Stuckenia, Persicaria and Triglochin, and introduced Rorippa and Rumex). Over 65% of the channel was covered by aquatic plants in spring in response to the presence of excessive nutrient loads, high sunlight levels and permanent water in the drain. The nutrient enriched conditions also supported a large growth of phytoplankton algae in autumn and spring, and a moderate growth of filamentous algae in spring.
The narrow riparian zone included no native trees, shrubs or understorey species and was dominated by introduced grasses and weeds. The surrounding vegetation at the site was largely cropping and grazing land.
Special environmental features
Drain 31 provides habitat for a threatened fish called the Dwarf Galaxias that was collected in 2009. A site sampled from further downstream on Drain 44 also supported Dwarf Galaxias and another threatened species called the Southern Pygmy Perch.
Pressures and management responses
|Livestock having direct access (causing sediment erosion and adding excessive nutrients).||Drains have been constructed since the 1860s as an engineering solution to support agricultural development and it is South Eastern Water Conservation and Drainage Board practice to lease drain reserves for grazing in certain circumstances. Not all drains are subject to grazing and leases for grazing are only approved following an engineering and environmental assessment. Lease conditions require the lessee to fulfil pest plant, pest animal and CFS management requirements, thereby relieving the Board of these responsibilities.|
|Limited riparian zone vegetation (reducing habitat quality, increasing sediment erosion).||The South Eastern Water Conservation and Drainage Board has undertaken a limited revegetation program at key locations, and has the ability to undertake further revegetation works when resources allow. Revegetation at biological hotspots is recognised as a mechanism to reduce nutrient input and soil erosion, and can be undertaken if it does not impede access for management and maintenance.|
|Wastewater discharge, adding excessive nutrients and organic matter (leading to algal growth and aquatic weeds).||
SA Water Millicent Wastewater Treatment Plant
SA Water assess and undertake scheduled process improvement actions at the wastewater treatment plant, with the aim to reduce environmental risk and ensure operations are compliant with EPA licence conditions.