A stormwater runoff collection system was installed on the Joint Levee Road which consisted of grated slot collection wells approximately one meter in width, which lead to a collection sump controlled by a tap for sampling. An overflow site was situated above the tap level to allow excess water to discharge and to ensure first-flush events are captured. The collect wells were located at ten (10) sites along Sams Road, with collection wells generally located at the Top, Middle and Bottom of the 8 to 10 m high road embankment.
The monitoring program undertaken by Catchment Solutions was run from the 2017 to the 2019 wet season, using the monitoring program as described above. Sample collection over the three years generally occurring from January to May, during the wet seasons. The stormwater quality samples were collected at ten sites along Joint Levee Road (Sams Road, Mackay) at different distances from the road shoulder. Soil samples were also collected at the base of these ten sites on two occasions at the
beginning and end of each wet season (2017 to 2019). Post event-based water quality sampling was completed following significant rainfall events such as >30mm within 48 hours or an intensive rainfall event occurred, e.g. >10mm/hour, when appropriate.
Following laboratory analysis by a NATA accredited laboratory, the combined results from all monitoring were analysed and compared between the Top, Middle and Bottom collection wells across the ten (10) sites. Water quality monitoring data was represented as median, mean, 20 percentile and 80 percentile results for comparison, with the median results used as the main comparison, to reduce the impact of any outlining or anomalous stormwater run-off results.
Based on three years of wet season monitoring, key improvements in stormwater run-off are demonstrated with the changes in the nitrogen compounds. The reduction in DIN shows that some level of denitrification is occurring (DIN median reduction – 17.1%). Similar median reductions in contaminants down the embankment were also shown with sediments (23.1% reduction), zinc (65.3% reduction) and hydrocarbons (44.4% reduction). An increase in organic nitrogen is recorded as the run-off moves down the embankment, which counteracts the reduction in DIN resulting in no overall change in total nitrogen down the embankment. As the majority of the DIN found in roadside stormwater run-off would be the result of engine combustion and DIN is targeted for reduction under the Reef 2050 Water Quality Improvement Plan, the DIN reduction associated with plant uptake in vegetated buffers should assist in meeting these water quality improvement targets.
This extended investigation over three years demonstrates that vegetated road embankments within low lying wetland and tidal areas have a key role to play in the management of stormwater run-off from elevated roadways. The reduction in nutrients, sediment and some metals through assimilation should be accounted for in any stormwater treatment design process. Where engineered stormwater improvement devices can’t be used due to site constraints, the appropriate design of vegetated road embankments (buffer strips) within the road corridor to treatment stormwater runoff should be considered. To maximise the ability of these vegetated road embankments to assimilate contaminants prior to entering waterways or tidal zones, the design should consider maximising flow paths using design changes such
as appropriate batter slopes and contour bunds (at about 3m) (if safe and practical). This design approach may widen the construction footprint but the long-term water quality improvement gains should be considered in any road project proposed in or adjacent to wetlands and tidal mangrove areas. To maximise the ability of these vegetated embankments to continue to assimilate nutrients and metals, the ongoing maintenance of the vegetation including harvesting of any cut vegetation is critical to ensuring the long-term removal of contaminants from the road corridor.
Catchment Solutions would like to acknowledge the Queensland Department of Transport and Main Roads for funding this project and undertaking repairs and maintenance on the stormwater monitoring collection system over the three years. Additionally, Tetra Consulting are thanked for their assistance in drafting up the stormwater collection figure. Thanks, is also extended to Timothy Dalton and Stephanie Kiem from the Department of Transport and Main Roads whose assistance and reviews made the successful completion of this program possible.