3.3 Timing of pollutant losses related to environmental impact
The timing of losses is a critical factor in determining the relative risk of pollutants to the receiving environments . For example , the highest risk periods for pesticide impacts on freshwater ecosystems are ( Davis et al ., 2015 ):
� Acute effects in main stream channels would be expected primarily during first flush events ; occurring soon after herbicide application .
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The first flush generally washes away much of the instream plant biomass making detection of herbicide effects unlikely in the main stream . However , large events may be the main avenue for delivery of herbicides ( and water ) to off-channel floodplain wetlands that are recharged ( but not necessarily flushed ) by floodwaters .
Irrigation tailwater runoff events during the dry season may present a special case , acting like first flush events , especially in cases where the receiving stream does not have strong natural baseflow ( see Davis et al ., 2013 ).
For coastal and marine ecosystems , any risk from pesticide runoff delivered from the Lower Burdekin areas is only going to occur in wet season rainfall events and is considered to be high to very low risk dependent on the distance of the ecosystem from the stream mouth ( Lewis et al ., 2013 ; Brodie et al ., 2013 ). As an example , coastal seagrass in Bowling Green Bay directly influenced ( within 5 kilometres ) of Barratta Creek may be at high risk from herbicides discharged in an event flow . In contrast for coastal seagrass in Cleveland Bay , ~ 80 km from the mouth of Barratta Creek the risk will be very low from herbicide discharge due to dilution .
These concepts are summarised in Table 3.4 . As an additional factor , groundwater recharge from deep drainage associated with increased irrigation has been a significant factor in the rise of groundwater levels in the Lower Burdekin ( DNRM , 2017 ). Where these irrigation practices and systems are inefficient the contribution to deep drainage is more pronounced .
Table 3.4 . The relative risk of DIN , pesticides and suspended sediment runoff from sugarcane in the Lower Burdekin catchment to receiving aquatic environments . High risk indicates that the pollutant load / concentration is likely to exceed ecological health thresholds at levels that may lead to severe ecological damage over an extended area ; Moderate risk indicates that the pollutant load / concentration is likely to cause degradation of ecosystems at local scales ; and Low risk indicates that the loads / concentrations are not likely to result in measurable impacts on ecological health .
Asset Nutrients – DIN 1 PSII herbicides 2 Sediment 3
Groundwater |
Uncertain ( limited evidence and confidence in existing data ) |
Uncertain ( limited evidence and confidence in existing data ) |
Low |
|
Freshwater reaches of rivers and freshwater / coastal wetlands
Estuarine reaches of the rivers
Coastal intertidal & subtidal seagrass
Coral reefs – inner shelf
|
Moderate to High ( limited evidence of effects on aquatic plants in region ) Dry season & first flush ( e . g . hypoxic events ) Moderate to Low ( limited understanding on effects on biota & WQ data ; trophic interactions ) Dry season & first flush High ( high flow conditions only ) Interaction with sediment >> flocs and increased turbidity Wet season Locally high ( high flow conditions only ) Interaction with sediment >> flocs and increased turbidity Wet season ; irrigation tailwater nutrient loss is low to no risk |
Very high to moderate ( depending on location ) Dry season & first flush events
High to low First flush events
High to low Wet season esp . first flush events
Low to very low Wet season
|
Low – drain erosion limited
Low
Low
Low
|
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