The development of a Relative Risk Method model based on the risk management of aquatic ecosystems influenced by construction activities
The water quality of Wilge River sub-catchment in the upper Olifants water management area is under threat from a number of land-use activities. The aims of this study is to use existing environmental monitoring and biomonitoring tools that are routinely applied in environmental assessments in South Africa and to interpret the results in a uniform risk-based format to allow for informed decision-making relating to the potential risk of impacts of construction activities on the aquatic resources. Water quality, toxicity, macroinvertebrates, fish and wetland status were evaluated for the period 2006-2014 for the Wilge River sub-catchment B20F area. The relationship between water quality, toxicity and the biological responses were evaluated using relevant multivariate(principal component and redundancy) statistical analyses and piper analyses. Water quality results showed that the combination of land-use impacts has affected the water quality in the Wilge River sub-catchment B20F area. The main sets of stressors therefore, is acidic water containing heavy and trace metal ions and sulphate that is attributable to abandoned mining and nutrient concentrations originating from agricultural and livestock runoff, and from untreated or poorly treated sewage. The careful management and mitigation of these pollutant sources are essential to ensure compliance to the Wilge River IWRMP RWQOs. The four-tiered toxicity assessments were found to be applicable and appropriate for measuring the change in toxicity hazards due to a range of land-uses and produced additional information when considering the relative health of a water resource under stress. The hazard categories of the sampling sites were found to have a predominantly moderate hazard to toxicity. Thus implying that the cumulative effects of the impacts, i.e. agriculture, livestock farming, mining, the construction site and the quarry are contributing to the increasing toxicity in the catchment. Comprehensive macroinvertebrate studies show that considerable variations occurred with regard to the families found between the various surveys and between each of the sampling sites. A dominance of families found had a preference for low to very low water quality, probably due to the changes in land-use. Macroinvertebrate assemblage within the Wilge River sub-catchment B20F area show that it is in a poor state of health and it is therefore imperative to maintain the ecological integrity of the Wilge River. The fish assessments showed that the highest species diversity was found at sampling reaches 4 and 5, with all fish species on the Fish Reference Frequency of Occurrence (FFROC) list being found. Anthropogenic factors such as, impeding structures i.e. dams, bridges and roads have affected fish migration in sampling reaches 1, 2 and 3 showing lower species diversity and higher fish species absences in these reaches. Fish assemblage structures were shown not to have altered due to changes in land-use as the ecological categories remained similar from assessments carried out from 2006 to 2014. WET-health assessments of the wetlands, indicated that the wetlands conditions were found to have deteriorated from March 2010 to December 2011 in the wetland complexes assessed, and can be attributed to the changing land-use. Improvements to the wetland ecological status from August 2012 to December 2014 can be due to a decrease in construction activities and an increase in wetland rehabilitation efforts implemented. The Bayesian Network-Relative Risk Method was applied as a tool to perform a regional?scale, multiple?stressor ecological risk assessment in the Wilge River subcatchment area in the Upper Olifants River catchment. The results of this study demonstrated that the bayesian network can be used to calculate risk for multiple stressors, and that they are a powerful tool for informing future management strategies for aquatic ecosystem management in the Wilge River sub-catchment. The evidence based outcomes can facilitate informed environmental management decision-making. The careful management and mitigation of pollutant sources are essential to ensure compliance to the Wilge River Integrated Water Resource Management Plan Resource Water Quality Objectives.