The applicability of a Water Quality Index (WQI) as an assessment tool for urban rivers : a case study in the Crocodile River

Abstract

The water quality of the Crocodile-West River is deteriorating and has affected the ability of the water resource to be utilised adequately. Mining, urban, and agricultural run-off have negatively impacted the water quality of the Crocodile-West River catchment and has caused water quality issues in the catchment. The Crocodile-West River is monitored by the Department of Water and Sanitation where complex water quality data are collected and stored in the water quality database called the Water Management System (WMS). The Canadian Council of Ministers of the Environment Water Quality Index (CCME WQI) was applied in the study to evaluate surface water quality data to protect aquatic ecosystems. In this study, the CCME WQI integrated physico-chemical variables collected in the Crocodile-West River to determine its applicability as an assessment tool for Department of Water and Sanitation (DWS) managers in evaluating the health of the catchment for decision-making purposes. The water quality data was sourced from the Water Management System. The water quality data consisted of annual seasonal data of DWS monitoring sites 90194, 90167, 90203, 90204 and 90233 on the Crocodile-West River from 1976 to 2018. The water quality variables chosen in the study were ammonia (NH3), nitrate/nitrite (NO3-/NO2-), phosphate (PO43-), calcium (Ca2+), magnesium (Mg2+), potassium (K+), chloride (Cl-), electrical conductivity (EC), sulphate (SO42-) and pH. Each data set was organised into average seasonal data for summer, autumn, winter and spring. This was done for each water quality variable per year for each site. The average seasonal data for each water quality variable was organised into a series of data from 1976 to 2018. The seasonal data was used to calculate the CCME WQI values and plot PCA biplots for each site from 1976 to 2018. A box-and whisker graph was plotted to determine the spatial differences in the WQIs of each site. A principal component analysis (PCA) was used to evaluate the success of the CCME WQI as an appropriate tool to evaluate water quality data of the Crocodile-West River. The overall status of the Crocodile-West River was good in the 1990s and early 2000s and deteriorated to marginal quality from 2010 to 2018. The CCME WQI was able to indicate the temporal and spatial water quality changes of the Crocodile-West River from 1976 to 2018. The trend in CCME WQI of sites situated upstream of Crocodile-West River deteriorated more than CCME WQI trends for sites situated downstream. The CCME WQI is sensitive to the changes in number (F1) and magnitude (F3) of water quality variables that exceeded the target water quality guidelines of the study. The years which have lower F1 scores and F3 scores have the highest CCME WQI scores than the years which have higher F1 scores and F3 scores.

The PCA results have indicated that the spatial distribution of the sites can influence the pollutants surveyed. The CCME WQI correlated with PCA biplot in flagging pH, PO43- and NH3 as problematic water variables that negatively impacted the water quality of each site for the period of 1976 to 2018. In addition, the PCA biplot showed that the contribution of pH, PO43- and NH3 to the pollution of each site was insignificant as compared to NO3-/NO2-, Cl- and SO42- other water quality variables in the study. The CCME WQI and PCA were implemented successfully in the Crocodile-West River. CCME WQI was flexible in integrating the water quality data and the water quality guidelines sourced from different water quality guidelines to interpret the overall water quality status of the Crocodile-West River. The CCME WQI can be enhanced by incorporating scientific and local knowledge of the river to ensure that the correct water quality variables appropriate with Crocodile-West River are included in the calculation of CCME WQI of the Crocodile-West River.