Assessing the relationship between water quality parameters and changes in landuse patterns in the Upper Manyame River, Zimbabwe

Abstract

For the past 30years, the increases in population pressure and external influences, such as economic growth, have accelerated the demand for land within the Upper Manyame River catchment in Zimbabwe which has caused substantial changes in landuse. The general objective of this research was to assess the impacts of landuse activities on the water quality of the Upper Manyame River which drains the rural and urbanised part of the catchment up to flow gauging station C21. Landcover data for the month of April in years of 1984, 1995, 2003 and 2011 were acquired from available Landsat TM and ETM images and were classified through the maximum likelihood digital image classification using the supervised classification approach. The status of water quality of the Upper Manyame River was also assessed through analyses of historical concentrations and pollution loads for TP, DO, COD, NH3-N, SS, Pb, NO3, BOD5, EC, PO4-P and TN at the Environmental Management Agency (EMA) gauging station CR21 sampling point for 1996, 2000/1 and 2008/9. Water quality of 15 monitoring sites comprising 25 water quality parameters were monitored monthly from January to June 2012. These locations were selected to reflect a wide array of landuse for both the dry and wet seasons. The results indicated that there was an increase in pollution load from 1995 to 2012; for TP from 130kg/day to 376kg/d, and for TN from 290kg/day to 494kg/d. This indicates high pollution levels which have severe impacts on downstream users and also severe sewage contamination. Significant deviations occurred in DO (0.1–6.8) mg/L, COD (11–569) mg/L, BOD5 (5–341) mg/L, PO4-P (0.01–4.45) mg/L, NH3-N (0.001–6.800) mg/L and EC (38–642) μS/cm. Hydrologic Response Unit and buffer analysis were used to determine the dominant landuse which contributes to a certain water quality. Results of digital image classification indicate that woodland/forest, grassland and bareland decreased between years 1984 to 2011 by 24.0%, 22.6% and 31.7% respectively. This was mainly due to expansion or increase of agriculture and urban areas by 24.4% and 41.6% respectively over the same time period. It was concluded that settlements and agricultural areas are the ones mainly affecting the water quality in Upper Manyame River with a Pearson’s Correlation Coefficient of r=0.97 for COD and r=0.78 for TSS respectively. It was also observed that the water quality status of the Upper Manyame River and its tributaries is very poor and the level of pollution generally increases from upstream to downstream. The results obtained provided baseline information which may be used in the development of appropriate water quality management systems in the catchment. Thus the study recommended a combined programme of point source control and landuse modification.