Assessment of water quality using GIS techniques and water quality index in reservoirs affected by water diversion

Abstract

This paper assesses the quality of the excess water diverted from the Gurara reservoir to the Lower Usuma reservoir, with declining water resources, based on the Nigerian Standard for Drinking Water Quality (NSDWQ) and World Health Organization (WHO) permissible limits. A near-laser analysis was carried out on different samples from each of the two reservoirs both in the dry and wet season using geographic information system (GIS) technique and water quality index (WQI). Samples collected were analyzed for pH Temperature, Taste, Odour, Electrical Conductivity (EC), Total Dissolved Solid (TDS), Dissolved Oxygen (DO), Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD), Sodium (Na⁺), Bicarbonate (HCO₃^-), Chloride (CI^-), Nitrate (NO₃^-), Calcium (Ca²⁺), Magnesium (Mg²⁺), Iron (F^-) and potassium (K⁺). The obtained values were evaluated and compared with the standard permissible limits. The result shows a link between the two reservoirs. The physiochemical determinants are within the maximum permissible limits with some slight variations at the Lower Usuma reservoir. The diverted water from the Lower Usuma reservoir was rated as a good water type with index of 62.5% in the dry and 49.7% in the wet season by WHO’s permissible limits, whilst the Nigerian Standard for Drinking Water Quality rates it at 83.3% at the dry season and 69.0% at the wet season. The water from the Gurara reservoir was rated as an excellent type with index of 43.23 % in the dry season and 45.7% in the wet season by WHO’s permissible limits whilst the Nigerian standard for drinking water quality equally rates it at 61.7% and 65.5% in the wet season. According to the correlation matrix, predominant determinants amongst others show both positive and negative correlation at seasonal basis. The spatial variation map shows higher concentration along downstream section at both reservoirs. The study outlined that the surface water of the area is impaired by the anthropogenic activities and needs proper management plan to control further contamination and pollution of the surface water of the area. Further, the surface water of the receiving region needs a proper treatment before its consumptive uses. The application of GIS techniques and water quality index in the assessment of water quality in the context of reservoir systems revealed to be a valuable contribution statistically to the understanding and management of connected water bodies. Future analyses will concentrate on broader applications of this methodology in the context of the understanding and management of reservoir systems in Nigeria. This is, therefore, a call on water managers to put in place routine mechanisms of monitoring the water from reservoirs affected by water diversion, due to the ever-increasing anthropogenic activities around the upstream of reservoir watershed in the coming decades.