Impact of effluent discharge from a medium-scale fish farm on the water quality of Odo-Owa stream near Ijebu-Ode, Ogun State, Southwest Nigeria

Abstract

This study aimed to assess the impact of effluent discharge from a medium-scale aquaculture fish farm on the water quality of a nearby receiving stream, Odo-Owa stream, near Ijebu-Ode (06° 49′ N, 003° 56′ E) in Ogun State, Southwest Nigeria. Some physicochemical water quality parameters of the effluent receiving stream were determined at seven selected sites, above and below the effluent discharge point into the stream to assess the effluent impact on the water body. The seven sampling stations comprised three stations (A–C) located upstream of the effluent discharge point (Station D), two stations (E and F) located downstream of Station D, while the seventh station (Station G) was a close-by reference groundwater source. The water quality parameters investigated include five physical parameters (water temperature, apparent color, true color, turbidity and transparency), seven general chemical parameters (pH, conductivity, TDS, total alkalinity, total hardness, ammonia and DO), seven major ions (Ca2+, Mg2+, Na+, K+, Cl−, SO42− and HCO3ˉ), three nutrient compounds (PO43−, NO3− and NO2−) and five heavy metals (Hg2+, Mn2+, Fe2+, Cu2+ and Zn2+). Water samples were collected from each sampling station twice per season: dry season (December 2013 and February 2014) and rainy season (April and July 2014), and analyzed in the laboratory using standard analytical methods. The result showed a significant difference (P < 0.05) in most of the investigated parameters between the impacted (Stations A–C) and unimpacted sections (Stations E and F) of the effluent receiving stream. Ammonia was about nine times higher; true color was more than five times higher; apparent color, depth, alkalinity, HCO3− and SO42− were each more than two times higher, while pH, Ca2+, Na+, K+, Cl− and PO43− were generally higher along the impacted section than the unimpacted section of the receiving stream. Except for Hg2+, the mean concentrations of all the heavy metals analyzed (Fe2+, Mn2+, Zn2+ and Cu2+) were also higher in the impacted portion than in the unimpacted portion. Seasonal variations across the water stretch showed that the mean (± SE) concentrations of TDS, turbidity, PO43−, Mn2+, Fe2+ and Hg2+ were significantly higher (P ≤ 0.05) in the rainy season than in the dry season. The WQI was more fair (26–50) at the unimpacted (48.42) section of the water stretch as well as in the rainy (48.36) than it was in the impacted section (47.62) as well as during the dry season (45.36). The effluent discharge point, D (40.50), recorded the least WQI. This study showed that effluent discharge from the investigated medium-scale fish farm had a significant negative impact on the water quality of the receiving Odo-Owa stream and the indicator parameters comprised of ammonia, color and alkalinity.