Integral Application of Chemical Mass Balance and Watershed Model to Estimate Point and Nonpoint Source Pollutant Loads in Data-Scarce Little Akaki River, Ethiopia

Zelalem Abera Angello,Beshah M Behailu,Jens Tränckner

doi:10.3390/su12177084

Abstract

The quality of Little Akaki River in Addis Ababa (Ethiopia) is deteriorating significantly due to uncontrolled waste released from point and diffuse sources. In this study, pollution load from these sources was quantified by integrating chemical mass balance analysis (CMB) and the watershed model of pollution load (PLOAD) for chemical oxygen demand, biochemical oxygen demand, total dissolved solid, total nitrogen, nitrate, and phosphate. Water samples monitored bimonthly at 15 main channel monitoring stations and 11-point sources were used for estimation of pollutant load using FLUX32 software in which the flow from the soil and water assessment tool (SWAT) model calibration, measured instantaneous flow, and constituent concentration were used as input. The SWAT simulated the flow quite well with a coefficient of determination (R2) of 0.78 and 0.82 and Nash-Sutcliff (NSE) of 0.76 and 0.80 during calibration and validation, respectively. The uncharacterized nonpoint source load calculated by integrating CMB and PLOAD showed that the contribution of nonpoint source prevails at the middle and downstream segments of the river. Maximum chemical oxygen demand (COD) load from uncharacterized nonpoint sources was calculated at the monitoring station located below the confluence of two rivers (near German Square). On the other hand, high organic pollution load, biochemical oxygen demand (BOD) load, was calculated at a station upstream of Aba Samuel Lake, whereas annual maximum total dissolved solid (TDS), total nitrogen (TN), and phosphate load (PO4-P) from the nonpoint source in Little Akaki River (LAR) were found at a river section near Kality Bridge and maximum NOX load was calculated at station near German Square. The integration of the CMB and PLOAD model in this study revealed that the use of area-specific pollutant export coefficients would give relatively accurate results than the use of mean and median ECf values of each land use.

Highlights

Nowadays, urban rivers of developing countries are heavily polluted due to the release of pollutants from the point and nonpoint sources where the determination of accurate pollution load to a river is often difficult due to combined factors of financial, data quality and availability, and technical capability making the river water quality management more challenging [1]
MAvaatihlaebmilaityicaolflyth, ethloecpaloplloultlauntatnlot aedxpuosritncgoeeffixpcoiernttcioseaffpicrieernetquwisitihte afonrianccclursaiotendoefteprrmecinipaittiaotnioonf pinodlluucteadntpsolloluadtisonincanwbaeteerxsphreeds.seHdobwyever, the study area did not have an established pollutant export coefficient (ECf) and determination for Little Akaki River (LAR) depends on the danta on another watershed elsewhere with nearly similar hydrological, topographical, land use, andLic,j=lim෍atሺiEcks,i×etA-uk,pj+.PTi,joሻ account for catchment variability and(4t)o select appropriate ECf values, we evaluated coke=ffi1 cients globally (Table 4)
The discussion mainly focuses on the major pollution hotspots in the watershed and the pollutant contribution of various land uses were quantified

Summary

Introduction

Urban rivers of developing countries are heavily polluted due to the release of pollutants from the point and nonpoint sources where the determination of accurate pollution load to a river is often difficult due to combined factors of financial, data quality and availability, and technical capability making the river water quality management more challenging [1]. Despite the wide range of challenges for the estimation of pollutant loads on different watersheds, scientists have tried to develop different approaches and come up with various best pollution management practices. The most common approaches for pollutants load estimation are based on watershed models that require extensive data, which were reported in the works of [5,6,7,8]. Researchers often recommend the study of pollutant loads rather than the concentration could ease the river pollution and pollutant load management in a river system [11]

Objectives

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Results

Conclusion