Assessment of groundwater vulnerability due to urban settlements: A case study of Temeke District in Dar Es Salaam City, Tanzania

Abstract

The world’s human population is increasing at an unprecedented rate with much of this growth taking place in towns and citiesof developing countries.Dar es Salaamin United Republic of Tanzania is one of the fastest growing cities in Africa with a population of more than 4.5 million people.The study was carried in the populous district Temeke, one of the three districts within Dar es Salaam City. The district is accommodating 30% of total populationof Dar es Salaam(estimated to reach 1.4 million people). Approximately, less than 15% of Temeke residents supplied with surface water from DAWASCO water supply system. The rest90% rely on groundwater as their main source of potable water supplied through public and private deep boreholes or shallow wells. The illegaland improper dumping of solid waste,leakage of effluentfrom industries, poor management of sewage from domestic and commercial activities aremajor common pollution sources identified in the area.The main objective of this study was to assess the impact of rapid expansion of the urban settlements on groundwater vulnerability in Temeke District. The study assessed the spatial and temporal changes of land use and land cover changes in the area, identifiedand assessed the groundwater quality status.The study further investigated and mappedgroundwater vulnerability areas to pollution using the DRASTIC Model.Land use andland coveranalysis of the study area through GIS and Remote Sensing application using Landsat imagery TM and ETM of 1989, 1998, 2009 and 2014 undersupervised classification was applied. Results showed largeexpansion of built up area by 62% due to urbanizationfrom 1989 to 2014. Population increasecausedexpansion of built up areaand reduction ofthe open land from 47% to 13% and woodlandcover from 33% to 18%. The high expansion of built-up area with rapid emerging of informal settlements and urban slums and use of onsite sanitation threatened the water quality in the area.In assessingthe groundwater quality status of Temeke District, the area was divided into four categories namely; industrial areas (Keko,Changómbe and Kurasini), residential areas (MbagalaKuu, Mtoni andKurasini), agricultural areas (Changómbe) and commercial areas (Tandika). Groundwater samples from boreholes and shallow wells inthe selected sampling sites in the study area were analysed for physico-chemical and microbiological parameters. The parameters were analysedusing standard methods. Faecal coliforms increased in concentrationfrom 0 CFU/100 ml at Keko,MbagallaKuu and Tandika in the dry season to 170 CFU/100 ml at Changómbeduringwet season. The mean value measuredwas26 ±37CFU/100ml in the dry season and56 ± 64CFU/100ml during the wet season. The total coliform count measured in the dry season ranged from 5 CFU/100ml to 257 CFU/100ml while in the wet season it ranged from 10 CFU/100ml to 310 CFU/100ml). The mean total coliform count ranged from 67 ± 95 CFU/100ml in the dry season to 115 ± 110 CFU/100ml in the wet season. Statistical results from ANOVA indicated significant difference of faecal coliforms among sampling sites and between seasons at significant level p = 0.05 (f = 10.036, p = 0.001). A TukeyPost-hoc Test confirmed the significant differences between Tandika and Changómbe (p = 0.001), Mtoni and Changómbe (p = 0.003) Kurasini and Changómbe (p = 0.021), Keko and Changómbe (p = 0.001) and MbagallaKuu and Changómbe (p = 0.005). Analysis of varianceshowed statistical significant differences of total coliforms count among sites and between season at significant level p < 0.05(f=20.447, p < 0.000. The TukeyPost hoc Test discovered the statisticalsignificant differences of the total coliforms means between Tandika and Changómbe (p = 0.000), Mtoni and Changómbe (p = 0.000) Kurasini and Changómbe (p = 0.001), Keko and Changómbe (p = 0.000) and MbagallaKuu and Changómbe (p = 0.000) at significant level p < 0.05.Results revealedalso high concentration inturbidity, total dissolved solids,nitrate, iron, manganese,faecal coliforms and total coliforms exceeded the Tanzanian Standards and World Health Organization (WHO) permissible limits for drinking water.The rapid population increasein the area coupled with high usage of on-site sanitary facilities(pit latrines and septic tanks) and poor waste management all contribute to elevated levels of some of the groundwater quality parameters.The DRASTIC method for mapping intrinsic aquifer vulnerability was applied to identify and delineate the groundwater vulnerable area to pollution based on hydrogeological parameters ofthe study area. Results from DRASTIC Model revealed that, about 49.39% of the total area in Temeke District found to lie between moderate to extreme high vulnerable of groundwater pollution. Thesewere allareas characterized with high population density with high use of onsite sanitary facilities mostly pit latrines, septic tanks, improper dumping of solid wasteand sewage from industries, commercial and residential areas. Also, hydrogeological parameters of the area like highporosity and permeability (influencing hydraulic conductivity) of the existing alluvial sandy aquifer, high rechargefrom surface pollution sources lowered the capacity of this aquifer to attenuate all the contaminants.The study recommends the need to have regular monitoring of groundwater in Temeke District and that intrinsic vulnerability maps should be incorporated into urban landuse planning and decision making to minimise the risk of groundwater contamination from pollution load.