Assessment of the impact of pit latrines on groundwater contamination in Hopley Settlement, Harare, Zimbabwe.

Abstract

It is estimated that proximately 1.77 billion people around the world use some form of pit latrines as their primary means of sanitation. Most countries in sub-Saharan Africa failed to meet the Millennium Development Goals (MDG) target of improved sanitation by 2015. Only 66% of the world’s population has access to improved sanitation far below the MDG target of 75% by year 2015. Hopley is a settlement in Harare that was established in 2005 and had an estimated population of 15 000 by 2012. Due to many reasons the responsible local authority (City of Harare) has failed to cope with Hopley’s infrastructural development needs including that for water and sanitation. As a result many developments including Hopley Settlement have been occupied without adequate water and sanitation infrastructure. Many of the residence have resorted to shallow underground water mainly from wells and a few boreholes. Poor water and sanitation status is believed to have resulted in the 2008-2009 cholera outbreaks in Zimbabwe, which resulted in 4300 deaths. Therefore, this study sought to assess the impact of pit latrines on groundwater quality in Hopley Settlement. A supervised landuse classification of Landsat images was performed to determine landuse changes and pit latrine density in Hopley Settlement using Geographical Information Techniques (GIS) techniques. Grab sampling was performed for groundwater source quality during the period of February to April 2015 in four sampling campaigns from 11 sampling sites comprising of 3 boreholes and 8 wells. The parameters that were studied included Electrical Conductivity (EC), pH, turbidity, nitrates, chlorides, ammonia, Dissolved Oxygen (DO) and Faecal Coliforms (FC). Student t-test was performed using groundwater quality data of 44 groundwater samples from 11 sampling locations in Statistical Package for Social Sciences (SPSS) software (v 16) to determine groundwater suitability for drinking by comparing mean values of analysed groundwater parameters with guidelines/standards to check if there was any significant difference. The Inverse Distance Weighting (IDW) technique was used in a GIS environment to estimate the spatial distribution of the groundwater contaminants in the environment. Five disturbed soil samples in the vicinity of pit latrines and groundwater sources were collected at a depth of 1.5 m for permeability tests. Out of the 4 km2 forming the study area, there was a 250 % growth in built up area from 1 km2 in year 2000 to 3.5 km2 in the year 2014. FC counts in groundwater sources increased with increase in pit latrine density, while nitrates, ammonia and chlorides showed a decrease in groundwater concentration. About 250 inhabitants of Hopley Settlement occupied a space of 0.014 km2 translating to 4286 persons/km2. There was an increase in the strength of the relationship between pit latrine density and groundwater contamination by faecal coliforms as the radius increased from 15 m to 100 m i.e. r (42) = 0.425, p < 0.01 to r (42) = 0.624, p < 0.01. Groundwater location W1 to the north in Hopley Settlement had the highest pit latrine density of 79 pit latrines in 100 m radius of the groundwater source. Descriptive statistics of the analysed groundwater parameters showed mean values of 6.58 (pH), 574.89 µS/cm (EC), 5.23 NTU (turbidity), 3.70 mg/L (DO), 80.38 mg/L (nitrates), 0.54 mg/L (ammonia), 90.40 mg/L (chlorides) and 81.55 cfu/100 mL sample (FC). Student t-test showed that 84%, 50%, 14%, 34%, 9%, 34%, 64% and 0% of the 44 groundwater samples had FC, nitrates, ammonia, turbidity, Electrical Conductivity (EC), pH, dissolved oxygen and chlorides respectively exceeding the World Health Organization guideline value for drinking water.. Using Principal Component Analysis, nitrates, chlorides and EC were found as principal components contributing to groundwater contamination. Groundwater source locations that had mean FC counts >100 cfu/100 mL were generally located to the north and west in Hopley Settlement (i.e. groundwater source W1, W2, W6 were 240 cfu/100 mL, 153 cfu/100 mL and 155 cfu/100 mL respectively). The soil indicated low permeability coefficient values that ranged from 2.3 × 10-6 m/s to 8.0 × 10-7 m/s. The results showed that groundwater source locations to the north and west in Hopley Settlement had FC counts >100 cfu/100 mL indicating a high chance of pathogenic contamination. Based on pathogen survival time in soil, the results generally suggested that the low soil permeability coefficients allowed for the attenuation of pathogens in soil before reaching groundwater sources. There was also a significant positive relationship between groundwater level depth from the surface and groundwater pollution i.e. r (18) = 0.764, p < 0.05 (chlorides), r (18) = 0.831, p < 0.05 (EC) and r (18) = 0.838, p < 0.05 (turbidity); while DO showed an inverse relationship, r (18) = − 0.486, p < 0.05. Nitrates, pH, ammonia and FC showed no relationship with water level depth from the ground surface. Nitrate levels posed a threat to human health. Raised and lined pit latrines and other low cost technologies should be considered to minimize the potential of groundwater pollution.