Assessment of groundwater vulnerability in Norton Town, Zimbabwe.

Abstract

The current threats of climate change have prompted the dependency on groundwater as a sustainable supply of domestic water. As a result, maintaining groundwater quality has remained a critical intervention for many local authorities especially in developing countries including Zimbabwe. This study assessed the vulnerability of groundwater quality within Norton Townin Zimbabwe, an urban area that dependents partly on groundwater sources for domestic water supplies.The town has a number of potential pollution sources. Potential pollution sources were digitized onGoogleEarth mapusing GIS techniques. Ground control points were collected to validate and improve the potential pollution source map. Fifteen (15) systematically selected groundwater points (8 boreholes and 7 wells) located close to potential pollution sources were chosen and water samples collected from them. Four (4) sampling campaigns were undertaken in January and February 2016. The water samples were analysed for selected water quality parameters using standard methods and compared to Zimbabwean and World Health Organization limits to assess drinking suitability. The parameters that were studied include temperature, turbidity, pH, DO, electrical conductivity, TDS, total hardness, iron, sulphates, chlorides, faecal coliform and total coliform. One-way Analysis of Variance was performed using SPSS version 23 to test for any significant differences between parameters and sites. In order to determine parameters that are important in assessing variation in groundwater quality data set, Principal Component Analysis was used. The Moving Average technique in Integrated Land and Water Information System was used to plotspatial and temporal variation of groundwater in the environment. The Aquifer Vulnerability Index Model was used for mapping the vulnerability of groundwater in Norton Town. Six parameters including hydraulic conductivity, soil media, depth to water level, aquifer media, slope and land cover were assigned weights and ratings using ILWIS Software. Statistical data grouping was implemented in order to differentiate five categorical index ranges.Results for mapping potential pollution sources showedthat, industrial activities and improper disposal of solid wastes and wastewater are the main causes of groundwater pollution in Norton. Descriptive statistics for the analysed groundwater parameters showed the mean values for temperature, turbidity, pH, DO, electrical conductivity, TDS, total hardness, iron, sulphates, chlorides, faecal coliform and total coliform were 25.7 °C, 6.8 NTU, 7.2, 3.66 mg/L, 580 μS/cm, 280 mg/L, 698 mg/L, 0.05 mg/L, 455 mg/L, 282 mg/L, 1015 cfu/100mL, 991 cfu/100mLrespectively. The resultsshowed that temperature, turbidity, DO, TDS, pH, chlorides, total hardness, electrical conductivity and sulphates had significant variation of parameters (spatial and temporal) explained by (p values <0.05). PCA components F1, F2, F3 and F4 had total variability of 80% with each one of the components having 36%, 24%, 11% and 9% respectively.The significant parameterswerechlorides, dissolved oxygen, electrical conductivity and feacal coliform. From the Aquifer Vulnerability IndexModel, five different vulnerability zones were established which were; very low vulnerability (index 63-73), low vulnerability (74-84), moderate vulnerability (85-95), high vulnerability (95-106) and very high vulnerability (107-126). The results showed that 17.8% of the area had very low vulnerability, 37.2% low vulnerability, 30% moderate vulnerability, 12.5% high vulnerability and 2.5% very high vulnerability. The study identified the main pollution sources as treatment plant, agriculture, landfill, onsite sanitation and industrial discharge. The study also revealed that potential pollution sources are the main causes of groundwater contamination. The results showed that groundwater sources situated in high density areas had faecal coliform counts greater than 100 cfu/100 mL which could be harmful to human health. Groundwater quality parameters (50 %) exceeded the Zimbabwean and World Health Organisation drinking water limits. At the present moment, the area shows a total of55 % very low to low vulnerability.It is therefore recommended that water from vulnerable sources be disinfected regularly before human consumption.