An investigation of groundwater vulnerability within the vicinity of a landfill: A case study of Pomona Landfill, Harare.

Abstract

Landfills are considered to be one of the major threats to groundwater quality. The study focused on Pomona Landfill in Harare which is the only official active landfill for Harare City Council. The main purpose of this study was to investigate groundwater vulnerabilitywithin the vicinity of the landfill. Groundwater and leachate samples were collected and analysed for selected water quality parameters and compared to Zimbabwe Standards of Drinking Water Quality and World Health Organization Guidelines. Groundwater samples were collected from eightpoints, two located upstream of the landfill and four located downstream of the landfill and two within the landfill. Leachate samples were collected from a leachate trench at the centre of the landfill and from a pond down-gradient where the leachate was drained out by gravity. Samples were collected and analysed, from February 2015 through June 2015 giving a total of thirty-two (32) groundwater samples and eight (8) leachate samples. The Hydrologic Evaluation of Landfill Performance (HELP) Model was used to estimate the quantity of leachate generated. In this study, the DRASTIC Model was also used for a part of Pomona area to generate a small-scale map of groundwater vulnerability to pollution.The results showed that,most parameters (87%) satisfied the Zimbabwe Standards of Drinking Water Quality and the stipulated World Health Organization potable water guidelines except turbidity and iron. The average volume of leachate discharged from Pomona Landfill during the period 1983 to 2014 was 94 486 m3/year. The average annual leakage from the landfill base was 13% of the average annual total precipitation of 708 140m3/year.Four different vulnerability zones were determined, namelylow vulnerability (38%), moderate vulnerability (58%), high vulnerability (3%)and very high vulnerability (1%). The current results show insignificant impact of the landfill operations on the groundwater resource. The existing soil stratigraphy at the landfill site consisting of clay and silt-clay is deduced to have influenced natural attenuation of leachate into the groundwater resource. It is however observed that in the absence of a properly designed leachate collection system, uncontrolled accumulation of leachates at the base of the landfill pose potential contamination risk to groundwater resource in the very near future. It is recommended that groundwater be monitored regularly and a properly engineered landfill be constructed.