Movement of contaminants in the unsaturated zone of the subsurface from a low flush on-site sanitation system in Ivory Park, Johannesburg

M.B Van Ryneveld,I.H Palmer,A.B Fourie

doi:10.4314/wsa.v42i4.01

Abstract

This study measured the movement of contaminants in the unsaturated zone of the subsurface from a low-flush on-site sanitation system (also known as an aqua-privy) at three sites in Ivory Park, which at the time that the fieldwork was done was a newly-established informal settlement in Johannesburg. The aim of the study was to provide field data on the movement of contaminants from this sanitation system in a low-income area, in these climatic and subsurface conditions, and to counter a perception that on-site sanitation will inevitably cause pollution. Subsurface conditions consisted nominally of a 1 m thick layer of silty sand with denser gravel at the bottom of it, underlain by a stiff sandy clay (residual granite), with the natural water table more than 15 m below ground surface. In summary, it can be deduced that movement in the unsaturated zone of bacterial indicators, nitrogen and phosphorus from low-flush on-site sanitation systems in lowincome areas is very limited, as long as the effluent remains in the subsurface. More specifically, the impact in terms of COD appears to be negligible, with substantial treatment apparently taking place within the soakaway or within the immediate vicinity thereof. With respect to (i) bacteriological indicators, (ii) nitrogen (in the form of ammonium, nitrite and nitrate) and (iii) phosphorus (in the form of Total Phosphorus), using the distilled water washout technique, there is a very rapid drop-off of contamination with distance from the soakaway. At horizontal distances of 3 m from the soakaway, levels of the measured contaminants had dropped to values consistent with the background levels. The results of this study are not confined to this particular sanitation system. By virtue of the higher hydraulic loading, these results provide an upper bound for contaminant movement from dry on-site systems under similar conditions. Keywords: contaminants, unsaturated zone, subsurface, on-site sanitation

Highlights

While good progress has been made in the provision of sanitation to all in South Africa since 1994, the target set in the Strategic Framework for Water Services for the provision of at least a basic sanitation service to all by 2010 (DWAF, 2003) remains some way from being achieved
Measurements of the vertical movement of contaminants were more limited, the rate of decrease of the contaminant concentrations in the vertical direction was slightly steeper than the rate of decrease in the horizontal direction: at 1.5 m below ground surface and 0.5 m from the soakaway, the contaminant concentrations were roughly the same as at 0.85 m depth and 2 m from the soakaway
Despite the large discrepancy between the upper- and lowerbound estimates of contaminants in the soil that might be transported, a comparison of results obtained using various experimental techniques indicated that the contaminant concentrations obtained using the distilled water washout technique were most representative of the level of contaminants which would travel in the subsurface

Summary

Introduction

While good progress has been made in the provision of sanitation to all in South Africa since 1994, the target set in the Strategic Framework for Water Services for the provision of at least a basic sanitation service to all by 2010 (DWAF, 2003) remains some way from being achieved. Sanitation backlogs, in informal settlements, continue to present a challenge even for large metropolitan municipalities such as the City of Johannesburg (CoGTA, 2009; City of Johannesburg, 2010; Kunene, 2010) Both of these points confirm the more general statement in a report prepared for the South African Human Rights Commission (SAHRC) by the Department of Performance Monitoring and Evaluation in The Presidency (DPME et al, 2012), which indicated that ‘... While higher levels of service remain the long-term objective, the high cost of construction, operation and maintenance of full waterborne sanitation systems compared to on-site sanitation systems (Van Ryneveld, 1994; Pujari et al, 2007; Still et al, 2009; Shivendra and Ramaraju, 2015) may well make on-site sanitation the most viable option for a portion of the population in South Africa for some years to come. In order to avoid this in the case of on-site sanitation, the movement of contaminants in the subsurface needs to be understood and given due consideration in the choice and design of the sanitation system (Van Ryneveld and Fourie, 1997; DWAF, 2007)

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Conclusion