IMPACT OF WASTEWATER TREATMENT WORKS ON GROUNDWATER

Roger Parsons

Parsons and Associates Specialist Groundwater Consultants PO Box 2606, , 7129 Tel (021) 855-2480; Fax (021) 855-2363; E-Mail [email protected]

ABSTRACT

The impact of waste disposal sites on groundwater is well documented. As a result, stringent measures are in place to reduce the impact of current and future waste sites on valuable groundwater resources. The impact of wastewater treatment works on groundwater, however, is less well documented. Geohydrological investigations and groundwater monitoring at such facilities have not been prerequisites for issuing operating permits. However, recent geohydrological studies in the vic inity of the WWTW, the Zandvliet WWTW and the Bellville South WWTW have shown these facilities result in levels of groundwater contamination comparable to that caused by waste disposal sites elsewhere on the Cape Flats Aquifer. It is hence argued requirements similar to those for waste disposal sites need to be prepared and implemented for WWTWs.

INTRODUCTION

The impact of waste disposal sites on groundwater is well documented. As a result, stringent measures are in place to reduce impacts of current and future waste sites on valuable groundwater resources (DWAF, 1998). Minimum requirements for landfill, monitoring and disposing hazardous waste were introduced in 1994, revised in 1998 and are currently being updated. These requirements resulted in a significant improvement in the siting, design, construction and operation of waste disposal sites in .

Impacts of wastewater treatment works on groundwater, however, are less well documented. Geohydrological investigations and groundwater monitoring at such facilities have not been prerequisites for the issuing of operating permits. Water quality management targets effluent quality, but not geohydrological issues. For example, groundwater investigations have been undertaken at very few of the 26 wastewater treatment works (WWTW) in the Cape Metropole, with regular groundwater quality monitoring implemented at only one. This is in contrast to all 6 operational waste disposal sites having been investigated geohydrologically and groundwater monitoring undertaken at least on a six monthly basis. The Department of Water Affairs and Forestry (DWAF) has also required a number of unsuitable waste disposal sites be closed.

Paper presented at the Biennial Conference of the Water Institute of Southern Africa (WISA) 19 – 23 May 2002, Durban, South Africa www.wisa.co.za ISBN Number: 1-86845-844-X CD-ROM produced by: Water Research Commission (WRC), www.wrc.org.za Organised by: Conference Planners

Groundwater investigations have recently been undertaken in the vicinity of 3 WWTWs in the Cape Metropole (Figure 1). A groundwater investigation was initiated at the Zandvliet WWTW following a spill from temporary sludge lagoons (Parsons, 1998). As part of the permit application procedure, an investigation at the Bellville South waste disposal site required impacts of historic wastewater disposal be considered (Parsons, 2000a). Investigation into groundwater contributions to Zeekoevlei revealed seepage from the adjacent Cape Flats WWTW accounted for 34% of nutrients in the vlei (Parsons, 2000b). A subsequent study south of the WWTW was then initiated (Parsons, 2001b). These studies clearly indicated the WWTWs have resulted in significant groundwater contamination. This paper presents the findings of investigations at the three sites and argues requirements similar to those set for waste disposal sites are required for WWTW.

AQUIFER DESCRIPTION

The WWTW described in this paper are located on the extensive Cape Flats Aquifer. This primary aquifer has been studied in detail by, amongst other, Henzen (1973) and Wright and Conrad (1995) and comprises unconsolidated sands of Tertiary to Quaternary age. In places, the permeable sands attain a thickness in excess of 50 m and are underlain by less permeable decomposed granites and argillaceous sediments. The aquifer is recharged by direct rainfall, which amounts to about 550 mm/a. Groundwater quality is general good, but can vary significantly over short distances. Ambient concentrations of potassium and nitrogen, both good indicators of contamination by waste disposal sites and WWTWs, are generally less than 2 mg/L. The aquifer is classified as a major aquifer vulnerable to anthropogenic impacts

ZANDVLIET WWTW

Following a spill from temporary sludge lagoons, a geohydrological investigation was initiated to assess the impact of the spill on groundwater quality (Parsons, 1998). The WWTW is located on the southeastern limb of the Cape Flats Aquifer where sand thickness ranges between 5 and 15 m. Depth to groundwater is generally less than 2 m.

It was found the sludge spill of 1998 had little impact on groundwater quality. This was ascribed to prompt remedial action by the responsible authorities. Impacts from a significantly smaller spill south of the works in 1996, which was not remediated, were readily detectable. However, the most noticeable impacts at the Zandvliet WWTW were those associated with sludge management practices, and particularly the disused sludge drying beds and temporary sludge lagoons (Figures 2). The old sludge drying beds were abandoned soon after being brought into operation in 1989 because of the shallow water table.

Using Piper diagrams it was possible to distinguish between contaminated and uncontaminated groundwater. However, potassium and nitrogen concentrations in excess of 80 mg/L and 160 mg/L provided an indication of the severity of contamination. These concentrations far exceed ambient levels, which generally are less than 2 mg/L.

BELLVILLE SOUTH WASTE DISPOSAL SITE

The Bellville South waste disposal site is located on the northeastern extremities of the Cape Flats Aquifer. The saturated thickness of the aquifer exceeds 20 m with depth to groundwater generally less than 3 m. The area in the vicinity of the waste disposal site has been used to dispose ’s sewage since the 1930s and as a waste disposal facility since the 1960s. Interpretation of old aerial photographs indicates the waste pile is located on a series of lagoons used to dispose of night soil transport to site by donkey cart, and later by train.

Detailed geohydrological investigations were undertaken at the site as part of the permit application for the operation and expansion of the waste disposal site (Parsons, 2000a). Very high levels of potassium and nitrogen were found to the south of the site (Figures 4). This was unexpected as groundwater flows in an easterly direction. Correlation of the position of historic sewage ponds and the plume clearly indicates the ponds to be the source of contamination. This was supported by earlier isotopic work undertaken by Levin et al. (1995) and Saayman (1999). Though the waste disposal site almost certainly impacts the underlying vulnerable aquifer, these effects could not be detected and are probably masked by contamination from historic sewage disposal.

Effluent from the Bellville WWTW discharged into the currently exceeds general effluent standards. In contrast, modelling the current position of the plume and the rate of advancement indicated the plume would only reach the Kuils River, located some 2 000 m east of the waste site, in about 15 years (Parsons, 2000a). It was also estimated contaminated groundwater discharging into the river would only exceed general effluent standards in the next 50 to 60 years.

CAPE FLATS WWTW

As part of an investigation by Southern Waters (2000) into remediation of Zeekoevlei and Rondevlei, the groundwater contribution to the vleis was assessed (Parsons, 2000b). In contrast to earlier findings that surface water accounted for total inflow, it was found groundwater contributes about 15% of the total inflow into the vlei and is the sole source of water during the dry summer months.

An important finding of the investigation was that about 35% of the annual phosphorus load discharged into Zeekoevlei is derived from subsurface seepage from the adjacent Cape Flats WWTW. Other major sources of phosphorus included the Big catchment (28%) and sediments trapped in the vlei (25%). The concentration of phosphate (as P) discharged from the WWTW into Zeekoevlei via the subsurface was set at 7.5 mg/L (Parsons, 2001a). Surface and subsurface concentrations of phosphate are presented in Figure 5. These levels of phosphate concentrations are considered very high as ambient concentrations are less than 0.1 mg/L and phosphate is immobile in the subsurface.

It is of interest to note that potassium and nitrogen contamination levels are lower than those recorded at the Bellville and Zandvliet works. This may relate to the impact at the Cape Flats WWTW being caused by the effluent ponds while that at the two other WWTW is the result of the sludge ponds. While reduction of the phosphorus contribution from the Big Lotus River catchment requires long-term land use planning and catchment management actions, reducing contributions from the WWTW and trapped sediment can be attained in the short term. For any meaningful improvement in the trophic state of Zeekoevlei to be attained, both sources of phosphorus need to be addressed.

A subsequent study of the impact of the Cape Flats WWTW on groundwater south of the works revealed the quantity of nutrients discharged into the sea is about 2.5 orders of magnitude more than anywhere else along the northern shore of (Parsons, 2000c). The impact of the WWTW on the quality of groundwater discharged into False Bay is illustrated in Figure 6. Because of juxtaposition of the WWTW to the presence and concentration of phytoplankton (brown water), the causal relationship with contaminated groundwater from the WWTW cannot be ignored and needs to be reassessed.

DISCUSSION

Investigations in the vicinity of 3 WWTWs in the Cape Metropole showed them to have significant impact on underlying groundwater quality. Potassium and nitrogen concentrations exceeded 160 mg/L while phosphate concentrations exceeded 10 mg/L (as P) in the vicinity of Zeekoevlei. Ambient concentrations of are generally less than 2 mg/L while phosphate concentrations are usually less than 0.1 mg/L.

Contamination monitoring at the now closed Atlantis waste disposal site indicated potassium and ammonia (as N) concentrations of contaminated groundwater ranged between 50 and 100 mg/L (Cave and Parsons, 1996). At Coastal Park, potassium and ammonia (as N) attain concentrations in the order of 400 mg/L and 200 mg/L. Both waste disposal sites are located on similar aquifers to those underlying the WWTW described above. Comparison of contamination concentrations recorded at the waste sites to those observed at the WWTW indicates the degree of contamination is similar.

Estimates of the rate of plume migration have been made. The rate of plume migration at the two waste disposal sites were respectively estimated to be 40 m/a (Cave and Parsons, 1996) and 6 m/a (Ball and Stow, 2000). At Zandvliet WWTW the rate of plume advance was observed to be 50 m/a (Parsons, 2001) while at Bellville the plume migrates eastward at a rate of about 20 m/a (Parsons, 2000).

While these rates appear comparable, they do not provide an indication of the area impacted. Careful study will be required to allow for valid comparison of the area impacted in relation to the size and duration of disposal. Nonetheless, investigation of the impact of WWTW on the underlying groundwater quality of the Cape Flats Aquifer clearly demonstrates they are of a similar order of magnitude to that resulting from waste disposal sites. It is hence illogical that stringent siting, design, operation and management requirements be enforced at the 6 waste dis posal sites in the Cape Metropole, but not at the 26 operational WWTW. In light of improvements resulting from implementation of DWAFs minimum requirements for landfill (DWAF, 1998), it is argued similar requirements and standards need to be prepared and implemented for WWTWs.

CONCLUSION

Geohydrological investigations in the vicinity of 3 WWTW in the Cape Metropole have shown that groundwater contamination resulting from these activities are at least comparable to impacts resulting from waste disposal sites. It is therefore illogical that stringent requirements be set for waste disposal sites and not WWTW. Given improvements resulting from implementation of DWAFs minimum requirements for landfill, it is argued similar requirements and standards need to be prepared and implemented for WWTWs.

ACKNOWLEDGEMENTS

Work described in this paper was funded by the (South Peninsula Administation and the Cape Metropolitan Council Administration Departments of Catchment Management, Waste Management and Waste Water). Permission to publish the results is gratefully acknowledged. However, views expressed in this paper are those of the author and do not necessarily reflect those of the City of Cape Town.

REFERENCES

BALL JM and STOW JG (2000) Pollution plume migration Coastal Park landfill. IWM Conf. Proc. 5 – 7 Sept 2000, Somerset West, pp 495 – 504.

CAVE L and PARSONS RP (1996) Groundwater monitoring Atlantis solid waste disposal site – Report No 8. Report No. ENVS/S-C96019, Environmentek, CSIR, Stellenbosch.

DWAF (1998) Minimum requirements for waste disposal by landfill. Second edition, Department of Water Affairs and Forestry, Pretoria.

HENZEN MR (1973) Die herwinning, opberging en ontrekking van gesuiwerde rioolwater in die Kaapse skiereiland. Council for Scientific and Industrial Research, Cape Town.

LEVIN M, ERASMUS DS and GROBBELAAR M (1995) Finale verslag oor die geohidrologiese ondersoek van die Bellville-Suid afvalstortingsterrein vir lisensieringsdoeleindes. Report No. 55007/G2/1995, GeoSpesialiste Ing., Pretoria.

PARSONS RP (1998) Zandvliet wastewater treatment works – Assessment of the impact of effluent spills on groundwater quality. Report 028/ZAND-1, Parsons and Associates, Somerset West.

PARSONS RP (2000a) Bellville South waste disposal site specialist geohydrological investigation. Report 049/CMC -2f, Parsons and Associates, Somerset West.

PARSONS RP (2000b) Zeekoevlei and Rondevlei restoration study: geohydrological specialist report. Report 060/SWAT-F1, Parsons and Associates, Somerset West.

PARSONS RP (2000c) Assessment of the impact of the Cape Flats WWTW on surrounding water bodies. Report 074/SPM-1, Parsons and Associates, Somerset West.

PARSONS RP (2001a) Assessment of remedial actoions to reduce phosphorus discharge from the Cape Flats WWTW into Zeekoevlei via the subsurface. Report 098/KAT-D1, Parsons and Associates, Somerset West.

PARSONS RP (2001b) Zandvliet sewage treatment works – Results of 2001 monitoring in and around the treatment works. Report 089/ZAND-1, Parsons and Associates, Somerset West.

SAAYMAN I (1999) Case study on the chemical characteristics of a pollution plume and determination of its direction of movement at the Bellville waste site, Cape Town, South Africa. Unpublished draft M.Sc. Thesis, University of the , Cape Town and the Royal Institute of Technology, Stockholm.

WRIGHT AH and CONRAD JE (1995) The Cape Flats aquifer - current status. Report No. 11/95, Groundwater Programme, CSIR, Stellenbosch. Atlantis wds

Bellville South

Zandvliet WWTW Coastal Park wds

Cape Flats WWTW

Figure 1: Location of WWTW and waste disposal sites

Legend Kuils River Current sludge pond

Old sludge drying beds

Approximate area 20 of spill

Informal 10 settlement

10 Sampling position 20 40 60 Canal 80

K contour 10 (mg/L) 40 20 60 80

0 500 10 m Figure 2: Potassium concentrations at the Zandvliet WWTW (November 1998) Legend

Currrent waste site WWTW

Sewage ponds 1968 1968

Historic disposal area

Natural vlei area

1988 Aerial photograph taken in April 1968

Natural vlei area

0 m 200 m 400 m Sometime between 1968 -1988 Approximate scale Figure 3: Areas used to dispose sewage and waste at Bellville South

Sacks Circle Legend

5 Potassium contour (mg/L)

Wellpoint position

Bellville South waste disposal site

Proposed extension Existing waste site Roads

R300

5 10 20 40 80 160

0 m 150 m 300 m 450 m 600 m

Figure 4: Potassium concentrations at Bellville South (January 2000)

Legend Effluent

0.8 Surface water

Zeekoevlei Groundwater 0.0 1.3 0.0 8.2 0.0 1.8 7.9 8.3 4.9 7.7 8.8 10.3 12.2 7.5 9.6 6.1 6.8 8.3 10.4 8.4 8.5 11.9 5.9 Cape Flats 6.8 11.4 5.7 12.4 0 40 80 0 0 m

Figure 5: Concentrations of soluble phosphoru s (as mg/L P) for waters sampled south of Zeekoevlei.

Swartklip Road Weltevrede Road

Strandfontein Road Nitrogen Phosphorus Silica Cape Flats WWTW Discharge through a meter width of aquifer (g/d) Capricorn Park

Figure 6: Discharge to sea of nutrients at various points along the coast