Abstract
M.Sc. (Aquatic Health)
The increased supply of water to areas targeted for development and the concomitant increase in land use and waste disposal activities has lead to the deterioration in the quality of water in South Africa‟s freshwater systems (DWA, 2011a). In order to manage this, the South African Department of Water Affairs (DWA) has developed numerous water quality policies and management approaches. One such approach is the determination and use of Resource Water Quality Objectives (RWQOs) (DWAF, 2006a). Resource Water Quality Objectives are a set of instream water quality objectives that aim to ensure that water that is fit for use is supplied to downstream users through the management and control of upstream impacts (DWAF, 2006b). In this study, site specific RWQOs were determined for the Modder and Riet Rivers as a means of providing a basis for reporting on instream water quality. As with many of the freshwater catchments in South Africa, the Modder-Riet catchment, located in the Free State and Northern Cape Provinces, is a water limited catchment (Slabbert, 2007). Historically most of the Modder and Riet Rivers would typically have had low water yields, with dry periods and pools forming in large endorheic areas (Grobler and Davies, 1981). As the need to supply water for crop production and domestic use grew, various transfer schemes and reservoirs were developed between the Caledon, Orange and Modder-Riet River systems (DWAF, 2006a). In order to distribute this water, various canal systems and operating rules for the storage dams were developed to transport water to the predominantly agricultural and domestic water users in the various parts of the catchment. Much of the water that is used in the Modder-Riet catchment therefore originates from reservoirs and from outside the catchment, completely transforming it from its natural state. This change in land-use and hydrology of the Modder-Riet catchment resulted in a concomitant change in the water quality. Salinisation, microbial contamination and eutrophication have, in particular, been recorded as the most significant problems affecting the fitness for use of the water in the Modder-Riet catchment. In order to manage these water quality problems and the effects on downstream users, a Catchment Management Strategy (CMS) for the Modder-Riet catchment was developed in 2006 which identified the users in the catchment and put forth a set of catchment-wide RWQOs (DWAF, 2006a). Further studies in the area have also subsequently highlighted that, given the current growth in water use, the associated negative water balance and the potential for serious deleterious effects of increased return flows on the water users, the need for improved efficiency of water use and the management of water quality in this catchment (as well as in those catchments which support the Modder and Riet Rivers) will become ever increasingly important (DWA, 2006a; DWA, 2012a; DWA, 2013a).
The aim of this study was to: 1) determine whether catchment characteristics, water quality and user requirements across the catchment differed enough to warrant the separation of the catchment into different Management Units (MUs) for which site specific instead of generic RWQOs should be set, 2) to set low confidence level 3 RWQOs for various sites which represent each MU and 3) to determine the implications for the future management of the water quality in the Modder-Riet catchment. Fourteen Level 3 RWQOs for the main constituents of concern, namely phosphate (PO4), ammonia (NH3), ammonium (NH4+), nitrate and nitrite (NO3 + NO2-N), Escherichia coliform (E. coli), Electrical Conductivity (EC), chloride (Cl), sodium (Na), the Sodium Adsorption Ratio (SAR), aluminium (Al) and fluoride (F), were determined at strategic sites within six water quality MUs. These MUs were roughly the same as those determined in the Modder-Riet CMS, with a few exceptions:
the Upper and Middle Riet MUs from the CMS were not found to be different enough in terms of land use and water quality to warrant their separation and in this study were lumped together to form the Upper Riet MU, and
the Lower Riet MU from the CMS did not take account of the operation of the Kalkfontein and Orange-Riet transfer schemes which, in this study, resulted in the delineation of the Middle Riet River MU from Kalkfontein Dam to the outlet of Kalkfontein Canal and the Lower Riet MU from below the Kalkfontein Canal end point on the Riet River and the transfer site on the Modder River (at Scholzburg Weir) to just above the zone of influence from the Vaal River on the lower Riet River below Soutpansdrift.
As with the delineation of the MUs, the location of the RWQO sites were determined in consultation with the Modder-Riet Catchment Management Forum (CMF) and were based on 1) their ability to exercise control over the quality of water entering a MU (resulting from upstream water uses) and 2) the need to provide water that is fit for use for the downstream users in areas of similar land uses. Within each MU, additional sites were added that would ensure fitness for use at specific strategic points. Since the RWQOs that were set at these sites were based predominantly on the current state of the water quality in the Modder and Riet Rivers they varied widely, with the strictest RWQOs being set for the transfers from the Caledon River via Novo Transfer Scheme, at Mazelspoort Weir on the Modder River and within the Kalkfontein Canal at the outlet of Kalkfontein Dam on the Riet River.
Whether the status quo should be maintained, improved or allowed to deteriorate was determined by considering the water quality management vision and goals for the catchment, the water quality needs of the downstream users and the potential impact on the upstream uses. Although the RWQOs in almost all cases were more stringent than those set for Modder and Riet Rivers in the.
CMS, of greater significance was the variation between the RWQOs set at each of the sites. By using the current state of water quality as a reflection of fluvial processes (such as assimilation, deposition and dilution), the RWQOs in this study took account of the differences in the effects of the various land uses and allowed for the determination of objectives which were tailored to specific areas of the catchment. They therefore more accurately represented the user requirements and the need for interventions. The RWQOs set by the CMS on the other hand were applied at catchment scale (i.e. there was one set of RWQOs for the entire catchment) and as such may have been too lenient in some areas and too strict in others. Thus although broad scale objectives set by the CMS may be useful for water quality assessments in the Modder-Riet catchment, the need to account for differences in local factors which influence water quality across the catchment (such as the presence of multiple transfer systems and large dams, and variations in land and water use) would warrant the determination of level 3 RWQOs. Based on an assessment of the compliance to the RWQOs over the past three to five years, PO4, NH3, EC, E.coli and Al were identified as the major water quality variables which have the potential to impact on the downstream users and for which Source Management Objectives (SMOs) and water quality allocation, management and implementation plans for the urban and agricultural sectors will be required. This is especially evident in the middle Modder and lower Riet catchments as well as below the numerous small urban areas which occur throughout the catchment. Key factors which have been highlighted for consideration in the future management of water quality in these catchments include inter alia: the consideration of appropriate water quality models, the development and implementation of agricultural Best Management Practices (BMPs) and the improved management of Sewerage Treatment Works (STWs). In addition, Persistent Organic Pollutants (POPs) and metals such as Al, chromium and manganese have been identified as potential variables which may affect users and for which further investigations are required.
In conclusion, the hypothesis that that “the characteristics of the Modder-Riet catchment were different enough to warrant the determination of level 3 RWQOs, where in the more impacted portions of the catchment, water quality stress would be evident and would require a number of source directed interventions” could be accepted. It was however also found that interventions to improve instream water quality should not only be based on Source Directed Controls (SDCs) in their strictest sense, but should also incorporate aspects of good governance and effective regulation. This includes improved co-ordination of water quality monitoring and data management and reporting, effective and sustained capacity building and community learning and the establishment of functional, multi-scale feedback mechanisms. The application of the principles of Strategic Adaptive Management (SAM) was also identified a key element in the future management of water quality in this catchment.