Nutrient reduction options in Hartbeespoort Dam catchments to lower in-dam eutrophication status
Botha, Frederick Johannes
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Eutrophication of surface water constitute a major threat to the provision of raw potable and irrigation water in South Africa, being largely dependent on impounded water in order to ensure water supply . South Africa is highly dependent on water stored in dams for socio-economic uses. Some of these dams, including the Hartbeespoort Dam, are experiencing excessive amounts of nutrients which are introduced by raw or partially treated effluents or run-off water from irrigated land that had fertilizer applied. Dams that have been affected by high levels of nutrients in the presence of abundant blue–green algae cause cyanobacterial blooms, which produce cyanotoxins. These cyanotoxins can pose a health-risk to both humans and animals (including aquatic animals) if ingested. The affected dams are classified as either hypertrophic (excessive levels of nutrients) or eutrophic (high levels of nutrients). Algal and cyanobacterial blooms and particularly surface scums that may result are unsightly and can have unpleasant odours. Eutrophication of water results in various symptomatic changes which can include the increased production of algae and aquatic macrophytes and the deterioration of water quality. The process occurs naturally over geological time, or may be accelerated due to allochothonous anthropogenic impacts, often referred to as “cultural” eutrophication. Phosphorous (P), and to a lesser degree nitrogen, have been identified as the major causes of eutrophication in surface waters. Eutrophication of dams is a major threat throughout South Africa since the early 1970’s. As a result of the eutrophication of the Hartbeespoort Dam this catchment was identified as a priority area for the implementation of the Waste Discharge Charge System (WDCS) to manage the phosphate load that is discharged from the various sub-catchments to the dam. The Hartbeespoort Dam has formed the basis of various studies undertaken by the Water Research Commission (WRC). It is factual that eutrophication is especially apparent in the inland areas of South Africa, especially around major urban areas such as the Johannesburg-Pretoria complex (Harding, 2008). Prominent limnologists recommended that in order to manage the dam effectively the eutrophication capacity for the Hartbeespoort Dam should not be exceeded. The eutrophication capacity refers to the capacity of the dam to assimilate phosphorus loads without trophic status thresholds being exceeded. The trophic status thresholds pertain to the phosphorus loading levels, translated to concentration of in-lake phosphorus that may result in increased frequency of problematic conditions such as algae blooms. The phosphate load entering the Hartbeespoort Dam is entirely due to anthropogenic activities that emanate from the urban areas in the upper reaches of the catchment. A large portion of the load is due to overflowing and leaking sewer systems, which is more difficult to address than the point sources. Due to this problem with water quality in the Hartbeespoort Dam, the catchment was selected for the research on nutrient reduction options in line with the Waste Discharge Charge System (WDCS) as included in the National Water Act (NWA) of 1998. Water quality standards (DWAF,1996) in the NWA are related to the South African Water Quality Guidelines (the WQGs) which were published in eight volumes, describing the acceptable level of substances or constituents for the different water users (Barnard, 1999:277). The objective of this dissertation is to determine the total load of phosphates that is discharged from the four sub-catchments and the area around the Hartbeespoort Dam in order to identify and quantify the different sources as accurately as possible. The load that enters the Hartbeespoort Dam can be related to the actual nutrient level present in the dam. This can be correlated by models such as the Vollenweider Model. A study by South African scientists was used to verify the relationship. From literature the threshold level of eutrophication in South African reservoirs have been determined to be 55 μg/ml P. The actual level in the Hartbeespoort Dam is about 250 μg/ml P. To achieve a level of 55 μg/ml P, the load entering the dam will have to be reduced by about 80%. This study investigated the level of load reduction that could realistically be achieved through various means. The performance of the relevant waste water treatment works (WWTW) and the non-point source contribution originating from agriculture and informal settlements, as well as storm water was calculated from available databases. Options were identified that could estimate realistic nutrient reduction levels based on current and more advanced technology. The research also touched on the nutrient reduction theory of using pre-impoundment to perform nutrient reduction coagulation before inflow of water into the Hartbeespoort dam. The problem of Acid Mine Drainage (AMD) decanting into the catchment is placed within the context of the eutrophication problem in the Hartbeespoort Dam. An intervention is investigated to convey AMD per pipeline to chemically reduce phosphate levels in the Crocodile River. WDCS and zero phosphate detergents are also discussed to optimise nutrient reduction options in the catchments. The nutrient reduction strategy is accepted as a long term goal that needs to be integrated with the National Water Resource Strategy.