Deterioration in the water quality of an urbanised estuary with recommendations for improvement

Deterioration in the water quality of an urbanised estuary with recommendations for improvement Adams JB1,2*, L Pretorius1 and GC Snow1,3 Department of Botany, Nelson Mandela University, Port Elizabeth, South Africa DST/NRF Research Chair in Shallow Water Ecosystems, Nelson Mandela University, Port Elizabeth, South Africa 3 School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa 1 2 ABSTRACT Water quality characteristics of the heavily urbanised and industrialised Swartkops River and Estuary in the Eastern Cape have been the focus of several studies since the 1970s. Overloaded and poorly maintained wastewater treatment works (WWTWs), polluted stormwater runoff and solid waste have all contributed to the deterioration in the water quality of the river and estuary. The objective of this study was to determine the current water quality status of the Swartkops Estuary, by investigating spatial and temporal variability in physico-chemical parameters and phytoplankton biomass and where possible relate this to historical water quality data. The present study found evidence suggesting that water is not flushed as efficiently from the upper reaches of the estuary as was previously recorded. Reduced vertical mixing results in strong stratification and persistent eutrophic conditions with phytoplankton blooms (> 20 µg chl a·L−1), extending from the middle reaches to the tidal head of the estuary. The Motherwell Canal was and still is a major source of nitrogen (particularly ammonium) to the estuary, but the Swartkops River is the primary source of phosphorus with excessive inputs from the cumulative effect of three WWTWs upstream. An analysis of historical water quality data in the Swartkops Estuary (1995 to 2013) shows that all recorded dissolved inorganic phosphorus measurements were classified as hypertrophic (> 0.1 mg P·L−1), whereas 41% of dissolved inorganic nitrogen measurements were either mesotrophic or eutrophic. If nutrient removal methods at the three WWTWs were improved and urban runoff into the Motherwell Canal better managed, it is likely that persistent phytoplankton blooms and health risks associated with eutrophication could be reduced. Keywords: Swartkops Estuary, nutrients, salinity, eutrophication, microalgae Introduction A threat facing many developing countries is the improper management of waste generated through various anthropogenic activities, combined with old and inadequate infrastructure, unskilled wastewater plant operators and financial constraints (WRC, 2013). Furthermore, urbanisation leads to increased stormwater volumes running off paved areas. Consequently, runoff that would previously have infiltrated the soil discharges directly into aquatic ecosystems, potentially harming the water quality and ecosystem health status more rapidly than diffuse sources (US EPA, 2003). Poor or inadequate stormwater management systems have also led to stormwater infiltrating the sewer system resulting in pump station overflows and overloading of sewage treatment plants not designed to cope with these loads. All these issues are relevant to the urbanised Swartkops Estuary (33.8604° S, 25.6221° E) in the Eastern Cape, where land use activities have measurably changed the water quality and health of the system (Enviro-Fish Africa, 2011; Lemley et al., 2017). The estuary, located 15 km north of the Port Elizabeth Central Business District, opens into Algoa Bay in the Indian Ocean. The estuary is 16.4 km long and the total length of the river is 155 km from the mouth to its origin (Baird et al., 1986). Swartkops is a nationally important estuary as it is one of the few systems that remain permanently open to the sea. The river and estuary meander through a highly urbanised and industrialised region of the Nelson Mandela Bay Municipality (NMBM). Anthropogenic activities affecting * To whom all correspondence should be addressed. e-mail: Received 22 January 2016, accepted in revised form 12 December 2018. https://doi.org/10.4314/wsa.v45i1.10 Available on website http://www.wrc.org.za ISSN 1816-7950 (Online) = Water SA Vol. 45 No. 1 January 2019 Published under a Creative Commons Attribution Licence the estuary include wastewater treatment works (WWTW), saltpans, sand/clay mining, brickworks, tanneries, the motor industry, wool industry, extractive/beneficiation processes as well as marshalling railway yards and depots (Baird et al., 1986). In the 60 years from 1939 to 1998, industrial and residential developments removed 35 ha of supratidal salt marsh, leaving only 5 ha (Colloty et al., 2000). The deterioration in water quality has affected recreation, for example the relocation of the Redhouse River Mile swimming event elsewhere, because of high faecal coliform bacteria counts in the water. Furthermore, a number of sites above the tidal limit are used for cultural ceremonies by traditional healers, but the water quality is now so poor that participants mentioned during an informal interview that they cannot drink from the river during their cleansing ceremonies and depend on tap water carried to the sites. Thus, the Swartkops Estuary is largely modified, with a Present Ecological State (PES) of Category D. A Recommended Ecological Category (REC) of a C is expected of this estuary (Van Niekerk et al., 2014) because of its national conservation importance. The estuary is ranked 11th out of over 280 estuaries nationwide, mainly as a result of its size, habitats and biodiversity (Turpie et al., 2002). Improvement to a C category can only be achieved through appropriate management interventions which require an understanding of the key drivers. Past studies have been of too short a duration to gather the long-term datasets needed for assessing historical patterns of change within the estuary. Our study in 2012–2013 measured water quality parameters to provide an assessment of the status of the estuary and to identify management actions and recommend monitoring requirements. River, sea and estuary sites were sampled as well as points of entry into the estuary (Chatty River, Markman Canal and Motherwell 86 Canal) to identify pollutant inputs. New data were important because outdated water quality information had been used in the Swartkops Integrated Environmental Management Plan (Enviro-Fish Africa, 2011). Bacterial measurements showed high Escherichia coli and enterococci levels, especially in the summer months, rendering the estuary unsafe for recreation. Faecal bacteria, originating from the Motherwell Canal, contaminated the middle reaches of the estuary, whereas the Swartkops River has intermediate effects because bacteria die off between the point of release from the WWTWs and the riverine reaches and the tidal limit of the estuary. Trace metal data for the water column suggested that copper, zinc, iron and cadmium concentrations have increased by at least 90% in the estuary, at the tidal limit of t (...truncated)