The impact of irrigation return flow on aspects of the water quality of the Upper Gamtoos Estuary, South Africa

Groundwater quality in the Locric Flats, and subsurface irrigation return f low to the upper Gamtoos estuary was monitored from November 1992 to April 1994. The nutrient loading of groundwater was highly variable both spatiall y and temporally, with elevated nitrate-N levels reaching 163 mg-l(-1). The elevated nutrient concentrations in the groundwater were directly related to the timing of land-surface application of fertilisers. The nutrient load ing to the estuary, via subsurface irrigation return flow was calculated us ing Darcy's equation together with the mean concentration of nutrients foun d in groundwater in the Loerie Flats area, Results indicate that an average of 0.52 t nitrate-N, 0.02 t nitrite-N and 0.10 t of total phosphorus enter the Gamtoos estuary from the 5.5 km(2) Loerie Flats area on an annual basi s. Such nutrient loads emanate from less than 20% of the total agricultural area bordering the estuary. Despite the elevated nutrient input to the est uary via groundwater discharge, the nutrient content of the estuary was gen erally low, at times elevated concentrations of nutrients were recorded in the upper estuary. It was found that estuarine mixing processes and dilutio n, with adequate flushing have prevented the periods of low quality water f rom persisting in the Gamtoos estuary.