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

Abstract

Groundwater quality in the Loerie Flats, and subsurface irrigation return flow to the upper Gamtoos estuary was monitored from November 1992 to April 1994. The nutrient loading of groundwater was highly variable both spatially 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 loading to the estuary, via subsurface irrigation return flow was calculated using Darcy's equation together with the mean concentration of nutrients found 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 basis. Such nutrient loads emanate from less than 20% of the total agricultural area bordering the estuary. Despite the elevated nutrient input to the estuary via groundwater discharge, the nutrient content of the estuary was generally low, but at times elevated concentrations of nutrients were recorded in the upper estuary. It was found that estuarine mixing processes and dilution, with adequate flushing have prevented the periods of low quality water from persisting in the Gamtoos estuary.


WaterSA Vol.27(3) 2001: 367-372