Abstract
South Africa is prone to droughts and is heavily dependent on stored water resources. Few studies in South Africa have investigated the impact of drought on water quality when rainfall resumes and reservoir sediments are rewet. Other research has shown that several negative impacts may occur with rewetting, including eutrophication, and acidification with increased metal solubility. In this study, the impact of drying and rewetting sediments on the mobility of nutrients and metals is explored using sediments from Loskop Reservoir. Rarely exposed sediments were collected from three sites, dried at 35°C for 36 days, and then rewet under aerobic conditions. Filtered water samples were collected on Days 1, 2 ,4, 7, 11 and 16 following rewetting, and analysed for NH 4 + , NO 3 - , NO 2 - , total N, total P, PO 4 3- , dissolved Al, Fe, Mn, and SO 4 2- . Results were expressed as mass/m 2 , and were compared to reference (non-drought) treatments that were maintained under saturated conditions. Acute toxicity tests were performed using Daphnia magna to establish if toxic effects were associated with changes in water quality following sediment rewetting. Following rewetting, elevated SO 4 2- indicated the mobilisation of acid from sulfide oxidation, with an associated reduction in pH. Nitrification was evident as high quantities of organic N and NH 4 + were initially released, followed by increases in NO 2 - + NO 3 - . Total P increased, but PO 4 3- reduced. This was attributed to complexes formed between PO 4 3- and Fe (FePO 4 ) and Al (AlPO 4 ). Amounts of Fe and Al released were lower following rewetting due to the formation of relatively immobile (hydr)oxides. In contrast, Mn increased substantially reaching maximum values > 2 300 mg/m 2 (14 000 μg/L). There were no acute toxic effects observed to D. magna. This study highlights the potential for acidification, mobilisation of nutrients, SO 4 2- , and Mn following rewetting of reservoir sediments under aerobic conditions. This information may be incorporated into reservoir and catchment management plans in the form of revised water quality guidelines and a better ability to predict how drought impacts water quality in our stored water resources. Keywords : sediment, drought, drying, rewetting, nutrient and metal release, Loskop Reservoir
Highlights
South Africa is prone to periods of drought and is heavily dependent on stored water resources
Water levels have reduced to this level and lower during previous droughts, these events are infrequent, and water level draw-down is not a standard management practice applied at Loskop Reservoir (LR)
The reduction in pH observed immediately following rewetting of dried sediments suggests the potential for mild acidification to occur following sediment rewetting in LR
Summary
South Africa is prone to periods of drought and is heavily dependent on stored water resources. Evidence from observed rainfall trends (Kruger, 2006) and model projections of climate change indicate that southern Africa may become hotter and drier, with a higher incidence of extreme events like heat-wave days and droughts in the future (Engelbrecht et al, 2009, 2015). This will place increasing pressure on stored water resources. Large areas of previously submerged sediment can be exposed for extended periods during drought until normal rainfall resumes.
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