Abstract
Water from the Blyderiver dam in the Mpumalanga province, South Africa (SA), is used for gravity-fed irrigation. Biofilm development in the irrigation pipelines causes an increase in pipeline surface roughness leading to reduction of hydraulic capacity, resulting in water delivery below design capacity for the production of a variety of produce. The potential role of manganese (Mn) concentration on biofilm development is of interest, since the water is currently extracted at depth near the bottom of the reservoir (approximately 45-50 m when at 100% capacity) where high Mn levels were measured during four sampling events spread over two years. In the water body, dissolved oxygen (DO) and Mn concentrations showed a strong, inverse correlation, with rapid decrease in DO at increased depth, mirrored by an increase in both total and soluble Mn. The depth of this inflection point was found to correlate with the reservoir’s water level. DO concentrations typically remained constant between 8 and 9 mg l-1 in the upper regions of the water column, followed by a rapid decline to lower than 2 mg l-1 at deeper depths. Similarly, Mn concentrations remained constant with increasing depth, ranging between 10 and 100 µg l-1, followed by a rapid increase once the depth is reached where DO levels started to decline, up to 8631 µg l-1 near the bottom. In the main, 1.5 m diameter pipeline, Mn concentrations decreased with distance; from 8631 µg l-1 at the extraction point to 134 µg l-1 at 23 km downstream in the bulk aqueous phase, while in the biofilm biomass, Mn concentrations decreased from 30105.4 mg kg-1 at 4.5 km to 23501.9 mg kg-1 at 12.5 km, and 13727.7 mg kg-1 at 28.4 km downstream. This decrease in Mn concentration with distance suggests that biofilm accumulation has not yet reached a steady state.
Highlights
The Blyderiver dam, located in Mpumalanga, South Africa, is a major source of water used for irrigation of a variety of agricultural produce
The water level of the dam was at ∼80% and it was expected that dissolved oxygen (DO) levels near the bottom of the dam would be similar to that of the epilimnion, due to sufficient mixing between the two layers typically observed during turnover between seasons (Gantzer et al, 2009; Willey et al, 2011)
The higher concentrations of Mn measured at deeper depths compared to that in the upper layers led to the conclusion that the extraction of water for the Lower Blyde Irrigation System (LBIS) network at the depth with Mn-rich water may select for microorganisms that derive energy from the Mn cycle, as Mn reducing microorganisms are capable of using oxidized Mn as an electron acceptor to obtain energy for anaerobic growth (Nealson and Myers, 1992)
Summary
The Blyderiver dam, located in Mpumalanga, South Africa, is a major source of water used for irrigation of a variety of agricultural produce. The dam with a capacity of ∼54 million cubic meters is fed primarily by two rivers, the Orighstad river and the Blyderiver, with some minor contribution from the Treur river. Water from this dam is supplied under gravity to farms in the Mpumalanga and Limpopo provinces via the Lower Blyde Irrigation System (LBIS). The main contributor to these changes was identified as biofilm accumulation in the pipe network (White, 1999; van Vuuren and van Dijk, 2012) This has serious implications for the local economy and agricultural output, as a notable share of the country’s mangoes, citrus, papaya and avocadoes are produced on the ∼7,000 hectares of land being irrigated
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