Potential role of dissolved oxygen and manganese concentration on the development of biofilms causing reduction in hydraulic capacity of a gravity-fed irrigation system.

Alno Carstens, Lee Bryant, Marelize Botes, Gideon Wolfaardt

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1 Citation (SciVal)

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 the reduction of hydraulic capacity, estimated to be as high
as 20%, resulting in water delivery below design capacity for the
production of a variety of produce such as mangoes, tomatoes, papayas and
citrus cultivated on nearly 7000 hectares. The potential role of
manganese (Mn) concentration on biofilm development is of interest, since
the water is currently extracted at depth near the bottom (approximately
45-50 m when the reservoir is at 100% capacity) of the reservoir 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 as high as 8631 μg l-1
near the bottom. In the main, 1.5 m diameter pipeline, Mn concentrations
decreased with distance; inductively coupled plasma mass spectrometry
(ICP-MS) analyses indicated a decrease 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.
Original languageEnglish
JournalScience of the Total Environment
DOIs
Publication statusPublished - 10 Sep 2020

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