Palustrine wetlands in Lesotho are vulnerable to vegetation loss due to overgrazing and
the nature of the topography, the latter leading to gully erosion exacerbated by a degraded
soil structure. Degraded soils are not able to adsorb pollutants; neither can they support
vegetation growth. The presence of degraded soils in wetlands thus contributes towards
leaching of pollutants into nearby streams and groundwater resources. Khubelu wetland
(which was the focus of this study) is a palustrine wetland that discharges water into the
Khubelu stream in Lesotho. The water purification function of this wetland is pertinent
since Khubelu River is one of the tributaries at the headwaters of the shared Orange-
Senqu basin. This function is threatened by vegetation loss and soil degradation through
overgrazing and environmental conditions like extreme climatic variations. Consequently,
water released into adjacent streams from the wetland could be of low quality, further
putting at risk the health of this ecosystem and users of these streams due to toxicity
caused by the polluted water from the wetland. With predicted floods and/or droughts and
intense heat, water temperatures may rise by up to 70% in the 21st century according to
researchers. It is believed that floods would lead to shorter residence time of water within
wetlands, washing away soil with pollutants into surrounding streams before any
geochemical processes that would sequester them occurs. Droughts on the other hand
would lead to failure of dilution of polluted waters. Excessive evaporation due to intense
heat would also leave pollutant-concentrated water behind. Since these wetlands are the
headwaters of an international river, the problem of water pollution and deteriorated water
resources might be regional.
The main aim of the study was to characterise the extent of soil degradation and water
quality in the Khubelu wetland and assess the water purification ability in an endeavour to
understand the role the wetland plays in the quality of water in rivers and streams fed by
the Khubelu wetland, and also to understand how changes in climate would impact on the
wetland characteristics. In situ analyses of soil and water were done followed by sampling
of the same for further analysis in the laboratory using standard methods. Surface water
samples were collected from two sampling points in the Khubelu stream, whereas water
in the wetland was sampled from seven piezometers installed in the wetland. Three
replicates of water samples were collected from each sampling point monthly over a
period of one year. The water properties determined included pH, Electrical Conductivity (EC), Dissolved Oxygen (DO), Biological Oxygen Demand (BOD), Chemical Oxygen
Demand (COD), cations (magnesium, calcium, potassium and sodium), Total Dissolved
Solids (TDS), nitrates, phosphates and chlorides. The data generated from these analyses
were subjected to various statistical tests and the Water Quality Index (WQI) of the
wetland and stream waters determined. The water quality drinking standards were
preferred in this study since the major beneficiaries of the stream that emanates from the
wetland are human populace. Prediction of water quality in the wetland in light of the
changing climate was done using the Water Evaluation and Planning (WEAP) model.
Soil samples were collected from the upper, middle and lower areas of the wetland,
referred to as upstream, midstream and downstream of the wetland in the report, at the
same sites where the piezometers were installed. At each site, three sampling points were
identified two metres apart from each other and samples collected at depths of 15 cm, 30
cm and 45 cm at each site. The soil samples were then characterised for their texture, pH,
Electrical Conductivity (EC), Cation Exchange Capacity (CEC), Total Carbon (TC), Total
Nitrogen (TN), Organic Matter (OM), exchangeable calcium, magnesium, potassium and
sodium, and available phosphorus, using standard procedures. The soil data generated
were then subjected to data analyses and the Chemical Degradation Index (CDI) of the
wetland soils determined. Determination of the wetland’s potential to purify water was
done by assessing its ability to retain nutrients, pollutants and sediments.
Results obtained in this study showed that the wetland and stream water had
circumneutral pH with values that ranged from 6.32 -7.69. The values for Na, Ca, K, Mg,
TDS, NO3, Cl and DO in the wetland and stream waters were below the WHO drinking
water standards thresholds of 200 mg/l for Na and Ca, 12 mg/l for K, 150 mg/l for Mg, 50
mg/l for TDS 50 mg/l for NO3, 5 mg/l for DO and BOD, and 250 mg/l for Cl. Food and
Agricultural Organisation (FAO) water standards for livestock drinking were: EC: <1.5
mS/cm (Excellent); 1.5 – 5.0 mS/cm (very satisfactory); < 250 mg/l of Mg for cows, 400
mg/l for beef cattle, and 500 mg/l for adult sheep. SA Irrigation water quality standards
were also used, and it was determined that pH was within the acceptable threshold of 6.5
– 8.4, 70 mg/l for sodium and 0.4 mS/cm for EC. EC of 0.41 mS/cm to 1.12 mS/cm in the
wetland and 0.67 mS/cm to 2.11 mS/cm in the stream was above the SA irrigation water
quality standards. Other water properties such as PO4 (0.06-1.26 mg/l in stream and 0.17-
0.61 mg/l in wetland), and COD (10.00 to 55.00 mg/l in stream and 48-140.80 mg/l in the wetland) were above the WHO permissible limits. The water quality in the Khubelu wetland
and stream ranged from very poor to unsuitable for drinking, with WQI values of 107 for
the stream and 93 for the wetland. Water quality simulation along the Khubelu stream
using the WEAP model shows that by the year 2025, BOD as one of the water quality
parameters, would be high, with DO declining further especially if temperature increases
and precipitation decreases. The wetland had sandy and acidic soils, with the TC and TN
content of the soil decreasing with depth. The CDI value for the soil was 3.29. Regarding
potential to reduce sediments, nutrients and organic pollutants, the wetland scored 7.09,
5.39 and 7.39 out of 10, respectively. This implies that there is moderate potential for the
wetland to purify water that is discharged into the stream.
The study concludes that the stream and wetland water qualities are unsuitable for human
consumption and usable for livestock drinking. However, there might be some risks
associated with evaporation that would leave the water saline. The wetland water presents
a threat to the water quality of the receiving stream. However, the wetland has moderate
potential to retain sediments, nutrients and toxic organics. This potential is threatened by
a predicted decrease in precipitation and increase in temperature since oxygen-depleting
contaminants and other pollutants whose behaviour in the environment are influenced by
climate are highly likely to increase in concentrations in both the wetland and the stream.
There is therefore a threat to the supply of water of good quality to the Senqu catchment,
which supplies neighbouring countries (South Africa, Namibia and Botswana). Similar
studies to this one need to be carried out for other wetlands in Lesotho on a regular basis
to come up with data that would aid policy development that seeks to protect water
resources. / Environmental Sciences / D. Phil. (Environmental Management)
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:unisa/oai:uir.unisa.ac.za:10500/27544 |
| Date | 01 1900 |
| Creators | George, Antoinette Maeti |
| Contributors | Ngole-Jeme, Veronica |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 1 online resource (xi, 183 leaves) : illustrations (mostly color), application/pdf |
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