Assessment of water quality, soil degradation and water purification ability of Khubelu wetland in Mokhotlong Lesotho, and the implications of climate change

Abstract

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, v 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 vi 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.