Effects of gaseous emissions from the Namakwa Sands Mineral Separation Plant near Lutzville on the adjacent succulent Karoo vegetation : a pilot study

[摘要] A pilot study was conducted at the Namakwa Sands Mineral Separation Plant, to investigate theeffects of acidic gaseous emissions from the Mineral Separation Plant on the adjacent Succulent Karoovegetation. Sulphuric acid fumes, a major gaseous emission of the mineral processing, was the subjectof investigation of the present study, due to the potential high negative impact of elevatedconcentrations thereof on vegetation in the ecosystem.Permanent sample plots along three transects radiating from the Mineral Separation Plant were laid outin the eastern, south-eastern and southern directions following the prevailing wind directions andpractical consideration of land accessibility.The ecological components assessed as indicators of possible pollution levels in the environmentincluded percentage plant mortality, foliar sulphur content of selected plant species, chemicalcomposition of solubles in mist and dust samples, and soil pH. In addition, the vegetation wasscreened for plant species suitable to be used as potential bioindicators.Potential bioindicator plant species were selected on the basis of their relatively wide distribution inthe study area and apparent sensitivity to the ambient air pollutants. The percentage of dead plants ofeach species that occurred on the sample plots was used as a criterion of the possible sensitivity of theplant species towards air pollution. The bioindicator plant species selected for potential monitoringpurposes were: Galenia fruticosa, Lampranthus suavissimus, Lycium ferocissimum and a Ruschia sp.(SP 9). Plant mortality was greater nearer the emission source, with 28 + 5 % dead plants at 400 m, 19+ 6 % at 800 m and only 10 + 4 % at 1,200 m from the Mineral Separation Plant. Data summed for allspecies recorded and pooled for all three transects per sampling distance.With the methods used in this study, in the case of all sample plots on the three transects, nosignificant difference was found between the mean pH values of soil samples collected from openspaces without plant cover (8.01 + 0.46) and those collected underneath shrubs (8.91 + 0.96).Subsequently only the pH values of soil samples collected on open spaces were used to investigate thevariation in soil acidity with distance and direction from the emission source. The means representtotal number of samples from open space versus those collected from underneath shrubs. The pH of soil samples increased with distance from the emission source along the transects to thesouth and south-east of the emission source. Eastward of the emission source, soil pH values remainedrelatively low at all sample distances. This pilot study could not determine whether the continuousacidity of the soil along the eastern transect in the direction of the prevailing wind, was caused byincreased deposition of gaseous emissions on the higher lying hilly terrain in this area, or by theunderlying geology.Ion chromatographic analysis of mist and dust samples collected on each sample plot indicated thepresence of several chemicals that had probably originated from the gaseous emissions from theMineral Separation Plant as well as wind blown constituents from the adjacent surroundings of thesample plots. Of these chemicals, only the sulphate concentrations of the mist and dust samples werefurther evaluated, since that could be related to the emission of sulphuric acid fumes by the MineralSeparation Plant. Results indicated that the mean sulphate concentration of mist and dust samplescollected from sample plots relatively close to the Mineral Separation Plant, 118.8 + 31.6 mg/litre (400m), were higher than further afield, decreasing to 57 + 30.1 mg/litre at 800 m and 43.1 + 19.6 mg/litreat 1,200 m. These values, representing the mean sulphate concentrations of mist and dust samples ateach sampling distance (data of the three transects pooled), differ significantly at the 85 % confidencelevel.Statistical evaluation of the data of the mist and dust pH measurements, pooled for the three transectson the basis of distance, indicated a gradual increase of the mean values from 400 m (7.3 + 0.26),through 800 m (7.7 + 0.34), to 1,200 m (8.2 + 0.83), although these values were not significantlydifferent.A decreasing trend in accordance with that in the case of the sulphate concentrations of mist and dustsamples with distance from the mineral processing plant, was also observed in the sulphur content ofthe leaves of selected plant species, with mean sulphur content higher at 400 m sampling distance(0.29 + 0.091 %) than at 800 m (0.264 + 0.086 %) and a further decline at 1,200 m (0.232 + 0.079 %),data of the three transects pooled. However, these values were also not significantly different. Although not significantly so, the decreasing trend in the results of the sulphate concentration of mistand dust samples, the sulphur content of plant leaf samples as well as plant mortality observed, andincreasing soil pH values with distance from the Mineral Separation Plant, suggest that the gaseousemissions from the Mineral Separation Plant could probably have had a detrimental effect on theadjacent Succulent Karoo vegetation. A more detailed study is necessary to confirm this trend. Inaddition it is recommended that in order to clarify the soil pH measurements outcome along theeastern transect that were contradicted by the results of the mist and dust pH measurements, a moreintensive survey over a greater distance (at least further than 1.2 km from the Mineral SeparationPlant), be conducted to quantify vegetation damage and acid deposition to the east of the emissionsource.

[发布机构] Stellenbosch University