Impact of SOx and NOx deposition on the leaching behaviour of soils and water quality in the South African highveld.

Abstract

Atmospheric emissions of sulphur and nitrogen oxides lead to the formation of acid rain, which, when deposited, can result in soil and water acidification. Cases of soil and water acidification have been reported in temperate climates, such as Europe, the United States and Asia. However, in semi-arid grassland areas, such as South Africa, where seasonal wetting and drying climatic cycles are experienced, the impacts of atmospheric deposition are expected to differ from those in temperate climatic zones. Hence, the purpose of this study was to determine the impacts of atmospheric deposition on soils and water in the semi-arid grassland areas of the South African Eastern Highlands. This was done by investigating the leaching characteristics and susceptibility to acidification, of soils exposed to atmospheric deposition in the Sandspruit Catchment, and to determine the influence of deposition on catchment water quality. Soil samples were collected from a farm located within the Sandspruit Catchment, in the Vaal River basin, from two hillslope transects (WS and WM) at the crest and toe slope positions. Soils from the four sampling positions were subjected to aerated and non-aerated column leach tests, using distilled water (pH 7.05) and a simulated acid rain solution (pH 4.35). Leachate from each column was collected as fractions of pore volume aliquots and measurements for EC, pH and redox potential were conducted immediately after collection. Further analysis was conducted for base cations, metals and anions and the data presented as breakthrough curves. Comparing the leaching characteristics of the two hillslope soils, both WS crest and toe soils result in leachate with higher EC values, indicating that there are more base cations and anions in WS soil column leachates than WM soil column leachates. The pH for WS soil column leachates remained in the alkaline to slightly acidic range, whilst that for WM soil columns was mostly in the slightly acidic range. This indicates that even after the addition of an acidic solution, the soil had the capacity to neutralise some of the acidity through CEC and exchangeable bases. Hence, the two invading liquids, at pH 4.35 and 7.05 resulted in similar leachate pH values ranging from 6.05 to 8.0. The level of aeration within the columns also determined the leaching characteristics of WS and WM soils. WS soil column leachates had lower redox potential values (below 0 mV), indicating reducing conditions, hence had a higher concentration of Mn in solution. On the other hand, the WM soil had higher redox potential values (greater than 150 mV), indicating oxidising conditions, hence the Mn concentrations in these columns was below detection limits. Surface water samples were obtained from the farm catchment and sampling was extended to the quaternary scale, comprising tributaries downstream of the research site that feed into the Sandspruit River. The samples were taken on a seasonal basis between 2011 and 2012. The pH and EC of the samples was measured upon collection and further analysed for sulphate, nitrate and base cations. The results showed that atmospheric deposition had not resulted in surface water acidification, as the pH of the water was in the alkaline zone (greater than pH 7). However, there was some evidence of sulphur loading to concentrations greater than those from an atmospheric input of 2.84 mg/L. The sulphate concentration in the surface water was greater than 10 mg/L across all sampling positions with the Sandspruit Catchment. The results of the column leach tests, together with catchment scale monitoring, show that leaching is a complex process that is influenced by the leaching solution, the chemical and physical characteristics of soil, as well as the hydraulic processes during leaching. The soil leaching behaviour differs with different slope positions and the effect of leaching on surface water quality is a summation of these different processes at each catchment position, as evidenced by the same chemical characteristics of the WM soil column leachates and water samples from the borehole (close to the windmill), and the WS soil column leachates and the spring (close to the weather station).