Abstract
The fate of soil contaminants, and thus associated risks as well as the functionality of different remediation techniques are widely accepted to be determined, to a large extent, by the relationships between environmental conditions and soil biogeochemical conditions at a given contaminated site. There have also been predictions on impending climatic changes in the near future, leading to a need to develop an understanding of the possible implications of these climatic changes on the management of soil contamination and the performance of remediation strategies..
The objective of this research work was to investigate the implications of a number of climate change scenarios on biological and chemical properties and processes in contaminated soils remediated with specific remediation techniques in order to aid in the development of adaptive sustainable responses. The remediation techniques considered include stabilisation with additives, namely zeolite and green compost, and bioremediation. An initial study was carried out on the effectiveness of these techniques, used singularly or in combination, in the immobilisation of heavy metals and promoting the biodegradation of organics. Another study was carried out using a laboratory simulation of projected climate change scenarios for 2080 over a real-time "two-year" period to observe the effect of climate change. This was combined with an extensive literature review of relevant information on the linkages between the different aspects of soil properties, soil contamination and climatic conditions. Different biogeochemical soil properties were monitored in nine different contaminated and remediated soil systems subjected to simulated baseline climate scenario, 1961-1990, and two 2080 climate change scenarios which differed in summer precipitation conditions namely dry and wet-dry summers. The nine soil systems included: 2 site soils contaminated with different levels of heavy metals, a hydrocarbon contaminated site soil which has been remediated, and six model soils which were subjected to different treatments with additives. The properties addressed include contaminant mobility and degradation, soil geochemical parameters; namely pH, cation exchange capacity, redox potential and soil biological properties, namely soil microbial enzyme and microbial population dynamics.
The literature review showed that, apart from the direct effect of climate parameters on certain soil properties, there is also a case for the interdependence between these soil properties. In addition, the relationship between climate parameters and soil properties can also be subject to confounding effect that may exist between different climate parameters. The results from studies on the development of remediation techniques showed that soil stabilisation and bioremediation treatments applied had good potential for the remediation of heavy metal and hydrocarbon contaminated soils. The soil stabilisation treatment when applied alone was able to reduce metal leachability by up to 60% while the bioaugmentation treatment increased microbial population by up to 1000%. However, the bioaugmentation treatment was observed to increase metal leachability in soil samples by up to 62% relative to corresponding soil samples which had no bioaugmentation treatment. The results from the experimental studies on climate change showed similarities with other studies in literature and the different soil properties fluctuated in response to seasonal conditions. Soil properties such as soil microbial activity, soil acidity, heavy metal leachability, rate of hydrocarbon contaminant degradation were shown to be higher in the summer season than in the winter season, while the soil CEC has been shown to be stable over the two year period of the study. With respect to differences between the climate scenarios, different soil properties such as soil pH, soil redox potential, soil CEC and cadmium leachability showed no variability. However other soil properties such as the soil microbial activity, copper leachability and the rate of hydrocarbon degradation were shown to be lower by up to 30%, 30%, and 150% respectively in the warmer and more arid climate change scenarios compared to the cooler and wetter baseline scenario. In the case of hydrocarbon soil contamination, these findings suggest a revisit of remediation guidelines and risk assessment procedures with the aim of putting into perspective the expected slower rate of hydrocarbon degradation in coming years under arid climate change scenarios. In the case of heavy metal contaminants, there may be a shift in the risk and contamination pathway due to possible reduction in ground water contamination and increase in contaminant concentration in soil dust particles. The application of remediation strategies such as revegetation that reduce both the transport of contaminated dust by wind erosion and also reduce leachability by sequestration in plant parts have been suggested as adaptation to these changes. Ultimately, due to the complex interactions between climate parameters and soil properties the best response to these contamination condition in view of the impending climate change conditions would be site specific, determined by perceived contamination pathways which would be influenced by the end use purposes for the sites; both at the present and in the foreseeable future.