Assignment 2 - Assessing the contamination plume with BioScreen
Introduction
The European Regulations of management of waste material and pollutants is not applied by China and the Chinese government. The Chinese regulations are less restricting, and are leading to long-term problems with pollutants and disruption of local eco systems.
Samples taken after 2010 display high concentrations of heavy metals, correlating with the areas practical problems with its agriculture and the disappearance of flora and wildlife. This study will focus on the high cadmium concentration, and the use of the Bioscreen tool as an initial analysis of how the contaminant may progress further landwards from the wetland, with a section of the most inner tidal shelf section as the focused infiltration point.
Study Site
The wetlands along the estuary are supplied with tidal water from the estuary and river system, which has an average composition of approximately 65% sand, 35% silt and less than 1% Clay. Studies by Zhang et al. (2012) present high concentrations of the heavy metals Cadmium (Cd), Copper (Cu) and Lead (Pb), indicating similar degrees of contamination within the wetlands compared to the estuary.
Bioscreen
The tool used for the analysis is an excel-macro program called Bioscreen. By defining boundaries and parameters for the pollutant and the medium the program execute a preliminary estimate of the dispersion of the pollutant within the system.
Parameters
The parameters influencing these calculations include: area, hydrogeological flow, dispersion range, sorption, degradation and temporal values.
The dispersion plume range were set to 250 feet, as the functions become more unreliable further away from the contamination source. The soil thickness is set as 10 feet as the study does not have data from a longer core. The hydraulic gradient has been set to 0.003. The simulation length is set as 20 years while the decay rate and half-life has been set to 1000 years as the Cadmium, which is the pollutant of focus in this study, is a stable element.
The parameters that has been altered for different scenarios are the retardation factor, due to the partition coefficient, and the seepage velocity due to the porosity.
Fig.1: Overview of the Bioscreen input user interface.
Scenarios
The different scenarios run by Bioscreen is based upon variations in the parameters: conductivity, porosity, and pH.
According to previous studies made by Zhang et al. (2012) the main parameters affecting the solubility of cadmium in these conditions are pH and cation exchange capacity, while total organic content and composition had an insignificant impact.
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The scenarios cover changes in composition of the soil from sand-dominated to clay-dominated, and changes in acidity due to rain or Industrial waste. Since the simulations involve elements with a low decay rate the equation in focus is the “No Degradation”-curve indicated by the red line.
Results & Conclusion
Sand-dominated environment
The Simulations show us that the partition coefficient, Kd, which is the ratio between the solutes in the two phases, indicate retaining properties of the pollutants as the value increase, enhancing the retardation/adsorption. This parameter should be read as an effect rather than a cause of the retardation, with the pH being the active factor. A more basic medium, with a possibility of a higher Kd, have the ability to induce a higher adsorption.
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Clay-dominated environment
The Simulations show us that the partition coefficient, Kd, which is the ratio between the solutes in the two phases, indicate retaining properties of the pollutants as the value increase, enhancing the retardation/adsorption. This parameter should be read as an effect rather than a cause of the retardation, with the pH being the active factor. According to Table 1 a more basic medium, with a possibility of a higher Kd, have the ability to induce a higher adsorption.
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Fig.2: Bioscreen array-modelling of the dispersion of Cadmium in the conditions of the Pearl River Wetlands after 20 years. Simulation with a low partition coefficient-parameter of 50 L/kg.
Fig.3: Bioscreen array-modelling of the dispersion of Cadmium in the conditions clay-dominated environment after 20 years. Simulation with a high partition coefficient-parameter of 4000 L/kg.
Fig.4: Bioscreen array-modelling of the dispersion of Cadmium in the conditions clay-dominated environment after 20 years. Simulation with a high partition coefficient-parameter of 4000 L/kg.