Introduction

The aim of this assignment was to combine both analytical and synthetic modelling to answer a question set for an environmental case study of our choice. The analytic modelling was done by defining parameters affecting the question, creating a cross-impact martix, a cause-effect diagram as well as creating a system-relationship graph. The second step, the synthetic modelling was dealt with by doing pair-wise weighting of the parameters, defining different scenarios, creating utility graphs and calculating the total utility (wi*ui). The case study was set to one of Sweden’s most polluted lakes, Ala Lombolo in Kiruna. The question that was modelled was defined as “How much mercury leaves Ala Lombolo by sediment transport through suspension”. Since the formulation of the question is focusing on the spread of the pollutant bound to sediments, the organic parameters such as biological leaching of Hg as well as mercury bound to organics, are excluded.

Analytical model

Parameters

Six parameters were assumed to be relevant to the sediment transport out of the lake:

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• Discharge, water flowing out from the lake

• Water level, of the lake

• Concentration, concentration of mercury bound to sediment particles

• Grain size, of bottom sediments

• Erosion, of bottom sediments

• Turbulence, close to the bottom resulting in stirring of the bottom sediments

 

Cross-impact matrix

From the six parameters, a cross-impact matrix was created (table 1). Each parameter was compared to all the other parameters and given a number representing how much impact the parameter has on each of the other parameters.

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Table 1. The cross-impact matrix shows that the parameters which affect the others the most are the discharge and the water level. The parameters which are affected the most by other parameters are the erosion, concentration ant the grain size. The average total is 6.33.

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Cause-effect diagram & systems relationship diagram

To visualize how the parameters in the system relate to each other a cause-effect diagram and a system relationship diagram were created (figure 2 & 3). The average and a line cutting origo was inserted to the diagram to distinguish which parameters are generally more dominant and which are more affected by other parameters.

 

 

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Figure 2 (left). The turbulence and the grain size are the most interactive parameters which are both influenced by other parameters as well as influencing others. The parameters which are most dominant in the system are the discharge and the water level. Finally the concentration and erosion are the parameters which influence the system the least but are affected by the system. 

Figure 3 (right). The water level and the discharge are the parameters which most of the high value arrows go from. The concentration is the one parameter which is not affecting the system but is only affected by the system.

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Synthetic model

Pair-wise weighting of parameters

To create an understanding for how important each parameter in the system is for the transport of mercury-hosting suspended sediments out of the lake, a pair-wise estimation chart was created (table 2). Each parameter was compared to another and given a certain value indicating that it is more or less important than the other parameter in relation to the defined question. 

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Table 2. The most important parameter affecting the suspended sediments being transported out of Ala Lombolo was the concentration in the sediments themselves. The erosion of the sediments as well as the turbulence of the bottom sediments were also very important. The least important parameter was the water level in the lake.

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Scenarions, utility functions & total utility

Two scenarios were created to see how the formulated question “how much mercury leaves Ala Lombolo by sediment transport through suspension” responded to them. A scenario with normal flow conditions (NFC) and a scenario with high flow conditions (HFC) were chosen. For each scenario a utility value was determined with the help of utility graphs. The utility value was then multiplied with the weight value of each parameter (table 2) to calculate the total utility (table 3).

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Table 3. The total utility is higher for the HFC than for the NFC. What differs, in the total utility calculation is the utility values for some of the parameters in the different scenarios. The weight is the same in both scenarios.

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Discussion

The table of total utility shows that the spreading of mercury through sediment transport is higher during HFC than during NFC which relates to the information on the website of the municipality of Kiruna (2016). This means that caution should be taken during HFC e.g. whit periods of high snow melt or heavy rains. The human activity e.g. the water regulation, by LKAB, of the upstream lake Luossajärvi should also be aware of this risk when changing the water level in Luossajärvi. To prevent the spreading of mercury by suspended sediment transport a calm aquatic environment in Ala Lombolo is optimal.