Ala Lombolo
- One of Sweden's most polluted lakes
Groundwater modelling
Bioscreen is a screening model based on the Domenico 3D analytical solute model (1987) which can be used to simulate the spreading of dissolved contaminants with groundwater flow (i.e. how extended the contaminant plume will be and for how long it will persist). It is primarily designed to model the transport of dissolved hydrocarbons with first-order decay and instantaneous biodegradation at petroleum fuel contamination sites. However, it can also be used to model solute transport without decay, i.e. non-degrading solutes such as heavy metals, which will be the application in this case of Ala Lombolo. In this model, the only processes considered are dispersion and adsorption of contaminants to the soil matrix, and not the solute degradation. (Newell, McLeod & Gonzales, 1996)
In Ala Lombolo, the contaminants are bond to the organic-rich, fine-grained sediment particles and is mainly spread when the sediment particles are stirred up by internal circulation in the lake and carried in suspension downstream Luossajoki (Kiruna kommun, 2013). However, there is a risk that the contaminants spread into the groundwater and to other terrestrial and aquatic environments in the area if the water level in Ala Lombolo were to be lowered, which is an actual threat since the mining company LKAB wants to drain its water supply. We made BIOSCREEN model simulations of how far the contaminants mercury and zinc would spread with groundwater flow in the scenario that LKAB do not keep its part of the agreement. An environmental sketch showing the setting of the lake is shown in figure 1.
In this modelling, we assume the extreme and unrealistic scenario that the entire lake is depleted in water and that the entire content of mercury and zinc is exposed and available for oxidation and infiltration into the ground with precipitation. Furthermore, BIOSCREEN presumes that the contaminants have already entered the saturated zone and is available for transport with the groundwater flow. The system has been drastically simplified and, due to the lack of data, the parameters have been roughly estimated and generalized. We are well aware that all of these assumptions are unrealistic, but the main objective of this model is to make a rough assessment of the area and to give rise to a better understanding of how the system works and which parameters have the greatest influence.
Two initial simulations were made, one evaluating the contamination plume of mercury and one the spreading of zinc. In addition, to see which parameters that influence the system the most, we ran some sensitivity tests by using alternate values for the hydraulic gradient, the fraction organic content and the concentration and soluble mass of the respective contaminant. The results of the model simulations are shown in the figures below.
According to these BIOSCREEN simulations of the contamination plume development for mercury and zinc in the Ala Lombolo area, the contaminants will not spread significantly far and the distance of contamination spread does not increase with time.
The foc seems to be the parameter which influences the contaminant spreading the most and it also seems to be the factor which impacts the linearity of the concentration with distance. When the foc was lowered from the initial 0.186 to 0.02, the extent of the contamination plume was tripled (Figure 5 to 6). Dissolved mercury and zinc are highly reactive and easily adsorb onto the charged surfaces of clay particles and organic matter. The high organic content in the Ala Lombolo sediments would thus attenuate the spreading of the contaminants.
Unsurprisingly, a steeper hydraulic gradient would also increase the extent of the contamination plumes. The extent doubled when the hydraulic gradient was steepened with a factor of 10. It is assumed that higher values of hydraulic conductivity and porosity would have a similar effect, since all of these three parameters affect the groundwater flow and thus the transport of dissolved contaminants.
The concentrations and soluble mass of the contaminants in the source zone do not seem to influence the extent of the contamination plumes, since the model simulations give the same results for both mercury and zinc and since it is not changing with alternation of the values. This appears a bit unrealistic to us.
The conclusion of these BIOSCREEN model simulations is that groundwater spreading of the contaminants is probably not the most critical environmental threat in Ala Lombolo, since the contamination plumes do not extend that far. The surface water spreading of contaminants bound to sediment particles in suspension ought to be a more acute problem. However, since we do not have access to any comparative studies, nor a detailed database for the input parameters, it is difficult to make a proper assessment of the situation. After all, BIOSCREEN just offers a rough simplification model of the reality.
using BIOSCREEN Natural Attenuation Decision Support System
Figure 2. Input data choices for the BIOSCREEN Natural Attenuation Decision Support System. This example shows the values we have chosen in the mercury contamination plume simulation.
Figure 3. Dissolved mercury concentration along plume centerline with distance from the contaminant source with the initial input parameters.
Figure 4. Dissolved zinc concentration along plume centerline with distance from the contaminant source after 10 years when the hydraulic gradient has been increased to 0.05 ft/ft.
Figure 5. Dissolved mercury concentration along the plume centerline with distance from the contaminant source after 10 years when the hydraulic gradient has been increased to 0.05 ft/ft, the foc set down to 0.02 and the concentration of mercury in the zone source increased to 50 mg/L.
Figure 1. The sketch shows how the groundwater gradient is sloping towards the SE where the outlet of Ala Lombolo is located. The areas which will be affected by a lowered water level are shown as the brown sediments above the low water level. When precipitation falls over the sediments in the oxidized environment Hg2+ will percolate with the rainwater and reach the groundwater.
