Bioscreen Model - Chrome mine
Figure 1.Closeup map of terrain in the interpreted catchment of the mine. (Google maps, 2017.)
Seyhan reservoir contamination of chromium VI related to chromite mining in Turkey.
The purpose of this study is to estimate the time it takes for chromium contamination caused by leakage from a fictional mining area to reach the Seyhan dam reservoir (Figure 1).
The computer program Bioscreen is utilized for the analysis.
The Seyhan reservoir is located in the south of Turkey in the outskirts of the city of Adana,
a dam built in 1956 in the Seyhan river created the reservoir. The low energy conditions of the reservoir has caused considerable contamination of the reservoir sediments due to increased sedimentation of fine particles and organic matter.
Contaminants are transported by the Seyhan river and other minor rivers and creeks in the region. The land use in catchments areas of these streams that contribute to contaminations are residential, agriculture and chromite mining.
The lithology of the area is called the Hendere formation. The uppermost layers consist of fluvial and flooding clastic sediments and form the shallow unconfined aquifer that is of interest for this study.
The contaminated groundwater will probably flow and reach the reservoir fastest in the section from 10 to 20 meters deep because it has the coarsest material, more than 50 % gravel. Therefore this section was used in the modeling parameter estimations.
In order to estimate for worst-case scenarios it would be assumed that the aquifer in the valley between the mine and the reservoir is composed mainly by either gravel(scenario 1) or sand(scenario 2) with almost no fine material, and these two scenarios would be simulated with bioscreen.
Figure 2. Scenario 1, Gravel aquifer. Contaminant plume 4 years simulation time.
The results for scenario 1 and 2 shows that the 2D form (fig 2 respective 3) of the contaminant plume has similar shape both for the gravely and for the sandy aquifer.
The spread of the contaminant is mainly perpendicular to the source in the direction of groundwater flow towards the reservoir. There is also lite lateral spread of the contaminant but the highest concentrations are still in the middle of the plume.
However there is significantly difference in the simulation time it takes for the contaminant to reach the reservoir in both scenarios.
For the gravelly layer it takes 3 years to found a concentration of 0,011 mg/L in the reservoir edge and for the sandy layer it takes 20 years to found a concentration of 0,001 mg/L.
Figure 3. Scenario 2. Sand aquifer. Contaminant plume 20 years simulation time
Discussion
After testing with different parameters the different simulations showed that the parameters that has the highest impact for the spreading are hydraulic gradient, the hydraulic conductivity and effective porosity. This is because chromium is assumed to be soluted in the groundwater and both the scenarios represent aquifers that have material with high permeability. Further stratification that could affect the flow direction is not taken to account.
There was no groundwater measurements for the aquifers so the hydraulic gradient were estimated from the topography and therefore it is great uncertainty on its true value.
Therefore only one value are showed in the results and it is a source of error.
The hydraulic conductivity values chosen for both scenarios could be lower in the reality because no account for the clay and silt portion in the sediments was used in the simulation, the smaller materials has a mayor influence lowering the hydraulic conductivity. Clays can also absorb chromium VI. There could also be stratification in the sediments or lenses of less impermeable material that could delay the contaminant spread.
The aquifer composition is was estimated from a sediment log that was from another place close and assumed that the sediments were the same at the site of the mine but there could be difference.
Further different parameters like pH and redox potential can affect chrome solubility in groundwater and these are not used in the modeling. Also organic matter from farm wastes or agricultural runoff may be present in the aquifers may contribute to immobilization of Chromium.
Assignment 2_ Bioscreen _ Miguel Cabrera
Assignment 2_Bioscreen_ Amie Remberg