The Matanza- Riachuelo River basin covers an area of about 2240 km2, the river itself is about 64 km long and stretches from the west of Buenos Aires through the city and drains in the estuary of Rio de la Plata. The densely populated catchment is home to approximately 5 million people and the river is the main provider of drinking water in the region (Auge, 1986). Apart from an extensive agricultural activity in the upper parts of the basin, the river is experience heavily pollutant discharge from more than 15.000 industries. Leather & tanning industries and car manufacturing are among the most common industrial activities in the region (The World Bank, 2014).
The sedimentary subsoil is divided into three sections based on hydrogeological characteristics.The puelche aquifer is the second aquifer from the top and is the most used aquifer in area for fresh groundwater. It is a semi-confined aquifer with fine sand topped with a greenish-gray clay, the thickness is approximately 25m and does not crop within the catchment area. It is only recharged from the sides and the above lying aquifer (Auge, 1986).
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The Puelche aquifer was modelled in Bioscreen and simulations for three scenarios were conducted with respect to the partition coefficient of the metals cadmium (Cd), lead (Pb) and zinc (Zn). Parameters selected for the modelling was found in previous studies, Puelche is a well investigated and documented aquifer and many investigations have been conducted by Autoridad de Cuenca Matanza Riachuelo (ACUMAR), which is the water district authority in the region but some values were found in other studies as well. The thickness of the aquifer is known to be approximately 25 m and the porosity to 10.8 % (ACUMAR), the hydraulic conductivity 1.3E-3 cm/s (Zabala, 2016), the soil bulk density 1.6 kg/L and the gradient is 0,045% according to Auge, (1986). The partition coefficient was estimated from top and bottom values measured in the aquifer and from this was a mean value calculated, oxygen (4.57 gO2/ g N-NH4), nitrate (33.15 mg/L), sulfate (70.25 mg/L) and iron (0.25 mg/L) (Zabala, 2016). The partition coefficients for the metals was Cd 46 L/kg, Pb 66 L/kg and Zn 251 L/kg Jakomin et la. (2015). The carbon fraction is taken from the database FOC. The other parameters needed such as the dispersion as well as the methane was estimated, since they could not be found in literature.
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Three simulations were done with respect to the partition coefficient of the metals Cd, Pb and Zn. This to get an insight in how different contaminates are spreading within the aquifer. The contaminate plume of the metals were tracked 1 year with the array function and 50 years with centerline to track the dispersion with time. Graphs showing the spreading after 1, 5, 10, 25 and lastly 50 years was created.
1 year
After one year lead is still by mass largest at the source and the plume mass is 45,2 kg.
The concentration after one year at the source is 13,371 mg/L and at 100 m the concentration is 0,708 mg/L.
5 years
After 5 years the contaminant at the source has not changed much and is still high at 12,204 mg/L but at 100 m the concentration is now 3,766 mg/L.
10 years
At the 10 year mark the contamination at the source has lowered to 10,887 and at 100 m the concentration is still around 3 mg/L.
25 years
25 years after the event the concentration at the source is below 8 mg/L and the concentration at 100 m is now 2,594 mg/L.
50 years
After 50 years the concentration at the source is below 5 mg/L and at 100m the concentration is at 1,466 mg/L.
1 year
The zinc plume is not as extended and after one year it reaches 40 meters.
The curve for no degradation is quite steep and it reaches no detectable values at 50 meters. The concentration at the sources is 13,371 mg/L and already at 33 meters the values are under 3 mg/L.
5 years
After 5 years the plume has started to disperse and now the pollutant has spread to the 100 meter mark and has a concentration of 3,592 mg/L the concentration at the source is still quite high with a value of 12,204 mg/L.
10 years
At the 10 year mark the source value has still not changed considerably and is 10,587 mg/L and at 100 m away from the source the value is 3,546 mg/L.
25 years
After 25 years a reduction can be seen in the concentration at the source 7,720 mg/L and the value at 100 m is 2,517 mg/L.
50 years
After 50 years the concentration at the source has been reduced to 4,367 mg/L and a slightly lower value is detected at 100 m 1,422 mg/L.
Zinc
Groundwater
1 year
After one year the cadmium plume will have spread approximately 70 m from the sources and 16 meters in with. It will contain 45,2 kg but most of the contaminant is still at the source.
After one year the contaminant has spread over a 100m from the source when you look at he no degradation curve after 1 year. The concentration at 100 m from the source is 1,610 Mg/L.
5 years
Five years after the event the concentration at the source is still high at 12,204 mg/L. At 100 m from the source there is a concentration of 3,766 mg/L which is the highest concentration at this point for cadmium over the 50 years the calculations were done.
10 years
After 10 years the contaminant at the source has lowered and is now 10,887 mg/L compared to after one year when the concentration was 13,371 mg/L. The contaminant concentration is still detectable at 100 m from the source with a slightly higher concentration 3,360 mg/L.
25 years
At 25 years from the contamination the concentration at the source is 8 mg/L. And 100 m from the source the concentration has decreased as well to a concentration of 2,358 mg/L.
50 years
After 50 years the cadmium has a concentration of 4,367 mg/L at the source and at 100 m the concentration is 1,348 mg/L
Lead
Cadmium
The three simulations were done with the precondition of no degradation, since the modelling was conducted with respect to metals (Cd, Pb and Zn). In the array modelling the plume was only calculated one year after contamination. However, the centerline modelling was calculated up to 50 years after contamination to track the dispersion with time. As seen in both the centerline and array graphs 1 year after contamination cadmium is the fastest spreading metal followed by lead and then zinc. This since, cadmium is transported furthest from the source during the first years and have less steep graphs if compared to lead and zinc. This could lead to the conclusion that remediation cost could be cut extensively if the pollutant are find within the first year of contamination. After 5 years the pollutant will have dispersed over a much larger area which will generate in a more cost some remediation process.
By conducting different BioScreen models for different metals, their individual properties and dispersion characteristics can be estimated.
BioScreen has its limitations as the program simplifies and approximates complicated natural conditions such as groundwater flow conditions. One thing that also is important to keep in mind is that the Matanza- Riachuelo River is constantly polluted by hazardous discharge. BioScreen is only modelling single event contaminations. So the models are showing a somewhat misleading result with decreasing metal concentration with time, in reality there is a constant discharge of metals.