Esercitazione su contaminazione continua e istantanea (monitoraggio e rimedio)

1. Continuous injection into a 2D flow field

1.1. The study case 3

In our site, there is an underground tank that is leaking at a constant rate of 0.11 m /d.

We can imagine that it could be an old steel tank since the corrosion can easily attack the walls of this type

of tank, causing the presence of holes.

The tank contains water mixed with a chemical known as 1,1,1-TCA or 1,1,1-Trichloroethane that have a

concentration of 15’000 µg/L.

TCA is an organic chemical introduced in the environment by human activity, and it can be found in

products such as paints, glues, cleaning solvent...

In Italy, the Dlgs 152/2006 has established a maximum contaminant level for different chemicals in water,

however, for 1,1,1-TCA there is no specific indication.

Hence, we can consider the statement of Istituto Superiore di Sanità (2003), who sets the limit value at 200

µg/L, in analogy with what stated by the US EPA (United States – Environmental Protection Agency).

We can easily notice that the TCA concentration in the tank water is way more above the minimum

accepted, so it becomes necessary to assess the impact of this contaminant in the area around the tank.

In particular, this study aims to evaluate the TCA concentration at a monitoring well positioned at

coordinates x=31 m and y=2.05 m after 155 days.

1.2. Initial Assumptions

On the field, the soil is characterized by heterogeneity, of pores dimension and grains material, which lead

to different punctual values of hydraulic conductivity and velocity.

Let’s consider a simplified approach and assume that the porous medium that characterise the site is

homogeneous, isotropic, and saturated.

Let’s also consider that the aquifer has a constant thickness (b) of 2 m through which we can assume a

uniform velocity field of 0.125 m/d in the x-direction (v ).

x

For what concerns the spill, as we said, we will consider a constant rate of continuous injection of TCA over

a punctual area and through the entire thickness b of the aquifer.

For what concerns the transport mechanism we assume that the diffusion can be neglected since the

diffusion is a very slow phenomenon and its contribution after 155 days would be very low.

Moreover, we can assume that the TCA is not a reactive chemical: so any form of decay or adsorption

transport mechanism will be considered.

1.3. The method

As we have anticipated, this study aims to evaluate the TCA concentration at a specific point (the

monitoring well) after a certain time (155 days).

Given the hypothesis listed in the previous paragraph, the 2D solution of Equation 1, at a generic time t is: