# HydroGeoSphere/Relative Concentration as Primary Variable

The following instruction can be used to enable a density-dependent flow and transport solution. Note that the single species involved in the transport process is described by RELATIVE CONCENTRATIONS.

## Density-dependent transport

1. rhomax The maximum fluid density (which corresponds to the maximum solute concentration).
2. toldens The minimum absolute difference in head and concentration for convergence of the non linear Picard loop.
3. maxitdens The maximum number of iterations for the Picard loop. If the maximum number of iterations is reached without obtaining convergence, the time step is cut in half and the loop is started over.
4. linear_rho_c A logical switch which determines how fluid density is calculated from relative concentration. If .TRUE, a linear relationship is used, otherwise nonlinear.
5. cmax The maximum relative concentration, corresponding to the fluid with maximum density. This instruction proved to be especially useful to simulate the lab experiments by Oswald and Kinzelbach (2004), where the concentration of the fluid with maximum density is not 1.

Note that you must set up the problem with relative concentrations. Therefore, rhomax is the fluid density for a relative concentration equal to 1.

As an example:

```1200.0    ! rhomax
0.02      ! tolerance
100       ! maximum iterations
.true.    ! linear rho-c relationship
1.0       ! maximum concentration
```

In this case, the maximum density is 1200.0 kg/m3, and the reference density is 1000 kg/m3, the tolerance on absolute head and concentration change to terminate the non-linear loop is equal to 0.02, and the maximum number of non linear iterations is 100. A linear relationship between fluid density and concentration is used and the maximum relative concentration (associated with the maximum fluid density) is 1.0.

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