# HydroGeoSphere/Specified Third-Type Concentration

This is also known as a third-type or Cauchy boundary condition. It is a distributed property so you should first choose the subset of faces for which you want to apply the condition and then issue one of the following instructions.
If the node was assigned a specified concentration by a previous instruction then it will not be modified by further specified mass flux instructions.
If the node was assigned a third-type concentration value by a previous instruction then third-type concentration fluxes assigned in subsequent instructions will be cumulative. This is because third-type concentrations are applied to faces, and any node common to two such faces requires a contribution from each face.

## Specified third-type concentration

* calcflux A logical switch which determines how fluid fluxes are handled for this third-type boundary condition. If .FALSE, read the following:
       (a) userflux Fluid flux value [L/T].
2. npanel Number of panels in the time-variable concentration function. For each panel, enter the following:
       (a) ton_val,toff_val, (bc_val(j),j=1,nspeciesmob) Time on [T], time off [T], and specified concentration [M L$^{−3}$] of each species.
Chosen nodes in the currently active media (see Section 5.8.1) are assigned a time-variable third-type concentration value unless they were previously assigned a first-type concentration.
If the variable calcflux is .TRUE, nodal fluxes are calculated by HydroGeoSphere from the flow solution and used to calculate the third-type boundary condition. Only positive fluxes (i.e. flowing into domain) are used. If .FALSE, HydroGeoSphere reads a flux value userflux which is used instead.
A panel is a point in time at which the specified third-type concentration is set to a new value. The first panel would normally start at time zero. The concentration given for the last panel will be maintained until the end of the simulation. You can assign a static concentration for the duration of the simulation by setting npanel to 1, ton_val to 0.0 and toff_val to a large number.
Note that if nspeciesmob is greater than 1, additional values of bc_val should be included.

## Specified third-type concentration from file

* fname Name of the file which contains the time-varying third-type concentration information.
This file should contain the following input data:
* nelem3 Number of elements.
* npanel Number of panels in the time-variable concentration function.
* calcflux A logical switch which determines how fluid fluxes are handled for this third-type boundary condition. If .FALSE, read the following:
       (a) userflux Fluid flux value [L/T].
       For each panel, enter the following:
       (a) ton_val,toff_val Time on [T] and time off [T].
For each of the nelem3 elements, enter the following:
* nel, n1, n2, n3, n4, (bc_val(i,j),i=1,npanel, j=1,nspeciesmob) Element number, 4 node numbers defining face and third-type concentration [M L$^{−3}$] history of each species.
Faces defined by the 4 nodes in the listed elements in the currently active media (see Section 5.8.1) are assigned a time-variable third-type concentration value unless they were previously assigned a first-type concentration. It is the responsibility of the user to ensure that the nodes define a face that is on the exterior of the domain.
For 3-node faces, enter a value of zero for node n4.
If the variable calcflux is .TRUE, nodal fluxes are calculated by HydroGeoSphere from the flow solution and used to calculate the third-type boundary condition. Only positive fluxes (i.e. flowing into domain) are used. If .FALSE, HydroGeoSphere reads a flux value userflux which is used instead.
If nspeciesmob is greater than 1, additional values of bc_val should be included.
Although all elements in the file share the same time on/time off panel information the concentration values in each panel can vary from element to element.

## Specified third-type concentration from face file

* fname Name of the file which contains the time-varying third-type concentration information.
This file should contain the following input data:
* nface3 Number of faces.
* npanel Number of panels in the time-variable concentration function.
* calcflux A logical switch which determines how fluid fluxes are handled for this third-type boundary condition. If .FALSE, read the following:
       (a) userflux Fluid flux value [L/T].
       For each panel, enter the following:
       (a) ton_val,toff_val Time on [T] and time off [T].
For each of the nface3 faces, enter the following:
* nfce, (bc_val(i,j),i=1,npanel, j=1,nspeciesmob) Face number and third-type concentration [M L$^{−3}$] history of each species.
Listed faces in the currently active media (see Section 5.8.1) and which are located on the exterior of the domain are assigned a time-variable third-type concentration value unless they were previously assigned a first-type concentration.
If the variable calcflux is .TRUE, nodal fluxes are calculated by HydroGeoSphere from the flow solution and used to calculate the third-type boundary condition. Only positive fluxes (i.e. flowing into domain) are used. If .FALSE, HydroGeoSphere reads a flux value userflux which is used instead.
If nspeciesmob is greater than 1, additional values of bc_val should be included.
Although all faces in the file share the same time on/time off panel information the concentration values in each panel can vary from face to face.