SOURCES and SINKS in MT3D can be specified as distributed (recharge) or point (wells, rivers, drains and specified head or head dependent flux boundaries) as in MODFLOW

Concentration is specified for sources and the
mass into the system = C * flow rate * time step length

Sinks remove water at the prevailing concentration, except fresh water is removed by evapotranspiration, allowing contaminant concentrations to build up in such areas.

INFLUENCE of SOURCEs and SINKs on CONCENTRATION is solved in the Eulerian step, with:

C_SSM= change in concentration due to a source/sink

q = volumetric flow rate of the source

R = retardation coefficient

= effective porosity

C = grid block concentration

C_S = source concentration n = the time step indicator

i,j,k = grid cell indicator

To prevent the concentration from exceeding the source concentration we need to ensure that the ratio of:



consequently we must limit the time step to: