MASS BALANCE in MT3D includes all solute flux in/out of boundaries as well as in/out of storage in the dissolved and sorbed mode



SOURCES - mass into ground-water flow system via external sources as well as released from storage due to decrease in solute and sorbed concentrations

SINKS - mass out of ground-water flow system via sinks plus mass accumulated in storage due to increase in solute and sorbed concentrations

Some erroneous results may be obtained in that the above equation does not account for the mass in the aquifer at any given time, so MT3D also reports an

ALTERNATIVE MASS BALANCE as:

where:

M_o - initial mass (dissolved and sorped)

M_t - total mass in system at time t (dissolved and sorped)

EULERIAN-LAGRANGIAN SCHEMES ARE NOT BASED ON CONSERVATION OF MASS the discrete nature of moving packets does not ensure the mass into a cell will equal the mass out of a cell

FOR MOVING PACKET METHODS AVERAGE MASS BALANCE OVER TIME IS A BETTER INDICATION OF ACCURACY THAN MASS BALANCE OVER ANY ONE TIME STEP

Items to consider for IMPROVING MASS BALANCE:

total number of packets

initial distribution of packets

packet tracking schemes

time step sizes

grid sizes