Title: Introduction Text: One method of calculating mass flux is to use contaminant concentration data from a transect of groundwater wells and estimated groundwater velocity data. The contaminant concentration data is gathered from recent groundwater sampling results and the groundwater velocity data is estimated from the hydraulic gradient measured within the aquifer. The calculation of mass flux for a specific cross-sectional area of the plume can then be performed manually or using computer software.

Title: Theory Text: One software program available to assist in this calculation is the Mass Flux Tool Kit. It was recently developed as part of the Department of Defense (DoD) Environmental Security Technology Certification Program (ESTCP). Click here to learn more about this software. This software follows these steps: Plume Characterization: For a given transect, groundwater data are entered that defines the contaminant distribution in the subsurface including the full width and depth of the plume at that location. Single-level wells can be used, but multi-level monitoring points will provide a more detailed, 3-dimensional characterization of the plume. Groundwater Flow Characterization: The specific discharge (q) is calculated for each transect given the hydraulic gradient (i) and the hydraulic conductivity (K) of the aquifer (where q = K x i). Subdivision of Transects: Transects are then divided into smaller subareas to better represent the different concentrations in the plume between each sampling location. Cumulative Mass Flux Calculation: The mass flux at a sampling location is the product of the contaminant concentration, specific discharge, and flow area. The total mass flux is the sum of all of the mass fluxes at all sampling locations across a given transect.

Title: Advantages and Limitations Text: The major advantage of this method is that mass flux can be calculated from readily available data. The major limitation of this method is the uncertainty in the mass flux calculation due to heterogeneity within the subsurface. Several parameters can impact the magnitude of the mass flux calculation including: (1) spatial distribution of the contaminant in the source zone, (2) contaminant release from non-aqueous contaminant phases into groundwater, (3) spatial variability in the conductivity of the aquifer, and (4) seasonal fluctuation of conductivity. However, averaging over spatial and temporal data can reduce error in the mass flux distribution results.