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### CONTAMINANT MOVEMENT SCENARIO & **NEW** GROUNDWATER VISTAS v5

A Contaminant Movement Scenario

In the following scenario, we will describe one method of determining the movement of a contaminant within groundwater using example field/lab data, and some of the calculators available from GroundwaterSoftware.com.

A plume of PCE (Tetrachloroethylene) has been found under a dry cleaning business.  The property boundary is located 75 m downgradient from the point source of the plume.  Your consulting firm has the contract to determine if contamination is expected above the U.S. EPA's MCL (maximum contaminant level) of 0.005 mg/L beyond the property boundary, at present.  The dry cleaner estimates that they have been polluting for 25 years.

The initial field work has determined that the horizontal groundwater gradient is approximately 0.005 m/m, and the aquifer is a silty sand, with a hydraulic conductivity of 1.7 x 10-5 m/s, a porosity of 0.40, an effective porosity of 0.35, a bulk density of 1.85 g/cm3, and is 0.35% organic carbon.  In addition, the point source concentration is estimated to be 5.2 mg/L, and the log Koc value of PCE is 2.56.  You have estimated the longitudinal dispersion to be 9.5 x 10-7 m2/s.

Step 1: Determine Average Groundwater Velocity

Use Calculator #1 (Darcy Flux Calculator)

Input:

Hydraulic Conductivity = 1.7 x 10-5 m/s

Effective Porosity = 35%

Output:

Average Groundwater Velocity = 2.4 x 10-7 m/s

Step 2: Determine Kd

Use Calculator #14 (Kd Calculator)

Input:

log Koc = 2.56

Fraction of Organic Carbon = 0.35%

Output:

Kd = 1.27

Step 3: Determine Retardation Coefficient

Use Calculator #13 (Retardation Coefficient Calculator)

Input:

Bulk density = 1.85 g/cm3

Porosity = 0.40

Kd = 1.27

Output:

Retardation Coefficient = 6.87

Step 4: Determine Concentration at Property Boundary

Use Calculator #5 (Ogata-Banks Calculator)

Input:

Distance = 75 m

Time = 788400000 s (25 years)

Initial Concentration = 5.2 mg/L

Average Groundwater Velocity = 2.4 x 10-7 m/s

Dispersion  = 9.5 x 10-7 m2/s

Retardation Coefficient = 6.87

Output:

Concentration = 0.003 mg/L

Therefore, we are not yet over the maximum concentration, however look at the difference one year can make (try this example with 26 years instead of 25).

There are several commercial programs that are also able to calculate the concentration downgradient, including MODFLOW and Groundwater Vistas.

References

Appelo, C.A.J., and Postma, D. (1996). Geochemistry, Groundwater and Pollution. Published by A.A. Balkema, Brookfield, VT.

Drever, J.I. (2002). The Geochemistry of Natural Waters; Third Edition. Published by Prentice-Hall, Inc. Englewood Cliffs, NJ.

Fetter, C.W. (1994). Applied Hydrogeology; Third Edition.  Published by Prentice-Hall, Inc. , Englewood Cliffs, NJ.

Langmuir, D. (1997). Aqueous Environmental Geochemistry. Published by Prentice-Hall, Inc., Englewood Cliffs, NJ.

**NEW** Groundwater Vistas Version 5

The newest version of Groundwater Vistas, version 5 (GWV5), will be released on June 4, 2007.  This newest version adds many more features to the already impressive Windows based modelling environment for the MODFLOW family of programs.  Some of the new features are detailed below:

• GWV5 supports PEST v11, MT3DMS v5, MODFLOW-SURFACT v3, MODFLOW2000 v1.17 and the new MODFLOW 2005
• PEST v5 comes with GWV5.
• The Improved PEST error checking checks for common errors in typical calibration runs (SVD and regularization).
• GWV5 will support at least 2 analytic element models SPLIT and GFLOW in the future.
• The charting capabilities have been rewritten to make it easier and more customizable.
• Surfaces can be created from triangular networks, surfer grid files, matrices, and other sources.
• Other New Features in Groundwater Vistas version 4.0 include:

• Import new MODFLOW2000 files
• New layout view that shows you what your plot will look like and allows you to customize
• Supports MODFLOW2005 Unsaturated Zone Flow Package
• New Analytic Elements; polyline and polygon
• Much, much more…