MODFLOW in action at a Superfund Site in California

Groundwater MODFLOW models have many practical uses, including aiding engineers in designing groundwater remediation systems.  Engineers use  models to place pumping and injection wells on a site to contain a plume of contaminated groundwater.  MODFLOW was appliedthis way in Los Angeles County, CA, to design a groundwater remediation system at adjacent Superfund sites, Montrose Chemical and Del Amo.  The planned well field will keep a plume of contaminated water from migrating into residential areas near the facility.

In the mid-1900s the Del Amo site served as a large industrial facility, originally built to manufacture rubber during World War II.  The site was sold to Shell in 1955, which continued using the site for rubber manufacturing until it was closed in 1972.  Contamination was discovered in a waste pit in 1984.¹

Right next door, the Montrose Chemical Plant manufactured chemical products from 1947 to 1983, including DDT pesticide.  Soils and groundwater in the vicinity are contaminated with DDT and the plant discharged DDT-laced waste water to local sewers, contaminating drainage channels and ocean sediments.²

Beneath these two facilities, soil and groundwater are polluted with industrial by-products, including chlorobenzene, DDT, parachlorobenzene sulfonic acid, benzene, ethylbenzene, naphthalene, trichloroethylene, perchloroethylene, and dichloroethylene. These substances are present beneath the facility as non-aqueous phase liquids (NAPL), dissolved in groundwater, and adsorbed to soils, and, if left untreated, these pollutants could pose a possible threat to the public.

In 1996 the EPA launched a joint groundwater modeling effort for both sites.  This MODFLOW model consisted of 5,229 rectangular cells, sized 200 x 200 feet in the primary area of interest and 200 x 400 feet in the peripheral areas.  The model contained 13 vertical layers representing 8 hydro-stratigraphic units that had been delineated and characterized through field investigations.  MODFLOW estimated cleanup time frames and identified down-gradient locations that could be impacted by pollution leaking from the site.  The model indicated that  NAPL plume could be contained through careful placement of extraction and injection wells and that it was possible to mitigate adverse downgradient migration of many of the tainted groundwater plumes.  MODFLOW also suggested that if no action was taken downgradient migration could become a problem.  In 1999, the Environmental Protection Agency (EPA) developed a remedial plan calling for the extraction of the most polluted material, extraction and injection to contain the plume on the site, and monitoring less polluted areas.³

Over the next ten years the groundwater model was refined, updated with new site data,  and calibrated with PEST.  PEST also aided in identifying data gaps and numerical uncertainty.  By 1998, a set of initial design specifications for the remedial well field had been developed based on the model.  The EPA and the responsible parties (Montrose and Shell) collaborated on oversight of the effort.  The final design included 17 extraction wells and 6 injection wells.  The modeling effort optimized pumping rates and well placement, and specified cleanup targets that would trigger decreases in the pumping rate or the shut down some of the wells.  Shut down targets were generated during modeling as containing the NAPL plume was more important than shutting down wells when the water surrounding them had reached an acceptable concentration.⁴

This example shows the power and importance of groundwater models in maintaining groundwater supplies and protecting the public from pollution.  Through the optimization of pumping and injection rates, and well placement, MODFLOW showed the engineers how to managed the cleanup in an efficient and cost-effective manner.

1. Environmental Protection Agency. (2010). Region 9: Superfund. Montrose Chemical Corp. http://yosemite.epa.gov/r9/sfund/r9sfdocw.nsf/ViewByEPAID/cad008242711?OpenDocument

2. Environmental Protection Agency. (2010). Region 9: Superfund. Del Amo Facility.   http://yosemite.epa.gov/r9/sfund/r9sfdocw.nsf/ViewByEPAID/CAD029544731?OpenDocument

3. Environmental Protection Agency. (1999). Record of Decision for Dual Site, Groundwater Operable Unit, Montrose Chemical and Del Amo Superfund Sites. Volume I: Declaration and Decision Summary. Region IX. EPA ID: CAD008242711 and CAD029544731. 3 to 4, 11-8 to 11-9.

4. CH2MHILL. (2008). Overall Operational Design Report Based On Remedial Wellfield Optimization: Dual Site Groundwater Operable Unit Remedial Design: Montrose Chemical and Del Amo Superfund Sites. Prepared for the USEPA: Region 9.  EPA Contract No. 68-W-98-225. 1-2, 4-2, and Table 1.