Mapping the flow of groundwater is a challenging, yet vital, aspect of monitoring and remediating subsurface contamination at commercial/industrial, chemical and military facilities. Correctly analyzing contaminant locations and concentrations with respect to subsurface geologic structures can help locate point sources of contamination, the direction of movement and predict yet-to-be impacted locations of contamination plumes.
Mabbett & Associates, Inc. relies on Surfer to quickly create water table elevation and contamination concentration contour maps based on sample data collected from soil borings and monitoring wells installed at project sites. Mabbett, a Bedford, Mass., environmental consulting and engineering company, works extensively with local and national business and governmental clients to assess, monitor and remediate sites where groundwater contamination is either suspected or known to have occurred.
“We collect environmental samples and analyze them for volatile organic compounds, polychlorinated biphenyls (PCBs), heavy metals, etc… based on the information we have regarding the history of the property and its current operations,” said Kayla Cox, Mabbett Project Geologist. “Some of our projects are state or federal Superfund sites, some are private, others are military bases or other past or present government facilities.”
Mabbett uses Surfer in two ways: (1) to map the direction of groundwater flow and (2) to map contaminant concentration plumes, consisting of industrial waste chemicals, metals, leachates dissolved in groundwater, at sites with known or suspected groundwater or soil contamination.
The data input to Surfer generally consists of water table elevations and contamination concentrations. To calculate the elevation of the water table, Mabbett uses the ground elevation surveyed in feet based on a national geodetic vertical datum (NGVD), and the measured depth-to-water from the ground surface. The water table elevation is equal to the ground elevation minus the depth-to-water. Like surface water, groundwater flows from areas of higher elevation to lower elevation.
By determining the contours of the water table or piezometric surface and assuming groundwater flows at an angle perpendicular to water table contours, technicians can predict the direction and presumed destination(s) of contaminants in groundwater.
Most of the remedial work Mabbett performs involves point source contamination, including leaking underground storage tanks, wastewater lagoons, or spilled drums of industrial waste chemicals. For example, leaks from underground fuel storage tanks (USTs) at gasoline stations are common point sources for subsurface contamination. Fuel leaks penetrate through the subsurface sediment and rock fissures and descend vertically onto the water table. To trace the spill, several soil borings and groundwater monitoring wells may be installed around the UST.
Samples of soil and groundwater are taken from the borings and wells and analyzed for petroleum products and/or other suspected contaminants. Analytical results derived from the samples are then mapped in Surfer to visualize “source areas” of contaminants. Heat maps are typically generated to visualize where the highest and lowest concentrations of contaminants reside and to predict the future migration path of the contaminant plume, in an effort to assess risk levels to human health and to the environment. This information is then overlaid on an aerial base map either in Surfer or the more traditional ArcGIS.
The number of monitoring wells drilled at a site depends on its size and other factors, but a typical project includes several wells. Mabbett usually takes water samples at each well on a quarterly or other frequency and then conducts chemical analyses to determine the presence of contaminants and their concentrations. The levels of contamination are monitored over time. Contour maps are generated in Surfer after each sampling event and compared to each other to assess any changes in the location of the contamination spatially and temporally.
One critical advantage of mapping the contaminants as a contour plan in Surfer is the ability to visualize where plumes or buildups of contaminants are located in one aquifer system and how it may potentially impact or communicate with another aquifer system either above or below. This may indicate the potential presence of a serious environmental situation that must be dealt with immediately.
The uses of the Surfer contour maps vary with the project. Some projects are currently active industrial plants where the site monitoring is focused on detecting new contaminant source areas, while other sites are decades-old projects that require ongoing monitoring and remedial efforts. In either case, contamination plumes can appear in new locations or change shape suddenly, requiring a modification in the remedial strategy.
“Ideally, we want to see contamination levels and plumes decrease significantly over time signifying the site is becoming cleaner,” said Cox.
Once the contour map has been generated in Surfer, Cox either exports it as a PDF for inclusion in a written report or exports it as a GIS-ready shapefile for importing into ArcGIS. Mabbett maintains a GIS database to study their projects and many of their clients request GIS files for their own evaluations.
Cox taught herself how to use Surfer after arriving at Mabbett. Although the environmental firm owns and uses ArcGIS on a regular basis, its scientists and engineers use Surfer for contour mapping because it is much more cost and time effective than generating similar maps using an ArcGIS platform.