A Civil Action - The Woburn Toxic Trial > Learning Modules > Movement of Contaminants

Module 8: Movement of TCE to Wells G and H

Module goals

Student assignment

Overview

The movement of contaminants through the subsurface is complex and is difficult to predict. Different types of contaminants react differently with soils, sediments, and other geologic materials and commonly travel along different flowpaths and at different velocities. One of the challenges for hydrogeologists is to obtain meaningful chemical data from water samples collected from observation wells and monitoring wells to use to map the distribution of specific contaminants and to use as targets for any models that may be constructed to predict forward or backward in time.

How does contamination move from the surface to the subsurface?

Most contaminants are introduced to the subsurface by percolation through soils. The interactions between a soil and a contaminant are important for assessing the fate and transport of the contaminant in the groundwater flow system. Contaminants that are highly soluble, such as salts (e.g. sodium chloride, NaCl) move readily from surface soils to saturated materials below the water table. This often occurs during and after rainfall events. Those contaminants that are not highly soluble may have considerably longer residence times in the soil zone. Some contaminants adsorb readily onto soil particles and slowly dissolve during precipitation events, resulting in dissolve fraction concentrations of contaminants migrating to groundwater. This mode of transport is common for trichloroethylene.

Liquids spilled onto surface soils can migrate downward or can evaporate, which limits their potential for reaching the water table. Once below the water table, contaminants are also subject to dispersion (mechanical mixing with uncontaminated water) and diffusion (dilution by concentration gradients). For additional information concerning the fate and transport of contaminants in soils, refer to Contaminant movement through soils, an overview provided by the the U.S. Department of Agriculture. The U.S. Geologic Survey also provides a helpful summary of the interaction between soils, streams, and groundwater. See the USGS Hydrology Primer for additional information.

Advective groundwater movement

The most common mode of contaminant migration in the subsurface is advective flow with groundwater. Advective flow velocities are based on the average (bulk) properties of the aquifer materials and the average hydraulic gradient causing flow. Darcy's Law is the basis for quantifying the rate of fluid flow through saturated subsurface materials. This simple approach does not take into account dispersion, diffusion or adsortion of, contaminants travel, which can increase or decrease the rate of groundwater flow calculated by advection. To learn more details, download the EPA Groundwater fact sheet.

Advection with dispersion

Contaminants like other dissolved solutes in groundwater also move on a smaller scale than calculated using Darcy's law and bulk values of aquifer properties. Contaminant movement is also controlled by the process of mechanical dispersion. Dispersive mixing causes some contaminant molecules to move ahead of (longitudinal to) the average advective velocity along the hydraulic gradient and some molecules to move laterally (tranverse) to the hydraulic gradient. The net effect is to spread (disperse) the contaminant plume about the advective front. The amount of spreading is related to the dispersivity of the rock or sediment, the advective velocity of groundwater flow, and the molecular diffusion of the contaminant in the water in the pore space. The amount of diffusion is a function of the concentration gradient and the porosity of the materials. The details of these physical and chemical processes are presented in groundwater hydrology textbooks by Schwartz and Zhang (2003), Fetter (2002), and Bair and Lahm (2006). Field tests show that the longer the distance of contaminant travel, the greater the amount of longitudinal dispersivity. The equations used to make these calculations are presented in the Excel spreadment that is part of the student assignment.

Advection with dispersion and chemical retardation

Chemical retardation occurs when a solute (contaminant) reacts with the porous media and its rate of movement is retarded relative the advective groundwater velocity. Retardation can occur by a variety of processes including adsorption and precipitation. Organic solvents like TCE and PCE can sorb onto particles of organic carbon that are present in minor amounts in the aquifer matrix. This is especially true in shallow aquifers of glacial-fluvial origin as at Woburn. Retardation rates of contaminants are highly variable and typically range from 0 to 10 times slower than the advective groundwater velocity. Expensive field tests are needed to pin down site-specific values. Literature values commonly are used with measured values of total organic carbon from field samples of soils, sediments, and bedrock. The expert witnesses in the Woburn Toxic Trial who addressed contaminant retardation used retardation rates for TCE that were 2 to 4 slower than the advective velocity and 6 to 8 times slower than the advective velocity for PCE. The details of this process is presented in groundwater hydrology textbooks by Schwartz and Zhang (2003), Fetter (2002), and Bair and Lahm (2006). The equations used to make these calculations are presented in the Excel spreadment that is part of the student assignment.

Looking at a specific example

Every contaminant has a unique set chemical properties and physical properties. Because TCE is the primary contaminant involved in the Woburn Toxic Trial, let's use it as an example. The migration of TCE is further described in this link. The TCP concentration contouring exercise presented in the student assignment explores the spatial distribution of TCE as well as a temporal variation.

Student assignment


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