Module 8: Movement TCE to Wells G and H
Module goals
- Understand how trichloroethylene (TCE) and perchloroethylene (PCE) migrate through subsurface materials.
- Review how the experts' illustrated the movement of these contaminants.
- Understand why the assessments of the fate and transport of TCE and PCE are critical to groundwater investigations.
Overview
The movement of contaminants through the subsurface is commonly complex and is always difficult to predict. Different types of contaminants react differently with soil and geologic materials and travel differently in the subsurface. One of the challenges for hydrogeologists is to obtain meaningful chemical data from water samples collected from observation wells and monitoring wells.
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 to projecting the "fate and transport" of the contaminant in the groundwater flow system. Contaminants that are highly soluble, such as salts (e.g. 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 some dissolved species precipitate in soils, whereas others may cause minerals in soils to dissolve.
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 created 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 Primer for additional information.
Advective groundwater movement
The most common mode of contaminant migration in the subsurface occurs as advective flow with groundwater. Advective flow can be described as the ability of water to flow through geologic materials and it is commonly expressed in terms of the velocity of groundwater flow. Darcy's Law is the basis for quantifying the rate of fluid flow through subsurface materials. Based on this simplistic approach, which does not take into account dispersion, diffusion or adsortion, contaminants travel at the same rate as the moving groundwater. To learn more details, click here.
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.