Monitoring Well Placement and Plume Behavior

 

Groundwater monitoring wells are often placed in, around, and downgradient from a dissolved contaminant plume for a variety of purposes including:

• Monitoring plume dynamics (concentration changes) in different areas of the plume following a chemical release as the plume grows
• Bounding the plume in the horizontal and vertical for regulatory compliance and remedy planning purposes
• Evaluating plume stability as a criterion for considering a Monitored Natural Attenuation (MNA) remedy
• Projecting plume reduction over time as the result of a source remediation and/or MNA remedy
• Proper placement of downgradient sentinel wells to protect potential receptors

 

Planning the layout of a groundwater network is highly source and aquifer specific, with well location distances and anticipated contaminant arrival times being dependent on groundwater velocity and source concentration. Placement and interpretation of concentration patterns are also related to the source type; i.e. whether the source is constant, slowly declining (flushing), or suddenly removed (excavation).

 

Simple analytical contaminant fate and transport modeling can be very helpful in identifying optimal locations for monitoring wells, and in interpreting sampling results obtained from those wells. The following examples illustrate the application of TS-CHEM for the design and analysis of monitoring well networks for differing source and aquifer conditions.

 

Example 1 – Constant Source: Plume Growth and Stabilization

 

For the case where a chemical spill creates a continuing constant concentration source to groundwater, a dissolved contaminant plume will form, grow, and eventually stabilize (stop growing) (see time series in Figure 1). Monitoring wells placed down the axis of the plume can show this progression as concentrations rise through time and then sequentially level off at each downgradient location (Fig 2 and Fig 3).

Figure 1. Plume growth and stabilization caused by the slow constant dissolution of a vinyl chloride source. Monitoring wells are located at the following distances from the source: MW-1  400 ft; MW-2  600 ft; MW-3  700 ft; MW-4  875 ft. The placement of MW-4 coincides with the farthest downgradient extent of the plume after it has stabilized about 7.5 years after the initial release.

 

Figure 2. The spreading plume reaches each downgradient monitoring well in turn, causing the vinyl chloride concentration at that location to rise. After some period of time following first impact at a well location, the plume stabilizes in that area as indicated by the leveling off of monitoring well concentration.

 

Figure 3. Zooming in on the lower concentrations measured at MW-3 and MW-4 shows that these downgradient wells exhibit the same concentration versus time pattern as the wells in the central area of the plume.

 

Example 2 – Instantaneous Source: Plume Drift and Dissipation

 

For the case where a small chemical spill enters groundwater and dissolves - - creating an initial slug source in a small localized area - - a dissolved contaminant plume will leave that area and drift downgradient with groundwater seepage, spreading and dissipating as it does so (see time series in Figure 4). Monitoring wells placed down the axis of the plume can show this progressive arrival at, passing through, and leaving each monitoring well area (Fig 5 and Fig 6).

 

Figure 4. Plume behavior resulting from initial dissolution of a vinyl chloride source, with subsequent downgradient drift. Monitoring wells are located at the following distances from the source: MW-1  400 ft; MW-2  600 ft; MW-3  700 ft; MW-4  875 ft. The plume initially expands as it drifts, but attenuation (dispersion and degradation) gradually reduce the plume concentrations and plume area, and eventually cause it to disappear.

 

Figure 5. The drifting and spreading plume reaches each downgradient monitoring well in turn, causing the vinyl chloride concentration at that location to rise, peak, and then decline. Notice that the velocity of the plume peak does not match the groundwater Darcy velocity, nor the retarded groundwater Darcy velocity.

 

Figure 6. Zooming in on the lower concentrations measured at MW-3 and MW-4 shows that these downgradient wells exhibit the same concentration versus time pattern as the wells in the central area of the plume.

 

For the ideal case (often described in textbooks) the downgradient movement of the plume peak (even as that peak concentration declines with time) would occur at the same rate as the calculated Darcy’s Law groundwater pore velocity, which is 146 ft/yr for the model depicted above. If the chemical of interest were affected by adsorption and retardation, one might expect the plume peak to move at the rate of the groundwater velocity divided by the retardation rate. For this vinyl chloride model, that rate is 146 ft/yr / 1.113 = 131.1 ft/yr.

 

Instead, the plots above demonstrate that the plume peak is moves downgradient from the source at a faster apparent rate of approximately 155 ft/yr - - almost 20 % faster than the retarded groundwater velocity; and faster even than the average groundwater linear pore velocity of 146 ft/yr. This is caused by the nonlinear interaction of all of the processes acting on the vinyl chloride plume: groundwater seepage velocity, retardation, dispersion, and degradation. Thus, a model can be very useful in developing a more accurate estimate of the rate of movement of a detached (from the source) contaminant plume than simple velocity calculations would provide.

 

To learn more about TS-CHEM and how it can be used to assist with the estimation of the extent and movement of contaminant plumes, or to download a FREE DEMO of the software, visit the TS-CHEM Website today!