Title: Introduction Text: Aquifer pump tests can be used to estimate the mass flux from an extraction well or series of extraction wells located downgradient of the NAPL source zone. These integral pumping tests (IPTs) are conducted using one or more pumping wells that are operated at a constant flow rate for several days during the pump test period. The wells should be configured to capture the entire contaminant plume. The concentration of the target contaminant is measured in each pumping well as a function of time. The concentration versus time data series is then used to determine the mobile contaminant mass flux using a mathematical model.

Title: Theory - Analytical Solution (1 of 2) Text: The figure at the left depicts the steps undertaken to determine the mass flux using the IPT method. This pumping method overcomes the problem of heterogeneous contaminant distributions, since a large aquifer volume is involved in the evaluation. The total mass discharged from a source zone is captured by pumping wells (wells 1, 2, and 3) located in a row that is perpendicular to the contaminant plume and groundwater flow direction. Plots are generated for contaminant concentrations versus time for each well. An analytical solution is then employed to determine the mass flux across the control plane (e.g. vertical cross section). In the presence of more than one control plane across a plume, the difference in mass fluxes between the respective control planes can be attributed to degradation, sorption, and volatilization of the target compounds. In general, it is difficult to determine which process(es) are contributing to the difference.

Title: Theory - Assumptions (2 of 2) Text: IPT assumes a homogeneous, isotropic, confined aquifer with uniform thickness under steady-state and uniform flow conditions. Certain assumptions made in defining a well capture zone include: (1) radially symmetric flow toward extraction wells, (2) little variation in porosity, hydraulic conductivity, and aquifer thickness within the well capture zone, and (3) little variation in concentration along the streamtubes within the well capture zone. The animation shown here illustrates typical concentration versus time curves collected from an IPT. For well 1, the pumping well is located on the outside edge of a contaminant plume. At the beginning of the pumping, the well capture zone doesn't tap the plume completely and the target compound concentrations in the well are below detection limit. As the well capture zone (isochrone) expands into the plume area, the contaminant concentrations in the well start to increase. Well 2, on the other hand, shows a relatively flat concentration versus time distribution since it is located along the centerline of a large plume.

Title: Application Text: General site information needed to determine mass flux using IPTs include: aquifer thickness (from drilling logs), transmissivity values (from pumping tests), hydraulic gradients (from differences in hydraulic potential between upgradient monitoring wells and extraction wells prior to any pumping), and groundwater velocity (from tracer analysis). An IPT study would require two control planes (when possible) and extraction wells approximately 50 meters apart. The number of wells across a control plane will depend on the transmissivity of the site and the estimated width of the plume. Depending on the transmissivity, the wells may need to be less than or can be more than 50 meters apart. The number of extraction wells needed for reliable mass flux estimates varies depending on the heterogeneity of the aquifer and the potential size of the contaminant plume. Natural attenuation rate constants based on degradation, sorption, and volatilization can be estimated for compounds which are detected at both control planes.

Title: Data Collection and Interpretation Text: The concentration versus time data collected from the IPT can be used to: Show the DNAPL distribution along the control plane Determine the presence of a DNAPL plume Compute flow rate along the control plane Determine the mean concentration in the well capture zone Interpretation of the mass flux data can allow you to: prioritize contaminated groundwater sites or zones for remediation; evaluate the effectiveness of source removal technologies or natural attenuation; and identify a source term for groundwater contaminant transport modeling.

Title: Streamtubes Text: Streamtube width is determined by the time increment between two consecutive sampling events. As seen in the graphic above, the overall number of streamtubes at each well is equal to twice the number of data points in the concentration-time series (e.g., at each sampling time, two streamlines are crossing the control plane, one at each side of the well). The theoretical concentration of each streamtube can be calculated based on the concentration of the two streamtubes that cross the control plane at each time interval.

Title: Performance Assessment Text: The IPT method provides limited information on the spatial distribution of contaminant flux, although mass discharge estimates may reflect less uncertainty because pumping allows a large "volume" of the aquifer to be analyzed and spatial averaging of point data is not performed. Factors affecting the accuracy of the plume delineation include: number of wells and distance between wells at a site, location of wells with regard to the plume, amount of time wells are pumped and number of samples collected while being pumped, and heterogeneity of the site.

Title: Advantages and Limitations Text: Advantages: Generates contaminant mass flow estimates Interrogates large volumes of water Can be used in deep aquifers Limitations: Costly due to wastewater disposal and pumping well installation (if not already present) Heterogeneous aquifers and asymmetrical well capture zones will require a large number of pumping wells A large DNAPL plume would also require a large number of pumping wells Requires lengthy time execution (depends on transmissivity of site) Does not estimate water flux It is difficult to quantify uncertainty in the estimates Subject to the limitations of sampling configuration Underestimates maximum resident contaminant concentrations

Title: Conclusion Text: More information on the use of IPTs and transects to calculate mass flux can be found in the Environmental Security Technology Certification Program (ESTCP) reference listed below. Farhat, S.K., Newell, C.J. and E.M. Nichols. 2006. User’s Guide: Mass Flux Tool Kit.