OBTAINING HIGH-RESOLUTION SITE DATA
Mass flux is the mass rate per unit area (grams/day/square meter) at a dis- crete location in a plume (Figure 8 below) and mass discharge is the total mass per unit time (grams per day, [g/d]) that flows through an entire cross section of a plume. The transect method was used to calculate mass flux and mass discharge in and just downgradient of the source area and along other tran- sects throughout the plume to evaluate remedy performance. Seepage velocities were estimated at individual wells along transects using equilibrium flow rates established during low-flow sampling activities. Corresponding TCE concen- trations in groundwater from those wells were used to calculate mass flux and mass discharge, as follows:
Mass Flux (J) = q0 · C = -K · i · C Where:
q0 = Darcy flux, L3/L2/t (e.g., volume/ area/d)
Figure 7 - Typical cross-section/ transect.
less concern since the project objective was to conduct thorough, plume-wide treatment to achieve rapid site closure. The plume was considered stable to contracting, based on the findings of the assessment.
Calculating Mass Flux and Mass Discharge. To better understand plume behavior, a site should not just be evalu- ated based on solute concentrations at discrete locations. The static nature of such an approach can be misleading. A mass-flux/mass-discharge approach factors in groundwater velocity (a.k.a.
Darcy flux q0 defined below), to better evaluate solute fate and transport. In essence, even if solute concentrations are high but groundwater velocity is very low (or even stagnant), then the mass flux is very low; hence, solute mobility is very low, ergo so is risk.
Assessing the risk of impact to down- gradient receptors is a common appli- cation of mass-flux/mass-discharge measurements. For this site, treatment was conducted plume wide and there was no evidence of off-site impacts; therefore, there was very little risk to third parties. The purpose of a mass- flux/mass-discharge analysis in this case was twofold. First, mass flux was used to identify the variability in solute con- centrations and the transmissive zones through which the bulk of the mass moved. Second, mass-discharge mea- surements over time were used to ensure that targeted levels (5 μg/L of TCE) were
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being met at the downgradient property boundary, even though a higher, risk- based concentration of TCE was allowed within the (on-site portion of the) plume. In other words, the property boundary needed to be the “zero flux line”.
It was important to understand plume architecture (solute distribution dictat- ed by heterogeneity) and plume strength (contaminant mass moving in ground- water per unit time – The Interstate Technology & Regulatory Council, 2010) to develop an appropriate treatment design. These metrics were evaluated by calculating mass flux within solute- bearing horizons along cross-sectional transects.
K = saturated hydraulic conductivity, L/t, (e.g., m/d)
i = hydraulic gradient, dimensionless (e.g., m/m)
C = contaminant concentration, M/L3 (e.g., mg/volume)
Mass discharge is the integration of the mass fluxes across a selected transect:
Where:
A = area of the transect, L2 (e.g., m2) J = spatially variable mass flux
(The Interstate Technology & Regulatory Council, 2010)
Note that mass flux (J) varies both spatially and temporally across tran- sects and these variations may be sig-
Figure 8 - Different solute strengths moving through the subsurface (Arcadis 2010).
Oct.Nov.Dec 2017 • TPG 13
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