Nonideal transport of reactive solutes in heterogeneous porous media: 5. Simulating regional‐scale behavior of a trichloroethene plume during pump‐and‐treat remediation

Abstract

Abstract. "Pump and treat" is widely used for containment and remediation of groundwater contaminant plumes. However, it is commonly observed that pump-and-treat systems begin to exhibit reduced efficiency at some point. A number of factors and processes may contribute to the reduced efficiency, among them being those associated with nonideal transport behavior, such as subsurface heterogeneity, nonlinear, rate-limited sorption/desorption, and rate-limited dissolution of immiscible liquid. We use numerical modeling to analyze the regional-scale (-49 km 2) nonideal transport behavior of trichloroethene in a contaminated aquifer undergoing pump and treat remediation. The pump-and-treat system has been in operation for -12 years, during which time the composite concentrations of trichloroethene in the treatment plant influent have decreased from >300 to -100/xg L -•. However, as is typically observed elsewhere, the system is exhibiting extensive concentration tailing, wherein the concentrations have remained relatively constant at -100/xg L -• for the past 8 years. Various factors that may be contributing to this tailing phenomenon are evaluated using a three-dimensional solute transport model specifically developed for the site, which is located in Tucson, Arizona. The values for almost all of the input parameters of the model were obtained independently of the historic concentration data being simulated. The hydraulic conductivity field was generated using information obtained from borehole logs and pumping tests, the sorption and local-scale mass transfer parameters were obtained from laboratory experiments conducted with aquifer material collected from the site, and the initial immiscible-liquid saturations were based on the results of partitioning tracer tests conducted in a representative source zone at the site. On the basis of our analyses we conclude that while rate-limited desorption and large-scale spatial variability of hydraulic conductivity have significant impacts on trichloroethene transport, the dissolution of immiscible-liquid saturation associated with the source zones is most likely the primary cause of the extensive concentration tailing observed at the site. The impact of nonlinear sorption and local-scale mass transfer on trichloroethene removal appears to be insignificant.

Relationship between mass-flux reduction and source-zone mass removal: Analysis of field data

“…As previously mentioned, such behavior is expected for a system controlled by non-ideal mass transfer and displacement, leading to inefficient source-zone mass removal. This is consistent with observations at the site, in particular the presence of high TCE concentrations within lower-permeability units, concentration rebound when the pump-and-treat system is offline, and the asymptotic decrease of the pump-and-treat system effluent concentrations, as well as the results of modeling-based analyses (Brusseau et al, 1999a;Zhang and Brusseau, 1999;Brusseau et al, 2007).Inspection of Figure 7 shows that the Borden and TIAA studies have similar fractions of mass removal. However, the mass flux reductions are significantly different.…”

“…The selected source zone is located on the edge of the large contaminant plume. Partitioning tracer tests were employed at this source zone and, based on their results, it was determined that 5600 kg of immiscibleliquid mass was present at this site (Nelson and Brusseau, 1996;Zhang and Brusseau, 1999). The total amount of sorbed and aqueous-phase contaminant mass was estimated to be approximately 100 kg, less than 2% of the total mass present in the source zone.…”

“…Comparison of the mass-flux-reduction/mass-removal relationships for three aqueous flushing field studies. Nelson and Brusseau, 1996Brusseau et al, 1999aZhang and Brusseau, 1999Brusseau et al, 2007 Table 3 Comparison of Site Variables …”

“…The fundamental concept of contaminant mass flux, its relationship to mass-removal processes and source-zone properties, and its impact on risk has long been established (e.g., Fried et al, 1979;Pfannkuch, 1984). The impact of subsurface heterogeneity, immiscibleliquid distribution, and mass-transfer dynamics on mass-removal behavior and aqueous concentration profiles (mass flux) has been examined for some time through laboratory, modeling, and field studies (e.g., Schwille, 1988;Dorgarten, 1989;Guiguer, 1991;Anderson et al, 1992;Brusseau, 1992;Guarnaccia and Pinder, 1992;Mayer and Miller, 1996;Berglund, 1997;Nelson and Brusseau, 1997;Powers et al, 1998;Unger et al, 1998;Broholm et al, 1999;Brusseau et al, 1999a;Frind et al, 1999;Zhang and Brusseau, 1999;Nambi and Powers, 2000;Zhu and Sykes, 2000;Brusseau et al, 2000Brusseau et al, , 2002 ; Saba and Illangasekare, 2000;Sale and McWhorter, 2001;Rivett et al, 2001;Enfield et al, 2002;Rao et al, 2002;Rao and Jawitz, 2003;Jayanti and Pope, 2004;Lemke et al, 2004;Parker and Park, 2004;Phelan et al, 2004;Soga et al, 2004;Falta et al, 2005a, b ;Jawitz et al, 2005;Feenstra, 2005, Fure et al, 2006;Lemke and Abriola, 2006;Suchomel and Pennell, 2006;Brusseau et al, 2007…”

Mass-removal and mass-flux-reduction behavior for idealized source zones with hydraulically poorly-accessible immiscible liquid

“…The impact of subsurface heterogeneity and non-uniform immiscibleliquid distribution on mass-removal behavior and associated aqueous-phase concentrations (mass flux) has been examined for some time through laboratory, modeling, and field studies (e.g., Schwille, 1988;Dorgarten, 1989;Guiguer, 1991;Anderson et al, 1992;Brusseau, 1992;Guarnaccia and Pinder, 1992;Mayer and Miller, 1996;Berglund, 1997;Nelson and Brusseau, 1997;Blue et al, 1998;Powers et al, 1998;Unger et al 1998;Broholm et al, 1999;Brusseau et al, 1999;Frind et al, 1999;Zhang and Brusseau, 1999;Brusseau et al, 2000;Nambi and Powers, 2000;Saba and Illangasekare, 2000;Zhu and Sykes, 2000;Rivett et al, 2001;Sale and McWhorter, 2001;Brusseau et al, 2002;Jayanti and Pope, 2004;Lemke et al, 2004;Parker and Park, 2004;Phelan et al, 2004;Soga et al, 2004;Falta et al, 2005;Jawitz et al, 2005;Rivett and Feenstra, 2005;Fure et al, 2006;Lemke and Abriola, 2006;Brusseau et al, 2007). An early effort to quantify the relationship between contaminant mass flux reduction and mass removal, and the resultant reduction in risk, was presented by Freeze and McWhorter (1997).…”