Hydrogeologic assessment of in situ natural attenuation in a controlled field experiment

Abstract

[1] An experiment to investigate the natural attenuation of three volatile organic compounds, toluene, carbon tetrachloride, and tetrachloroethene ($1-10 mg L À1 ) was performed in a 3 m deep, sandy aquifer isolated within a 24 m long, 2 m wide, three-sided sheet pile alleyway (hereafter referred to as the gate). A constant flow was maintained in the test volume by pumping a well at the closed end of the gate at 130 mL min À1 . The test compounds were introduced to the aquifer using diffusive emitters installed inside 25 cm diameter wells located at the open end of the gate. Monitoring was performed by sampling along six multilevel fences (consisting of 12 sampling points each) ranging in distance from 1 to 22 m from the source wells. A bromide tracer experiment established that there were no significant hydraulic leaks, nor was there any continuous channeling through the gate. Degradation of the test compounds was assessed by mass balance calculations between fences located 1 and 7 m from the source, and the results were compared with degradation rate estimates from snapshot analyses and the analysis of fluxes. There was reasonably good agreement between rates estimated by these different methods. Toluene degraded with a half-life of 58-62 days, carbon tetrachloride degraded with a half-life of $11 -13 days, and tetrachloroethene degraded too slowly for a reliable estimate of rate to be made. Transformation products identified in the gate included acetate, possibly from toluene degradation, chloroform, trichloroethene, and cis-1,2, dichloroethene. The latter two compounds only appeared in trace quantities and could not be assessed for continuing degradation. However, chloroform degradation was assessed with the snapshot data and using the flux estimates and was found to degrade with a half-life in the range of 10-34 days. No additional chlorinated methanes were detected in the gate, suggesting that the carbon tetrachloride was completely dechlorinated by natural processes within 10 m of the source wells. This experiment demonstrated that degradation of chlorinated solvents occurs naturally at the Borden site but that the ethenes are more resistant to biodegradation than the methanes. In addition, the flux calculations were found to be the most robust in terms of estimating degradation rates.

Monitoring in situ biodegradation of benzene and toluene by stable carbon isotope fractionation

“…An average groundwater flow velocity of 1.9 ± 0.5 m/d was estimated to be representative for the area of the monitoring profile. The retardation of toluene due to interaction with the aquifer matrix was assumed to be 1.2 [37], that is, travel time ( t ) of the toluene along the water flow path is prolonged by a factor of 1.2 in comparison to the travel time of water. With these assumptions, the zero‐order biodegradation rate constant was calculated by plotting theoretical concentrations (μM) over travel time (d) according to Equation 6 (Fig.…”

Contaminant Plume Classification System Based on Mass Discharge

“…Traditional remediation efforts have been based on site investigations that typically focus on both chemical concentration and subsurface flow data. However, an increasing number of research and industry groups (ASTM 1998; Einarson and Mackay 2001a; CTDEP 2002; Devlin et al 2002; Rao et al 2002; Kavanaugh et al 2003; Newell et al 2003; Farhat et al 2006; Chapman et al 2007; Brooks et al 2008; Basu et al 2009; USEPA 2009) suggest that combining contaminant and hydrology into a mass discharge metric may provide key insights about plume behavior. Although some practitioners have used the term mass flux to denote mass per time associated with a plume, terminology acceptance is now consolidating along traditional chemical engineering nomenclature, where mass discharge is mass per time and mass flux is defined as a flux in units of mass per time per area.…”

Nitrate removal in a first-order stream: reconciling laboratory and field measurements

“…Alternatively, ionic salts such as potassium bromide (KBr) can be used to artificially elevate concentrations in one of the contributing waters. This approach has been widely used in various hydrological and hydrogeological studies (e.g., (Hill et al 1998;Hendriks et al 1999;Burns and Nguyen 2002;Devlin et al 2002).…”