Rapid Approach to Evaluate NSZD at LNAPL Sites G. Todd Ririe (
[email protected]) (BP La Palma, CA, USA) Robert E. Sweeney (Petroleum & Environmental Geochemistry, Etna, California) ABSTRACT: A sub-surface approach has been developed for rapid evaluation of the rate of natural source zone depletion (NSZD) using easily obtainable field measurements and existing site data. This rapid approach is based on the gradient method for determining the fluxes of O2 and CO2 used to calculate the rate of NSZD. Until recently, the gradient method has been limited for use in NSZD studies due to the need to intrusively place soil gas probes in the soil down to the depth of the contamination. This limitation, however, can be eliminated by obtaining the ‘source’ soil gas composition by measuring CO2 ,O2 and CH4 from vapor samples collected at the base of the vadose zone in existing monitoring wells that are screened across this interval. The next step is to collect a vertical profile temperature profile in the monitoring well to evaluate change in temperature from biodegradation of hydrocarbons. These data can be used to help establish the depth of the aerobic/anaerobic interface. Because not all areas of large LNAPL sites have monitoring wells in locations needed we tested a soil gas method to supplement these measurements. We collected CO2 concentrations at 1 and 2 feet depth in the soil, with the objective to identify areas with a positive CO2 gradient below the active surface soil respiration processes. Based on the results, locations were chosen to place traps to measure surface CO2 efflux, and shallow vapor probes to monitor CO2 concentrations in the soil. Concentration gradients determined from monitoring well or vapor well measurements can then be used to calculate the gas flux, the rate of hydrocarbon removal, and the rate of NSZD as a function of the effective diffusivity in the soil. The effective diffusivity in vadose zone soil needs to be measured in the field, or estimated on the basis of the texture of the soil, or vapor and moisture porosities. A nomogram approach is then used to plot the relationship between the concentration gradient and the rate NSZD as a function of effective diffusivity. For O2 or temperature measurements, the nomogram approach is used to relate the depth of the aerobic/anaerobic interface and the rate of NSZD. INTRODUCTION LNAPL sites typically have a large amount of data and have usually been the focus of a variety of remedial actions. Because product recovery is limited in its ability to reduce risk and achieve regulatory end points at these sites, other approaches need to be considered when product recovery has achieved its technical limits. For this purpose, natural source zone depletion (NSZD) has become recognized as an important process especially at large LNAPL sites that have achieved, or are near achieving, asymptotic levels of free product recovery. A comparison of NSZD methods (Table 1) has been modified to incorporate recent updates in the use of the temperature approach. The main concerns for the gradient method since it was first developed (Johnson and Lundegard, 2006; and Lundegard and Johnson, 2006) have been that it is an invasive method with the need to install vapor wells, analytical results may take weeks to obtain, and there is a need to determine accurate effective diffusion coefficients.
Recent developments in sampling of soil gas vapor and temperature from existing monitoring wells (Jewel and Wilson, 2011; Sweeney and Ririe, 2014) has now led to the ability to obtain non-intrusive sub-surface data necessary for NSZD studies by using the existing infrastructure at a site. The rapid approach to NSZD was developed to take advantage of these sampling methods. Besides cost savings, field measurements of O2, CO2, CH4, and temperature allows for useful results to be obtained while in the field. Though there is still need to determine the effective diffusivity of the soil, a nomogram approach can be used to bracket reasonable diffusivity ranges on the basis of soil texture and porosity data. The collection of vapor samples from monitoring wells is a key component of the rapid assessment approach. The concept and EPA method for collecting vapor samples are discussed in Jewell and Wilson (2011). Application of the method to obtain gas composition of the ‘source’ for PVI studies (Wilson et al., 2014) and NSZD (Jewell and Wilson, 2011) have also been documented. The EPA method has been modified to minimize purging (small purge method), and to simplify field procedures. For NSZD studies, the absence of O2 (
