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if-soil gas organic vapor and soil core a show contami- dimensional radial flow (, the extraction well is shown in lu nation, but microbial respiration has not yielded 02 uptake equation 2 [221: and CO2 production rates above background soil levels, con- ; ditions within the contaminated soil have resulted in soil mi- crobial toxicity or severe inhibition, or significant nutrient or PQ= —0.5772--]n{4kP rteµ +In(t)J (2) t moisture limitations exists at the site. Unless soil moisture is 4xm the cause of this limitation, bioremediation has limited appli- cation at the site, and alternative remediation schemes should be considered. where P'="gauge" pressure(g/cm-s�measured at the vapor If soil contamination exists and microbial activity above probes some radial distance r(em) from the vent well at time background levels is evident from soil gas measurements,quan- t(s),in=vent well screen interval(cm),k=soil gas permeability 1 tification of maximum respiration rates under field conditions (cm), µ=air viscosity(1.8 x 10-`g/cm-s ® 18'C),e=soil air 1 can be carried out utilizing in situ respiration measurement filled porosity(decimal 1/6), Q=volumetric air flow rate at the techniques described by H.inchee et al. (20, 21]. This method vent well (cm3/s), and P.,.=atmospheric pressure (1 entails the oxygenation ofcontaminated and uncontaminated atm=1.013 x 106 g/cm-s2). background subsurface sr oil around a soil gas probe via air Soil gas pressure or vacuum data collected over time at injection for a 16 to 24 hr period,followed by the measurement various vapor probe locations following initiation of vent well of 02 uptake and CO2 production at the soil gas probe over pumping allow the determination of in situ soil gas permeability time. The collected soil gas data are analyzed using either a and its variability throughout the site. Vapor probe readings zero or first order reaction rate model to generate either zero are plotted as a function of the natural log of time,generating or first order respiration rate values (vol%/hr or I/h, respect a straight line with a slope equal to equation 3 [221: lively) from the slope of these linear regression relationships. [ The background soil values are used to correct contaminated soil values for basal soil respiration taking place at the site. Slope= Q (3) An inert gas tracer can be",injected during soil aeration so that 4-XM/k1 respiration rate measurements can also be corrected for dif- fusion of 02 away from;F or CO2 diffusion to the sampling probe during respiration rate determinations. Rearrangement of this equation allows the determination of f. Using these respiration"data, in situ contaminant biodegra- k directly as: dation rates can be estimated assuming the 3:1 O2:hydrocarbon stoichiometry presented in Equation 1. in addition, required }} oxygen transfer rates can'lbe estimated, and the feasibility of k= Qt` (4) i. in situ bioventing, and the estimated time for remediation 4Slope7rm under prevailing site conditions can be assessed. This approach to data reduction will not be possible if the assumption of radial flow is not maintained at the field site. In Situ Air Permeability Determinations Radial flow will not occur if a significant vertical air velocity component exists due to shallow contamination and subse- Once bioactivity at they site has been verified, the rate of quently a small well screen interval(e 10 ft), and if the soil is transfer of the electron acceptor to the contaminated soil re- coarse grained. Under these conditions, pressure/vacuum mains to be determined- This can be readily accomplished by measured in the vapor sampling points will reach constant obtaining in situ air perrtieabiliry measurements at several lo- values very quickly, requiring that the data be reduced using cations throughout the site.The approach that has become the equations 5 and 6 [221: recommended standard for to situ soil air permeability meas- urements was described in I990 by Johnson et al. 1221 and:isw based on Darcy's Law anQpm(R d equations for steady-state radial Qp flow at a vent well. The method entails the use of a single vent VR—,) i = l well with sail vapor probes placed radially and vertically away for vacuum wells k (5)rIP z from it to monitor soil gas pressure or vacuum throughout the HaP„L i - !P field site when air is extracted or injected at a constant rate at the well head. A schematic of the instrumentation necessary f for a typical in situ permeability field study is shown in Figure m(RRI 3 [221. Q /I he governing equation for such a system assuming one- ,p for extraction wells k= / \ (6) t HaP,,,,, 1— { ] vanar Rtarrnt LL \P / J P:esrene Vapor:�aeenear Unit 037VLC=.n ao�Q,-� r_... where R„=the radius of the vent well (cm), H=the depth to \ :h T �� the top of the well screen (cm), Rt=the minimum radius of` �� rN vent well influence under steady-state flow conditions, and + y Pw=the absolute pressure at the well head (g/cm-s). Ri can AirPtrmeaiSdhry be estimated from inspection of field data,or by extrapolating { TesrScasap the relationship of vapor probe vacuum/pressure versus 109(r) to a 0 vacuum/pressure value.- - :• Integration of Field Data prersme S—?1b%&prober Bioventing system design can now be carried out by esti- FIGURE 3. Schematicof an in situ permeability field mating the equivalent daily oxygen demand and vent air flow study. From Johnson et al. [22] rate as determined from in situ respiration measurements, and 48 February, 1993 Environmental Progress (Vol. 12, No. 1)