ARCHIVED REPORTS XR0011902 - Laserfiche WebLink

Home Browse Search

v -� boo A S S O C I A T E S I N C Equation 1 describes the biodegradation of a shrinking plume where the concentrations of a contaminant at a specific monitoring well are observed to decline with time. Equation 2 describes the biodegradation of a stable plume where the concentrations of contaminants remain the same over time. Using Equation 2, the biodegradation decay rate is calculated using contaminant concentrations observed in three or more wells that are located near the source and on the downgradient side of the plume. The plume at the site is a stable plume as evidenced by the relatively consistent chemical analytical results seen the monitoring wells over the past four years, and as such Equation 2 is used to calculate the biodegradation decay rates for benzene and TPHg. The most recent benzene and TPHg results for wells MW-2, MW-3 and MW-4 were used for this analysis. Figures 11 and 12 show the exponential fit and the resulting equations for April 1999 data. Using a groundwater advective velocity of 11.4 feet per year (derived from geophysical laboratory tests and site measurement data), the decay rate (k) for TPHg and benzene is 0.0012 and 0.0017 per day, respectively. These decay rates are conservative estimates because the TPHg and benzene data for MW-3 lie below the exponential curve. As a result, the biodecay rates for gasoline and benzene are probably even greater than those calculated. The biodecay half-fife (ty,) for a chemical is the time required for the chemical in question to degrade to one-half its original concentration. The equation for calculating the half-life of a chemical is shown below (Dominico and Schwartz, 1990). t%_(In 2)/k As a result, the half-lives for benzene and TPHg at the site are 578 days and 408 days, respectively. bw Given the biodecay rates for TPHg and benzene calculated above, the time required to biodegrade TPHg presently in groundwater to 100 ppb (taste, odor threshold) and benzene presently in groundwater to 1 ppb (MCL) can be calculated by rearranging equation 1 as shown below. In (C/C;)_-kt Using 1,600 ppb for C; as TPHg and 970 ppb as C; for benzene, the time required to biodegrade these analytes to the regulatory levels stated above is 6.3 and 11.1 years, respectively. As a result, potential petroleum hydrocarbons that leach from the vadose zone and become dissolved in V groundwater beneath the site are intrinsically biodegraded to these regulatory levels within about 11 years. 4.2.2 Assimilative Capacities for Biodegradation of BTEX The assimilative capacity is a semi-quantitative estimate of the potential mass of BTEX •- compounds that can be degraded by aerobic and anaerobic biodegradation processes. The assimilative capacity is calculated using biodegradation process stoichiometric relationships between the mass per unit volume of each electron acceptor (and biodegradation byproduct) to wA95122\repotts\ReVest for NFA.doc 12 r