Dense Nonaqueous Phase Liquids (DNAPLs)

Calculating Degradation Rates

Abstracts of Journal Articles

A Critique of the Internal Tracer Method for Estimating Contaminant Degradation Rates
Martian, Peter; Kent S. Sorenson Jr.; Lance N. Peterson. Ground Water, Vol 41 No 5, Sep/Oct 2004

The internal tracer method for estimating contaminant degradation rates separates the attenuation effects not associated with degradation by using a co-disposed recalcitrant internal tracer to normalize the degrading contaminant concentration. The remaining attenuation between the internal tracer and degrading contaminant is attributed to degradation, and the degradation rate half-life is estimated from the first-order decay equation. An analytical solution of the advection/dispersion equation was used to evaluate flow-and-transport conditions that could result in incorrect estimates of contaminant degradation rate constants, to estimate the magnitude of error associated with using the internal tracer method at an example site, and to explain different degradation rates estimated using tracers with different decay rate constants.

Determining In Situ Degradation Rates for Various Electron Acceptors via Bio-Trap™ Samples and Push-Pull Tests: A Case Study at a Petroleum Distribution Facility
Raes, E.J. and A.V. Callaghan (Engineering and Land Planning Associates, Inc., Clinton, NJ); K. Sublette, J. Busch-Harris, and E. Jennings (Univ. of Tulsa, OK); J. Istok (Oregon State Univ., Corvallis); A. Peacock (Univ. of Tennessee, Knoxville); G. Davis (Microbial Insights, Inc.). IPEC 2006: 13th Annual International Petroleum Environmental Conference, 17-20 October 2006, San Antonio, TX. [abstract only]

When applied together, BioTrap™ samplers and push-pull tests can be used to assess the biological component of natural attenuation in situ. The biodegradation kinetic data generated from this combined approach can provide key information for assessing the feasibility of monitored natural attenuation as a stand-alone remedy versus a biostimulation strategy. A case study utilizing this approach at a large petroleum distribution facility in New Jersey is presented. Using baited BioTraps™, microbial responses to a suite of electron acceptors (nitrate, sulfate, and dissolved oxygen) were measured in 3 monitoring wells with different geochemical environments. Each BioTrap™ was amended with a different electron acceptor and a mixture of 12C-benzene and 13C-labeled benzene. Control BioTraps™ were included, amended only with benzene. Total benzene loss and incorporation of 13C into microbial biomass were measured for each BioTrap™ condition. The relative biodegradation rates under amended conditions (based on total benzene loss) were compared to the relative degradation rates in the unamended controls to determine whether biodegradation is enhanced under certain terminal electron-accepting conditions. Push-pull tests were conducted to verify the preliminary BioTrap™ data and to segregate biodegradation from abiotic processes, such as dilution. Using the results of the push-pull tests, the plume behavior was modeled to predict the plume's migration and persistence due to abiotic processes, unamended conditions, and biostimulated conditions. Collectively, these techniques were used to calculate mass reduction per dollar rate estimates for natural attenuation (unamended conditions) versus biostimulation (amended conditions) to develop the most cost-effective and successful remedial strategy.

Emerging and Under Utilized Assessment Technologies for Determining Degradation Rates for In Situ Natural Attenuation Capacity
Raes, E.J. (Engineering and Land Planning Assoc., Inc., Clinton, NJ), K. Sublette, J. Istok, J. Fields, J. Jones, A. Peacock, and G. Davis. IPEC 2005: 12th Annual International Petroleum Environmental Conference, 7-11 November 2005, Houston, TX. [abstract only]

In situ degradation rates are typically determined through calculating decreases in chemical concentrations over time from long-term groundwater monitoring programs, which can be a costly and time-consuming approach. A field study was conducted using existing but underutilized remedial investigative tools to assess the feasibility of applying these techniques to determine degradation rate kinetics more quickly and cheaply than with the conventional approach. The field study was conducted at a petroleum distribution facility in New Jersey, where gasoline and an off-site source of tetrachloroethene (PCE) comingled. The objective was to create a protocol of sequential tests to determine if unamended natural attenuation was a sufficient remedial strategy for the site or if bio-stimulation or bioaugmentation was necessary, and then to quickly and cost-effectively screen these processes to select the remedial strategy with the greatest probability of success for this site. The following techniques were assessed. (1) Bio-Traps(TM) coupled with microbial analyses (PLFA and rPCR-DNA) were used to establish background microbial conditions. This novel, in-well sampling device is often referred to as an in-well microcosm. Bio-films, which rapidly form within the bio-traps, are analyzed for a presence/absence and abundance determination for known microbial groups documented to degrade, in part or fully, the compounds of interest. (2) Bio-Traps(TM) augmented with electron acceptors (oxygen, nitrate and sulfate) were used to determine the target contaminant degradation rates when exposed to these different electron acceptors (biostimulation). (3) A series of single-well, push-pull tests were used to determine in situ kinetics and microbial processes in the same wells. A test solution contained the same electron acceptors (oxygen, nitrate and sulfate) used in the Bio-Trap study in addition to a conservative tracer (bromide) to segregate abiotic and biotic in situ degradation rates of the chemicals of interest over time (short term) when exposed to different biostimulation substrates. Baited with electron acceptors, the Bio-traps were loaded with a 13C-labeled benzene and passive flux meters to quantify mass reduction of the 13C-labeled benzene. This allowed comparison of the biostimulated degradation rates to unamended degradation rates to determine whether natural attenuation could stand alone as a remedy or if bio-stimulation produced significantly better results. The different substrates were screened to identify which one had the highest probability of favorable results for a full-scale implementation. The biodegradation rates from the Bio-trap and push-pull studies were compared for similarities and contrasted with the overall site degradation rates generated from years of groundwater monitoring at the site.

Estimation of Anaerobic Biodegradation Rate Constants at MGP Sites
Lewandowski, G. and G. Mortimer, New Jersey Inst. of Technology, Newark. Ground Water, Vol 42 No 3, p 433-437, May-June 2004

Field data at six former manufactured gas plant sites in New Jersey were used to estimate the biodegradation rate constants for the anaerobic processes naturally occurring within the groundwater contaminant plumes. Those rate constants turned out to be about an order of magnitude smaller than values reported for benzene and naphthalene at fuel sites.

Case Studies

Detecting and Quantifying Reductive Dechlorination During Monitored Natural Attenuation at the Savannah River CBRP Site
Istok, J.D. and J.A. Field (Oregon State Univ.); E. Raes (Engineering and Land Planning Associates); M.R. Millings (Savannah River National Lab); A.D. Peacock (Microbial Insights, Inc.). Report No: WSRC-STI-2006-00340, 50 pp, Jan 2007

Researchers used field sampling and testing in 10 monitoring wells to investigate the relationship between baseline geochemical and microbial community data and in situ reductive dechlorination rates at a site contaminated with trichloroethene (TCE) and carbon tetrachloride (CTET). The 10 monitoring wells represented conditions along a ground-water flow path from the contaminant source zone to a wetlands ground-water discharge zone. Analytical results from background ground-water samples for a suite of geochemical and microbial parameters and the results of push-pull tests with fluorinated reactive tracers were used to measure in situ reductive dechlorination rates. Geochemical data provided some evidence of the occurrence of reductive dechlorination at the site, and microbial data confirmed the presence of known dechlorinating organisms, as well as sulfate reducers, iron reducers, and methanogens. A principal component analysis identified three groups of wells with similar geochemical and microbial characteristics. Push-pull tests were conducted using trichlorofluoroethene (TCFE) as a reactive tracer for TCE and trichlorofluoromethane (TCFM) as a reactive tracer for CTET. Injected TCFE was transformed in situ to cis- and trans-dichlorofluoroethene and chlorofluoroethene. In one test, TCFE completely dechlorinated to fluoroethene. Injected TCFM was transformed in situ to dichlorofluoromethane and chlorofluoromethane. Zero-order TCFE transformation rates ranged from < 0.05 to 1.00 nM/hr (< 0.44 to 8.76 µM/yr). TCFE reduction rates differed among the three groups of wells identified by principal component analysis, providing preliminary evidence that geochemical, microbiological, and in situ reductive dechlorination rates may provide complementary information. A single TCFM transformation rate was estimated as < 0.05 nM/hr (0.44 µM/yr). This study demonstrated that push-pull tests with reactive tracers can be used to detect and quantify reductive dechlorination of chlorinated ethenes and methanes under monitored natural attenuation conditions.