Inverse modeling of BTEX dissolution and biodegradation at the Bemidji, MN crude-oil spill site

Abstract

The U.S. Geological Survey (USGS) solute transport and biodegradation code BIOMOC was used in conjunction with the USGS universal inverse modeling code UCODE to quantify field-scale hydrocarbon dissolution and biodegradation at the USGS Toxic Substances Hydrology Program crude-oil spill research site located near Bemidji, MN. This inverse modeling effort used the extensive historical data compiled at the Bemidji site from 1986 to 1997 and incorporated a multicomponent transport and biodegradation model. Inverse modeling was successful when coupled transport and degradation processes were incorporated into the model and a single dissolution rate coefficient was used for all BTEX components. Assuming a stationary oil body, we simulated benzene, toluene, ethylbenzene, m,p-xylene, and o-xylene (BTEX) concentrations in the oil and ground water, respectively, as well as dissolved oxygen. Dissolution from the oil phase and aerobic and anaerobic degradation processes were represented. The parameters estimated were the recharge rate, hydraulic conductivity, dissolution rate coefficient, individual first-order BTEX anaerobic degradation rates, and transverse dispersivity. Results were similar for simulations obtained using several alternative conceptual models of the hydrologic system and biodegradation processes. The dissolved BTEX concentration data were not sufficient to discriminate between these conceptual models. The calibrated simulations reproduced the general large-scale evolution of the plume, but did not reproduce the observed small-scale spatial and temporal variability in concentrations. The estimated anaerobic biodegradation rates for toluene and o-xylene were greater than the dissolution rate coefficient. However, the estimated anaerobic biodegradation rates for benzene, ethylbenzene, and m,p-xylene were less than the dissolution rate coefficient. The calibrated model was used to determine the BTEX mass balance in the oil body and groundwater plume. This article is a U.S. government work, and is not subject to copyright in the United States.Dissolution from the oil body was greatest for compounds with large effective solubilities (benzene) and with large degradation rates (toluene and o-xylene). Anaerobic degradation removed 77% of the BTEX that dissolved into the water phase and aerobic degradation removed 17%. Although goodness-of-fit measures for the alternative conceptual models were not significantly different, predictions made with the models were quite variable. D

Reactive transport modeling of geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN

“…He further established that partial equilibrium [Jakobsen and Postma, 1999] or fully kinetic reactions with free energy dependencies successfully avoid these unrealistic simulations. Two-dimensional cross section modeling studies by Essaid et al [1995], Essaid et al [2003], Amos [2006], and Amos and Mayer [2006] provided additional insights. Essaid et al [1995] simulated sequential aerobic, Mn(IV)-coupled, Fe(III)-coupled, and methanogenic biodegradation of dissolved organic carbon (DOC) using multiple Monod kinetics with microbial growth.…”

“…The Bemidji site has been the subject of multidisciplinary studies by the U.S. Geological Survey Toxic Substances Hydrology program, many of which are described in the review by Essaid et al [2011]. A number of geochemical modeling studies have been conducted to assess processes behind the hydrocarbon attenuation Baedecker et al, 1993;Essaid et al, 1995;McNab and Narasimhan, 1995;Curtis et al, 1999;Curtis, 2003;Essaid et al, 2003;Amos, 2006;Amos and Mayer, 2006], making the Bemidji site an ideal test case with which to develop a model to investigate SWQIs.…”

Influence of Petroleum Deposit Geometry on Local Gradient of Electron Acceptors and Microbial Catabolic Potential

“…In case of NAPLs in groundwater, substantial field evidence exists in support of presence of multiple electron acceptors within one contaminated site [72,74,75]. In presence of multiple TEAs microbes tend to utilize them sequentially, in increasing order of their Gibbs free energy (Table 2.2).…”

“…Most of the current models take in account the sequential use of TEAs and transport and uptake of nutrients necessary for microbial growth [75]. As explained in Essaid et al (2003), these models are of varying complexity depending on the extent to which they represent: 1) physical, chemical, and biological processes;…”

“…Experimental information on dissolution characteristics will be useful for existing models [74], however it is difficult to determine experimentally because of quick onset of biodegradation, which quickly degrades any petroleum hydrocarbon in dissolved phase. It will be interesting to understand the dissolution rates and dissolution limit of different components of hydrocarbons, when present together in a complex mixture as found in natural reservoirs [43] and compare it to the values used in existing groundwater flow models.…”