title: A mass balance approach to investigating geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN.
abstract: Secondary water quality impacts can result from a broad range of coupled reactions triggered by primary groundwater contaminants. Data from a crude-oil spill research site near Bemidji, MN provide an ideal test case for investigating the complex interactions controlling secondary impacts, including depleted dissolved oxygen and elevated organic carbon, inorganic carbon, CH4, Mn, Fe, and other dissolved ions. To better understand these secondary impacts, this study began with an extensive data compilation of various data types, comprising aqueous, sediment, gas, and oil phases, covering a 260m cross-sectional domain over 30years. Mass balance calculations are used to quantify pathways that control secondary components, by using the data to constrain the sources and sinks for the important redox processes. The results show that oil constituents other than BTEX (benzene, toluene, ethylbenzene, o-, m- and p-xylenes), including n-alkanes and other aromatic compounds, play significant roles in plume evolution and secondary water quality impacts. The analysis underscores previous results on the importance of non-aqueous phases. Over 99.9% of the Fe(2+) plume is attenuated by immobilization on sediments as Fe(II) and 85-95% of the carbon biodegradation products are outgassed. Gaps identified in carbon and Fe mass balances and in pH buffering mechanisms are used to formulate a new conceptual model. This new model includes direct out-gassing of CH4 and CO2 from organic carbon biodegradation, dissolution of directly produced CO2, and sorption with H(+) exchange to improve pH buffering. The identification of these mechanisms extends understanding of natural attenuation of potential secondary impacts at enhanced reductive dechlorination sites, particularly for reduced Fe plumes, produced CH4, and pH perturbations.
authors.affiliation: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States. Electronic address: gcng@umn.edu.
authors.forename: G-H Crystal
authors.initials: GH
authors.lastname: Ng
authors.affiliation: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States. Electronic address: babekins@usgs.gov.
authors.forename: Barbara A
authors.initials: BA
authors.lastname: Bekins
authors.affiliation: U.S. Geological Survey, 12201 Sunrise Valley Dr, Reston, VA 20192, United States. Electronic address: icozzare@usgs.gov.
authors.forename: Isabelle M
authors.initials: IM
authors.lastname: Cozzarelli
authors.affiliation: U.S. Geological Survey, 12201 Sunrise Valley Dr, Reston, VA 20192, United States. Electronic address: mjbaedec@usgs.gov.
authors.forename: Mary Jo
authors.initials: MJ
authors.lastname: Baedecker
authors.affiliation: University of Texas at Austin, Dept. of Geological Sciences, 1 University Station C1100, Austin, TX 78712, United States. Electronic address: pbennett@jsg.utexas.edu.
authors.forename: Philip C
authors.initials: PC
authors.lastname: Bennett
authors.affiliation: University of Waterloo, Dept. of Earth and Environmental Sciences, 200 University Ave. W, Waterloo, ON N2L 3G1, Canada. Electronic address: ramos@scimail.uwaterloo.ca.
authors.forename: Richard T
authors.initials: RT
authors.lastname: Amos
mesh_terms: D001673:Biodegradation, Environmental; D004784:Environmental Monitoring / Q000379:methods*; D060587:Groundwater / Q000737:chemistry*; D008910:Minnesota; D008956:Models, Chemical; D010578:Petroleum; D059392:Petroleum Pollution / Q000032:analysis*; D014874:Water Pollutants, Chemical / Q000032:analysis*; D060753:Water Quality
chemical_list: D010578:Petroleum; D014874:Water Pollutants, Chemical
keywords: BTEX; Biodegradation; Groundwater; Hydrocarbon; LNAPL; Oil spill; Secondary water quality impacts