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, CH 4, 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 260 m cross-sectional domain over 30 years. 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 Fe2 + 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.
Bibliographical noteFunding Information:
This work was funded by SERDP (grant ER-2131) aand also supported by the USGS Toxic Substances Hydrology Program and the USGS National Research Program. The authors thank Doug Kent, Gary Curtis, Bob Borden, Jason Tillotson, Bill Herkelrath, and Hedeff Essaid for their insightful discussions. Helpful comments were provided by Bob Eganhouse and two anonymous reviewers. Jeanne Jaeschke provided analytical support, and Ean Warren helped with the production of Fig. 1 . Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. government.
- Oil spill
- Secondary water quality impacts