User:Debra Tabron/sandbox

A long-term study of a 40-year-old crude oil spill provides insights about petroleum hydrocarbon natural attenuation processes and rates. In the source zone, fermentation coupled to methanogenesis is the dominant natural source zone depletion (NSZD) process, and most of the carbon mass exits the surface as CO₂ efflux. Monitored natural attenuation (MNA) of the groundwater plume shows that benzene degradation is coupled to iron reduction and that the benzene plume is stable. A plume of hydrocarbon oxidation products measured as nonvolatile dissolved organic carbon (NVDOC) expanded ~20 m in 20 years. Most of the NVDOC is biodegraded by 200 m from the source, but optical data suggest there are components that persists for 300 m. Biological effects screening indicates decreasing biological effects with distance from the source.

Related Article(s):

• Natural Source Zone Depletion (NSZD)
• Monitored Natural Attenuation (MNA)
• Biodegradation - Hydrocarbons

CONTRIBUTOR(S): Dr. Barbara Bekins

Key Resource(s):

• Crude oil at the Bemidji site: 25 years of monitoring, modeling, and understanding
• doi: 10.1111/gwat.12419 Crude Oil Metabolites in Groundwater at Two Spill Sites

Introduction

The purpose of this article is to provide an overview of a petroleum release research site and to summarize key results relating to natural source zone depletion (NSZD) and to anaerobic degradation of hydrocarbon plumes. A final section briefly describes ongoing research projects at the site.

The Bemidji crude oil research site was established by the U. S. Geological Survey’s Toxics Substance Hydrology Program in 1983 at the location of an oil release which occurred in 1979 when a high-pressure pipeline ruptured. The goal of the research is to improve understanding of monitored natural attenuation (MNA) of petroleum hydrocarbon spills with transformative research on the source, transport and fates of the major contaminant classes. To date, 274 publications by interdisciplinary teams working at the site have covered physical, biological, and geochemical processes controlling these contaminants. Since 2009, the pipeline owner, Enbridge Energy LLC, has provided supplemental funding for site management and research seed money through a collaborative agreement between USGS, Minnesota Pollution Control Agency, and Beltrami County.

Site Description

Figure 1. Map of the site located near Bemidji, MN, showing oil pipelines, location of 1979 rupture, oil bodies at the water table, area sprayed by oil, direction of flow, the 10 ppb BTEX contour and the down-gradient lake. Figure reprinted from McGuire, et al., 2018.

The site (Figure 1) is located 16 km northwest of Bemidji, MN. An oil release occurred on August 20, 1979, when a high-pressure pipeline ruptured along a seam, spilling 10,700 barrels (1.7 million liters) of light, low sulfur crude. The immediate cleanup removed about 70% of the oil by pumping, excavation, land farming, and burning. An area of 6,500 m² that was sprayed by oil from the pipeline contains oil in the top few centimeters of soil. The remaining oil infiltrated into the aquifer forming three oil bodies at the water table designated as the north, middle and south pools. Additional remediation by a dual-pump recovery system from 1999 through 2003 removed an estimated 36-41% of the oil from the three oil pools.

Most studies of the Bemidji source zone and plume have been focused on the north pool. The oil body at the water table is 1-2 m thick and spans an area of 100 m × 25 m. Oil saturations are 10-70%, with the highest saturation in a coarse-grained layer below the water table. Residual oil saturations of 15-30% are present from the land surface to the water table.

The depth to the water table near the north pool is 6-8 m. The aquifer is a glacial outwash deposit of medium to coarse variably sorted sand with <0.1% organic carbon. It is underlain by poorly permeable till at 18-27 m depth. The range of estimated hydraulic conductivities is 0.56-7 x 10^-5 m/s. The background groundwater is aerobic with dissolved oxygen of 9 mg/L. Average pore water velocity estimates range from 0.004 to 0.056 m/day (0.5-67 ft/year, 0.1-20.5 m/year).

Figure 2. Conceptual cross section of the site illustrating microbial processes, modified from Ng et al., 2015 and Sihota et al., 2016.

Natural Source Zone Depletion (NSZD)

Natural Source Zone Depletion (NSZD) refers to the changes over time in the composition of spilled oil or fuel that occurs naturally by dissolution, volatilization and biodegradation. Figure 2 shows a conceptual model of NSZD processes at the Bemidji site. Biodegradation within the oil body occurs primarily by fermentation coupled to methanogenesis. The biogenic methane produced in the oil body diffuses toward the land surface where it is oxidized in the middle of the vadose zone by methanotrophic bacteria. The CO₂ produced by methane oxidation together with some produced in the source zone exits as surface efflux over the oil body. Garg et al. present a useful summary of the important NSZD results from the site. This brief section will focus on how NSZD has been measured at the north pool and what has been discovered. Estimates of NSZD at Bemidji have been based on a variety of indicators, including vadose zone gas concentrations, compositions of oil samples from wells, surface CO₂ efflux, temperature measurements, and modeling.

Gas concentration data from 1985, six years after the spill, indicated that volatilization accounted for 97% of NSZD and biodegradation only 3%. The volatile fraction was dominated by C3-C5 alkanes and was oxidized in the vadose zone before reaching the surface. Total losses were estimated to be 10.5 kg/day. By 1997, volatilization losses were 10 times lower and although biodegradation losses increased, total losses had dropped to 2 kg/day. The total average mass loss (Table 1) was estimated from the gas data to be 0.6% per year over the period 1979-1997.

Analyses of oil samples collected in 2010 from 13 wells in the north oil pool showed that 31 years after the spill, n-alkanes, toluene, and o-xylene were the most depleted hydrocarbons, while n-C_10–24 cyclohexanes, tri- and tetra- methylbenzenes, acyclic isoprenoids, and naphthalenes were the least depleted. Benzene was still present at every sampling location. The technique of normalizing composition to a conserved compound yielded estimates for spatially variable mass losses ranging from 18% to 30% or an overall average rate of 0.8% per year (Table 1).

Table 1. Oil Loss Measurements*

Reference	Method	Time Interval	Equivalent CO2 flux	Total Oil Loss Rate		Annual Loss	Comments
			μmol/m2-s	kg/m2-d	gal/acre-y	%/y
Chaplin et al. (2002)	Gas data and 1D model	1979-1997	0.75	9.33×10-4	429	0.63	Published estimate of 0.2% was based on total oil rather than N. Pool; Loss given is average of 3 oil sites in their Table 4.
Baedecker et al. (2018)	Oil composition	1979-2010	0.93	1.16×10-3	533	0.79	Average using four conservative compounds. Computed from published values as (7×30.1+6×17.8)/13÷31 years.
Sihota et al. (2016)	Surface CO2 efflux	2011-2013	1.1	1.37×10-3	627	0.93	Value given is annual average efflux June 2011-July 2013 from their Table 1.
Molins et al. (2010)	Gas and water data with 2D model	1979-2007	2.15	2.67×10-3	1,226	1.81	Value given is total flux. Model lower boundary methane flux of 1.3 mol/m2-d was 0.7 of total. Published value based on only VZ degradation was 1% y-1.
Ng et al. (2015)	2D model using oil, gas, and water data	1979-2008	1.46	1.81×10-3	831	1.23	Oil loss rate computed from losses given in their Figure 7, masses in Table 3, and 82 m length of oil body from Figure 2.
* Note: The values in bold are taken directly from the listed reference. The other columns are computed from this column using the following values: oil specific gravity = 0.85 , carbon fraction of oil = 0.835, north pool oil body area = 2,323 m2 and total mass of oil 124,950 kg.

The CO₂ efflux technique involves measuring the flux of CO₂ leaving the surface above the source zone and comparing it to an average rate from natural soil respiration in uncontaminated locations. The difference in efflux between the source zone and background is assumed to reflect the amount generated by biodegradation of the source, but the estimate can be complicated by spatial variation in background rates. A refinement of the technique is based on the principle that degradation of oil produces ancient carbon, and analyzing for ¹⁴C content of CO₂ provides more precise estimates of the efflux attributable to contaminant degradation. Monthly data from the site shows that surface efflux and subsurface gas concentrations vary seasonally, with 2-3 times higher efflux values in the summer and fall compared to the spring and winter. The CO₂ effluxes are typically expressed in units of micromoles of CO₂ per meter-squared per second (µmol m ² s ¹). The average annual rate measured for the north pool from June 2011 until July 2013 was 1.1 ± 0.4 µmol m ² s ¹. This corresponds to an average oil biodegradation rate of 0.9% per year (Table 1).

The temperature method for evaluating NSZD involves measuring vertical profiles of vadose zone temperatures above the source zone and at a background site subject to the same external heating. The source of the heat at the Bemidji site is the combined effect of methane oxidation in the vadose zone above the oil body and the active oil pipelines passing through the site. Temperature increases are used to estimate NSZD rates by converting the excess heat to reaction rates using the reaction enthalpy. Estimates using average annual temperature increases were comparable to NSZD rates estimated from the average annual surface efflux of CO_2.. However, modeling indicates that half of the excess heat measured in the aquifer came from the pipelines, underscoring the importance of subtracting temperature effects of nearby infrastructure before using excess temperatures to estimate NSZD rates. The method has been carefully tested at a refined petroleum products terminal, where researchers found 8% higher estimated NSZD rates using CO₂ flux measurements compared to temperature based estimates.

Two comprehensive modeling studies have focused on NSZD at the site. Molins et al. modeled the NSZD processes of volatilization, biodegradation and dissolution in the vadose zone. They used gas and aqueous concentration data with estimates of vadose zone transport properties, but did not have surface efflux or oil composition data to constrain fluxes. Their calibrated methane efflux was 2.15 µmol m ² s ¹, of which 30% came from inside the model and 70% from the smear zone. Within the model domain the modeled loss was 1% per year during the period 1979 – 2007. Including the flux and oil outside the model domain gives a loss of 1.8% per year (Table 1). An important conclusion of this paper is that soil diffusion properties are poorly constrained, and surface efflux measurements might be an effective strategy to constrain gas flux. The model by Ng et al. included biodegradation and dissolution constrained by surface efflux and oil composition data but not volatilization. Their overall loss was 1.8x10^-3 kg/m²-d during 1979 – 2008 or 1.2% per year (Table 1). A major finding of this study is that 85% of the carbon loss from the oil occurs through CO₂ gas efflux from the surface over the source zone.