Environ. Sci. Technol. 1996, 30, 2626-2633
Hydrologic and Microbiological Factors Affecting Persistence and Migration of Petroleum Hydrocarbons Spilled in a Continuous-Permafrost Region JOAN F. BRADDOCK* Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska 99775-7000
KATHLEEN A. MCCARTHY U.S. Geological SurveysWater Resources Division, 10615 SE Cherry Blossom Drive, Portland, Oregon 97216
Fuel spills, totaling about 1300 m3, occurred between 1976 and 1978 adjacent to Imikpuk Lake, a drinking water source near Barrow, AK. Substantial contamination of soils and groundwater near the lake persists. We examined the magnitude and direction of groundwater flux and the microbial activity at this site to understand the persistence of contamination and its effect on the lake. We found that groundwater flux is small due to shallow permafrost, which restricts the cross-sectional area available for flow, and to the short annual thaw season (ca. 90 days). The small flux and limited depth also constrain contaminant transport and dispersion, resulting in persistent, shallow contamination. The numbers of hydrocarbonoxidizing microorganisms and their laboratory mineralization potentials for benzene (at 10 °C) were higher in samples from contaminated areas than in reference samples. Benzene mineralization potentials in groundwater samples were comparable to more temperate systems (0.1-0.5 mg of benzene mineralized L-1 day-1) and were stimulated by nutrient additions. Field measurements of dissolved oxygen, nitrate, ferrous iron, and sulfide in groundwater provided evidence that biodegradation of petroleum hydrocarbons is occurring in situ. Despite evidence of an active microbial population, microbial processes, like contaminant transport, are likely limited at this site by the short annual thaw season.
Introduction During the years of operation of the Naval Arctic Research Laboratory (NARL) near Barrow, AK, accidental releases of approximately 1300 m3 of various types of fuel occurred in the vicinity of the NARL airstrip (1), and groundwater and * Corresponding author telephone: (907) 474-7991; fax: (907) 4746967; e-mail address:
[email protected].
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soils still show evidence of petroleum-hydrocarbon contamination. Fuel remaining in the subsurface as a result of these spills is of particular concern because of the proximity of Imikpuk Lake, the drinking-water source for the NARL facility. We began an evaluation of the hydrology and microbiology at the airstrip site in 1993. The purpose of the study was to assess the local hydrologic conditions and the implications for subsurface migration of contaminants. It is predicted that “a considerable portion of the world’s oil and gas production in the foreseeable future will come from hydrocarbon deposits in the Arctic and Subarctic” (2). Therefore, there is a continuing need to understand the fate of contaminants in these systems. Our approach was to examine the groundwater flow system and estimate the magnitude and direction of groundwater flux; measure the concentrations of petroleum hydrocarbons in groundwater to determine the distribution of contaminants in the system; assay microbial populations and their hydrocarbon mineralization capacities; and evaluate physical properties and chemical constituents in groundwater as indicators of naturally occurring biodegradation. The results of this study are being used to design a subsurface hydraulic containment system that will function to intercept upgradient groundwater and reduce the flux of contaminated groundwater into Imikpuk Lake. In addition, downgradient of the containment system, the hydraulic gradient toward the lake will be reduced, further decreasing groundwater flux into the lake and increasing the time available for biological degradation of contaminants before discharge to the lake.
Background The study site is located along the northeast shore of Imikpuk Lake, 6 km northeast of Barrow, AK, and 10 km southwest of Point Barrow, the northernmost extremity of the Arctic Coastal Plain (Figure 1). Three documented fuel spills occurred between 1976 and 1978 including about 180 m3 of leaded gasoline near the south corner of Hanger 136, about 95 m3 of JP-5 in the “north depression”, and about 1050 m3 of gasoline in the “south depression” (see Figure 1). Permafrost in the region is continuous and extends from near the surface to depths of up to 300 m (3). A shallow layer, referred to as the active layer, overlies the permafrost. This active layer, which thaws each summer and refreezes each winter, contains a transient groundwater system. Shallow soils in the study area are predominantly poorly sorted gravels and coarse sands, but areas of finer grained material exist, particularly near the shores of Imikpuk Lake and North Salt Lagoon. Mean monthly air temperatures in the study area rise above freezing only during the months of June-August, resulting in an annual hydrologic cycle that can be described in terms of four periods: winter, snowmelt, summer, and freeze-up (4). The winter season in the study area typically begins during late September and continues through early to mid-June. Subfreezing temperatures prevail throughout this period. As a result, streamflow does not occur, freshwater lakes less than approximately 2 m in depth freeze to the bottom, and the subsurface is frozen from land surface to depths of up to 300 m (3). The snowmelt period, which marks the transition from winter to summer, is the
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1996 American Chemical Society
elevation at each site was determined using trigonometric and differential field leveling (Table 1). The benchmark for the leveling was a monument referenced to mean sea level for the Point Barrow tidal station. The locations of most wells were determined by trigonometric surveying and referenced to horizontal control stations established by the National Ocean Survey. All sites were resurveyed during each site visit to determine whether well casings had shifted vertically as a result of the freeze-thaw cycle. No significant changes in the elevations of well casings were noted over the course of the study.
FIGURE 1. Map showing sampling site locations. Solid symbols indicate sites where hydrologic data were collected; open symbols indicate sites where microbiological and chemical data were collected.
dominant hydrologic event of the year. The brief thaw season generally begins during early to mid-June, and most snowmelt occurs within approximately 2 weeks. Surface runoff is considerable during the snowmelt period, but recharge to the groundwater system and groundwater flow are limited by incomplete thawing of the subsurface active layer. The brief summer extends through late August or early September, resulting in a thaw season that lasts approximately 90 days. Thawing and deepening of the active layer continues throughout the summer, but the rate of thaw penetration generally decreases considerably over the course of the season. During the freeze-up period, which is the transition from summer to winter, temperatures drop below freezing, the ground surface cools and freezes, and freezing gradually progresses downward through the active layer. Precipitation during the late summer and early freeze-up periods is considerable, and infiltration and recharge to the active layer occur during this time.
Materials and Methods Hydrologic Data Collection. Surface-water drainage patterns throughout the Imikpuk Lake and North Salt Lagoon basins were observed and mapped in June 1993, during the snowmelt period, and again during both July and August. Subsurface hydrologic data were collected from 40 wells at the site (Figure 1) during the summers of 1993-1995. Three wells (including well US7AS) were installed as part of the current study; all other wells were installed during previous studies (5-10). After measuring the depth to static groundwater level in each well, the well was pumped using a manual peristaltic pump, and the response of the water level was monitored to assure that the well was hydraulically connected to the surrounding groundwater system. The static water level in each well was then remeasured at least one additional time. The vertical extent of the groundwater zone at each well was determined by measuring the depth of thaw either by augering to the frost table with an 8-cm manual auger or by driving a metal probe into the ground to the depth of refusal, which was assumed to be the surface of the frost. At several sites, thaw depth was measured using both methods, and agreement between the methods was good (( 5 cm). All data collection sites were surveyed, including the water surfaces of Imikpuk Lake and North Salt Lagoon. The
Microbiology and Chemistry Data Collection. (A) Sampling Protocol. Seven wells along the shoreline of Imikpuk Lake were sampled in early (9, 10) and late (2527) August 1994 and in July (13-15) 1995 (Figure 1). Sampling followed guidelines established by the U.S. Geological Survey (USGS) (11). The wells were purged using a peristaltic pump until measurements of pH, specific conductance, and temperature stabilized (after about 1520 L was pumped). Water samples from purged wells were then collected in disposable bailers. Soil (primarily fine gravel) from the unsaturated zone just above the water table (sample depths ranged from 5 to 20 cm) was also collected from triplicate 7-cm (2.75-in.) diameter cores within a radius of about 1 m from each well. Water samples were either analyzed in the field as described or were collected and stored on ice for shipment to the laboratory for processing. Soil samples were collected in sterile plastic sampling bags and stored on ice. (B) Chemistry. Measurements of dissolved oxygen, pH, specific conductance,and temperature were made in the field using a flow-through meter (Hydrolab Monitor with Scout II data logger, Hydrolab Corporation; Austin, TX). Readings for dissolved oxygen were occasionally verified using the azide modification of the wet chemistry Winkler method (12). Ferrous iron and sulfide concentrations were measured by colorimetric field assays (Hach Company; Loveland, CO). Samples for hydrocarbon analyses were collected in Teflon bailers and transferred to precleaned 40-mL amber septa vials (I-Chem Research; Hayward, CA). Two drops of HCl (1:1 in hydrocarbon-free water) were added to each vial before sealing. The samples were immediately placed on ice and shipped to the USGS National Water Quality Laboratory (NWQL; Denver, CO). Total recoverable volatile organic hydrocarbons (schedule 1390; similar to EPA 524.2) were analyzed using purgeand-trap gas chromatography/mass spectrometry. Samples for nutrient analyses were filtered through 0.45-µm membrane filters (Millipore, Bedford, MA) and stored in 125mL amber polypropylene bottles for shipment to the NWQL where P-PO4, N-NO3, N-NO2, and N-NH3 were measured colorimetrically. (C) Microbial Population Estimates. The most probable number (MPN) technique was used to assay three populations of microorganisms: crude oil emulsifiers, gasoline degraders, and total heterotrophs. The number of crude oil emulsifying microorganisms in each sample was estimated using the Sheen Screen MPN method (13, 14). The plates were incubated at 10 °C for 3 weeks before being scored for emulsification of the oil sheen. Gasoline degraders were detected by use of a redox dye in the medium as an indicator of substrate use (method adapted from ref 15). A six-tube miniaturized MPN procedure was developed using 96-cell well plates (Corning; Corning, NY). Ten-fold
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TABLE 1
Land-Surface Elevations and Subsurface Flow-System Measurements for Selected Wells, 1993 July 10-11, 1993 site AFW1 AFW4 ASW2 ASW3 B14 B16 MW1 MW4 MW6 MW11 MW14 TB2 TB13 IMIKPUK
water table elevation (m)
frost depth (m)
saturated zone thickness (m)
water table elevation (m)
1.08 1.21 1.49 1.04 1.95 2.28 2.65 2.31 1.19 2.79 1.60 1.39 2.65
1.06 1.05 1.09 1.05
0.8 0.9 1.0 0.5
0.8 0.7 0.6 0.6
1.89 1.56 1.04 1.97 1.10 1.30 2.10 1.03
1.0 1.0 0.9 1.0 0.9 0.9 0.9
0.2 0.3 0.7 0.2 0.4 0.8 0.3
0.87 0.88 0.97 0.90 1.01 1.12 1.58 1.19 0.88 DRY 0.90 1.19 1.80 0.84
serial dilutions were made in Ringer solution (16) in a 24well plate. A multi-channel pipetter was used to transfer 100 µL of each dilution from the 24-well plate to replicate wells in a 96-well plate each containing 100 µL of 2/5 strength Bushnell-Haas (BH) medium (17) amended with 0.025 g/L tetrazolium violet (2,5-diphenyl-3-[R-naphthyl]tetrazolium chloride). Each well was then spiked with 1 drop of filter-sterilized gasoline. The plates were incubated for 14 days at room temperature before being scored for positive growth. Color change was measured as the difference in absorbance between a reference wavelength of 655 nm and a measurement wavelength of 595 nm using a BioRad (Philadelphia, PA) Model 3550-UV microplate reader. A well was considered positive for growth if the final absorbance was four times the value obtained in control (uninoculated or inoculated but killed) wells. The final MPN for the six-“tube” assay was determined by an algorithm (18). The numbers of heterotrophic microorganisms were estimated in a similar manner except that the growth medium was 2/5 strength R2A medium (17) and the plates were incubated for 10 days before being scored for turbidity. (D) Mineralization Potentials. Radiorespirometry (with [U-14C]benzene) was used to assay the benzene mineralization potential of microorganisms under aerobic conditions in soil slurries and groundwater (19). To determine whether nutrient availability limited benzene mineralization, soil and water samples were diluted in both a nutrientcontaining mineral salts medium (BH) and in a saline solution containing no added nutrients (Ringer). Soil was diluted (1:10) in sterile BH or Ringer. After shaking vigorously by hand for 1 min, 10-mL samples were pipetted into 40-mL precleaned and sterilized glass vials fitted with Teflon-lined septa (I-Chem Research; Hayward, CA). Groundwater was diluted by adding 5 mL of groundwater to either 5 mL of BH or Ringer in 40-mL septa vials. Replicate vials of the 10-mL slurries or the diluted groundwater were injected with 50 µL of a 2 g/L solution of radiolabeled benzene (in acetone; specific activity 6.8 µCi/ mL). At the end of the designated incubation period, radiolabeled CO2 was recovered and counted as previously described (19). “Zero-time” values for each isotope served as controls and were averaged and subtracted from each mineralization potential sample to yield a corrected dpm value.
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land-surface elevation (m)
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frost depth (m) 1.1 1.2 1.2 >1.8 1.4 1.2 1.4 1.1 1.3 1.3 1.2 1.1
saturated zone thickness (m) 0.9 0.8 0.7 0.2 0.2 0.2 0.8 0.6 1.0 0.2
FIGURE 2. Map showing generalized groundwater flow directions for July and August 1993 and surface-water drainage divides. Dashed/ dotted lines indicate surface-water drainage-basin divides; symbols show data collection sites.
Results Hydrology. Because of the slight topographic relief of the study site (approximately 2.7 m overall), even very subtle topographic highs can act as drainage divides between basins. Observations of surface-water flow patterns and land-surface elevation measurements indicate that the site spans parts of three surface-water drainage basins (Figure 2). The northwestern part of the site drains to the Arctic Ocean, the southwestern part of the site drains to Imikpuk Lake, and the eastern part of the site drains to North Salt Lagoon. Water table elevations indicate that drainage divides in the groundwater system generally coincide with surface-water drainage divides (Figure 2, Table 1). The depth of thaw in July 1993 ranged from 0.3 to more than 1.2 m at those sites measured. Subsurface flow system data from selected wells are presented in Table 1. By August 1993, the thaw had penetrated to more than 1 m at most sites, but the range of depths was still considerable (0.5>1.8 m). Because the thaw penetration defines the lower boundary of the active-layer saturated zone, the thickness of the saturated zone also varied considerably across the site. In July 1993, the saturated zone ranged in thickness from less than 0.1 to 1.0 m. At most sites, the thickness increased from July to August 1993 but remained less than
TABLE 2
Physical and Chemical Characteristics of Groundwater at NARL Airstrip Wells Sampled in 1994 and 1995a well
B56 8/9/94 7/13/95 US7AS 7/13/95 MW14 8/9/94 7/13/95 MW6 8/9/94 7/13/95 ASW3 8/9/94 AFW4 8/9/94 7/13/95 AFW1 8/9/94 7/13/95 a
N-NO3 + NO2 (mg/L)
P-PO4 (mg/L)
benzene (mg/L)
toluene (mg/L)
ethylbenzene (mg/L)
xylene (mg/L)
temp (°C)
pH
sp. cond (µS/cm)
N-NH4 (mg/L)
1.2 1.7
7.1 7.1
527 476
0.08 0.30
1.50 1.20
0.04 0.03
