Effects of crude oil on water and tracer movement in the unsaturated and saturated zones.

A tracer test was conducted to aid in the investigation of water movement and solute transport at a crude-oil spill site near Bemidji, Minnesota. Time of travel was measured using breakthrough curves for rhodamine WT and bromide tracers moving from the soil surface through oil-contaminated and oil-free unsaturated zones to the saturated zone. Results indicate that the rates of tracer movement were similar in the oil-free unsaturated and saturated zones compared to the oily zones. These results are somewhat surprising given the oil contamination in the unsaturated and saturated zones. Rhodamine tracer breakthrough in the unsaturated and saturated zones in general was delayed in comparison to bromide tracer breakthrough. Peak tracer concentrations for the lysimeters and wells in the oily zone were much greater than at the corresponding depths in the oil-free zone. Water and tracer movement in the oily zone was complicated by soil hydrophobicity and decreased oil saturations toward the periphery of the oil. Preferential flow resulted in reduced tracer interaction with the soil, adsorption, and dispersion and faster tracer movement in the oily zone than expected. Tracers were freely transported through the oily zone to the water table. Recharge calculations support the idea that the oil does not substantially affect recharge in the oily zone. This is an important result indicating that previous model-based assumptions of decreased recharge beneath the oil were incorrect. Results have important implications for modeling the fate and transport of dissolved contaminants at hydrocarbon spill sites.

Loss of volatile hydrocarbons from an LNAPL oil source.

The light nonaqueous phase liquid (LNAPL) oil pool in an aquifer that resulted from a pipeline spill near Bemidji, Minnesota, was analyzed for volatile hydrocarbons (VHCs) to determine if the composition of the oil remains constant over time. Oil samples were obtained from wells at five locations in the oil pool in an anaerobic part of the glacial outwash aquifer. Samples covering a 21-year period were analyzed for 25 VHCs. Compared to the composition of oil from the pipeline source, VHCs identified in oil from wells sampled in 2008 were 13 to 64% depleted. The magnitude of loss for the VHCs analyzed was toluene≫o-xylene, benzene, C(6) and C(10-12)n-alkanes>C(7)-C(9)n-alkanes>m-xylene, cyclohexane, and 1- and 2-methylnaphthalene>1,2,4-trimethylbenzene and ethylbenzene. Other VHCs including p-xylene, 1,3,5- and 1,2,3-trimethylbenzenes, the tetramethylbenzenes, methyl- and ethyl-cyclohexane, and naphthalene were not depleted during the time of the study. Water-oil and air-water batch equilibration simulations indicate that volatilization and biodegradation is most important for the C(6)-C(9)n-alkanes and cyclohexanes; dissolution and biodegradation is important for most of the other hydrocarbons. Depletion of the hydrocarbons in the oil pool is controlled by: the lack of oxygen and nutrients, differing rates of recharge, and the spatial distribution of oil in the aquifer. The mass loss of these VHCs in the 5 wells is between 1.6 and 7.4% in 29years or an average annual loss of 0.06-0.26%/year. The present study shows that the composition of LNAPL changes over time and that these changes are spatially variable. This highlights the importance of characterizing the temporal and spatial variabilities of the source term in solute-transport models.

Methane oxidation in a crude oil contaminated aquifer: Delineation of aerobic reactions at the plume fringes.

High resolution direct-push profiling over short vertical distances was used to investigate CH(4) attenuation in a petroleum contaminated aquifer near Bemidji, Minnesota. The contaminant plume was delineated using dissolved gases, redox sensitive components, major ions, carbon isotope ratios in CH(4) and CO(2), and the presence of methanotrophic bacteria. Sharp redox gradients were observed near the water table. Shifts in δ(13)C(CH4) from an average of -57.6‰ (±1.7‰) in the methanogenic zone to -39.6‰ (±8.7‰) at 105m downgradient, strongly suggest CH(4) attenuation through microbially mediated degradation. In the downgradient zone the aerobic/anaerobic transition is up to 0.5m below the water table suggesting that transport of O(2) across the water table is leading to aerobic degradation of CH(4) at this interface. Dissolved N(2) concentrations that exceeded those expected for water in equilibrium with the atmosphere indicated bubble entrapment followed by preferential stripping of O(2) through aerobic degradation of CH(4) or other hydrocarbons. Multivariate and cluster analysis were used to distinguish between areas of significant bubble entrapment and areas where other processes such as the infiltration of O(2) rich recharge water were important O(2) transport mechanisms.

Crude oil at the bemidji site: 25 years of monitoring, modeling, and understanding.

The fate of hydrocarbons in the subsurface near Bemidji, Minnesota, has been investigated by a multidisciplinary group of scientists for over a quarter century. Research at Bemidji has involved extensive investigations of multiphase flow and transport, volatilization, dissolution, geochemical interactions, microbial populations, and biodegradation with the goal of providing an improved understanding of the natural processes limiting the extent of hydrocarbon contamination. A considerable volume of oil remains in the subsurface today despite 30 years of natural attenuation and 5 years of pump-and-skim remediation. Studies at Bemidji were among the first to document the importance of anaerobic biodegradation processes for hydrocarbon removal and remediation by natural attenuation. Spatial variability of hydraulic properties was observed to influence subsurface oil and water flow, vapor diffusion, and the progression of biodegradation. Pore-scale capillary pressure-saturation hysteresis and the presence of fine-grained sediments impeded oil flow, causing entrapment and relatively large residual oil saturations. Hydrocarbon attenuation and plume extent was a function of groundwater flow, compound-specific volatilization, dissolution and biodegradation rates, and availability of electron acceptors. Simulation of hydrocarbon fate and transport affirmed concepts developed from field observations, and provided estimates of field-scale reaction rates and hydrocarbon mass balance. Long-term field studies at Bemidji have illustrated that the fate of hydrocarbons evolves with time, and a snap-shot study of a hydrocarbon plume may not provide information that is of relevance to the long-term behavior of the plume during natural attenuation.

A regression model to estimate regional ground water recharge.

A regional regression model was developed to estimate the spatial distribution of ground water recharge in subhumid regions. The regional regression recharge (RRR) model was based on a regression of basin-wide estimates of recharge from surface water drainage basins, precipitation, growing degree days (GDD), and average basin specific yield (SY). Decadal average recharge, precipitation, and GDD were used in the RRR model. The RRR estimates were derived from analysis of stream base flow using a computer program that was based on the Rorabaugh method. As expected, there was a strong correlation between recharge and precipitation. The model was applied to statewide data in Minnesota. Where precipitation was least in the western and northwestern parts of the state (50 to 65 cm/year), recharge computed by the RRR model also was lowest (0 to 5 cm/year). A strong correlation also exists between recharge and SY. SY was least in areas where glacial lake clay occurs, primarily in the northwest part of the state; recharge estimates in these areas were in the 0- to 5-cm/year range. In sand-plain areas where SY is greatest, recharge estimates were in the 15- to 29-cm/year range on the basis of the RRR model. Recharge estimates that were based on the RRR model compared favorably with estimates made on the basis of other methods. The RRR model can be applied in other subhumid regions where region wide data sets of precipitation, streamflow, GDD, and soils data are available.

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

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. 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.

Effects of surface run-off on the transport of agricultural chemicals to ground water in a sandplain setting.

An experiment was conducted at a depressional (lowland) and an upland site in sandy soils to evaluate the effects of surface run-off on the transport of agricultural chemicals to ground water. Approximately 16.5 cm of water was applied to both sites during the experiment, representing a natural precipitation event with a recurrence interval of approximately 100 years. Run-off was quantified at the lowland site and was not detected at the upland site during the experiment. Run-off of water to the lowland site was the most important factor affecting differences in the concentrations and fluxes of the agricultural chemicals between the two sites. Run-off of water to the lowland site ppears to have played a dual role by diluting chemical concentrations in the unsaturated zone as well as increasing the concentrations at the water table, compared to the upland site. Concentrations of chloride, nitrate and atrazine plus metabolites were noticeably greater at the water table than in the unsaturated zone at both sites. The estimated mass flux of chloride and nitrate to the water table during the test were 5-2 times greater, respectively, at the lowland site compared to the upland site, whereas the flux of sulfate and atrazine plus metabolites was slightly greater at the upland site. Results indicate that matrix flow of water and chemicals was the primary process causing the observed differences between the two sites. Results of the experiment illustrate the effects of heterogeneity and the complexity of evaluating chemical transport through the unsaturated zone.