Weathering of Oil in a Surficial Aquifer

Abstract

The composition of crude oil in a surficial aquifer was determined in two locations at the Bemidji, MN, spill site. The abundances of 71 individual hydrocarbons varied within 16 locations sampled. Little depletion of these hydrocarbons (relative to the pipeline oil) occurred in the first 10 years after the spill, whereas losses of 25% to 85% of the total measured hydrocarbons occurred after 30 years. The C6‐30 n‐alkanes, toluene, and o‐xylene were the most depleted hydrocarbons. Some hydrocarbons, such as the n‐C10–24 cyclohexanes, tri‐ and tetra‐ methylbenzenes, acyclic isoprenoids, and naphthalenes were the least depleted. Benzene was detected at every sampling location 30 years after the spill. Degradation of the oil led to increases in the percent organic carbon and in the δ 13C of the oil. Another method of determining hydrocarbon loss was by normalizing the total measured hydrocarbon concentrations to that of the most conservative analytes. This method indicated that the total measured hydrocarbons were depleted by 47% to 77% and loss of the oil mass over 30 years was 18% to 31%. Differences in hydrocarbon depletion were related to the depth of the oil in the aquifer, local topography, amount of recharge reaching the oil, availability of electron acceptors, and the presence of less permeable soils above the oil. The results from this study indicate that once crude oil has been in the subsurface for a number of years there is no longer a “starting oil concentration” that can be used to understand processes that affect its fate and the transport of hydrocarbons in groundwater.

Influence of Ambient Temperature, Precipitation, and Groundwater Level on Natural Source Zone Depletion Rates at a Large Semiarid LNAPL Site

“…This is consistent with observations for near‐surface microbial activity in uncontaminated settings (Yuste et al ), including semiarid environments (Huxman et al ), although the timeframe for observing microbial response is shorter for such shallow environments (Kim et al ; Ford et al ). Alternatively, the additional soil moisture may not be important per se, but rather the percolating water may carry additional nutrients and electron acceptors downward to the LNAPL body for use in biodegradation (Bekins et al ; Baedecker et al ). Irianni‐Renno et al () identified the top of the LNAPL body and bottom of the vadose zone in a localized portion of this site as having a diverse microbiological community where such electron acceptor utilization may occur.…”

Application of Four Measurement Techniques to Understand Natural Source Zone Depletion Processes at an LNAPL Site

“…As such, the LNAPL depletion ranged from 5 to 90% for each carbon number bucket, with an average of 30%. Having an average of 30% of the LNAPL deplete over only a 9-year period may be too high, based on the longevity of LNAPL at the Bemidji site (Baedecker et al 2018); possible reasons include: (1) the Douglas model assumes that these longer-chained compounds (>nC 11 constituents) are totally recalcitrant but there is likely some biodegradation of these compounds at the site, thereby make the Douglas model overpredict LNAPL depletion; (2) the monitoring well purging may have been incomplete so some of the LNAPL weathered more rapidly while in the monitoring well, and therefore does not completely represent the LNAPL in the subsurface;…”

Infiltration of Sulfate to Enhance Sulfate‐Reducing Biodegradation of Petroleum Hydrocarbons

“…A number of studies have also demonstrated the important role of natural surface recharge in transporting rate‐limiting nutrients and EAs to groundwater. This process has been shown to exert an important control on natural attenuation of dissolved contaminants (McGuire et al ; van Stempvoort et al ), as well as methanogenic biodegradation of long chain alkanes in a crude oil body located in the shallow subsurface (Bekins et al ; Baedecker et al ).…”