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Large lakes provide a variety of ecological services to surrounding cities and communities. Many of these services are supported by ecological processes that are threatened by the increasing prevalence of cyanobacterial blooms which occur as aquatic ecosystems experience cultural eutrophication. Over the past 10 yr, Lake Erie experienced cyanobacterial blooms of increasing severity and frequency, which have resulted in impaired drinking water for the surrounding communities. Cyanobacterial blooms may impact ecological processes that support other services, but many of these impacts have not been documented. Secondary production (production of primary consumers) is an important process that supports economically important higher trophic levels. Cyanobacterial blooms may influence secondary production because cyanobacteria are a poor-quality food resource and cyanotoxins may be harmful to consumers. Over 3 yr at 34 sites across the western basin of Lake Erie, we measured three indices of secondary production that focus on the dominant bivalve taxa: (1) growth of a native unionid mussel, (2) the size of young-of-year dreissenid mussels, and (3) the mass of colonizing animals on a Hester-Dendy sampler. Associations between these indices and cyanobacterial data were estimated to assess whether cyanobacteria are associated with variation in secondary production in the western basin of Lake Erie. The results suggest cyanobacterial abundance alone is only weakly associated with secondary production, but that cyanotoxins have a larger effect on secondary production. Given recurring late-summer cyanobacterial blooms, this impact on secondary production has the potential to undermine Lake Erie's ability to sustain important ecosystem services.

Manure application is a source of pathogens to the environment. Through overland runoff and tile drainage, zoonotic pathogens can contaminate surface water and streambed sediment and could affect both wildlife and human health. This study examined the environmental occurrence of gene markers for livestock-related bacterial, protozoan, and viral pathogens and antibiotic resistance in surface waters within the South Fork Iowa River basin before and after periods of swine manure application on agricultural land. Increased concentrations of indicator bacteria after manure application exceeding Iowa's state bacteria water quality standards suggest that swine manure contributes to diminished water quality and may pose a risk to human health. Additionally, the occurrence of HEV and numerous bacterial pathogen genes for Escherichia coli, Enterococcus spp., Salmonella sp., and Staphylococcus aureus in both manure samples and in corresponding surface water following periods of manure application suggests a potential role for swine in the spreading of zoonotic pathogens to the surrounding environment. During this study, several zoonotic pathogens were detected including Shiga-toxin producing E. coli, Campylobacter jejuni, pathogenic enterococci, and S. aureus; all of which can pose mild to serious health risks to swine, humans, and other wildlife. This research provides the foundational understanding required for future assessment of the risk to environmental health from livestock-related zoonotic pathogen exposures in this region. This information could also be important for maintaining swine herd biosecurity and protecting the health of wildlife near swine facilities., Graphical abstract Image 1, Highlights • Assessment of manure application as a source of pathogens to the environment. • Overland runoff and tile drainage facilitates pathogen transport to surface waters. • Detected hepatitis E virus in surface water following manure application. • Increased pathogen gene detections and indicator bacteria concentrations post-manure. • Manure application can potentially impair water quality and environmental health.

Effluent-impacted surface water has the potential to transport not only water, but wastewater-derived contaminants to shallow groundwater systems. To better understand the effects of effluent discharge on in-stream and near-stream hydrologic conditions in wastewater-impacted systems, water-level changes were monitored in hyporheic-zone and shallow-groundwater piezometers in a reach of Fourmile Creek adjacent to and downstream of the Ankeny (Iowa, USA) wastewater treatment plant (WWTP). Water-level changes were monitored from approximately 1.5 months before to 0.5 months after WWTP closure. Diurnal patterns in WWTP discharge were closely mirrored in stream and shallow-groundwater levels immediately upstream and up to 3 km downstream of the outfall, indicating that such discharge was the primary control on water levels before shutdown. The hydrologic response to WWTP shutdown was immediately observed throughout the study reach, verifying the far-reaching hydraulic connectivity and associated contaminant transport risk. The movement of WWTP effluent into alluvial aquifers has implications for potential WWTP-derived contamination of shallow groundwater far removed from the WWTP outfall.

Pharmaceutical contamination of shallow groundwater is a substantial concern in effluent-dominated streams, due to high aqueous mobility, designed bioactivity, and effluent-driven hydraulic gradients. In October and December 2012, effluent contributed approximately 99% and 71%, respectively, to downstream flow in Fourmile Creek, Iowa, USA. Strong hydrologic connectivity was observed between surface-water and shallow-groundwater. Carbamazepine, sulfamethoxazole, and immunologicallyrelated compounds were detected in groundwater at greater than 0.02 m gL �1 at distances up to 6 m from the stream bank. Direct aqueous-injection HPLC-MS/MS revealed 43% and 55% of 110 total pharmaceutical analytes in surface-water samples in October and December, respectively, with 16% and 6%, respectively, detected in groundwater approximately 20 m from the stream bank. The results demonstrate the importance of effluent discharge as a driver of local hydrologic conditions in an effluentimpacted stream and thus as a fundamental control on surface-water to groundwater transport of effluent-derived pharmaceutical contaminants. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://

Little information exists on the co-occurrence of fecal indicator bacteria (FIB), bacterial pathogens, and organic wastewater-associated chemicals (OWCs) within Great Lakes tributaries. Fifteen watershed sites and one beach site adjacent to the Little Calumet River–Portage Burns Waterway (LCRPBW) on Lake Michigan were tested on four dates for pH, dissolved oxygen, specific conductance, chloride, color, ammonia- and nitrate-nitrogen, soluble phosphorus, sulfate, turbidity, and atrazine; for concentrations of FIB; and for genes indicating the presence of human-pathogenic enterococci (ENT) and of Shiga-toxin producing Escherichia coli (EC) from various animal sources. Nineteen samples were also tested for 60 OWCs. Half of the watershed samples met EC recreational water quality standards; none met ENT standards. Human-wastewater-associated OWC detections were correlated with human-influence indicators such as population/km 2 , chloride concentrations, and the presence of WWTP effluents, but EC and ENT concentrations were not. Bacterial pathogen genes indicated rural human and several potential animal sources. OWCs of human or ecosystem health concern (musk fragrances AHTN and HHCB, alkylphenols, carbamazepine) and 3 bacterial pathogen genes were detected at the mouth of the LCRPBW, but no such OWCs and only 1 pathogen gene were detected at the beach. The LCRPBW has significant potential to deliver FIB, potential bacterial pathogens, and OWCs of human or ecosystem health concern to the nearshore of Lake Michigan, under conditions enhancing nearshore transport of the river plume. Nearshore mixing of lake and river water, and the lack of relationship between OWCs and FIB or pathogen genes, pose numerous challenges for watershed and nearshore assessment and remediation.

Predictive models, based on environmental and water quality variables, have been used to improve the timeliness and accuracy of recreational water quality assessments, but their effectiveness has not been studied in inland waters. Sampling at eight inland recreational lakes in Ohio was done in order to investigate using predictive models for Escherichia coli and to understand the links between E. coli concentrations, predictive variables, and pathogens. Based upon results from 21 beach sites, models were developed for 13 sites, and the most predictive variables were rainfall, wind direction and speed, turbidity, and water temperature. Models were not developed at sites where the E. coli standard was seldom exceeded. Models were validated at nine sites during an independent year. At three sites, the model resulted in increased correct responses, sensitivities, and specificities compared to use of the previous day's E. coli concentration (the current method). Drought conditions during the validation year precluded being able to adequately assess model performance at most of the other sites. Cryptosporidium , adenovirus, eaeA ( E. coli ), ipaH ( Shigella ), and spvC ( Salmonella ) were found in at least 20% of samples collected for pathogens at five sites. The presence or absence of the three bacterial genes was related to some of the model variables but was not consistently related to E. coli concentrations. Predictive models were not effective at all inland lake sites; however, their use at two lakes with high swimmer densities will provide better estimates of public health risk than current methods and will be a valuable resource for beach managers and the public.

Animal waste, stream water, and streambed sediment from 19 small (

Pharmaceutical contamination of contiguous groundwater is a substantial concern in wastewater-impacted streams, due to ubiquity in effluent, high aqueous mobility, designed bioactivity, and to effluent-driven hydraulic gradients. Wastewater treatment facility (WWTF) closures are rare environmental remediation events; offering unique insights into contaminant persistence, long-term wastewater impacts, and ecosystem recovery processes. The USGS conducted a combined pre/post-closure groundwater assessment adjacent to an effluent-impacted reach of Fourmile Creek, Ankeny, Iowa, USA. Higher surface-water concentrations, consistent surface-water to groundwater concentration gradients, and sustained groundwater detections tens of meters from the stream bank demonstrated the importance of WWTF effluent as the source of groundwater pharmaceuticals as well as the persistence of these contaminants under effluent-driven, pre-closure conditions. The number of analytes (110 total) detected in surface water decreased from 69 prior to closure down to 8 in the first post-closure sampling event approximately 30 d later, with a corresponding 2 order of magnitude decrease in the cumulative concentration of detected analytes. Post-closure cumulative concentrations of detected analytes were approximately 5 times higher in proximal groundwater than in surface water. About 40% of the 21 contaminants detected in a downstream groundwater transect immediately before WWTF closure exhibited rapid attenuation with estimated half-lives on the order of a few days; however, a comparable number exhibited no consistent attenuation during the year-long post-closure assessment. The results demonstrate the potential for effluent-impacted shallow groundwater systems to accumulate pharmaceutical contaminants and serve as long-term residual sources, further increasing the risk of adverse ecological effects in groundwater and the near-stream ecosystem.

Studies of sulfidic springs have provided new insights into microbial metabolism, groundwater biogeochemistry, and geologic processes. We investigated Great Sulphur Spring on the western shore of Lake Erie and evaluated the phylogenetic affiliations of 189 bacterial and 77 archaeal 16S rRNA gene sequences from three habitats: the spring origin (11-m depth), bacterial-algal mats on the spring pond surface, and whitish filamentous materials from the spring drain. Water from the spring origin water was cold, pH 6.3, and anoxic (H 2 , 5.4 nM; CH 4 , 2.70 μM) with concentrations of S 2− (0.03 mM), SO 4 2− (14.8 mM), Ca 2+ (15.7 mM), and HCO 3 − (4.1 mM) similar to those in groundwater from the local aquifer. No archaeal and few bacterial sequences were >95% similar to sequences of cultivated organisms. Bacterial sequences were largely affiliated with sulfur-metabolizing or chemolithotrophic taxa in Beta -, Gamma -, Delta -, and Epsilonproteobacteria . Epsilonproteobacteria sequences similar to those obtained from other sulfidic environments and a new clade of Cyanobacteria sequences were particularly abundant (16% and 40%, respectively) in the spring origin clone library. Crenarchaeota sequences associated with archaeal-bacterial consortia in whitish filaments at a German sulfidic spring were detected only in a similar habitat at Great Sulphur Spring. This study expands the geographic distribution of many uncultured Archaea and Bacteria sequences to the Laurentian Great Lakes, indicates possible roles for epsilonproteobacteria in local aquifer chemistry and karst formation, documents new oscillatorioid Cyanobacteria lineages, and shows that uncultured, cold-adapted Crenarchaeota sequences may comprise a significant part of the microbial community of some sulfidic environments.

The objective of this study was to compare fecal indicator bacteria (FIB) (fecal coliforms, Escherichia coli [EC], and enterococci [ENT]) concentrations with a wide array of typical organic wastewater chemicals and selected bacterial genes as indicators of fecal pollution in water samples collected at or near 18 surface water drinking water intakes. Genes tested included esp (indicating human-pathogenic ENT) and nine genes associated with various animal sources of shiga-toxin-producing EC (STEC). Fecal pollution was indicated by genes and/or chemicals for 14 of the 18 tested samples, with little relation to FIB standards. Of 13 samples with

The interdependence between geoelectrical signatures at underground petroleum plumes and the structures of subsurface microbial communities was investigated. For sediments contaminated with light non-aqueous-phase liquids, anomalous high conductivity values have been observed. Vertical changes in the geoelectrical properties of the sediments were concomitant with significant changes in the microbial community structures as determined by the construction and evaluation of 16S rRNA gene libraries. DNA sequencing of clones from four 16S rRNA gene libraries from different depths of a contaminated field site and two libraries from an uncontaminated background site revealed spatial heterogeneity in the microbial community structures. Correspondence analysis showed that the presence of distinct microbial populations, including the various hydrocarbon-degrading, syntrophic, sulfate-reducing, and dissimilatory-iron-reducing populations, was a contributing factor to the elevated geoelectrical measurements. Thus, through their growth and metabolic activities, microbial populations that have adapted to the use of petroleum as a carbon source can strongly influence their geophysical surroundings. Since changes in the geophysical properties of contaminated sediments parallel changes in the microbial community compositions, it is suggested that geoelectrical measurements can be a cost-efficient tool to guide microbiological sampling for microbial ecology studies during the monitoring of natural or engineered bioremediation processes.

Manure spills into streams are relatively frequent, but no studies have characterized stream contamination with zoonotic and veterinary pathogens, or fecal chemicals, following a spill. We tested stream water and sediment over 25 days and downstream for 7.6 km for the following: fecal indicator bacteria (FIB), the fecal indicator chemicals cholesterol and coprostanol, 20 genes for zoonotic and swine-specific bacterial pathogens by presence/absence PCR for viable cells, one swine-specific Escherichia coli toxin gene (STII gene) by quantitative PCR (qPCR), and nine human and animal viruses by qPCR or reverse transcription-qPCR. Twelve days postspill, and 4.2 km downstream, water concentrations of FIB, cholesterol, and coprostanol were 1 to 2 orders of magnitude greater than those detected before, or above, the spill, and genes indicating viable zoonotic or swine-infectious Escherichia coli were detected in water or sediment. STII gene levels increased from undetectable before or above the spill to 10 5 copies/100 ml of water 12 days postspill. Thirteen of 14 water (8/9 sediment) samples had viable STII-carrying cells postspill. Eighteen days postspill, porcine adenovirus and teschovirus were detected 5.6 km downstream. FIB concentrations (per gram [wet weight]) in sediment were greater than in water, and sediment was a continuous reservoir of genes and chemicals postspill. Constituent concentrations were much lower, and detections less frequent, in a runoff event (200 days postspill) following manure application, although the swine-associated STII and stx 2e genes were detected. Manure spills are an underappreciated pathway for livestock-derived contaminants to enter streams, with persistent environmental outcomes and the potential for human and veterinary health consequences.

We investigated the bulk electrical conductivity and microbial population distribution in sediments at a site contaminated with light nonaqueous-phase liquid (LNAPL). The bulk conductivity was measured using in-situ vertical resistivity probes; the most probable num­ ber method was used to characterize the spatial distri­ bution of aerobic heterotrophic and oil-degrading mi­ crobial populations. The purpose of this study was to assess if high conductivity observed at aged LNAPLimpacted sites may be related to microbial degradation of LNAPL. The results show higher bulk conductiv­ ity coincident with LNAPL-impacted zones, in contrast to geoelectrical models that predict lower conductivity in such zones. The highest bulk conductivity was ob­ served to be associated with zones impacted by resid­ ual and free LNAPL. Data from bacteria enumeration from sediments close to the resistivity probes show that oil-degrading microbes make up a larger percentage (5–55%) of the heterotrophic microbial community at depths coincident with the higher conductivity compared to �5% at the uncontaminated location. The coincidence of a higher percentage of oil-degrading microbial popu­ lations in zones of higher bulk conductivity suggests that the higher conductivity in these zones may result from increased fluid conductivity related to microbial degra­ dation of LNAPL, consistent with geochemical studies that suggest that intrinsic biodegradation is occurring at the site. The findings from this study point to the fact that biogeochemical processes accompanying biodegra­ dation of contaminants can potentially alter geoelectri­ cal properties of the subsurface impacted media.

A simple screening method was developed to detect in situ biosurfactant production by exploiting the relationship between surface tension (ST) and surfactant concentration. Filtered groundwater from contaminated wells with ST values of 60 to 70 dynes/cm decreased to 29 dynes/cm after being concentrated 10 to 15 times in a rotary evaporator, indicating that biosurfactants in the sample reached the critical micelle concentration (CMC). Samples from uncon-taminated groundwater concentrated 25 times showed no decrease in ST below 72 dynes/cm, suggesting that biosurfactants were not present. Microorganisms from soil cores were cultured on diesel fuel and identified using fatty acid methyl ester (FAME) analysis. Pseudomonas aeruginosa was found at very low numbers in uncontami-nated soil but was the dominant species in contaminated soil, indicating that hydrocarbon release impacted microbial diversity significantly. High-performance liquid chromatography (HPLC) was used to quantify rhamnolipids, biosurfactants ...

A soil core collected in a tidal freshwater marsh in the Waccamaw National Wildlife Refuge (Georgetown, SC) exuded a particularly strong odor of cow manure upon extrusion. In order to test for manure and determine its provenance, we carried out microbial source tracking using DNA markers for Bacteroides, a noncoliform, anaerobic bacterial group that represents a large proportion spectrum of the fecal population. Three core sections from 0–3 cm, 9–12 cm, and 30–33 cm were analyzed for the presence of Bacteroides. The ages of core sediments were estimated using 210Pb and 137Cs dating. All three core sections tested positive for Bacteroides DNA markers related to cow or deer feces. Because cow manure is stockpiled, used as fertilizer, and a source of direct contamination in the Great Pee Dee River/Winyah Bay watershed, it is very likely the source of the Bacteroides that was deposited on the marsh. The mid-points of the core sections were dated as follows: 0–3 cm, 2009; 9–12 cm, 1999, and 30–33 cm, 1961. The presence of Bacteroides at different depths/ages in the soil profile indicates that soils in tidal freshwater marshes are, at the least, capable of being short-term sinks for Bacteroides and, may have the potential to be long-term sinks of stable, naturalized populations.

Field geophysical studies have identified anomalously high conductivities in and below the free product zone at many sites with aged contamination by light, non-aqueous phase liquids (LNAPL). Laboratory experiments were conducted to test the hypothesis that these anomalously high conductivities can result from products of LNAPL biodegradation. Soil from a hydrocarbon-impacted site with anomalously high conductivities was washed repeatedly to remove soluble constituents, recontaminated with diesel fuel (DF), and the pores filled with water to simulate a saturated smear zone. Nutrients were provided at levels observed at the site, which resulted in anaerobic conditions due to DF biodegradation. Within [Formula: see text], the increase in specific conductivity from microbial activity was [Formula: see text], caused by an increase in total dissolved solids (DS) of over [Formula: see text]. The increase in DS was due to mineral (mostly carbonate) dissolution and to the production of organic acids and biosurfactants. Under aerobic conditions (i.e., without added nutrients) products of DF biodegradation increased the total DS and conductivity by [Formula: see text] and [Formula: see text], respectively. The results show that products of LNAPL biodegradation can drastically increase the conductivity at impacted sites.

The relation of bacterial pathogen occurrence to fecal indicator bacteria (FIB) concentrations used for recreational water quality criteria (RWQC) is poorly understood. This study determined the occurrence of Shiga-toxin producing Escherichia coli (STEC) markers and their relation to FIB concentrations in Michigan and Indiana river water. Using 67 fecal coliform (FC) bacteria cultures from 41 river sites in multiple watersheds, we evaluated the occurrence of five STEC markers: the Escherichia coli (EC) 0157 antigen and gene, and the STEC virulence genes eaeA, stx1, and stx2. Simple isolations from selected FC cultures yielded viable EC 0157. By both antigen and gene assays, EC 0157 was detected in a greater proportion of samples exceeding rather than meeting FC RWQC (P < 0.05), but was unrelated to EC and enterococci RWQC. The occurrence of all other STEC markers was unrelated to any FIB RWQC. The eaeA, stx2, and stx1 genes were found in 93.3, 13.3, and in 46.7% of samples meeting FC RWQC and in 91.7, 0.0, and 37.5% of samples meeting the EC RWQC. Although not statistically significant, the percentage of samples positive for each STEC marker except stx1 was lower in samples that met, as opposed to exceeded, FIB RWQC. Viable STEC were common members of the FC communities in river water throughout southern Michigan and northern Indiana, regardless of FIB RWQC. Our study indicates that further information on the occurrence of pathogens in recreational waters, and research on alternative indicators of their occurrence, may help inform water-resource management and public health decision-making.

Field and analytical methods, mercury and ancillary water-quality data, and associated quality-control data are reported for eight streams in Oregon, Wisconsin, and Florida from 2002 to 2006. The streams were sampled as part of a U.S. Geological Survey National Water-Quality Assessment Program study of mercury cycling, transport, and bioaccumulation in urban and nonurban stream ecosystems that receive mercury predominantly by way of atmospheric deposition.

The U.S. Geological Survey (USGS) conducted a survey of water quality and quantity in Huron County, Michigan from 1988-1990 (Sweat, 1992). In that study atrazine, a widelyused agricultural herbicide with possible endocrine-disrupting effects was detected in 9 of 9 surface-water samples. Metolachlor, metribuzin, prometone, simazine, 2,4-dichlorophenoxyacetic acid (2,4-D), and dichlorprop (2,4-DP) were also detected at some surface-water sites. One pesticide, 2,4,5- trichlorophenoxyacetic acid (2,4,5-T), was detected in two monitoring wells. Additionally, routine USGS monitoring through the early 1990 s indicated the Pigeon River in Huron County to have some of the highest levels of nitrate-nitrogen and dissolved solids of any monitored river in Michigan (Blumer and others, 1992). Recently, new methods of chemical and biological analysis have been developed that allow better evaluation of the nature and source of contamination of water. Eleven Huron County townships have passed a resolution calling for evaluation of water quality in Huron County. In 2004 Huron County desired a survey of current water quality in their county. In cooperation with Huron County, the USGS collected water samples for analysis of microbiological, chemical and physical constituents from the Pinnebog and Pigeon River watersheds in Huron County from April through November, 2004 to assess the current state of Huron County s water quality.

The U.S. Geological Survey (USGS) sampled multiple stream sites across the St. Joseph and Galien River Basins to detect and quantify the herbicide atrazine using a field enzyme-linked immunosorbent assay (ELISA) triazine test. In May 2001, July 2001, April 2002, August 2002, August 2003 and September 2003, composite samples were collected across streams at USGS streamflow-gaging stations. Concentrations and instantaneous loading for atrazine sampled in stream water throughout the St. Joseph River and Galien River Basins in Michigan and Indiana ranged from nondetection (< 0.05 part per billion (ppb)) with an associated load less than 0.001 kilogram per day (kg/d) to 6 ppb and a maximum load of 10 kg/d. Atrazine concentrations were highest in May 2001 just after the planting season. The lowest concentration was found in April 2002 just before planting. Atrazine concentrations in streambed-sediment pore water were not spatially connected with atrazine concentrations in stream-water samples. This study showed that atrazine concentrations were elevated from May to July in the St. Joseph and Galien River Basins. At many sites, concentrations exceeded the level that has been shown to feminize frog populations (0.2 ppb). There were 8 sites where concentrations exceeded 0.2 ppb atrazine in May 2001 and July 2001.

[1] Bulk electrical conductivity of sediments during microbial mineralization of diesel was investigated in a mesoscale laboratory experiment consisting of biotic contaminated and uncontaminated columns. Population numbers of oil degrading microorganisms increased with a clear pattern of depth zonation within the contaminated column not observed in the uncontaminated column. Microbial community structure determined from ribosomal DNA intergenic spacer analysis showed a highly specialized microbial community in the contaminated column. The contaminated column showed temporal increases in bulk conductivity, dissolved inorganic carbon, and calcium, suggesting that the high bulk conductivity is due to enhanced mineral weathering from microbial activity. The greatest change in bulk conductivity occurred in sediments above the water table saturated with diesel. Variations in electrical conductivity magnitude and microbial populations and their depth distribution in the contaminated column are similar to field observations. The results of this study suggest that geophysical methodologies may potentially be used to investigate microbial activity.

The results of a field and laboratory investigation of unconsolidated sediments contaminated by petroleum hydrocarbons and undergoing natural biodegradation are presented. Fundamental to geophysical investigations of hydrocarbon impacted sediments is the assessment of how microbial degradational processes affect their geoelectrical response. Therefore, the primary goal of this study was to understand how microbially mediated processes in hydrocarbon impacted sediments influence the geoelectrical response of this impacted zone. The field and laboratory results showed higher bulk conductivity in sediments impacted by petroleum hydrocarbons. The impacted sediments also showed increased populations of alkane degrading microbes and elevated dissolved cations (e.g. Ca). The elevated cations in the contaminated sediments relative to uncontaminated sediments suggest enhanced mineral dissolution related to the microbial degradation of the hydrocarbon. Both the laboratory and field data showed the highest bulk conductivities occurring within zones impacted with the free-phase and residual phase hydrocarbon and not within the water saturated zone. A model using a simplified form of Archie's Law suggests highly elevated estimated pore water conductivities within this conductive zone (~4 to 6 times background bulk conductivity) for both the laboratory and field data. The similar results for hydrocarbon contaminated sediments in laboratory experiments and field settings suggest that the mechanism for the high bulk conductivity in the contaminated zone is related to the microbial metabolism of the hydrocarbon and the resulting geochemical alterations within the contaminated zone. This study demonstrates that the higher bulk conductivity measured by geoelectrical methods at hydrocarbon impacted sites may be in part related to the microbial mineralization of the hydrocarbon.