William A. Sauck

Investigations of geoelectrical signatures at a hydrocarbon contaminated site

Abstract This study provides an evaluation of the utility and resolution of different geoelectrical methods in mapping contaminant distribution in the subsurface, and provides a window into the processes that may control their response at a site in Central Michigan. In situ and 2D surface resistivity, ground penetrating radar (GPR), and electromagnetic methods (EM) constrained by soil boring data were used to investigate the electrical properties of a light nonaqueous phase liquid (LNAPL) contaminant plume that resulted from 50 years of leakage into a glacio-fluvial geologic setting. Overall, the electrical signature from the in situ resistivity measurements were best able to image the subsurface stratigraphy and the associated contamination zone. GPR also mapped the subsurface stratigraphy. In particular, the GPR recorded a reflector that is subparallel to the water table, and occurs a few meters above the current free product level, which is coincident with the top of an oil-stained, light-gray sand layer. Further, regions of attenuated GPR reflections (shadow zones) due to enhanced conductivities were found to be coincident with low apparent resistivities. 2D geoelectrical measurements successfully imaged the top of the saturated zone and the underlying clay layer, but was unable to resolve any anomalous region that could be attributed to the hydrocarbon contamination. Likewise, the EM results provided no evidence of the presence of the free product plume at depth. Throughout this investigation, geoelectric measurements consistently recorded low resistivities (high apparent conductivities) associated with zones containing the free/residual product plume instead of high resistivities as has been suggested by the simple intuitive model. From this, it is inferred that substantial modification of the geochemical characteristics of the plume, surrounding media, and associated groundwater has occurred as a result of biogeochemical reactions. It is evident from this study that in situ resistivity measurements combined with surface geoelectrical measurements can characterize the distribution of conductive zones that may be associated with the biodegradation of LNAPL in the subsurface. Thus, the application of these techniques to hydrogeologic, contaminant monitoring, and remediation studies are far reaching.

A model for the resistivity structure of LNAPL plumes and their environs in sandy sediments

Abstract Geophysical site characterization investigations at fuel spill sites have been generally guided by a working hypothesis which assumes that the light non-aqueous phase liquids (LNAPL) are a fully saturating phase of intrinsically very high electrical resistivity. Using observations from other related sciences, and contrary geophysical observations, a different model is developed which treats these spills as dynamic, changing systems dominated by surprisingly low resistivities. The major geophysical response of a mature or established spill of this type is due to an anomalously low resistivity zone in the lower vadose zone and upper portion of the aquifer. This zone is produced by a high total dissolved solids (TDS) leachate which is aperiodically flushed down from the volume of intimately mixed hydrocarbon, water, oxygen and soil near the base of the vadose zone where microbial activity is a maximum. This leachate is a result of acidification by organic and carbonic acids of the water-filled capillaries in the heterogeneous mixing zone at the free/residual product level, and is produced by the leaching and etching of the native mineral grains and grain coatings. This conductive inorganic plume is generally coincident with the uppermost part of the anaerobic dissolved hydrocarbon plume as defined by hydrochemical studies, but is thin and most concentrated at the top of the aquifer. It has been best detected and mapped by virtue of the amplitude shadow it causes on ground penetrating radar (GPR) profiles, and more recently by direct measurement using vertical resistivity probes (VRP) with readings every 5.08 cm from the surface to more than 7.5 m in depth. Other surface electrical geophysical methods (VES, electromagnetic method (EM), and multi-spaced horizontal resistivity profiling) can define this zone only if conditions are optimal. The conductive zone has been known for some years by hydrochemists and hydrogeologists, especially at sites where water samples are collected from short screens at multi-level wells.

The Effects of LNAPL Biodegradation Products on Electrical Conductivity Measurements

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 120days, the increase in specific conductivity from microbial activity was 2,100μS∕cm, caused by an increase in total dissolved solids (DS) of over 1,700mg∕L. The increase in DS was due to mineral (mostly carbonate) dissolution and to the production of organic acids and biosurfactants. Under aerobic conditions...

Geophysical Investigation of Vadose Zone Conductivity Anomalies at a Hydrocarbon Contaminated Site: Implications for the Assessment of Intrinsic Bioremediation

Electromagnetic (EM-31), dc resistivity, and ground penetrating radar (GPR) methods were used to investigate part of a former refinery site in Kalamazoo, Michigan, USA. The study area is underlain by about 9–12m of heterogeneous glacial drift deposits overlying a sandy outwash unit. Three large above ground storage tanks (ASTs) formerly occupied the study area and were removed by the U.S. Environmental Protection Agency (USEPA) in 1997. EM-31 surveys revealed eight remnants of buried pipe ranging from 20m to more than 100m in length. Emanating from some of these abandoned pipe segments are extensive near surface conductivity anomalies, with values ranging from 16mS∕m to greater than 100mS∕m vs. background values of less than 15mS∕m. GPR and dipole-dipole resistivity surveys performed along selected profiles with anomalous conductive zones revealed a channel form within the shallow subsurface. Shallow hand augured soil borings into these conductive zones encountered hydrocarbons perched on thin clay lenses...

Ground-penetrating radar profiles over multiple steel tanks: Artifact removal through effective data processing

This paper describes some ground-penetrating radar (GPR) results to characterize steel tanks buried in the subsoil of an urban center. Tanks were installed in the first Brazilian geophysical test site located at the Institute of Astronomy, Geophysics, and Atmospheric Science (IAG) of the University of Sao Paulo (USP). This paper also presents an effec-tive procedure for removal of hyperbolic artifacts associated with GPR reflections between multiple steel tanks. One hundred sixty-five GPR profiles of 200 MHz , 400 MHz , and 500 MHz (shielded bistatic antennae) were measured. The work was done in two distinct places: on the geophysical test site of the IAG/USP in the city of Sao Paulo, Brazil, and in an area formerly occupied by a truck cleaning, refueling, and maintenance facility, located in Oscoda, northeastern Michigan, USA. Predictive deconvolution and Kirchhoff migration were first used to improve vertical and lateral resolution. To minimize the hyperbolic artifacts caused by reflections between tank...

Integration of GRACE (Gravity Recovery and Climate Experiment) data with traditional data sets for a better understanding of the time-dependent water partitioning in African watersheds

Monthly (71 months) Gravity Recovery and Climate Experiment (GRACE) gravity field solutions acquired over North and Central Africa (August 2002–July 2008) were destriped, smoothed (250 km; Gaussian), and converted to equivalent water thickness. These data were analyzed in a geographic information system environment together with relevant data sets (e.g., topography, geology, remote sensing) to assess the utility of GRACE for monitoring elements of hydrologic systems on local scales. The following were observed over the Niger, Congo, and Nile Basins: (1) large persistent anomalies (standard deviation, SD > 10 cm) on SD images over periods of 2–7 yr; (2) anomalous areas originate at mountainous source areas that receive high precipitation, extend downslope toward mountain foothills, and often continue along main channels, wetlands, or lakes that drain these areas; (3) time-series analyses over anomalous areas showed that seasonal mass variation lags behind seasonal precipitation; and (4) seasonal mass variations showed progressive shift in phase and decrease in amplitude with distance from the mountainous source areas. Results indicate that (1) the observed temporal mass variations are largely controlled by elements of the hydrologic cycle (e.g., runoff, infiltration, groundwater flow) and have not been obscured by noise, as previously thought; and (2) it is possible to use GRACE to investigate the temporal local responses of a much larger suite of hydrologic systems (watersheds, lakes, rivers, and marshes) and domains (source areas and lowlands) within watersheds and subbasins worldwide.

Temporal geophysical signatures from contaminant-mass remediation

We have previously documented changes in bulk electrical conductivity, self-potential (SP), and ground-penetrating-radar (GPR) reflections in a field setting caused by biogeochemical transformations of hydrocarbon-contaminated media. These transformations are associated with hydrocarbon biodegradation. The results of surface geophysical surveys acquired in 1996, 2003, and 2007 document changes in geophysical signatures associated with removing hydrocarbon mass in the contaminated zone. Initial investigations in 1996 showed that relative to background, the contaminated area was characterized by higher bulk electrical conductivity, positive SP anomaly, and attenuated GPR reflections. Repeated surveys in 2003 and 2007 over the contaminated area showed that in 2007, the bulk electrical conductivityhad reverted to near-background conditions, the positive SP anomaly became more negative, and the zone of attenuated GPR reflections showed increased signal strength. Removal of hydrocarbon mass in the vadose zone o...

Red Sea rifting controls on aquifer distribution: Constraints from geochemical, geophysical, and remote sensing data

Highly productive wells in the Central Eastern Desert of Egypt are tapping groundwater in subsided blocks of Jurassic to Cretaceous sandstone (Taref Formation of the Nubian Sandstone Group) and Oligocene to Miocene sandstone (Nakheil Formation), now occurring beneath the Red Sea coastal plain and within the proximal basement complex. Aquifer development is related to Red Sea rifting: (1) rifting was accommodated by vertical extensional displacement on preexisting NW-SE– to N-S–trending faults forming a complex array of half-grabens and asymmetric horsts; and (2) subsided blocks escaped erosion accompanying the Red Sea–related uplift. Subsided blocks were identifi ed and verifi ed using satellite data, geologic maps, and fi eld and geophysical investigations. Interpretations of very low frequency (VLF) measurements suggest the faults acted as conduits for ascending groundwater from the subsided aquifers. Stable isotopic compositions (δD: –19.3‰ to –53.9‰; δ 18 O: –2.7‰ to –7.1‰) of groundwater samples from these aquifers are interpreted as mixtures of fossil (up to 70%) and modern (up to 65%) precipitation. Groundwater volumes in subsided blocks are large; within the Central Eastern Desert basement complex alone, they are estimated at 3 × 10 9 m 3 and 10 × 10 9 m 3 for the

New least-squares algorithm for model parameters estimation using self-potential anomalies

We have developed a new least-squares minimization approach to depth determination from self-potential (SP) data. By defining the anomaly value at the origin and at any two symmetrical points around the origin on the profile, the problem of depth determination from the residual SP anomaly has been transformed into finding a solution to a nonlinear equation of the form f(z)=0. Procedures are also formulated to estimate the polarization angle, amplitude coefficient and the shape of the buried structure (shape factor). The method is simple and can be used as a rapid method to estimate parameters that produced SP anomalies. The method is tested on synthetic data with and without random errors. It is also applied to a field example from Turkey. In all cases, the model parameters obtained are in good agreement with actual ones.

Integrated solutions for hydrologic investigations in arid lands

Hydrological assessment studies across vast regions of the arid world are often hindered by the inaccessibility of these areas and the paucity of data sets, as well as the high expenses and diffi culties entailed in acquiring these data sets, their unpublished nature, and their varying scales, projections, and datum. Using the Eastern Desert (ED) of Egypt (225,000 km 2 ) and the Sinai Peninsula (61,000 km 2 ) as test sites, we demonstrate practical and cost-effective integrated (geochemistry, geophysics, and modeling) solutions that utilize web-based geographic information system (GIS) (http://www.esrs .wmich.edu/webmap) technologies and take advantage of readily available global remote sensing data sets. Adopted methodologies allowed: (1) development of conceptual models for hydrogeologic settings conducive to groundwater entrapment and augmentation, including groundwater in fractured basement aquifers, groundwater impounded by dike swarms crosscutting alluvial aquifers, and groundwater residing in alluvial aquifers associated with ascending deep-seated fossil waters; (2) selection of criteria to identify and validate the preferred distribution of each of these aquifer types and usage of the selected criteria and observations from the GIS data sets to identify, test, and refi ne potential well locations; and (3) construction and calibration of hydrologic models to estimate average annual recharge over the major watersheds in the Sinai (463 ◊ 10 6 m 3 /yr) and ED (171 ◊ 10 6 m 3 /yr) and the average modern contributions to Nubian fossil aquifers (Sinai: 13 ◊ 10 6 m 3 /yr), and to model the partitioning of precipitation as a function of precipitation amounts. The successful application of the integrated and cost-effective methodologies developed for the study areas should invite similar applications in arid regions elsewhere.