Estella A. Atekwana

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.

Wetland vegetation colonization and expansion in small impoundments in Yaounde, Cameroon, West Africa

Temporal changes in the pattern and distribution of tropical wetland vegetation in four small impoundments over a 28-year period were documented for a wetland ecosystem in the Olezoa drainage basin in Yaounde, Cameroon, West Africa. These impoundments were constructed along the Olezoa stream for the purpose of aquaculture and ranged in size from 1.4 to 3.0 ha, had depths between 200 to 400 cm, and were devoid of vegetation. Evaluation of the impoundments and the surrounding catchment was accomplished using low altitude aerial photographs for the years 1964, 1974, and 1986 and field surveys for the periods 1985 through 1992. The results reveal a progressive decline in open water surface area of the impoundments ranging from 70 to 100% due to plant colonization and expansion. Measured impoundment depths along profiles show more than 50% reduction from original depths of 300–400 cm to less than 150–200 cm in 1992. We suggest that vegetation colonization and expansion in the impoundments is a function of the reduction of water depth due to siltation accompanying increasing urbanization. During the period of evaluation, urbanization in the surrounding catchment tripled. Siltation in the impoundments resulted mainly from the accumulation of soil eroded from the upland accompanying devegetation and construction activities. The shallowing of the impoundments by siltation provided a favorable water-depth regime and substrate for colonization by aquatic and emergent wetland vegetation. Continued shallowing resulted in development of vegetation zones along a water-depth gradient, and the migration of these zones toward the interior of the impoundments. The history of degradation of the catchment area and siltation of the impoundments is thus recorded by temporal changes in vegetation.

A generic automated/semiautomated digital multi-electrode instrument for field resistivity measurements

The objective of this work is to design and test a digital multi-electrode acquisition system for use in geophysical investigations utilizing direct current electrical (geoelectric) methods. The system is a 64-electrode modular system, which utilizes field electrodes with individual wires that lead back to a digital switch box and then to a data acquisition instrument (Iris Syscal R2). The electronic design allows for different arrays or geometrical switching configurations of the field electrodes in order to allow for rapid data reading and yield essentially real-time results. Additionally, the electronic switch box has the capability to be connected in series to similar switch boxes and, therefore, has the ability to switch between an unlimited number of electrodes. The acquisition control software, developed as an interface for the hardware, controls operation of both the digital switch box and the resistivity acquisition instrument.

A Coupled Hydrogeological‐Petrophysical Analysis of Geophysical Variations in the Vadose Zone

This study compares fluid content profiles derived from high-resolution geophysical data acquired before and after a controlled light non-aqueous phase liquid release experiment with those from hydrogeological models (Van Genuchten and Brooks-Corey) based on laboratory saturation-capillary pressure data for the porous medium. Standard petrophysical relationships (e.g., Archie’s equation, Topp’s equation and Complex Refractive Index Method) were used to convert in situ geophysical properties to fluid contents and saturations. The total fluid content profiles estimated from neutron probe data using a linear relationship is a good overall match to the hydrogeological prediction for both uncontaminated and contaminated conditions. In contrast, the water saturation and content profiles estimated from the vertical resistivity probe and time domain reflectometry data, respectively, differ appreciably from the hydrogeological model predictions of water content. Further, these differences are greater for the three-phase contaminated case. The results of this study suggest that care should be exercised when estimating fluid content from geoelectrical properties using standard petrophysical relationships. The inference of water content from geoelectrical probes therefore requires more study necessitating the development of petrophysical relationships that are applicable over a wider range of vadose zone conditions. Introduction A major objective in hydrogeophysics is the extraction of hydrogeological parameters from geophysical data. In the vadose zone, geophysical data can be used to obtain water content and saturation profiles from which saturation-capillary pressure relationships can be inferred. Subsequently, these relationships can be used to estimate relative permeability and the vertical distribution of immiscible contaminants. For example, Binley et al. (2002) used geophysically-inferred water content estimates as the input to the hydrogeological relationship of van Genuchten (1980) to predict hydraulic conductivity in the vadose zone. The important elements of this process are the hydrogeological models that describe the fluid content-capillary pressure relationships in two and three phase systems and the petrophysical equations that connect fluid contents with geophysical properties. In this study, we examine the nature of these elements by analyzing detailed geophysical profiles acquired before and after a light non-aqueous phase liquid (LNAPL) release within a controlled vadose zone and comparing them with hydrogeological model predictions based on the laboratory characterization of the saturation-capillary pressure relationship. Field Experiment This study analyzes high-resolution geophysical data acquired by DeRyck (1994) during a controlled LNAPL release experiment where kerosene was injected into a large-scale test cell containing repacked Borden sand. During that experiment, vertical profiles were obtained using a neutron soil moisture probe, a vertical resistivity probe (VRP) and a time domain reflectometry (TDR) array. The