A New Approach to Predict Hydrogeological Parameters Using Shear Waves from the Multichannel Analysis of Surface Waves Method

For near-surface contaminant characterization, the accurate prediction of hydrogeological parameters in anisotropic and heterogeneous environments has been a challenge since the last decades. However, recent advances in near-surface geophysics have facilitated the use of geophysical data for hydrogeological characterization in the last few years. A pseudo 3-D high resolution P-wave shallow seismic reflection survey was performed at the P Reactor Area, Savannah River Site, South Carolina in order to delineate and predict migration pathways of a large contaminant plume including trichloroethylene. This contaminant plume originates from the northwest section of the reactor facility that is located within the Upper Atlantic Coastal Plain. The data were collected with 40 Hz geophones, an accelerated weight-drop as seismic source and 1 m receiver spacing with near- and far-offsets of 0.5 and 119.5 m, respectively. In such areas with near-surface contaminants, a detailed subsurface characterization of the vadose zone hydraulic parameters is very important. Indeed, an inexpensive method of deriving such parameters by the use of seismic reflection surveys is beneficial, and our approach uses the relationship between seismic velocity and hydrogeological parameters together with empirical observations relating porosity to permeability and hydraulic conductivity. Shear wave velocity ( Vs) profiles were estimated from surface wave dispersion analysis of the seismic reflection data and were subsequently used to derive hydraulic parameters such as porosity, permeability, and hydraulic conductivity. Additional geophysical data including core samples, vertical seismic profiling, surface electrical resistivity tomography, natural gamma and electrical resistivity logs allowed for a robust assessment of the validity and geological significance of the estimated Vs and hydrogeological models. The results demonstrate the usefulness of this approach for the upper 15 m of shallow unconsolidated sediments even though the survey design parameters were not optimal for surface wave analysis due to the higher than desired frequency geophones.

Assessment of Leachate Contamination of Groundwater at Some Waste Disposal Sites in the Southern Guinea Savannah Ecological Zone of Nigeria Using Geoelectric Processes

This study aims at determining the leachate contamination of the groundwater resource at selected domestic wastes disposal sites in Minna, Nigeria for a population about 2.1 million, to locate aquifers and hydraulically active structures by tracing the movement of contaminant plumes and seepages in ground at the selected locations. Resistivity data was collected using a terrameter (SAS4000) while the Vertical Electrical Sounding (VES) mode was deployed using the Schlumberger array to enable investigation of the depth penetration of contaminant plume. The induced polarization (IP) was used to determine the level of contaminant plume. The VES readings measured at 50m intervals along each profile line and 100m inter-profile distance, with a maximum current electrode separation of 200m and potential electrode separation of 30m. There are equal numbers of three and four layers observed on the profile, which has ten VES points. The first layer has a resistivity range between 48.4 Ωm & 428 Ωm and thickness between 0.65m & 3.83m. However, isolated resistivity area such as VES; N5 (287Ωm), N6 (295Ωm) and N8 (428Ωm) also suggested sandy/soil rich in organic matter (humus material/soil). The second and third layer is the fractured basement which has very low resistivity values for most VES (N1–48.5Ωm, N2–38.7Ωm, N3–41.6Ωm, N5–61.5Ωm, N7–49.6Ωm, N8–60.7Ωm, N9–108Ωm and N10–97.6Ωm) that indicated leachate presence and contamination, which results from increased ionic concentration. In conclusion, it was discovered that the study area had high conductivity values for some of the locations using the resitivity determination method. This indicated the presence of water within the study area. It was also concluded that the IP which indicated high concentration of metals caused the lowering of the resisitivity values at some of the locations, thus indicating the presence of metals within the study area.

Transport of Contamination under the Influence of Sea Level Rise in Coastal Heterogeneous Aquifer

This paper provided for the first time an experimental study on the influence of sea level rise on transport of contamination in the heterogeneous unconfined aquifer of the coastal zone. The experiments were conducted using the tank, considering the difference between sea level and inland head 1 cm for Case 1 and 2 cm for Case 2. Observed data were validated using the numerical model, which matched well with the toe length of seawater wedge and the shape of the contaminant plume. The results showed that the observed and simulated values of Cl− concentration at the sampling points increased sharply at the initial time, and then they increased slowly and tended to be stable. The seawater wedge migrated inland with time under the effects of the hydraulic gradient toward the inland and the density difference between saltwater and freshwater. The steady state length of the 50% isoline of the seawater wedge was 167 cm in Case 2, which was larger than that of Case 1. The maximum area of plume in Case 2 was 0.13 m2, larger than that in Case 1, which indicated that the velocity of diffusion of the contaminant plume increased as the sea level increased. As the velocity of diffusion increased, the time for pollutant migration to the intersection between seawater and freshwater became shorter. The maximum area and vertical depth of pollutant plume were sensitive to the hydraulic conductivity, dispersivity, and contamination concentration. The infiltration depth and range of the contaminant plume in the heterogeneous aquifer were greater than those in the homogeneous aquifer of the actual beach.

ASSESSING AQUIFER VULNERABILITY AND CONTAMINANT PLUME AT ARTISANAL REFINING SITES IN PARTS OF OKRIKA AND OGU-BOLO LOCAL GOVERNMENT AREAS, RIVERS STATE, NIGERIA

This study aims at the assessment of aquifer vulnerability and contaminants plume where artisan refining of crude is taking place, which is a threat to availability of potable water. The ArcGIS version 10.3, ENVI version 4.7, Surfer 10, SPSS 22 and Microsoft Enterprise were used for the interpretation and the water and soil samples were analyzed in the laboratory using standard methods. A total number of sixteen (16) sampling points were selected using random sampling techniques for the water points and soil samples within Ogu Bolo and Okrika. The Digital Elevation Model was created from the elevation data obtained from SRTM (Short Radar Thematic Mapper) satellite image and contour extracted from the topographic map. The influence of the soil type, slope, flow accumulation, flow direction of the study area was used to delineate the level of contaminant plume. This was generated from the DEM using the ArcGIS 10.3 3-D analyst tool function. A water Quality index rating of 1 was measured in the study area which is an indication that the water is very bad. Also, the physiochemical analysis on soil and water revealed poor water and soil. Water analysis showed high concentration of Fe and Zn which made the water in the area unsuitable for drinking. Also, the soil samples recorded high levels of crude content from 1m, with concentration reducing with depth up to 3m. A general contamination map of Okrika/Ogu-bolo was modelled, indicating the contamination rating of the total land mass of the study area, with 23.59km (5.71%) rated very good, while 85.65km (20.71%) were rated good. Also, 140.37km (33.95%) had only showed moderate level of contamination while 112.56km (27.22%) recorded high level of contamination, with 51.29km (12.40%) of the total land mass having a very bad contamination record.

The Relevance of Fluid and Porous Media Properties for DNAPL Migration and Entrapment: A Numerical Evaluation of Laboratory Experiments

<p>A large number of sites worldwide are subjected to contamination by dense non-aqueous phase liquids (DNAPLs). This group of typically highly persistent chemicals arise tremendous threats to ecosystems and humankind, especially for groundwater abstraction and usage. In particular, chlorinated solvents have great risk profiles due to their toxic and carcinogenic properties, posing essential needs for appropriate risk assessment and site management strategies. Once released into the subsurface, DNAPLs form so-called source zone geometries (SZGs), i.e., physical shapes containing multiple phases, which represent long-term sources for contamination of downstream groundwater. The complex geometrical and chemical properties of such sources are, together with subsurface characteristics and hydraulic conditions of the aquifer, the most sensitive factors in controlling contaminant plume propagation. As locations of DNAPL sources are widely unknown and subsurface phase exploration methods are limited by technical and financial constraints, in most site assessments, dissolved contaminant plumes are detected only. This fact has led to numerous sites where remediation efforts have been inefficient or even failed, or exceeded economical pre-calculations. Here, improved knowledge on factors controlling source zone formation would lead to better predictions for corresponding SZGs and, therefore, better estimations of contaminant plume evolution and prediction.</p><p>A quasi-two-dimensional tank setup formed the basis for generating experimental measurement data of DNAPL migration and entrapment at a Darcy scale under defined laboratory conditions. Three different types of single-size fraction materials (glass beads, filtering glass, and natural sand) were used as homogeneous porous media. DNAPL release into the initially fully water-saturated tank was realized by means of a falling-head boundary condition. Both the aqueous and non-wetting phases were marked for better optical visibility using colorization tracers. All experimental scenarios were conducted under equal ambient conditions (e.g., constant temperature, homogeneous light source). Raw data collection was performed by serial image acquisition from one tank side. A set of customized image analysis and processing approaches was used for the automatized calculation of DNAPL saturation distributions, which served as experimental data for calibrating the base case model scenarios.</p><p>For each base case scenario, a representative numerical multiphase flow model was set up using the software codes TMVOC and OpenGeoSys. Starting parameters for calibration were selected based on the tank layout, technical data sheets and hydraulic characterization of the porous media. Each model setup was then run with a specified range of variation for each parameter after successful calibration, whereby parameter ranges were chosen to coincide with physically plausible values at laboratory scale. Through this semi-automatized parameter sensitivity analysis, controlling factors of source zone formation could be identified and ranked along their strength of impact on SZGs. Furthermore, the comparison between results of each software code could identify strengths and weaknesses of each one.</p>