Hydrogeological Site Characterization Research Articles

Ensemble Kalman inversion of induced polarization data

SUMMARY This paper explores the applicability of ensemble Kalman inversion (EKI) with level-set parametrization for solving geophysical inverse problems. In particular, we focus on its extension to induced polarization (IP) data with uncertainty quantification. IP data may provide rich information on characteristics of geological materials due to its sensitivity to characteristics of the pore–grain interface. In many IP studies, different geological units are juxtaposed and the goal is to delineate these units and obtain estimates of unit properties with uncertainty bounds. Conventional inversion of IP data does not resolve well sharp interfaces and tends to reduce and smooth resistivity variations, while not readily providing uncertainty estimates. Recently, it has been shown for DC resistivity that EKI is an efficient solver for inverse problems which provides uncertainty quantification, and its combination with level set parametrization can delineate arbitrary interfaces well. In this contribution, we demonstrate the extension of EKI to IP data using a sequential approach, where the mean field obtained from DC resistivity inversion is used as input for a separate phase angle inversion. We illustrate our workflow using a series of synthetic and field examples. Variations with uncertainty bounds in both DC resistivity and phase angles are recovered by EKI, which provides useful information for hydrogeological site characterization. Although phase angles are less well-resolved than DC resistivity, partly due to their smaller range and higher percentage data errors, it complements DC resistivity for site characterization. Overall, EKI with level set parametrization provides a practical approach forward for efficient hydrogeophysical imaging under uncertainty.

How can the role of leachate on nitrate concentration and groundwater quality be clarified? An approach for landfills in operation (Southern Italy)

Where the unique natural water resource is groundwater, the attention and the susceptibility of local communities and authorities to groundwater quality degradation risks can be so high to determine relevant problems to waste management, especially for landfills in operation or to be realised. A multi-methodological approach was suggested with the purpose to clarify the role of landfill leakage on groundwater quality degradation.The selected study area (SSA) hosts some landfills in a narrow portion of a wide and deep coastal karstic aquifer, for these characteristics to be considered a case of high hydrogeological complexity and vulnerability. News concerning nitrate and secondly iron groundwater concentration anomalously high caused concern in the population and strong local opposition to landfills.The multi-methodological approach includes: the hydrogeological site characterization; the chemical study and the multi-isotope characterization of groundwater and leachate; the land use analysis and the estimation of nitrogen contributions deriving from fertilizers; the mineralogical study of groundwater suspended particles to define the role of natural soil substances.The hydrogeological site characterisation highlighted the local peculiarities of the aquifer. The chemical study was used to define geochemical features, groundwater and leachate characteristics and their macroscopic mixing.The environmental isotopes of hydrogen, carbon, nitrogen, and oxygen were used to investigate the groundwater origin, the most relevant geochemical reactions, the existence of groundwater-leachate mixing, and the sources of anthropogenic NO3−. The land use analysis highlighted quantity and type of used fertilizers permitting to compare these with groundwater in terms of isotopic signature. The mineralogical study demonstrated the role of suspend natural particles due the presence of terre rosse (red or residual soils) in groundwater.The approach confirmed that there are not the groundwater quality degradation effects of landfills, contributing to reassure population and institutions, simplifying the waste management.

Model-based classification of CPT data and automated lithostratigraphic mapping for high-resolution characterization of a heterogeneous sedimentary aquifer

Cone penetration testing (CPT) is one of the most efficient and versatile methods currently available for geotechnical, lithostratigraphic and hydrogeological site characterization. Currently available methods for soil behaviour type classification (SBT) of CPT data however have severe limitations, often restricting their application to a local scale. For parameterization of regional groundwater flow or geotechnical models, and delineation of regional hydro- or lithostratigraphy, regional SBT classification would be very useful. This paper investigates the use of model-based clustering for SBT classification, and the influence of different clustering approaches on the properties and spatial distribution of the obtained soil classes. We additionally propose a methodology for automated lithostratigraphic mapping of regionally occurring sedimentary units using SBT classification. The methodology is applied to a large CPT dataset, covering a groundwater basin of ~60 km2 with predominantly unconsolidated sandy sediments in northern Belgium. Results show that the model-based approach is superior in detecting the true lithological classes when compared to more frequently applied unsupervised classification approaches or literature classification diagrams. We demonstrate that automated mapping of lithostratigraphic units using advanced SBT classification techniques can provide a large gain in efficiency, compared to more time-consuming manual approaches and yields at least equally accurate results.

How to chase a tracer – combining conventional salt tracer testing and direct push electrical conductivity profiling for enhanced aquifer characterization

Tracer testing is a well-established technique in hydrogeological site characterization. However, certain a priori knowledge of the hydraulic regime is required beforehand to avoid test failure, e.g. miss of tracer. In this study, we propose a novel tracer test concept for the hydraulic characterization of shallow unconsolidated sedimentary deposits when only scarce a priori information on the hydraulic regime is available. Therefore, we combine conventional salt tracer testing with direct push vertical high resolution electrical conductivity logging. The proposed tracer test concept was successfully tested on coarse, braided river deposits of the Tagliamento River, Italy. With limited a priori information available two tracer tests were performed in three days to reliably determine ground water flow direction and velocity allowing on-site decision-making to adaptively install observation wells for reliable breakthrough curve measurements. Furthermore, direct push vertical electrical profiling provided essential information about the plume characteristics with outstanding measurement resolution and efficiency.

Site investigation and design of the monitoring system at the Sant`Alessio Induced RiverBank Filtration plant (Lucca, Italy)

Innovative water management strategies such as the storage of reclaimed water or excess water from different sources in Managed Aquifer Recharge (MAR) schemes can greatly increase water availability and therefore improve water security in the Mediterranean area. The Sant'Alessio well field will be used, within the framework of the MARSOL EU FPVII-ENV-2013 project, to demonstrate the sustainability, by a technical, social and market point of view, and the benefits of managing Induced Riverbank Filtration, one of the most common MAR techniques, versus the unmanaged option. Along with a detailed hydrogeological and hydrogeochemical site characterization a Wireless Sensor Network to monitor quantitatively and qualitatively the hydrologic variables in the river water, in the aquifer and in the well field has been designed. In this paper we present the test site, the investigations performed and the design of the monitoring system along with a brief overview of the Decision Support System (DSS) under development to control the whole monitoring devices. The DSS along with the monitoring system will constitute a prototype whose potential market exploitation will be tested.

Radionuclides Transport Phenomena in Vadose Zone

Radioactive waste management is fundamental to safeguard population and environment by radiological risks. Environmental assessment of a site, where nuclear activities are located, allows to understand the hydro geological system and the radionuclides transport in groundwater and subsoil. The use of dedicated software allows the investigation of transport phenomena and to predict accidental scenarios dynamic; this allows to understand the evolution of the accidental contamination, but at the mean time the potentiality of the software itself can be verified. The aim of this paper is to perform a numerical analysis by means of HYDRUS 1D code, so as to evaluate radionuclides transport dynamics in a nuclear site in Piedmont region (Italy). In particular, the behaviour in vadose zone has been investigated. An iterative assessment process has been performed for risk assessment of radioactive contamination. The analysis therein developed considers the following aspects: i) hydro geological site characterization; ii) individuation of the main factors influencing water flow and radionuclides transport phenomena; iii) software potential for radionuclides leakage simulation purposes

Modeling contaminant transport and remediation at an acrylonitrile spill site in Turkey

The August 1999 earthquake in Turkey damaged three acrylonitrile (AN) storage tanks at a plant producing synthetic fiber by polymerization. A numerical modeling study was carried out to analyze the groundwater flow and contaminant (AN) transport at the spill site. This study presents the application of a numerical groundwater model to determine the hydrogeological parameters of the site, where such data were not available during the field surveys prior to the simulation studies. The two- and three-dimensional transient flow and transport models were first calibrated using the first 266days of observed head and concentration data and then verified using the remaining 540-day observed data set. Off-site migration of the contaminant plume was kept under control within the site boundaries owing to the favorable geology of the site, the characteristics of the local groundwater flow regime and the pumping operations. As expected, the applied pump-and-treat system was effective at high-permeability zones, but not fully effective at low-permeability zones. The results of long-term simulations for unconfined aquifer showed that the size of the plume in the high permeability zone shrank significantly due to the dilution by natural recharge. However, in the low permeability zone, it was not significantly affected. The study showed that accurate and sufficient data regarding the source characteristics, concentration and groundwater level measurements, groundwater pumping rates and their durations at each of the extraction points involved in the pump-and-treat system along with the hydrogeological site characterization are the key parameters for successful flow and transport model calibrations.

Hydrogeological site characterization of marly-silici-calcareous rocks through surveys of discontinuities and pumping tests

One of the challenges in the characterization of fractured aquifers is to define whether the approximation to a porous medium is valid or not. In this context, the present study concerns the hydrogeological characterization of a small area constituted by densely fractured marly-silici-calcareous rocks. The hydrogeological investigations included fracture network surveys and pumping tests. The survey of the outcrops showed a rock mass characterized by a high number of persistent discontinuities with a wide spatial distribution, high frequency and a wide range of values of the aperture in the different systems. These characteristics imply the possibility of considering the rock mass as an equivalent porous medium at the outcrop scale. Pumping tests highlighted a consistent reaction of the various monitoring points and a similar trend of the drawdown-time curve, resulting in considering the aquifer as continuous medium at the scale of the volume influenced by pumping. The drawdown maps seems to be related to a weak anisotropy of the aquifer in the horizontal plane with the major axis of the ellipse aligned along the direction NE-SW, consistent with the directions of the principal components of the hydraulic conductivity resulting from the hydraulic model of the rock mass. Although the aquifer can be considered an equivalent anisotropic porous medium at the scale of the study, the comparison between the results of pumping tests and of the rock mass survey indicates that this approach would be not sufficient to characterize groundwater movement.

Radionuclide migration at experimental polygon at Red Forest waste site in Chernobyl zone. Part 2: Hydrogeological characterization and groundwater transport modeling

Abstract This article represents the second of two articles, which review the main results of the international radioecological projects: Chernobyl Pilot Site Project (1999–2003) and Experimental Platform in Chernobyl (2004–2008). These projects studied radionuclide migration from the near-surface radioactive waste trench at the Red Forest waste dump in the Chernobyl zone, which contained nuclear fuel particles. This article presents results from the comprehensive hydrogeological site characterization program including the following issues: geological structure of the study site, hydraulic properties of the deposits, tracer tests in the aquifer, results of groundwater monitoring and unsaturated zone regime studies, as well as data on the 90Sr distribution in the unsaturated zone and aquifer, and analyses of 90Sr sorption behavior. The derived parameters were used to develop and calibrate 1D (flow tube) and 2D (cross-section) models describing the migration of 90Sr from the studied waste trench to the unsaturated zone and aquifer over a 16-a period (1986–2002). The models involved the following sub-models: (1) the geostatistical (structural) model for radioactivity distribution in the trench (using GSLIB); and (2) the radionuclide source term model (STERM1D) describing dissolution of fuel particles and a 1D of radionuclide redistribution in the trench body and unsaturated zone. The MODFLOW – MT3D codes were used to model the 2D 90Sr transport in the aquifer cross-section. Calibration of the 1D model with respect to Kds and dispersivities allowed quite accurate reproduction of 90Sr migration behavior for the early period (1995–1998). The less perfect fit between the 1D and 2D modeling results and monitoring data for the later period (1999–2002) suggests the need to improve the conceptual radionuclide migration model (i.e. to account for transient hydraulic and geochemical regimes of the waste site).

Kalman filter updating of possibilistic hydraulic conductivity

Preferential sampling regimes too often leave hydrogeological site characterization in need of hydraulic conductivity data. Furthermore, faith in a kriged hydraulic conductivity field (K-field) wanes when viable data are few. A straightforward solution to this problem is the application of expert knowledge to the identification of hydraulic conductivity at specified locations. However, the nature of this supplementary data suggests that its use should be limited to the initial estimation of a K-field. Available hydraulic conductivity measurements are used to update the initial estimate of the K-field in a fuzzy Kalman filtering procedure. The results are fuzzy and can be defuzzified into traditional crisp estimates. An application of these protocols to the CIBA-Geigy site in New Jersey is provided to illustrate the ease of the procedure and the impact of viable expert input.

Characterization of Crystalline Bedrock Contaminated by Dense Nonaqueous Liquid

AbstractA systematic hydrogeologic site characterization has been completed in a fractured rock flow system, with the objective of identifying contaminant migration and fate pathways from a historical release of 1,1,1‐trichloroethane (TCA). The study integrated hydrogeologic analysis techniques such as borehole geophysical logging, pumping test analysis, and hydrochemical facies analysis to study the impact of a dense nonaqueous phase liquid (DNAPL) in a sparsely fractured crystalline bedrock. The assessment methodology can be divided into two parts: (1) characterization of the source area, where DNAPL is acting as a residual source of TCA, and (2) characterization of the downgradient plume. Reduction in DNAPL mass in the source area has resulted in significant and sustained reductions in downgradient concentrations, suggesting that remediation of fractured crystalline bedrock contaminated with DNAPL is possible and not “technically infeasible.”

Inversion of tracer test data using tomographic constraints

We have developed a methodology for inverting tracer test data using zonation information obtained from two‐dimensional radar tomograms to improve the (typically overly smooth) hydraulic conductivity fields obtained from conventional inversion of tracer test data. The method simultaneously yields two‐dimensional estimates of hydraulic conductivity as well as petrophysical relationships that relate hydraulic conductivity to radar velocity; these relationships can be assumed to be stationary throughout the area of investigation or to vary as a function of zonation. Using a synthetic three‐dimensional hydraulic conductivity field, we apply the developed inversion methodology and explore the impact of the strength and stationarity of the petrophysical relationship as well as the impact of errors that are often associated with radar data acquisition (such as unknown borehole deviation). We find that adding radar tomographic data to tracer test data improves hydrogeological site characterization, even in the presence of minor radar data errors. The results are contingent on the assumption that a relationship between radar velocity and hydraulic conductivity exists. Therefore the applicability of the proposed method may be limited to field sites where this condition is partially or fully satisfied.

Characterization of hydrogeologic properties for a multi-layered alluvial aquifer using hydraulic and tracer tests and electrical resistivity survey

A multi-layered aquifer, typical of riverbank alluvial deposits in Korea, was studied to determine the hydrologic properties. The geologic logging showed that the subsurface of the study site was comprised of four distinctive hydrogeologic units: silt, sand, highly weathered and fresh bedrock layers. The electrical resistivity survey supplied information on lateral extension of hydrogeologic strata only partially identified by a limited number of the geologic loggings. The laboratory column tracer test for the recovered core of the sand layer resulted in a hydraulic conductivity of 5.00×10−2 cm/s. The slug tests performed in the weathered rock layer yielded hydraulic conductivities of 4.32–7.72×10−4 cm/s. Hydraulic conductivities for the sand layer calculated from the breakthrough curves of bromide ranged between 2.08×10−3 and 2.44×10−2 cm/s with a geometric mean of 6.89×10−3 cm/s, which is 7 times smaller than that from the laboratory column experiment. The trend of increasing hydraulic conductivity with an increase in tracer travel length is likely a result of the increased likelihood of encountering a high conductivity zone as more of the aquifer is tested. The combined hydrogeologic site characterization using hydraulic tests, tracer tests, and column test with geologic loggings and geophysical survey greatly enhanced the understanding of the hydrologic properties of the multi-layered alluvial aquifer.

Modified total variation methods for three-dimensional electrical resistance tomography inverse problems

Images from geophysical surveys and data inversion can be valuable information for hydrogeologic site characterization and monitoring. This paper focuses on solving nonlinear electrical resistance tomography (ERT) inverse problems; the methods are applicable to other inverse problems, such as groundwater flow. The total variation (TV) method is used to constrain ERT inverse solutions. The classic TV functions are modified to obtain differentiability with respect to parameters, which allows the use of gradient search optimization methods. The modified total variation (mTV) method constrains ERT inverse solutions effectively. Sometimes the mTV constraints cause important parameter features to be smoothed out. To eliminate excessive smoothing, two successive partial variation relaxation (SPVR) methods are devised for use with the mTV method. Computational experiments demonstrate that mTV works well in identifying very sharp discontinuities and smoothly distributed parameters and that SPVR improves mTV solutions for both problem types. The mTV and SPVR methods are applied to synthetic layered systems with and without measurement error, and a comparison of results for a large three-dimensional ERT problem with block anomaly presented by Zhang et al. [1995] demonstrates the superiority of the mTV and SPVR methods. A final example examines a smoothly varying case and explores the impact of two computational parameters introduced in the development of mTV and SPVR. The example shows also that SPVR can be used directly to invert data and not necessarily as an adjunct to mTV.

Bayesian Method for hydrogeological site characterization using borehole and geophysical survey data: Theory and application to the Lawrence Livermore National Laboratory Superfund Site

A stochastic Bayesian approach for combining well logs and geophysical surveys for enhancing subsurface characterization is presented. The main challenge we face is in creating the bridge to link between ambiguously related geophysical surveys and well data. The second challenge is imposed by the disparity between the scale of the geophysical survey and the scale of the well logs. Our approach is intended to integrate and transform the well log data to a form where it can be updated by the geophysical survey, and this tends to be a convoluted process. Our approach starts with generating images of the lithology, conditional to well logs. Each lithology image is then used as the basis for generating a series of shaliness images, conditional to well log data. Shaliness images are converted to resistivity images using a site‐specific petrophysical model relating between shaliness, resistivity, and lithology, to create the necessary interface with the cross‐well resistivity survey. The lithology and resistivity images are then updated using cross‐well electromagnetic resistivity surveys. We explored the limits of the approach through synthetic surveys of different resolutions and error levels, employing the relationships between the geophysical and hydrological attributes, which are weak, nonlinear, or both. The synthetic surveys closely mimic the conditions at the LLNL Superfund site. We show that the proposed stochastic Bayesian approach improves hydrogeological site characterization even when using low‐resolution resistivity surveys.

A Comprehensive Hydrodynamic Modeling Approach for the Hydrogeological Site Characterization and for Deriving Input to Performance Assessment

ABSTRACTThis paper gives an overview of the numerical models employed for the simulation of ground-water flow at the Wellenberg site, the runs carried out and the kind of products which were specified as results of the modelling calculations. With respect to more conventional modelling approaches, based mainly on fully deterministic and steady-state schemes, this new approach integrates both deterministic and stochastic aspects (hydraulic conductivity distribution, geometry of the fracture network, etc.) in three different groundwater models, accounting for different scale levels. Moreover, this new approach is as close as possible to a realistic representation of the in situ conditions, since it is based on plausible distributions of transmissivity and hydraulic conductivity at all scales and takes the transient nature of flow fully into account. This latter aspect is fundamental since, at Wellenberg, both the underground facilities and a natural head anomaly in the host rock cause pressure disturbance in the flow field. Finally, and because boundary conditions (especially topographic surface and overburden) vary over time, model predictions based on current topography and climatic conditions are of limited duration and hence a validity duration has to be defined.

Facies Architectural Analysis of the Shallow Subsurface: A Strategic Component of an Interdisciplinary Approach to Hydrogeologic Site Characterization: ABSTRACT

Facies Architectural Analysis of the Shallow Subsurface: A Strategic Component of an Interdisciplinary Approach to Hydrogeologic Site Characterization: ABSTRACT

Delineation of Near-Surface Preferential Pathways of Contaminant Transport within the Unsaturated Zone Using Soil-Gas: A New Method for Hydrogeological Site Characterization and Evaluation

Delineation of Near-Surface Preferential Pathways of Contaminant Transport within the Unsaturated Zone Using Soil-Gas: A New Method for Hydrogeological Site Characterization and Evaluation

Application of a coupled adjoint sensitivity and kriging approach to calibrate a groundwater flow model

A new technique is employed to calibrate a regional‐scale groundwater flow model to an extensive data base of undisturbed (i.e., assumed to be steady state) and transient heads. The methodology presented in this study is similar in concept to one presented by de Marsily et al. (1984) in that an adjoint sensitivity technique is coupled with a kriging algorithm to calibrate a flow model. The notable difference of the methodology presented in this paper is that it directly identifies the regions where modification of the model's kriged transmissivity or boundary pressure values will directly improve the overall fit between measured and model‐calculated heads at selected wells. At the locations identified as most sensitive to transmissivity changes, synthetic transmissivity values, referred to as pilot points, are added to the transmissivity data base and used as input for kriging the transmissivity field. An application of the methodology to data originating from approximately 10 years of regional hydrogeologic site characterization efforts that have been conducted in the Culebra dolomite at the Waste Isolation Pilot Plant in southeastern New Mexico is presented.