Aquifer Tracer Research Articles

Collected Rain Water as Cost-Efficient Source for Aquifer Tracer Testing.

Locally collected precipitation water can be actively used as a groundwater tracer solution based on four inherent tracer signals: electrical conductivity, stable isotopic signatures of deuterium [δ2 H], oxygen-18 [δ18 O], and heat, which all may strongly differ from the corresponding background values in the tested groundwater. In hydrogeological practice, a tracer test is one of the most important methods for determining subsurface connections or field parameters, such as porosity, dispersivity, diffusion coefficient, groundwater flow velocity, or flow direction. A common problem is the choice of tracer and the corresponding permission by the appropriate authorities. This problem intensifies where tracer tests are conducted in vulnerable conservation or water protection areas (e.g., around drinking water wells). The use of (if required treated) precipitation as an elemental groundwater tracer is a practical solution for this problem, as it does not introduce foreign matters into the aquifer system, which may contribute positively to the permission delivery. Before tracer application, the natural variations of the participating end members' tracer signals have to be evaluated locally. To obtain a sufficient volume of tracer solution, precipitation can be collected as rain using a detached, large-scale rain collector, which will be independent from possibly existing surfaces like roofs or drained areas. The collected precipitation is then stored prior to a tracer experiment.

Origin of halogens and their use as environmental tracers in aquifers of Thriassion Plain, Attica, Greece

The processes that control halide concentrations in coastal aquifers of the Thriassion Plain in Greece include both natural and anthropogenic factors as follows: evapotranspiration, dissolution of evaporites, domestic sewage as well as dissolution of minerals containing fluoride. Groundwater monitoring data indicate that the Cl−/Br− mass ratio ranges between 43 and 1510. Ratios ranging round the value 292 are attributed to sea intrusion; 300–600 to domestic sewage; and 900–1510 to dissolution of evaporites. The evaporation of irrigated water plays an important role as well. Fluoride concentrations ranging from 0.1 up to 9.94 mg/l possibly originate from Holocene as well as Plio–Pleistocene sediments which contain organic material. It is also likely that fluoride originates from fluorite dissolution or comes from Palaeozoic-Middle Triassic volcano-sedimentary rocks which occur around the wider area and constitute the basement of the study area. In those rocks, the alteration of biotite to chlorite takes place under the influence of hydrothermal fluids, as the Thriassion Plain is close to South Aegean volcanic arc where volcanic activity exists.

Many‐objective groundwater monitoring network design using bias‐aware ensemble Kalman filtering, evolutionary optimization, and visual analytics

This study contributes the Adaptive Strategies for Sampling in Space and Time (ASSIST) framework for improving long‐term groundwater monitoring decisions across space and time while accounting for the influences of systematic model errors (or predictive bias). The new framework combines contaminant flow‐and‐transport modeling, bias‐aware ensemble Kalman filtering (EnKF), many‐objective evolutionary optimization, and visual analytics‐based decision support. The ASSIST framework allows decision makers to forecast the value of investments in new observations for many objectives simultaneously. Information tradeoffs are evaluated using an EnKF to forecast plume transport in space and time in the presence of uncertain and biased model predictions that are conditioned on uncertain measurement data. This study demonstrates the ASSIST framework using a laboratory‐based physical aquifer tracer experiment. In this initial demonstration, the position and frequency of tracer sampling was optimized to (1) minimize monitoring costs, (2) maximize the information provided to the EnKF, (3) minimize failures to detect the tracer, (4) maximize the detection of tracer fluxes, (5) minimize error in quantifying tracer mass, and (6) minimize error in quantifying the centroid of the tracer plume. Our results demonstrate that the forecasting, search, and visualization components of the ASSIST framework represent a significant advance for observation network design that has a strong potential to innovate our characterization, prediction, and management of groundwater systems.

A novel meshing and finite element flow model for porous media

This work presents a new fully-unstructured meshing scheme for generating simulation meshes, coupled with a novel simulation method for fluid flow models in porous media. We use unstructured tetrahedral meshing algorithms to generate meshes that rigorously adhere to structural and depositional heterogeneity in geological reservoir models at multiple scales. Geological heterogeneity is represented using numerous surfaces, in contrast with traditional pixel- or grid-based methods. This approach allows the generation of meshes which capture heterogeneity more efficiently and accurately than structured or partially-structured grids. The flow simulations are performed using a new hybrid discontinuous Galerkin/control volume based method which combines continuous pressure and discontinuous velocities. Our approach rigorously enforces material balance equations while ensuring crucial scalar fields such as pressure and saturation remain positive and bounded. This research has direct application to aquifer flow systems. We report the results of single-phase flow simulations on unstructured meshes for complex geological systems. These simulations accurately model the spread of groundwater contaminants such as methyl tertiary butyl ether (MTBE) ormetals such as arsenic that behave as tracers in aquifers. Initially, simulations are run on simple anisotropic geological systems for two-dimensional flow to benchmark against analytical solutions and validate the accuracy of the simulator for simple flow problems. Next, simulations for tracer flow are performed on 3D channelized and fractured reservoir systems to demonstrate the robustness and accuracy of the new grids and simulator in complex reservoir geology. We present numerical simulations of single-phase flow for a variety of geological systems of increasing complexity, culminating in systems which would be impossible to mesh and solve with structured meshes. This is also a first step towards a more complex multi-phase and multi-component implementation of the novel methods for flow through porous media.

A general real-time formulation for multi-rate mass transfer problems

Abstract. Many flow and transport phenomena, ranging from delayed storage in pumping tests to tailing in river or aquifer tracer breakthrough curves or slow kinetics in reactive transport, display non-equilibrium (NE) behavior. These phenomena are usually modeled by non-local in time formulations, such as multi-porosity, multiple processes non equilibrium, continuous time random walk, memory functions, integro-differential equations, fractional derivatives or multi-rate mass transfer (MRMT), among others. We present a MRMT formulation that can be used to represent all these models of non equilibrium. The formulation can be extended to non-linear phenomena. Here, we develop an algorithm for linear mass transfer, which is accurate, computationally inexpensive and easy to implement in existing groundwater or river flow and transport codes. We illustrate this approach by application to published data involving NE groundwater flow and solute transport in rivers and aquifers.

Addressing model bias and uncertainty in three dimensional groundwater transport forecasts for a physical aquifer experiment

This work contributes a combination of laboratory‐based aquifer tracer experimentation and bias‐aware Ensemble Kalman Filtering (EnKF) to demonstrate that systematic modeling errors (or bias) in source loading dynamics and the spatial distribution of hydraulic conductivity pose severe challenges for groundwater transport forecasting under uncertainty. The impacts of model bias were evaluated using an ammonium chloride tracer experiment conducted in a three dimensional laboratory tank aquifer with 105 near real‐time sampling locations. This study contributes a bias‐aware EnKF framework that (i) dramatically enhances the accuracy of concentration breakthrough forecasts in the presence of systematic, spatio‐temporally correlated modeling errors, (ii) clarifies in space and time where transport gradients are maximally impacted by model bias, and (iii) expands the size and scope of flow‐and‐transport problems that can be considered in the future.

Determination of longitudinal dispersivity in an unconfined sandy aquifer

AbstractStudy of the mobility of contaminants in an aquifer is an important issue for the proper remediation of contaminated groundwater. Determination of associated solute transport parameters therefore is essential for investigation of the extent to which groundwater can be contaminated. This study aimed at determining solute transport parameters for an unconfined sandy aquifer at a laboratory scale through various tracer tests using a conservative solute as a tracer. Tracer tests consisted of both well‐tracer tests (single and double wells) and an aquifer tracer test using a plume‐capturing device such as time domain reflectometry (TDR). The results showed that longitudinal dispersivities estimated from the single and double well‐tracer tests were 2·2 cm and 13·5 cm for a travel distance of 9·3 cm and 13·5 cm from the injection point respectively. These results agreed reasonably well with the results of the aquifer tracer test. The solute transport parameters obtained at multiple points in the aquifer through the aquifer tracer test revealed that the dispersivity length was proportional to the travel distance by a factor of 0·3, which was moderately higher than the value of 0·1 given in the literature. Copyright © 2002 John Wiley & Sons, Ltd.

The modern water interface: recognition, protection and development — advance of modern waters in European aquifer systems

Abstract Modern groundwater that has recharged aquifers within the past 50 a shows the influence of humans globally, either by the presence of small concentrations of environmental tracers or in some cases by severe pollution. This study describes important environmental tracers (e.g. 3 H, 85 Kr, chlorofluorocarbons, SF 6 ) and contaminants (e.g. NO 3 − }, pesticides, chlorinated solvents) for modern groundwater dating and recognition of human impacts. Some applications of the described tracers in aquifers investigated in the PALAEAUX study are presented in order to illustrate the advance of modern waters in European aquifer systems. The study shows that the location of the modern water interface varies within a range of between c . 10 and c . 100 m in the investigated aquifers due to variations in hydrogeological setting, climate and exploitation of the groundwater resource. The subsurface distribution of the modern water indicators and contaminants demonstrate that the advance of modern groundwaters and the fate of harmful substances in them have important implications for protection and development of the water resources. Contaminants that do not degrade or degrade only very slowly will advance further into the aquifers and may eventually contaminate even deep groundwater systems.

Correction to “Convergent Radial Dispersion: A Laplace Transform Solution for Aquifer Tracer Testing”

Correction to “Convergent Radial Dispersion: A Laplace Transform Solution for Aquifer Tracer Testing”

Difluorobenzoates as Nonreactive Tracers in Soil and Ground Water

AbstractOver the past 10 years, several fluorinated derivatives of benzoic acid have proven their usefulness as nonreactive water tracers. In this study, the suitability of four previously untested isomers of difluorobenzoate (DFBA) as saturated‐ and unsaturated‐zone tracers was examined in a series of laboratory and field experiments. Experimental conditions included batch shaking tests with two different soils, laboratory soil column tests under saturated and unsaturated conditions, a three‐month aquifer tracer test, and a month‐long tracer test in a field soil. Bromide, which is considered nonreactive in most hydrological environments, was included as an index tracer in each experiment. Under every condition, each DFBA isomer snowed the same concentration distribution and mass recovery as Br. These and other previously tested fluorobenzoates were ranked according to their suitability as nonreactive soil and ground‐water tracers.

Optimal Configuration and Scheduling of Ground‐Water Tracer Test

A technique for jointly configuring and scheduling a monitoring network for an aquifer tracer test is presented. A dynamic programming (DP) algorithm is used to select among competing designs for a test to provide data for estimating aquifer model parameters. A maximal information criterion is used to evaluate competing designs that satisfy a particular budget constraint. Decision variables in DP formulation are sampling locations (configuration variables) and sampling initiation times (scheduling variables). A forward DP solution method is used; optimal configuration and scheduling are determined by maximizing information without exceeding a budget. Maximization of information is argued to be equivalent to minimizing total cost of installation as well as sampling and analysis subject to an information demand function constructed from the trace of the covariance matrix of the estimated parameters. A tracer test for a confined aquifer is simulated using a finite difference scheme. A short injection period followed by a monitoring period is simulated. The DP algorithm is applied to this model to design a monitoring network and schedule to estimate aquifer characteristics. The sampling network and schedule is designed with estimation of transmissivity, retardation, and dispersivities in mind. Resulting designs are reasonable, and the method can be extended to regional-sized problems with little modification.

Convergent radial dispersion: A Laplace transform solution for aquifer tracer testing

A Laplace transform solution was obtained for the injection of a tracer in a well situated in a homogeneous aquifer where steady, horizontal, radially convergent flow has been established due to pumping at a second well. The standard advection‐dispersion equation for mass transfer was used as the controlling equation. For boundary conditions, mass balances that account for mixing of the tracer with the fluid residing in the injection and pumped wells were used. The derived solution, which can be adapted for either resident or flux‐averaged concentration, is of practical use only for the pumped well. This problem is of interest because it is easily applied to field determination of aquifer dispersivity and effective porosity. Breakthrough curves were obtained by numerical inversion of the Laplace transform solution. It was found that tracer mixing with fluid in the pumped and injection wells, especially in low‐porosity aquifers, may have a significant influence on the shape of the tracer breakthrough curves.