Tracer methods have been widely used in many fields of environmental and natural sciences, and also in human health sciences. In particular, tracers are used in the study of karst hydrogeology, ...typically focusing on phenomena such as sinkholes, sinking rivers and large karst springs. It is known that tracers have been used since antiquity. The aim of tracer tests has been to investigate underground flow paths, transport processes and water–rock interactions, and to get an insight into the functioning of a karst aquifer. In karst hydrogeology, tracer methods are the most important investigation tools beside conventional hydrological methods. In early times, tracer methods were applied only to investigate underground flow-paths. Later they were also used to elucidate transport processes associated with water flow, and today they are often the basis, together with detailed hydrological information, of groundwater protection investigations and aquifer modelling. Many substances (spores, microspheres, bacteriophages, salt tracers, fluorescent dyes, radioactive substances) have been investigated for their properties and potential usage in environmental investigations, in particular the often unknown and inaccessible underground systems of karst areas. A great number of analytical techniques is available. This includes instrumentation for laboratory applications and direct online, on-site or in-situ field measurements. Modern instruments have a high capability for data acquisition, storage and transmission in short intervals, as a basis for quantitative evaluation and modelling. This enables research on the hydrological and hydrochemical dynamics of aquifers and their response to different natural or anthropogenic impacts.
•Hyporheic zones cause longer retention of pollutants within the river-bed sediments.•Hybrid Cells In Series Model Simulates stream flow, hyporheic zone interactions.•A useful tool for water quality ...monitoring and management.
Issues of pollution pervade the eco-environment, including water sources, resulting from constant pollutant release into river channels from varying sources. The presence of hyporheic zones in rivers play a role in the fate of pollutants in water systems. This paper studies pollutant transport in a natural river with hyporheic zone exchange and non-point source inflows. A non-point source, hyporheic exchange, Hybrid Cells in Series model is proposed and derived analytically. The proposed model is simulated under varying time delays between the hyporheic zone and the overflow channel. The Breakthrough curves produced from the simulations are in good agreement with field tracer test results, as presented in most studies. The model adequately captures the elongated arrival and trailing tails of field data breakthrough profiles. The proposed model's overall performance shows that the model adequately predicts pollutants' transport and fate along a natural river.
Identifying the location, orientation and characteristics (e.g. aperture and density) of fractures in aquifers is crucial for the simulation of flow, mass and heat transport with discrete fracture ...network models. Yet, it is often considered a highly challenging problem due to data limitations, representativity issues, and the difficulty in distinguishing between open and closed fractures, or connected and nonconnected fractures. This paper proposes a methodology for explicit characterization of fractures based on combining harmonic pumping with tracer injection (HPTI) in an experimental setting. In this method, sinusoidal excitation through two-spot, variable-rate pumping and injection is combined with a conservative solute tracer (i.e. chloride as sodium chloride) injection at the injection well. A customized Kalman filtering technique, termed the ‘constrained restart dual ensemble Kalman filter’ (CRD-EnKF), is then employed to estimate fracture characteristics using pressure and tracer concentration data from the HPTI experiment. The effectiveness of the proposed methodology is evaluated using two hypothetical test cases of fractured aquifers dealing with two cross fractures and a regular network of fractures, respectively. Comparison of HPTI-based fracture characterization results with those obtained from three previously used methods—harmonic pumping/injection without tracer (i.e. harmonic hydraulic tomography), constant-rate pumping without tracer (i.e. conventional hydraulic tomography), and tracer injection with constant pumping (i.e. conventional forced-gradient tracer test)—shows that the HPTI data can significantly reduce the estimation error and uncertainty compared to the other methods, as the absolute percentage estimation errors are lower than 10% for all estimated fracture characteristics.
Deuterated water has been applied in hydrogeological tracer tests in recent years. However, there is a contradiction in the conservativeness of the artificial deuterium (D/
2
H). In this study, what ...circumstances HDO behaved truly conservatively were investigated through the laboratory-scale sand tank by comparing with the widely used tracer chloride (Cl
−
). In addition, the advection-dispersion equation (ADE) and dual-domain mass transfer (DDMT) equation were employed to describe the breakthrough curves (BTCs) of tracers. Reasons for the non-conservativeness of HDO were discussed comprehensively for the first time. HDO behaved conservatively when transported in the porous media with high permeability, and ADE could describe BTCs successfully. While HDO presented the phenomenon of retardation in media with low permeability, especially in the clay medium. DDMT was more suitable for demonstrating delayed BTCs. Hydrogen bonds between HDO and H
2
O, the isotopic exchange effect, and the dual-domain of the media were used to explain the retardation of HDO. The retardation factor (
R
= 1.27) was used to describe transporting behaviors of HDO in clay first. The relatively large molecular coefficient of HDO could make the proportion of immobile regions of HDO greater than that of Cl
−
. Tracers tend to be delayed when transported in the media with the larger porosity, smaller hydraulic gradient, and lower permeability. This work provides more considerations for using deuterium as artificial tracers in hydrogeological tests.
An identification method for determining the aquifer’s mass transport parameters is proposed, based on data from field tracer tests with a pulse or a continuous source and an arbitrary position of ...the observation well in respect to the tracer entry point. The method is also applicable in the presence of a representative set of data on changes in the concentration of pollutants at different points in the aquifer around a short-term (instantaneous) or a continuous surface or underground source. The identification procedure is based on the automated comparison of the observations data with a series of theoretical curves by varying the required parameters in order to achieve maximum compliance. The tracer transport is represented by analytical solutions of the partial differential equation for mass transfer in a homogeneous and isotropic two-dimensional porous media. The developed computer programs include numerical optimization using the Levenberg-Marquardt algorithm. Results from tests performed in order to assess reliability and errors of detection and identification are presented. Using the programs, the mass transport parameters: active porosity n0, effective (sorption) porosity nS, longitudinal dispersivity αL, transverse dispersivity αT and rate constant γ can be determined.
We perform a salt tracer experiment, monitored by time‐lapse electrical resistivity tomography, in a quasi‐two‐dimensional sandbox with the aim of determining the hydraulic conductivity distribution ...in the domain. We use sodium chloride as a tracer, together with cochineal red for visual monitoring. The time series of observed resistance for each electrode configuration is characterized by its temporal moments. We invert the mean arrival time of electrical potential perturbations and a few steady state hydraulic head measurements using the fully coupled hydrogeophysical approach recently introduced by Pollock and Cirpka (2010). This is the first application of the approach to experimental data. The results obtained show a reasonable agreement between the estimated hydraulic conductivity field and the pattern of the actual sandbox filling. Using this estimation, a transient simulation is performed to compute the propagation of the salt tracer plume through the sandbox. The latter is compared to pictures taken during the experiment. These results show an even better agreement, indicating that the lenses of different sand types are not entirely homogeneous and some unexpected preferential flow paths are present. We conclude that temporal moments of potential perturbations obtained during salt tracer tests provide a good basis for inferring the hydraulic conductivity distribution by fully coupled hydrogeophysical inversion.
Key Points
Use temporal moments to invert ERT monitoring data of salt‐tracer experiments
Application to laboratory experiments has been successful
Inverted results may be better than intended zonation of filling pattern
Understanding the groundwater flow in carbonate aquifers represents a challenging aspect in hydrogeology, especially when they have been struck by strong seismic events. It has been proved that large ...earthquakes change springs hydrodynamic behaviour showing transitory or long-lasting variations and making their management much more difficult. This is the case of Sibillini Massif (central Italy), which has been hit by the well-known 2016–2017 seismic period. This work aims to improve the knowledge of carbonate aquifers groundwater circulation and their possible changes in the hydrodynamic behaviour, during and after a series of strong seismic events. The goal has been achieved by comparing long-time tracer tests and transient time-series analysis, based on a sliding-window approach. This approach allowed investigating transient variations in the carbonate aquifers recharge system, highlighting the changes of relationships between the inflow contributions to the spring discharge in the area. As a result, the seismically triggered pore pressure distribution, and the hydraulic conductivity variations, because of the ground shaking and the fault systems activation, account for all the mid- and long-term modifications in the recharge system of Sibillini aquifers, respectively. These outcomes provide valuable insights to the knowledge of aquifer response under similar hydrogeological conditions, that are vital for water management.
Tracer test is an effective tool for characterizing the complex conduits and exploring the solute transport law for karst aquifers. In this study, three generalized karst conduit models were ...established, including auxiliary side conduit, cave, and waterfall. A series of tracer tests were conducted to obtain tracer residence time distribution curves. These tracer curves were used for performing temporal moment analysis to estimate fluid flow characteristics and solute transport process of karst conduit models. The results demonstrate that the residence time distribution curves are mainly constrained to the length and path differences of auxiliary side conduits, the lengths and geometries of pools, and the lengths and numbers of waterfalls. Although the multi-peaked residence time distribution curve is the basis for judging the existence of multiple karst conduits with path differences, the number of paths cannot be determined directly. Moreover, it can be presumed that there are some caves or waterfalls when the residence time distribution curve has a long tail. This study provided some useful references for the development of solute transport theory in karst conduits.
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