Push-pull Tracer Tests Research Articles

Best practices for characterization of High Temperature-Aquifer Thermal Energy Storage (HT-ATES) potential using well tests in Berlin (Germany) as an example

Application of Aquifer Thermal Energy Storage with High Temperatures (HT-ATES) ranging from 60–90 °C is a promising technique to store large amounts of energy in urban areas. However, these areas typically lack information on hydrogeological and thermal parameters of the subsurface to determine the potential for energy storage. Moreover, conventional exploration methods as pumping tests do not account for the variation in density caused by the high temperature gradients or changes in salinity as encountered in HT-ATES operation. The objective of this study is therefore to develop best practices for characterizing the hydrogeological and thermal properties of groundwater wells and their surrounding formation that determine the potential performance of HT-ATES-systems. In addition to conventional pumping tests, a set of Push–Pull tracer Tests (PPTs) with cold and hot water are proposed and scrutinized using Berlin as case study. There, the research well Gt BChb 1/2015, which is characterized by a reservoir temperature of 17 °C at a depth between 220 und 230 m below ground surface was tested.In 2017, seven Slug-Withdrawal Tests (SWTs), a Step-Rate-Test (SRT), a production tests, and two Push–Pull tracer Tests (PPTs) with hot and cold water were performed during a period of 40 days. These tests were accompanied by Distributed-Temperature-Sensing (DTS) monitoring. The temperature measurements provide indications of injection areas based on the warmback period during a PPT with 81 °C hot water.The determined aquifer transmissibility T=3.2×10−5m2/s, the related Productivity Index (PI=2.0m3/(h bar)), and maximum flow rates of about 5m3/h indicate that the aquifer has potential for HT-ATES. However, the PPT and the DTS monitoring revealed cross flow between the target aquifer and an overlying aquifer. Thus, a new well with a design avoiding cross flow is required to utilize the aquifer’s energy storage potential. A set of best practices for characterizing HT-ATES potential was derived from the experiences in this study.

Nitrate attenuation potential in karst conduits and aquifer matrix

A critical determinant of the effective denitrification potential in karst aquifers is the relative contribution of matrix and non-matrix (e.g., conduits) to the total groundwater flow. This work tests the hypothesis that observed karst aquifer-scale denitrification rates represent the superposition of the effects from matrix zones, which have low denitrification rates and long residence times, and non-matrix zones, which have higher denitrification rates and short residence times. To better evaluate karst whole-aquifer denitrification capacity, this study examined groundwater chemical data from 69 individual wells across the springshed of Silver Springs, FL, and conducted five push–pull tracer tests (PPTTs) in matrix and non-matrix aquifer sections. Mean residence times and mean zero-order denitrification rates (K0) were determined using reduced-complexity analytical models for matrix and non-matrix zones. Significant nitrate degradation at the local scale was observed in wells that sampled matrix portions of the aquifer (K0 ≈ 5 to 7 × 103 μmol N/L/d), indicating denitrification hotspots comparable to previous results employing the same PPTT method. However, PPTTs in matrix portions of the aquifer found no evidence of aquifer denitrification, likely due to the short residence times experienced during PPTTs. The analytical models coupled with the measured nitrate concentrations and excess N2 data in groundwater discharged at the spring vents showed whole-aquifer estimates of residence times tavg = 4.2 years and aquifer denitrification rate K0,avg = 0.01 ± 0.01 μmol N/L/d that were consistent with previous studies. However, the average denitrification rate for non-matrix zones K0,nm = 1.29 ± 0.92 μmol N/L/d was two orders of magnitude higher than that for matrix zones K0,m = 0.002 ± 0.001 μmol N/L/d. Whole-aquifer denitrification measures thus reflect the weighted contributions of low denitrification in non-matrix zones, which constitute the majority of the groundwater flow, and high-denitrification but low-flow matrix zones. The modeling approach with non-matrix networks from this study can provide insight into the catchment-scale N budget and transport in karst aquifers.

Biased Estimation of Groundwater Velocity from a Push-Pull Tracer Test Due to Plume Density and Pumping Rate

The single-well push-pull tracer test is a convenient and cost-effective tool to estimate hydrogeological properties of a subsurface aquifer system. However, it has a limitation that test results can be affected by various experimental designs. In this study, a series of laboratory-scale push-pull tracer tests were conducted under various conditions controlling input tracer density, pumping rate, drift time, and hydraulic gradient. Based on the laboratory test results, numerical simulations were performed to evaluate the effects of density-induced plume sinking and pumping rate on the proper estimation of groundwater background linear velocity. Laboratory tests and numerical simulations indicated that the actual linear velocity was underestimated for the higher concentration of the input tracer because solute travel distance and direction during drift time were dominantly affected by the plume density. During the pulling phase, reasonable pumping rates were needed to extract the majority of injected tracer mass to obtain a genuine center of mass time (tcom). This study presents a graph showing reasonable pumping rates for different combinations of plume density and background groundwater velocity. The results indicate that careful consideration must be given to the design and interpretation of push-pull tracer tests.

Long-term mass flux assessment of a DNAPL source area treated using bioremediation

This study assessed the long-term effectiveness of bioremediation as a remedial strategy for a chlorinated, ethene dense, non-aqueous phase liquid (DNAPL) source area, consisting of a higher- and a lower-permeability zone at Alameda Point, California. The evaluation was performed over 3.7 years after cessation of active source area bioremediation using passive flux meters (PFMs), push-pull tracer tests, and soil cores. PFMs showed that total chlorinated ethene molar discharge emanating from the source area remained relatively unchanged pre-and post-bioremediation, but molar discharge compositions shifted from trichloroethene (TCE) and cis-1,2-dichloroethene (cis-DCE) to vinyl chloride (VC) and ethene dominated during post-remedial monitoring. First-order rate constants, derived from PFM data at the edge of the source area and describing the complete dechlorination of TCE at 3.7 years following active bioremediation, were approximately 1.05 yr−1, which was over three times lower than the rate 3.6 yr−1 determined using compound stable isotope analysis (CSIA). Soil cores and push-pull tracer test data showed that DNAPL volume estimates were relatively unchanged pre- and post-bioremediation due to the remaining presence of DNAPL in the lower-permeability zone. These data suggest biotransformation processes are continuing in the higher-permeability zone, whereas DNAPL in the lower-permeability zone continues to serve as a significant source of groundwater contamination. The results suggest that it will take many years under current conditions to attain the United States Environmental Protection Agency (EPA) Maximum Contaminant Levels (MCLs) cleanup objectives.

Probabilistic inference of fracture-scale flow paths and aperture distribution from hydrogeophysically-monitored tracer tests

Fracture-scale heterogeneity plays an important role in driving dispersion, mixing and heat transfer in fractured rocks. Current approaches to characterize fracture scale flow and transport processes largely rely on indirect information based on the interpretation of tracer tests. Geophysical techniques used in parallel with tracer tests can offer time-lapse images indicative of the migration of electrically-conductive tracers away from the injection location. In this study, we present a methodology to invert time-lapse ground penetrating radar reflection monitoring data acquired during a push-pull tracer test to infer fracture-scale transport patterns and aperture distribution. We do this by using a probabilistic inversion based on a Markov chain Monte Carlo algorithm. After demonstration on a synthetic dataset, we apply the new inversion method to field data. Our main findings are that the marginal distribution of local fracture apertures is well resolved and that the field site is characterized by strong flow channeling, which is consistent with interpretations of heat tracer tests in the same injection fracture.

Nitrate reduction mechanisms and rates in an unconfined eogenetic karst aquifer in two sites with different redox potential

AbstractThis study integrates push‐pull tracer tests (PPTT) with microbial characterization of extracted water via quantitative polymerase chain reaction (qPCR) and reverse transcriptase qPCR (RT‐qPCR) of selected functional N transformation genes to quantify nitrate reduction mechanisms and rates in sites with different redox potential in a karst aquifer. PPTT treatments with nitrate (AN) and nitrate‐fumarate (ANC) were executed in two wells representing anoxic and oxic geochemical end‐members. Oxic aquifer zero‐order nitrate loss rates (mmol L−1 h−1) were similar for AN and ANC treatment, ranging from 0.03 ± 0.01 to 0.05 ± 0.01. Anoxic aquifer zero‐order nitrate loss rates ranged from 0.03 ± 0.02 (AN) to 0.13 ± 0.02 (ANC). Microbial characterization indicates mechanisms influencing these rates were dissimilatory nitrate reduction to ammonium (DNRA) at the anoxic site with AN treatment, assimilatory reduction of nitrate to ammonium (ANRA) with ANC treatment in the water column at both sites, and additional documented nitrate reduction that occurred in unsampled biofilms. With carbon treatment, total numbers of microbes (16S rRNA genes) significantly increased (fourteenfold to thirtyfold), supporting stimulated growth with resulting ANRA. Decreased DNRA gene concentrations (nrfA DNA) and increased DNRA activity ratio (nrfA‐cDNA/DNA) supported the assertion that DNRA occurred in the anoxic zone with AN and ANC treatment. Furthermore, decreased DNRA gene copy numbers at the anoxic site with ANC treatment suggests that DNRA microbes in the anoxic site are chemolithoautotrophic. Increased RT‐qPCR denitrification gene expression (nirK and nirS) was not observed in water samples, supporting that any observed NO3‐N loss due to denitrification may be occurring in unsampled microbial biofilms.

A Straightforward Random Walk Model for Fast Push-Pull Tracer Test Evaluation.

In this article, we present a straightforward random walk model for fast evaluation of push-pull tracer tests. By developing an adaptive algorithm, we overcome the problem of manually defining how many particles have to be used to simulate the transport problem. Beside this, we validate the random walk model by evaluating a push-pull tracer test with drift phase and confirm the results with MT3DMS. The random walk model took less than 1% of computational time of MT3DMS, thus allowing a remarkable faster evaluation of push-pull tracer tests.

Use of single-well push–pull tracer tests to simulate a dynamic saltwater–freshwater interface

Two single-well injection–extraction (“push–pull”) tracer tests were performed in a groundwater monitoring well located in the Horonobe coastal sedimentary basin, Japan. The aim of the experiment was to investigate the in situ behavior of groundwater constituents when different mineralized fluids intrude into the aquifer (e.g., through a dynamic saltwater–freshwater interface). Artificially produced brackish water was used as the test fluid in the first experiment and deionized water in the second. Results from mixing models strongly suggest rapid mobilization and demobilization of ions within the aquifer as a reaction to the intruding fluids. Hydraulic modeling of the conservative chloride ions yielded effective porosities of about 1 %. These results show that the transport of ions is dependent on the composition and mineralization of the fluid. The refreshening of an aquifer leads to increased mobilization of ions and, in addition, to an increase of dispersivity due to clay swelling. This has consequences not only for dynamic saltwater–freshwater systems (aquifers in coastal areas and aquifer storage and recovery systems), but also for all systems in which chemically altered fluids come into contact within an aquifer (as, for example, can occur during geothermal heat/energy generation, or during CO2 storage).

Dissolved gas dynamics in wetland soils: Root-mediated gas transfer kinetics determined via push-pull tracer tests

Gas transfer processes are fundamental to the biogeochemical and water quality functions of wetlands, yet there is limited knowledge of the rates and pathways of soil-atmosphere exchange for gases other than oxygen and methane (CH4). In this study, we use a novel push-pull technique with sulfur hexafluoride (SF6) and helium (He) as dissolved gas tracers to quantify the kinetics of root-mediated gas transfer, which is a critical efflux pathway for gases from wetland soils. This tracer approach disentangles the effects of physical transport from simultaneous reaction in saturated, vegetated wetland soils. We measured significant seasonal variation in first-order gas exchange rate constants, with smaller spatial variations between different soil depths and vegetation zones in a New Jersey tidal marsh. Gas transfer rates for most biogeochemical trace gases are expected to be bracketed by the rate constants for SF6 and He, which ranged from approximate to 10(-2) to 2 x 10(-1) h(-1) at our site. A modified Damkohler number analysis is used to evaluate the balance between biochemical reaction and root-driven gas exchange in governing the fate of environmental trace gases in rooted, anaerobic soils. This approach confirmed the importance of plant gas transport for CH4, and showed that root-driven transport may affect nitrous oxide (N2O) balances in settings where N2O reduction rates are slow.

Study of the effects of the chaser in push–pull tracer tests by using temporal moment analysis

“Push–pull” tracer tests are a suitable tracer test method for hydrochemical characterization of an aquifer in a single-well setting (e.g. in deep geothermal systems). A known amount of selected solutes as conservative and reactive tracers is injected into the aquifer (“push”) and afterwards extracted (“pull”). In many cases, a so-called “chaser”, which is just original groundwater without any added solutes, is injected directly after the injection of the test solution. Its objective is to push the test solution out of the borehole into the aquifer and therefore to minimize the influence of the gravel pack on the shape of the breakthrough curve. The influence of the chaser on the tracer breakthrough curve is unknown so far. Also, the determination of the appropriate volume for the chaser is a difficult task if at all applied. A first experiment was conducted with the objective to compare three push–pull tests with similar injection volumes, two tests with and one without a chaser. Results show that the application of a chaser lowers the main peak concentration. However, it does not alter the tailing of the breakthrough curve nor does it have a negative influence on tracer mass recovery. In a second experiment, a new method was developed to determine the optimal chaser volume by testing seven different chaser injection volumes combined with temporal moment analysis. As a result, the application of a chaser is recommended, when reactions of injected solutes within the open well or the gravel pack should be avoided. If a chaser is used, the new method mentioned above can easily be used to determine the required chaser injection volume. The experiments were conducted at the Hamasato test site in Horonobe (Hokkaido, Japan).

Enhanced aqueous dissolution of a DNAPL source to characterize the source strength function

Simplified analytical solutions, developed as source strength functions (SSFs), are capable of describing the temporal dissolution of nonaqueous phase liquids in groundwater, which is useful for predicting source longevity and can serve as a guide for remedial activities. Here, SSF parameters were estimated by fitting enhanced aqueous dissolution data from a flow cell consisting of three injection and four extraction wells to analytical dissolution models (power law model (PLM) and equilibrium streamtube model (EST)) at a trichloroethene (TCE) contaminated site, Alameda Point, California. Both the PLM and the EST model were able to characterize the observed aqueous TCE dissolution during enhanced water flooding. Additional field activities conducted at the site included soil core collection, a recirculated partitioning tracer test, passive flux meter transects, and push–pull tracer tests. The additional site characterization data were used to independently estimate the observed SSF parameters using information such as the TCE mass, distribution and porous media heterogeneity. The exponential decay model (a subset of the PLM) accurately predicted the enhanced dissolution, likely because the site was significantly aged (most of the mass in the plume rather than in the source zone) or middle stage, and the mass in the source zone could be approximately estimated. The EST tracer-based model, when combined with data from the recirculated partitioning tracer test, soil cores, and the push–pull tracer test, was capable of accurately predicting the observed aqueous dissolution. The mass in the source zone and the fraction of contaminated flowpaths were the most important site characteristics, requiring the greatest accuracy to predict aqueous dissolution. Establishing steady state dissolution was essential to provide a more accurate estimate of the fraction contaminated and high resolution data from soil cores in the source zone were needed to estimate the mass present.

Fluorescent dye imaging of the volume sampled by single well forced-gradient tracer tests evaluated in a laboratory-scale aquifer physical model

This study presents a new method to visualise forced-gradient tracer tests in 2-D using a laboratory-scale aquifer physical model. Experiments were designed to investigate the volume of aquifer sampled in vertical dipole flow tracer tests (DFTT) and push–pull tests (PPT), using a miniature monitoring well and straddle packer arrangement equipped with solute injection and recovery chambers. These tests have previously been used to estimate bulk aquifer hydraulic and transport properties for the evaluation of natural attenuation and other remediation approaches. Experiments were performed in a silica glass bead-filled box, using a fluorescent tracer (fluorescein) to deduce conservative solute transport paths. Digital images of fluorescein transport were captured under ultraviolet light and processed to analyse tracer plume geometry and obtain point-concentration breakthrough histories. Inorganic anion mixtures were also used to obtain conventional tracer breakthrough histories. Concentration data from the conservative tracer breakthrough curves was compared with the digital images and a well characterised numerical model. The results show that the peak tracer breakthrough response in dipole flow tracer tests samples a zone of aquifer close to the well screen, while the sampling volume of push–pull tests is limited by the length of the straddle packers used. The effective sampling volume of these single well forced-gradient tests in isotropic conditions can be estimated with simple equations. The experimental approach offers the opportunity to evaluate under controlled conditions the theoretical basis, design and performance of DFTTs and PPTs in porous media in relation to measured flow and transport properties. 2 2 DFTT = Dipole Flow Tracer Test, PPT = Push Pull Tracer Test.

Circadian methane oxidation in the root zone of rice plants

In the root zone of rice plants aerobic methanotrophic bacteria catalyze the oxidation of CH4 to CO2, thereby reducing CH4 emissions from paddy soils to the atmosphere. However, methods for in situ quantification of microbial processes in paddy soils are scarce. Here we adapted the push–pull tracer-test (PPT) method to quantify CH4 oxidation in the root zone of potted rice plants. During a PPT, a test solution containing CH4 ± O2 as reactant(s), Cl− and Ar as nonreactive tracers, and BES as an inhibitor of CH4 production was injected into the root zone at different times throughout the circadian cycle (daytime, early nighttime, late nighttime). After a 2-h incubation phase, the test solution/pore-water mixture was extracted from the same location and rates of CH4 oxidation were calculated from the ratio of measured reactant and nonreactive tracer concentrations. In separate rice pots, O2 concentrations in the vicinity of rice roots were measured throughout the circadian cycle using a fiber-optic sensor. Results indicated highly variable CH4 oxidation rates following a circadian pattern. Mean rates at daytime and early nighttime varied from 62 up to 451 μmol l−1 h−1, whereas at late nighttime CH4 oxidation rates were low, ranging from 13 to 37 μmol l−1 h−1. Similarly, daytime O2 concentration in the vicinity of rice roots increased to up to 250% air saturation, while nighttime O2 concentration dropped to below detection (<0.15% air saturation). Our results suggest a functional link between root-zone CH4 oxidation and photosynthetic O2 supply.