Single Well Tracer Tests in Aquifer Characterization

When the purpose of aquifer testing is to yield data for modeling aqueous mass transport, pumping tests and gradient measurement can only partially satisfy characterization requirements. Effective porosity, ground water flow velocity, and the vertical distribution of hydraulic conductivity within the aquifer are left as unknowns. Single well tracer methods, when added to the testing program, can be used to estimate these parameters. A drift, and pumpback test yields porosity and velocity, and point-dilution testing yields depth-discrete hydraulic information, A single emplacement of tracer into a test well is sufficient to conduct both tests. The tracer tests are facilitated by a simple method for injecting and evenly distributing the tracer solution into a wellbore, and by new ion-selective electrode instrumentation, specifically designed for submersible service, for monitoring the concentration of tracers such as bromide.     

Comparison of Hydraulic Conductivity Values Obtained from Aquifer Pumping Tests and Conservative Tracer Tests

Afield site was established in an area of glacial outwash near Des Moines, Iowa. Hydraulic conductivity (K) of the outwash was measured in various ways including six pumping tests and two natural-gradient Cl^- tracer tests. The velocity of the conservative tracer was converted to K using measured gradients and effective porosity determined from two radial-convergent Cl^- tracer tests.
K values measured from the conservative tracer tests are approximately one-tenth to one-twentieth the average pumping-test value. Thus the K relevant to solute transport does not reflect the K measured by pumping tests. This discrepancy may be caused by the different scale and dimensionality of the two test types. Dispersion may prevent solutes from flowing exclusively within smaller high-conductivity paths which strongly affect the K measured by pumping tests.     

Measurement of LNAPL Flux Using Single‐Well Intermittent Mixing Tracer Dilution Tests

The stability of subsurface Light Nonaqueous Phase Liquids (LNAPLs) is a key factor driving expectations for remedial measures at LNAPL sites. The conventional approach to resolving LNAPL stability has been to apply Darcy's Equation. This paper explores an alternative approach wherein single‐well tracer dilution tests with intermittent mixing are used to resolve LNAPL stability. As a first step, an implicit solution for single‐well intermittent mixing tracer dilution tests is derived. This includes key assumptions and limits on the allowable time between intermittent mixing events. Second, single‐well tracer dilution tests with intermittent mixing are conducted under conditions of known LNAPL flux. This includes a laboratory sand tank study and two field tests at active LNAPL recovery wells. Results from the sand tank studies indicate that LNAPL fluxes in wells can be transformed into formation fluxes using corrections for (1) LNAPL thicknesses in the well and formation and (2) convergence of flow to the well. Using the apparent convergence factor from the sand tank experiment, the average error between the known and measured LNAPL fluxes is 4%. Results from the field studies show nearly identical known and measured LNAPL fluxes at one well. At the second well the measured fluxes appear to exceed the known value by a factor of two. Agreement between the known and measured LNAPL fluxes, within a factor of two, indicates that single‐well tracer dilution tests with intermittent mixing can be a viable means of resolving LNAPL stability.     

Estimation of Biodegradation Rates Using Respiration Tests During In Situ Bioremediation of Weathered Diesel NAPL

Respiration tests were carried out during a seven month bioremediation field trial to monitor biodegradation rates of weathered diesel non-aqueous phase liquid (NAPL) contaminating a shallow sand aquifer. Multiple depth monitoring of oxygen concentrations and air-filled porosity were carried out in nutrient amended and nonamended locations to assess the variability of degradation rate estimates calculated from respiration tests.
The field trial consisted of periodic addition of nutrients (nitrogen and phosphorus) and aeration of a 100 m² trial plot. During the bioremediation trial, aeration was stopped periodically, and decreases in gaseous oxygen concentrations were logged semi-continuously using data loggers attached to recently developed in situ oxygen probes placed at multiple depths above and within a thin NAPL-contaminated zone. Oxygen usage rate coefficients were determined by fitting zero-and first-order rate equations to the oxygen concentration reduction curves, although only zero-order rates were used to calculate biodegradation rates. Air-filled porosity estimates were found to vary by up to a factor of two between sites and at different times.
NAPL degradation rates calculated from measured air-filled porosity and oxygen usage rate coefficients ranged up to 69 mg kg^-1 day^-1. These rates are comparable to and higher than rates quoted in other studies, despite the high concentrations and weathered state of the NAPL at this test site. For nutrient-amended sites within the trial plot, estimates of NAPL degradation rates were two to three times higher than estimates from nonamended sites. Rates also increased with depth.     

In Situ Measurement and Numerical Simulation of Oxygen Limited Biotransformation

Enhanced subsurface biorestoration is rapidly becoming recognized as a valuable tool for the restoration of hydrocarbon-contaminated aquifers and sediments. Previous field and laboratory studies at a former wood creosoting facility near Conroe, Texas, have indicated that insufficient oxygen is the primary factor limiting the biotransformation of polynuclear aromatics (PNAs) in sediments and ground water at this site. A series of laboratory experiments and field push-pull injection tests were performed as part of this project to: (1) study the effect of low oxygen concentrations on the biotransformation of PNAs; (2) identify the minimum concentration of PNAs that could be achieved through the addition of oxygen alone; (3) confirm that enhanced subsurface biorestoration is feasible at this site; and (4) test an existing numerical model of the biotransformation process (BIOPLUME). The laboratory studies demonstrated that biotransformation of the PNAs was not inhibited at dissolved oxygen concentrations as low as 0.7 mg/L although this work did suggest that there may be a minimum PNA concentration of 30 to 70 μg/L total PNAs below which biotransformation was inhibited. The field push-pull tests confirmed that addition of oxygen was effective in enhancing the subsurface biodegradation of the PNAs. The minimum concentration achieved using oxygen alone was approximately 60 μg/L total PNAs. Minimal biotransformation of these compounds was observed without oxygen addition. The numerical model BIOPLUME was tested against monitoring data from the field experiments and appears to provide a good approximation of the biodegradation process.     

A Method for Simultaneous In Situ Measurement in the Vadose Zone of Field-Saturated Hydraulic Conductivity, Sorptivity and the Conductivity-Pressure Head Relationship

The Guelph Permeameter (GP) method for simultaneous, in situ measurement in the vadose zone of field-saturated hydraulic conductivity (K_fS), sorptivity (S) and the conductivity-pressure head relationship [K(Ψ)] is described and discussed.
The method involves measuring the steady-state liquid recharge, Q, necessary to maintain a constant depth of water, H, in an uncased, cylindrical well of radius, a, finished above the water table. An "in-hole" Mariotte bottle device is used to maintain H and to measure Q.
Step-by-step procedures with example calculations are given for obtaining K_fS, S and K( Ψ ). Techniques for assessing the results are also given.
Although detailed field verification of the GP method is still incomplete, the studies conducted so far are quite encouraging.
Theoretical and practical advantages afforded by the GP method make it a desirable and cost-effective means for in situ measurement of K_fs, S and K( Ψ ), which are three of the most important parameters governing the flow of water and other wetting liquids in the vadose zone.     

Characterization of High-K Pathways by Borehole Flowmeter and Tracer Tests

This paper reviews different borehole flowmeter analysis methods and evaluates their applicability to a test site composed of fluvial deposits. Results from tracer and aquifer tests indicate that the aquifer is highly heterogeneous and that low-K skin effects exist at the wells. Borehole flowmeter tests were performed at 37 wells. An appropriate method for calculating borehole flowmeter K values was developed based on results from multiwell pumping tests, single-well pumping tests, and slug tests. The flowmeter data produced 881 K values. The trends and the magnitude of the K values are consistent with results from geologic investigations, recirculating tracer tests, and large-scale multiwell pumping tests. The field tests illustrate that high-K deposits can significantly affect ground-water flows in some heterogeneous fluvial aquifers.