Single Borehole Tracer (SBT) Logging

Basic Concept

Single-borehole tracer (SBT) methods measure the concentration of a tracer material within the borehole fluid over time and space. The concentration of tracer within the water column is most significantly influenced by the flow of groundwater that occurs naturally or is induced within the borehole. The most common SBT methods employ and monitor dye-type tracers to estimate the velocities of the horizontal and/or vertical components of groundwater flow through a borehole.

SBT methods can estimate the vertical velocity component by monitoring the displacement of tracer concentration peaks throughout a borehole. Though, the novelty of SBT is its use in estimating the horizontal component of flow velocity, which is otherwise difficult or impossible to detect. Measuring the rate at which borehole inflow dilutes a tracer at several depths allows for the identification and differentiation of horizontal flow throughout the saturated borehole (Zhang and others, 2020).

Theory

Tracers are used to track groundwater movement by “labelling” the water at a location in a manner that allows that same water to be identified elsewhere. Tracers include naturally occurring and artificially introduced substances (e.g., radioisotopes, deionized water, dyes, contaminant spills, salts) that typically become dissolved or suspended within the water column. Because they are foreign (i.e. unlikely to occur naturally in a borehole environment), fluorescent dyes are a well-liked tracer and can provide highly reliable flow data.

Fluorescent dye tracers involve the absorption and conversion of exciting light energy into emitted light energy. Each dye has unique excitation and emission spectra, whereby the fluoresced energy typically has a longer wavelength/lower frequency than that of the exciting light. For a given sample solution, fluorescent light intensity is proportional to the amount of dye contained within it. Thus, measuring fluorescence allows for estimating dye concentration (Wilson and others, 1986).

Using a light source and photodetector, a borehole fluorometer induces fluorescence and measures the relative intensity of light emitted by the fluid within its measurement chamber. The measured signals are transmitted to the land surface via a cable that also allows the fluorometer to be suspended at known depths within the borehole. Because fluorescence is a relative value, raw data are calibrated to fluorescent solute concentration using standards or prepared solutions with known concentrations prior to logging (Flynn and others, 2005).

Many SBT methodologies have been successfully applied in the field for use in groundwater flow studies. In general, however, the data logger (i.e., fluorometer) collects timeseries concentration data after the tracer (i.e., dye) has been appropriately (with respect to method) introduced into the system. Depending on both the nature of the study and equipment, concentration data may be collected at a stationary position within or throughout a segment of the borehole.

Applications

Two of the most applied SBT methods are the dilution test and arrival test, which are often used to characterize horizontal and vertical flow, respectively. Both methods measure the tracer concentration repeatedly over a sufficient period of time such that concentration profiles are recorded to produce a sequential series of breakthrough curves. However, these two components are usually detected separately, and complications arise when vertical and horizontal flow both occur at a single point within a borehole (Zhang, 2020).

The dilution method can estimate horizontal-flow velocity and requires water moving freely through the borehole and its sidewalls (i.e., borehole is open or screened). The method is based on the idea that tracer-bearing water will become diluted with tracer-free water in zones of active horizontal flow (Flynn, 2005). Furthermore, there is a linear relationship between the natural logarithm of the tracer concentration over time and apparent (i.e. borehole-affected) filtration velocity (Pitrak, 2007; Ogilvi, 1958).

Ideal dilution test conditions involve a tracer that is evenly distributed within an isolated borehole segment through which there is no vertical-flow component. In such circumstances, the lowest tracer concentrations observed in the breakthrough curve will correspond to zones of active horizontal flow (Flynn, 2005). If flow is steady, the horizontal-groundwater velocity can be calculated using two measurements of tracer concentration at two times at a certain depth. If flow is unsteady, then velocity estimation requires further data analysis (Zhang, 2020).

Vertical flow will often occur in boreholes that intercept aquifers with different water heads and likely influence the horizontal flow information derived from a dilution test. Vertical flow is evidenced by the downward or upward movement of the tracer plume relative to the point at, or segment in, which it originated. Though employing tracer mixers and packers can improve dilution test accuracy by creating near-ideal conditions, neither are a guarantee (Zhang, 2020).

A tracer within vertical-flow zones will be advected by water entering and exiting the borehole and disperse through the segment across which the flow occurs. In contrast to the dilution test, tracer concentrations are greatest in zones of active vertical flow, which can be monitored by employing the tracer arrival test. In this case, tracer concentration peaks are identified and tracked to determine the direction and magnitude of vertical-flow gradients (Flynn, 2005).

The peak-arrival test involves injecting the tracer at a known depth and measuring the concentration over time either continuously or at a fixed separation interval. These data can be used to produce tracer concentration breakthrough curves for each depth increment within the length of measured borehole segment. The vertical flow velocity can be calculated using the distance between two peaking values of the tracer concentration along with the time elapsed between the peaks (Zhang, 2020).

Single-borehole tracer data, calculations, and interpretations are subject to interference, namely from field procedures and borehole environment. Such factors include multi-directional flow, molecular tracer-water interactions, sample homogenization by detector, sample collection mechanics, temperature, and borehole diameter. However, these may be negligible or compensated for by data collection or processing. In addition to their potential as alternatives to estimate groundwater velocity and ability to complement conventional flowmeter data, SBT methods have successfully aided the following:

Locating transmissive fractures/preferential flow zones
Assessing contaminant transport
Identifying/characterizing horizontal and vertical flow
Substituting traditional vertical flowmeter in fast-flowing systems
Determining volumetric flow rate and direction of vertical flow
Estimating hydraulic conductivity

Examples/Case studies

Brainerd, R.J., and Robbins, G.A., 2008, A Tracer Dilution Method for Fracture Characterization in Bedrock Wells: Groundwater, v. 42, no. 5, p. 774-780, doi:10.1111/j.1745-6584.2004.tb02731.x.

Abstract: This investigation was undertaken to develop an integrated method of downhole fracture characterization using a tracer. The method presented can be used to locate water‐bearing fractures that intersect the well, to determine the ambient fracture flow rate and hydraulic head, and to calculate fracture transmissivity. The method was tested in two fractured crystalline bedrock wells located at the University of Connecticut in Storrs. The method entails injecting a tracer (uranine dye) into the well, while at the same time water is pumped out of the well. After steady‐state conditions are reached, a borehole tracer concentration profile is developed. The dilution of the tracer is used to locate the inflowing fractures and to determine their flow rate. The fracture flow rate, plus the drawdown in the well, is then used to determine the fracture hydraulic head, transmissivity, and ambient flow rate.

Flynn, R.M., Schnegg, P., Costa, R., Mallen, G., and Zwahlen, F., 2005, Identification of zones of preferential groundwater tracer transport using a mobile downhole fluorometer: Hydrogeology Journal, v. 13, p. 366-377, doi:10.1007/s10040-004-0388-3.

Abstract: A mobile downhole fluorometer was used to detect zones of preferential groundwater tracer transport into an observation well. Identification of such zones is not possible if individual samples are collected over the well’s entire screened interval. Laboratory-based tests using the fluorometer, and a purpose-built apparatus demonstrated that the fluorometer could be used with tracers to characterise well water flow regimes. During field investigations in a porous aquifer, the fluorometer monitored tracer concentrations in an observation well with a 12-m-long screen, 10 m down the hydraulic gradient from a fully penetrating injection well. Test results showed that the tracer occurred in the observation well over a discrete 2.5-m-thick interval. Single-well dilution test and vertical-flow data indicated that water entered the well at additional depths, but no tracer was detected at these levels. A numerical model reproducing dilution test concentration profiles indicated that water entered the well in many of these horizons at comparable velocities to those in the tracer-bearing zone. These data suggest that groundwater flow direction varied with depth in the aquifer under investigation. Moreover, simulations of tracer arrival indicated that the tracer distribution observed in the observation well was derived from a horizon that may be no thicker than 0.5 m.

Harte, P.T., Anderson, J.A., Williams, J.H., and Fuller, A., 2014, Observations From Borehole Dilution Logging Experiments In Fractured Crystalline Rock Under Ambient And Pump Test Conditions, in Proceedings, 27th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems: Boston, Massachusetts, European Association of Geoscientists and Engineers, doi:10.3997/2214-4609-pdb.400.166.

Abstract: Identifying hydraulically active fractures in low permeability, crystalline-bedrock aquifers requires a variety of geophysical and hydrogeophysical borehole tools and approaches. One such approach is Single Borehole Dilution Tests (SBDT), which in some low flow cases have been shown to provide greater resolution of borehole flow than other logging procedures, such as vertical differential Heat Pulse Flowmeter (HPFM) logging. Because the tools used in SBDT collect continuous profiles of water quality or dye changes, they can identify horizontal flow zones and vertical flow. We used SBDT with a food grade blue dye as a tracer and dual photometer-nephelometer measurements to identify low flow zones. SBDT were conducted at seven wells with open boreholes (exceeding 300 ft). At most of the wells HPFM logs were also collected. The seven wells are set in low-permeability, fractured granite and gneiss rocks underlying a former tetrachloroeythylene (PCE) source area at the Savage Municipal Well Superfund site in Milford, NH. Time series SBDT logs were collected at each of the seven wells under three distinct hydraulic conditions: (1) ambient conditions prior to a pump test at an adjacent well, (2) mid test, after 2-3 days of the start of the pump test, and (3) at the end of the test, after 8-9 days of the pump test. None of the SBDT were conducted under pumping conditions in the logged well. For each condition, wells were initially passively spiked with blue dye once and subsequent time series measurements were made. Measurement accuracy and precision of the photometer tool is important in SBDT when attempting to detect low rates of borehole flow. Tests indicate that under ambient conditions, none of the wells had detectable flow as measured with HPFM logging. With SBDT, 4 of the 7 showed the presence of some very low flow. None of 5 (2 of the 7 wells initially logged with HPFM under ambient conditions were not re-logged) wells logged with the HPFM during the pump test had detectable flow. However, 3 of the 5 wells showed the patterns of very low flow with SBDT during the pump test including pumping induced changes of inflow and outflow patterns at one well.

Lewis, D.C., Kriz, G.J., and Burgy, R.H., 1996, Tracer dilution sampling technique to determine hydraulic conductivity of fractured rock: Water Resources Research: v. 2, no. 3, p. 533-542, doi:10.1029/WR002i003p00533.

Abstract: Groundwater in foothill and mountain watershed areas commonly occurs in fractured rock. The small well diameters and low apparent groundwater velocities in fractured rock require modification of normal techniques for the investigation of unconfined groundwater movement. The determination of hydraulic conductivity by the tracer dilution method normally employs injected radioisotope tracers. The dilution is determined by monitoring the isotope activity in the well with a scintillation probe. A modification of this method, using fluorescent dye tracers and physical sampling and analysis to determine dilution, has been applied in small wells with consistent results. Hydraulic conductivities of 0.02 to 0.5 ft/day have been determined in sixteen wells. Where comparison is possible, the values agree favorably with hydraulic conductivities determined by pumping tests.

Pitrak, M., Mares, S., and Kobr, M., 2007, A Simple Borehole Dilution Technique in Measuring Horizontal Ground Water Flow: Groundwater, v. 45, no. 1, p. 89-92, doi:10.1111/j.1745-6584.2006.00258.x.

Abstract: Borehole dilution techniques use repeated fluid column profiling after establishment of an initial uniform condition to monitor the rate at which ambient ground water moves into a borehole. Application of the dilution technique in a monitoring well makes it possible to estimate the horizontal Darcy flow velocity of ground water in the aquifer surrounding the borehole. Previous investigators have demonstrated the technique using either relatively concentrated saline solutions or deionized water to produce a fluid column with properties distinctly different from those of local ground water. We present a new dilution technique using the food color Brilliant Blue FCF (Euro code E‐133) to mark the fluid column and using a specially constructed photometric sensor to characterize the dilution of this dye over time. The effective application of this technique is documented by two practical examples.

Rollinson, J., Rees-White, T., Barker, J.A., and Beaven, R.P., 2010, A single borehole dilution technique to measure the hydrogeological properties of saturated landfilled waste, in Proceedings, Waste Conference: Stratford-upon-Avon, United Kingdom, University of Southampton Faculty of Engineering and the Environment, p. 105-112.

Abstract: A single borehole dilution test is a relatively simple hydrogeological technique used to determine the volumetric flow rate of groundwater through a borehole. The technique potentially provides a means to obtain hydrogeological properties without the need to undertake a pumping test, avoiding the logistical difficulties of such testing. The use of the technique in landfills has been explored by application in two landfills in south-east England, providing data to inform activities such as in-situ remediation. The tests used fluorescent dye tracers, rhodamine WT and sodium fluorescein, with concentrations monitored using submersible fluorimeters. Results from nine dilution tests gave average Darcy velocities (volumes per unit area per unit time) ranging over two orders of magnitude: 2.6 x 10-3 to 2.8 x 10-1 m/day. Hydraulic conductivities, inferred using estimated hydraulic gradients, ranged from 6.7 x 10-2 to 7.1 m/day (7.7 x 10-7 m/s to 8.3 x 10-5 m/s) and compared favourably with pumping test data. The tests revealed zones of preferential flow and zones of negligible flow.

Zhang, Y., Wang, H., Zhang, X., Dong, H., and Mao, C., 2020, Groundwater velocity determination by single-borehole dilution test: IOP Conference Series: Earth and Environmental Science, v. 525, 5 p., doi:10.1088/1755-1315/525/1/012175.

Abstract: The single-borehole dilution test involves the injection of a tracer into a borehole following by repeated water column profiling to monitor groundwater velocity within the vicinity of the borehole. Based on the concept that a tracer's concentration decreases as a consequence of the groundwater dilution, the groundwater velocity can be determined by analysing the measured tracer concentration curves over time. The borehole dilution test allows the determination of both horizontal and vertical velocities in a single well. The presence of vertical flow in a borehole may greatly affect the evaluation of horizontal velocity since the decline of tracer concentration is caused by both horizontal and vertical flow simultaneously. Two approaches presented provide the estimation method of horizontal velocity without using packers to prevent vertical flow.

References