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DIVISION S-1-SOIL PHYSICS

Method for In Situ Field Calibration of Fiber Optic Miniprobes

Division of Ecosystem Sci., Dep. of Environmental Science, Policy, and Management, Univ. of California, Berkeley, CA 94720-3110 USA

ghodrati{at}nature.berkeley.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion Conclusions REFERENCES

 
A fiber optic miniprobe (FOMP) system, based on remote fluorometry, has been recently developed for in situ real-time measurement of solute transport processes in soil. In order for the FOMP output light intensity measurements to be converted to fluorescent tracer concentration, an in situ calibration is necessary for each probe. The conventional calibration method consists of leaching the entire soil unit of interest with several pore volumes of tracer solution for each calibration step. The steady signal measured by the probes at each step is then related to tracer concentration. This procedure works well for typical laboratory soil columns but is impractical for fiber optic calibration in longer soil columns or field conditions because of the large amount of time and tracer required. As a result, a "point" calibration method has been developed that consists of injecting standard solutions directly into the small soil volume surrounding the measurement tip of the probe. Comparisons of the conventional leaching method with the injection calibration techniques confirmed the new procedures work well in both silica sand and clay loam soil columns. As a result, the new calibration technique was tested in a heterogeneous field soil and provided a simple and accurate approach to calibrating FOMPs for in situ real-time measurement of solute transport processes in the field.

Abbreviations: BTC, breakthrough curve • FOMP, fiber optic mini-probe

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion Conclusions REFERENCES

 
BASED ON THE PRINCIPLES of remote fiber optic fluorometry, a new methodology has been developed to provide in situ real-time measurements of solute transport processes in soil (Ghodrati, 1999). The system consists of a constant beam of light passing through the input leg of a bifurcated FOMP to a location of interest within the soil matrix. At the probe's tip, the incoming light interacts with the soil matrix, where it is partially absorbed and partially reflected back into the probe. The reflected signal is transmitted through the output leg of the fiber optic cable to a photodetector and quantified. At steady state the output light intensity is constant and then changes when a plume of fluorescent tracer passes the probe. Since the output signal is related to both the tracer concentration and the overall reflective properties of the soil matrix, an in situ calibration procedure is necessary for each probe to define the relationship between output light intensity and tracer concentration.

The conventional method to calibrate FOMPs consists of leaching the entire soil unit of interest with several pore volumes of a tracer at a few different concentrations and measuring the resulting steady signals. This procedure has been used in numerous laboratory studies to calibrate fiber optic sensors and measure solute transport processes in glass beads and repacked soil columns (Kulp et al., 1988; Nielsen et al., 1991; Ghodrati, 1999; Campbell et al., 1999). This procedure, which works well for laboratory soil column studies, is impractical in long soil columns and field applications because of the large amount of time and tracer required.

Calibration procedures for other fiber optic probes have been described for pH measurements of clay interstitial water (Montellier et al., 1995), hydrocarbon concentration in water (Lawford et al., 1990; Apitz et al., 1992), Cl^- concentrations in soil columns (Consentino et al., 1995), and even suspended sediment in breaking ocean waves (Beach et al., 1992). In most cases, these calibration procedures were performed in the laboratory before the sensors were applied elsewhere. However, laboratory calibrations are often not accurate for field applications, particularly when applied to data from heterogeneous soils. For example, Theriault et al. (1998) discussed the effect of point-to-point variations of the particle-size distribution and water content on field measurements of contaminant (cations) concentrations. The authors stressed the necessity for in situ sensor calibration for all measurements performed in soil. For example, time domain reflectometry probes require an in situ calibration procedure for both soil electrical conductivity and water content measurements (Dirksen and Dasberg, 1993; Campbell et al., 1999).

We have developed a localized injection procedure for site-specific calibration of FOMPs. This approach allows in situ, point, and fast calibration, without requiring a significant volume of tracer. The performance of the injection procedure is compared with the conventional leaching calibration method in silica sand and a clay loam soil and then tested in a heterogeneous field soil.

	Material and methods

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion Conclusions REFERENCES

 
Fiber Optic Miniprobe System
The FOMP system used in this study has been developed based on remote fiber optic fluorometry techniques (Krohn, 1988) and is described in detail in Ghodrati (1999). A general schematic of the system may be seen in Fig. 1A . The common end of a bifurcated fiber optic bundle encased in a stainless steel cylindrical tube (3 and 5 mm o.d. in this study) constitutes the fiber optic miniprobe. The measurement arena in front of each probe consists of a disc-shaped soil volume with an approximate thickness of 1 to 3 mm (Ghodrati, 1999). These probes have a measurement volume on the order of 10 mm³.

View larger version (28K): [in this window] [in a new window]   Fig. 1 (A) Schematic of the fiber optic miniprobe (FOMP) measurement system, (B) a diagram of the jacket point tracer injection method used in silica sand, and (C) the external tube point tracer injection method used in clay loam soil

External Jacket
This consists of a 6-mm-o.d. stainless steel sheath that the FOMP snugly fits inside (Fig. 1B). About 1 mm away from the beveled outlet edge a number of holes were drilled homogeneously spread all around the tube surface to allow solution to exit the sheath. On the other end (the inlet) an injection tube was attached to allow the solution to enter the sheath. An o-ring was placed at the inlet end between the FOMP body and the calibration tube to stabilize the FOMP within the sheath and to prevent the solution from leaking out. The solution travels freely in the empty volume between the outside wall of the probe and the inside wall of the calibration tube. With this device, the cross-sectional shape of the FOMP does not change, although its diameter increases slightly. This increase in the probe size could be a limitation in those experiments where the soil sample volume is especially small.

External Tube
A 1.8-mm-o.d. 1.3-mm-i.d. stainless steel tube was attached on the outside of the FOMP with heat shrink plastic tubing (Fig. 1C). The inlet side of the tube was connected to either a peristaltic pump or simply to a syringe for tracer injection. This calibration device changed the cross-sectional cylindrical shape of the probe slightly, but not the total size of the probe. The change in probe shape was not found to produce any problems for probe installation.

Neither of these modifications altered the normal functioning of the fiber optic system itself. In both cases, the calibration system and the FOMP were installed as a solid unit, resulting in no extra installation problems. In order to minimize the impact of the injection process on the probe's measurement volume, a distance of 3 mm was kept between the outlet of the injection device and the probe's tip (Fig. 2 and 3) .

View larger version (31K): [in this window] [in a new window]   Fig. 2 (A, D) Step calibration data for the conventional calibration procedure, (B, E) point injection calibration, and (C, F) the associated calibration curves for replicated experiments in silica sand

View larger version (37K): [in this window] [in a new window]   Fig. 3 (A, D, G) Step calibration data for the conventional calibration procedure, (B, E, H) point injection calibration, and (C, F, I) the associated calibration curves for replicated experiments in clay loam soil

The soil columns were leached with 6 mM CaCl₂ solution at steady fluxes of 25.2 and 1.2 cm h^-1 for silica sand and clay loam soil, respectively (80% of their corresponding saturated hydraulic conductivity) using a multichannel drip irrigation system connected to a peristaltic pump. Two different tracers were used in this study, uranine in silica sand and pyranine in the clay loam soil, as suggested by Ghodrati (1999).

Conventional Calibration Procedure
Once at the steady-state flux, a calibration using the conventional leaching procedure was performed. The procedure consisted of consecutively applying tracer solutions of known concentrations through the surface irrigation system to the column until the tracer solution had completely displaced the leaching solution and a steady signal was reached (2–5 pore volumes each step). The process was repeated for four to five different concentration steps and the corresponding steady light intensity values were used to correlate concentration to light intensity. Once the calibration step was finished, the entire column was leached with 6 mM CaCl₂ until the dye had been completely flushed from the column. Light intensity values were normalized against the corresponding steady-state background intensity signal (i.e., no tracer is in the matrix). Uranine was applied to the silica sand column in concentrations of 10, 20, 40, 80, and 160 mg L^-1, and pyranine was used for the clay loam soil columns in concentrations of 250, 500, 1000, and 2000 mg L^-1.

Injection Calibration Procedure
The injection procedure started from steady-state water content and constant flux conditions in the column producing a constant output light intensity. Then, 1 to 2 mL of solution of lowest concentration was injected into the calibration tube with a syringe. After the entire volume was injected, the output light signal was allowed to stabilize before proceeding to the next solution. In a similar manner as performed with the conventional method, the stabilized value of light intensity corresponding with known concentrations was used to calibrate the FOMP. To avoid pushing air bubbles into the soil surrounding the FOMP's tip, the tube was kept full of either background or tracer solution, leaving the syringe attached to the tube. The injection flow rate was kept approximately constant at 3 mL min^-1 for every front injected. After injecting the last tracer solution, 6 mM CaCl₂ solution was sent to wash the entire injection tube volume. Once the calibration step was finished, leaching with the irrigation system was started again.

Two independent repacked silica sand columns were used to test the external jacket calibration device and three more repacked clay loam soil columns to test the external tube. While the experiments were in process, output light intensities were recorded by the data logger each 30 s.

Two approaches were taken to compare the new injection calibration procedures with the conventional procedure. First, the calibration steps and resulting calibration curves for the injection methods and conventional leaching procedure were plotted together for qualitative comparison. The second approach was the comparisons of the mass balances for breakthrough curves (BTCs) measured with the probes for both procedures. Calculating the mass balance depends on the concentration values, so if the calibration procedures produced significantly different mass balances, then techniques would not be successful. Procedures for measuring a BTC are discussed below.

Miscible Displacement
In order to fully examine the difference between the conventional and injection systems to calibrate the FOMP, a series of miscible displacement studies was performed in each column following calibration procedures. Once the soil column was completely washed, a 1-cm pulse (i.e., tracer pulse volume = column surface area x 1 cm; 160 mgL^-1 in the case of uranine and 2000 mg L^-1 for pyranine) of the same fluorescence tracer was applied to the soil surface using the same irrigation system and then leaching resumed until all the tracer passed the FOMP. For each miscible displacement study, a corresponding BTC was constructed by plotting normalized concentration vs. normalized travel time (i.e., pore volumes) to calculate the mass balance resulting from using both conventional and injection calibration methods.

Field Testing
Once the applicability of the calibration procedures was established in the laboratory, a field test of the point calibration technique was performed in a Botella clay loam at the University of California Russell Reservation near Lafayette, California. There was clear evidence of soil macropores from macrofauna and cracks at the soil surface, demonstrating the heterogeneous natural structure of this soil. A drip irrigation system was used to wet the soil with 6 mM CaCl₂ solution to a steady state near saturation. Two FOMPs with the external calibration tube were inserted vertically into the wet soil at depths of 10 and 20 cm. Steps of pyranine tracer (5–7 mL) at five concentrations (125, 250, 500, 1000, and 2000 mg L^-1) were added through the external tube following the procedures developed in the laboratory. A calibration curve was developed for each probe. The irrigation was started again and after flushing the injected tracers with 6 mM CaCl₂ solution a 2-cm pulse of 4000 mg L^-1 pyranine tracer was added to measure a miscible displacement.

	Results and discussion

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion Conclusions REFERENCES

 
The alternative calibration procedures were generally successful, while some important considerations and limitations must be discussed. Neither of the modifications derived from the injection devices altered the normal functioning of the fiber optic system. However, one limitation of these devices is the potential clogging of the tracer delivery holes during installation into the soil. Although this problem did not happen during these experiments, it is reasonable to think that it might happen, especially in clayey soils. Another consideration is that the external jacket adds extra rigidity to the FOMP, whereas the heat shrink plastic tube might break in field applications. However, both air gaps and the injection flow rate are easier to control using the external tube than the external jacket.

Calibration curves from both the conventional and injection systems performed in silica sand are shown in Fig. 2. Concentration steps from the conventional procedure, on the left of Fig. 2, are reasonably similar to those produced by the injection procedure. These experiments performed in silica sand produced an excellent match in the first four steps (concentrations from 10 to 80 mg L^-1) between the calibration curves from both the conventional and the injection systems. The conventional calibration curves and calibration curves from the injection system are also shown on the right of the figure and are, speaking qualitatively, the same. However, the external jacket did not perform as well in a clay loam soil, possibly because the lower injection rate coupled with a longer and relatively more tortuous path in the sheath system resulted in sloppy calibrations without crisp concentration step transitions.

The external tube calibration device was tested in three independent repacked clay loam columns. Fronts of 250, 500, 1000, and 2000 mgL^-1 pyranine concentration from the laboratory in clay loam soil columns are shown in Fig. 3. In this case, this second injection device produced delineated concentration steps due to a more uniform and relatively smaller transport pathway that was more like "piston" flow with little internal mixing. The correlation coefficients (r²) of all the calibration curves are listed on the figures and are all >0.99. The conventional calibration procedure took from 600 to 1200 min (i.e., 10–20 h) depending on the soil type. Each calibration step required 200 mL of tracer solution. One can easily see how fast the injection procedure was relative to the conventional method; none of the experiments needed more than 30 min and only 1 to 2 mL of tracer solution per step. Each conventional calibration curve required 800 mL of the tracer solutions, whereas the point calibrations required 8 mL.

This difference is even more important in the field soil as the amount of time and tracer volume for the new injection calibration is independent of the probe's depth. Using the injection procedure to calibrate a probe placed at the 10-cm depth would require the same amount of tracer as a probe 20 cm deep. However, in the field the amount of tracer and time necessary for the conventional calibration steps is depth dependent. Applying a pore volume of solution at the field irrigation rate of 2.5 cm h^-1 would require on the order of 2 h, and increases to 4 h for the 20-cm depth. As we use four pore volumes of each tracer concentration for each calibration step (usually 4–5 steps), the depth dependence requires a large amount of time and tracer. Using the point injection calibration in the field took 15 to 20 min and only 30 to 50 mL of tracer.

The fact that the output light signal returns to the same background after the calibration procedure demonstrates the injection process by itself does not modify the soil particle arrangement in front of the probe. Comparison of the conventional leaching method with the injection methods did reveal that the calibration curve for the conventional method is slightly shifted to the right at higher tracer concentrations (i.e., greater light intensity corresponds to a given concentration). It may actually be that the leaching solution within the small soil volume in front of the probe is not fully displaced with the dye solution using the injection leaching procedure producing lesser output light intensity, while the conventional procedure produces a more complete displacement. Probably, with a higher injection flow rate or a larger volume of dye solution, this disagreement between both calibration methods would have been lessened.

The mass balances for all the BTCs corresponding with the miscible displacement experiments performed in both the silica sand and the clay loam soil calculated by means of the conventional and injection systems are shown in Table 1 . Taking the mass balance from the conventional method, the relative difference between both calibration procedures are 1 and 3% for both silica sand column miscible displacement studies.

View larger version (28K): [in this window] [in a new window]   Fig. 4 Breakthrough curves (BTCs) of normalized concentration from the conventional and point injection methods in (A) silica sand and (B) clay loam soil

View larger version (27K): [in this window] [in a new window]   Fig. 5 (A) Step calibration, (B) calibration curve, and (C) measured BTCs from FOMP 1 and the (D) Step calibration, (E) calibration curve, and (F) measured BTCs from fiber optic miniprobe (FOMP) 2 using the point injection method in the field clay loam soil

	Conclusions

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion Conclusions REFERENCES

The external calibration procedure was applied to a field soil to determine if a calibration curve could be constructed and a miscible displacement measured by the FOMPs. Both calibration and measurements of solute transport were successful in the field. As a result, these studies have shown that the external localized injection calibration provides results that enhance the applicability of the FOMP system to field and long soil column experiments.

	ACKNOWLEDGMENTS

 
The authors wish to thank the Ministerio de Educacion y Cultura (Spain) and the Kearney Foundation of Soil Science for their support of this research.

Received for publication April 12, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION Material and methods Results and discussion Conclusions REFERENCES

 

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