D2O-Na24 Method for Tracing Soil Moisture Movement in the Field1

C. C. HASKELL AND R. H. HAWKiNS2

ABSTRACT

A unique method is described for measuring unsaturatedflow of water in the field with D2O tracer, a Na24 gammasource, and a slow-neutron detector. The interaction ofgamma radiation of energy greater than 2.23 Mev. anddeuterium produces photoneutrons. The Na24 gammasource and the slow-neutron detector are contained in amodified neutron-type soil moisture probe. The methodsuccessfully measured vertical movement of deuteriumtracer in a Gilead sandy loam with the aid of access tubes.

THE STUDY OF SOIL MOISTURE movement has shifted in-creasingly from the field to the laboratory in recent

years partly because of the inadequacy of existing fieldmethods. In the more familiar field methods, soil moisturemovement is inferred from changes in soil moisture contentas measured by neutron scattering (1) or tensiometers(8,11). A suitable tracer method would have definite ad-vantages over these indirect methods.

Heavy water (D2O) has been used effectively as a watertracer in moisture diffusion studies (4,10). Moreover, withits molecular weight of 20, D2O differs only slightly fromtritium oxide (mol. wt. 22) which has been reported asa satisfactory tracer for mass flow of ground water (2,3).

A nuclear reaction not previously applied in soils studiesis the production of photoneutrons from the interactionof high energy gamma radiation with deuterium in D2O(9). This paper describes a unique field method for observ-ing movement of deuterium tracer through soil by meansof a modified neutron-type soil moisture probe containinga high-energy gamma source.

METHOD AND EQUIPMENT

The method is based on the reaction: iH2 + y (>2.23 Mev.)-> iH1 + on1. Gamma radiation of 2.75 Mev. is emitted by aNaM gamma source contained, along with a slow-neutron detec-tor, in a modified Nuclear-Chicago Model P-19 neutron-typedepth moisture probe. When the probe is lowered into anaccess tube in the soil, the high-energy gamma radiation inter-acts with deuterium in D-O previously injected in the soil asa moisture tracer. The nonradioactive deuterium emits photo-neutrons which undergo scattering and energy loss in the soil.Some of these photoneutrons return to the detector as slowneutrons and are recorded as counts on the attached sealer.The count rate is proportional to the amount and distributionof deuterium tracer within the gamma field.

The depth moisture probe was modified by removing theouter metal shell containing the Ra-Be neutron source fromthe detector preamplifier. This shell was replaced with a poly-styrene shell containing 232 g. of solid reagent-grade NaOHin a sealed hollow cylinder (Fig. 1 and 2). The polystyreneshell was attached to the detector-preamplifier unit with theadapter shown, permitting rapid attachment or removal bymeans of a partial turn and snap-lock.

Contribution from Savannah River Plant, E. I. duPont deNemours and Company, Aiken, South Carolina. Information inthis article was developed during the course of work underContract AT(07-2)-l with the U.S. Atomic Energy Commission.Presented before Div. S-l, Soil. Sci. Soc. Am., Nov. 18, 1963,at Denver, Colo. Received Dec. 9, 1963. Approved May 25,1964.

2Engineer and Chemist, Health Physics Section, E. I. du-Pont de Nemours & Company, Aiken, S.C. The senior author isnow Engineer, Glidden Paint Co., Jacksonville, Fla.

Fig. 1—Disassembled Na24 probe.

Annulus Filled withPrecast NaOH Filler

1-1/8"ID X 1-1 2"OD

2-3 8" ID X 2-3, 4"OD

3/4" FPT, 1,2" Deep

Fig. 2—Polystyrene shell assembly.

725

726 SOIL SCIENCE SOCIETY PROCEEDINGS 1964

The NaOH in the probe was activated by attaching a thread-ed rod to the base of the polystyrene shell and inserting theshell in an irradiation port of a nuclear reactor at the Savan-nah River Laboratory. The specific activity (me. Na24 per g.NaOH) induced in NaOH during irradiation is given by theequation:

S = (2.27 X 10-1(» [1 — exp(— 0.00077t)]

where:S = specific activity, me. Na24 per g. NaOH,(/> — thermal neutron flux, n per (cm2) (sec.), andt = irradiation time, minutes.

When the irradiated NaOH was removed from the reactor itcontained approximately 114 me. of Na24 (half life of 15 hours).With this amount of Na24, it was possible to obtain accuratedata throughout a 3-day period. Approximately 9 me. of Na24

remained on the afternoon of the third day. No appreciableradioactivity is induced in polystyrene and the NaOH in theshell can be reactivated as often as desired without buildingup long-lived sources of radioactivity.

To protect personnel using the Na24 probe from gammaradiation, a cylindrical lead shield housed the probe whenabove ground. The 90-pound shield, having a wall thicknessof 1 inch, was mounted on a hand truck so that it could bepositioned directly over an access tube in the field beforelowering the probe (Fig. 3). Radiation exposure to personnelusing the Na24 probe remained below 10 millirontgens (mR.)per 8-hour day.

In reporting data, correction was made for the radioactivedecay of Na24 and measurements were expressed as counts perminute per millicurie (counts per min. per me.) of Na24. Tofacilitate this a standard was calibrated as follows: Severalhours after the probe was removed from the reactor, gammaradiation intensity 3 feet from the Na24 was measured with aVictoreen R-Meter. Based on this measurement, and assuming

Fig. 3—Shield and Na24 probe positioned over access tube.

| 20

y, 10

20 30 40 50

Depth of probe, inches below surface

Fig. 4—Neutron profile obtained with the Na24 probe.

y, *28 °">'I \

I \

z O- 30

...-;•-\

/ \

Doys

1^44 Doys

/ \

InitialMaximum

Final (169 days)

40 50 60 70 80 90 100 110 120 130

Depth of probe, inches oelow surface

Fig. 5—Initial, maximum, and final tracer peaks in 3different plots.

HASKELL AND HAWKINS: D2O-NA24 METHOD FOR TBACING SOIL MOISTURE MOVEMENT 727

2.15 mR. per hr. per me. 3 feet from the unshielded Na24

source (7), the amount of Na24 in the probe was calculated tobe 90 me. The probe was then placed in a double-wall coppercylinder whose annular space contained a sealed volume of600 g. D2O (99.78% D2O by weight was used throughout thestudy). This yielded 12,150 counts per min. on the sealer, or135 counts per min. per me. Na24. Using the cylinder of D2Oas a calibrated standard, with its factor of 135 counts per min.per me., the me. content of Na24 was determined followingeach subsequent irradiation of the NaOH. The amount ofNa24 remaining at each 15-minute period of probe use wascorrected for radioactive decay.

The background count associated with the D2O-Na24 methodwas due to two sources. The first was gamma radiation fromthe Na24 itself. Input sensitivity of the detector was such thata gamma exposure rate of greater than 50 R per hr. at thedetector tube produced a detectable background count. Radi-ation intensity at the detector tube was always greater than50 R. per hr. for practical working quantities of Na24, andthe background due to gamma radiation averaged 0.08 countsper min. per me. A second source of background was thenatural occurrence of deuterium in H2O (0.0148 mol. %)resulting in a background which varied in proportion to soilmoisture content. During this study the total background variedfrom 0.4 to 0.6 counts per min. per me., while tracer peaksyielded up to 30 counts per min. per me. Measurements above6.5 counts per min. per me. had less than 5% statistical errorwith 90% confidence (5).

RESULTS AND DISCUSSIONPreliminary Test

A preliminary test was conducted to determine thefeasibility of detecting deuterium tracer in the vicinity ofan access tube by the D2O-Na24 method. To maintain pre-cise spacing, four 125-ml. polyethylene bottles each con-taining 110 ml. of D2O were attached to "Plexiglas" armsextending outward 3 inches from the wall of a 3.5-inch-diameter aluminum access tube. The assembly was placedupright in a large pile of medium sand (5% moisturecontent) and covered to near its top. The midpoint of theD2O was located 36 inches below the top of the 6-foot-long access tube.

Counts of 2 minutes duration were made with the Na24

probe in the access tube at 1-inch vertical intervals. Back-ground was counted at the access tube under the sameconditions with no D2O present. Results were correctedfor background and are shown in Fig. 4.

Results of this preliminary test with the D2O-Na24

method, indicated good sensitivity for determining thevertical location of the tracer. The plotted counts per min.per me. versus depth showed a sharp maximum at 36inches, the depth of the D2O. The tracer in this test wasconfined in four small bottles only 3 inches from theaccess tube. The effect of this concentration may have

Days

Fig. 6—Downward movement of tracer peaks versus rainfall and time at 3 plots.

728 SOIL SCIENCE SOCIETY PROCEEDINGS 1964

caused a sharper and higher maximum than was seen inthe subsequent field test.

Field TestA field test with the D2O-Na24 method was made under

conditions of natural rainfall and evaporation on a devege-tated Gilead sandy loam. Soil texture graded to a sandyclay at 12 inches, with distinct mottling below 20 inches.The surface slope averaged 2%. Vegetation since 1952was primarily bermudagrass, which was removed fromthe area 8 months before the test was begun.

The test was conducted between February 19 andAugust 7, 1963 on three plots, each 6 feet by 6 feet,located within a 1/4-acre test area. An aluminum accesstube 2.875-inches i.d. and 0.063-inch wall thickness wasinstalled to a depth of 20 feet in a 4-inch-diameter drilledhole in the center of each plot. Soil was carefully packedaround the access tubes, and each plot was surroundedwith a soil berm 6 inches high.

At each plot, four equally spaced 1.5-inch-diameterholes were augered parallel to and 3 inches from theaccess tube to a depth of 4 feet. The D2O tracer (110 ml.D2O) was placed at the bottom of each hole by means ofa plastic tube. The holes were backfilled and carefullypacked with the soil taken from them.

Weekly measurements were made with the Na24 probeat 2-inch vertical intervals on each plot. Initial and finaltracer peaks and maximum counts per min. per me. at eachplot are shown in Fig. 5. Downward movement of thetracer peak at each plot, and the cumulative rainfall, areshown in Fig. 6.

Two aspects of the field test merit discussion. First, thepronounced decrease in amplitude (counts per min. per me.)and increase in width of the tracer peaks during the courseof the field test, seen in Fig. 5, are attributed to the com-bined effects of tracer diffusion and mixing. (6) Maximumchange between initial and final tracer peaks was observedat plot 3, where total downward movement was approxi-mately twice that of plots 1 and 2 during the 169 days ofthe test. This indicates mixing may have been somewhatmore important than diffusion in accounting for tracerpeak degeneration.

Secondly, Fig. 6 shows the total downward movementof the tracer peaks at plots 1 and 2 was quite similar,averaging 0.94 inch per inch of rainfall; at plot 3 down-ward movement of the tracer was more rapid, averaging1.99 inches per inch of rainfall. A probable explanationfor this difference is that plot 3 was located on the edgeof a former ditch, backfilled about 2 years earlier. Althoughsoil physical properties were not measured, the variationin density and structure between disturbed and undis-turbed soils might easily account for the different ratesof tracer movement that were observed.

A phenomenon not possible in the preliminary test withthe confined D2O was observed in the field test. This wasthe apparent lateral movement of tracer toward the accesstube during the 28 to 44 days following tracer installation.

The result was a gradual increase in the peak height dur-ing this period to the maximum indicated in Fig. 5.

CONCLUSIONThe D2O-Na24 method is a reliable and practical means

for observing the mass movement and semiquantitativevertical distribution of D2O tracer in soil. To the degreethat D2O is a satisfactory water tracer, the method pro-vides a new and useful technique applicable to both fieldand laboratory studies of soil moisture.

Multiple placements of tracer at different depths abouta single access tube offer a means for studying soil mois-ture movement over a considerable vertical distance ata single site. Through the use of a horizontal access tubethe D2O-Na24 method appears practicable also for thestudy of predominantly lateral soil moisture movement.

Further development is needed before the D2O-Na24

method can be considered entirely quantitative for ratesof soil moisture movement.

An obvious improvement in the method would be thesubstitution of a long lived gamma source for Na24.Thallium208 has the necessary gamma energy (2.62 Mev.)and is present as a radioactive daughter of Th228 (half-life1.9 years) in aged, high level, radioactive waste. AlthoughTl2"8 has not yet been made available for this use, in-creased demand for an improved gamma source shouldhasten its availability.