Ground Water Quantity Measurement on the Foot of Mt. Fuji by the Use of Radioisotopes Toshlro OCHIAI and V. C. RODRIGUEZ* (Agricul. Eng. Res. Station, Ministry of Agriculture and Forestry, * Filipino IAEA trainee in Japan) (Received May 25, 1962) In the present report, the authors describe the method and the results of the measure ment with an improved radioisotope velocity meter, and gamma-gamma logging with radioisotopes, as an example, applied to ground water investigation in Fujinomiya near Mt. Fuji. With the new radioisotope velocity meter velocity and direction of ground water flow can be determined at the same time. In order to determine the depth to which the isotope velocity meter should be placed and to compare the geological column of two test borings, radioactive logging (60Co 1 mc) was made. Based on these results, the authors concluded that the lava strata with medium scattering intensity are of medium porosity and have the average ground water velocity, so it was decided to put the velocity meter at 42 meters depth. The computed ground water velocity was 1.316 ~10-3m/sec for No. 1 boring and 1.310 ~10-3m/sec for No. 2 boring. Radioisotope 131I solution (0.018 Đc/ml) was used. Direction of ground water flow was determined by the dilution ratio. On the basis of the data obtained by the present measurement, the amount of the ground water which flows 1 m wide section was estimated. Introduction Foot of volcanic regions consists of permeable strata where shallow ground water is very difficult to obtain. In such regions, if the existing water supply for agriculture, industrial, and home consumption can not meet the demand, deep ground water must be taken out to supplement the existing water supply. Detailed and comprehensive study of ground water in such a place then becomes an important problem. Up to now calculation of ground water quantity makes use of hydrological measurement, pumping test, and use of tracer materials. Hydrological measurement method of measuring discharge requires a very long time to collect the necessary data and the results obtained can not be applicable to an specific water vein. Field measurement of ground water velocity could be done by the use of boreholes where dye could be introduced at the upper borehole (higher water level borehole) and then observing the time for it to appear at the other borehole (lower water level). From the information gathered from this, the ground water velocity could be obtained.
374 Radioisotopes Vol. 11, No. 4 Using the formula Q=VAP where V is the average velocity, A is the cross sectional area, P is the porosity, the ground water discharge Q can be obtained. But, due to too much amount of dye necessary to neglect the effect of dispersion and molecular diffusion the density current increases giving a very low accuracy in measuring the porosity. Such tracer technique when examined in detail proves to have serious limitations. Pumping test may be the most accurate method of determining permeability in the field. Knowing the permeability and the existing gradient approximate discharge Q can be computed from the formula Q=KiA where K is the permeability, i the gradient, and A the cross section area. However, it is not yet a very simple one. Big diameter boring is necessary in order to conduct pumping test. Making big diameter boring to conduct pumping test is not practical because of its high drilling cost. Recently, a new device for measuring the velocity of ground water was designed by the senior author. The difficulties encountered in estimating ground water quantity could be overcome by using this device (radioisotope velocity meter). Previous test* shows that the computed permeability obtained from the results of this test is approximately the same as that in pumping test. After its improvement the velocity and direction of ground water flow can be determined at the same time. * Proceedings of the 2nd Conference on Radioisotope pp. 627-32, Feb. 1958, Japan Atomic Industrial Forum Inc. Tokyo, Japan. Radioisotope Velocity Meter a. Description of parts The radioisotope velocity meter consists of two main parts, the isotope room, and the rotating mechanism (Fig, 1). The isotope Fig. 1 Diagram of the radioisotope velocity meter room consists of the inner and the outer cylinders with a thin clearance betweem them. It is made of plastics to prevent the adsorption of the radioactive materials used. The inner cylinder is divided into three compartments. Each compartment has two windows lying opposite each other. The outer cylinder is also provided with
T. OCHIAI, et al.: Ground Water Quantity Measurement 375 three windows arranged one above another and forming an angle of 60 between each other. Ring grooves are also provided on the outside of the inner cylinder. These grooves house the silicon oil which keeps the isotope room waterproof when the windows are not open. The rotating mechanism consists of a switch, a small motor, and a series of gear system connected to the upper end of the inner cylinder by means of a shaft. This mechanism rotates the inner cylinder when the motor is switched on. b. Operation of the isotope velocity meter This device makes use of the radioisotope dilution ratio. The activity of the sample (cpm per ml) is measured before it is injected into the meter and after it is diluted by the ground water. Absolute activity measurement is not necessary. Before the isotope could be injected into the isotope room, the inner cylinder is pushed down a little till the injection holes appear. Each isotope compartment has an injection hole. Through these holes the isotope is injected by means of a syringe. After injection the inner cylinder is pushed up to its original position and the stopper is attached to the bottom. The meter is now ready for use. At the time of inserting the meter into the borehole the direction to which each of the window faces must be noted. This is very necessary in determining the direction of ground water flow. While lowering down the meter twisting the cable must be avoided so as not to change the direction to which the window originally faced. When the meter is already at the proper depth (pry viously determined aquifer) it is kept steadily and the motor is switched on. As the inner cylinder rotates slowly at the rate of 4 ~ 10 meter per minute the windows of the two cylinders coincide for some time. At this coincident time the isotope room is opened and the ground water dilutes the isotope. It requires about 10 minutes to open and perfectly close the isotope room. After the isotope room is closed the meter is taken out from the borehole and one ml sample from each compartment is taken. The activity (cpm) of the sample is measured. Knowing the activity of the sample before and after dilution, the dilution ratio D can be obtained D=SA/SO from the formula where SA is the activity after dilution, and SO the activity of the original sample. c. Calibration of the radioisotope velocity meter. For calibration of the radioisotope velocity meter use is made of a tank filled with gravel into which a pipe strainer is inserted (Fig. 2). Fig. 2 Schematic diagram of the calibration tank used in the calibration of the radioisotope velocity meter
376 Radioisotopes Vol. 11, No.4 Water is made to pass through this gravel layer in a similar way that ground water flows in a gravel aquifer. The velocity of the water passing through this aquifer model is varied by changing the height of the water source. The water velocity V is determined from the formula V=Q/t E1/A where Q is the total discharge during a time t, and A is the mean cross section. The isotope used in the calibration must be the same as that used in test borings. Calibration of the meter is done after the test in the test boring is finished. This is so in order to place the meter into the pipe of the aquifer model in the same way that it was placed in the test boring (This refers to the direction to which the window faces with respect to the determined direction of water flow). It is tested (at least four times) in the mentioned aquifer model at different known velocity. The smallest dilution ratio (among the three compartments) obtained from each of the trials is plotted against its corresponding velocity. The resulting curve is the calibration curve of the radioisotope velocity meter. The smallest dilution ratio (among the three compartments) obtained from the test boring is referred to this curve. Results of the calibration of the radioisotope velocity meter during the test in Fujinomiya (foot of Mt. Fuji) is shown in Fig. 3. The isotope used was the same as that used in test borings (131I, 0.018 Đc/ml). The equation D=-6.728V+0.8862 where D is the dilution ratio and V is the velocity, is the equation of the calibration curve. Temperature and chemical properties of water vary from one place to another. These Fig. 3 Calibration curve of the radioisotope velocity meter in the Fujinomiya test two factors have influence on the characteristic of the calibration curve. Due to this influence it is necessary to calibrate the meter during each test using water flow similar to that in test boring in which the velocity is to be measured, Radioactive Logging and Electrical Resistivity Measurement In order to determine the depth to which the isotope velocity meter should be placed and to compare the geological column of the two test borings, radioactive logging (gammagamma method) was made. The plane view of the two test borings is shown in Fig. 4. The radioactive logs of the two test borings are shown in Fig. 5. The radioactive logs of the upper layer of the two test borings are identical. At the depth from 11 to 12 meters both logs show high scattering intensity. These layers are silt and compact lava respectively as determined by the driller's well log. From 17 to 18 meters in borehole No. 1 there is another high scatter-
T. OCHIAI, et al.: Ground Water Quantity Measurement 377 borehole No. 2. The high scattering intensity stratum from 24 to 26 meters in borehole No. 2 has its corresponding strata in No. 1 att the depth from 28 to 30 meters, while the stratum from 39 to 40 meters in No. 1 has. no definite corresponding strata in No. 2. According to Ochiai's previous experiments there is a linear correlation between the porosity of gravel aquifer and its gammaray scattering intensity: Fig. 4 Plane view of the test boring place and the ground water contour P=54.0I/I0-8.47 where P is the per cent porosity, I is the gamma-ray scattering intensity in gravel aquifer and Io is the gammaray scattering in water was derived from such experiment. In the same manner lava layer has, the same gamma-ray scattering characteristic as gravel aquifer. If we compare the radioactive log of the column with its corresponding layer in the drillers well log, high scattering intensity corresponds to a porous lava and low scattering intensity to a compact lava. Based on these results, we concluded that the lava strata with a mediumm scattering intensity are of mediumm Fig. 5 Driller's well log of the test boring and its corresponding radioactive logging results ing intensity stratum. In No. 2 the corresponding stratum is thicker. The stratum, therefore, seems to increase its thickness forming a wedge-shape as it approaches porosity and have the average ground water velocity; so we decided to put the velocity meter at such strata. This was 42 meters deep in this case. Although the total depth of the test boring was 70 meters, radioactive logging was made up to 44 meters only. Beyond 44 meters the volcanic gravel collapsed and the radioactive logging up to 70 meters was impossible. To have an idea of the nature of the column not covered.
378 Radioisotopes Vol. 11, No. 4 by radioactive logging, electrical resistivity measurement was made on the ground surface. Results of the electrical resistivity measurement revealed that the strata from 40 to 70 meters belong to the same lava bed. Their resistivity was more or less 410 ohmmeters. Based on the radioactive log from 40 to 44 meters, the strata (40 to 70 meters deep) were considered a medium porous lava. per second in borehole No.1 and 1.310 ~ 10-3 meter per second in No.2. The dilution ratios of other isotope rooms range from 0.477 to 0.902. The vectors of the dilution ratio are shown in Fig. 6. Measurement of Ground Water Velocity and Direction of Flow The meter was prepared in the same :manner as described in its operation. It was lowered down to 42 meters deep in each test boring with the window of the top -isotope room facing directly north.the isotope used was 131I. Results of the test are -shown in Table 1. Table 1 Results of the Fujinomiya test Fig. 6 Vector diagram of the dilution ratios in the Fujinomiya test As shown in Table 1, the most dilute isotope solution was that on the topmost room in both borings. The dilution ratio was 0.00063 in borehole No. 1 and 0.00424 in No. 2. To compute the velocity of the ground water these dilution ratios were substituted to D in the equation of the calibration curve of the velocity meter. The computed From the vector diagram we can see in both cases that the most dilute isotope room was the one with windows facing due north followed by the room with windows facing north-west. This result implies that the water is coming from north with a little deflection to the west. This direction is the same as the direction determined by the ground water contour (Fig. 4). ground water velocity was 1.316 x 10-3 meter
T. OCHIAI, et al.: Ground Water Quantity Measurement 379 Calculation of Ground Watcr Quantity in Lava sec. Summary Based on the results of radioactive log and electrical resistivity measurement, the thickness of the aquifer was determined to be 32 meters (from 38 to 70 meter deep). The other aquifers above the 38 meter depth were excluded to supply water to shallow wells and hand-operated pitcher pumps. Knowing the ground water velocity and the thickness of the aquifer, the approximate discharge Q of a certain cross section can be determined Q=D ~V ~L from the formula where D is the thickness of the aquifer, V is the ground water velocity, and L is the width of the section. For a one meter wide section, the discharge Q is equal to 32 ~1.31 ~10 ~1= 0.04192 m3/ In volcanic regions where ground water is usually deep, estimation of discharge quantity by pumping test or tracer tech niques is very difficult to undertake. How ever, if we make use of the newly-designed radioisotope velocity meter, combined with radioactive logging and electrical resistivity measuring devices, discharge approximation could be made easy. From the results of radioactive log and electrical resistivity the thickness of the aquifer can be determined. Knowing the aquifer thickness and the ground water velocity, the discharge quanti ty can be calculated. More accurate results would be obtained from this method if further tests are made within the thickness of each aquifer to get the mean velocity.