Artificial radio-tracers in groundwater flow measurements

Transcription

1 Groundwater Monitoring and Management (Proceedings of the Dresden Symposium, March 1987). IAHS Publ. no. 173, Artificial radio-tracers in groundwater flow measurements INTRODUCTION J. BULLA, R. RAVINGER, P. TURCEK Slovak Technical University, Department of Geotechnics, Radlinského 11, Bratislava, Czechoslovakia H. JORDAN Mining Academy Freiberg, Department of Hydrogeology, Freiberg, German Democratic Republic Abstract Single-borehole radio-indicator methods have proved to be of value in solving hydrogeological and geotechnical problems. A tracer is discharged into a screened borehole at a chosen depth and its vertical displacement or horizontal dilution is measured by a submerged detector. In the Quaternary deposits, filtration parameters were determined from measurements of natural vertical velocities and water inflows during pumping. Inactive lower horizons were detected in measuring an older water well. New pumping wells were suggested with a shortened depth. Sealing elements of foundation pits were controlled by extensive measurements in the network of boreholes. Sliding surfaces were localized in landslide aquiferous slopes. Artificial radioactive tracers have already been used to a large extent in studying parameters affecting groundwater movement. Under field conditions, quantifiable data on aquifers may be established and the influence of flow on the natural environment or engineering works can be studied. The present studies have as their objective the extension of application and an increase in measurement precision by single-bore-hole methods related to solving protection problems, optimum use and influence of groundwaters. Measuring procedures were made more efficient, measurement equipment and evaluation of results modernized. Methods of solution and interpretation of measured results on specific problems of hydrogeological investigation and geotechnical engineering were worked out. BASIC PRINCIPLES Measurements by radio-indicators may be carried out in the more permeable parts of observation boreholes constructed in aquifer environments or in proximity to controlled engineering objects. Two single-borehole methods are mainly used from among several available. Radioactive Geiger-Muller tubes 131

2 /. Huila et al. 132 are placed in submerged detectors. The vertical water movement in boreholes and its changes are then determined by operating the equipment. In the case of negligible flow rates (v < 5 x 10" 5 m s" 1 ), the dilution method is applied to the measurement of filtration velocities. A small dosage of radioactive indicator with an initial volume V Q = 1-9 x 10" 7 m 3 will form an active cloud. Dosage depends on the sensitivity of detectors and the extent of initial volume. In measuring natural movement, a radionuclide as low as 0.04 MBq ( ftci) 131 I is sufficient and is not considered as an emitter by hygienic standards. At an activity of 2 MBq, the radioactive tracer is detectable at distances of several metres in measuring the vertical velocity v v = 1 x 10" m s" 1. The detection of the radio-indicator is achieved with a submerged doser with electric or electromagnetic action of the piston. Service controls spray through a 500-m-long coring cable. The most recent design of equipment comprises computers with doser in one watertight sleeve connected to evaluation and registration equipment as shown in Fig. 1. Ng. 1 Equipment for measuring vertical water movement, a strengthening member, b Geiger-MWer tube, i radio-indicator container, j spray piston, e spray dosage control, f evaluation equipment, g recording equipment, h perforated tube, c concentration, d tube diameter, t time, t y time of peak, A detail submerged equipment, B width of water flow. Among the wide spread radioactive counts, 5 Cr-EDTA (with half-life decomposition of 27.8 days), 82 Br (35.87 h), 86 Rb (18.66 days) and 131 I (8.07 days) which fulfill most technical and health requirements. Compared with classical indicators, they do not affect by their salts and colours the physical and chemical properties of water, which enables precision of measurement

3 133 Artificial radio-tracers in groundwaterflowmeasurements results. The range of registered ionizing radiation in the vicinity of the submerged detector is up to 0.10 m, which enables the registration of travel curves of the tracer around the detector (vertical movement) or changes of impulse frequency (dilution method) in time relationships. After a single discharge of radioactive tracer, its vertical movement around the detector appears on the registration record with an obvious maximum. By means of calibration one can obtain average values for vertical flow velocity between the places measured. From several measurements along the borehole depth of positions of change of vertical movement and borehole drainage influence â = one can calculate also filtration velocities of water movement in soil or disturbed rock around the borehole according to the relationship: v ff = = (1) fi ad-ah; in which: Aq vi is the increase or decrease of vertical water travel in the borehole section of Ah t height, and d is the borehole internal diameter. In the dilution method, water movement parameters in soils are calculated from the plotting of tracer movement: c o V m vfi K = oca : t In c W where V is the volume of indicated water in the observation perforated tube; A the vertical passage surface of tube; a the borehole drainage effect; c the initial impulse frequency; proportional to the initial volume activity of tracer; and c the impulse frequency in time t. The average value of filtration velocity of the whole borehole is calculated from the relationship: Vf, = ± (3) Processing the results is undertaken by means of a 'computer. Methodological bases and some examples of application were published by Halevy et al. (1967), Klotz et al. (1979) and Huila et al. (1983). Artificially induced vertical movement of water in boreholes due to pumping has a similar method of measurement and use of equipment. The advantage of radio-tracers is in offering a wider range of application even

4 /. Huila et al. 134 against hydrometric meters, by which the vertical velocities (v^) < 0.05 is not registered. PRACTICAL APPLICATIONS Problems of hydrogeological investigation can be solved with precision by measurement of natural or artificially-induced water movement in boreholes constructed in unconsolidated sediments or jointed rock environment. Such measurements confirm the fact that longer perforated boreholes interconnect aquifers and allow intense vertical flow to take place in them. The objective of these measurements was to establish inflow, outflow and travel zones of water within the borehole, respectively parts of the total yield. Parameters of groundwater movement were obtained from measurements made under conditions of natural movement or during pumping by calculation from relationships in equations (1) to (3). For the environment adjacent to the boreholes, depth relationships for flow velocities were established, which can be used according to Darcy's law. By comparing results from several localities it was found that differences in average values of filtration coefficients determined from measurements of natural movement and from classical pumping tests are no larger than ±50%, which are acceptable for practical purposes. Radio-tracer measurements can be used to establish the vertical movement of water in the borehole or shaft in which pumping it carried out. Detectors are placed in the flow direction close to the pump inlet. DEPTH VERTICAL WATER PASSAGE IN PUMPING Q = 0,0176 m 3? 0 = 0,047 m3"s 1 h m] NATURAL WATER PASSAGE COEFICIENTS OF FILTRATION X h 60 0, q v t m 3 s 1 ) kflms 1 Fig. 2 Measurements results and their evaluation in borehole in induced and natural vertical water passage.

5 135 Artificial radio-tracers in groundwaterflowmeasurements Increments in the mass curve of vertical inflow are not uniform in the Quaternary deposits, but they keep their division of permeability (yield) in changes of pumped quantities. Figure 2 shows measured results and evaluations obtained in borehole ST 14 near Bratislava, where the filtration coefficients values calculated were comparable under different artificial movement. In further boreholes in the locality, a greater concordance was obtained between the depth relationship of filtration coefficients k f from natural and artificial movement. From the results in the layer of sandstone illustrated in Fig. 3, it could be judged that from a greater depth than 100 m, under pumping q v = m 3 s" 1 the inflow to the borehole was less than 10% of the total quantity. This fact could be shallower by 40 m, representing a considerable economic benefit. The measurements of natural vertical movements confirmed that there is little flow activity below 110 m depth. In using the value «= 50 for the borehole drainage influence, similar values for filtration coefficients were obtained as from the evaluation of pumping tests. VERTICAL WATER PASSAGE BOREHOLE IN 0 PUMPING NATURAL MOVEMENT 0 imi: 25; 50 : U , 1Ô 7 1Ô S 1Ô s qv!*1ô J ] qvmo-nvyi vf[msl] kf msi] Fig. 3 Measurements results of vertical movement in a borehole with permeable part in sandstone. Depth division of natural vertical water travel were used to identify locations for withdrawal of samples for tritium and radio-carbon analyses. In order to obtain a water composition withdrawn from boreholes close to that in the surrounding strata, suitable water withdrawals are at depths h = 110, 117, 125 and 133 m where there are more intense inflows. Analysis carried out by staff at the Mining Academy of Freiberg found that this water was several thousands of years old. Measurements in a flooded mine shaft in the GDR served to verify vertical movement down to a 445 m depth below the level and it was proved there existed strong inflows and outflows from flooded horizontal shafts and an intensely increased inflow at deeper horizons.

6 /. Huila et al. 136 The inequality of groundwater inflows at different depths to boreholes constructed in cherry soils, which was detected by radio-tracer methods, makes possible the recommendation of depth and number of boreholes necessary to drain foundation pits. Based on the measured results of vertical movement in the observation well or borehole it is possible to ensure an economic depth proposal and an average efficiency for the entire system of drainage wells. The construction of engineering works below the groundwater level currently requires in the bigger foundation pits the building of seal elements. The peripheral sealing walls are keyed to relatively impermeable layers or an artifically sealed base. The described radio-tracer methods were applied to the controlled sealing of foundation pits in the Gabcrkovo hydraulic works on the Danube. Based on facts from measurements in the verification sector, considerable financial savings were realized in designing the foundation hydroelectric power plant and float chambers. Quality control of sealing elements was carried out by extensive measurements made during a pumping test before starting the excavation works. From the measurements made in observation boreholes, values for vertical and filtration velocities were established. The results served to determine the division of water inflows to the sealed foundation pit, which provided the basis for calculation of permeabilities of the sealing walls and bottom. They also made possible the location of the more permeable positions and degree of risk to the filtration I i :!! :! i ; i r ' r T m 0 1 2[x1Ô b ms 1 ] FILTRATION VELOCITIES Fig. 4 More permeable positions and the greatest vectors of average filtration velocities in the sealed pit of the Gabcikovo hydroelectric power plant.

7 137 Artificial radio-tracers in groundwater flow measurements stability. The diagram of the foundation pit sealing in Fig. 4 was obtained from the evaluation of measured parameters and their comparison with the suggested design. Further application of radio-tracer measuring methods were made in identifying the groundwater flow regime and seepage waters of dams levees. Flow parameters can be measured during the investigation phase well before the construction during the investigation, which serves as a basis to judge the measured effects from building the sealing elements, dam impoundment influence and changes in long-term operation. The efficiency of sealing and drainage elements and the entire stability of the engineering work can be judged by the analyses made in actual cases from the measurements. Measurements made during the investigation of groundwater flow in landslide areas confirmed their suitability for identifying positions of sliding surfaces. Weakened places identified by zones of maximum values of filtration velocities were established on the basis of measurement. It was possible to verify also the effect of remedial drainage measures. For the Lfskovâ locality, Fig. 5 clarifies the places of sliding surfaces, which appeared by marking seepages or inflows of water in the measured boreholes. The radio-tracer methods helped to rectify the coverage in areas of a less risk of landslide occurrences, which are a frequent phenomena in the area of the Carpathian flysch and neo-volcanites of Slovakia SLIDING SURFACE id'm 5 vf[ms 1 ) Fig. 5 Cross section of the principal landslide flow in Lfskovâ. Measurements of groundwater flows by using artificial radioactive tracers have helped to clarify many actual problems of hydrogeological investigation and building of structures. By gaining further facts on the correlation between the values of filtration parameters calculated from measurements of natural and artificial movement by radioactive tracers in boreholes and those processed from classical pumping test experiments it is assumed that in the future there will result a still greater extent of application of the described methods. Further investigation will be directed towards extending the possibilities of interpretation of measured quantities by artificial radio-tracers to problems of utilization and protection of groundwaters.

8 /. Huila et al. 138 REFERENCES Halevy, E., Moser, H., Zellhofer, O. & Zuber, A. (1967) Borehole dilution techniques. In: Isotopes in Hydrology, p IAEA, Vienna, Austria. Huila, J., Ravinger, R. & Turcek, P. (1983) Radio-indicator tests in hydrogeological boreholes. Bull. Int. Ass. Engng Geol , Klotz, D., Moser, H. & Trimborn, P. (1979) Single borehole Techniques. In: Isotope Hydrology 1978vol. I, p IAEA, Vienna, Austria.