Safety_Series_015_1965 - gnssn - International Atomic Energy ...

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This publication is not longer validPlease see http://www-ns.iaea.org/standards/Where direction also is required, more "positive" results areattainedby introducing a radioactive tracer at one point and collectingit as it passes another sampling point suitably installed downstream.When a tracer is injected into an aquifer it causes a disturbance toground water which may be significant to the results if the measureddistances are small. With large distances and deep aquifers it maybe adequate to introduce large quantities of a tracer into a well andto encourage its diffusion into the surrounding soil by hydrostaticpressure. On small-scale measurements this is not possible; smallquantities of tracer solution may be introduced into the soil by usinga water sampler, such as that shown in Fig. 13, in reverse. If theinjection pressures are low and the introduction continued over anappropriately longer period any disturbance to the natural flow maybe acceptably small. If prior experiments have indicated the directionof flow, several injection points may be aligned closely at rightangles to the flow path so that the plume of tracer may advance in abroad "front" toward the sampling points. In this way there is lessrisk that the sampling point downstream is by-passed by an unexpectedtortuous streamline.The choice of a suitable tracer will depend on the local soil andground water. Although tritium is recognized as the most preferableowing to its low health hazard and virtually identical behaviour withthat of the natural water, it may be more practicable to use anotherradioisotope. Sulphur-35 as sulphate, has proved a valuable betatracerin waters that have a naturally low sulphate content.Iodine-131may be useful for short experiments provided that no organic materialis present in the soil. Cobalt-60 is useful when present asan anion (K3Co60(CN)6); its long-life makes it adaptable for long experiments,and its detection is simple, but its use may be limitedby the pollution hazard if used in concentrations exceeding the maximumpermissible concentrations in drinking water. Many of theseadvantages are obtained with less hazardusing chromium-51-E.D.T.A.(half-life 28 days) which has been demonstrated [105] as a stable reliabletracer in concentrations down to 0.01 ppm. Other nuclidesthat havebeen successfully applied are rubidium-86, phosphorus-32,calcium-45, brom ine-82, and the nitro-com plexes of nitrosylruthenium-106[106].To minimize any disruption to the streamlines of the groundwater the "frozen" source [19] has beendeveloped and used experimentallyonly at shallow depths (2 m). A mixture of active solutionand sand is frozen in a tube; it is then lowered or driven to the re98
This publication is not longer validPlease see http://www-ns.iaea.org/standards/quired level and the contents are extruded or driven from the tubeinto the soil. As the source slowly thaws, a plume of radioactivetracer disseminates into the neighbouring soils without disruptingthe streamlines, although problems of density and viscosity differencesmay be more pronounced. Similarly the detection of the tracer maydisrupt the streamlines of flow downgradient from the injection point.For strong gamma-emitters the disruption may be negligible if thedetecting counters are lowered into dry tubes previously installedin the estimated flow path or if counters are buried beforehand atappropriate points. For beta-emitters and tritium it is necessaryto sample the water and the mere withdrawal encourages the movementof replacement ground water to that point. However, for shallowdepths the withdrawal of 100-ml samples into evacuated flaskshas not caused any noticeable loss in accuracy. The copper-rod methoddispenses with liquid sampling [19], The method is based on thechemisorption reaction between the tracer and a metal. It has beensuccessfully applied with I131 being adsorbed on copper rods drivenin the soil. By installing a network of bars in the estimated path ofa plume and removing them later, the boundary of the plume may beaccurately delineated.An alternative to permeability tests on samples or the directmeasurement of water velocity by tracers is the pumping test in whichan aquifer is subjected to an artificially induced flow pattern and measurementsare taken of flow and pressure to deduce the permeabilityof the surrounding soil.When water is pumped from a well the water level drops in thecasing and in the surrounding annulus of soil and more water flowsin radially from the neighbouring soil to replace that previously removed.The hydraulic impetus for this radial flow is observed inthe profile of the water table, which in gravity aquifer may be closebelow the surface in the surrounding area, becoming progressivelydeeper towards the well. This profile is called the draw-down curveand may be plotted from the water levels in observation wells suitablyplaced around the pumped well. In a pumping test the dischargefrom the pumped well is continuously recorded and pumping is sustained,if possible, until the draw-down curve and the pumping dischargeapproach an apparent equilibrium. For these conditions theThiem equation relates the permeability of the formation with theprofile of the draw-down curve and the aquifer thickness. However,since "equilibrium" is seldom, if ever, achieved it is more usualto use the Theis non-equilibrium formula99
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