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• �J • 5,605,634 3 The unit 43 contains a peristaltic pump 45 or other means for circulating the fluid through the tubing and around the pipes in an accurately controlled manner. Peristaltic pumps are of course well-known for their capability, once set up, of metering small quantities of material in an accurate and 5 controlled manner. The pump 45 circulates the fluid around the pipes and the diffusion tube, and through a stock tank 47. A sampling point 49 includes a means (which may be conventional) for determining the composition of the fluid in the circulation to system, and especially for determining the concentration of the remedial or treatment substance within the fluid, If the concentration should drop too low, provision is made within the structure of the unit 43 for make-up quantities of the treatment ingredients to be added into the fluid, e.g from a is replenishment reservoir 50. Such checking of the concentration, and making up where necessary, can be done more or less entirely automatically. The functions can be performed either by a simple rimer, and/or in response to requirements as derived from the 20 concentration measurements. As such, it will be understood that the system, once set up, can be left to operate without attention (other than occasional inspection) for periods that can be measured in weeks or months. The circulation of the treatment fluid through the diffusion 25 tube is important. If the fluid were not circulated, the concentration or the remedial substance within the fluid in the diffusion tube would gradually become reduced. There is usually no need for the circulation of the fluid through the diffusion tube to be continuous. Circulation for 30 15 or 30 minutes once a day, or once every two days, typically will suffice, at feast when the flux or flow rate required of the remedial substance into the groundwater is quite small. If larger quantities of remedial substance are required, the greater will be the tendency for the concentra- 35 lion of the remedial substance in the diffusion tube to become depleted more quickly, and the more often the fluid should be circulated. Indeed, continuous circulation is called for in some cases. However, the intention generally is that continuous circulation would be well in excess of what is 40 required: the system is mainly intended, not for injecting fluids at maximum flux rates, but for maintaining accurate control of small concentration densities, evenly over large areas of groundwater, and over long periods The concentration will eventually drop in the diffusion 45 tube, as the remedial substance is consumed. However, if the volume capacity of the stock tank is large enough, it may be possible to avoid the need for periodic make-up of the concentration of the remedial substance, and hence to avoid 5o the need for the replenishment reservoir 50. It should be noted that the volume of the remedial substance contained in the diffusion tube itself will not, in general, be enough for the concentration of the substance to be maintained for an adequate period of time without 55 circulation. That is to say: if the fluid in the diffusion tube 30 were not circulated at all, the concentration in the tube would fall off too quickly, whereby control of the concen- tration of the remedial substance in the groundwater could not be accurately maintained. 60 Use of the system is advantageous when the fluid which is circulated through the diffusion tube comprises the treat- ment or remedial substance dissolved in a carrier liquid, such as water. Preferably, the solution of the remedial substance in the 65 water is far from being saturated, i.c is at a Iow or dilute concentration. Again, it is noted that the benefit of the system as described lies not so much in achieving a maxi- mum flux nate of the remedial substance, but rather in the accurate control of a small flux rate. Placing the remedial substance, at low concentrations, evenly, into a wide front of slowly -moving groundwater is a quite different task from the more common task of injecting substances, when the main aim is to achieve a maximum flux rate, It is contemplated that the remedial substance may be a gas. if the gas can be dissolved in a carrier, such as water, so much the better. If the remedial substance is a gas of a type that cannot be dissolved, the gas may be passed through the diffusion tube in the gaseous phase. However, the liquid phase is preferred. The reason the liquid phase is preferred is that the flux rate at which a substance diffuses through a permeable wall is affected by the pressure differential across the wall. If the engineer desires the flux rate to be constant, he should ensure the pressure differential is constant. However, the pressure of the groundwater lying outside the diffusion tube is not constant, but of course varies with depth. If the fluid being circulated in the diffusion tube has the same density as the groundwater, the pressure in the fluid also varies with depth, and parallels the pressure of the groundwater --and any induced excess of pressure obtains independently of depth, over the height of the diffusion tube. But when the fluid being circulated is a gas, the gas is at constant pressure throughout the circulation system, whereas the pressure of the groundwater varies with depth, and as a result the pressure differential at the top end of the diffusion tube, where the groundwater pressure is low, is different from the pressure differential at the lower end of the diffusion tube, where the groundwater pressure is high. When the fluid being circulated is a gas, therefore, a considerable variation in concentration with depth must be expected. However, in some cases this can be allowed for, and the use of a gas as the circulation fluid is not ruled out. Another reason liquid -phase fluids are preferred to gas - phase is that the liquid -phase can be diluted, e.g with water. Dilution/concentration is a parameter that can be accurately and readily controlled. In the system as described, the flux rate of diffusion of a dissolved liquid substance can be fine-tuned by controlling the concentration of the solution, which can be done simply, yet accurately. As mentioned, it is preferred, for reasons of flexibility of control, that the liquid substance being injected is not saturated, but is dilute, in solution in the carrier liquid. The carrier liquid need not be water, although usually water will be the most convenient. The treatment liquid need not in fact be a solution in the carrier liquid. When the treatment liquid is a non -aqueous -phase liquid, it can still be advantageous to carry the NAPL in water, and similarly to keep the propor- tion of NAPL low. In the system as described, a substance inside the diffu- sion tube 30 diffuses through the walls of the tube and enters water lying outside the tube, in the borehole. The rate at which the diffusion takes place is determined by the molecu- lar structure of the material of the wall of the tube. When the material is LDPE, as manufactured for the common LDPE tubing used in very many applications, the variations in the crystalline structure of the wall material, and of course the thickness of the wall, can be expected to vary somewhat, at different locations along the length of the tube. That being so, is a reason the circulation of the fluid within the diffusion tube is preferably of a periodic nature. ne interruptions in the circulatio"c fluid then being stationary serve a useful purpose, as follows.