ARCHIVED REPORTS XR0003043 - Laserfiche WebLink

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• • 5,605,634 E through the borehole, ie laterally from side to side. The screens 29 should be such that the water inside the borehole 20 is largely free from soil and dirt particles. The remedial substance, upon being released from the diffusion tube, enters the water in the borehole. It may be s expected that the substance will diffuse quickly through the water in the borehole, whereby the concentration of the remedial substance will be substantially constant over the whole volume of water in the borehole. As the groundwater moves through the boreholes, the to remedial substance is carried in plumes 56, downstream of the borcholes. The remedial substance can be expected to spread laterally and vertically from these plumes. However, generally, in most aquifer materials, there is some horizontal layering, whereby the vertical spread will be quite small. 15 However, in the system as described, this poor vertical spreading characteristic is of no consequence, because the diffusion tube extends over the whole vertical height of the contaminant plume. Lateral spread will more readily arise, and it can be 20 expected that the plumes of remedial substance from adja- cent boreholes will meet and join, laterally, a short distance downstream of the boreholes. The engineer should specify the spacing of the boreholes according to prior determina- tions of the lateral spreading characteristics of the particular aquifer material, so that the adjacent plumes do meet before 25 the remedial substance starts to become depleted. Thus, even though emitted from discrete points, the remedial substance can move downstream in the groundwater as a complete front. As the remedial substance acts on the contaminant in the 30 groundwater, the remedial substance starts to become depleted. It is the engineer's task to make sure the concen- trations and other parameters are {just) enough to enable proper and complete treatment of the contaminant. 35 In a typical case, the contaminant may be, for example, TCE (trichloroethylene). The solubility of TCE is such that TCE can be dissolved in groundwater at concentrations of several milligrams per liter of groundwater. However, the drinking water standard is only a few micrograms per liter. 40 TCE in light concentrations can be treated by microbial action, but one difficulty is that heavy concentrations of TCE can be toxic to the microbes. The difficulty could be addressed by maintaining a high population density of microbes. This can be done by supplying a nutrient, such as 45 toluene, upon which the microbes feed, and which enables the microbes to build up their numbers until the population density is large enough to break down the TCE. If the toluene were injected jerkily or patchily, the microbe popu- lation would be patchy, and the TCE would not be broken 50 down evenly. The toluene must be injected evenly, and at small concentration densities, over the whole arta of the plume. The system as described is very well-suited for that task. The treated water can be expected to contain less than the allowed concentrations of TCE and of toluene— 55 whereas, if the injection of the toluene were not done accurately, smoothly, and evenly, traces of both those sub- stances might be present, at least in patches. As described, the diffusion tubes 30 are placed inside the borcholes 20, and the remedial substance diffuses out of the 6o diffusion tube, into the body of water present in the borehole. It would not be possible to use the borehole itself (that is to say, the lining or casing of the borehole) as the diffusion tube. A key feature of the system as described is that the concentration is so tightly and accurately controlled, and 65 that feature arises because the remedial substance is circu- lated through the diffusion tube. Circulation is only possible when both ends of the diffu- sion tube are accessible and available for conveying the remediation substance. FIG. 3 shows another system for injecting small concen- tration densities evenly over a large body of water, but where the flux rate is rather too large to be handled conveniently by LDPE tubing. In place of the length of LDPE tubing, the diffusion tube in this case comprises a length of dialysis tubing 60. The dialysis tubing has a typical thickness of 0.04 mm, and is rather flimsy mechanically in the context of a down- hole installation. The dialysis tubing is therefore housed in a protective sheath 63. The sheath comprises a slotted screen, made of PVC. The dialysis tubing is sealed to the sheath 63, and inlet and outlet pipes 65,67 are fed into the tubing, as shown. The materials used are selected so as to be unaffected by the particular contaminant and remedial substance. The tubing 60 as employed was SpcctraTor 5 (TM of Spectrum Medical Ind Inc) rcgcncratcd cellulose -impreg- nated dialysis tubing. Suitable forms of dialysis tubing, for use in the system as described, can be obtained in thick- nesses of 16-60 micrometers. The concentration gradient over the whole length of the tube is kept constant by periodic circulation of the stock solution of the remedial substance, in the manner as described previously. It should be understood that the expression borehole as used herein is not confined to a drilled well of cylindrical profile. Thus, for example, a cavity in the ground that was formed by inserting sheet piling into the ground to form an enclosure, then excavating soil material from inside the enclosure, would still be a borehole within the context of the invention. Similarly, the expression diffusion tube is not intended to be narrowly construed as referring only to circular -profile manufactured tubing. So long as the ends of the tube are available for circulation of the fluid through the tube, the profile of the diffusion tube is of little importance. The systems as described are intended for the accurate control of small and very small concentration densities, spread evenly over a large volume of groundwater, which is extensive as to area, and over the depth of the plume. The invention is aimed at minimising the need for expen- sive close control, the intention being to provide a system that can be left for a maximum period without intervention. We claim: 1. Procedure for placing small quantities of a substance into groundwater moving through an aquifer, comprising the steps of; in a borehole in the material of the aquifer, placing a diffusion tube down from the ground surface into the depth of the borehole; the borehole being of the kind into which groundwater can enter from the aquifer, can pass through, and can re-enter the aquifer, laterally; providing a source of the substance in fluid form; the diffusion tube is made of a material of the kind through which the fluid substance is capable of diffus- ing molecularly; placing the diffusion tube in the borehole, whereby the diffusion tube is located in the groundwater in the borehole; the diffusion tube having an entry end and an exit end, and forming means for passing fluid through the diffusion tube, from the entry end to the exit end thereof.