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<br />C]
<br />5,605,634
<br />through the borehole, ie laterally from side to side The
<br />screens 29 should be such that the water inside the borehole
<br />20 is largely free from sod and dirt particles
<br />The remedial substance, upon being released from the
<br />diffusion tube, enters the water in the borehole It may be
<br />expected that the substance will diffuse quickly through the
<br />water in the borehole, whereby the concentration of the
<br />remedial substance will be substantially constant over the
<br />whole volume of water in the borehole
<br />As the groundwater moves through the boreholes, the
<br />remedial substance is earned in plumes 56 downstream of
<br />the boreholes The remedial substance can be expected to
<br />spread laterally and vertically from these plumes However,
<br />generally, in most aquifer materials, there is some horizontal
<br />layering whereby the vertical spread will be quite small
<br />However, to the system as described, this poor vertical
<br />spreading characteristic is of no consequence, because the
<br />diffusion tube extends over the whole vertical height of the
<br />contaminant plume
<br />Lateral spread will mon: readily arse, and it can be
<br />expected that the plumes of remedial substance from adja-
<br />cent boreholes will meet and join, laterally, a short distance
<br />downstream of the boreholes The engineer should specify
<br />the spacing of the boreholes according to prior determtna
<br />tions of the lateral spreading characteristics of the particular
<br />aquifer material, so that the adjacent plumes do meet before
<br />the remedial substance starts to become depleted Thus, even
<br />though emitted from discrete points, the remedial substance
<br />can move downstream in the groundwater as a complete
<br />front
<br />As the remedial substance acts on the contaminant in the
<br />groundwater, the remedial substance starts to become
<br />depleted It is the engineers task to make sure the concen-
<br />trations and other parameters are (lust) enough to enable
<br />proper and complete treatment of the contaminant
<br />In a typical case, the contaminant may be, for example
<br />TCE (trichloroethylene) The solubility of TCE is such that
<br />TCE can be dissolved in groundwater at concentrations of
<br />several milligrams per liter of groundwater However, the
<br />drinking water standard is only a few micrograms per liter
<br />TCE in light concentrations can be treated by microbial
<br />action, but one difficulty is that heavy concentrations of TCE
<br />can be toxic to the rrucrobes The difficulty could be
<br />addressed by maintaining a high population density of
<br />microbes This can be done by supplying a nutrient, such as
<br />toluene, upon which the microbes feed, and which enables
<br />the microbes to build up their numbers until the population
<br />density is large enough to break down the TCE If the
<br />toluene were injected jerkily or patchily, the microbe popu-
<br />lation would be patchy, and the TCE would not be broken
<br />down evenly The toluene must be injected evenly, and at
<br />small concentration densities, over the whole area of the
<br />plume The system as described is very well-suited for that
<br />task The treated water can be expected to contain less than
<br />the allowed concentrations of WE and of toluene—
<br />whereas, if the injection of the toluene were not done
<br />accurately, smoothly, and evenly, traces of both those sub-
<br />stances might be present, at least in patches
<br />As described, the diffusion tubes 30 are placed inside the
<br />boreholes 20, and the remedial substance diffuses out of the
<br />diffusion tuho, into the body of water present in the borehole
<br />It would not be possible to use the borehole itself (that is to
<br />say, the lining or casing of the borehole) as the diffusion
<br />tube A key feature of the system as described is that the
<br />concentration is so tightly and accurately controlled, and
<br />that feature arises because the remedial substance is circu-
<br />lated through the diffusion tube
<br />le
<br />Circulation is only possible when both ends of the diffu-
<br />sion tube are accessible and available for conveying the
<br />remediation substance
<br />FIG 3 shows another system for injecting small concen-
<br />s [ration densities evenly over a large body of water, but where
<br />the flux rate is rather too large to be handled conveniently by
<br />LDPE tubing In place of the length of LDPE tubing, the
<br />diffusion tube in this case comprises a length of dialysis
<br />tubing 60
<br />io The dialysis tubing has a typical thickness of 004 mm
<br />and is rather flimsy mechanically in the context of a down-
<br />hole installation The dialysis tubing is therefore housed in
<br />a protective sheath 63 The sheath comprises a slotted
<br />screen, made of PVC The dialysis tubing is sealed to the
<br />15 sheath 63 and inict and outlet pipes 65,67 are fed into the
<br />tubing, as shown
<br />The materials used are selected so as to be unaffected by
<br />the particular contaminant and remedial substance
<br />The tubing 60 as employed was Spectra/Por 5 (TM of
<br />20 Spectrum Medical Ind Inc) regenerated cellulose impreg-
<br />nated dialysis tubing Suitable forms of dialysis tubing, for
<br />use in the system as described, can be obtained in thick
<br />nesses of 16-60 micrometers
<br />The concentration gradient over the whole length of the
<br />25 tube is kept constant by periodic circulation of the stock
<br />solution of the remedial substance, in the manner as
<br />described previously
<br />It should be understood that the expression borehole as
<br />used herein is not confined to a drilled well of cylindrical
<br />3D profile Thus, for example, a cavity in the ground that was
<br />formed by inserting sheet piling into the ground to form an
<br />enclosure, then excavating soil material from inside the
<br />enclosure, would still be a borehole within the context of the
<br />invention
<br />35 Similarly, the expression diffusion tube is not intended to
<br />be narrowly construed as referring only to circular -profile
<br />manufactured tubing So long as the ends of the tube are
<br />available for circulation of the fluid through the tube, the
<br />profile of the diffusion tube is of little importance
<br />40 The systems as described are intended for the accurate
<br />control of small and very small concentration densities
<br />spread evenly over a large volume of groundwater, which is
<br />extensive as to area, and over the depth of the plume
<br />The invention is armed at mimmising the need for expcn-
<br />45 sive close control, the intention being to provide a system
<br />that can be left for a maximum period without intervention
<br />We claim
<br />1 Procedure for placing small quantities of a substance
<br />into groundwater moving through an aqutfer, comprising the
<br />so steps of,
<br />in a borehole in the material of the aquifer, placing a
<br />diffusion tube down from the ground surface into the
<br />depth of the borehole,
<br />the borehole being of the kind into which groundwater
<br />ss can enter from the aquifer, can pass through, and can
<br />re enter the aquifer, laterally,
<br />providing a source of the substance to fluid form,
<br />the diffusion tube is made of a material of the land
<br />60 through which the fluid substance is capable of diffus-
<br />ing molecularly,
<br />placing the diffusion tube in the borehole, whereby the
<br />diffusion tube is located in the groundwater in the
<br />borehole,
<br />the diffusion tube having an entry end and an exit end, and
<br />forming means for passing fluid through the diffusion
<br />tube, from the entry end to the exit end thereof,
<br />
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