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5,605,634 whereby Because the walls of the diffusion tube are not exactly the variations in coefficient, the same flux rate can uniform (not uniform, that is to say, as regards the ratio of be maintained from each tube amorphous to crystalline structure), the substance leaf out In some cases, it may be preferred, in fact, to vary the flux more quickly through the more porous areas of the tube wall rare gni the various diffusion tubes Many plumes are more If the fluid is stagnant, a local volume of the Hurd would then s concentrated in the centre than at the edges, for example, and become depleted—and the more permeable the local area of the engineer may wish to tailor the flux rates tube to tube, the wall, the more the local volume of the Hurd would be across the width of the plume, accordingly depleted The rate of diffusion being concentration-depen Equally, some plumes vary in concentration over the dent, that depletion would therefore slow down the ensuing vertical depth of the plume In that case the engineer may diffusion through that area of the wall Therefore, the system io specify for instance, that the portion of the diffusion tube has at least a marginal capability to self-correct, and to keep that will diffuse remedial substance into the topmost and concentrations of the remedial substance constant within the bottommost margins of the plume be of a thicker wall groundwater, even though wall-perme-ability may vary The thickness than the tubing which diffuses remedial substance capability arises provided the water is left, without moving, into the central band of the plume, where the contarmnants for long periods 15 may be more concentrated Thus, although periodic circulation is important for main- The pipes and conduits which convey the remedial sub- taining overall levels of concentration over a long time, the stance in its carver fluid need not be all LDPE tubing, like periods of non-circulauon are important also, because they the diffusion tube For example, the pipes may be of metal smooth out variations in concentration doe to variations in tubing, through which no discernible diffusion can take wall-permeability zo place As mentioned, many types of plastic tubing tend to allow In order for circulation to take place, the engineer must migration or diffusion of molecules through the wall of the provide a down-tube and a return-tube The engineer may tubing, LDPE being well-suited in many cases However, arrange that both the down tube and the return-tube are both LDPE tubing, as available commercially, may not always be diffusion tubes, of LDPE or other suitable plastic Or, he the same as to its crystalline structure, which is the factor 25 may arrange that only one of the tubes is a diffusion tube, that mainly sets the diffusion coefficient Wall thickness and the other tube is of metal The entry end of the diffusion tube would be located at the bottom of the diffusion tube if variations also cannot be ignored the diffusion tube is the return tube The engineer should therefore carry out laboratory tests on samples of the particular batch of tubing, in order to It is possible for back-diffusion to take place That is to determine the diffusion coefficient for that batch Knowing 30 say, it can happen that molecules of groundwater lying the diffusion coefficient and knowing the extent, nature, and outside the diffusion tube can diffuse into the tube However, . concentration, of the contaminant in the groundwater, the if tlus should occur, its effects on the dilution-concentration the remedial substance in its earner fluid would be picked inground velocity of the groundwater the positioning of the boreholes, the nature of the remedial substance, and so on, of up by the routine sampling, and the effects could be adjusted the engineer can compute what the concentration of the 35 for automatically If the remedial substance is a gas, perhaps remedial substance in its tamer fluid should be, and can set it could not be ruled out that a small quantity of back- diffusing water might collect on the inside of the diffusion up the fluid circulation means appropriately to maintain that tube, but the volume of water thus collected would hardly be required concentration The engineer adjusts the concentration of the remedial significant enough to affect the performance of the system substance in its earner fluid as the main agency by which the 40 Thus, especially if a pressure differential is engineered flux rate of the remedial substance is finely and accurately whereby the fluid on the inside of the diffusion tube is at a higher pressure than the surrounding groundwater, back- controlled He can make gross adjustments to the flux rate by selecting, as the diffusion tube, a tube with a different wall diffusion can largely be ignored thickness or diameter Of course, a finer degree of control is One of the reasons back-diffusion can be ignored is that, required for the dision flux rates than can be achieved 45 ffu in the system as desenbed, the ratio of diffusion area of the merely by altering wall thickness tube to the volume of fluid contained in the tube is small If The prudent engineer should check the diffusion coeffi- the diffusion tubes were small-Bore, such that the area-to- cient of each batch of LDPE tubing, because significant volume ratio were larger, then the phenomenon of back- diffusion could start to become significant. In the example of variations can occur between batches However, the coef i- 5l1 cient, once determined, does not change with time The FIG 1, the diffusion tube is LDPE of typically 32 mm coefficient is simple enough to determine to a laboratory, overall diameter, and 3 2 mm wall thickness The preferred range of tube diameters that may be suitable are 5 mm to 50 involving setting up the lengths of tubes, standardising the conditions of entry concentration, circulation characteristics, min diameter etc, and noting the exit concentration 55 Back-diffusion, not just of the water, but of the contami- Although each diffusion tube should be individually funis in the groundwater can also occur sometimes (although the type of contaminants that are hazardous in tested, the testing is not onerous, and the number of diffusion tubes per installation is not large, at least in the context of the small concentrattons are often organic, such as chlorinated task of cleaning up a toxic plume moving through an aquifer hydrocarbon solvents, etc, the molecules of which are fairly If it is found that the diffusion tubes have reasonably 6o large, and are thus not so able to diffuse through plastic as identical diffusion coefficients, they may all be fed from a molecules of water) FIG 2 is a plan view of several of the boreholes 20, single rareulation control unit, which would be adjusted to supply the (single) concentration of remedial substance arranged in a line across the path of the oncoming plume of appropriate to the average of the coefficients In other cases, contiumnant. The diffusion tubes 30 are mounted in the it might be preferred to have a circulation control unit for 65 boreholes in down-and-return pairs 0 each diffusion tube, and to tailor the concentrations in each The boreholes comprise wells, typically of 25 cm dram tube of the remedial substance in its carrier fluid to cater for eter The boreholes contain groundwater, which flows