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5,605,634 5 Because the walls of the diffusion tube are not exactly uniform (not uniform, that is to say, as regards the ratio of amorphous to crystalline structure), the substance leaks out more quickly through the more porous areas of the tube wall. If the fluid is stagnant, a local volume of the fluid would then become depleted—and the more permeable the local area of the wall, the more the local volume of the fluid would be depleted. The rate of diffusion being concentration -depen- dent, that depletion would therefore slow down the ensuing diffusion through that area of the wall. Therefore, the system has at least a marginal capability to self -correct, and to keep concentrations of the remedial substance constant within the groundwater, even though wall -permeability may vary. The capability arises provided the water is left, without moving, for long periods. Thus, although periodic circulation is important for main- taining overall levels of concentration over a long time, the periods of non -circulation are important also, because they smooth out variations in concentration doe to variations in wall -permeability. As mentioned, many types of plastic tubing tend to allow migration or diffusion of molecules through the wall of the tubing, LDPE being well-suited in many cases. However, LDPE tubing, as available commercially, may not always be the same as to its crystalline structure, which is the factor that mainly sets the diffusion coefficient. Wall thickness variations also cannot be ignored. The engineer should therefore carry out laboratory tests on samples of the particular batch of tubing, in order to determine the diffusion coefficient for that batch. Knowing the diffusion coefficient, and knowing the extent, nature, and concentration, of the contaminant in the groundwater, the inground velocity of the groundwater, the positioning of the boreholes, the nature of the remedial substance, and so on, the engineer can compute what the concentration of the remedial substance in its carrier fluid should be, and can set up the fluid circulation means appropriately to maintain that required concentration. The engineer adjusts the concentration of the remedial substance in its carrier fluid as the main agency by which the flux rate of the remedial substance is finely and accurately controlled. He can make gross adjustments to the flux rate by selecting, as the diffusion tube, a tube with a different wall thickness or diameter. Of course, a finer degree of control is required for the diffusion flux rates than can be achieved merely by altering wall thickness. The prudent engineer should check the diffusion coeffi- cient of each batch of LDPE tubing, because significant variations can occur between batches. However, the coeffi- cient, once determined, does not change with time. The coefficient is simple enough to determine in a laboratory, involving setting up the lengths of tubes, standardising the conditions of entry concentration, circulation characteristics, etc, and noting the exit concentration. Although each diffusion tube . should be individually tested, the testing is not onerous, and the number of diffusion tubes per installation is not large, at least in the context of the task of cleaning up a toxic plume moving through an aquifer. If it is found that the diffusion tubes have reasonably identical diffusion coefficients, they may all be fed from a single circulation control unit, which would be adjusted to supply the (single) concentration of remedial substance appropriate to the average of the coefficients. In other cases, it might be preferred to have a circulation control unit for each diffusion tube, and to tailor the concentrations in each tube of the remedial substance in its carrier fluid to cater for 6 the variations in coefficient, whereby the same flux rate can be maintained from each tube. In some cases, it may be preferred, in fact, to vary the flux rate from the various diffusion tubes. Many plumes are more 5 concentrated in the centre than at the edges, for example, and the engineer may wish to tailor the flux rates tube to tube, across the width of the plume, accordingly. Equally, some plumes vary in concentration over the vertical depth of the plume. In that case, the engineer may to specify, for instance, that the portion of the diffusion tube that will diffuse remedial substance into the topmost and bottommost margins of the plume be of a thicker wall thickness than the tubing which diffuses remedial substance into the central band of the plume, where the contaminants 15 may be more concentrated. The pipes and conduits which convey the remedial sub- stance in its carrier fluid need not be all LDPE tubing, like the diffusion tube. For example, the pipes may be of metal tubing, through which no discernible diffusion can take 20 place.. In order for circulation to take place, the engineer must provide a down -tube and a return -tube. The engineer may arrange that both the down -tube and the return -tube are both diffusion tubes, of LDPE or other suitable plastic. Or, he 25 may arrange that only one of the tubes is a diffusion tube, 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 the diffusion tube is the return tube. It is possible for back -diffusion to take place. That is to 30 say, it can happen that molecules of groundwater lying outside the diffusion tube can diffuse into the tube. However, if this should occur, its effects on the dilution -concentration of the remedial substance in its carrier fluid would be picked up by the routine sampling, and the effects could be adjusted 35 for automatically. If the remedial substance is a gas, perhaps it could not be ruled out that a small quantity of back - diffusing water might collect on the inside of the diffusion tube, but the volume of water thus collected would hardly be significant enough to affect the performance of the system. ao Thus, especially if a pressure differential is engineered whereby the fluid on the inside of the diffusion tube is at a higher pressure than the surrounding groundwater, back - diffusion can largely be ignored. One of the reasons back -diffusion can be ignored is that, 45 in the system as described, the ratio of diffusion area of the tube to the volume of fluid contained in the tube is small. If the diffusion tubes were small-bore, such that the area -to - volume ratio were larger, then the phenomenon of back - diffusion could start to become significant. In the example of 50 FIG. 1, the diffusion tube is LDPE of typically 32 mm overall diameter, and 3.2 mm wall thickness. The preferred range of tube diameters that may be suitable are 5 mm to 50 mm diameter. 55 Back -diffusion, not just of the water, but of the contami- nants in the groundwater can also occur sometimes (although the type of contaminants that are hazardous in small concentrations are often organic, such as chlorinated hydrocarbon solvents, etc, the molecules of which are fairly 60 large, and are thus not so able to diffuse through plastic as molecules of water). FIG. 2 is a plan view of several of the boreholes 20, arranged in a line across the path of the oncoming plume of contaminant. The diffusion tubes 30 are mounted in the 65 boreholes in down -and -return pairs. The boreholes comprise wells, typically of 25 cm diam- eter. The boreholes contain groundwater, which flows