a preferred embodiment of the invention is shown in fig1 and comprises a flexible wall tank 10 situated in a receiving body of water 11 such as a lake . effluent material such as runoff water flows through a delivery pipe 12 extending from the shoreline 13 and leading to a central inlet compartment 15a of the tank 10 . the tank further includes a plurality of concentric compartments 15b - 15f surrounding the central inlet compartment 15a . each respective concentric compartment 15a - 15f is enclosed by flexible side walls 16a - 16f which extend generally vertically from the bottom of the receiving body of water 11 to the surface thereof and are preferably formed of waterproof plastic sheet material . it will be appreciated that , as is well known in the art , support structure will be necessary to hold the flexible walls of the present : invention in a vertical orientation . for purposes of clarity , this support structure has not been shown in the drawings . the compartment side walls 16a - 16f each include respective gates 17a - 17f for allowing effluent material to pass from the central inlet compartment 15a sequentially into each of the remaining compartments 15b - 15f and finally from the outermost compartment 15f into the receiving body of water 11 all as shown by the arrows designated &# 34 ; a &# 34 ;. since the gates contained in adjacent compartment side walls are located diametrically opposite to one another , complete dilution within each compartment 15a - 15f takes place before the effluent material leaves one compartment and enters another compartment or the receiving body of water . moreover , effluent material entering compartment 15a from delivery pipe 12 can only pass into the next sequential compartment 15b whether flow is through the intended gate 17a or through an imperfect lower seal in the compartment side wall 16a . similarly , effluent material contained in all of the remaining concentric compartments 15b - 15f can only pass into an adjacent compartment in the sequence or , in the case of water contained in the outermost compartment 15f , into the receiving body of water 11 . thus , for example , effluent material cannot pass out of sequence from compartment 15b into compartments 15d , 15e , or 15f , or into the receiving body of water 11 . as further shown in fig1 an immersible pump 19 is preferably located in the central inlet compartment 15a and supplies a treatment plant 14 via a conduit 19a . the treated water is taken from the treatment plant 14 via a discharge conduit 14a to the receiving body of water 11 . the function and purpose of the pump 19 is well known and disclosed , for example , in u . s . pat . no . 4 , 377 , 477 to dunkers which is expressly incorporated herein by reference . the concentric design of the tank 10 eliminates the need for difficult shoreline seals and guarantees sequential flow through all compartments 15a - 15f . the term &# 34 ; concentric &# 34 ; as used herein to describe the arrangement of the compartments 15a - 15f is intended to encompass overall shapes of the tank 10 other than the circular design shown in fig1 . these alternative shapes may be square , rectangular or other polygonal shapes , for example , depending on the particular application requirements . in any case it is only necessary that the arrangement of compartments be such that the inner compartment is completely surrounded by outer compartments which assure sequential flow of the effluent material to the receiving body of water 11 . an alternative tank design 110 is illustrated in fig2 . the tank 110 is situated in a receiving body of water 11 such as a lake effluent material such as runoff water is fed through a delivery pipe 12 extending from the shoreline 13 into a central inlet compartment 115a . the central inlet compartment 115a is enclosed on all sides except one by a vertical side wall 116a . the inlet compartment 115a is surrounded on all sides but one by further compartments 115b - 115d . each of the compartments 115a - 115d includes one common vertical side wall 118 which may abut and seal against the shoreline 13 . the compartments 115a - 115d further include gates 117a - 117d which allow sequential passage of effluent material between the compartments 115a - 115d in the direction of the arrows &# 34 ; b &# 34 ;. like the embodiment of fig1 the inlet compartment 115a includes an immersible pump 19 preferably located in the central inlet compartment 115a and supplies a treatment plant 14 via a conduit 19a . the treated water is taken from the treatment plant 14 via a discharge conduit 14a to the receiving body of water 11 . although this design would normally be susceptible to nonsequential flow of effluent material by leakage beneath the common side wall 118 , the tank 110 assures sequential flow of effluent material through the tank 110 where a positive shoreline seal of the common wall 118 is possible . thus , when such a positive shoreline seal is easily attainable it may be desirable to utilize this adaptation of the concentric design of fig1 . such a positive seal between the lower edge of the common side wall 118 and the floor of the receiving body of water 11 may , for example , be obtained by using one of the improved compartment wall lower edge constructions described below in connection with fig3 - 5 . it will be appreciated that once a positive seal between the lower edge of the common vertical side wall 118 and the floor of the receiving body of water or shoreline has been established , the arrangement of compartments 115a - 115d assures sequential flow of effluent material from the inner compartment 115a to the receiving body of water 11 . in this regard , effluent material entering compartment 115a from delivery pipe 12 can only pass into the next sequential compartment 115b whether flow is through the intended gate 117a or through an imperfect bottom seal in the compartment side wall 116a . similarly , effluent material contained in all of the remaining concentric compartments 115b - 15d can only pass into an adjacent compartment in the sequence or , in the case of water contained in the outermost compartment 115d , into the receiving body of water 11 . thus , for example , effluent material cannot pass out of sequence from compartment 115a into compartments 115c or 115d , or into the receiving body of water 11 . the gates 117a - 117d are located , as in the first embodiment , such that maximum dilution occurs before the effluent material leaves one compartment and sequentially enters either the next compartment or the receiving body of water 11 . that is , since the gates contained in adjacent compartment side walls are located a significant distance from one another , complete dilution within each compartment 115a - 115d takes place as a result of the effluent material having to travel a significant distance within the compartment before leaving the compartment . fig3 illustrates one embodiment of a compartment side wall , which may be any compartment side wall of either tank 10 or 110 but for illustrative purposes is shown as side wall 16a , having an improved sealing lower edge 20 . the lower edge 20 is weighted to provide a seal between the wall 16a and the floor 24 of the receiving body of water 11 . the lower edge 20 is also constructed to be highly flexible and compliant such that it more readily follows the irregular contour of the floor 24 and the contour of debris such as rocks 25 which project upwardly from the floor 24 . the lower edge 20 includes alternating weighted segments 21 and highly flexible segments 22 . the weighted segments 21 may be preformed from metal , concrete or other nonbuoyant heavy materials , or alternatively , may be sand , gravel , rock , or bottom silt used in bulk form contained within a seam at the lower edge 20 . the highly flexible segments 22 are preferably formed from open celled foam contained within a seam at the lower edge 20 . one or more apertures 23 are formed in the seam at the lower edge 20 to allow for escape of entrapped air which would reduce the effect of the weighted segments 21 by providing unwanted buoyancy , and to permit saturation of the open celled foam segments 22 with water . the compliancy of the lower edge 20 allows a more positive bottom seal to be obtained by significantly reducing the size of or eliminating gaps 28 , 29 which would otherwise form when the lower edge 20 lays on an irregularly shaped bottom floor of the receiving body of water 11 , for example , having large debris such as rocks 25 . fig4 shows another embodiment of the lower edge portion of a compartment wall 16a , which may form any one or all of the compartment side walls of the tanks 10 , 110 of the present invention . in this embodiment a flexible skirt 31 is attached along a junction 32 extending along a lower portion of the wall 16a . the skirt 31 is preferably formed of plastic sheet material and attached to the wall 16a by , for example , heat sealing along the junction 32 . the lower edge 20 of the compartment wall 16a and the lower edge 34 of the skirt 31 each include weight material 21 which may be contained within respective lower seams of the wall 16a and the skirt 34 . each of these weighted lower edges 20 , 34 are also preferably constructed according to the embodiment of fig3 to include flexible segments 22 formed of open celled foam so as to make the lower edges 20 , 34 highly compliant and flexible to conform to the irregular contour of the floor 24 of the body of water 11 . a buffer volume or zone 37 of water is defined between a lower portion of the compartment side wall 16a and the skirt 31 to further reduce leakage . another manner of preventing leakage of effluent material from a compartment is illustrated in fig5 . again , although only three compartments 15a - 15c are shown for illustrative purposes , it will be understood that the concepts employed in any flexible tank design . according to this embodiment , lower edges 40a , 40b of the respective compartment walls 16a , 16b are sealed to flexible bottom walls 41a , 41b which include ballast or weight material 42 for maintaining the bottom walls 41a , 41b on the floor 24 of the receiving body of water 11 . the compartment side walls 16a , 16b are preferably double sheet walls . by way of this design , one sheet 43 of wall 16a is sealed to the bottom wall 41a of compartment 15a at the lower edge 40a thereof and the other sheet 44 is sealed to the bottom wall 41b of compartment 15b at the lower edge thereof . in the same manner , one sheet 43 of wall 16b is sealed to the bottom wall 41b of compartment 15b along the lower edge 40b thereof and the other sheet 44 is sealed to the bottom wall 41c of compartment 15c at the lower edge thereof . ballast or weight material 42 is preferably sealed between an upper surface 45 and a lower surface 46 of each bottom wall 41a , 41b , 41c . although preferred embodiments of the invention have been shown and described above in detail , numerous structural and material substitutions and other design modifications fully encompassed by the teachings of the present invention will become readily apparent to those of ordinary skill . for example , the number of compartments in the tanks 10 , 110 may , of course , be varied according to the specific needs of the locale . also , the individual compartments of the tanks 10 , 110 could be broken up into subsections with inner subsections being in fluid communication with outer , adjacent subsections . although less efficient , the leakage problem addressed by the present invention would be solved as long as the subsections of a given compartment in the tank 10 aggregate to fully surround the adjacent , inner compartment and so on . in the case of the tank 110 , the subsections of a given compartment would aggregate to surround the adjacent , inner compartment except around the side connected to the common wall 118 . numerous other modifications will become readily apparent and applicant therefore intends to be bound not by the specific details disclosed herein as preferred embodiments but by the scope of the claims appended hereto .