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first use and each subsequent use, percolation filters and traps fine sediments and thereby continually reduces the permeability of the basin interior. Thus, although some minor seepage should be expected , particularly during the basin ' s first use, the basin floor and dike walls are, within limits, self-sealing . Third , elements of the facility design and its operational guidelines incorporate additional precautions against the off- site migration of saltwater. Operational precautions, most notably the implementation of a groundwater monitoring program , are discussed in Sections 2 .4 , 3 . 5 , and 4 . 5 . Design features to control the off-site migration of basin seepage include a perimeter ditch , discussed below . A perimeter ditch, constructed at a 20-ft setback from the outside toe of the containment dike and surrounding the basin (Figure 2 . 1 ), will serve several purposes . First, as discussed above, the ditch must interdict the lateral spread of saline seepage through the dike ' s side slopes and foundation . To accomplish this purpose , the mean invert of the ditch must fall at or below the mean excavated grade elevation of the basin interior. If inspection of the dike during dredging and decanting identifies excessive basin seepage , its control may require that all water entering the perimeter ditch be continuously pumped back to the basin until all ponded water is released over the weirs. Dike inspection requirements during dredging and decanting arc discussed in Sections 3 . 2 . and 3 . 3 . Additional design measures to control basin seepage (e . g . , underdrains) require site- specific geotechnical information not presently available , and thus must be deferred to the facility ' s final design phase . In addition to intercepting seepage from the basin , the perimeter ditch must serve two other functions . First, the ditch must maintain the drainage and conveyance capacity provided by the east-west ditch that presently bisects the site . Facility construction requires that this ditch be rerouted around the containment basin . As shown in Figure 2 . 1 , the perimeter ditch will connect to the existing ditch on the basin ' s western side, redirect the ditch ' s flow around the basin ' s northern and southern sides, then reconnect to the existing ditch on the basin ' s eastern side before continuing to the Indian River. Thus, the perimeter ditch must match the existing ditch ' s invert elevation at both intersection points . By these criteria, the perimeter ditch ' s preliminary design includes a mean invert elevation of approximately 45 ft NGVD, or 4 . 2 ft below the existing mean grade elevation within the basin ' s footprint, 1 V ; 214 side slopes and a bottom width of 3 ft, to yield a mean top width of 20 ft. The slope of the existing ditch between the prescribed intersection points dictates the mean slope within the perimeter ditch . Second , the perimeter ditch must control stormwater runoff from the exterior face of the containment dike, the perimeter road , and portions of the buffer area . 22