The present invention relates to means for containing and detecting leaks in storage tanks.  More particularly, the present invention relates to apparatus and methods for constructing such apparatus, for containing multiple or repeated leaks of hazardous, polluting, or otherwise undesirable fluids or solids from storage tanks, and for quickly detecting and signaling the presence of such leaks, thereby minimizing the dangers posed by storing such fluids and solids and complying with regulations requiring such containment and detection. 
Storage of hazardous liquids and solids used in numerous industries requires storage tanks of all sizes.  Chemical process plants, refineries, oil and gas production sites, manufacturing plants, and the like require storage of a variety of materials for processes used in such facilities.  The materials so stored, whether gases, liquids, or solids, may include chemicals and compounds that could endanger the environment or pose significant health risks in the event of leakage into areas surrounding these storage systems.  Heightened awareness in recent years over the quality of the environment has increased and tightened the rules, regulations, and requirements governing storage of such materials.  Growing concern with public health issues has further emphasized the need to prevent leaks of hazardous materials into the environment to prevent, for instance, contaminating drinking water or exposing humans or wildlife to hazardous compounds. 
Among the regulations governing the storage discussed above, by way of example, are the rules promulgated by the Environmental Protection Agency ("EPA") for hazardous waste management systems.  See, e.g., 40 C.F.R. .sctn..sctn.260-65 and .sctn.268 (1988).  The EPA rules govern, among other matters, tank systems that store hazardous wastes.  Id., .sctn.260.10.  Hazardous wastes subject to these regulations include a host of residues.  byproducts, and wastes that are generated or used in any of a lengthy list of chemical, manufacturing, and other processes.  Id., .sctn..sctn.261.3-ff.  Under these regulations, what is designated "secondary containment" must be provided on all new tank systems storing hazardous wastes, and on existing hazardous waste systems as of various effective dates subsequent to Jan. 12, 1987.  See id., .sctn.264.193 and .sctn.265.193.  Such a secondary containment system must permit spilled or leaked waste to be removed from that secondary containment system within twenty-four (24) hours after a spill or leak.  Id. Furthermore, the EPA regulations deem a tank system "unfit for use" if it is no longer capable of storing or treating hazardous wastes without itself posing a threat of release of hazardous waste to the environment.  Id., .sctn.260.10 Finally, the EPA regulations require that any hazardous waste materials released into a secondary containment system must be removed within twenty-four (24) hours, or in as timely a manner as is possible.  Id., .sctn.264.196 The tank system cannot be used until the released waste is removed and any necessary repairs to the system are made. ( Although reference is made in the present application to EPA regulations and the definitions used in those regulations, those definitions are not intended to, and do not, generally govern the use of terms in this application.  Except as may be expressly noted to the contrary, all terms used in this application are to have their common and accepted meanings.) 
Therefore, an acceptable secondary containment system under these EPA regulations must, in general terms, be capable of collecting and accumulating liquids that leak from a tank, detecting such a leak or the presence of the accumulated liquids in the system, and permitting removal of such liquids, all within twenty-four (24) hours of the leak.  Id. As can be seen, therefore, the EPA regulations, as well as increasing safety and health concerns, have imposed stringent requirements for containing, detecting, and removing leakage of hazardous materials from storage tank systems.  Effective, economical, and safe double containment and leak detection systems, therefore, are not only desirable but also mandatory, both for new and existing tank systems. 
The cost of building new systems or converting old systems to comply with the EPA regulations could be astronomical if not performed with a minimum of alteration to tank systems built under previous requirements.  Prior attempts at meeting the EPA regulations have encountered problems and proven unsatisfactory, for a variety of reasons.  For example, various plastic liners, both internal and external, have been used in trying to meet the secondary containment requirement of the EPA regulations.  Such liners, however, have split at their seams and would lead to contamination of the soil in t he event of external tank leaks.  To meet the requirements for removal of wastes, the contaminated soil then has to be removed and disposed of, which requires either removing the tank bottom or lifting the entire tank, to permit digging up the soil.  This process of lifting or dismantling the tank and removing soil (which generally needs to be replaced) is very expensive and time-consuming.  Another unsuccessful attempt to meet the EPA secondary containment regulations has utilized double-walled and double-bottom tanks, generally made of steel, with the annular space between the walls and bottoms filled with sand or other filler material.  This latter technique has also proven to be unacceptable, because it fails to allow for removing, cleaning and disposing the filler material should a leak occur.  It is important to realize that any tanks which are of double bottomed construction are susceptible to leaks in two different directions.  First, it is possible for the baffle plate supporting the stored material to form a leak so as to allow the stored material to flow through the leak.  It is also possible for ground water, and other materials, to corrode the tank bottom to the extent that the very bottom of the double-bottom tanks will corrode and leak before the material-supporting baffle plate leaks.  It is possible for either the stored material to leak into the containment space or it is possible for ground water to seep from under or around the tank system into the containment space.  It is also possible for leaks to occur along the tank shell in the area between the double bottoms.  Whenever a leak occurs, it is possible for spillage to occur.  As such, it is both important to detect the presence of any stored materials in a containment space and it is important to detect any breaches in the containment space.  As such, it is very important to provide a dual system of leak detection and prevention. 
The present inventor has had considerable experience with the subject of U.S. Pat.  No.  4,939,833, the parent to the present application.  U.S. Pat.  No.  4,939,833 describes a double containment and leak detection apparatus which includes a tank, a tank bottom, and a surrounding shell.  A containment baffle is provided above the bottom of the tank.  A suitable leak detection means, such as fiberoptics, gas detectors, and/or valves, are provided so as to interact with this containment space.  In practical use, it was found that the leak detection means (of whatever form) was very effective in detecting the presence of any stored material found in the containment space.  This original configuration, however, lacked the ability to detect leaks or breaches in the integrity of the tank bottom.  Under such circumstances, there would be no leakage from within the tank to detect in the containment space.  However, if the tank bottom had a breach in its integrity, then any liquid that would leak from the tank into the containment space would also leak through the breach in the tank bottom.  Under this possible situation, the leak detector would not necessarily determine that a leak had occurred.  As such, it was felt imperative to develop a system which would effectively analyze and determine whether a breach in the integrity of the tank bottom and/or tank shell, had occurred.  Furthermore, it was felt that back-up systems and dual detection would effectively prevent the situation where undetected failure in a component of the systems would prevent effective leak detection. 
It can be seen, therefore, that a need exists for meeting EPA regulations and satisfying environmental and safety concerns in general by providing economical, effective, and reliable double containment and leak detection systems for storage tanks, for both new and existing storage tank systems.