Permanent, covered storage is recommended, but some agencies still store salt in outdoor stockpiles on bituminous or concrete pads. This low-cost method provides maximum storage space and easy access. Whether stored inside or outside, salt must be on an impermeable pad, or base.
See more:
http://www.jcw.org/Env_gm_ip_wql/ip_pdfs/ipSaltStorage.pdfCe...

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Note added at 3 hrs (2013-09-19 11:00:58 GMT)
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STORAGE/LOADING PADS
All salt should be stored, mixed, and loaded on an impervious pad to prevent salt from
infiltrating into the subsurface. Pads should be large enough to contain the salt and provide
maneuvering room for trucks, loaders, and other equipment. Pad thickness and material should
be capable of supporting the pile and the associated heavy equipment, thereby preventing
cracking. Asphalt or concrete are preferred. If concrete is used, it should be of the high quality,
air-entrained variety and treated with sealants, asphalt-type coatings or other treatments to
repel salt and prevent chipping and crumbling.
The pad/loading pad should be sloped (1-2%) to let water drain away in accordance with any
measures in place for storm water management (see Storm Water Control section). There
should be no floor drains. Around the exterior of the structure, the pad should be curbed or
sloped away to keep out run-on.
http://www.ohiodnr.com/Portals/23/pdf/Draft OWRC Salt Storag...