As you are finding out, the IRS dictates the "Recovery Period" (number of years to depreciate) based on the asset, and it doesn't matter if you expect it to last that long or not. You are stuck with those amounts.
Yes, the well pump would qualify for the Special Depreciation Allowance. I'm not sure if it would be considered part of the well itself (a "Land Improvement", depreciated over 15 years) or considered a separate appliance/tool/machine (probably depreciated over 7 years). But either way it would qualify for the Special Depreciation Allowance
