How / where do I deduct lead abatement on a rental property?

The 2008 proposed regulations required a taxpayer to capitalize an amount paid to ameliorate a material condition or defect that existed at the time the taxpayer acquired or produced the property. The rule follows the general principle that a taxpayer must capitalize costs incurred to correct a pre-existing defect in acquired property regardless of whether the taxpayer was aware of the defect at the time of acquisition. 

If value increases in this context, then the remediation costs must be capitalized.

However,26 U.S. Code § 198 - Expensing of environmental remediation costs

YOU MUST have a  statement from  your State environmental agency for this election.

The treatment depended on each individual circumstances and you must have proper documentation as a "qualified contaminated site." from the State environmental agency.