There was an underground oil tank on the property & removed years ago, but then had the remediation to remove the residual oil contamination under the garage(the house had to be supported on helicial piers as crews dug down 10 feet to the water table).
You can not deduct repairs or improvements on your primary residence. You can add the cost of improvements to the value of your home. That way, when you sell it, the potential taxable value will be less. So save the documentation for this oil remediation in case you decide to sell the place.
An improvement (or betterment) is work that leads to a permanent change to the property that improves its value or extends its useful life. More common examples would be new windows or a new heating and cooling system, but remediating the impact of an oil tank would certainly count as improving the property value. You can add the cost of improvements to the cost of the property when you sell, which may reduce your capital gains tax.