ACC - Xco is an accrual basis taxpayer with multiple lines of businesses

Xco is an accrual basis taxpayer with multiple lines of businesses. One business is a gas station. The land underneath the gas station did not appear contaminated when Xco purchased it. However, the land now has potential soil and groundwater problems (environmental liabilities). Xco engaged in a section 351 tax free exchange transferring the gas station to a new subsidiary Sco in exchange for the stock of Sco and the assumption of the environmental liabilities. (Xco controls Sco)

Before the transfer, Xco did not take any environmental remediation efforts to clean up the land’s
soil and groundwater problems. (No change in basis perhaps?)

How is the basis of Xco’s land determined? (Sco takes the same basis as Xco, but I don't know
how Xco determined their basis. Historical?)