The subsurface soil at a site was impacted by an accidental spill of gasoline. A soil sample, taken $10$ days after removal of the polluting source, showed a $($total petroleum hydrocarbon$)$ TPH concentration of $1,200$ mg$/$kg$.$ The second sample taken at $25$ days showed a drop of concentration to $1,100$ mg$/$kg$.$ Assume that a combination of all the removal mechanisms, including volatilization, biodegradation, and oxidation, shows first$-$order kinetics. Estimate how long it will take for the concentration to drop below $100$ mg$/$kg without any remediation measures taken