Step 1: Think about how contaminants travel underground.
A pollutant moves through groundwater by following whatever open pathways the rock or soil offers.
Step 2: Compare the pathway types.
Fractured rocks are laced with interconnected cracks and joints that act like open highways for water and dissolved contaminants, while clay, shale, and peat have tiny, poorly connected pores that slow flow down a lot.
Step 3: Draw the conclusion.
Because fractures offer far less resistance to flow, contamination spreads noticeably faster through fractured rock than through those low-permeability formations.
\[ \boxed{\text{Fractured rocks}} \]