Flow in bedrock is bedrock aquifers is generally more rapid than in unconsolidated sediments, so it not surprising that microbial contamination in bedrock aquifers is often two to three times more common than in unconsolidated sediments, as shown for instance by the figure, based on data from >50,000 wells in New Jersey, USA .
A serious incident of bacterial contamination occurred in the carbonate aquifer at Walkerton (Ontario, Canada) following heavy rain, and resulted in 2300 illnesses and seven deaths. Subsequent intensive monitoring of wells showed that bacteria concentrations can change by an order of magnitude over periods as short as hours, especially following rain, as explained here.