Shales generally form &quot;confining layers&quot; in the bedrock, confining water in intervening aquifers The lowermost aquifers in an area will contain the oldest and most pure groundwater This groundwater is old because it takes a long time to recharge the deep aquifers by infiltration of water at the surface This groundwater is often more pure because it comes from a time before industrial use of the land introduced pollutants This groundwater is often more pure because it has had more time for pollutants to be removed from the water by interaction or exchange with the sediment particles which surround the pore spaces Winona's lack of glacial cover Winona lies in the &quot;driftless area&quot; and lacks a cover of glacial sediment Bedrock is very near the surface, beneath a thin cover of soils Water infiltrating the surface will find its way to bedrock aquifers in a short period of time Without glacial drift covering the surface, the infiltrated water has no opportunity to be cleansed by interaction with this sediment The bedrock at the surface of the upland in the Winona area is dominated by 
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old because it takes a long time to recharge the deep aquifers by infiltration of water at the surface This groundwater is often more pure because it comes from a time before industrial use of the land introduced pollutants This groundwater is often more pure
pollutants
by
the pore spaces Winona's lack of glacial cover
T:from a time T:before industrial use of the land introduced pollutants
from the water by interaction or exchange with the sediment particles
very near the surface
no opportunity to be cleansed by interaction with this sediment
the oldest and most pure groundwater This groundwater is old L:in the bedrock
more time for pollutants to be removed from the water by interaction or exchange with the sediment particles
a cover of glacial sediment
often more pure because it has had more time
by interaction with this sediment
T:often more pure because it comes from a time before industrial use of the land introduced pollutants
its way to bedrock aquifers T:in a short period of time
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