Lecture Set 14

Groundwater in Southeastern Minnesota

Days 23 and 24

Water Cycle

• surface runoff
  • rainfall intensity > infiltration rate = surface runoff
  • controls on infiltration rate
    • texture of soil and bedrock - porosity and permeability
    • vegetation
    • condition of soil - baked or frozen?
• Water that does not run off the surface will soak into or infiltrate the surface and become part of the groundwater system

Water Table

• The zone of the subsurface where pore spaces within the materials are partly filled with air and partly filled with water percolating downward is called the undersaturated zone
• The zone of the subsurface where pore spaces within the materials are completely filled with water is called the saturated zone
• The boundary between the undersaturated and the saturated zones is called the water table

Flow Paths of Groundwater

• Geometry of the water table
  • The water table rises to higher elevations beneath hills and drops to lower elevations in valleys, where it intersects the level of permanent streams or lakes.
  • If for some reason the water table intersects the surface of the earth above a valley bottom, then springs will be located at that position along a hillside
  • The level of the water table is determined by the amount of water that infiltrates the surface, which is determined by amount of rainfall in a given interval of time, together with the infiltration rate of surface material
• Flow Paths beneath the water table
  • Beneath the water table, water will flow from high places on the water table, toward low places which are generally valley bottoms
  • Note that water forced downward under its own weight can actually flow upward into the bottom of streams or lakes. This is how permanent streams are kept flowing, in spite of prolonged dry spells. The flow from springs into the bottom of a stream is called base flow
  • The position of the deepest stream valley in a area is therefore a major control on the pattern of ground water flow

Winona Aquifers

• An aquifer is a porous and permeable body of rock or unconsolidated material that can both hold and transmit water
• Aquifers are often developed in sandstones or sand and gravel deposits or porous and permeable carbonate rocks
• Shales will retard the movement of ground water because they are impermeable.
• Shales generally form "confining layers" 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 "driftless area" 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 soluble carbonate rocks
  • These rocks have sinkholes developed at the surface, along with caverns beneath the surface
  • The bedrock is therefore very porous and permeable, and the sinkholes act as holes in a giant sieve to conduct water downward to storage in shallow aquifers
  • Water there seeps rapidly into aquifers from the surface, carrying surface pollutants such as animal waste, fertilizers and pesticides with it.
  • The upper most aquifers in the carbonate bedrock therefore become polluted rapidly in situations where improper land use is practiced.
• Fortunately, lower aquifers are somewhat protected from pollution by the confining layers. However, wells penetrating the lower aquifers could bring pollutants downward if the wells are not properly constructed or maintained

Regional Ground Water Flow

• Controlled by the locations of deeply incised valleys
• Mississippi River controls flow in lowermost Mt. Simon aquifer
• Smaller streams such as Garvin Brook control flow in higher aquifers

Karst Topography in Southeastern Minnesota

• A landscape dominated by sinkholes, dry valleys, sinking creeks and caverns is called karst topography
• Karst topography creates problems for groundwater quality which can only be solved by proper land use