Geology 101 Lab Hydrogeology Groundwater Groundwater the water held in the pore spaces of bedrock also found in fractures and caves Groundwater quantity and availability depend on porosity and permeability Porosity the fraction of the volume of a rock or sediment that is open void space Porosity volume of pores total volume 1 Which one of the sediments below has a higher porosity has more void space that could be filled with water 2 a Think back to the rock groups igneous sedimentary and metamorphic discussed in the first half of the semester What rock group would typically have rocks with the highest porosity and thus be a good source for groundwater b Name a rock type within that group that you think would have a high porosity 3 In rocks that have very little or no porosity what features may be present that could make them a good source for groundwater Permeability the measure of ease of water flow through bedrock Permeability is dependent on porosity and connectivity of pores Rocks or sediment may have high porosity and high permeability ex sandstone sand or high porosity and low permeability ex clay Rocks and sediments with both high porosity and high permeability are ideal and economically important sources of groundwater 4 Both of the sediments below have high porosity but one is more permeable than the other Which one of the sediments below is more permeable Why A B 5 Imagine a well was put into the sediments shown in A well A and the sediments shown in B well B Which well would you be more likely to extract a consistent supply of water from How Fast Does Groundwater Flow 1856 Henri Darcy determined that water flows downhill and thus the velocity of groundwater must be related to 1 hydraulic gradient the slope of the water table o Determined by h1 h2 L where h is head the height of the water surface water table at location 1 h1 and location 2 h2 over L the distance between h1 and h2 L and 2 hydraulic conductivity k the permeability of rock or sediment and the density and viscosity of the fluid Darcy s Law describes the flow of fluid through a porous medium the law is based on the results of experiments conducted by Henri Darcy on the flow of water through beds of sand Darcy s Law states where h1 h2 V k L V velocity k hydraulic conductivity h head and h1 h2 L is the hydraulic gradient 6 Discuss a few reasons why you think geologists may think it s important to know how fast groundwater is flowing 7 Consider why we might be concerned about the proximity of a leaky landfill to a large river Problem landfill leachate that gross liquid stuff that s in the bottom of your trash bag after you ve forgotten to take the trash out for weeks and things have begun to decompose This leachate often contains many toxic chemicals and metals that leak into the groundwater and may eventually end up in well water drinking water as well as in rivers and lakes What would you be concerned about if you lived near a site like this Case Study The Norman Landfill in Norman OK The Norman Landfill a municipal landfill that operated from 1922 until 1985 the landfill accepted unrestricted solid waste the landfill was unlined modern landfills have an impermeable clay liner at the base to prevent leaking closed in 1985 and covered with clay and vegetation Aerial view West Cell Transect East Cell Oklahoma Norman Landfill 8 Examine the map below of the Norman Landfill showing contours of the water table surface in the area Draw several arrows indicating the direction groundwater is flowing 331 0 330 0 A 329 0 Scholl et al 1999 9 If the mean hydraulic conductivity k in this area is 5 4m day how long will it take landfill leachate at point A to reach the Canadian River 1km away hint use Darcy s Law 10 What would the risk be for people living downstream of the Canadian River if the groundwater flow in this area was faster