Groundwater • All water found underground, frequently in: • Joints and cracks in rocks • Open space between sediment grains • Cave systems http://www.ctecomputer.com/fountains/
Porosity and Permeability Porosity –volume of open space within a body of rock or sediment (all space not occupied by solid material) Permeability – how easily fluid can move through the rock or sediment, which is frequently dependent on how much connectivity there is between pores.
Porosity Volume of open space within a body of rock or sediment. Usually expressed as a percentage of the total volume of rock or sediment. http://www.epa.gov/
Aquifers • Bodies of rock and/or sediment with high permeability through which groundwater flows. • Bodies of rock with low permeability are called aquicludesoraquitards. • Unconfined aquifer- aquifer with open to the surface • Confined aquifer - aquifer confined at the top by an aquiclude or aquitard
Unsaturated zone saturated zone Unconfined aquifers • Groundwater in unconfined aquifers is found in two underground zones: • Unsaturated zone - pore space primarily filled with air • saturated zone - pore space filled with water • The top of the saturated zone is a surface called the water table.
The weight of the overlying rocks puts pressure on the water in a confined aquifer. Potentiometric surface - level to which water in a confined aquifer will rise if given the chance. Artesian well - a well in which water rises without pumping due to a high potentiometric surface. If the potentiometric surface is above the well head (the top of the well), water will flow to the surface, making a flowing artesian well.
Pollution in groundwater is worse than pollution in surface water because it: • Has a longer residence time • Is harder to clean up • Is harder to trace to source
Point Source Pollution The contaminant goes with the flow – wherever the water goes, the pollution follows.
Point Source Pollution If the leak is caught early and flow within the contaminated aquifer has been well studied, it may be possible to force the pollution out with a combination of pressure (to concentrate the contaminant) and pumping. If the flow of that aquifer at that spot is not well understood, it is possible to make the situation worse. Pumping out the contaminant is not a viable option for most groundwater pollution.
well 1 well 2 stream or lake underwater storage tank Water Flow Through Model unconfined aquifer aquiclude confined aquifer Well 1: taps the upper, unconfined aquifer Well 2: taps the lower, confined aquifer
Water Table and Potentiometric Surface unconfined aquifer aquiclude confined aquifer Water table - in unconfined aquifer, the border between the saturated and unsaturated zones Potentiometric Surface - level to which water from a confined aquifer will rise if given a chance. Determined by pressure in confined aquifer.
Water Table and Potentiometric Surface Both the surfaces are dynamic, changing with the amount of water flow through the system
contaminant in underground tank Groundwater Contamination Contaminant - any material that can be moved through groundwater
Groundwater Contamination In groundwater with no flow, the contaminant will move through water by diffusion (from high concentration to low concentration)
Groundwater Contamination In more realistic model (e.g., in which water flows out at stream) the contaminant will flow with the water, making a pollution plume.
cone of depression Groundwater Contamination Well 1 Pumping of Well 1 results in a cone of depression in water table, and thus the pumping of the contaminant up the well.
HYDROLOGIC CYCLE P = R + ET + I Precipitation = Runoff (to streams) + EvapoTranspiration(from soil/plants to atmosphere) + Infiltration (to groundwater)
GROUNDWATER 1 year March/ April March/ April WATER LEVEL October 1 year TIME Why the fluctuations (with no pumping)? Time of years? 1 year cycle, with a high and low of groundwater levels. Where does groundwater come from?
HYDROLOGIC CYCLE P = R + ET + I Growing season (March – September) More Transpiration (T) More Evaporation (E) Less Infiltration (I) to groundwater Lower groundwater levels Plus, possible irrigation, removal of groundwater.
ACTIVITIES Groundwater Model (in groups of 3-4) 12.4 All (as a group)