Chap 17 ground water 1

Groundwater • Hydrogeology- the study of groundwater • Groundwater is water stored beneath the Earth’s surface in pores and fractures in the rocks– pores are the space between grains in a rock– fractures are an open crack in a rock

Groundwater • 96% of the fresh water available to humans is groundwater

Porosity versus Permeability • porosity- void space in the rock • permeability- ability of rock to transmit water – proportional to how fast water will flow in the rocks– A measure of how connected the pores are (for this reason a high porosity does not necessarily mean high permeability) • if pores are not interconnected they will not transmit water (vesicular basalt) • if the water is held in pores by electrostatic charge on mineral grains the water will not flow (clays)

Porosity Demonstration

Examples of Porosity versus Permeability • Uncemented Sandstone high porosity and high permeability • Cemented Sandstone – high porosity, but pore pathways blocked by cement, so moderate to low permeability • Vesicular Basalt – high porosity, but pores not connected, so very low permeability

Unfractured Shale - very low permeability Fractured Shale moderate permeability

Aquifer and Aquitard - determined by Permeability • Permeability is the ability of a material to transmit a fluid and depends on the connectivity between pores • An aquitard is an impermeable or prevents water movement layer that hinders • Example: Clay, shale or limestone (without caves) • An aquifer is a permeable rock strata or sediment that transmits groundwater freely • Example: sand, gravel, sandstone, cave-filled limestone Source of Groundwater • Rainwater that infiltrates (soaks) into the ground (rather than running of into streams) is the source of groundwater– This is referred to as groundwater recharge– Any surface-water impoundment (lakes, streams, swamps, and wetlands) will enhance rainwater infiltration (groundwater recharge).– For this reason wetlands are so important, they enhance groundwater recharge. • Wetlands are disappearing fast because of real estate development, government regulations have been enacted to prevent this.

Surface water impoundments allowing rainwater infiltration to groundwater

Distribution of underground water

soil moisture-shallow surface layer wetted by each rain • zone of aeration–the zone between the surface and the water table where there is both air and water in the pore space • water table-the top of the zone of saturation. Where groundwater begins. • zone of water saturation There is only water (no air in in the pore spaces). Water in this zone is groundwater

Zone of Aeration

Zone of Saturation

Water Table Features • the water table shape is a subdued replica of the ground surface, because of the slow infiltration of rainwater • The depth of water table goes up in the rainy season, when there is more rainwater recharge, and down in the dry season, when there is little to no rainwater recharge. • the level of water in a surface water impoundment, such as a lake, stream, swamp or marsh, marks the level of the adjacent water table

Water Table wet period

Water Table dry period

Water Table wet period

Interaction between Groundwater and Streams • Gaining streams – gain water from the inflow of groundwater through the streambed– The adjacent water table rises above than the stream surface • Losing streams – lose water to groundwater by outflow through the streambed– The adjacent water table drops below the stream surface if connected to the water table– In dry areas the water table can be disconnected • Most streams are gaining in some sections and losing in others

Unconfined versus Confined Aquifers • unconfined aquifer– the aquifer material extends continuously to the ground surface– The water table marks the level of water in an unconfined aquifer • confined aquifer– is an aquifer that is sealed by aquitards above and below, so the water can be in the aquifer under artesian pressure .– Recall an aquitard is an impermeable layer such as clay or shale that will not transmit water. – artesian pressure is elevated pressure so when the aquifer in punctured by a well the water in the well will rise above the top of the aquifer.– flowing artesian is when the water will flow above the ground surface.

Unconfined aquifer recharge is along the entire ground exposure Confined aquifer recharge is only in the small outcrop area

How Artesian Pressures develop in Confined Aquifers– Artesian pressures are a result of the high elevation of the recharge zone and the aquitards confining the water so the pressure cannot be released.

• The water table is the surface that marks the level of water in an unconfined aquifer • A pressure surface marks the level water will rise in a well in a confined aquifer • artesian well when water in the well will rise above the top of the aquifer. • flowing artesian well is when the water will flow above the ground surface.

nonflowing artesian well- water in well rises above the top of the aquifer, but does rise up to the ground surface so it does not flow out of the well flowing artesian well- water rises above the ground surface and flows out of the well

Artesian well Flowing Artesian Well

Classic Artesian System Flowing artesian well in South Dakota