Chapter 19

The ground has

1) Porosity- the volume of potential water storage Porosity = Volume of pores in a sample / Volume of the sample 2) Permeability- the ease with which water moves through the ground. High permeability means that water can disperse through the medium easier.

The smaller the grains of soil/rock, the harder it will be for water to disperse through it (low permeability). What does this mean for cones of depression?

Coarse sand and gravel: has high permeability --> shallow cone of depression Fine sand: has moderate permeability --> moderate cone of depression Sandy silt: has low permeability --> deep cone of depression

What does Darcy's law tell us about how the hydraulic gradient and permeability affect discharge?

Darcy's law/equation reveals that steeper slopes deliver more water (or have a higher discharge), and more permeable materials deliver more water (or have a higher discharge).

How does excessive pumping affect the local water table?

Excessive pumping lowers the water table.

How does the rate of groundwater flow compare with that of moving ocean water or river currents?

Groundwater movement is slow relative to surface water. Typical rates of flow: 1) Ocean water- 3 km/hr 2) River currents- 30km/hr 3) Groundwater flow- 0.00002 km/hr

How does the chemical composition of groundwater change with time? Why is "hard water" hard?

Groundwater reacts as it flows through an aquifer. Concentrations of dissolved ions generally increase with time. Hard water is hard because of the amount of Ca2+ (ionized calcium) and Mg2+ (ionized magnesium) ions. It's hard because carbonate minerals precipitate from it.

Is groundwater a renewable or nonrenewable resource? Explain how the difference in time frame changes this answer.

Nonrenewable because it can't necessarily be replenished quickly.

How do porosity and permeability differ? Give examples of substances with high porosity but low permeability.

Porosity: total amount of open space within a material Permeability: ability of a material to allow fluids to pass through an interconnected network of pores A material whose pores are isolated from each other is an example of a substance with high porosity but low permeability.

What factors affect the level of the water table? What factors affect the flow direction of the water below the water table?

Water table- the horizon that separates the unsaturated zone above from the saturated zone below. It is affected by-the water supply from above, rainfall, and location. Flow direction of water is affected by gravity and pressure.

How is an artesian well different from an ordinary well?

Wells are holes excavated or drilled to obtain water. An artisan well taps confined, tilted aquifers, which causes upland recharge to pressurize the aquifer, causing a well casing below to by the potentiometric surface to flow without pumping. An ordinary well penetrates an aquifer below the water table and water from the pore space in the aquifer seeps into the well and fills it to the level of the water table.

Confined aquifer

is overlain by an impermeable layer or an aquitard.

Water Table

is the surface of the saturated zone underground.

Groundwater

is water in the saturated zone underground.

Darcy's law states that groundwater flux per unit cross-sectional area of an aquifer is equal to the slope of the water table multiplied by the hydraulic conductivity.

q (discharge rate) = k (permeability or hydraulic conductivity) x i (hydraulic gradient (which is the slope of the water table)) x A (cross sectional area perpendicular to flow) q = k x i x A

Aquifer

sediment or rock that contains and transmits water easily.

Aquitard

sediment or rock that hinders water flow.

Safe yield

the rate at which you can take water out of the ground without depleting groundwater. It is measured with the conservation of mass equation: Change in Storage = I - O Change in groundwater storage = Recharge - Pumping + Discharge (plants, streams) Safe yield = Recharge - Discharge In order to not deplete groundwater, pumping rate must be less than the recharge rate.

Unconfined aquifer

when local rainwater percolates directly from land surface to the zone of saturation.

Cone of depression

when you lower the water table by placing a well and pumping the water out.

Spring

where the water table intersects the land surface, groundwater issues directly onto the surface as a spring. A spring emerges between geologic units of different porosity and permeability.