Sub-Surface Movement of Groundwater

Unsaturated zone

• Water moves primarily downward due to gravity
• Water infiltrates from surface and moves downward to water table or ponds on impermeable surface (clay layer, etc.) to form a Perched water table

Saturated zone

1. Water seeks its own level
2. Water will flow from high water levels to low water levels
3. Water levels can be measured by wells
4. Contour map of water levels can provide information on groundwater flow directions
5. In most cases, water flow direction is perpendicular to the water level contour lines (from high to low values of water level)

RECHARGE AND DISCHARGE AREAS

• Surface water and groundwater are generally connected with flows in and out of the subsurface
• Surface areas where water flows from surface into groundwater are called recharge areas
• Surface areas where water flows from groundwater out onto ground surface are called discharge areas

WATER LEVEL CONTOUR MAPS

1. Contours are lines on 2-dimensional maps representing equal values of a parameter
2. You are probably used to looking at topographic maps which show contour lines of ground surface elevation
3. When a map is made with equal interval contour lines (every 1 ft, or every 2 ft, or every 5 ft, etc.), the spacing of contour lines provides visual clues to the change in slope
4. Closely spaced contour lines would represent steep slopes
5. Widely spaced contour lines would represent gentle slopes
6. Water level contour maps provide the same information on water level slopes (hydraulic gradients)

HYDRAULIC GRADIENT

1. Hydraulic gradient is the slope of the water level
2. Hydraulic gradient = [water level at point A - water level at point B]/Distance between point A and B
3. Large hydraulic gradient = steep slope in water level change
4. Most regional groundwater flow has values of 0.001 to 0.0001
5. Hydraulic gradient of 0.001 = 0.1 ft (1.2 inches) drop in water level over 100 ft distance
6. Hydraulic gradient of 0.0001 = 0.01 ft (0.1 inch) drop in water level over 100 ft distance

DARCY'S LAW - GROUNDWATER FLOW RATE OR DISCHARGE

1. Henri Darcy was a French engineer in late 1800s
2. Darcy studied the flow of water through sand filters for water treatment
3. He measured the flow rate or discharge (with units of volume of water per unit time; similar to stream discharge) through porous medium (sand)
4. Q is the abbreviation for flow rate
5. He found Q increased with increasing hydraulic gradient (steeper sloping water level)
6. He found Q increased with cross-sectional area of flow (similar to streams)
7. He defined an empirical constant for geologic material called hydraulic conductivity

HYDRAULIC CONDUCTIVITY (K)

1. Hydraulic Conductivity is term used for permeability of geologic material to water flow
2. K is abbreviation for hydraulic conductivity
3. Units are length/time
4. Q = K * Cross-Sectional Area (A) * Hydraulic Gradient

DARCY'S LAW - HOW FAST DOES GROUNDWATER FLOW?

• Velocity of water through porous medium can be calculated with Darcy's Law
• Velocity = (K * Hydraulic Gradient)/Effective Porosity

STREAMS AND GROUNDWATER

• Losing streams occur when surrounding groundwater levels are lower than the stream's water level
• Gaining streams occur when surrounding groundwater levels are higher than the stream's water level
• Flow in streams provided by groundwater is called baseflow

SPRINGS

• Water flows freely from ground surface
• Occur where water table intersects ground surface
• Springs seen along cliff faces generally occur when downward percolating water collects on an impermeable rock layer è this perched water table will flow out of cliff face above the impermeable layer
• Springs occur when fractures in impermeable rocks bring water to surface
• Springs occur when solution channels in limestone bring water to surface

WELLS

• Hole bored or drilled into the saturated zone
• Can measure water level in aquifer
• Can obtain water samples
• Can pump water to surface for water supply

PUMPING GROUNDWATER WELLS

• Pumping water from a well causes a lowering of the water level in the well (drawdown)
• Water will flow from surrounding geologic material (high water level) to the well (low water level)
• Cone of depression in the water surface produced by pumping

HIGH PLAINS OR OGALLALA AQUIFER

1. High Plains aquifer occurs in the states of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas and Wyoming
2. Ogalalla Formation (also referred to as Ogalalla Aquifer) is the principal aquifer in the High Plains aquifer
3. Other permeable formations (sandstones, siltstones) are also part of the High Plains Aquifer
4. Ogalalla Aquifer is an unconfined aquifer consisting of alluvium (sand and gravel)
5. The alluvium was deposited by streams draining from the eastern Rocky Mountains
6. Aquifer is recharged by rainfall (16 inches annually in western part of aquifer, up to 28 inches annually in eastern part of aquifer)
7. Potential evapotranspiration ranges from 60 to 105 inches annually
8. You do the math - how much recharge could occur!
9. Potential evapo transpiration refers to maximum possible evapo-transpiration and generally actual evapotranspiration is lower
10. Estimates of annual recharge to the aquifer are 0.024 to 6 inches per year
11. Saturated thickness of alluvial aquifer ranges from 0 to 1000 ft and averages about 200 ft thick
12. Prior to development, 3.42 billion acre-feet of drainable water was in storage in the aquifer
13. Starting in the late 19th century, the aquifer was tapped by wells for irrigation
14. By 1978, 170,000 wells pumped 23 million acre-ft per year
15. In some areas, pumping rate exceeded aquifer recharge rate resulting in water table declines up to 100 ft
16. Still have 3.25 billion acre-feet of drainable water in aquifer
17. However, increased cost of pumping from deeper depths prohibits the use of this water for farming