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Groundwater Hydrology Rachel Clark, P.E. Environmental Compliance Coordinator KPB Risk Management.

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Presentation on theme: "Groundwater Hydrology Rachel Clark, P.E. Environmental Compliance Coordinator KPB Risk Management."— Presentation transcript:

1 Groundwater Hydrology Rachel Clark, P.E. Environmental Compliance Coordinator KPB Risk Management

2 Groundwater Basics - Definitions What is Groundwater? What is Groundwater?  Groundwater is water below the water table where all of the pore spaces are filled with water.  The area above the water table where the pore spaces are only partially filled with water is called the capillary fringe or unsaturated zone.

3 Groundwater Basics - Definitions

4 Unconfined or water table aquifer – an aquifer whose upper boundary is formed by the water table. Unconfined or water table aquifer – an aquifer whose upper boundary is formed by the water table.

5 Groundwater Basics - Definitions Confined or artesian aquifer – an aquifer that is confined both above and below by an aquifer confining layer. The aquifer is under pressure. Confined or artesian aquifer – an aquifer that is confined both above and below by an aquifer confining layer. The aquifer is under pressure.

6 Groundwater Basics - Definitions Aquifer Confining Layer or Aquitard Aquifer Confining Layer or Aquitard  A layer of relatively impermeable material which restricts vertical water movement from an aquifer located above or below.  Typically clay or unfractured bedrock.

7 Wells Static water level will be the level of the water table in an unconfined aquifer and the potentiometric surface for a confined aquifer. Static water level will be the level of the water table in an unconfined aquifer and the potentiometric surface for a confined aquifer.

8 Wells Well depth will be lower than static water level (water table). Well depth will be lower than static water level (water table). Pumping water level will be between the two due to drawdown. Pumping water level will be between the two due to drawdown. The radius of influence is the maximum horizontal distance where drawdown of the water table occurs. The radius of influence is the maximum horizontal distance where drawdown of the water table occurs.

9 Aquifer Characteristics Groundwater Elevation Groundwater Elevation  Distance to the static groundwater table from the ground surface or preferably, a vertical datum.  Measured in wells (existing or while drilling) or test holes. To a vertical datum requires a surveyor.  Expressed in units of feet – either below ground surface or above a vertical datum.  Values from ground surface can range from inches to more than 100 feet.

10 Groundwater Flow Maps  lines of equal elevation (equipotential).  Must have vertical datum. Flow Direction  Perpendicular to equipotential lines.

11 Aquifer Characteristics Hydraulic gradient Hydraulic gradient  Steepness of the slope of the water table.  Groundwater flows from higher elevations to lower elevations (i.e. downgradient).  Measured by taking the difference in elevation between two wells and dividing by the distance separating them.  Expressed in units of ft/ft or ft/mi.  Typical values for groundwater are.0001 to.01 ft/ft.

12 Aquifer Characteristics Porosity Porosity  The ratio of pore/void volume to total volume, i.e. space available for occupation by air or water.  Measured by taking a known volume of material and adding water.  Usually expressed in units of percent.  Typical values for gravel are 25% to 45%.

13 Aquifer Characteristics Hydraulic Conductivity Hydraulic Conductivity  Measure of the ease with which water can flow through an aquifer.  Higher conductivity means more water flows through an aquifer at the same hydraulic gradient.  Measured by well draw down or lab test.  Expressed in units of ft/day or gpd/ft 2.  Typical values for sand/gravel are 1 to 100 ft/day.  Typical values for clay are.001 ft/day. That is why is is an aquifer confining layer.

14 Aquifer Characteristics Groundwater Velocity Groundwater Velocity  How fast groundwater is moving.  Calculated by conductivity multiplied by gradient divided by porosity.  Expressed in units of ft/day.  Typical values for gravel or sand are.5 to 50 ft/day.  Typical value for Kenai River at flood stage is more than 10 ft/sec or 864,000 ft/day.

15 Ground/Surface Water Interactions The Water Cycle The Water Cycle

16 Ground/Surface Water Interactions Homer P = 621 mm, E = 385 mm Homer P = 621 mm, E = 385 mm

17 Ground/Surface Water Interactions Lakes - act as buffers to moderate fluctuations. Lakes - act as buffers to moderate fluctuations. Gains water when groundwater table is higher than lake surface. Gains water when groundwater table is higher than lake surface. Loses water when groundwater table is lower than lake surface. Loses water when groundwater table is lower than lake surface. Both gains and loses when water table is at a gradient. Both gains and loses when water table is at a gradient.

18 Excavating Into the Water Table What happens when we excavate into the water table? What happens when we excavate into the water table?  Initially, water table lowered by volume of material excavated multiplied by 1 minus porosity, i.e. excavating 5 feet into the water table will lower it initially by (1-.3)*5 = 3.5 ft.

19 Excavating Into the Water Table However, the pit will refill with water from the groundwater table. However, the pit will refill with water from the groundwater table. Q s = W ~ (r i 2 -r p 2 ) Q b = 4 K r p (h-d) During this time, the pit acts as a sink, and water flows towards it from all directions. During this time, the pit acts as a sink, and water flows towards it from all directions.

20 Excavating Into the Water Table Will the pit ever fill to pre-mining levels? Will the pit ever fill to pre-mining levels?  Lake surface will have more evaporation than pre- mining conditions. In Alaska we have relatively low evaporation rates, ( 15 in/year in Homer) so the difference in infiltration rates does not necessarily result in a measurable difference to the water table.  Terminal sink is a lake where the water balance is at a deficit to the surrounding groundwater – it takes in groundwater but never releases it.  Water balance is very difficult to accurately calculate.

21 If pit fills to pre-mining levels Surface will be horizontal, rather than at the hydraulic gradient. Surface will be horizontal, rather than at the hydraulic gradient.

22 Terminal Sink Lake never fills to pre-mining levels. Lake never fills to pre-mining levels.

23 Excavating Into the Water Table What is significant? What is significant?  Darcy’s Law Q = KAiQ = KAi Discharge = conductivity*area*gradient.Discharge = conductivity*area*gradient.  So aquifer capacity is reduced in direct proportion to aquifer depth, i.e. if the aquifer is 60 feet deep and it is permanently reduced by.5 feet, capacity is reduced by.00833 or less than one percent.  Well capacity is usually limited by the well pump, not aquifer capacity.

24 Excavating Into the Water Table How much horizontal distance to recovery? How much horizontal distance to recovery?

25 Sample Pit

26 Sample Pit – Time to fill

27 Sample Pit – Recovery distance

28 Changes in Variables VariableChange Recovery Dist. Fill Time Pit Radius IncreasesIncreasesIncreases DecreasesDecreasesDecreases Depth of Ex. IncreasesIncreasesIncreases DecreasesDecreasesDecreases Hydraulic Gradient SteeperIncreasesDecreases ShallowerDecreasesIncreases Hydraulic Conductivity IncreasesIncreasesDecreases DecreasesDecreasesIncreases

29 Any Questions?


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