Aquifer Test Analysis Carter Lake, Iowa

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Aquifer Test AnalysisCarter Lake, Iowa

• Luca DeAngelis, P.E., P.G.
• Hydrologist
• Layne Western

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Introduction

• The community of Carter Lake plans to pump
  groundwater from an alluvial aquifer to maintain
  a desired level of a lake near Kiwanis Park.
• Layne Christensen Company (Layne) was retained by
  Carter Lake to estimate percentage of water from
  current production well that is obtained from
  induced infiltration from lake.
• Objective
• Estimate distance from lake for future wells that
  will be least affected by infiltration from the
  lake

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Outline

• General Carter Lake Water Balance
• Layne Activities
• Conclusions

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Water Balance
Streamflow (Qi)
Equation Form
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Water Balance

• Inflows
• Stream flow into lake 0
• Mean annual precipitation 31 inches (precip
  onto lake)
• Runoff Drains an area of 1,930 acres
• Groundwater flux into ?
• Outflows
• Groundwater flux out ?
• Lake evaporation 40 inches per year
• Overall balance
• Runoff function of precip USGS (5 in/year)
• Groundwater flux -220 acre feet (72 million
  gallons)
• Loss to lake evap -1,082 acre feet (300
  million gallons)
• from USACE 84 - 85 study
• USGS publication (1951-1980 data)

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Layne Field Activities

• Rehabilitation of the existing production well.
• Installation of three (3), two (2) inch diameter
  observation wells.
• Implementation of a seven (7) day aquifer test.

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Aerial photograph Carter Lake, Iowa showing
Carter lake and location of production well.
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Aerial photograph of Carter lake indicating wells
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Site Geology

• 0 7 feet Silty clay
• 7 15 feet Fine sand
• 15 20 feet Blue clay
• 20 43 feet Fine sand
• 43 58 feet Gravel and coarse sand
• 58 60 feet Blue clay (confining unit)
• 60 93 feet Fine to coarse sand, with some
  small gravel
• 93 feet Bedrock (Limestone).
•  
• Depth to water was measured in the test well and
  the observation wells at approximately 15 feet
  below ground surface (bgs). Depth to water was
  consistent thought the Site.

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Pump Test

• Pump Well 82-1 at rate of 1,070 gpm
• Pumped continuously for 7 days
• Monitored water levels in wells (pump and
  observation)
• Used pressure transducers and manual measurements

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Analytical Methods

• Cooper-Jacob (1946) distance drawdown/time
  drawdown method. This method is applicable for
  confined or unconfined aquifers and was developed
  with the following assumptions
• The aquifer is confined
• The aquifer has a seemingly infinite areal
  extent
• The aquifer is homogeneous, isotropic, and of
  uniform thickness over the area influenced by the
  test
• Prior to pumping, the potentiometric surface of
  the aquifer is nearly horizontal
• The aquifer is pumped at a constant discharge
  rate
• The well penetrates the entire thickness of the
  aquifer and receives water by horizontal flow
  and
• Drawdown data is corrected for the dewatering of
  the aquifer (unconfined systems only).

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Plot of Observation Well OW50
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Constant Rate Example
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Drawdown Response in Observation Wells(Lin-Log
Scale)
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Aquifer Test Analysis Results

• Pumping rate 1,070 gallons per minute
• Transmissivity 21,500 square feet per day
• Hydraulic conductivity 715 feet per day
• Storativity coefficient .001
• Distance to source of recharge 400 feet
• Percentage of water derived from
• source of recharge 45 percent

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Aerial photograph of Carter lake indicating wells
and calculated recharge boundary.
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Conclusions

• Using USACE numbers lake needs to add 400
  million gallons per year of water to maintain
  level (runoff or supplemental pumping)
• Existing production well estimated to be at 400
  feet from a source of recharge boundary (lake).
• Current pumping at existing production well
  results in induced infiltration from lake of 45
• New wells should be located further south of
  existing well at a greater distance from lake to
  reduce infiltration of lake water resulting in
  increased recharge of lake.
• 200 feet south Induced infiltration 30
• 400 feet south Induced infiltration 15

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QUESTIONS?