Transcript Aquifer Test Analysis Carter Lake, Iowa

Aquifer Test Analysis
Carter Lake, Iowa
Luca DeAngelis, P.E., P.G.
Hydrologist
Layne Western
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
Outline
 General
Carter Lake Water Balance
 Layne Activities
 Conclusions
Water Balance
Streamflow (Qi)
Equation Form:
Is  Ig  Qi  P   (Os  Og  E )  dS
dt
Water Balance

Inflows





Outflows



Stream flow into lake = 0
Mean annual precipitation = 31 inches (precip onto lake)
Runoff – Drains an area of 1,930 acres
Groundwater flux into = ?
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)
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.
Aerial photograph Carter Lake, Iowa showing Carter
lake and location of production well.
Aerial photograph of Carter lake indicating wells
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.
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
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).
Plot of Observation Well OW50
5.
Obs. Wells
OW50
Aquifer Model
Confined
4.
Solution
Cooper-Jacob
Displacement (ft)
Parameters
T = 1.783E+4 ft2/day
S = 0.001451
3.
2.
1.
0.
0.001
0.01
0.1
1.
10.
Adjusted Time (min)
100.
1000.
1.0E+4
Constant Rate Example
Figure 1
Distance Drawdown
Q = 1,070 gpm
Carter Lake, Nebraska
Distance from pumped well (feet)
1
10
100
1,000
0.00
5.00
Slope of best fit line
y = -1.5119Ln(x) + 10.202
R2 = 0.9643
obs wells
pump well
Log. (obs wells)
Drawdwon (feet)
10.00
Plot time =
9,660 min
15.00
20.00
25.00
30.00
35.00
Drawdown Response in Observation Wells
(Lin-Log Scale)
Aquifer Test Analysis Results






Pumping rate
Transmissivity
Hydraulic conductivity
Storativity coefficient
Distance to source of recharge
Percentage of water derived from
source of recharge
1,070 gallons per minute
21,500 square feet per day
715 feet per day
.001
400 feet
45 percent
Aerial photograph of Carter lake indicating wells and
calculated recharge boundary.
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
+400 feet south
Induced infiltration 30%
Induced infiltration 15%
QUESTIONS?