Shawn Oliver Characterization of Groundwater System under Chico State Abstract Under the campus of Chico exists a dynamic groundwater system in interaction with Big Chico Creek. Through well excess this system was explored. Six wells were all measured for ground water level twice during two separate events. During one of these events hydraulic conductivity values were found in all but one well through data analyses of slug tests. Synthesis of data characterizes an unconfined aquifer system which is currently being recharged by Big Chico creek.
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Shawn Oliver
Characterization of Groundwater System under Chico State
Abstract
Under the campus of Chico exists a dynamic groundwater system in interaction with Big Chico Creek.
Through well excess this system was explored. Six wells were all measured for ground water level twice
during two separate events. During one of these events hydraulic conductivity values were found in all
but one well through data analyses of slug tests. Synthesis of data characterizes an unconfined aquifer
system which is currently being recharged by Big Chico creek.
Works Cited.................................................................................................................................................7
Tables and Figures.......................................................................................................................................8
Introduction
Ground water models are standard tools used in characterizing groundwater systems and designing
policy which can have impact on a natural hydraulic system. Vulnerable unconfined aquifers which
interact with an anthropogenic urban type system and stream ecological systems are home to critical
water resources for both the human and natural worlds. The aquifer underneath Chico state campus is
one of these critical flow systems and its monitoring provides data useful for its modeling.
Methods
Data collection involved two ground water level monitoring events using a sounder punker at a variety
of Chico state monitoring wells and slug testing where drawdown was measured with time after
insertion of a water slug. Slug testing data was analyzed using the Bower Rice Slug Test analysis method
to find Hydraulic conductivity values for wells. Ground water level measurements yielded depth to
water from well top length data on two separate occasions during the spring season. These length
measurements and their change between measurement events can be referenced in table 1. Well
locations were referenced from a field map used to find wells. This map was then used to create well
lace marks in Google Earth to give geographic significance to wells and their measured data Figure 1.
Slug Test Data Collection Methods
Slug testing involved the measurement of well structure parameters and observation of well response to
slug insertion in the form of well head change with respect to time; measurement of well head with time
was recorded with a pressure transducer. This measured data was then used to estimate hydraulic
conductivity within the vicinity of the well using the Bower and Rice Slug Test Method documented in
fetter page 197-200.
Bower Rice Data Analysis Method (Fetter, 2004)
Uses a conceptual model a of a slug of water drawing down within a well by a rate controlled by well
construction parameters, the effective area over which head is displaced and an aquifers hydraulic
conductivity. With this model Hydraulic conductivity of an aquifer region can be determined if a well is
present, with known dimensions and proper construction, and draw down is measured with time
according to equation 1:
K=rc ln(¿Re/R)2Le
1tln (H 0
H t
¿)¿¿
Where
K =hydraulic Conductivity
Rc=¿¿ radius of well casting
R= radius of gravel envelope
Re= Effective Radial Distance over witch head is dissipated
Le= length of well screen
H 0= drawdown at time t=0
H t= drawdown at time t=t
t = time since H=H 0
Because the variable Re is impossible to measure in the field software is used to determine this
variable’s value and calculate hydraulic conductivity. Aqua solve, is able to determine this parameter
through the use of equations relating Re to: well aquifer and aquifer parameters, coefficients linked to
well parameters in charts and drawdown vs. time trend line analysis.
It should be noted that Bower Rice Method is not valid for any kind of slug test. For data to be valid
testing bust be done in a well with a static water level above the well screen. Also, slugs during a test
should be large and inserted as instantly as possible in order to produce data relevant data.
Well level Data Collection Methods
Well water levels were measured using a sounder plunker. A sounder plunker is a device that measures
distance to water in a well in a process where circuit break is lowered into a well attached to a wire
displaying length until the break comes into contact with water and sounder produces noise and length
can be recorded. This data was then used with known well elevation data and well location data to find
spatial and temporal aquifer head data.
Results
Studied groundwater system is dynamic and within geology showing intrinsic permeability spatial
variability.
Hydraulic head data shown in table 2 slows, when ignoring well 10, evidence that ground water flow
does not change direction during the spring. This is because wells do not change their rank of hydraulic
head magnitude. This is evidence of system of constant flow direction because of property of
groundwater to flow from high to low hydraulic head. Data in table 1 also shows that the water table
adjacent to Little Chico creek has greater head variability because head changes calculated are greater in
wells adjacent to big Chico creek.
Data shows that aquifer conductivity decreases logarithmically with respect to a proximity to big Chico
creek south west and across the stream from Holt Hall. Conductivity measured east of Holt Hall is in
conclusive other than conductivity showing to be relatively constant. Stream shape visible in Figure 1 is
evidence that the wells 14, 15 and 19 penetrating into a point bar deposit. This would validate the
conductivity results for the wells because decreasing clast size with respect to channel proximity is a
pattern found in point bars.
Table 3 shows how head change is increasing among wells 14, 15 and 19. According to Darcy’s law this
could mean that recharge rates have increased between measurement events.
Drawdown data showed well 10 to be problematic. Absence of drawdown measured by pressure
transducer shows that well is clogged and unable to transmit water into aquifer.
Discussion
Head level measurement and slug testing of wells within an unconfined aquifer groundwater system
underneath Chico State produced data sufficient for very basic geographic and temporal
characterization of the system. Characterization of the system is as follows: Data definitively shows that
groundwater system is subject to head variability and operates in geologic material with variable
hydraulic properties. Data also matches what would be expected in an active alluvial groundwater
system in interaction with geology undergoing active surficial processes; evidenced by what has been
characterized as a point bar through geographic hydraulic conductivity value synthesis. Analysis of data
also shows that stream water was recharging the unconfined aquifer during both measurement events
and that this recharge rate was increasing.
Works Cited
(n.d.).
Fetter, C. W. (2004). Applied Hydrogeology (Fourth ed.). Upper Saddle River , New Jersey : Prentice Hall .
Tables and Figures
Table 1
well 8 well 9 well 10 well 14 well 15 well 19
K in april 0.000529 0.000221 n/a 0.01217 0.001025 0.000997
depth to water april 12.46 3.8 13.35 5.55 10.42 10.8
Depth to water mar 12.75 4 13 5.82 10.63 10.87
well elevation 199.04 191.02 198.13 190.54 195.23 195.05