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.

Contents

Abstract.......................................................................................................................................................1

Introduction.................................................................................................................................................3

Methods......................................................................................................................................................3

Slug Test Data Collection Methods..........................................................................................................3

Bower Rice Data Analysis Method (Fetter, 2004)....................................................................................4

Well level Data Collection Methods........................................................................................................5

Results.........................................................................................................................................................5

Discussion....................................................................................................................................................6

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

Groundwater head Mar

186.29 187.02 185.13 184.72 184.6 184.18

Groundwater head April

186.58 187.22 184.78 184.99 184.81 184.25

Change in head 0.29 0.2 -0.35 0.27 0.21 0.07

Table 3

Head differencees March April

well 14-15 0.12 0.18

well 15-19 0.42 0.56

Table 2

rank from highest to lowest head

Rank Mar

Rank April

well 9 well 9

well 8 well 8

well 10 well 14

well 14 well 15

well 15 well 10

well 19 well 19

Figure 1