Aquifer Test and Estimation By : Putika Ashfar .K May 2016

• Fetter, C.W. (2001): Applied Hydrogeology, Prentice Hall,

New Jersey; 598pp. • Todd, D.K. and Mays L.W., (2005): Groundwater Hydrology.(3rd

Edition). John Wiley & Sons, New York • Freeze, R.A. and Cherry, J.A. (1979): Groundwater, Prentice

Hall, New Jersey,604pp.

Introduction

Hydrologic properties are usually determined by means of

aquifer test. In an aquifer test, a well is pumped and the rate of decline of the water level in nearby observation

Objectives 1. Pumping test used to measure the aquifer response by

monitoring hydraulic parameters of the aquifer as a function of time.

2. Determine well efficiency and amount of groundwater avaible in the aquifer

3. Determine the suitable depth of pump 4. Carry out the information about water quality with time

variablity

Several things must be considered before starting a pumping test 1. Literature review or previous report regarding geologic

and hydrogeologic system for the proposed area 2. Pumping test should be carried out within the range of

proposed or designed rate 3. Determine the neraby wells that will be used during the

test if they will be affected. R0 = 1/2 [(2,25 x T x t /S)] 4. Avoid back flow phenomena with open-end discharge

pipe 5. Measure groundwater levels in both the pumping test

well and nearby wells 24 hours before pumping starts.

Before we start..

Hydraulic Properties • SWL / static water level (H0) The equilibrium of water level before

pumping • PWL / pumping water level (H) The equilibrium of water level during

pumping • Drawdown (s = H0-H) The amount of water level decline in a well due to

pumping. Usualy measured relative to static (non-pumping) conditions • Well Yield (Q) Volume of water pumped per unit time • Specific Capacity (Q/S) • Time drawdown graph • Transmittivity (T), • Hydraulic Conducticity (K), • Storativity (S) The volume of water release from storage per unit surface

area per unit change in head

Equipment Requirements

• Water level indicator to be used to measuring static and

dynamic water level • Stopwatch • Personal requirements

Flow meter is used to measure flow rate

A well pad is placed on the surface to hold up the well. A blank casing is used from the surface down to the aquifer. Clay or concrete fills the space outside the casing. A screened casing is used in the aquifer. Sand or gravel fills the space outside the casing A submerged turbine pump lifts the water to the surface. The motor that drives the pump is either on the surface or also submerged.

Duration of Pumping Test

• In some test, 24-72 hours testing is enough to produce diagnostic data and enable the ramaining well test.

• Recovery test : Rises of water level after the pump

has been shut down. If the pumping rate was not constant throughout the pumping test, recovery test are more reliable than drawdown data because the water table recovers at a constant rate.

Aquifer test Analysis Method Confined Aquifer

1. Theis 2. Theis recovery 3. Cooper-Jacob 1 (time drawdown) 1. Cooper-Jacob 2 (distance

drawdown) 2. Cooper Jacob 3 (time-distance drawdown)

Unconfined Aquifer 1. Theis with Jacob

correction (isotropic) 2. Neuman (Anisotropic) 3. Boulton (Anisotropic)

Theis Equation

Assumption 1. The T in the limits of ‘cone of depression’ is constant 2. Water withdrawn is derively discharge and instanously recharge 3. For unconfined aquifer, the aquifer is fine grained

Theis Equation

Q = constant pumping rate h = the hydraulic head (PWL) / pumping water level h0 = the initial hydraulic head (SWL)/ static sater level s= h0-h = the drawdown ( difference between PWL and SWL) t = time since pumping has began r = radial distance from pumping well S = aquifer storativity coefficient The finite series has been called the well function is generally called as W(u)

Original potentiometric surface

Land surface

potentiometric surface at time = t

b v

h

ho

Confining bed

aquifer

Q

Data required for Theis Solution • Drawdown vs Time Data at an observation well, • Distance from the pumping well to the observation

well, • Pumping rate of the well.

Theis Equation to determine T and S

s= h0-h = the drawdown ( difference between PWL and SWL) T= transmittivity r = radial distance from pumping well S = aquifer storativity coefficient Theis Equation is limited by cone of depression. It calculate

the drawdowns in the outer part of the cone

Theis Equation..(application example)

Given : A Confined aquifer with Q = 1500 m3/day, T = 600 m2/day, S = 4.10-4

Find the drawdown 1 km from well after 1 year

Find W(u) by table u vs W(u) or by graph W(u) / Well function = 7,12

The drawdown = 1,42 m

The Non-equilibrium Reverse Type Curve (Theis Curve) For A Fully Confined Aquifer. Theoretical curve W(u)

versus 1/U is plotted on a log-log paper.

The Non-equilibrium Reverse Type Curve (Theis Curve) For A Fully Confined Aquifer

The field measurements are similarly plotted on a log-log plot with (T) along the X-axis and (Sw) along the Y-axis

Match field measurements graph to Theis teoritical curves

The Cooper-Jacob Equation (Multiple Wells)

Assumption 1. Confined, homogenous, isotropic aquifer 2. Constant rate pumping 3. Well is fully penetrating, diameter is small 4. Piezometric surface is horizontal prior to pumping

The Cooper-Jacob Equation I (Time Drawdown Method)

t0 = time at the point where the straight line is intersect the zero drawdown line r0 = distance by intersect of the zero drawdown and straight line through data point

The Cooper-Jacob Equation II (Distance Drawdown Method)

The Cooper-Jacob Equation I (Time – Distance Drawdown Method)

Theis Recovery Test The rise in water level after pumping test when the pump is

shut down is known as residual drawdown (s’)

s’ = residual drawdown S’= recovery storativity values T’ = elapsed times endingof pumping

S’ and S are constant and equal , T = constant, the equation can be reduced to

Theis Recovery Test

Theis with Jacob Correction Assumption

1. Unconfined, isotropic aquifer 2. Transmittivity is no longer constant, decrease with increasing

drawdown

Scor = the correction drawdown S = measured drawdown D = original saturated aquifer thickness

Neuman Equation Delayed yield ; when most water has been derived from its

additional source, the time-drawdown curve become relatively steep again

Kz = vertical hydraulic permeability Kr = horizontal hydraulic permeability Kz, Kh = vertical / horizontalhydraulic permeability D = original saturated aquifer thickness

Boulton Equation Assumption

1. Unconfined, isotropic,anisotropic aquifer 2. Both fully or partially penetrating wells Procedure

1. Fitted test data to Theis curve 2. Keep T and adjust S to determine Sy 3. Calculate SD and adjust Boulton Curve

b = thickness of unsaturated zone H = average head along saturated thickness

Programs for aquifer analysis (example) And its an open source software

“Essentially, all models are wrong, but some are useful“ - George Edward Pelham Bo-