In this Final Year Project, Sustainable yield and Yield Reliability will be obtained by using large scale of the well diameter and higher pumping rate. Water Budget Analysis will be also be done and updated by rainfall event from Jurong Island Fire Station.
CHAPTER 1 - INTRODUCTION
1.1 Background
Jurong Island an artificial island formed from the amalgamation of 7 offshore islands, receives an average of 2.5m of rainfall each year (based on daily rainfall records on the island from the period from 1991 to 2007). Presently, she has a total land area of approximately 32 km2, with original land area of 10 km2 and 22 km2 of reclaimed land. In order to develop a holistic groundwater supply system utilizing reclaimed sand aquifers on Jurong Island, hydrological and hydraulic investigations shall be carried out.
1.2 Objectives
This project would be carried out with the following objectives:
To find out natural infiltration rate or recharge rate in order to find out recharge percentage in yearly basis, with different kin
To estimate the sustainable yield from abstraction of groundwater from well hydraulic analysis
To figure out suitable abstraction methods
To conduct groundwater abstraction yield reliability analysis
CHAPTER 2 - LITERATURE REVIEW
2.1 Water hydrology
The Sun, which drives the water cycle, heats water in oceans and seas. Water evaporates as water vapor into the air. Ice and snow can sublimate directly into water vapor. Evapotranspiration is water transpired from plants and evaporated from the soil. Rising air currents take the vapor up into the atmosphere where cooler temperatures cause it to condense into clouds. Air currents move water vapor around the globe, cloud particles collide, grow, and fall out of the upper atmospheric layers as precipitation. Some precipitation falls as snow or hail, sleet, and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Most water falls back into the oceans or onto land as rain, where the water flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff and groundwater are stored as freshwater in lakes. Not all runoff flows into rivers, much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers, which store freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge. Some groundwater finds openings in the land surface and comes out as freshwater springs. Over time, the water returns to the ocean, where our water cycle started.
Processes are shown in the figure below:
Figure 1: Process of hygrology
1.jpg
Precipitation
Condensed water vapor that falls to the Earth's surface . Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet.
Canopy interception
The precipitation that is intercepted by plant foliage, eventually evaporates back to the atmosphere rather than falling to the ground.
Runoff
The variety of ways by which water moves across the land. This includes both surface runoff and channel runoff. As it flows, the water may seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.
Natural infiltration (Recharge)
The flow of water from ground surface into ground. Once infiltrated, the water becomes soil moisture or groundwater. This will be further discussed in section 2.2 as this is the scope and objective of this project.
Subsurface flow
The flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (e.g. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.
Evaporation
The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration.
Transpiration
The release of water vapor from plants and soil into the air. Water vapor is a gas that cannot be seen.
2.2 Recharge (Richard W. Healy "Estimating Groundwater Recharge", page 3 onwards)
It is defined as the downward flow of water reaching the water table, adding to groundwater storage. (This definition is similar to those given by Meinzer (1923), Freeze and Cherry (1979), and Lerner et al. (1990)). Strictly speaking, this definition does not include water flow to an aquifer from an adjoining groundwater system (such as water movement from an unconfined aquifer across a confining bed to an underlying aquifer); it is refered as interaquifer flow. Recharge is usually expressed as a volumetric flow, such as m3 /d, or as a flux, (in terms of volume per unit surface area per unit time, mm/yr.
There are two types of recharge mechanisms, i.e. diffused and focused mechanisms.
Diffuse: is recharge that is distributed over large areas in response to precipitation infiltrating the soil surface and percolating through the unsaturated zone to the water table or direct recharge. It is a localized recharge defined as concentrated recharge from small depressions, joints, or cracks
Focused: is the movement of water from surface-water bodies, such as streams, canals, or lakes, to an underlying aquifer. It is recharged from rivers, canals, and lakes.
Generally, diffuse recharge dominates in humid settings.
Different climatic regions are referred to throughout the text. Climatic regions are classified on the basis of annual precipitation. An arid climate is one with annual precipitation of less than 250 mm; a semiarid region has precipitation rates between 250 and 500mm/yr; a subhumid climate refers to precipitation rates between 500 and 1000mm/yr; and humid climates have annual precipitation rates that exceed 1000mm.
Singapore has an average annual rainfall between 2.5m~2.7m; therefore this report will mainly focus on percentage of rainwater direct recharge into aquifer.
2.2.1 Spatial and temporal variability in recharge.
Recharge rates vary in space in both systematic and random fashions. This is true for both focused and diffuse recharge. Systematic trends often are associated with climatic trends, but land use and geology are also important. The concept of recharge rates increasing with increasing precipitation rates is certainly intuitive-recharge cannot occur if water is not available. The random factor in recharge variability can be viewed as local-scale variability that can be attributed, for example, to natural heterogeneity in permeability in surface soils or variability in vegetation. Any of the factors addressed below can contribute to apparent random variability.
Recharge also varies temporally. Seasonal, multiyear, or even long-term trends in climate affect recharge patterns.
2.2.2 Climate
Precipitation, the source of natural recharge, is the dominant component in the water budget for most watersheds. Seasonal, year-to-year, and longer-term trends in precipitation, as well as frequency, duration, and intensity of individual precipitation events also affect recharge processes. Conditions are most favorable for water drainage through the unsaturated zone to the water table when precipitation rates exceed evapotranspiration rates.
2.2.3 Soils and geology
Permeabilities of surface and subsurface materials can greatly affect recharge processes. Recharge is more likely to occur in areas that have coarse-grained, high-permeability soils as opposed to areas of fine-grained, low-permeability soils.
Coarse-grained soils have a relatively high permeability and are capable of transmitting water rapidly and promote recharge because water can quickly infiltrate and drain through the root zone before being extracted by plan roots.
Fined-grained sediments are less permeable, but are capable of storing greater quantities of water. Thus, in areas of ined-grained sediments, one would expect decreased infiltration, enhanced surface runoff, increased plant extraction of water from the unsaturated zone, and decreased recharge relative to an area of coarser-grained sediments.
Subsurface geology influences discharge processes as well as recharge processes. If the rate of discharge from an aquifer is less than the recharge rate, water storage within the aquifer increases.
Aquifer storage can reach a maximum at which point additional recharge cannot be accepted, regardless of the amount of precipitation. This condition typically leads to enhanced runoff.
2.2.4 Surface topography
Land-surface topography plays an important role for both diffuse and focused recharge. Steep slopes tend to have low infiltration rates and high run off rates. Flat land surfaces that have poor surface drainage are more conducive to diffuse recharge; these conditions also favor flooding.
Small, often subtle depressions can have a profound influence on infiltration rates. Even with uniform surface characteristics, apparent infiltration rates increase in the downslope direction along a long hill slope. Ti is because downslope portions of the hill are exposed to runoff from upslope portions as well as precipitation.
2.2.5 Hydrology
The depth to the water table also is important. If the unsaturated zone is thin, infiltrating water can quickly travel to the water table; recharge may be largely episodic, occurring in response to any large precipitation event. However, shallow water tables are also susceptible to groundwater discharge by plant transpiration. Therefore, water that recharges shallow subsurface systems may only reside in the saturated zone for a short time before it is extracted by plan roots and returned to the atmosphere.
Thick unsaturated zones are less likely to have episodic recharge events; recharge would be expected to be seasonal or quasi-steady because wetting fronts moving through the unsaturated zone tend to slow with depth and multiple fronts may coalesce and become indistinguishable from each other.
2.2.6 Vegetation and land use
Vegetation and land use can have profound effects on recharge processes. A vegetated land surface typically has a higher rate of evapotranspiration (less water available for recharge) than an unvegetated land surface under similar conditions.
The depth to which plant roots extend influences the efficiency with which plants can extract water from the subsurface. Trees, for example, are capable of drawing moisture from depths of several meters or more than shallow-rooted crops. Thus, enhanced recharge in areas with shallow-rooted vegetation is practical.
Urbanization brings about many land surface changes that can have significant ramifications for recharge processes. Roads, parking lots, and buildings all provide impervious areas that can inhibit recharge. Runoff diversions are common features in urban landscape. Diversions may lead to surface-water bodies or to infiltration galleries. In the former case, overall recharge for the area is reduced. In the latter case, recharge may not necessarily be reduced, but at the very least it is redirected and may change from a diffuse source to a focused source. Runoff diversions maybe important in terms of aquifer vulnerability to contamination because they can quickly funnel contaminants to the subsurface.
Delivery Systems for water supply and treatment are additional artifacts of urbanization that can affect recharge processes, both in terms of water supply and potential for contamination. These systems consist of open channels or water pipes and sewers. Invariably, there is leakage associated with any delivery system.
2.3 Water-budget methods
2.3.1 Introduction
A water budget is an accounting of water movement into and out of, and storage change within, some control volume. Universal concept of mass conservation of water is used in this project because it is applicable over any space and time scales.
There are 3 characteristic about water budget equations:
The equation is an integral component of atmospheric general transmission models used to predict global climates over periods of decades or more.
Equations can be easily customized by adding or removing terms.
Equations are generally not bound by assumptions on mechanisms of water ( moves into, through and out of the control volume)
Water-budget methods represent the largest class of techniques for estimating recharge; therefore this chapter is limited to discussion of the "residual" water-budget approach.
The following section presents a general analysis of water-budget equations include measuring and estimating components such as precipitation, evapotranspiration, runoff and water storage.
Temporal and spatial scales play an important role; for convenience, methods are organized by spatial scales.
2.2.2 Water budget
A control volume (can be a volume of earth or atmosphere or a hydrologic structure) is selected for study. This is to estimate the recharge in a control volume system also equate in the water budget equation.
When selecting a control volume to estimate recharge, factors to be considered as:
Where and when recharge occurs,
focused or diffuse,
what types of data are available,
Locations where fluxes are known or can be easily measured or estimated.
For estimating recharge, watersheds, aquifers, and one-dimensional soil columns are widely used control volumes.
A simple water-budget analysis used in many hydrological studies is based on a soil column that extends downward from land surface to some depth, L (figure)as the control volume:
(Control volume figure)
P = precipitation
ET = evapotranspiration
= change in water storage in the column
Roff = is direct surface runoff
D = is drainage out the bottom of the column (It can be concluded as Natural Infiltration)
Note:
All components are given as rates per unit surface (mm/day)
Drainage, D, is equivalent to recharge, R, only if the bottom of the column extends to the water table. Distinguishing between drainage and recharge may be important if temporal trends of recharge are of interest. Land-use and climate changes can result in current drainage rates ahtat are substantially different from historical rates of reacharge. But if recharge is being estimated for input to a steady-state groundwater flow model, the two terms can be considered synonymous.
Evapotranspiration can be divided on the basis of the source:
Where
ETsw = evaporation or sublimation of water stored on land surface
ETuz = bare soil evaporation and plant transpiration of water stored in the unsaturated zone
ETgw = evapotranspiration of water stored in the saturated zone.
Change in water storage can be written as;
Where
=storage in surface depressions, in plants, and in the plant canopy.
= changes in water storage in the unsaturated zone at depth equal to or less than the zero-flux plane (ZFP)
= refers to changes in storage in the saturated zone; or zone at depth greater than ZFP.
Note:
Zero-flux plane, ZFP, which is a plane at some depth in the subsurface where the magnitude of the vertical hydraulic gradient is 0. Water above this plane moves upwards, vice versa. It is also sometimes at the bottom of the root zone.
2.3.0 The water-table fluctuation method ( Physical methods for saturated zone)
2.3.1 Introduction
A very straight forward method that is used based on measurement of groundwater over time and space. It uses fluctuations in groundwater levels over time to estimate recharge for unconfined aquifers.
2.3.2 Specific yield (http://www.co.portage.wi.us/groundwater/undrstnd/soil.htm)
Not all the water stored in pore spaces becomes part of flowing or moving groundwater. Just as water clings to a glass, water also clings to soil particles due to surface tension, cohesion, or adhesion. It forms a thin film around a particle.
Specific yield is the ratio of volume of water that drains from a saturated rock due to gravity to the total volume of rock. It is treated as a storage term, independent of time that accounts for the instantaneous change in water storage upon a change in total head.
There are several methods to obtain Specific yield. The volume-balance method can be considered to be used. It is suitable for a traditional aquifer test with a water budget of the cone of depression created by the withdrawal of water from the pumping wells.
Specific Yield (%)
Material
Maximum
Minimum
Average
coarse gravel
26
12
22
medium gravel
26
13
23
fine gravel
35
21
25
gravelly sand
35
20
25
coarse sand
35
20
27
medium sand
32
15
26
fine sand
28
10
21
silt
19
3
18
sandy clay
12
3
7
clay
5
0
2
Table specific of types of soils (Johnson (1967) as quoted by C.W. Fetter 2).
2.3.3 Water-table fluctuation method
Simple water budget for some part of a groundwater system written by Schicht and Walton (1961) as:
Where
= change in storage in saturated zone
R = recharge
= base flow
= evapotranspiration from groundwater
= subsurface flow away from area of interest
= subsurface flow into from area of interest
The water-table fluctuation (WTF) method is based on the premise that rises in groundwater levels in unconfined aquifers are due to recharge water arriving at the water table. With the assumption that the amount of available water in a column of unit surface area is equal to specific yield times the height of water in the column, recharge can be calculated from:
Assume:
Water arriving at the water table goes immediately into storage and that all other components in equation are zero during the period of recharge. Therefore, this method is suitable over short periods of time (hours or a few days).
The method may not be appropriate if water is transported away from the water table at a rate that is not substantially slower than the rate at which recharge water arrives at the water table.
2.1.1Sustainable yield ( page 6)
Sustainable yield (safe yield) refers to the rate at which water can be withdrawn from an aquifer without causing adverse impacts. Those impacts could be in the form of decreased discharge to streams and wetlands, land subsidence, or induced contaminations of groundwater, for example, by seawater intrusion.
(Groundwater, R.Allan Freeze/John A. Cherry) Todd (1959) defines the safe yield of a groundwater basin as the amount of water that can be withdrawn from it annually without producing an undesired result. Any withdrawal in excess of safe yiled is an overdraft. Domenico (1972) and Kazmann (1972) notes that the "undesired results" mentioned in the definition are not only the depletion of the groundwater reserves, but also the instrusion of water of undesirable quality, the contravention of existing water rights, and the deterioration of the economic advantages of pumping.
From an optimization viewpoint, groundwater has value only by virtue of its use, and the optimal yield must be determined by the selection of the optimal groundwater management scheme from a set of possible alternative schemes.
The optimal scheme is the one that best meets a set of economic and/or social objectives associated with the usese to which the water is to be put.
Recharge rates are sometimes incorrectly equated with the sustainable yield of and aquifer (Meinzer, 1923; Bredehoeft et al.,1982; Bredeheft, 2002; Alley and Leake, 2004). The notion that recharge is equivalent to sustainable yield is based on an incomplete or incorrect conceptual model of hydrologic system. Knowledge of recharge rates is important for determining sustainable yields in many groundwater systems (Sophocleous et al.,2004; Devlin and Sophocleous, 2005,) But recharge rates by themselves are not sufficient for determining sustainability (Bredehoeft et al., 1982; Bredehoeft, 2002). The effects of changes in groundwater levels on groundwater discharge rates and aquifer storage must also be considered.
Pilot Test
A pilot test well study was carried out on Jurong Island to investigate the feasibility of abstracting groundwater, mainly from the reclaimed parts of the island. A well of 1.2m diameter and 4m deep was constructed at a reclaimed site near Bayan Avenue. The test was conducted from August to October 2012 (3 months) with a pumping rate of 1~2 l/s. The test was also monitored continuously in terms of daily rainfall, pumping rate, well water level, and groundwater table outside the well.
Groundwater flow parameters
Permeability and hydraulic conductivity
Craig (2004) stated in one dimension, Water flows through a fully saturated soil in accordance with Darcy's empirical law:
Or
q = volume of water flowing per unit time
A= cross-sectional area of soil corresponding to the flow q,
k= the coefficient of permeability,
i= the hydraulic gradient,
Also it can be found by using borehole test or pumping well test (unconfined condition)
(graph of pumping test well)
Figure 2: Illustration of well pumping
The values of k for different types of soil are typically within the ranges shown in table1. For sands (material used for Jurong Island reclamation), Hazen showed that the approximate value of k is given by:
D10 = effective size in mm.
Table 1: Coefficient of permeability (m/s) (BS 8004:1986)
As a result, we can estimate k (permeability) by making use of K (conductivity).
γw = unit weight of water
η = the viscosity of water
The coefficient of permeability also varies with temperature, upon which the viscosity of the water depends. If the value of k measured at 20 ÌŠ C is taken as 100% then the values at 10 and 0 ÌŠ C are 77 and 56%, respectively.
Michael (1989) stated that hydraulic conductivity K, as applied to an aquifer, is defined as the rate of flow of water, in litres per day, through a horizontal cross-sectional area of one square metre of the aquifer, under a hydraulic gradient of one metre per metre at the prevailing temperature of water.
Groundwater head
Rushton (2004) found that the ground water head for an elemental volume in an aquifer is the height to which water will rise in a piezometer (or observation well) relative to a consistent datum. Figure 3 shows that the groundwater head :
Figure 3: Groundwater Head
head1.gif
where
p= the pressure head,
Ï = the density of the fluid
z = the height above a datum (Elevation Head).
All pressure are relative to atmospheric pressure.
(figure explaining the head)
Direction of flow of groundwater
Groundwater flows from a higher to a lower head (or potential) (Rushton,2004) . Typical examples of the use of the groundwater head to identify the direction of groundwater flow are shown in figure 4.
Figure 4: Illustration of different head and flow of groundwater.
head2.gif
Abstraction methods
Open wells
Open wells are dug down to the water-bearing strata. They derive water from the formations close to the ground surface (Michael, 1989).
Tube wells
Tube wells are constructed by fixing a pipe below the ground surface and passing through different geological formations consisting of water-bearing and non-water- bearing strata (Michael,1989).
Aquifer
Types of aquifer are explained by Department of Primary Industries, Parks, Water and Environment's (2012) website, shown in figure 5:
Confined Aquifers
Confined aquifers are permeable rock units that are usually deeper under the ground than unconfined aquifers. They are overlain by relatively impermeable rock or clay that limits groundwater movement into, or out of, the confined aquifer.
Groundwater in a confined aquifer is under pressure and will rise up inside a borehole drilled into the aquifer. The level to which the water rises is called the potentiometric surface. An artesian flow is where water flows out of the borehole under natural pressure.
Confined aquifers may be replenished, or recharged by rain or streamwater infilitrating the rock at some considerable distance away from the confined aquifer. Groundwater in these aquifers can sometimes be thousands of years old.
Unconfined Aquifers
Where groundwater is in direct contact with the atmosphere through the open pore spaces of the overlying soil or rock, then the aquifer is said to be unconfined. The upper groundwater surface in an unconfined aquifer is called the water table. The depth to the water table varies according to factors such as the topography, geology, season and tidal effects, and the quantities of water being pumped from the aquifer.
Unconfined aquifers are usually recharged by rain or streamwater infiltrating directly through the overlying soil. Typical examples of unconfined aquifers include many areas of coastal sands and alluvial deposits in river valleys.
Figure 5: Various types of aquifer.
fig5.gif
Jurong Island reclaimed land is filled by sand and has a conductivity of K= 35m/day which is assumed and measured during pilot test is said to be an unconfined aquifer.
Steady Radial Flow to a Well
Michael (1989) also found that the flow is said to be steady when no change occurs with time, i.e.
where ,v=velocity of flow, m/s
t= time,s
Flows in aquifer
Steady state flow occurs when there is equilibrium between the discharge of the pumped well and the recharge of the aquifer by an outside source. Flow condition differ for unconfined and confined aquifers, and need to be considered separately.
Since the case at Jurong Island reclaimed land is a sand filled material and conductivity K=35m/day (assumed and measured in previous report), therefore it is considered as unconfined aquifer. Thus, this report will be focusing at Steady State Flow to Wells in Unconfined Aquifers.
Environmental Issues (seawater intrusion)
In "Water Treatment Solutions Lenntech" (2008) web page stated that( shown in figure 6):
Seawater intrusion is the movement of seawater into fresh water aquifers due to natural processes or human activities. Seawater intrusion is caused by decreases in groundwater levels or groundwater head or by rises in seawater levels. When you pump out fresh water rapidly, you lower the height of the freshwater in the aquifer forming a cone of depression. The salt water rises during freshwater depression and forms a cone of ascension. See the following pictures. Intrusion can affect the quality of water not only at the pumping well sites, but also at other well sites, and undeveloped portions of the aquifer.
Figure 6: Seawater intrusion
gw-sewater-intrusion1.gifgw-sewater-intrusion2.gif
Well Hydraulic Analysis.
The following well hydraulics equations can be derived from the continuity equation and Darcy's law. From continuity equation, radius of divide or ridge radius RD (m) is a function of the pumping rate Q (m3/day) and net rainfall recharge rate N (m/day).
Radius of divide/ radius of influence of the well field, RD
When the water is pumped out of the well (Rushton, 2004), it gets supply from the surrounding formations, and the water table or piezometric surface, depending on the type of aquifer, is lowered. There is, thus an imaginary inverted cone formed around the well having the static water level as base and pumping level as apex figure 2. The conical shape is known as cone of depression. The area which gets affected by the pumping of the well is called the area of influence. The boundary of the area of influence is called the circle of influence. The radius of the circle of influence is called the radius of influence (RD).
The well discharge Q at any distance r is expressed as (from Dupuit formula)
Where
= the recharge from natural infiltration.
= the drawdown of the well.
Methodology
Assumption
Based on previous report and other researched from Literature Review
1) The flow is assumed 2-dimensional to a well centered.
2) Aquifer of reclaimed land is a homogeneous and isotropic aquifer.
3) Open well method is used as water table is ~2 m below the ground surface, thickness of H is~ 6m
4) The aquifer is unconfined
5) The aquifer has infinite aerial extent
6) The water table is horizontal prior to pumping
7) The aquifer is pumped at a constant discharge rate.
8) The well penetrates the full thickness of the aquifer and thus receives water from the entire saturated thickness of the aquifer.
Based on current researched
1) K=35 m/day = 0.000405 m/s (assumed and measured during last report)
As = 9.81 kN/m2
(Dynamic viscosity of water at 20 ÌŠC=1.002 Ns/m2 x 103
According to Hazen's diagram, the aquifer is classified as "Clean sands and sand-gravel mixtures".
2) Diameter = 1.6m, 1.8m, 2.0m
3) Pumping rate= 1 l/s, 2 l/s, 3 l/s
4) Natural Infiltration % = 20,25, 30, 35,40
5) Recovery Rate= 70%, 80% if possible based on Reduced Level.
6) Reduced Level =100.5 m
Well hydraulic Analysis
1) To prevent seawater intrusion, maximum draw down will be fixed at the maximum of 0.5m
2) With the assumptions of natural infiltration rate and diameter, pumping rate Q will be determined.
3) Therefore Sustainable Yield will be obtained.
4) Water Budget Analysis will be done and updated based on daily rainwall.
5) Yield reliability will be obtained.
