GSA Study module 1 – Basic groundwater theoryMSS025006A
Diploma of Environmental Monitoring & Technology
Study module 1
Basic groundwater theory
MSS025006A
Groundwater sampling & analysis
Completion RecordStudent name / Type your name here /
Available marks / 31
Final mark / Marker to enter final mark /
Completion date / Click here to enter a date. /
Introduction
Reasons for ground water sampling
Introduction
Natural zonation of groundwater
Water pressure, the saturated zone, aquifers, and aquitards
The geology of groundwater
Aquifer properties
Transmission of water
Hydraulic conductivity
Groundwater charging
Groundwater use in Australia
Groundwater extraction
Threats to groundwater quality
Difficulty of remediation
Assessment task
Assessment & submission rules
Answers
Submission
Penalties
Results
Problems?
Resources
References
Introduction
As with any sampling event, the aim is to obtain a representative sample to ensure that any information collected from the sample represents the population as a whole. This is hard under any conditions but becomes almost impossible with ground waters due to the nature of the beast (i.e. physical, chemical and hydraulic equilibria).
Reasons for ground water sampling
As with most environmental sampling, groundwater sampling events are commonly associated with compliance monitoring programs (typically contaminated site assessments), and although there are many other reasons ground water is monitored, it is the compliance angle that we shall focus on in these notes.
The three main triggers for groundwater monitoring in Australia are;
◗Compliance with National Environmental Protection Measures
◗Compliance with State Environmental Protection Authority (EPA) licences
◗To assist with State level environmental planning legislation
The specifics of each trigger are beyond the scope of this unit and can be found in the course notes for Environmental Management Systems (EMS) and EnvironmentalImpact Assessment (EIA).
Introduction
To most people, the two main bodies of water on our planet are the oceans and the polar ice caps yet these stores of water are of little use because the oceans are too salty for our use, and we cannot afford the ‘costs’ of melting the ice caps. Fortunately, potentially useful freshwater is almost always within sight, in the form of rain, rivers, or lakes.
By far the most abundant usable freshwaters are largely hidden from our gaze, in the ground beneath our feet. Subsurfacewater or groundwater accounts for just fewer than 99% of the total volume of freshwater presently circulating on our planet. Surface water, that is all of the readily-visible water present in rivers, lakes or wetlands, amounts to less than 1% of the total, with the balance (~0.16%) being present in the form of atmospheric moisture.
So what is groundwater? The simplest definition would propose to equate groundwater with all subsurface water that is any and all water beneath the ground surface. While such a simple definition does find use in some legal worlds, in technical circles more complicated definitions are generally preferred. Well diggers have long observed that water will only flow spontaneously into a well below a certain horizon, usually termed the water table. The depth to this water table varies from one locality to another, though it is generally found to form a relatively flat horizon over short distances between two neighbouring wells.
Figure 1.1 – Hydrological cycle showing our focus on groundwater [Ref 1 - Younger]
Natural zonation of groundwater
How does water exist under ground? Underground streams or rivers are very rare, and most groundwater occurs in the small openings in soil or rock known as pores, which may either correspond to the gaps between sediment grains, or else be due to the presence of fractures.
Pore space may be partially or completely filled with water. Thanks to gravity, the distribution of water underground is not random, but tends to be vertically zoned. The zone in which pores are completely filled with water is termed the saturated zone (or phreatic zone), and its upper surface is the water table.
Above the water table the pores are only partly filled with water; this is generally called the unsaturated zone (or vadosezone), and it is the zone in which water is referred to as soil moisture or, less commonly, “vadose water”. It is possible to further subdivide the unsaturated zone into the soil and sub-soil zones. The soil zone is the uppermost layer of earth, which typically supports plant life. In most climatic zones, the soil zone will tend to remain unsaturated for most of the year, as the root systems of the plants remove moisture from the soil for use in metabolic processes.
Figure 1.2 – Example of the different horizons and their effect on groundwater [source]
Excess water taken up by plants is not usually returned to the soil zone, but rather is lost to the atmosphere from the leaves, by a process termed transpiration. The base of the soil zone is defined by the maximum depth from which water can be removed by root suction; we can therefore term it the root-suction base.
Between the root-suction base and the water table is the sub-soil zone, comprising unsaturated soils and/or rocks in which the soil moisture is slowly seeping downwards, destined eventually to replenish the store of groundwater below the water table.
While we have already defined the uppermost surface of the saturated zone to be the water table, which is simply the level to which water will settle in a well dug into the saturated zone, in reality the pores tend to be completely filled with water for a short height above that level. In effect, the water table is surmounted by a thin mantle of fully saturated pores, which is usually referred to as the capillary fringe. The height of this fringe depends on the size of the pores; while it may be only a fraction of a millimeter in coarse gravel, it may reach several meters in clays, silts, and rocks with small narrow pores.
Figure 1.3 – Detailed description of the groundwater/soil interactions.
As mentioned, true underground streams are very rare features, being almost wholly restricted to terrains composed of three types of rock;
◗Limestone rocks which are rocks composed of readily soluble minerals in which cave systems have developed over geological time This is most notably the case in relation to calcite (CaCO3), the main constituent of limestone, and gypsum (CaSO4·2H2O), which forms thick and extensive beds in some sedimentary sequences.
◗Volcanic rocks, especially basaltic lavas containing caves.
◗Sandy or silty soils and rocks which have been subjected to particular modes of weathering known as “piping” and “sapping”, which cause caves systems comparable to those commonly found in limestone country.
This leaves us with thefar more pertinent questions than presence or absence of groundwater are;
◗How far below ground level does the water table lie?
◗In what kind of soil or rock is the groundwater present?
◗How readily will these soils or rocks yield groundwater?
◗For how long can we depend upon the supply?
◗Is the groundwater of good or poor quality (i.e. contaminated)?
Water pressure, the saturated zone, aquifers, and aquitards
If we construct a borehole through the unsaturated zone, we are working our way through ground that contains much water in the form of soil moisture (which you should remember from soil science calluses). It would at first seem that nothing should be preventing the soil moisture from entering the hole, but in reality, atmospheric pressure (which is around 10 tonnes per square meter) is the controlling factor at this stage. Unless the pressure of water in pores is greater than the atmospheric pressure, the water will never flow laterally into the hole.
Eventually, after much digging, we reach the water table. By definition, any water present in the ground below this level will flow freely into our hole. As it does so, it displaces the air, which moves up and out of the hole and the reason for this behaviour the pressure of water in the pores of the saturated zone is greater than atmospheric pressure.
Now we can offer the following definition of the water table, it is that subsurface horizon upon which the pore water pressure exactly equals atmospheric pressure.
Fluctuations in atmospheric pressure (a few millimeters at most) in the precise elevation of the water table can therefore be expected as atmospheric pressure changes. The capillary fringe is always at hand to “lend” water to, or “borrow” it from, the true saturated zone. So, the tendency for groundwater to flood an excavation is down to the fact that its pressure exceeds that of the air.
The tendency for groundwater to flow into a well or borehole varies dramatically from one piece of ground to another. It is possible to distinguish between those soils and rocks which release copious quantities of groundwater very rapidly and those which release it so slowly it may be unnoticeable over time-scales of interest to humanity. The following terms help us in making these distinctions;
Aquifer
Thisis a body of saturated rock that both stores and transmits important quantities of groundwater.
Aquitard
Thisis a saturated body of rock that impedes the movement of groundwater.
The term “aquitard” is probably the most widely used descriptor internationally for lowpermeability rocks. However, alternative terms have found favour such as the US Geological Survey prefer the term “confining bed”. Other alternative terms, including “aquifuge” and “aquiclude” are now essentially obsolete. It is very important to note that aquitards impede groundwater flow, rather than stopping it altogether.
Figure 1.4 – Examples of aquifers and aquitards affecting the flow of groundwater [Younger]
Unconfined aquifer
This is an aquifer in which the upper limit of saturation (neglecting the capillary fringe) is the water table, so that unsaturated soil or sub-soil lies between the upper boundary of the aquifer and the ground surface. Confined aquifers are also called ‘Artesian’ (such as the Great Artesian Basin in Eastern Australia), but that name is used only in non-specialist terms.
Confined aquifer
This is an aquifer lying below an aquitard, in which there is no unsaturated zone between the base of the aquitard and the groundwater within the aquifer. In most cases, groundwater within a confined aquifer is under sufficient pressure that, in a well penetrating to the confined aquifer, it will settle to a level higher than the base of the overlying aquitard.
Piezometric surface
This is the horizon formed by the levels to which groundwater in a confined aquifer would rise were it to be penetrated by a number of wells.This can be seen in the image below;
Figure 1.5 – Example of a piezometric surface
In certain geological settings, aquifers can switch between the confined and unconfined states over time. Confinement can be lost if the water pressure in the aquifer drops over time (due to pumping, for instance) such that the piezometric surface drops below the contact between the aquifer and its overlying aquitard.At this point the piezometric surface becomes, by definition, a water table and the aquifer becomes unconfined.
However, in most cases, only the shallowest two or three aquifers (and any intervening aquitards) will be of practical interest from a water resources perspective. The only other aquifer of interest is those known as “perched aquifer” conditions.
Figure 1.6 – Examples of perched aquifers [Younger]
The geology of groundwater
To discuss groundwater is to discuss rock, or at least the type of soil and geology involve, especially when discussing interrelations between aquifers and aquitards. Unfortunately, a discussion of the coexistence of rocks in the ground falls under the area of “stratigraphy”, which is study of the formation, composition, sequence and correlation of the stratified rocks of the earth’s crust, all of which can interact with groundwater, and results in another science called hydro-stratigraphy. Hydro-stratigraphy involves the identifying, naming, and specifying the properties of the aquifers and aquitards in a given geographical area.
We use groundwater data from the local area, such as well volume yields, to identify which of the local geological elements act as aquifers or aquitards. The local stratigraphic column, and therefore the local geological map, can then be re-labelled to provide a useful guide to the relative positions of aquifers and aquitards in the landscape.
To this end, the main focus is usually on understanding the direction and magnitude of any key features displayed by the local geology, especially where this varies from one place to another, as well as any breaches in the continuity of geology arising from the presence of faults or other geological phenomena.
Aquifer properties
Aquifer properties
We define an aquifer as a body of saturated rock that both stores and transmits quantities of groundwater. Understanding groundwater systems depends on quantifying the factors that control the ability of the aquifer to store and transmit groundwater. Indeed measurement (or estimation) of the storage and transmission properties of aquifers is a major routine task for hydro-geologists. Both storage and transmission properties are controlled fundamentally by geological factors which for any given rock mass determine;
◗the volume and sizes of the pores it contains
◗the strength of the rock mass when subjected to compression by the weight of overlying ground
Pores and effective porosity
Characterization of pore space is an important activity in many areas of science and engineering, and many specialist laboratory techniques exist for measuring the dimensions and volumes of pores in rock samples.
Less accurate field estimation methods for these characteristics also exist, which require the use of sophisticated geophysical tools that measure the density of the surrounding rocks as they are lowered down boreholes. The most common measure of pore occurrence is porosity, which is the proportion of a given volume of rock that is occupied by pores.
We commonly talk in terms of effective porosity, which is the ratio of the volume of interconnected pores to the total rock volume. Effective porosity arises from a range of rock properties. In unconsolidated sands and gravels, much of the effective porosity will be intergranular in nature. It is important to note that grain size does not in itself correlate with effective porosity: a skip full of ball-bearings will have the same effective porosity as a skip full of ten-pin bowling balls. Rather, effective porosity tends to correlate to other aspects of the sediment fabric, such as those listed below.
Grain shape
The more platy or more angular the grains, the closer they can pack together, and therefore the lower the effective porosity will be.
Grain sorting
Sediments composed of grains with a relatively uniform grain size tend to be more porous than those composed grains of a wide range of sizes; in the latter case, the small grains tend to occupy spaces that would be open pores in the uniform sediment.
Grain packing
Where depositional processes have tended to align the long axes of grains parallel to one another, the effective porosity will be lower than if the same sediment were dumped with grains orientated chaotically.
As unconsolidated sediments undergo burial, the weight of overlying strata tends to increase the packing density of grains. Various geochemical and mineralogical changes collectively referred to as diagenesis can result in changes in effective porosity, be this destruction (e.g. by precipitation of mineral “cements” in pores) or creation (dissolution of soluble minerals to create new pores).
Transmission of water
There is no general correlation between the effective porosity of a given rock and its permeability (i.e. its ability to transmit water, to use the term informally).
Effective porosity tells us the proportion of a given rock mass that is occupied by interconnected pores. However, beyond satisfying the condition that at least some effective porosity must exist if there is to be any permeability, the proportion of pores is unimportant. The real control on permeability is the size of the pore necks, i.e. the sizes of the openings which connect each pore to its neighbours.
The relationship between pore neck size and permeability is well illustrated by the fact that, for a range of samples of well-sorted sandstones which have the same effective porosity but differ in grain size, those with the largest grain size will also tend to exhibit the highest permeability. The bigger the grains, the broader will be the pore necks that remain after the grains are packed together.
However, any observed proportional relationship between effective porosity and permeability would be expected to break down if rocks other than sandstone were added to the sample suite, as these are likely to have different grain shape and packing characteristics. So beyond the requirement for at least some effective porosity if a rock is to transmit water, no general relationship between effective porosity and permeability exists.
Storage properties of aquifers
The storage properties of unconfined aquifers are directly related to their porosities. To understand why this is so, we must first consider what is meant by a change in storage in an unconfined aquifer.
Let’s say we have a very simple unconfined aquifer, which is entirely surrounded by impermeable bedrock. Although most aquifers are not this simple, such a scenario can occur in a desert setting, for instance, where wind-blown sand has accumulated to a considerable depth within a hollow in granite bedrock. Consider the following sequence of events: