Field Hydrogeology
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About this ebook
This third edition has been fully revised and updated to bring the book into line with developments in environmental regulations. The order of the chapters reflects the structure of a hydrogeological project and the development of a conceptual model up to completion of a report. The focus is on current practical applications of hydrogeological investigations using new case histories and a new chapter on specialist techniques has been included.
- Handy pocket-size for field research
- Features case histories
- Focuses on practical applications
- Contains a new chapter on groundwater investigations
Field Hydrogeology, Third Edition is an invaluable resource for undergraduate and postgraduate students of geology, hydrogeology, environmental sciences and engineering, as well as a wide range of professionals working in the water resources and environmental protection fields.
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Field Hydrogeology - Rick Brassington
1
INTRODUCTION
Groundwater provides an important source of drinking water over much of the world. It also has the fundamental importance of maintaining river and stream flows during periods without rain and also supporting wetland sites. Groundwater is under threat worldwide from over abstraction and by contamination from a wide range of human activities. In many countries activities that may impact on groundwater are regulated by government organizations, which frequently require hydrogeological investigations to assess the risks posed by new developments.
Pumping from new wells may reduce the quantities that can be pumped from others nearby, cause local spring flows to dwindle or dry up wetlands. The hydrogeologist will be expected to make predictions on such effects and can only do so if he has a proper understanding of the local groundwater system based on adequate field observations. It is equally important to evaluate groundwater quality to ensure that it is suitable for drinking and for other uses. Groundwater commonly provides the flow path that allows pollutants to be leached from industrially contaminated sites, landfills, septic tanks, chemical storage areas and many more. Hydrogeological studies are needed to define groundwater systems in order to prevent such contamination or manage its cleanup. This book is concerned with the field techniques used by hydrogeologists to evaluate groundwater systems for any or all of these purposes and with the primary interpretation of the data collected in the field.
1.1 Groundwater systems
Groundwater is an integral part of the hydrological cycle, a complex system that circulates water over the whole planet and is illustrated in Figure 1.1. The hydrological cycle starts as energy from the sun evaporates water from the oceans to form large cloud masses that are moved by the global wind system and, when conditions are right, precipitate as rain, snow or hail. Some of it falls on to land and collects to form streams and rivers, which eventually flow back into the sea, from where the process starts all over again. Not all rainfall contributes to surface water flow, as some is returned to the atmosphere by evaporation from lakes and rivers, from soil moisture and as transpiration from plants. Water that percolates through the soil to reach the water table becomes groundwater. Groundwater flows through the rocks to discharge into either streams or rivers. In coastal areas groundwater discharges into the sea and the aquifer contains seawater at depth. The volume of water percolating into the aquifers defines the groundwater resources that both support natural systems and are available for long-term water supply development. In most groundwater studies it is necessary to consider the other components of the cycle as well as the groundwater itself in order to understand the groundwater system. Consequently hydrogeological investigations usually include a range of field measurements to assess these parameters.
Figure 1.1 The hydrological cycle
Groundwater flow through saturated rock is driven a hydraulic gradient, which, in unconfined aquifers, is the water table. Rocks that both contain groundwater and allow water to flow through them in significant quantities are termed aquifers. Flow rates that are considered as significant will vary from place to place and also depend on how much water is needed. Water supplies to individual houses require small groundwater flows compared to wells supplying a town. In pollution studies even small groundwater flow rates may transport considerable amounts of contaminant over long periods of time. A critical part of the definition is that the rock allows a flow of water, rather than simply containing groundwater. Some rocks such as clays have a relatively high water content, although water is unable to flow through them easily. Other rocks may not be saturated but still have the property to permit water to flow, and therefore should be regarded in the same way as an aquifer, a clear example being the part of an aquifer formation that lies above the water table.
Unless groundwater is removed by pumping from wells, it will flow through an aquifer towards natural discharge points. These comprise springs, seepages into streams and rivers, and discharges directly into the sea. The property of an aquifer that allows fluids to flow through it is termed permeability, and this is controlled largely by geological factors. Properties of the fluid are also important, and water permeability is more correctly called hydraulic conductivity. Hydrogeologists often think of hydraulic conductivity on a field scale in terms of an aquifer’s transmissivity, which is the hydraulic conductivity multiplied by the effective saturated thickness of the aquifer.
In both sedimentary rocks and unconsolidated sediments, groundwater is contained in and moves through the pore spaces between individual grains. Fracture systems in solid rocks significantly increase the hydraulic conductivity of the rock mass. Indeed, in crystalline aquifers of all types, most groundwater flow takes place through fractures and very little, if any, moves through the body of the rock itself. Some geological materials do not transmit groundwater at significant rates, while others only permit small quantities to flow through them. Such materials are termed aquicludes and aquitards respectively, and although they do not transmit much water, they influence the movement of water through aquifers. Very few natural materials are completely uniform and most contain aquiclude and aquitard materials. Figure 1.2 shows how the presence of an aquiclude, such as clay, can give rise to springs and may support a perched water table above the main water table in an aquifer. The top diagram (a) shows a lower confined aquifer and an upper water table aquifer. The upper aquifer includes low-permeability material that supports a perched water table. The diagram shows the rest water levels in various wells in both aquifers. The lower diagram (b) shows how both confined and unconfined conditions can occur in the same aquifer. In zone A, the aquifer is fully confined by the overlying clay and is fully saturated. The groundwater in this part of the aquifer is at a pressure controlled by the level of water at point p, and water in wells would rise to this level above the top of the aquifer. In zone B, the overlying clay will prevent any direct recharge, although it is unconfined, like zone C. The aquifer in zone C is unconfined and receives direct recharge. Seasonal fluctuations in the water-table levels will alter the lateral extent of zone B along the edge of the aquifer. It is likely to be a minimum at the end of the winter and at its greatest extent in early autumn, before winter recharge causes groundwater levels to rise.
Where impermeable rocks overlie an aquifer, the pressure of the groundwater body can be such that the level of water in wells would rise above the base of the overlying rock (i.e. the top of the aquifer). In such instances the aquifer is said to be confined. Sometimes this pressure may be sufficiently great that the water will rise above the ground surface and flow from wells and boreholes without pumping. This condition is termed artesian flow, and both the aquifer and the wells that tap it are said to be artesian.
Figure 1.2 (a) A lower confined aquifer and an upper water table aquifer. (b) Both confined and unconfined conditions in the same aquifer
A groundwater system, therefore, consists of rainfall recharge percolating into the ground down to the water table, and then flowing through rocks of varying permeabilities towards natural discharge points. The flow rates and volumes of water flowing through the system depend upon the rainfall, evaporation, the geological conditions that determine permeability, and many other factors. It is this system that a hydrogeologist is trying to understand by carrying out field measurements and interpreting the data in terms of the geology. The four key factors in achieving a successful investigation are to understand the geology; to interpret the groundwater level data in terms of the three-dimensional distribution of heads that drive all groundwater flow systems; to remember that groundwater and surface water systems are interdependent and to use a structured iterative approach to developing your understanding of the groundwater system you are investigating.
1.2 Conceptual model
The foundation of all hydrogeological investigations is to gather sufficient reliable information to develop an understanding of how a particular groundwater system works. Such an understanding is usually called a conceptual model and comprises a quantified description encompassing all aspects of the local hydrogeology. The term ‘conceptual model’ has a different meaning in surface water hydrology, where it is specific to a type of computer modelling work.
Developing a conceptual model is an iterative process, as it is usual to review your ideas several times during the investigation progresses as more data is gathered from existing records or field measurements. This process is the basis of all hydrogeological investigations. The various steps in this process are illustrated by a flow diagram in Figure 1.3, which generally follows the stages of an investigation as described in this book. The objectives must be defined before the study is started. An essential part of each stage is to consider both the available data and the information needs so that the next stage of the fieldwork programme can be planned. The process is iterative and involves re-evaluation of each aspect of the model as new data is obtained. For example, differences in groundwater chemistry may imply the existence of a barrier such as a fault and new boreholes may be needed to obtain geological evidence to support these ideas. The final part of the conceptual modelling process is to describe each component in words and with values given to all the quantifiable elements. The conceptual model can then be used to assess environmental impacts or as the basis of a computer-based numerical model.
Figure 1.3 This flow diagram shows how a conceptual model is developed from existing information and then gradually improved as field evidence is collected
1.2.1 Stages of an investigation
A typical hydrogeological investigation can be divided into a number of separate stages, each of which builds on the previous one and forms the steps in developing the conceptual model. You will need to tailor the investigation to the needs of your study; however, the bare bones of an average investigation are as set out below.
Desk study – the existing available information is assembled to provide an early opportunity to get a ‘feel’ for the groundwater system and start the conceptual modelling process.
Initial reconnaissance – it is important to get to know the study area at first hand so that you can plan your fieldwork programme.
Monitoring programme – defines the variation in groundwater levels, groundwater chemistry, rainfall, spring and stream flows etc both across the area and seasonally.
Data management – a systematic way of noting data in the field and examining it as it is collected to determine its reliability and if it represents the groundwater in your study area.
Exploration – may include drilling boreholes, pumping tests and geophysical investigations.
Water balance – quantifies the volumes of water that are passing through the groundwater system. Computer simulations may be used in this process to help define recharge and flows through the aquifer.
Completion of the conceptual model and providing a quantified description of the groundwater system.
1.2.2 Quality control
It is important to ensure that the field measurements are made in such a way that the data are as reliable as possible and that making the measurement has had the minimum possible effect on the data. This means following standard protocols, and is usually best if these are written down and ideally part of widely accepted standards. This approach of formal quality management is used by individual organizations in most countries and includes specifications for performing key activities and for making and maintaining records. In hydrogeological work these may include such tasks as taking groundwater samples, recording data during a pumping test, checking that field equipment is working properly before (and sometimes, after) you have used it in the field, examining data sets to decide that they reflect the conditions in the aquifer and getting someone to check your report. Of course, having a written method describing how these activities should be carried out does not guarantee that they will be done properly. Neither does it mean that these checks were not a part of routine hydrogeological work before the modern concept of quality control was introduced.
Defining a comprehensive quality control system is beyond the scope of this book. However, the basic checks on equipment are discussed in the appropriate chapters as well as methods for checking that field readings are taken and recorded properly. If your employer has a formal system, you must ensure that you follow it. Where no formal system is in place ensure that your personal professional standards include taking proper care in your work and that you ask others to review or audit your work when appropriate. For example, have a colleague read your reports before they are sent out. Use appropriate national standards or codes of practice that are published by bodies such as the British Standards Institution in the UK or the American National Standards Institute in the USA.
Table 1.1 Template for planning the contents of a report
1.3 Hydrogeological report writing
A description of your investigation, the conclusions that you have drawn and the recommendations that follow from them is as important as any other part of the work. After all, it is likely to be the only tangible thing that is produced by your efforts. Most hydrogeological studies are intended to answer specific questions such as ‘Will a new well on my property produce enough water for my needs?’ or ‘What environmental impact will my new quarry have on water resources?’ and so the report should be written to answer the relevant question. It should have a logical structure and be consistent with the objectives of the study. It should be written in a clear, straightforward way that the reader will be able to understand.
All reports should consist of three parts, namely a beginning, a middle and an end: in other words, an introduction, setting out the objectives of the study; the main body of the report, which may extend to several chapters, and the conclusions and recommendations. Make sure that you always include these three elements. Table 1.1 sets out a possible structure for a report that provides a logical sequence for you to develop the description of the groundwater system you have been studying.
2
DESK STUDY
A desk study is the preliminary examination of the hydrogeology using all the available information before additional data are obtained from the field. It is usually carried out in an office, hence the name. The desk study is also the first step in developing your conceptual model and the methods described in this chapter are likely to be the first steps in any hydrogeological assessment. By defining what is already known about an area and identifying which questions need answering, a desk study provides a very useful way of planning a fieldwork programme. An early appreciation of the different field measurements needed will enable equipment to be assembled and plans to be made, so that the fieldwork can be completed over the minimum period possible. Some measurements can only be made during a particular season and planning will avoid missing opportunities and delaying the completion of your work.
You will need information on a wide variety of topics including geological maps and reports, topographical maps, borehole records rainfall and river flows. The availability of this information will be very variable. Some useful data may have been collected by government departments, local government organizations and a variety of other bodies for a large number of different purposes, some of which may have little directly to do with groundwater studies. In most cases you will have to supplement existing information with your own field measurements.
If you are unable to obtain information from organizations within a particular country, you may be able to obtain it from American (USA) or European Union (EU) organizations. The geological survey university departments from these countries have undertaken overseas work for many years. Besides providing maps and reports, these organizations may help you to locate a source of information in the country where you are working. Often the easiest and fastest way of finding a data source or even the data itself is to search the Internet using one of the commonly available search engines. There are a large number of on-line bibliographic databases that can help you look for references and companies that will conduct literature searches on specified topics. Your local librarian is likely to have information on these services.
Use a systematic approach to the desk study, methodically examining all the available information. Where little information is available on an area, the desk study will be completed quickly and only produce a simple idea of how the groundwater system works. The desk study should be completed nevertheless, as it plays an essential part in planning what may be an extensive field investigation.
2.1 Defining the area
Topographical and geological maps are the basic information source to define the extent of the area you need to include in your study. The size of the area depends on the reason for the project. If you are assessing possible impacts of an abstraction of less than 20 m³ d−1 from a new borehole an area with a radius of 1 km may be adequate, except in some fissured formations, where a larger area may be needed. If the work is being carried out as part of a waste disposal investigation, extend the study area well beyond the immediate area of the site. Never regard the boundaries as rigid and always take account of geological boundaries. Where wells or springs lie just outside the periphery of the study area, for example, it is usually best to include them.
When assessing the overall groundwater resources of an aquifer you should include the whole aquifer formation and extend it on to neighbouring strata to include any cross-boundary flows. Attention must be paid to stream catchment boundaries, which are fixed by the topography and may not coincide with groundwater or aquifer boundaries. Such investigations may extend over an area less than 1 km² in the case of a minor aquifer such as a river terrace to several thousand square kilometres for major aquifers.
2.2 Identifying the aquifers
One of the first steps in a hydrogeological study is to identify the aquifers and aquitards. Use all the available geological information to classify the rocks in the area as good aquifers, poor aquifers or non-aquifers, even preparing a simplified version of the geological map using these categories. Next, consider the aquifer boundaries. There are many sorts of boundary that in broad terms can be classified as permeable or impermeable. Figure 2.1 shows a few examples of each type of aquifer boundary. The boundary between the two aquifers faulted against each other (a) may be permeable and allow flow across it, with permeability being locally increased if rock fracturing has occurred. Alternatively, the boundary may be impermeable due to secondary minerals deposited along the fault plane or the presence of a fault gouge. An impermeable boundary may be detected by differences in groundwater chemistry or from pumping test analysis. The faulted boundary shown in (b) is essentially impermeable, as the aquifer is faulted against an aquiclude. In example (c), aquifer A lies unconformably over a sequence that includes two separate aquifers, B and C. Conditions are similar to example (a), except that groundwater quality may be different in all three aquifers. In example (d), the groundwater conditions in the aquifer are similar to those in (b) in that the aquifer overlies a non-aquifer. In example (e), an unconsolidated aquifer, such as a river terrace, partly overlies a lithified aquifer (such as limestone or sandstone) that forms an escarpment. Groundwater flow is possible between the two aquifers and, because of the topographic differences, it is most likely to be from aquifer B to aquifer A. In these circumstances, scarp-slope springs may not be seen. In example (f), a gravel aquifer overlies low-permeability bedrock. No groundwater flow takes place across the boundary, but the aquifer recharge will be greatly enhanced by surface runoff from the upland area.
Figure 2.1 Different types of aquifer boundary are shown in this diagram
Decide whether it is possible for groundwater to flow across the aquifer boundary, as this will affect groundwater recharge and flow directions, and there are also implications for groundwater quality and the pumping characteristics of wells.
Next look at the thickness of the aquifer and how it varies across the area, as this has a significant affect on the transmissivity. Borehole information will be particularly useful in this exercise and helps define the base of the aquifer. Some formations, such as mudstones, igneous and metamorphic rocks, may act as minor aquifers only when they are fractured or weathered. Borehole logs may provide information on the depth of weathering. In high latitudes perennially frozen ground (termed permafrost) has a