Ground Water Resources of British Columbia
Chapter 4 — Geology and Its Importance in the Search for, and Assessment of, Ground Water Resources in British Columbia
by
E. Livingston
In any examination of the ground water resources of British Columbia, it is important to keep in mind that our knowledge and understanding of ground water resources of any part of the Province is usually related to the local population density or, in certain rural areas, the amount of ground water which is being used for irrigation. In some fairly densely populated areas, where there is plenty of surface water to supply gravity supply systems, very little is known about ground water. An example of this is the City of Penticton which has, in the past, obtained water from large creeks and from Okanagan Lake.
Thus, because of the uneven distribution of population and development, the ground water resources of the Province are very poorly known. Certainly there are many large and highly productive aquifers which have not been discovered and which at present, because of their location, are of no interest. Productive aquifers are likely located in the basalt plateaus of the Cariboo and Chilcotin areas and in the valleys of most of the northern rivers. In contrast, an area where the ground water resources are quite well known is the Saanich Peninsula north of Victoria. This area was largely dependent on ground water from the time of settlement. Many wells in the area were drilled by contractors who kept excellent records. Surface water from the Victoria system now serves much of the area's population.
Other factors also have a bearing on how much is known about the ground water resource in a specific area. An important factor on the north side of the Fraser River in the Maple Ridge area is difficult drilling conditions. This area is largely underlain by a very compact till containing many large, hard boulders. Drilling is extremely difficult and therefore expensive, with many unsuccessful and abandoned holes. Productive aquifers may exist in this area which have not been found because of drilling problems.
Complex surficial geology, with a very erratic and uneven distribution of aquifers has also discouraged ground water exploration. This is particularly true in areas of extensive ice contact deposits. Once the presence of such deposits is known, it is then possible to explore for ground water in a rational way. Examples where complex geology has made ground water development difficult are found on the east side of Vancouver Island and in the valley of the Columbia River between Castlegar and Waneta.
Much of the previous discussion about ground water resources can be applied to the surficial geology of British Columbia. Where there is sparse population and little development, there has been little study of the surficial geology at least in the degree of detail which is helpful in the assessment of ground water resources. Large scale studies of glacial geology are a first step in the understanding of surficial geology. An example is the Geological Survey of Canada Bulletin 196, Glacial Geomorphology and Pleistocene History of Central British Columbia by H.W. Tipper. A number of engineering studies, mostly by B.C. Hydro in the 1950s, have also added to our understanding of the surficial geology of several major valleys.
In considering the ground water geology and hydrology of British Columbia, it is necessary to realize that the surficial geology of this Province is extremely complex, certainly more complex than the areas to the south and east of British Columbia. This is the result of several glaciations in an area of complex topography including mountain ranges, islands and a long sea coast with fjords. For this reason, it is not possible to make the type of general statements about the geology of aquifers which have been made about the Northwest States. It is not possible to describe the complex ground water geology of the Lower Fraser Valley on one page. An exception to this is an important aquifer in the coastal areas of Georgia Strait called the Quadra Sand, as described in Chapter 9.1.2. The surficial geology may also be somewhat simpler in the northeastern corner of the Province which is part of the Northern Canadian Prairie.
In the search for small sources of ground water suitable for domestic use, geology is often less important. Assuming that drilling and well construction are carried out by a competent contractor, it is usually possible at almost any site to construct a drilled or dug well which can supply enough water for domestic purposes. In an area where a cover of glacial debris overlies rock, it is often possible to get water from a dug well in till or a drilled well in glacial sediments. If no water is found in the overburden, some water can usually be found in fractured rock.
A working knowledge of the local geology is important to water well drillers. The most productive aquifers are typically identified from local experience. This information is generally available to others where there is an government agency compiling and storing driller's logs.
When larger amounts of water are required for community supply, for industry or for irrigation, an understanding of geology is essential. Exploration methods which involve such unimaginative techniques as drilling on a grid or hiring dowsers have often produced results in some instances. The advantages of using the geologic approach are that exploration usually costs less and takes less time. It may also be successful where other methods fail. It may also prevent the implementation of expensive exploration programs where there is virtually no chance of success.
Geophysical techniques are often used in ground water exploration, especially outside of North America. The most popular method is ground resistivity in which an array of 4 electrodes is either "expanded" or is moved over the ground with a fixed spacing. The data are then interpreted by use of model curves or by computer calculation to indicate whether or not water-bearing material is likely to be present. This is often done where there is virtually no knowledge of the local geology. Interpretation of "soundings," as the use of the expanding array is often called, can now be done by a computer using a suitable program. The resistivity technique and other geophysical methods can be useful but must be tied to the local geology. They are useful in tracing or extending certain geologic conditions which are already known from drilling and/or careful field mapping.
In British Columbia, the most productive aquifers are related to glaciation. The surficial geology, which is dominantly glacial geology, is therefore important in exploration for, and evaluation of, high capacity aquifers. Many glacial deposits are limited in extent and must be studied in the local area where the search for water is taking place. This is especially true in areas of medium to high relief. At the same time, in order to work out complex local geology, it is usually very helpful to understand what happened in late Pleistocene time on a regional scale. For example, to be able to deal with local problems involving the silty lake beds which are a major unit in the surficial geology of the Okanagan Valley, it is helpful to have an understanding of how they were deposited and the conditions which prevailed while deposition was taking place.
In studying glacial geology where little subsurface information is available from drilling, land forms are often a major source of information. Air photos are extremely valuable and should be used whenever possible. In densely wooded areas may smaller features, and even some larger features, may be missed on air photos. Ground reconnaissance must not be neglected. Remote sensing, particularly side-scan radar imaging, is likely to be helpful but is not available at reasonable cost, for most areas at this time.
In the study of geology, drillers' logs are very useful. In order to make the best use of the logs, it is necessary to know who drilled the hole, how it was drilled and the objective of the drilling. In considering the objective, we want to know whether the owner was looking for a domestic water supply, a large supply of water or foundation conditions. Terminology is often a problem; for example, What did the driller mean by "hardpan"?, Did the driller differentiate between silt and clay? In working with logs, it is often possible to get to know which contractors or which individual drillers produced the best logs. Even the poorest logs may be valuable in identifying such things as ice contact deposits which are often quite difficult to identify without subsurface information.
A misinterpretation of subsurface geology may have an important bearing on the success of exploration. Exploration in ice contact deposits is best carried out using different procedures than exploration in fluvial or deltaic deposits. The chances of success in ice contact deposits are less than in alluvial fans. Exploration, or even production drilling, should be started with a geologic hypothesis which the geologist must be prepared to change as work proceeds and more information comes to light.
An understanding of geology and geologic processes prevents the assumption of impossible or very improbable conditions. For example, in a situation involving buried drainage, the drainage must be rational. It must flow in one direction with a downstream gradient. Interpretation of geophysical surveys must be geologically possible.
The preceding discussion about geology may also apply to the hydrology of ground water. This is particularly true with flow systems in fractured rock where the amount of water in storage is usually very small. Rock wells near the discharge end of a large active flow system will usually respond to long term pumping in a much different way than similar wells in the recharge zone of a slow moving, poorly recharged flow system. A correct assessment of the flow system has an effect on the interpretation of pump tests of such wells.
In other words, a well or a well field or even a ground water flow system should not be considered in isolation but should be related to the geology and hydrology of the area to the extent that this is possible. A correct geologic and hydrologic assessment results in a better understanding of the situation and avoidance of both unreasonable conservatism or the unpleasant surprises which come with aquifer depletion.
Water quality is usually less easily related to geology. There are very obvious situations; for example, certain shales contain water with dissolved iron and highly mineralized water may be found at the discharge end of large, slow moving flow systems. In other cases, it is more difficult to relate water quality to geology. Much early work on ground water did not include chemical analyses. More analyses are now available and a detailed study of water quality in the Lower Fraser Valley is being carried out by Environment Canada.
It is often quite difficult to sell the idea of systematic ground water exploration which really includes geologic mapping, examination of air photos and may involve test drilling for geologic and hydrologic information. Yet, systematic exploration will increasingly become necessary in order to keep ahead of the demand for ground water in many areas.
Up to the present, systematic exploration for ground water has been confined to small areas usually with many constraints imposed by existing distribution systems. Such constraints are imposed by budgets, political boundaries, land ownership and other considerations. Much more expensive surface water schemes, which may have persistent problems caused by sediment, treatment problems, low flows, ice problems and the risk of contamination and pollution are much more popular politically because they involve little or no initial risk - the water is there and can be seen. As long as a large part of the population believes in dowsers and the mystique of ground water, this situation is likely to persist.
Little work is being done by government agencies on surficial geology in British Columbia at this time. Some excellent early work was done in the Fraser Valley and in the south coastal area by the Geological Survey of Canada. More recent G.S.C. reports deal with the Lower Fraser Valley and parts of the Interior. The Department of Mines of the Provincial Government published a report on the surficial geology of the Okanagan Valley, (Nasmith, 1962). More recently, the Ministry of Lands has published several reports in the form of terrain analysis maps which are concerned with an assessment of the suitability of land for development and other uses.
Where a single, widespread formation often serves as an aquifer, a geologic report and map showing the occurrence and extent of the formation and a discussion of its origin are very useful. A good example of this is The Geological Survey of Canada report on the Quadra Sand, (Clague 1977). This can seldom be done in British Columbia with surficial aquifers which are very local in extent. It is necessary, then, to map the geology of small areas. Such maps may show units which are actually aquifers but generally they are most useful in pointing out the possibility of the existence of aquifers where no drilling has been done.
In British Columbia the most important as well as the most widespread and productive aquifers are of glacial origin. Western Canada was glaciated several times within the past 3 million years. The advance of ice during each glaciation removed most of the sediments deposited during previous glaciations. Much of the removed material was "reworked" by the ice and deposited under the ice as till. Only glaciations of the most recent glacial period, the Wisconsinan, have left much sediment in British Columbia. Even within the Wisconsinan, only the most recent ice advances were important in aquifer deposition.
A very important effect of glaciation is the production of vast quantities of gravel sand, silt and clay. One of the major differences between a glaciated area and one which was not glaciated, is the amount of granular material, especially gravel. This is particularly true in mountainous areas where melting ice in upland areas supplied huge quantities of sediment-laden water to streams with a high gradient. In contrast, much of the breakdown of rock in areas which have not been glaciated is by chemical processes to produce saprolite, clays and other fine-grained weathering products.
The most productive aquifers are deposits of gravel and sand, mostly products of glacial erosion. Deposition of much of the sand and gravel took place during the retreat phase of glaciation when melting ice was producing streams of water capable of moving gravel and sand. Much drainage was disrupted and partially blocked by ice and glacial debris. This formed various types of traps which retained much of the sediment in the upper part of the drainage basins. Huge glacial lakes in the upper reaches of the major river systems of British Columbia trapped huge volumes of fine-grained sediment in the form of lake beds and raised deltas. Sudden releases of water from these huge lakes were responsible for reworking and deposition of torrential gravels to form some of our most productive aquifers. Much sediment of all kinds was trapped and deposited in and around melting ice to form complex ice contact deposits in which many important aquifers are located.
The glaciation of upland areas, especially in the southern part of the Province, left a very youthful topography in which much water is trapped in lakes and swamps. These areas are important in recharging major ground water flow systems which contribute to the base flow to streams. Much of the irrigation water used in the dry Okanagan Valley comes from the glaciated uplands.
Extensive areas of British Columbia are underlain by relatively unaltered volcanic rocks of Tertiary age. These are mostly basaltic in composition and are in the form of flat-lying flows often with total thicknesses of hundreds of metres. These rocks usually underlie plateau areas at high enough elevations so that only marginal agriculture is possible because of climatic limitations. These volcanic plateaus are rather sparsely populated and are generally well supplied with surface water. Since the demand for, and exploration of, ground water has been negligible, very little is known about these ground water resources. Experience in similar areas in Washington, Idaho and Oregon where there is intensive agricultural activity, indicates that there may be large quantities of good quality ground water in the volcanic rocks, mostly in interflow zones.
In parts of North America, carbonate rocks (mostly limestone) are important aquifers. Dissolution of carbonate minerals makes open space in the rock for storage and passage of water. Weathering at or near surface may produce large, continuous, cavernous openings through which very large scale recharge can take place and from which springs discharge. Springs of this type may have very high discharge rates.
There are several fairly extensive areas of British Columbia which are underlain by limestone. These include the western side of the Rocky Mountains, the Marble Mountains near Clinton, part of the Lardeau area north of Kootenay Lake, and parts of central Vancouver Island. However, little is known about the ground water resources of these areas because of the reasons stated earlier - there has been virtually no demand and, therefore, no interest in ground water.
There are a number of known occurrences of warm and hot ground water in British Columbia. Most of these are small and temperatures are usually not high. Study of the geology of these occurrences shows that the sources of heat for most of these is the normal geothermal gradient. In other words, the water has travelled deep enough below the surface to be warm. However, in a few areas; for example, near Pemberton, the source of heat is igneous activity. In such areas it may be possible to construct steam wells to serve as sources of power as has been done in New Zealand, in Italy and in California. Some exploratory drilling was done in the recent past by B.C. Hydro but no exploration is going on at this time. If the demand for geothermal power or heat increases, the study of geology of such areas will be encouraged.
In general, geology is important in dealing with ground water problems whether they be problems of exploration for water, evaluation of ground water use data, problems of water quality, prevention of ground water contamination, design of drainage works or exploration for geothermal steam.
Back to Table of Contents
| 
