Ground Water Resources of British Columbia
Chapter 11 — Ground Water Resources of the Mountain Ground Water Regions
11.1 INSULAR AND COASTAL MOUNTAINS, INTERIOR MOUNTAINS
M.L. Parsons and O. Quinn
The mountains of British Columbia occupy vast areas throughout the province as illustrated in Figure 8.8. For the purposes of this discussion the mountains are grouped into major geographic regions, each of which has relatively distinctive geological and geographical features. The mountain regions of the province can be conveniently grouped as follows:
- Insular and Coastal Mountains comprising the mountains of the Queen Charlotte Islands and Vancouver Island as well as the Coast Mountains and Cascade Mountains.
- Interior Mountains comprising the Columbia Mountains of the southern interior and the Omineca, Cassiar, Skeena and Hazelton Mountains of the northern interior.
- Rocky Mountains and Foothills of the eastern part of the province.
Figure 8.8 Mountain Ground Water regions
These mountainous regions collectively form more than half of the geographic area of the province, and thus in areal terms at least, represent a significant component of the province's ground water resources. However, these regions comprise some of the remotest, least populated and least developed parts of the province. In these areas, ground water is of limited consequence for water supply and has received little attention and study. As a result, much of this chapter is of necessity based on inference and conjecture.
Notwithstanding the relative unimportance of ground water as a water resource in the province's mountainous regions, it plays an important role in the hydrologic cycle and in sustaining streamflow during periods of low surface runoff. Thus the environmental significance of ground water mandates that it not be ignored in the remote regions, and that it be conserved and protected as it should be in the more populous, developed parts of the province.
Insular and Coastal Mountains
The Coast and Cascade Mountains comprise the Coastal Mountain region which extends in a continuous belt from the US border to the Yukon. The Coast Mountains consist primarily of granite intrusions and the Cascades primarily of sedimentary and volcanic rocks. The Insular Mountains of Vancouver Island and the Queen Charlotte Islands are also composed mainly of sedimentary and volcanic rocks, intruded in places by granitic plutons.
Glacial processes during and since the Pleistocene ice age have had a significant impact on the hydrogeology of the region. The most important effect has been the deposition of unconsolidated sediments in many of the valleys which dissect the mountain ranges. These deposits include significant thicknesses (commonly hundreds of metres) of silts, sand, gravel and till, all of which have an important bearing on the occurrence and behaviour of ground water. In some coastal areas such as Kitimat, located at the head of the Douglas Channel, lowlands may be infilled with marine clays deposited at a time when the sea level was relatively higher.
The Columbia Mountains, which include the Cariboo, Monashee, Selkirk and Purcell ranges, are composed mainly of metamorphic rocks including highly metamorphosed schist, gneiss, amphibolite and quartzite as well as unaltered siltstone, sandstone, conglomerate, limestone and dolomite.
The Columbia Mountains are drained by major river systems — the Columbia, Fraser, Kootenay and Thompson Rivers. These rivers were deeply scoured by glaciers during Pleistocene time and subsequently infilled with deposits of silt, sand, gravel and till. These deposits generally represent the most important aquifers in the region.
Cassiar and Omineca Mountains
The Cassiar and Omineca Mountains constitute a continuous belt of largely plutonic rocks, in places associated with sedimentary strata of shale, greywacke and limestone. The granitic plutons of Jurassic and Cretaceous age deformed and altered much of the older sedimentary rocks of the area.
Although much of the Cassiar and Omineca mountain region was glaciated during Pleistocene time, glacial deposits apparently are not extensive in occurrence. The rivers of the area are actively eroding and downcutting and the valleys tend to lack the thick valley fill deposits common in the Columbia Mountains and other mountainous regions.
Skeena and Hazelton Mountains
The Skeena Mountains are largely formed of sedimentary strata (shale, siltstone, coal, greywacke and conglomerate) deposited in the Bowser Basin during the Jurassic and Cretaceous periods. The mountain ranges are closely spaced, separated by a network of northwesterly trending main valleys and northeasterly trending transverse valleys. The area was glaciated during the Pleistocene but the distribution of glacial deposits and nature of valley-fill is largely unknown.
GROUND WATER FEATURES
Ground water occurs in the mountain bedrock masses and the intervening valley fill deposits. In the mountains, ground water occupies voids and fractures within the rock, often with irregular distribution because of the heterogeneous nature of rock fractures. Spatial variations in bedrock permeability are common, and in combination with topographic relief, have a strong effect on determining direction of ground water flow. Mountain springs often occur where downward movement of ground water through fractured bedrock is impeded by a reduction in rock permeability, forcing ground water to move in a lateral direction toward a slope or stream. In limestone terrain, such as is encountered in parts of Vancouver Island, karst features (natural underground channels and caverns) originally formed by circulating ground water now control its distribution.
In contrast to the mountain masses, ground water in the valleys occurs in porous granular deposits, most notably sand and gravel. Sand and gravel often has a widespread distribution within valleys and such deposits provide favourable conditions for storage and extraction of ground water for water supply development. However, these valley deposits also often include silts, clays and tills typically of low permeability, and not suitable as sources of ground water supply. However, these low permeability deposits have a significant bearing on the ground water regime generally, and play an important role in ground water resource management and protection.
Recharge and Flow
Management of ground water resources depends on an understanding of ground water recharge potential and direction of flow. For practical purposes the amount of ground water recharge is commonly determined as a percentage of mean annual precipitation. Throughout the mountainous regions of the province, mean annual precipitation varies considerably as reported by Farley (1979) and summarized below:
|Insular and Coastal Mountains
||1500 to more than 3500 mm
||500 to 2500 mm
|Omineca and Cassiar Mountains
||400 to 750 mm
|Skeena and Hazelton Mountains
||500 to 750 mm
Estimates of recharge range from a few percent to as much as 30% or 40% in the extreme. However, little research on recharge in mountainous terrain has been completed.
Lawson (1968) in his study of ground water flow in Trapping Creek (a mountain basin east of Kelowna) estimated recharge of local scale flow systems to be about 55% of precipitation. Jamieson (1981) in his study of the Lillooet basin in the Coast Mountains estimated that 17% of precipitation enters the ground water system. Because most precipitation in the mountains falls during the colder months, it is likely that ground water recharge occurs predominately in associated with snowmelt. In any case infiltration in the mountain masses is generally dependent on the presence of rock fractures, and ground water flow is associated with the network of rock discontinuities. The majority of rock discontinuities and thus flow systems are concentrated at shallow depths (<100 m). Such systems primarily discharge to mountain streams, springs and lakes.
In addition to local systems, larger scale flow systems originate in mountainous recharge areas, penetrate to hundreds and even thousands of metres in depth through fractured bedrock networks, eventually discharging along major river valleys. The geometry of such flow systems in mountainous regions, and particularly the influence of geological and topographic variation on their configuration at depth is well illustrated by Hodge and Freeze (1977).
The chemical composition of ground water normally reflects the mineral composition of the rock or soils through which ground water moves. Thus, in crystalline bedrock of low solubility, such as the granitic plutons of the Coast Mountains, ground water typically has low concentrations of dissolved chemical constituents and is likely of good chemical quality. In areas where base metals mineralization (Cu, Po, Zn, Fe, etc.) occurs, ground water may contain relatively high concentrations of those metals and be unacceptable for drinking water. Still elsewhere, where limestone and dolomite formations predominate, ground water will have high calcium and magnesium carbonate content which imparts hardness to the water. However, ground water in both crystalline bedrock and sand and gravel aquifers throughout the mountainous regions of the province is normally of good chemical quality and suitable for drinking and other uses.
Ground water flow systems that penetrate to considerable depths are influenced by the geothermal gradient of the earth and sources of heat associated with centres of volcanic activity. Geothermal effects include increased temperature as well as increased concentration of dissolved minerals in the ground water.
The Coastal Mountains contain a number of warm and hot springs, (less than and greater than 32° C, respectively), generally coinciding with regional fault zones associated with areas of relatively recent volcanic activity. However, Clark (1985) has identified and investigated 15 sites between Bella Coola and the Yukon Border within the Coast Mountains as well as on the Queen Charlotte Islands, where springs are primarily associated with intrusive rocks, and are unrelated to centres of recent volcanism. These waters have temperatures ranging from 23° C to 85° C, and generally discharge at elevations near sea level.
In geothermal areas such as Meagher Mountain, circulating ground water often incorporates dissolved geothermal gases and encounters geothermally mineralized zones. Under these conditions, and with the long residence time associated with deep percolation, ground water may substantially increase its dissolved mineral content. Upon discharge to the surface in the form of hot springs, precipitation of minerals commonly produces deposits of travertine.
GROUND WATER USE
Ground water usage in mountainous regions of the province is not well documented, however, ground water is used as a source of supply by many rural residents as well as by a number of communities. These include for example, Blackcomb and Whistler Mountain Resorts in the Coast Mountains and the City of Greenwood in the Monashee Mountains. In most cases, ground water sources are sand and gravel aquifers located in valleys, along which most communities have developed and where agriculture is carried out. In mountainous areas beyond the valleys, where bedrock terrain is predominant, the availability of ground water is somewhat unpredictable and often limited in quantity, reflecting the hydrogeologic nature of fractured rock. Water supplies in the mountains are often obtained from streams and springs rather than from wells.
The subsurface plays an important role in the hydrologic cycle in terms of collecting and storing ground water, which is then slowly released to lakes, streams and rivers. During drier months, ground water often constitutes the total flow of smaller streams and rivers, referred to as base flow. Stream base flow, much of which in British Columbia originates in mountainous regions, is important to the province's river systems, particularly in the drier areas and during low rainfall periods. During these periods, baseflow is essential for maintaining fish and other aquatic populations as well as water supplies for communities and agriculture. Thus, ground water of mountainous regions has significance far beyond the boundaries of a mountain mass itself.
The water balance in mountainous terrain is in large part dependent on the preservation of a stable watershed. Ground water recharge in such areas is enhanced by the absorption and storage of precipitation by soil and vegetation cover on the mountain slopes. Lack of such cover through deforestation increases soil erosion and reduces the potential for ground water recharge.
Taking into account the dynamic nature of ground water flow systems, the potential for contamination of ground water through introduction of chemicals into the environment is apparent. Potential sources of chemical contaminants in mountainous regions include mining developments, forestry and agricultural operations (which use pesticides, herbicides and fertilizers) and highway and railway right-of-ways along which herbicide spraying, salting of roads and accidental spillage of hydrocarbons and chemicals may occur.
Mountainous terrain is particularly vulnerable to contamination because soil cover with its natural contaminant attenuating capacity is generally limited. In addition, fractured crystalline bedrock has little potential for retarding contaminant movement. Once pollutants enter fractured bedrock, little attenuation is likely to occur and rates of migration can be relatively rapid. Thus, management of ground water resources in mountainous regions should include preservation of vegetation and soil cover.
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