Ground Water Resources of British Columbia
Chapter 9 — Ground Water Resources of the Basins, Lowlands and Plains
9.1.2 NANAIMO AND GEORGIA LOWLANDS
by
K. Ronneseth, W. Hodge, and A. P. Kohut
PHYSIOGRAPHIC SETTING
The Georgia Depression (Figure 9.8) is partly submerged beneath the Strait of Georgia and Puget Sound, and includes the Georgia Lowland along the mainland coast and the Nanaimo Lowland along the east and south coast of Vancouver Island (Holland, 1964).
The Georgia Lowland south of Sayward includes parts of numerous islands as far south as the Malispina Peninsula south of Powell River. From there southeastward a narrow strip of land 3 to 16 km wide together with a few offshore islands constitute the Lowland. The lowlands rise eastward merging with the Coast Mountain Range. High areas between Sayward and Menzies Bay, and on Quadra and Texada Islands, rise as monadnocks above the prevailing lowland surface (Holland, 1964).
The Nanaimo Lowland is a strip of low lying country, below 600 metres elevation, which extends for 280 km along the east coast of Vancouver Island from Sayward on Johnstone Strait to Jordan River west of Victoria. The Lowland is flanked on its western side above the 600 metre contour line by the Vancouver Island Ranges and reaches its maximum width of 32 km between Galiano Island and Shawnigan Lake (Holland, 1964). (Note: The Gulf Islands adjacent to Vancouver Island, though part of the lowland, are discussed separately in Section 9.1.3).
The major geomorphic features of the study area are the result of structural, erosional and depositional processes. Folding and faulting of the bedrock, erosion and repeated glaciation, isostatic and eustatic changes of sea level have all contributed to the physiographic features of the Georgia and Nanaimo Lowlands.
Figure 9.8 Physiographic setting, Nanaimo and Georgia Lowlands
The climatic uniqueness of this region enhances the importance of ground water as a source of freshwater supply. Kerr in 1951-52 divided the Georgia Depression into three climatic regions (Figure 9.9); the Cool Mediterranean, the Transitional (the most extensive) and the Maritime (or West Coast Marine). Cyclonic storms are responsible for the majority of the precipitation in the area with winter months usually being the wettest. An increase in high pressure systems in the summer in conjunction with the rain shadow effect caused by the mountains of the Olympic Peninsula and Vancouver Island result in less measureable precipitation. Precipitation increases both northwestward in the Georgia Basin away from the influence of the Olympic rain shadow and southwestward or northeastward from the centre axis of the Georgia Strait. Mean annual precipitation ranges from 50 to 250 cm in the region. Conversely, evapotranspiration and irrigation requirements tend to decrease from south to north in the Georgia Basin and away from the central axis of the Georgia Strait. Based on the Thornthwaite classification there can be a moisture surplus of 40 - 160 cm in winter but a moisture deficit of 5 - 20 cm in summer. This lack of precipitation in the summer season is the prime factor for many water deficiency related problems encountered in the Georgia and Nanaimo Lowlands.
Stream erosion under moist temperature climatic conditions has been the dominant process in developing British Columbia landforms (Holland, 1964), especially during the recent past when the relief was greatly increased through the uplift of the land. Rivers within the study area, especially those on Vancouver Island, show a seasonal pattern that suggests that they are predominately rain fed, the discharge being much higher in the fall and winter months than in the summer when the rainfall is modest. Baseflow is usually experienced in late summer and early fall. The water temperature of the rivers found in the area vary widely throughout the year. Turbidity is considered low with the exception of flood times. These variations (discharge, temperature and turbidity) limited the uses of river water as a supply source.
Ground water supplies in the region often have the advantage of consistent year round availability, low temperature variation and low turbidity. Rivers can either augment ground water supplies or be augmented by ground water. One example is in the lower reaches of the Cowichan River near Duncan. The abnormally high base flow conditions of Nile Creek for example are believed to be the result of ground water discharge (pers. comm. J. S. Mattison, Ministry of Environment, 1984). At present all major and most minor sources of surface water in coastal areas are licensed to their full capacity. Any additional sources of fresh water supplies must come from distant inland lakes, damming of more streams and rivers or from underdeveloped or unused ground water aquifers.
Figure 9.9 Climatic regions, Nanaimo and Georgia Lowlands
GEOLOGY
Glacial and Surficial Geology
In British Columbia the second most important agent of erosion has been glacial ice (Holland, 1964). "Glaciation within the Georgia Depression was intense. Ice pouring westward from the Coast Mountains and eastward from the Vancouver Island Ranges coalesced in the strait to form a composite glacier which flowed southeastward and southward, and escaped to the sea westward through Juan de Fuca Strait. The depression in part is of structural origin, but in part was over deepened by ice erosion. Low lying rock surfaces were stripped by weathered materials and were shaped, while elsewhere glacial materials were deposited as ground moraines, or as outwash of which Herrando, Savory and Harwood Islands are remnants" (Holland, 1964).
Most of the unconsolidated materials found in the study area may be attributed to the regimen and wasting of glacial ice during the Late Pleistocene. Though some of the unconsolidated deposits are the result of older glacial (Dashwood Drift) and interglacial (Mapleguard and Cowichan Head Sediments) activity, the majority of the deposits are from the Fraser Glaciation. The Fraser Glaciation probably represents the same geologic-climatic time period as the Classical Late Wisconsin Glaciation of the mid-continent region (Alley and Chatwin, 1979).
The unconsolidated deposits of Pleistocene and Holocene age, comprised of marine, fluvial and glacial materials are quite extensive and locally may exceed 100 metres in thickness. Elsewhere these unconsolidated deposits are found to be thin or absent with bedrock being widely exposed.
A stratigraphic framework of unconsolidated sediments and a chronology of Late Pleistocene and Holocene environments in the study area is shown in Figure 9.10.
Figure 9.10 Chronology of Holocene and Late Pleistocene environments and
stratigraphic framework, Nanaimo and Georgia Lowlands
BEDROCK GEOLOGY
Geologically the study area is situated between the St. Elias Insular Belt on the west and the Coast and Cascade Belt on the east. These belts form the two western most tectonic regions of the Canadian Cordillera.
The different bedrock geology on either side of the Strait of Georgia suggests that the strait between the two areas lies over a boundary between two structural regions. The Strait of Georgia and Johnston Strait essentially follow the contact between the granitic rocks of the coast intrusions of Jurassic age and older rocks of the Vancouver Group and other assemblages which lie to the west (Figure 9.11).
Figure 9.11 Generalized geology, Nanaimo and Georgia Lowlands
The Georgia Lowland (Holland, 1964) is underlain by granitic rocks as well as by inliers of older formations. Accordant summits represent remnants of a dissected late Tertiary erosion surface, which is warped and rises gradually eastward from the Georgia Strait until it is sufficiently high in the Coast Mountains to be completely dissected and destroyed. Below the 600 meter contour in the Georgia Lowland remnants are more extensive and are to be seen as gently sloping upland surfaces. The Georgia Lowland contains Paleozoic rocks (granitic rocks associated with the Coast Plutonic Complex which range in age from the Paleozoic Era up to the Early Tertiary Period of the Cenozoic Era); Upper Paleozoic rocks (consisting of sedimentary and basaltic rocks); Lower Mesozoic rocks (consisting of volcanic and sedimentary rocks); Middle Mesozoic (consisting of volcanic and sedimentary rocks); and Upper Mesozoic rocks (consisting of volcanic and sedimentary rocks).
The Insular Belt which includes the Nanaimo Lowlands, contains a Middle Paleozoic and a Jurassic volcanic-plutonic complex, both apparently underlain by gneiss-migmatite terrains and overlain respectively by Permo-Pennsylvanian and Crestaceous clastic sedimentary rocks. A thick shield of Upper Traissic basalt, overlain by carbonate-clastic sedimentary rocks separates these two complexes in space and time. Post orogenic Tertiary clastic sedimentary rocks fringe the west coast of Vancouver Island. The Pacific Belt on the western and southern rim of Vancouver Island contains in its inner (eastern) part an assemblage of Late Jurassic to Cretaceous slope and trench deposits, deformed to melange and schist, and an outer part of Eocene oceanic basalt and subjacent basic crystalline rock. The major rock type found in the Nanaimo Lowlands are the Upper Cretaceous carbonate-clastic sediments of the Nanaimo Group. They consist of upward fining sequences of conglomerate, sandstone, shale and coal of non-marine or near deltaic origin, succeeded by marine sandstone, shale and thin bedded, graded shale-siltstone sequences (Muller and Jeletzky, 1970).
The Nanaimo Lowland "consists of many low cuesta-like ridges separated by narrow valleys. The northwesterly elongation of the ridges is the result of differential erosion of Upper Cretaceous sedimentary rocks. The ridges are underlain by hard sandstone and conglomerate beds and the valleys are eroded in shales and softer rocks or along fault zones. In the south between Saanich Inlet and Jordan River, the lowland is underlain by granitic and older rocks, which are more resistant to erosion. This fundamental difference in bedrock is reflected in somewhat greater elevations and in different topographic forms", (Holland, 1964).
Figure 9.12 Major sand and gravel deposits, Nanaimo and Georgia Lowlands
GROUND WATER POTENTIAL
Bedrock
Ground water within the bedrock can be found in fractures, along bedding plane partings, lithologic contacts in the inter-flow zones of lava, in the intergranular openings of the rock, and in the case of limestone, in the channels formed by the dissolution of the rock by water.
The majority of bedrock wells in the Nanaimo Lowland portion of the study area are generally completed in rocks of the Nanaimo Group, principally sandstones, shales, coal and to a lesser extent conglomerates. The rapid accumulation of the sediments which make up these shales, sandstones and conglomerates, accounts for their being poorly sorted, massive and in general, lacking in highly permeable pore spaces for the transmission of water (Halstead and Treichel, 1966). Water wells drilled in these sediments indicate that fractures, bedding place partings and lithologic contracts are probably the main sources of ground water flow.
Water wells reported to be completed in sandstone generally produced higher yields than wells completed in shales. Existing data reports wells completed in sandstones generally yield water supplies greater than 5 litres/second.
A number of wells are reported to be completed in granitic rocks of the Island Intrusives (found principally in the southern east coast and Saanich Peninsula regions of Vancouver Island. Clapp (1913) reported that all intrusive rocks on Saanich Peninsula are highly fractured and that the Saanich granodiorite has regular and large joints and fracturing. These large, open fractures in the granodiorite extend to depths beyond 200 metres (Al-Mooji, 1982). It is through this network of joints and fractures that ground water is stored and transmitted. Based on water well log information, the yields from wells constructed in the granitic bedrock of the Saanich Peninsula are generally low (< 1 L/s) and large capacity wells are the exception (Zubel, 1980). In places, however, the fracturing can be so great the rock is unfit for building purposes (Clapp, 1913). Some wells for example must be screened the way sand is screened in unconsolidated deposits (Brown, et al., 1976).
According to Al Mooji (1982), in the Island Intrusives of the Saanich Peninsula there tends to be a general increase of well yields with depth, with the greatest yields observed between 40 and 80 metres, the second highest yields reported between 80 and 140 metres and the lowest yields between 0 and 40 metres. Some wells constructed in granitic rocks of the Saanich Peninsula and Mill Bay area are capable of yielding sufficient supplies of ground water for irrigation purposes. One well on the Saanich Peninsula is reported to yield over 15 L/s. Eight other wells on the Saanich Peninsula have been reported to yield over 3 L/s. Further studies would be required to verify if other granitic rocks found on Vancouver Island, other Islands and the Georgian Lowlands are of similar hydrogeologic character to the granitic rocks founds on Saanich Peninsula where locally high sustainable withdrawal rates maybe available.
UNCONSOLIDATED DEPOSITS
Most of the ground water extracted on Vancouver Island comes from aquifers within the unconsolidated deposits which are recharged by infiltration of either precipitation or surface water sources. The amount of water that can be extracted by individual wells constructed in these aquifers, depends on the permeability of the aquifer materials, the thickness and extent of the aquifer, the rate of aquifer recharge and on well construction.
The unconsolidated deposits which are hydrogeologically the most significant in terms of ground water potential are primarily comprised of sand and/or gravel. The deposits which fall into this category are discussed and listed below.
The shore, deltaic, fluvial and alluvial deposits of the Salish Sediments are hydrogeologically significant. These deposits range up 20 metres in thickness and 3000 metres across. The glacial fluvial deposits which include hummocky, knob and kettle, ridged, esker, terrace and pitted terrace, kame terrace, kame delta and ice contact alluvial fan deposits of the Vashon Drift. These deposits are found resting on the ground moraine of the Vashon Drift. Usually located within a few kilometers of the mountain slopes, these deposits may range up to 2 km wide and 7 km long and locally may exceed 15 metres in thickness. Lenses of sand and/or gravel are associated with the ground moraine deposits of the Vashon Drift. Though these deposits are potential aquifers, their location and viability must be confirmed by drilling, which can be both expensive and time consuming.
The primary targets for ground water development (Ronneseth, 1984a, 1984b, 1984c, 1985a, 1985b, 1986a, 1986b) from the above listed deposits, would include the fluvial deposits near present day stream channels and deltas, the deltaic terraces, the kame terraces and the kame deltas. The location of the major deposits can be seen in Figure 9.12.
Additional unconsolidated deposits which are hydrogeologically significant in terms of ground water potential include the glaciofluvial deposits of the Quadra Sediments (Fyles, 1963) known as the Quadra Sand (Clague, 1977). The ground water potential of this geologic unit merits an expanded understanding of its distribution and origin. Overlain by glacial sediments (mainly till) of the Fraser Glaciation and underlain by sediments of the Olympia Interglacial interval, the current theory on the origin of Quadra Sand is summarized by Clague (1977) below:
"The sand was deposited, in part, as distal outwash aprons at successive positions in front of and perhaps along the margins of glaciers moving from the Coast Mountains into the Georgia Depression and Puget Lowland during Late Wisconsin time. After deposition of the unit at a site, but before burial by ice, the sand was dissected by meltwater and the eroded detritus was transported farther down the basin to sites where aggravation continued. The sand was also eroded extensively by glaciers during the Fraser Glaciation."
Refer to Figure 9.13 (after Clague, 1977) for inferred distribution of Quadra Sand and the underlying Cowichan Head formation. Quadra sand was deposited in the northern region of the study area about 29,000 years ago and in the Saanich Peninsula region about 22,000 years ago. The extensiveness of the Quadra Sand, its thickness and locally proven yields of 6 L/s make these deposits a primary target for ground water exploration. Quadra sand may be located elsewhere in the study area. Aquifers consisting primarily of sand have been encountered but it is unknown if these sands are of the Quadra Formation.
OBSERVATION WELLS IN THE NANAIMO AND GEORGIA LOWLANDS
B.C. Environment operates a network of approximately 145 ground water observation wells in British Columbia. Of this number, 33 are located within the Georgia Depression. This number does not include 16 observation wells presently established on the Gulf Islands.
The majority of these observation wells are 152 mm (6-inch) diameter and equipped with Stevens type F automatic water level recorders which allow for continuous monitoring of ground water movement. Most of these wells have been established to assist in the evaluation, inventory and management of the ground water resource. Although a few wells have water level records going back to 1966, most active wells have been established since 1976.
Observation wells, both bedrock and surficial, show a seasonal water level response to precipitation or show additional erratic fluctuation caused by pumping thereby effecting the response of water levels to precipitation. A few observation wells show a response to river stage fluctuation (Observation wells constructed near the Nanaimo and Cowichan Rivers). Water wells that respond to precipitation simply reflect the changes in ground water storage. When recharge exceeds discharge, storage increases and water levels rise. This is normally evident in wells during the wet winter months when precipitation is greatest. The reverse is normally true in the late summer/early fall period when precipitation is lowest.
Observation wells located on the Saanich Peninsula, Cassidy, Lantzville, Qualicum, Coombs and Comox have shown an overall water level decline from 1984 to 1988 due mainly to the below normal annual precipitation experienced over this period. Many of these wells recorded historic low water levels in 1987.
Depending on the vertical permeability of material in the zone of aeration and the depth to water table or bedrock fracturing, a time lag can exist where peak water levels in wells lag behind precipitation periods. Kohut et al (1982) have shown a two and three month time lag exists in some surficial wells on the Saanich Peninsula near Victoria. A time lag also exists in bedrock wells and varies in length depending on factors such as rock type, degree of fracturing and depth to water bearing fracture zone.
Figure 9.13 Inferred distribution of Quadra Sand and Cowichan Head Formation
in the Georgia Depression
GROUND WATER USAGE
In more than l10 water well records it was reported that water yields greater than 3 L/s were possible in the Nanaimo and Georgia Lowland region. Over 25 water wells had reported yields of greater than l0 L/s.
Municipal usage accounts for 26 water wells with yields greater than 3 L/s. Some of the municipalities or towns which have developed large capadty wells include: Duncan, Nanaimo Regiona1District, Village of Qualicurn, Village of Parksville, Village of Comox, Sunshine Coast Region and Village of Gibsons.
Water users which are isolated from municipal water supplies have developed their own sources. Water districts, larger subdivision developments, trailer parks and individual property owners account for most of the domestic consumption of ground water outside the municipalities. There are over 12,000 water wells drilled in this region.
Industrial usage includes forest nurseries, dairies and other farming operations, fish hatcheries, gravel pit operations, pulp and paper mills and green houses.
Irrigation usage is confined primarily to farms and golf courses. Other users indude parks, schools, Indian reserves, nursing homes, airports and marinas.
GROUND WATER QUALITY
Natural ground water quality found in unconsolidated and bedrock aquifers of the Nanaimo and Georgia Lowlands, is generally acceptable for most uses. The ground waters from the unconsolidated sand and/or gravel aquifers were generally found to be of higher quality than the ground waters from the bedrock aquifers. The quality of the ground waters from the granitic rocks were usually of higher quality than those found in the shale rocks (Ronneseth, 1984a, 1984b, 1985a, 1985b, 1986).
Though the quality of the ground water is generally considered good in this region, it is sometimes unfit for domestic, irrigation or other purposes. Naturally occurring chemical constituents sometimes exist at unacceptable levels. Another area of concern is human induced pollution to ground water. Human activity, such as the placement of landfill sites, sometimes produces a negative impact in these same sand and/or gravel deposits on Vancouver Island which contain productive aquifers. Other ground waters are contaminated with sea water, resulting from poor well development and excessive ground water withdrawal rates. Examples of sea water intrusion can be found in a number of locations along the coast.
Ground Water Quality for Specific Regions
Natural water quality is expected to be acceptable for most uses from the ground waters of unconsolidated deposits. Analyses of these waters in the Saanich region (Ronneseth 1986a) indicate a low salinity hazard, low chloride levels and a low proportion of sodium to other cations. The analyses of ground waters from bedrock aquifers, also in the Saanich region, were generally reported to be suitable for irrigation and domestic consumption. However, it was reported (Ronneseth 1986a) that the salinity and sodium levels were sometimes very high. These waters may not be suitable for all uses. Chloride levels for iii of IN analyses were reported to be less than 150 mg/L. The remaining analyses reported chloride levels which range between 150 and 1500 mg/L.
In the Duncan to Nanoose Bay region the water quality information is based on nine laboratory analyses of ground waters from unconsolidated aquifers and 12 laboratory analyses from bedrock aquifers (Ronneseth,1985bl. Natural water quality is expected to be acceptable for most uses from the ground waters of unconsolidated deposits. Analysis of waters of unconsolidated deposits in the Duncan-Nanoose region indicate a low to medium salinity hazard which should not affect most irrigation practices. Electrical conductivity ranged from 41 to 340 micromhos/cm. The few analyses of ground waters from bedrock aquifers report the salinity and sodium hazards ranging from low to very high. When levels are high enough then special agricultural practices would be required. The electrical conductivity ranged from 312 to 6900 micromhos/cm. The very high saline readings found in two of the bedrock wells are probably the result of sea water intrusion. A salty taste in the ground water was reported in 22 water wells. This indicates chloride levels greater than 300 mg/L. Wells with a salty taste were constructed primarily in shales, though some wells were reported to be constructed in sandstone and clays.
In the Parksville area of Vancouver Island, the water quality information is based on nine Hach field test kit analyses and 18 laboratory analyses (Ronneseth 1984b). These analyses show ground waters are generally favourable for domestic or irrigation purposes. However, some of the bedrock ground waters may contain high levels of sodium relative to the levels of calcium and magnesium. Waters high in sodium relative to the level of calcium and magnesium may be undesirable for some crops.
In the Qualicum River to Union Bay region of Vancouver Island, the water quality information is based on just six laboratory analyses of ground waters from unconsolidated aquifers (Ronneseth1984c). These six analyses show ground waters are either the calcium bicarbonate type or the calcium-magnesium bicarbonate type based on equivalent per million (epm) percentages. The electrical conductivity values in these six samples ranged from 51 to 183 micromhos /cm. Total dissolved solids (TD5) ranged from 50 to 128 mg/L.
In the Union Bay to Oyster River region of Vancouver Island, the water quality information is based on just six laboratory analyses of ground waters from unconsolidated aquifers (Ronneseth1985a). Natural water quality is expected to be acceptable for most uses from the ground waters of unconsolidated deposits. Five of these six analyses show electrical conductivity values ranging from 14 to l12 micromhos/cm. These readings indicate low salinity levels and these waters would be suitable for most purposes. A salty taste in the ground water was recorded on 12 water well records which implies a chloride reading greater than 300mg/L. All twelve of these wells were completed in shales. A petroleum taste was also reported in the water from two wells.
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