Ground Water Resources of British Columbia
Chapter 9 — Ground Water Resources of the Basins, Lowlands and Plains
9.1.3 GULF ISLANDS
by
A.P. Kohut, J. Foweraker and W. Hodge
GENERAL SETTING
The Gulf Islands comprise a number of islands, situated over a distance of 160 kilometres in the Strait of Georgia off the eastern coast of Vancouver Island (Figure 9.14). The Gulf Islands as popularly known (Williams and Pillsbury, 1958) include a northern group of two islands (Hornby and Denman) and a southern group of 11 major islands (Gabriola, Valdes, Thetis, Kuper, Galiano, Saltspring, Prevost, Mayne, North and South Pender and Saturna) plus several smaller islands (Figure 9.15). The islands lie within the Nanaimo Lowland physiographic region (Holland, 1964). Topography of the lowland exhibits moderate relief up to 600 m and is primarily controlled by northwesterly trending bedrock structures. Elevations below 500 m are characterized by a cool Mediterranean type of climate in which precipitation falls primarily as rainfall during the period September to May. The coastal areas are dominated by the flow of air from the Pacific Ocean and coastal winters are mild and wet. Mean total annual precipitation ranges from 811 mm at Victoria to 1506 mm at Courtenay (Environment Canada, 1975). The summer months, June to September are normally very dry and subject to drought conditions.
Apart from a small number of freshwater lakes used for water supplies on some islands, ground water has been developed extensively and in some cases is the only viable source of potable water supply available. Although ground water availability is of on-going concern in this area, present and future ground water quality is of particular importance. Apart from natural water quality variations which can be significant, the effects of man's activities have in some cases resulted in ground water quality deterioration.
GEOLOGY
The area was extensively glaciated during the Quaternary period. Deglaciation was progressive and relatively rapid with the area becoming free of ice about 12,000 years ago (Halstead, 1967b). During deglaciation, sea level reached a maximum of 75 m above present levels near Victoria and 150 m near Courtenay (Fyles, 1963). Following deglaciation, isostatic uplift and eustatic changes caused a relative lowering of sea level to its present position. The unconsolidated deposits of Pleistocene and Holocene age, comprised of marine, fluvial and glacial material are generally thin (less than 18 m in thickness) or absent on the islands (Halstead, 1967b). Bedrock consequently is widely exposed and hence the major target for ground water exploration. A relatively thick section of unconsolidated deposits is found along the northeast portion of Denman Island.
Figure 9.14 Location plan, Gulf Islands
Bedrock formations underlying the Gulf Islands consist primarily of clastic rocks of the Nanaimo Group of Late Cretaceous age (Muller and Jeletzky, 1970). The Nanaimo Group (Figure 9.16) consists of a sequence of indurated marine and non-marine sedimentary rocks comprised of conglomerate, sandstone, shale and coal. Regionally the Cretaceous strata are gently folded with a uniform direction of dip towards the northeast into the Georgia Basin. Older rocks belonging to the Sicker Group are also found on Saltspring Island. These consist of Permian and older volcanic, sedimentary and metamorphic rocks comprised mainly of massive tuffs, volcanic breccias, limestone, argillite, quartzite and greenschist (Muller, 1977).
Figure 9.15 Location of specific Gulf Islands
Faulting believed to be related to late Cretaceous and Tertiary differential uplift of Vancouver Island and the simultaneous depression of the Georgia Basin has affected most of the rocks in the region (Muller and Jeletzky, 1970). Major longitudinal reverse faults trending northwesterly occur on Vancouver Island, parallel to the margins of the Georgia Basin and subsidiary transverse faults striking northeasterly have also been mapped. The major faults mapped by Muller (1977) which comprise geologic contacts are shown in Figure 9.16. Other numerous and widely distributed structural lineaments probably reflecting bedding planes and joint systems are readily observed on air photographs of the region. Outcrops of sandstone units often exhibit closely-spaced joints normal to bedding planes. It is most likely that the joint systems owe their origin to the post-Tertiary faulting, tilting and uplift that has affected the region. Isostatic rebound following glacial unloading and stress relief may have resulted in further development of bedrock fractures particularly flat-lying or sheet-type fractures. Open fracture zones in the bedrock are important as they constitute zones for ground water storage and movement.
Figure 9.16 Bedrock geology, Gulf Islands
HYDROGEOLOGY
The dominant factor governing ground water availability is secondary porosity due to structurally controlled fracturing while intergranular porosity is generally of minor importance. Bedding plane partings and geologic contacts between different rock types also constitute important water-producing zones. Systematic fracture traces (lineaments) observed on air photographs are widely distributed and appear interconnected suggesting the possibility of hydraulic continuity of ground water in fractured bedrock on a regional scale. Widespread fracturing in the bedrock appears to be more important than individual major fault zones in governing the regional availability and movement of ground water. Major faults however may play an important hydrogeologic role locally, either as preferred conduits or as relative barriers to ground water flow resulting in short circuiting of deep and shallow ground water flow systems.
Ground water from bedrock wells is an important source of water supply for individual domestic use, small communities and for small mixed farming operations in the region. Well depths range from 30 to 150 m. Confined and unconfined aquifer condition exist with non-pumping water levels generally within 5 m of the ground surface. Well yields range from less than 5 to as much as 250 litres per minute (L/min) but normally vary from 10 to 20 L/min. Ground water demand is greatest along waterfront areas and interior valleys where subdivision density is high. The majority of wells completed since 1970 have been drilled by the air rotary method. This method enables well drillers to identify the depth and approximate yield and water quality of individual major water-producing fracture zones. In most instances the bedrock wells are unlined except for a shallow surface casing completed in overburden materials. Where unlined, the wells may interconnect deep and shallow ground water flow systems.
Regionally, areas of ground water recharge and discharge have been recognized in the sedimentary terrain (Foweraker, 1974; Dakin et al, 1983). Areal distribution of composite hydraulic head data and spatial water quality variations suggest that upland areas are ground water recharge areas. In these areas water levels generally lie several metres below ground surface. Water levels in low lying regions on the other hand are generally close to the ground surface or under flowing artesian conditions indicative of ground water discharge regimes. These latter areas are for the most part localized along coastal shorelines, near the toe of slopes and within valleys. Figure 9.17 depicts the generalized ground water flow conditions which are believed to exist on most of the islands.
Data from numerous bedrock observation wells in the region (Kohut et al, 1984) indicate ground water levels respond cyclically on a seasonal basis to climatic variations. Ground water recharge occurs as water levels rise in response to fall and winter precipitation. Water levels decline during the dry summer and early fall months reaching a seasonal minimum between October and December. Long term (10-year) hydrograph records (Kohut et al, 1984) show a similar seasonal response in wells completed in different areas, in different rock types and different depths in various positions in ground water flow systems. This suggests that there is significant hydraulic continuity within the fractured flow systems regionally.
Figure 9.17 Generalized ground water flow in the
Gulf Islands (modified after Dakin et al, 1983)
GROUND WATER QUALITY
Significant spatial and temporal ground water quality variations occur at shallow depth (<150 m) within the fractured sedimentary bedrock of the Upper Cretaceous Nanaimo Group underlying the Gulf Islands (Kohut et al, 1986). Information on water quality variations within the older rock units is sparse. Natural ground water quality is found to vary relative to sampling position within evident ground water flow systems. Recharge areas are characterized by low mineralized (low specific conductance), calcium and sodium-bicarbonate type ground waters while deeper portions of flow systems and discharge areas are dominated by brackish sodium - choride type ground waters. Magnesium is not found to be a dominant cation in ground waters on the islands. TDS values range from less than 100 mg/L in recharge areas to 15,000 mg/L in discharge areas. Some representative water quality analyses for recharge and discharge areas are shown in Table 9.6. Unfortunately the poorer quality ground waters occur in coastal discharge zones and interior valleys which are the most attractive for residential land development. Ground water quality deterioration in these areas can be further accelerated by ground water development.
Figure 9.18 General areal relationship of high well density and
poor ground water quality on Hornby Island
Figure 9.18 shows the general areal relationship existing on Hornby Island where poorer quality ground waters high in total dissolved minerals and characterized by a high specific conductance (> 1000 S) and high chloride content (> 100 mg/L) are associated with areas of high well density. Ground waters with a sulphurous odor attributed to the presence of hydrogen sulphide gas and also high iron (> 0.3 mg/L) concentrations, are often associated with these high density areas. The presence of hydrogen sulphide gas, high mineralization, high chloride and iron content contribute to undesirable taste and odor problems and nuisance problems such as corrosion of pipes, and staining of toilet fixtures and laundry. The association between quality problems and high density development is also evident on Gabriola and Galiano Island (Hodge, 1978; Hodge and Mordaunt, 1983).
Fluoride concentrations up to 13.4 mg/L have been found in bedrock wells at a number of localities in the Gulf Islands (Kohut and Hodge, 1985). The high concentrations appear associated with water-producing zones in shale and clay strata and may be attributed to the release of fluoride ion from fossiliferous marine shales containing fluorapatite, through dissolution and anion exchange processes under elevated pH conditions. The high fluoride ground waters are generally of the sodium-chloride, sodium-calcium-chloride or sodium-bicarbonate-chloride types characterized by a moderately high pH value in the range 8.2 to 9.4 pH units, TDS in the range 250 to 1800 mg/L and negligible sulphate content (<10 mg/L). Fluoride concentrations in excess of 1.5 mg/L (British Columbia Ministry of Health, 1982) may produce dental fluorosis (tooth mottling) particularly among children ingesting these waters.
Since most bedrock wells are not lined and open throughout their length to water-producing zones located at different depths the resultant water quality of a sample taken at an arbitrary depth often represents a mixture of water from different depths and zones of differing lithology. For these reasons it is not always possible to relate ground water quality variations with changes in lithology. Structural variations (folding and faulting) further complicate assessment of the role of rock type on water quality. Samples collected during drilling often indicate quality variations with depth with higher mineralized sodium-chloride type waters often underlying lower mineralized, near surface bicarbonate type ground waters. Dakin et al (1983) have compared for example Na+ and Cl- concentrations in ground water with the corresponding element concentrations in rock samples at various depths for a 153 m deep borehole on Mayne Island. Highest concentrations were found in the rock and ground waters at depth near the bottom of the hole while in the upper 100 m of the hole the concentration of Cl- in the rock and water was very low. Heisterman (1974) has compared TDS values for samples from wells completed to three depth ranges on Mayne Island. This information (Figure 9.19) shows the general relationship of increasing mineralization with depth in broad upland recharge areas and localization of more mineralized ground waters in discharge areas. Evident in Figure 9.19 is the association of highly mineralized ground waters adjacent to a major northwest-southwest striking transverse fault in the western portion of the island. Dakin et al (1983) also report the association of saline ground waters on Mayne Island in discharge zones along fault or fracture zones. On Mayne Island, these saline ground waters were found (Dakin et al, 1983) to be of meteoric origin based on analyses of natural stable isotopes (018 and deuterium) and not the result of seawater intrusion. The salt content of the saline ground waters has been attributed to the slow release through molecular diffusion of Na+ and Cl- from the low permeability matrix of shale beds to the active paths of ground water flow in fractures in the shale and sandstone strata (Dakin et al, 1983). Na+ and Cl- in the shales may have originated as trapped marine waters during Tertiary time or from penetration of seawater during episodes of glacial loading and erosion during Pleistocene time when much of Mayne Island was submerged below sea level (Dakin et al, 1983).
Figure 9.19 Comparison of TDS values for three depth ranges on
Mayne Island (modified after Heisterman, 1974)
Brine springs having TDS concentrations up to 72,000 mg/L (Table 9.6) occur on Saltspring Island associated with a major fault. These discharges of probable deep sedimentary basin origin (Dakin et al, 1983) are however very localized and do not have a significant effect on the overall quality of shallow ground waters on the island.
Sea water intrusion caused by pumping of wells also occurs locally within some coastal communities where well density is high but generally is not extensive. Upconing of brackish ground waters (Todd, 1980) into wells completed into upper freshwater zones underlain by saline waters occurs in many areas as evidenced by seasonal ground water quality deterioration in pumped wells during the summer months. Reduced pumping demands and ground water recharge from infiltration of precipitation in the winter months generally results in improvement of water quality.
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