Ground Water Resources of British Columbia
Chapter 9 — Ground Water Resources of the Basins, Lowlands and Plains
9.1.5 ALBERNI BASIN AND ESTEVAN COASTAL PLAIN
by
J. C. Foweraker
ALBERNI BASIN
This basin, located on Vancouver Island, extends north-westward from Port Alberni for about 40 km and is 8 to 13 km wide (see Chapter 8, Figure 8.3).
The basin is drained by the Ash, Stamp, Sprout and Somass Rivers into the Alberni Canal. The abrupt fault-line scarp with the Beaufort Range forms the eastern boundary of the basin which is surrounded on the other sides, for the most part, by mountainous terrain. The basin is underlain by Upper Cretaceous sedimentary rocks (Holland, 1964).
The largest ground water reserves are likely to be contained in recent alluvial deposits, terraced fluvial and deltaic deposits and possibly other sediments beneath the upper drift sequences (Ronneseth, 1986).
There is potential according to Ronneseth for locating ground water supplies capable of meeting some irrigation requirements in the Alberni Basin. Ronneseth reports surficial geology maps indicate extensive deposits of sand and/or gravel are located at surface, and that a few wells constructed into these deposits show aquifer thicknesses greater than 7 metres and yields greater than 3 L/s. Many of the existing well records in the Alberni Basin, however, are for single family dwellings where the demand is often for only 0.3 L/s.
Site specific assessments are required to prove up the potential for larger yielding wells in the Alberni Basin.
According to Ronneseth, there are reports for seventy wells completed in bedrock in the Alberni Basin, including 30 in granite and 24 in sedimentary rocks. Ground water is stored in fractures or along bedding plane partings in the rock.
Ronneseth believes the possibility of obtaining adequate supplies of ground water in bedrock for irrigation purposes are generally considered to be poor and although a few bedrock wells constructed in sandstone and granite rock types in the Basin have reported yields of between 1 and 9 L/s, long duration pumping tests would be required to verify if the bedrock aquifers are capable of a sustained high withdrawal.
Based on very limited information, ground water quality for wells completed in unconsolidated deposits would appear to be acceptable. However, further data is required.
ESTEVAN COASTAL PLAIN
The Estevan Coastal Plain (Holland, 1964) is a narrow plain divided into sections by various inlets and generally 1.6 to 3.2 km wide and less than 46 m above sea level which extends along the West Coast of Vancouver Island from a point east of Brooks Peninsula south eastward for almost 270 km to a point south east of Carmanah Point (see Chapter 8, Figure 8.3). Where the Coastal Plain is underlain by softer Tertiary or Pleistocene and Recent deposits its surface is almost featureless, elsewhere harder rocks of the Vancouver Group have produced bluffs and hummocks (Holland, 1964).
Information on ground water potential is available from a limited number of well records and confidential ground water reports by consultants which are on file in the Ground Water Section. This information is restricted to the Tofino-Ucluelet area. Bordering Florencia Bay and extending back towards Kennedy Lake is a coarse sand and gravel aquifer approximately 18 m deep in which test wells producing 15.1 L/s have been constructed for the Pacific Rim National Park (Brown, 1979). Test well drilling over the remaining Long Beach area of the plain has shown a veneer of silts, sands and gravel and till overlying a considerable thickness of clay possibly 30 - 90 m thick. Marginal wells have been completed in these overlying thin deposits of sand and gravel above 7.6 m and produce up to 4.5 to 9 L/min. In 1974 a system of seven shallow sandpoints yielding a total of 59 L/min was designed to extract ground water from thin sand layers below near surface till, overlying the thick clay sequence.
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