Ground Water Resources of British Columbia
Chapter 10 — Ground Water Resources of the Plateaus and Highlands Ground Water Regions
10.1.3 FRASER PLATEAU
by
J.C. Foweraker
GENERAL SETTING
The Fraser Plateau (Figures 8.7 and 10.6) is located within the Interior System of the province. Over half of the area of the plateau is located on the west side of the Fraser River. This area includes the Chilcotin country which is traversed by the road from Williams Lake to Anahim Lake. The northern boundary with the Nechako Plateau is defined west of the Fraser river by the West Road River, while the eastern part of the plateau is bounded on the north by the Fraser Basin. The Fraser Plateau is bounded on the south by the Thompson Plateau. The south west boundary of the plateau is flanked by the Coast Mountains, while on the east side, the plateau meets the Quesnel Highlands, which Holland (1964) has drawn at approximately the 1525 m contour.
Figure 10.6 Fraser Plateau
A number of population centers and settlements are located within the Fraser Plateau which include the southern boundary centers at Lytton, Lillooet and Clinton and north along Highway 97 through 70 Mile House, 100 Mile House to Lac La Hache. Towards the east side of the plateau there are settlements at Canim Lake, Horsefly, Big Lake and Likely, while along Highway 20 west of the Fraser River and Williams Lake there are several settlements including Hanceville, Chilanko Forks, Tatla Lake and Anahim Lake.
PHYSIOGRAPHY
The Fraser Plateau is described by Holland (1964) as a flat and gently rolling country having large areas of undissected upland lying between 1220 and 1525 m. In the north western part of the plateau, Tipper (1971) describes three prominent volcanic centers, the Rainbow, Ilgachuz and Itcha Mountain Ranges. In the southern corner of the Fraser Plateau are located the Camelsford, Marble and Clear Ranges which rise prominently above the plateau surface and consist of older rocks. The central part of the Fraser Plateau has been deeply incised by the broad valleys of the Fraser River and its tributaries, the Quesnel River and the San Jose River which together form the southern portion of the Fraser Basin as defined by Holland (1964).
GEOLOGY
A large part of the Fraser Plateau is underlain by volcanic lava rocks which have steep escarpments along rivers and creeks and almost flat upper surfaces. Tipper (1971) refers to the widespread accumulations of Tertiary volcanic rocks of several ages within the plateau. He notes that the younger Tertiary lavas flooded the lower Chilcotin River Valley and the southeastern part of the plateau and are generally much less dissected than the earlier Tertiary volcanics. Today the Fraser and Chilcotin Rivers have incised into the younger lavas.
The Fraser Plateau is covered to a great extent by glacial drift which has been molded into elongated drumlin forms by the movement of ice across the plateau. Holland (1964) describes the direction of ice movement across the plateau as eastward and north eastward from the vicinity of Anahim Lake and north eastward to northward from the area between Chilko Lake and the mouth of the Chilcotin River. Major rivers such as the Chilcotin and Fraser flow in deep valleys within the plateau and the rivers have eroded into the glacial and outwash materials including till, silts, sands, gravels and lacustrine deposits which fill the older valleys. Kettles, eskers and other ice contact features are scattered along the valleys and according to Tipper (1971) indicate that at one time meltwaters associated with melting ice occupied these valleys within the plateau area.
GROUND WATER RESOURCES
It should be mentioned here that the well record information used in completing the following sections was supplied gratuitously to the Province by well drilling contractors and others, it may be incomplete and its accuracy and reliability has not been independently confirmed. The information summarized in the following sections should be used with caution.
The majority of the water wells completed within the Fraser Plateau are lower yield wells of less than 1 L/s. However, within the heterogeneous unconsolidated deposits which make up the materials filling the broad valleys of the major rivers within the plateau, there are some highly productive aquifers in which some high yield wells have been completed. Also a few deep higher yield wells have been completed in the basalt lavas and brecciated zones of the plateau bedrock.
There are almost 2000 recorded wells located in the Fraser Plateau if the population centers at the southern border of the plateau, namely Lytton, Lillooet and Clinton are included. It should also be recognized that there may be many additional wells within the plateau for which no records are presently available. Approximately 54% or 1070 of the available records on file are for wells completed in unconsolidated deposits. About 82% or 880 of these wells have either no reported yield or the well yields are reported as being less than 1 L/s. These wells are mainly used for domestic or stock requirements. The remaining 18% or 190 well records are for higher yield wells which fall into one of three groups based on reported potential yield. Group 1 consists of 170 wells which range in yield from 1 L/s to less than 4 L/s. Group 2 consists of 7 wells which range in yield from 4 L/s to less than 7 L/s, and the third group consists of 12 wells which have recorded yields of over 7 L/s.
The wells in the third groups of over 7 L/s are located at Lillooet, Deadman River, Canhim Lake, 108 Mile Road, Horsefly Lake, Chilanko Forks and also other locations. For example, wells completed in productive sand and gravel aquifers have been developed at depths below 30 m to produce well yields of 15.1 L/s at Lillooet, 37.9 L/s at Deadman River and 15.1 L/s at 108 Mile Ranch. At Horsefly Lake a 22.7 L/s well was completed in a gravel aquifer located between 23 and 30 m deep, while at Chilanko Forks, three wells with yields of over 7.6 L/s have been completed at depths between 46 and 61 m in sands and gravels.
A number of wells in the second group with yields reported between 4 L/s and less than 7 L/s are located in the general area west of 100 Mile House, where more productive sand and gravel zones within the unconsolidated deposits have been found.
Approximately 46% or 900 records are on file for bedrock wells located in the Fraser Plateau and 88% or almost 800 of these wells have either no known recorded yield or have recorded yields of less than 1 L/s. Over 10% or over 90 bedrock wells have recorded yields between 1 L/s and 4 L/s, while less than 2% or 10 bedrock wells have recorded yields of over 4 L/s.
Examples of higher yield bedrock wells include a 9.1 L/s bedrock well north of Lillooet, a test well producing 11.1 L/s completed for the Ground water Section to a depth of 152 m in basalt lava flows and brecciated zones at a location near Mile 80 on Highway 97 and two wells completed in basalt lava flows and brecciated zones to depths of approximately 183 m at 100 Mile House which produced 12.1 and 7.6 L/s respectively. There are also reports of high yield wells being completed in bedrock at a depth of about 107 m at 108 Mile Ranch.
GROUND WATER QUALITY
Available records for ground water quality are mainly confined to the south east portion of the Fraser Plateau between Clinton and Canim Lake. Generally the analyses show the ground water to be very hard, highly mineralized and of a calcium-magnesium-sodium- bicarbonate-sulphate type. Apart from hardness, other constituents at some locations, do exceed the recommended limits set by the Guidelines for Canadian Drinking Water Quality, 1989.
A discussion of some specific ground water quality analyses from the Fraser Plateau are as follows: North of Clinton a 46 m deep well completed in sands and gravels provides hard water which is high in sodium. Further north from Clinton, off Highway 97, analyses of a 14.3 m deep well showed the water to be hard but of acceptable quality for domestic use. Near 70 Mile House an analysis of a 20 m deep bedrock well showed the water to be high in sodium and iron and it would require treatment before use. An analysis of a 152 m deep bedrock test well constructed for the Ground water Section at 80 Mile on Highway 97 showed the water to be hard and high in iron. Similarly at Lone Butte an analysis from a 9.4 m deep well completed in unconsolidated material also showed the water to be hard and high in iron. At 100 Mile House analyses of two high yield bedrock wells about 183 m deep indicated the water to be hard, highly mineralized and high in sodium. This water was of the calcium-magnesium-sodium-bicarbonate-sulphate type. An analyses of a high yield 61 m deep well completed in sands and gravels at 108 Mile Ranch showed that the ground water was slightly alkaline, very hard and highly mineralized. The water was of the calcium- magnesium-sodium-bicarbonate-sulphate type. At Forest Grove and Canim Lake, well analyses showed the ground water to be very hard and highly mineralized with bicarbonates and sulphates of magnesium, calcium and sodium. In addition, the well analyses at Canim Lake showed phosphate concentrations of 3.7 mg/L PO4.
At Deadman River within the Fraser Plateau area, two high yield test wells completed in sands and gravels at about the 30 m depth, contained hard, highly mineralized ground water with mercury levels up to 0.005 mg/L which are above the maximum acceptable concentrations of 0.001 mg/L set by the Guidelines for Canadian Drinking Water Quality. The mercury is likely associated with the bedrock in the general area and tests showed the mercury concentrations increased with pumping of the wells. At Big Lake, a field analysis of a 9.1 m deep dug well in unconsolidated deposits showed the ground water to be highly mineralized with a high Ph and a high sodium concentration.
Back to Table of Contents
| 
