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This report presents results of water budget analyses of two hydrologically diverse aquifer areas in BC at Mayne Island and Cache Creek. Mayne Island is a fractured bedrock aquifer system in a humid coastal environment, and Cache Creek is a sand and gravel aquifer system in a dry climatic setting. Results include a preliminary assessment of groundwater availability for licensing, and identification of data gaps and monitoring activities for improving estimates of groundwater availability.
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Author: Hy-Geo Consulting
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Date Published: Aug 2015
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Report ID: 48989
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Audience: Government and Public
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This report presents the results of preliminary water budget analyses of two hydrologically diverse aquifer areas in BC at Mayne Island and Cache Creek. Mayne Is. represents a fractured bedrock aquifer system in a humid coastal environment while the Cache C. aquifer system includes alluvial and glacial sand and gravel deposits situated within an Interior dry climatic setting. The Cache C. aquifer system is hydraulically connected to the Bonaparte River and Cache C. The Cache C. aquifer system includes Aquifer 133 and 134 in the Bonaparte River Valley and Aquifer 135 in the Semlin Valley. Findings for Mayne Island indicate the annual recharge amounts over the years 2012-2014 ranged from 14.8 to 30.4 % of the annual precipitation. Recharge amounts equivalent to about 25 to 30 % of the annual precipitation likely represent the maximum potential amounts available for allocation purposes. Given the uncertainties involved and lack of specific data on surface water runoff on the island it would be prudent to apply an interim safety factor of at least 50% (approx 12 to 15 % of normal year precipitation) for future groundwater allocation decisions. Currently, groundwater use appears to be only a small percentage (1.9 to 5.0 percent) of the potential recharge that is occurring on an annual basis. While overall groundwater use is relatively low it is not uniform but concentrated in areas of high density lots in proximity to coastal areas where deteriorating water quality, well interference and salt water intrusion issues can occur. These conditions may also be exacerbated during periods of drought that often correspond with periods of highest water demand. In licensing new production wells and springs, additional factors such as water demands, proposed water use, proximity to neighbouring wells and surface water sources, potential well and stream interference effects, proximity to salt water and other water quality considerations may need to be taken into account. More detailed water budget investigations in some densely developed areas could provide a better perspective and clarity of the groundwater situation on Mayne Is.
Findings for the Bonaparte River Valley aquifers indicate that Aquifer 133 (confined) and Aquifer 134 (unconfined) are considered to be part of one interconnected aquifer system having various lithologic, structural and hydrogeologic heterogeneities. Groundwater levels in both aquifers follow an annual cycle, rising during the freshet season, declining thereafter and then rising slightly during the early winter months. Both the unconfined and confined aquifers show a similar response indicating they are both physically and hydraulically interconnected. The Thornthwaite-type water budget analysis conducted for the years 2010, 2011 and 2012 resulted in net soil moisture deficit amounts of 418.9, 406.2 and 393.2 mm respectively for surface water runoff and groundwater recharge indicating that precipitation over the aquifers does not contribute significantly to aquifer recharge. Surface water use, principally for irrigation during the late spring and summer months, is the largest use of water in the Bonaparte River Valley. Surface water use during the years 2010, 2011 and 2012 ranged from 2.5 to 4.4% of the annual outflow of the Bonaparte River with highest monthly use (July or August) ranging from 8.0 to 9.4 percent. During the summer months, return flow from irrigation contributes a significant portion to groundwater inflow (recharge). Groundwater use in the Bonaparte River aquifers is about 10 % of surface water use on an annual basis, peaking in the July August period following the freshet period. Groundwater use appears to comprise 50 to 60 % of the estimated groundwater inflow on an annual basis. There are critical times, however, during the year, e.g. April and October when groundwater use approaches groundwater inflow. Currently there is no indication that the aquifers in the valley are adversely stressed.
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Report Type
Subject
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Region - Province Wide |
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Water Information - Groundwater |
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