Cusheon Lake is located on Salt Spring Island (Figure 1). The watershed for this small lake is 7.24 km2. Domestic consumption, primary (e.g., swimming) and secondary (e.g., canoeing and angling) recreation, irrigation, aquatic life and wildlife are the water uses for Cusheon Lake.
This report assesses water quality data for 1974-95 (1974-99 for total phosphorus). We concluded:
· There was weak evidence of an increasing trend in spring overturn total phosphorus during 1975 to 1999. Total phosphorus values were above the upper guideline for aquatic life for 6 of 14 years, and above the guideline for drinking water and recreation for 10 of 14 years between 1975 and 1999.
· Chlorophyll a values were collected in 1980 and exceeded the guidelines for aquatic life, drinking water, and recreation.
· Phosphorus was the limiting nutrient for algal growth in Cusheon Lake.
· Increases in sodium, chloride, and specific conductivity indicated possible disturbances within the watershed.
· Total organic carbon values exceeded the guideline for raw drinking water. Chlorinating the water may produce trihalomethanes that may exceed the drinking water guideline.
· The Capital Health Region determined that the public beach on Cusheon Lake was suitable for bathing between 1981 and 1995.
· True colour values exceeded the guideline for drinking water aesthetics in 33% of the samples from Cusheon Lake.
· Dissolved oxygen did not meet the guideline for protecting adult and juvenile salmonids from production impairment in 29% of the samples during 1974-94. The guideline for protecting adult and juvenile salmonids from moderate production impairment was not met in 2% of the samples during this period.
· Water temperature exceeded the drinking water aesthetics guideline at the deep station in Cusheon Lake in October 1980. The guideline may also have been exceeded during the summer months.
· Total calcium values show that the lake had a low sensitivity to acid inputs (the lake was well buffered).
· Total iron values exceeded the guideline for drinking water aesthetics and aquatic life in two samples collected in 1974. Total manganese exceeded the guideline for drinking water aesthetics in samples collected in 1993 and 1994.
· One extinction depth value collected in October 1980 did not meet the guideline for swimming and may indicate that the guideline was not met in the summer months when swimming would most likely have occurred.
· Turbidity exceeded the aesthetics objective for drinking water (with disinfection only) in 20% of the samples collected between 1974 in 1995. The drinking water health guideline was exceeded in 80% of the lake samples. The turbidity levels in the Cusheon Lake were such that treatment processes to remove it are required prior to drinking.
We recommend that a remediation plan be developed and implemented to improve water quality in Cusheon Lake. The focus of the remediation plan would be:
· to determine the suitability of Cusheon Lake for recreation, drinking water, and sustaining aquatic life;
· to identify the sources of nutrients and contaminants in the watershed and lake;
· to identify what is required to improve the water quality in Cusheon Lake to make it suitable for recreation, drinking, and sustaining aquatic life; and
· to evaluate remediation options and recommend the most efficient approach to improving water quality in the lake.
The plan should be developed and implemented by a Cusheon Lake stewardship group in conjunction with the Islands Trust.
We recommend monitoring:
· to establish site-specific water quality objectives to protect water uses;
· to identify the sources of nutrients and contaminants in the watershed and lake; and
· to identify changes in water quality due to biological activity in the lakes, activities within the watershed such as urbanization, and changes in non-point discharge.
A Cusheon Lake stewardship group could implement this monitoring program with assistance from the Ministry of Environment, Lands and Parks.
Other monitoring is also recommended:
· to determine whether the public beach is suitable for bathing. The Capital Health Region will continue to continue monitor and assess the suitability of the public beach for bathing.
· to determine trihalomethane levels in chlorinated water from the water works. The water works licensee (Beddis Water Works District) and Capital Health Region are implementing this monitoring.
· to determine the quality of drinking water at the 35 domestic water licenses on Cusheon Lake. A Cusheon Lake stewardship group could implement this monitoring program with assistance from the Capital Health Region, and/or the Ministry of Environment, Lands and Parks.
Scale 1: 120,000
Figure 1 Cusheon Lake Watershed
Table of Contents
EXECUTIVE SUMMARY i
INTRODUCTION 1
QUALITY ASSURANCE 4
STATE OF THE WATER QUALITY 4
CONCLUSIONS - STATE OF WATER QUALITY 9
RECOMMENDATIONS FOR WATER QUALITY MANAGEMENT 10
REFERENCES 41
List of Figures
Figure 1 Cusheon Lake Watershed iv
Figure 2 Bathymetric map of Cusheon Lake 3
Figure 3 Total phosphorus (average in the water column before stratification)
from Cusheon Lake 13
Figure 4 Total dissolved phosphorus from Cusheon Lake 14
Figure 5 Dissolved Ortho-phosphorus from Cusheon Lake 15
Figure 6 Dissolved ammonia from Cusheon Lake 16
Figure 7 Kjeldahl nitrogen from Cusheon Lake 17
Figure 8 Nitrate/Nitrite from Cusheon Lake 18
Figure 9 Total nitrogen from Cusheon Lake 19
Figure 10 Ammonia:nitrate ratio from Cusheon Lake 20
Figure 11 N:P ratio from Cusheon Lake 21
Figure 12 Total calcium from Cusheon Lake 22
Figure 13 Total organic carbon from Cusheon Lake 23
Figure 14 Total inorganic carbon from Cusheon Lake 24
Figure 15 Dissolved chloride from Cusheon Lake 25
Figure 16 True colour from Cusheon Lake 26
Figure 17 Extinction depth from Cusheon Lake 27
Figure 18 Total iron from Cusheon Lake 28
Figure 19 Total magnesium from Cusheon Lake 29
Figure 20 Total manganese from Cusheon Lake 30
Figure 21 Dissolved oxygen from Cusheon Lake 31
Figure 22 pH from Cusheon Lake 32
Figure 23 Total residues from Cusheon Lake 33
Figure 24 Dissolved silica from Cusheon Lake 34
Figure 25 Dissolved sodium from Cusheon Lake 35
Figure 26 Specific conductivity from Cusheon Lake 36
Figure 27 Water temperature from Cusheon Lake 37
Figure 28 Turbidity from Cusheon Lake 38
List of Tables
Table 1 Summary of water quality data for Cusheon Lake 39
Table 2 Summary of Capital Health Region fecal coliform data for Cusheon Lake 40
Author
Holms, G. B. Water Quality Section,
Water Management Branch,
Environment and Resource Management Department,
Ministry of Environment, Lands and Parks,
Victoria. B.C.
Acknowledgments
Drafts of this report were reviewed by Lloyd Erickson and John Deniseger, Pollution Prevention in Nanaimo, Robert Bradbury, Capital Regional District, Barry Boettger, Public Health Protection, Roland Rocchini, Rick Nordin and Larry Pommen, Water Management Branch, and Wayne Hewitt and Tom Gossett, Salt Spring Island Water Protection Association. Valuable comments were provided and incorporated into this report.