Denman Island Natcrsheds
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c -- 11 . '. /I Province of British Colunbio I Ministry of Er!vi.r.cnment I Water Investigations Branch I . I I i- c t I I 1 F. Chwojka Te c hni ci an Groundwater Section I Hydro1 ogy Di vi s ion I I I P I Victoria, B.C., ?day I979 li TABLE OF CONTENTS 1 I Page LF I Table of Coriterlts ...................... ii List of Figures .................... .-.. iii 1 List of Tables ........................ iii I 1 1. Introduction ..........................I . 2. Location, Size and Climate ............... 1 I 3. Topography a~c! Drainage ................ 1 4. General Geology ..................... 2 8 I 5. Bedrock Geology ............' ......... 2 1 6. Surficial Deposits ................... 3 7. Groundwzter Occurrence ................ 3 ..8. Well Inventory ...................... 3 I I 9. Groundwater Recharge .................. 4 1 10. Well Distribution 2nd Groundwater Use in Each Watershed ................... 6 I 11. Gromchater Chemistry ................. 7 .. 12. Concltlsions ...................... 8 I 13. Recommendations .................... 9 I 14. References ...................... 10 1 E I I 1 r -a I LIST OF FIGURES t Figure 1 Location Map ..................W 11 Figure 2 Denman Island Natcrsheds ........... 12 Figure 3 Geological Map ................Pocket Figure 4 Water Well Lccati.cn Map ...........Pocket Figure 5 Distribution of 1974 Water Sampling Sites . 13 Figure 6 Distribution of 1973 Water Sampling Sites . 14 Figure 7 Distrlbuticn of Total Dissolved Solids in Groundwater ................. 15 Figure 8 Distributjon of LCh1ori.de i.n GrounGwater ... 16 Figure 9 Distribution of Hardness in Groundwater ... 17 LIST OF TABLES -..--r-Paee Table 1 Precipitation Kerns for the Period 1941-19?0 for Penman Islad ...............,. 18 Table 2 Estimates of Groundwater Recharge and Usage . 19 Table 3 Result of 1973 Water Sampling (from Cooper 1973) ..............; 20 Table 4 1973 Water Sampling Sites .......... 21 LIST OF APPENDICES '* Appendix 1 Well Inventory - Denman Island (Pzges 1-9) Appendix 2 Chemical Analysis (Sites 2490589-1400626) c I PRELIMINARY REVIEW OF I I GROUNDiVATER CONDITIONS AND AVAI LAB1 LITY ou I . DENMAN ISLAND, BRITISH COLUMBIA I 1. INTRODUCTION With population growth increasing in the Gulf Islands, the demand I for fresh water becomes very important. This preliminary review deals with the known groundwater conditions of Denman Island. It was prepared to show present groundwater use and potential groundwater supplies. .A list of all known wells, a well location map and a watershed map for the Island are included in this preliminary report. Estimates of ground- water recharge and usage within each watershed have been calculated, bsed on average precipitation considerations. Available .water quality informsticm on groundwaters for the Island has also been reviewed. * 2. LOCATION, SIZE AND CLIMATE .. Denman Island is located on the east side of Vancouver Island, 14 miles south of Courtenay (Figure 1). It can be,reached by scheduled ferry from Vancouver Island and has approximately 500 residents. In 1976, there were 795 people living on Denman Island and Hornby Island, with about 300 on Hornby. Denman Island covers 13,000 acres or 2C square miles. Its climate is warm in summers with mild, wet winters. The average annual precipitation is 53 inches. The yearly distribution of rainfall and snowfall are shown in Table 1. Precipitation rr.zans for the period 1941 to 1970 are from Environ- ment Canada (1970). I 3. TOPOGRAPHY AND DRAINAGE Denman Island is a Lorthwest trending island with a dominant ridge I paralleling its west coast (Figure 2). rilaximum elevations of 400 feet occur along this topographic feature. Local relief is generally less than 300 feet. I Seven watersheds make up the island's drainage system (Figure 2): Denman, i 1 'I '. 2 I Boyle, Hornby, Central , Roirias, Chicadee and Henry. L. I The largest watershed areas are Denman on the west side coverhg 24% of the island, Central in ths middle of the island taking up 27%, and I Horriby on the southern half with 18%. 1 4. GEENER4L GEOLOGY Denman Island is irnderlain by three sedimentary bedrock formations, I all part of the Nariaimo Group of Upper Cretaceous age,. These formations hav*e been designated the Cedar District, de Courcey and Northurnherland 1 Formations (Cooper 1973). Two unconsolidated surficial units are also recognized on the islaid. I These include the Quadra Sediments and the Salish Sediments. The Quadra Sediments cover about i/6 of the northern portion of Denman Island, while 1 the Salish Sediments occur only at the northern- tip of the island (Figure 3). I 5. --EEDROCK GEOJXGY The Cedar District Fomation is found on almost the mtire west coast: of Denman Island and measures up to 1,000 feet in thickness (Muller 1977). 1 It is made up of marine shales cut by sandstone dykes. The shales include clay, silt and fine sand size particles and are dark grey or dark greenish I in colour. The sandstone dykes range in thickness from 2 'to 8 inches in width and are dark to light brown. From earlier studies this fornation was I interpreted as an offshore marine deposit be.cause the fine laminations suggest it was deposited in deep waters free from wave action (Muller 1970). I *The de Courcey Formation overlies the Cedar District formation and is comprised of massive beds of brown-grey sandstone with interbedded, poorly I sorted conglnmerate. Total thickness is estimated to be 900 to 1,000 feet. S The sandstone ranges from medium- to coarse-grained and the conglomerate 1 fragments rag? Trcrn fine pebbles to boulders in a fine-grained sand matrix. This formation is believed to be a deltaic deposit because of the presence i of interbedded conglomerates ad sandstones. I - I 3 I I The Northimberland Formation is found on the nid-east coast of Denma? Island and overlies the de Courcey Formstion. The finc-grkkd sandstone, I siltstone and shale beds are said to be 300 feet thick [Nuller 1970). Like the Cedar District Formation, this unit is also believed t~ be a marine a depos it . I 6. SURFICIAL DEPOSITS The Quadra Sediments consist of sand, silt, gravel and clay up to 300 feet thick and are dated at more than 49,000 years before present I (Dyck et al, 1966). I The Salish Sediments a-e marine deposits and are comprised of reworked Quadra Sediments and consist of boulders, gravels, sands, silts and clays. I 7. GROUNDWATER OCCURRENCE On Denman Island groundwater can be found in both the bedrock and 1 the surficial deposits. Good to excellent sources of groundwater are found in the unconsolidated deposits. In the Salish and Quadra Sediments in the I northeast portion of the island, springs form adequate year-round fresh water supplies. Other springs occurring locally above clay beds indicate perched I water tables in some areas. Fair to good sources of groundwater occur alc#iig faults in the bedrock and may also be found at conglomerate-sandstone or I shale-sandstone ' contact bedding planes. Bedrock areas which are not suffi- ciently fractured or where structural conditions are unfavourable msy only I yield poor to fair quantities of groundwater (Cooper 1973). I a. WELL .INVENTORY From Groundwater Section files to September 1977, records are avail- aLle of 35 dug walls with an average depth of 14 feet and 78 drilled wells $ I with an averege depth of 130 feet. The deepest dug well is 32 feet and the deepest drilled well is 430 feet. Total footage for dug wells is 1,226 feet I and for drilled wells, 10,157 feet. Locations of wells and springs are I shown in Figure 4. All wells on file are listed on pages 1 to 9 in Appendix 1. I I i 4 I i Tne oldest entry is a spring recorded in 1920 and the.& oldest drilled well dates back to 1959 with many entries without coinpletion dates. From I * the 171 sources of water, 35 or 20% of them have site identific2.tion numbers for sampling purposes. Bemistry results are attached; see computer forms i for sites between numbers 1400588 and 1400626 in Appendix 2. The best producing well on the island has been reported at 30,gpn:. - I The average for drilled wells with reported yields is 4-1/2 'gprn. Ancngst the dug wells and springs there is one reported to yield 15 gpm. The average 1 for dug wells with reported yields is 7 gpm. These figures are based on dri'llers' reports and reported yields by residents. They can be pumping I test results, bailer test yields or open pipe flow estimates. Distance to water in dug wells ranges from 0 to 25 feet with afi I average of 9.5 feet. Distances to water in drilled4wells vary from 2 to 50 feet with one recorded at 218 feet. Average distance to water is 19.5 feet I on the island. 9. G ROW! D\'!ATE R RE CHARGE i From Environment Canada precipitation data, (Table 1) the average amount of precipitation for Denman Island for the period 1941 to 1970 was I 52.7 inches per year. This ,total is made up from 48.6 inches of rainfall and 41.0 inches of snowfall. Snowfall, however, is taken as only 10% or I 4.1 inches of water equivalent. Estimated potentia! recharge from precipitation (Foweraker, 1975) 1 was calculated for Maj-ne Island on the basis of 1 inch per year. However, Foweraker found that his calculations exceeded by several times the esti- I mated storage availablc for groundwater within the fractured bedrock media, leading him to believe that on an annual basis storage and permeability, and I not precipitation, appear as *he more immediate limiting factors controlling ? groundwater availability on the islands, particularly during the dry summers I and early fall periods. Table 2 has been prepared to show the estimated potential recharge I (based on this 1 inch of precipitation a year) and the groundwater usage for watersheds on Denmaii Island on which only a thin overburden cover exists over I 1 5 I I bedrock.