Salt Spring Island Appendix 8, OCP Bylaw No. 434

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CAPTURE ZONES, COMMUNITY WATER SUPPLY WELLS R. POTTER, 1996.

Bylaw Referenced in Volume 1: Section D - Island Resource Management BYLAW NO. 434 APPENDIX 8 – CAPTURE ZONES, COMMUNITY WATER SUPPLY WELLS

I i TO : LINDA ADAMS , ISLANDS TRUST , GANGES BC " CAPTURE ZONES " , COMMUNITY WATER SUPPLY WELLS . FOR THE ISLANDS TRUST , SALTSPRING ISLAND B . C . « BY ROBERT POTTER P . Eng . GULF ISLANDS GROUNDWATER 184 HILLCREST DRIVE SALTSPRING ISLAND B . C . V 8 K 1 Y 4 GULF ISLANDS GROUNDWATER Tel : 604 653 - 9729 JAN 26 , 1996 FILE 2408 ROBERT POTTER MScPEng GEOLOGICAL ENGINEER 184 Hillcrest Drive

CONTENTS f 1.0 INTRODUCTION 2.0 LOCAL HYDROGEOLOGY 3.0 MODELS 3.1 3.2 4.0 RESULTS 4.1 4.2 4.3 4.4 4.5 4.6 4.7 5.0 CONCLUSIONS 6.0 RECOMMENDATIONS HORIZONTAL MODEL SLOPING MODEL SCOTT POINT CEDAR LANE HARBOUR VIEW MARACAIBO ESTATES CEDARS OF TUAM ERSKINE HEIGHTS MT . BELCHER ILLUSTRATIONS Figure 1 : Figure 2 : Figure 3 : Figure 4 : Figure 5 : Figure 6 : Figure 7 : Scott Point ; Modeled head values : Plan Initial , 10 day , 100 day Scott Point ; Modeled head values : Profile , Well 1 Initial Scott Point ; Modeled head values : Profi 1 e , „ Wei 1 1 10 day • Scott Point ; Modeled head values : Profile , Well 1 100 day Cedar Lane , ; Well 1 100 day capture zone at n = . 001 Cedar Lane ; Well 1 100 day capture zone at n = . 0001 Sloping Site Model Figure 8 : Scott Point ; Capture Zone on Topo and Geology Base Figure 9 : it li , / Capture Zone on Cadastral Base Figure 10 : Cedar I . ane ; Capture Zone on Topo and Geology Base Figure 11 : ft if , t Capture Zone on Cadastral Base Figure 12 : Harbour View ; Capture Zone on Topo and Geo 1 ogy Rase Figure 13 : ti » * . Capture Zone on Cadast ral Base Figure 14 : Maracaibo ; Capture Zone on Topo and Geology Base Figure 15 : II r Capture Zone on Cadastral Base Figure 16 : Cedars of Tuam ; Capture Zone on Topo and Geology Base Figure 17 : f fl M . / Capture Zone on Cadastral Base . Figure 18 : Erskine Heights ; Capture Zone on Topo and Geology Base Figure 19 : If f l . t Capture Zone on Cadastral Base Figure 20 : Mt Belcher ; Capture Zone on Topo and Geology Base Figure 211 II 1 » t / Capture Zone on Cadastral Base ) ) I

1 1.0 INTRODUCTION I Gulf IslandsGroundwater was requested by The Islands Trustto carry out a preliminary assessment ofthe capture zones of seven community water supply well systems on Saltspring Island . The systems are : Scott Point Cedar Lane Harbour View Maracaibo Estates Cedars of Tuam Erskine Heights Mt . Belcher The assessment was based on data from published geological and topographic maps and on published hydrogeological data for the rock rock types found within each community area . Available site specific dataincluded approximate well locations , drillerswell logs and bail test capacities for some wells , and average pumping rates for each system . No field work was carried out during the course ofthis project . Definition : Capture zone is herein considered to be that volume of an aquifer from which water i . s supplied ( to a well or system of we ] Is , . The output ofthis report will be surface projections of capture zone estimates . The proceedure followed was that of defininq the limits of capture zones as per the following steps . 1 / Define local geological and topographic boundaries . 2 / Sellect expected values of effective hydraulic conductivities and porosities for each system from published ranges of values for the lithologies of the respective aquifers . 3 / Modelthe flow to each system during an active recharge period treating each system as a continuum with respect to aquifer parameters and assuming homogeneity and isotropism . 4 / Modelflow during the assumed 100 day zero recharge period of summer . 5 / Consider how these idealized models would differunder real world conditions in which flow is controlled by discrete fracture zones of unknowndimensions with hydraulic conductivities which are ordersof magnitude greater than the assumed values .

2 It was found that the systems could be grouped into two types ; one in which flow is largely governed by gradients induced by pumping ( relatively flat topography ) , and one in which flow is largely induced by topographic gradients . Due to the time constraints of this exercise and the major uncertainties of data , the modelling procedure outlined above was carried out for these two cases and the results applied to each of the seven community well systems . Type 1 ( flat topography ) was applied to Scott Point , Cedar Lane , Harbour View , and Maracaibo Estates . Type 2 ( sloping topography ) was applied to Cedars of Tuam , Erskioe Heights , and Mount Belcher . Computer modeling for this study was carried out . with the use of the " Visual Modflow " package of Waterloo Hydrogeologic of Waterloo Ontario . This is an integrated 3 - D modelling platform which utilizes the USGS ' s MODFLOW and MODPATH finite difference codes . Basemaps for the study were provided by Jacqueline Booth and Associates from the TRIM data base . 2.0 LOCAL HYDROGEOLOGY The bedrock geology of northern Saltspring Island is that of Cretaceous age shale , sandstone , and conglomerate . These rocks are folded along northwesterly trending axes which gives rise to the characteristic ridge and valley topographic pattern of the area . At the time of deposition the Cretaceous rocks would have had high intergranular porosities . Subsequent infilling by cementing material has , however , all but eliminated these voids . Folding and faulting at later times has produced the fracture systems which currently exist . Fractures are generally larger and more prevalent in the relatively brittle sandstones and conglomerates than in the more ductile shales . Below the water table the voids of these fractures are water filled and constitute the geometrically complex aquifers of northern Saltspring Island . The geology of the southern part of the island is dominated by Palaeozoic volcanic and intrusive rocks . These have been subjected numerous periods of deformation and in general are . more intensely fractured than the sedimentary rocks to the north . The level of the water table and therefore the quantity of water stored in the subsurface varies with the seasons of the year . A typical annual cycle would see the water table at . or near the topographic surface during the high rainfall months of midwinter . As precipitation decreases and eventually ceases in spring and summer the water table will fall gradually in response to gravity flow , evapotranspiration , and withdrawal from wells . It will l

3 reach its lowest level at tjie end of the summer dry period . With the heavy precipitation of autumn the system will be rapidly recharged to complete the c . Jcle about year end . The amount of fluctuation between the extremes of high and low water table levels and the variation of this parameter from place to place are functions of the geometry and hydraulic conductivities of the fracture system . 3.0 MODELS 3.1 Horizontal Model Figure 1 shows the configuration of head values within the modeled sector of the Scott Point area with 2 wells pumping at 8.33 id / day . Assumed aquifer constants : Hydraulic conductivity K = lxlO ' m / sec Porosity n = . 001 1 A : Steady state head with an infiltration rate ( R ) of 400 mm / yr . IB : Transient head values with an infiltration rate of 0 mm / yr at time = 10 days . 1 C : Transient head values with an infiltration rate of 0 mm / yr at time = 100 days . Note that the areas contributing water to the pumping wells , as defined by areas having head gradients toward the wells , increase as the water table declines with time . These areas reach their maximum sizes at t = 100 days ( end of the dry period ) and will begin to decrease as the aqu , ifer is recharged . Figures 2 , 3 , and 4 show the effects of gravity induced drainage and pumping on the head value ? on a section through well 3 . These show clearly the limited areas which are contributing water directly to the well . Figure 5 shows the 100 day pathlines of flow to a pumping well in a flat aquifer with no recharge ( Cedar Lane , summer ) . Pumping rate Q = 12 nl / day Assumed aquifer constants : K = lxl 0 ' m / sec n = . 001 The pathlines here have an average length of about 10 metres . The same model run with a porosity of . 0001 ( Figure 6 ) shows the 100 day pathlines to be about 50 meteres long . The capture zone for this well during the dry period can be considered as that of a circular area of radius between 10 and 50 metres depending on the porosity of the system . It can be concluded that the capture zones surrounding these wells , given the various simplifying assumptions , are probably not more than 100 metres in diameter .

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4 If one introduces the reality of planar fracture zones the problem becomes indeterminate . A single fracture zone intersected by a well can extend the capture area of that well over a broad area of the fracture surface which in plan would show as an elongate feature . Without any structural data to indicate the expected ' " direction and width of these features one can only assume a * random distribution and an arbitrary length of influence . For this exercise fracture influence will be considered to be radial for a distance of 200 metres . 3.2 Sloping Model In the case of a well pumping from a sloping homogeneous isotropic aquifer , the capture zone is an elongate feature which extends upslope to the nearest subsurface drainage divide ( which in most cases will coincide with a topographic divide ) . The width ( w ) of this feature will be a function of head gradient ( i ) , the pumping rate ( Q ) , the thickness of the flow system ( h ) , and the hydraulic conductivity ( K ) as shown in figure 7 . The head gradient here is assumed to be the slope on surface . Figure 7 When highly conductive fracture zones of unknown dimensions are added to this simple model it is not possible to define a unigue capture zone . It is reasonable to assume however that the zone will be elongated up slope . For this exercise it is assumed that the effects of fracture flow on capture zone geometry can be accommodated by extending a zone ' 1 for 100 metres beyond that predicted by the simple model . ' i

5 4.0 RESULTS 4.1 SCOTT POINT SOURCE OF SUPPLY * : Three wells : Well # Depth ( m ) Rated capacity ( IGPM ) 1 35 9 2 43 2 3 13 3 PUMPING RATES : IGPD Average daily withdrawal : 3500 Maximum daily withdrawal 6000 Dry Summer withdrawal 5500 GEOLOGY : Cretaceous age Nanaimo Group sediments . Aquifercomprised of fracturedsandstones of the Geoffrey Formation LIMITS OF AQUIFER : Narrow peninsula . Seawater constant head boundaries . Approximate dimensions 200 x 1800 metres . ASSUMED VALUES OF AQUIFER PARAMETERS : Hydraulicconductivity K = lxl 0 7 m / sec , Isotropic Porosity n = . 001 MODEL : Horizontal * Data from Watson 1995

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6 4.2 CEDAR LANE SOURCE OF SUPPLY ' : two wells Depth ( m ) 63 37 Hell # 1 2 Rated capacity ( IGPM ) 4.6 7 PUMPING RATES ' : IGPD Average daily withdrawal : 3500 Maximum daily withdrawal : 6000 Dry summer withdrawal : 5000 GEOLOGY : Cretaceous age Nanaimo Group sediments . Aquifer comprises fractured sandstones of the Geoffrey Formation . LIMITS OF AQUIFER : The fractured sandstone is bounded to the north and south by significantly less permeable shales of the Spray and Northumberland Formations . Approximate dimensions 400 x 1000 metres . ASSUMED VALUES OF AQUIFER PARAMETERS : Hydraulic conductivity K = lxl 0 ' ? m / sec , Isotropic . Porosity n = . 001 MODEL : Horizontal ' Data from Watson 1995

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7 4.3 HARBOUR VIEW i SOURCE OFSUPPLY * Well # , P Threewells Depth ( m ) : Rated capacity ( IGPM ) 1 2 3 18 27 122 3.5 3.5 7 + PUMPING RATES ' : IGPD Average daily withdrawal : 1 , 000 to 1 , 500 Maximum daily withdrawal : 4 , 000 Dry summer withdrawal : 2 , 800 GEOLOGY : Cretaceous age Nanaimo Group sediments . Aquifer comprises fractured sandstones of the De Courcy Formation . LIMITS OF AQUIFER : A relativelyimpermeable shale unit forms the boundary to the northeast ; the southwest limit is a constant head seawater boundary . Approximate dimensions 200 x 1000 metres . ASSUMED VALUES OF AQUIFER PARAMETERS : Hydraulic conductivity K = lxlO - m / sec , Isotropic . Porosity n = . 001 MODEL : Horizontal ' Data from Watson 1995

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8 4.4 MARACAIBO ESTATES SOURCE OF SUPPLY Well # : Five wells Depth ( m ) Rated capacity ( IGPM ) 77 - 5 7 40 - 50 79 - 6 7 8 89 - 1 7 12 89 - 2 7 25 93 - 2 7 3 to 9 PUMPING RATES * : IGPD Average daily withdrawal : 6 , 000 Maximum daily withdrawal : 16 , 000 Dry stimmer withdrawal : 10 , 000 GEOLOGY : Cretaceous age Nanaimo Group sediments . Aquifer comprises fractured sandstones of the . Geoffrey Formation . LIMITS OF AQUIFER : Narrow peninsula . Seawater constant head boundaries . Approximate dimensions 400 x 4000 metres . ASSUMED VALUES OF AQUIFER PARAMETERS : Hydraulic conductivity K = lxlO ' 1 m / sec , Isotropic . Porosity n = . 001 MODEL : Horizontal ' Data from Watson 1995 * . ' I f ) i

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I 9 ) 4.5 CEDARS OF TUAM | SOURCE OF SUPPLY * : one well Well # Depth ( m ) Rated capacity 1 24 800 TGPD ( dry season ) PUMPING RATES * : IGPD Average daily withdrawal : 1 , 400 Maximum daily withdrawal : 2 , 000 Dry summer withdrawal : 1 , 500 GEOLOGY : Lower Devonian Myra Formation . The aquifer comprises fractured silicic tuffs and breccias LIMITS OF AQUIFER : A broad , but as yet undifined area of the east side Mt . Tuam . Approximate dimensions 3000 x 4000 metres . ASSUMED VALUES OF AQUIFER PARAMETERS : Hydraulic conductivity K = lxl 0 ' f m / sec , Isotropic . Porosity n = . 001 / MODEL : Sloping * Data from Watson 1995 of

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J O 4.6 ERSKINE HEIGHTS SOURCE OF SUPPLY Well # 1 2 : Two wells Depth ( m ) Rated capacity ( JGPM ) 5.5 , 5.5 1 PUMPING RATES ' : IGPD Average daily withdrawal : 1 , 800 Maximum daily withdrawal : 3 , 500 Dry summer withdrawal : 2 , 800 GEOLOGY : Cretaceous age Nanaimo Group sediments . Aquifer comprises fractured shale and siltstone of the Haslam Formation and fractured sandstone and conglomerate of the Extension - Protection Formation . LIMITS OF AQUIFER : A broad ill defined area on the west side of Mt . Erskine . Approximate dimensions 800 x 1500 metres . ASSUMED VALUES OF AQUIFER PARAMETERS : Hydraulic conductivity K = 1 x 10 ' ' m / sec , Isotropic . Porosity n = . 001 MODEL : Sloping . Data from Watson 1995

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11 4.7 MT . BELCHER SOURCE OF SUPPLY * : Four pumping wells , three standby wells Well # Depth ( m ) Rated capacity ( IGPM ) Depths and capacities unknown . PUMPING RATES : IGPD Average daily withdrawal : 4 , 000 Maximum daily withdrawal : 8 , 000 Dry summer withdrawal : 5 , 700 GEOLOGY : Cretaceous age Nanaimo Group sediments . Aquifer comprises fractured conglomerates and sandstones of the Extension - Protection Formation . LIMITS OF AQUIFER : A broad area on the northeast flank of Mt . Belcher . Approximate dimensions 800 x 2000 metres . ASSUMED VALUES OF AQUIFER PARAMETERS : Hydraulic conductivity K = lxl 0 ' T m / sec , Isotropic . Porosity n = . 001 MODEL : Sloping ' Data from Watson 1995 I

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12 5.0 CONCLUSIONS The results of this study show capture zones for the respective community water supply systems which are conservative estimates . More precise definitions of these areas would require the inclusion of pump test data and most importantly the data from programs of well system and aquifer monitoring which would include the metering of pumping rates and the periodic measuring of water table elevations . These would encompass both the wells of the respective systems and any other wells within the capture zones . 6.0 RECOMMENDATIONS 5.1 Restrict the level of any contaminant producing activities within the capture zones . Septic systems may be significant sources of contaminants . 5.2 The requirements of any major proposed development within a capture zone should include a study sufficient to determine the possible effects of the development on the local aquifer . Robert Potter ' P . Eng January 26 , 1996 REFERENCES Anderson , M . Pand Woessner , W . W . 199 ? . Applied Groundwater Modeling . Academic Press . Domenico , P . A . , and Schwartz , F . W . 1990 . Physical and Chemical Hydrogeology . Wiley . Driscol 1 , G . P . 1986 . Groundwater and Wells . Johnson Filtration Systems Inc . Golder and Associates Ltd . Fraser River Action Plan . Groundwater Quality Protection Practices . Report for Environment Canada . Watson , R . 1995 . Saltspring Island Water Supply . Unpublished report for the Saltspring Island Official Community Plan Review . »

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