COWICHAN VALLEY REGIONAL DISTRICT
SOUTH COWICHAN WATER PLAN STUDY
V19830100 : Rev B : 11 February 2009
Page 59
surface area of the aquifers and estimated groundwater recharge rates. These calculations are presented
in Table 35 and a narrative description of the groundwater balance components is provided below.
Recharge from Natural and Art if ic ial Sources
The water budget calculations (Table 35 below) indicate that total annual groundwater inflows from natural
recharge may range between about 25 million m
3
(
lower limit estimate) and 110 million m
3
(
upper limit),
with a best estimate of about 45 million m
3
.
Considerable uncertainty therefore exists in natural recharge
rates and the volume of the groundwater resource. It is expected that some of this uncertainty can be
removed through the proposed groundwater model development.
Artificial recharge may include return flows from irrigation and domestic water use. Agricultural water use
in the South Cowichan is substantial but a portion of this water use (less soil evaporation and plant
uptake) will end up as groundwater recharge. Most sewage treatment in the region consists of ground
disposal fields. A large portion of this consumed water will also eventually end up a groundwater
recharge, particularly in the case of the Mill Bay area, where wastewater management systems are
making increasingly greater use of rapid infiltration basins.
Table 35 Estimated annual inflows from natural recharge to surficial and bedrock aquifers
Aquifer
Surficial
Bedrock
Surface Area (km
2
)
50
150
Best Estimate
300
200
Lower Limit
195
102
Recharge Rate (mm/yr)
Upper Limit
585
533
Total
Best Estimate
15,000,000
30,000,000
45,000,000
Lower Limit
9,750,000
15,300,000
25,050,000
Annual Inflow to Aquifers (m
3
)
Upper Limit
29,250,000
79,950,000
109,200,000
Notes
Surface areas based on Table 5. Bedrock aquifer surface area excludes area where surficial aquifers are present
Recharge rates are based on annual precipitation of 1300 mm for Shawnigan Lake station (Table 1)
Groundwater Uti l izat ion
Estimates of total current water consumption within the Study Area are about 10.7 million m
3
for
residential and other urban consumption, and about 15.4 million m
3
for agricultural uses for a total of about
26
million m
3
.
While precise data are lacking, it is estimated from the information review that about half
(50 %)
of this total water usage originates from a groundwater source. Based on the best estimate value
for total natural groundwater inflows (45 million m
3
),
this would suggest that for the Study Area as a whole
about 30% of annual groundwater inflows may be allocated for water consumption, with the remainder
being available for other uses (e.g., ecological receptors). Taking uncertainty in natural groundwater
inflows into account, this allocation level could be substantially higher or lower. As indicated in the above,
a portion of irrigation and domestic water use is expected to end up as groundwater return flow (artificial
recharge). This is likely not the case for urban water and sewage treatment systems.
