Protocol for the Derivation of Environmental and Human ... - CCME

Appendix D
2.3.3 Hydraulic Gradient
The hydraulic gradient is a dimensionless quantity describing the steepness of the water potential
gradient. In unconfined aquifers it is roughly equivalent to the gradient of the water table. The hydraulic
gradient is assumed to vary between 0.5 and 5% at most contaminated sites. In the absence of a
systematic data review the modal gradient was assumed equal to the mean of the bounding estimates.
2.3.4 Recharge
Methodology
Recharge is rarely measured directly but can be calculated by a water balance approach (Hillel 1971):
S = P - ET - R - U [10]
where
S = storage in soil
P = precipitation
ET = evapotranspiration
R = recharge
U = surface runoff
Over long periods of time (e.g., years) S = 0 and
R = P - ET - U [11]
Meteorological monitoring stations throughout Canada make direct measurements of precipitation and
these have been compiled into thirty-year means (Environment Canada 1982), which provide an
excellent source of data for water balance purposes. Actual evapotranspiration estimates computed
from class A pans and local-to-regional water balances are also available (Fisheries and Environment
Canada 1978). Unfortunately, runoff calculated from gauging rivers includes both recharge and surface
runoff (in this case runoff is defined for the entire drainage basin and the storage term includes
subsurface drainage). Such runoff estimates are also affected by heavy precipitation in mountainous
regions that do not represent contaminated sites, which typically occur at lower elevations.
Because surface runoff in cultivated areas is an erosion hazard, it is normally managed against and
represents a small proportion of the total water balance (Hillel 1971). Therefore, problems caused here
by lack of surface runoff measurements can be circumvented by computing recharge and surface runoff
(R + O) estimates from precipitation and evapotransportation and allowing for a maximum surface
runoff:
R + U = P - ET [12]
Surface runoff is not likely to represent more than a quarter of the total amount of recharge and surface
runoff at a majority of populated locations in Canada and may be close to zero in some locations (C.
Maulé, pers. comm.). Factors affecting the relative proportions of drainage and surface runoff include
vegetal cover, surface soil condition, slope, intensity of precipitation events, and extent of ice lensing
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