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

Appendix D
Finally, estimates of the Darcy velocity are usually obtained by considering the saturated hydraulic
conductivity and gradient:
q = K sat × i [8]
where K sat = saturated hydraulic conductivity, m yr -1
i = hydraulic gradient (dimensionless)
A working formula for estimating expected steady state dilution for an organic contaminant in
groundwater is therefore:
DF = (B × K × i/R × L) + 1 [9]
The basis for the general estimation of dilution effects in groundwater observed at the down-gradient
boundary of a contaminated site is formed in equation 9. Appropriate ranges for effective mixing depth,
hydraulic conductivity and gradient, recharge, and site length are developed below.
2.3 Parameter Ranges
2.3.1 Effective Mixing Depth
On the lower end of the range, the effective mixing depth (B) is restricted by the practical consideration
that a generic unconfined aquifer targeted for protection at the human drinking water guideline must have
sufficient yield to be exploitable as a potable source. Taken alongside well design requirements, it is
assumed that B must equal or exceed 1 m.
An upper limit for B will be set by the balance between rates of vertical dispersion and lateral transport
over the length of the contaminated site. Effective mixing depth would be expected to be maximized by
high recharge, low groundwater velocity, high permeability and large site lengths. However, these
conditions do not necessarily co-occur — high recharge and permeability usually are associated with
high velocity. A further practical constraint on the observed concentration of a contaminant in water
drawn from a potable well is the effect of the slotted or perforated zone on "depth averaging" of well
water. Slotted or perforated zones of potable wells can be expected to exceed the thickness of the
contaminated zone in many instances. In such cases the well will tend to function as a mixing cell over
the depth of the slotted or perforated zone.
These considerations suggest that an effective mixing depth between 1 and 5 m is reasonable. Because
high yield aquifers (which minimize requirements for slotted zone thickness) are sought as potable
sources a modal effective thickness is nominated downrange at 2 m.
2.3.2 Hydraulic Conductivity of Aquifer
Common Canadian aquifers have textures ranging from silty sands (fine) to intermediate gravels (coarse)
(Brown 1967). Freeze and Cherry (1979) give typical hydraulic conductivity values for silty sands to
intermediate gravels in the range of 10 -6 to 10 -2 m sec -1 . A majority of productive aquifers are sands to
fine gravels (Brown 1967). Such aquifers would have a hydraulic conductivity (K sat ) in the region of 10 -
4 m sec -1 .
147