Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 93

(K) multiplied by the hydraulic gradient (step 2). Groundwater, however, does not migrate
in open space; it travels through aquifer materials impeding its velocity. To account for
this, the effective porosity of the saturated geologic strata (specific yield) is required to
accurately estimate the velocity of groundwater flow (step 3).

Step 1 Step 2 Step 3

dH
dL

dH
dL

K
n

dH
dL

Av k A v K v (3.6)

= × × → = × → = ×

e

To illustrate the difference in the velocity of groundwater flow through two different mate-
rials, aquifers composed of sand and clay with the same hydraulic gradients are compared:

Sand aquifer example:
K = 0.1 cm/s
dH/dL = 1 cm/1000 cm
n e = 0.22
v

K
n

dH
dL

= ×

e

2 2 5

−

v = 0 1 . cm s / × 1 0 22 / . × 1 1000 / = . cm s /

Clayey aquifer example:
K = 0.000001 cm/s
dH/dL = 1 cm/1000 cm
n e = 0.20
v

K
n

dH
dL

= ×

e

5 0 9

−

v = 0 000001 . cm s / × 1 0 20 / . × 1 1 000 / , = . cm s /

Given the same hydraulic gradient, the velocity of groundwater flow in the sand aquifer
is approximately 10,000 times greater than the velocity of groundwater flow in the clay
aquifer.
The residence time of water as groundwater can range from days to thousands of years
and is a function of several variables. The dominant influence is depth, manifested by the
strong correlation between increasing groundwater depth and residence time. Other fac-
tors influencing residence time include

• The composition of the aquifer material
• Effective porosity
• Hydraulic gradient
• Distance to point of discharge

The span of residence times for groundwater is depicted in Figure 3.33, and the residence
times for water in other environments are listed in Table 3.4.