Movement of Light, Heat, and Chemicals in Water - Freshwater Ecology: Concepts and Environmental Applications - page 33

EXAMPLE 3.1.
Calculating Diffusion Flux into a Stream from a Groundwater Source

Nitrate is a common contaminant of drinking water that leads to serious
health problems when present in excessively high concentrations. Ground-
water is present below a feedlot and contains 100 mg liter 1 NO 3 -N
(i.e., the number of grams per liter of nitrogen in the form of nitrate) at a
distance of 10 m from a stream, where the concentration is 10 mg liter 1
NO 3 -N and the diffusion coefficient for NO 3 , D, has been measured as
1.85 10 3 cm 2 s 1 . Assume that D does not change from the feedlot to

the stream. What is the daily flux of nitrate into the stream per square me-
ter of stream bottom?
First, convert the nitrate concentration and distance into units consis-
tent with the diffusion coefficient, so 100 mg liter 1

NO 3 -N

0.1 mg

cm 3 , 10 mg liter 1 NO 3 -N 0.01 mg cm 3 , and 10 m 1000 cm.

Then,

10 3 cm 2 s 1 (0.1-0.01 mg cm 3 )/1000 cm J 1.67

10 7 mg cm 2 s 1 1.83

0.14 g NO 3 -N m 2 d 1

the diffusion coefficient, D ) because small particles are more likely to be
moved by collisions with water molecules. Thermal energy is a property of
the average velocity of dissolved ions and water molecules. Consequently,
molecular diffusion rates are greater as temperature increases (Fig. 3.3).

2.0

1.6

1.2

0.8

0.4

0.0

-5 5 15 25 35

Temperature ( 0 C)

FIGURE 3.3 Effect of temperature on rate of diffusion of chloride.