Environmental Engineering Reference
In-Depth Information
BOX 1.2
MASS BALANCE
To see just how useful the tool of mass
balance can be, suppose we are trying to
evaluate whether a lake ecosystem is taking
up or releasing phosphorus. We could try
to measure all the exchanges between parts
of the ecosystem (e.g., the uptake of phos-
phorus by phytoplankton and rooted
plants; the consumption and excretion of
phosphorus by the animals that eat phyto-
plankton and plants; the release of phos-
phorus during the decay of phytoplankton,
plants, and animals; and dozens of other
exchanges), then simply sum up all of
these measurements. It would take an enor-
mous amount of work to measure all the
exchanges, and our final answer would
be fraught with large uncertainties. Alterna-
tively, we could define a lake ecosystem
that was bounded by the lakeshore, the
overlying air, and the bedrock deep beneath
the lake sediments. Using mass balance, we
note that the amount of phosphorus being
retained by the lake ecosystem is simply
the amount of phosphorus going into the
lake minus the amount that is leaving
the lake. Now we just have to measure the
exchanges across the ecosystem boundary
(stream water and ground water going into
and out of the lake; rain, snow, and parti-
cles falling on the lake; and any animals
entering and leaving the lake; hard
enough!) to calculate whether the lake is
taking up or releasing phosphorus. In the
case of Mirror Lake, New Hampshire
(
Figure 1.2
), almost 40% of incoming phos-
phorus is retained by the lake.
FIGURE 1.2
Average phosphorus
inputs and outputs in kilograms/year to
Mirror Lake, NH. Total average inputs
Precipitation
3.7
5
6.7
kilograms/year; total average outputs
5
4.1
kilograms/year.
2.7
kilograms/year or 39.7% retention of phos-
phorus in the lake. (Data from
Winter and
Likens 2009.)
Inputs
outputs
5
Tributary inlet
1.4
Outflow
1.7
Ground water in
0.3
Ground water out
2.4
