First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 64

Table 2.3 Composition of Fresh Waters (River) and Marine Waters for
Some Important Ions

Composition River Water Salt Water

pH 6-8 8

Ca 2 + 4 × 10 − 5 M 1 × 10 − 2 M

Cl − 2 × 10 − 4 M 6 × 10 − 1 M

HCO − 1 × 10 − 4 M 2 × 10 − 3 M

K + 6 × 10 − 5 M 1 × 10 − 2 M

Mg 2 + 2 × 10 − 4 M 5 × 10 − 2 M

Na + 4 × 10 − 4 M 5 × 10 − 1 M

SO 2 − 1 × 10 − 4 M 3 × 10 − 2 M

Source : K. A. Hunter, J. P. Kim, and M. R. Reid, “Factors influencing the
inorganic speciation of trace metal cations in freshwaters,” Marine Freshwater
Research , 50, 367-372, 1999; R. R. Schwarzenbach, P. M. Gschwend, and D. M.
Imboden, Environmental Organic Chemistry , Wiley-Interscience, New York, 1993.

as benzene or acetone, will require fire-extinguishing substances other than water.
For substances heavier than water, such as carbon disulfide, water may be a good
choice. Thus, a good green design properly segregates and labels materials based
in part on their densities.
Another important comparison in Table 2.2 is that of pure water and seawater.
The density difference between these two water types is important for marine and
estuarine ecosystems. Salt water contains a significantly greater mass of ions than
that in fresh water (see Table 2.3). The denser saline water can wedge beneath
fresh waters and pollute surface waters and groundwater (see Fig. 2.7). This
phenomenon, known as saltwater intrusion , can significantly alter an ecosystem's
structure and function and threaten freshwater organisms. It can also pose a huge
challenge to coastal communities that depend on aquifers for their water supply.
Part of the problem and the solution to the problem can be found in dealing with
the density differentials between fresh and saline waters. Thus, fluid density is a
constraint in green design.

Figure 2.7 Saltwater intrusion
into a freshwater system. The
denser salt water is submerged
under the lighter freshwater
system. The same phenomenon
can occur in coastal aquifers.

Direction off low
of fresh water

Saltwater intrusion

Tidal
river

Estuary Marine

system