Environmental Engineering Reference
In-Depth Information
and on the residence time of the water because some
reactions are very slow. Groundwater in areas with
thick layers of mineral-poor sand often shows a very
typical evolution in water composition (Fig. 3.4). Both
rainwater and groundwater of upper layers are gen-
erally very poor in dissolved solutes. When going deeper
this water reaches layers where soluble minerals have
not been washed out and these go into solution and
increase the mineral load. Quantitatively the most
important minerals are several Ca
2+
compounds, espe-
cially CaCO
3
and CaSO
4
, but other minerals can also
be found. If groundwater flows from different depths
to the surface alongside each other this can result in
a steep hydrochemical gradient at the surface.
Transport of matter by water flows
Apart from water, rivers do also transport large
amounts of solutes, sediments and sometimes biomass.
The latter is deposited on floodplains during flooding
periods and this causes natural fertilization in other-
wise nutrient-poor landscapes. As long ago as several
millennia
BC
the agriculture in Egypt was based on
regular flooding by the river Nile and this could sus-
tain a large human population in an environment which
was otherwise very hostile. Other ancient cultures
also knew the value of water. Nabateans, Persians and
Carthaginians harnessed ephemeral rivers and used
these for water harvesting.
Both surface and groundwater contain also dissolved
substances such as nutrients (soluble compounds of
N, P and K) and other ions. The most common ones
are called major ions (Hem 1959) and they constitute
up to 99% of all solutes in water. Which ions belong
to this category differs somewhat from region to
region but on a worldwide scale the most common
cations are Na
+
, K
+
, Mg
2+
and Ca
2+
whereas HCO
−
,
Cl
−
and SO
4
2−
are the most important anions. The
exact ion content depends on the source of the water
and is sometimes used as a fingerprint to trace its
origins (Stuyfzand 1989, Komor 1994). In the case of
groundwater it depends on the chemical composition
of the soil layers that the water has flown through
Transport of matter by human activities
Apart from sometimes quite significant human impact
on natural flows, huge amounts of matter are also
transported on purpose by humans. Certainly the
most important objective for doing so is agriculture.
At present, in quantitative terms the most important
flows are transport of water for irrigation and of fer-
tilizer to increase crop production in less fertile areas.
Water transport is practised mainly in warmer climates
and consists of a redistribution of water from areas
with sufficient precipitation, for example mountains,
to sites where there is not sufficient water for crop
Sandy ridge
Sandy ridge
Fig. 3.4
Changes in groundwater
composition along its way through
the subsoil. Mineral-poor rainwater
infiltrates the subsoil and takes
up solutes along the way. Arrows
indicate direction of water flow,
darkness increases with mineral
content of the groundwater.
water flow pattern
mineral-poor
boulder clay
clay
mineral-rich
