Rain and Water Quality
Pure natural water is formed mainly by evaporation from seas and
lakes. As warm water vapour rises it meets cooler air and condenses into
water droplets (just like hot water vapour rising from a bath and
condensing into droplets on the cool bathroom walls). The water droplets
begin to dissolve a whole range of substances in the atmosphere, such as
gases, airborne dust particles and salt from sea spray. When this water
eventually falls to earth as rain it acquires other substances from rock
weathering and by drainage through fields and woods. Unfortunately,
man's misuse of the waterways to dispose of industrial wastes and sewage
also allows other, often undesirable, substances to affect the
composition of the water.
Many atmospheric gases dissolve in water to some extent and carbon
dioxide is one of the most important. It dissolves readily to form
carbonic acid, giving a weakly acidic solution. This chemical reaction
has important consequences, not only for the eventual chemical content
of water, but also for other chemical and biological reactions that
occur in the aquatic environment. The naturally acidic nature of rain
allows it to bring other, less reactive, substances into solution. (The
pH of naturally formed rainwater is about 5.6 but, as we shall see,
local atmospheric and landscape variations can alter it significantly
before it gets anywhere near our fish ponds.) The affinity between
carbon dioxide and water has many far-reaching effects on water quality,
playing a major role in plant and animal respiration and pH buffering.
Sea spray and local dust particles can have a major influence on the
composition of rain. Coastal areas may have quite high levels of
dissolved sea salt, with substantially higher levels of sodium and
chloride ions than inland areas. Likewise, dusty land surfaces can
affect rain. For instance, rain from chalky areas near the coast often
have higher than normal levels of calcium caused by the chalky soils in
these areas. Indeed, during long dry spells the amount of calcium is
often sufficient to neutralise the naturally acidic rain, leading,
unusually, to slightly alkaline rain!
As we are all aware, atmospheric pollution can have a major effect on
the composition of rainwater, typically caused by heavy metals such as
lead, zinc or chromium from industrial processes, although such effects
are more likely to be local, not widespread. Fossil-fuel burning is a
more global cause of atmospheric pollution, the two most important gases
being sulphur dioxide (mainly from coal burning) and nitrous oxides from
vehicle exhausts and oil-burning power stations. These gases can react
with rain to form strongly acidic solutions, giving acid rain. Some
parts of the world, including the UK have recorded pH values as low as
2.1, due to acid rain. Some fish keepers outside the UK have started to
cover their ponds to protect them from acid rain.
Rain picks up significant amounts of nitrogen, sulphur and sodium in
the atmosphere. At this stage it is likely to be very soft. So we see
that rain - which we like to think of as pure water - is already a
complex chemical mixture before it hits the earth (or our fish ponds).
Without going into details at this stage, let me advise that you do not
collect soft acidic rain for fish pond water changes.
Most of the inorganic substances in tapwater come from rock
weathering. As the nature of the local rock varies, so does the
content of the local water. The degree and rate of rock weathering
depends on several factors. There are broadly speaking two types of rock
of interest -igneous and sedimentary. (I have not included metamorphic
rock as it weathers in much the same way as igneous rock.)
Igneous rocks have
been formed by volcanic action and are common in mountainous regions.
They are extremely hard and dissolve slowly, helped by the slight
acidity of naturally occurring rain. Water draining from igneous rock
will contain most of the common metals, such as magnesium, calcium,
sodium and potassium, but usually only in small amounts. The degree or
amount of rock weathering that occurs has two important effects on the
character of the drainage water.
Water hardness is determined mainly by the amount of dissolved
magnesium and calcium ions there are. Soft water is described as such
because of the small amounts of dissolved "water hardness"
forming ions. This has important consequences for the fish-keeper. When
these ions are dissolved they tend to neutralise the acidity of rain. If
only small amounts of rock are dissolved (as with igneous rocks), the
drainage water tends to still be acidic -although the local water board
may well adjust this at the water purification plant.
Sedimentary rocks, on the other hand, are built from a jumble of
rocks and some organic debris. Chalk or limestone are common examples.
These rocks usually dissolve fairly easily so drainage waters contain
high concentrations of the major ions, such as calcium and magnesium.
Again, the slight acidity of rain is important in this process. Most of
these types of rock have an abundance of calcium carbonate which, being
alkaline, raises the pH of the water. The water draining from such rocks
is usually neutral or alkaline. The high level of dissolved ions thus
makes the water 'hard'.
The action of rain on natural rock and the subsequent weathering
plays a major role in determining the overall content or chemistry of
the water going into our fish ponds. Rock weathering is an important
factor in determining both the pH (acidity /alkalinity) and 'hardness'
of water. Both of these parameters are important to fish health.
Water dissolves many more substances as it drains from land to river
or reservoir, the number and type of substances depending on the land
usage. In an agricultural area there will be run-off from fields. Often,
water draining from such areas is high in nitrates and phosphates from
fertilisers and may contain man-made pesticides and
herbicides. Intensive stock-rearing units may yield 'slurry' which
penetrates the streams and groundwaters. Water treatment plants often
release treated sewage into streams and rivers. A typical effluent will
contain high levels of ammonia, nitrate and phosphate. Stream flora
usually oxidise most of the ammonia, but significant amounts of nitrate
and phosphate will remain. Industry still releases many wastes into
fresh waters, the nature and amount of which depend on the industry.
Unbelievably, we are still pumping toxic substances such as cyanide,
oil, heavy metals and formaldehyde into our waters.