Rainwater: Find out what's really in it

The composition of the rainwater is determined by the source of water vapour and the ionic composition acquired or lost by this water on its journey through the atmosphere. Over the oceans, rainwater appears similar to strongly diluted seawater with a Chloride composition of between 10 - 15 mg/L. Given the predominance of oceanic water vapour in the atmosphere, this composition will basically be that of the rainwater that we encounter on our golf courses, even if they are thousands of kilometres away from the sea.

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The sea salt content of rainwater comes from marine aerosols that are formed by gas bubbles bursting at the sea surface, Monahan, 1986; Blanchard and Syzdek, 1988. The evaporation  from seawater droplets produces aerosols of different sizes, with particle sizes larger than 10 µm being very similar to seawater. The importance of the contribution of seawater to the composition of rainwater can be seen through the concentration of chlorides.

The relative composition of seawater and rainwater can be compared by means of the fractionation factor; thus we have:

F Na = (Cl/Na)rain/(Cl/Na)sea water

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Rainwater is the most important source of groundwater, but before rainwater becomes the well water we use in some of our facilities, different processes occur in the ground that affect its ultimate composition, such as dust particles and dissolving gases. In addition, there is evapotranspiration which concentrates salts, mineralisation processes and our lawns which absorb certain nutrients by temporarily storing them in their biomass.

The movement of air masses and clouds collects continental dust and gases of natural or industrial origin, which modifies the composition of rainwater.

Soil dust, which has calcium carbonate particles, is the source of most of the calcium in rainfall, along with fossil hydrocarbon combustion. Much of the sodium in rainfall, in turn, comes from dust in arid climates Nativ and Issar, 1983.

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Differences between seawater and rainwater analyses are shown below:

Concentration (mmol/L)       Seawater Rainwater

Na 485 13

K 10.6 5

Mg 55.1 5

Ca 10.7 16

NH4 2 x 10_6 6

Cl 566 11

SO4 29.3 21

NO3                                             5 x 10_6 5

HCO3 2.4 21