Page 237 - Materials Chemistry, Second Edition

10 Life Cycle Impact Assessment 223

In a tripartite division of environmental impact categories into global, regional
and local, eutrophication is considered a local to regional impact. As a consequence
of the above explanations, impact potentials are highly dependent on local condi-
tions, e.g. whether the recipient of the emission will support the requisite conver-
sion of the emission (e.g. mineralisation of organic nitrogenous compounds), or
whether the recipient is limited in nitrogen or phosphorus, while both elements are
always considered potential contributors to eutrophication.
The calculation of characterisation factors for a nutrient enriching substance
consists of an assessment of the number of moles of nitrogen or phosphorus which
can be released into the environment from one mole of the substance emitted. This
can be expressed in the form of two nutrient enrichment equivalents, as kg
N-equivalents and kg P-equivalents. The possible consequences of eutrophication
are often irrespective of whether nitrogen or phosphorus is the causing agent. In
some situations it can therefore be desirable to reduce the complexity of the results
of the environmental assessment by expressing eutrophication as one equivalent, so
that the contributions for nitrogen and phosphorus are aggregated. In this case the
impact potential may also be expressed as an equivalent emission of a reference

substance (e.g. NO 3 as one of the most important nutrient enriching substances).
Aggregation of N and P potentials requires an assumption concerning the magni-
tude of the ratio N/P between these two elements in living organisms. As explained
above a molar ratio of 16 can be used for nitrogen:phosphorus in living material.
One mole of phosphorus (in an area where the availability of phosphorus limits
growth) therefore contributes as much to eutrophication as 16 mol of nitrogen (in
an area where the availability of nitrogen limits growth). The aggregate nutrient
enrichment potential for nitrogenous substances is then calculated as the emission’s
N potential multiplied by the gram/mol molecular weight of the reference substance

(e.g. NO 3 of 62 g/mol). The P potential for phosphorous-containing substances is
multiplied by 16 times the gram/mol molecular weight of the reference substance.
The primary receiving compartment for agricultural emissions is mainly fresh-
water where some of the nitrogen may be removed on the way to the marine
systems by denitrification in rivers and lakes converting the nitrogen into molecular
N 2 which is released to the atmosphere. Loading of freshwater with nitrogen is thus
greater than the quantity conveyed to the marine areas via rivers and streams.
Phosphorous compounds do not undergo this kind of conversion but phosphate
forms insoluble salts with many metals and this may lead to some removal through
accumulation of phosphorus in lake sediments. Phosphorus accumulated in the
sediments of rivers and streams during drier periods may later be washed out into
the marine environment when the water flow increases, e.g. after a thunderstorm.

10.9.3 Emissions and Main Sources

Due to the use of inorganic fertilisers and manure, agriculture is a significant source
of phosphorus and nitrogen emissions in the form of phosphates and nitrates,