Twenty-eighth Report Adapting Institutions to Climate Change Cm ...

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oxygen demand), dilution and temperature. These relationships were recognised at the beginning
of the 20th century by the Royal Commission on Sewage Disposal. Warmer waters brought on by
climate change will have a lower capacity for dissolved oxygen, and biochemical activity will be
greater. This means that more restrictions will have to be placed on the organic material present
in discharges.
Warmer lowland waters, enriched with nitrate and phosphate, are more prone in sunny conditions
to algal blooms, which have potential health and aesthetic implications, and can cause problems
for aquatic ecosystems. Water supply reservoirs are particularly prone to this phenomenon, which
is likely to become a more frequent event in the future. Treatment costs to produce drinking water
may rise as a result.
Another important factor is the presence of nitrogen compounds. Ammonia in untreated sewage
and from farm waste run-off is noxious, particularly to fish, and affects the dissolved oxygen
regime through nitrification. Again, changes to temperature and flow, and the sudden discharge
of untreated sewage during storms, will need even greater attention as climate changes.
Nitrates present in, or produced from, discharges of untreated sewage and farm waste add to that
present in the diffuse run-off from agricultural land and in discharges of treated sewage effluent
from sewage works. Agricultural nitrate arises from nitrification of organic nitrogen in the soil
and from fertiliser application; it will be washed into water bodies by rainfall and some will find
its way into groundwater as well as rivers. The interface between soil and water is a major factor in
water quality control, and climate change will affect this complex interface. Biochemical processes
of denitrification, in which nitrates can be reduced to nitrogen in anoxic environments, will be
just one of the processes affected. The Commission was provided with evidence of the operational
challenges of nitrate in groundwater in North Norfolk.
Reduced aquifer levels and freshwater flows, coupled with rising sea levels, may allow the ingress
of salt water upstream in estuaries, potentially affecting freshwater environments where excess salt
concentrations can affect the usability of water for irrigation and for potable water supplies. Rising
salt concentrations will also impact on the biodiversity of affected freshwater ecosystems.
Sediment can also be a major problem; it may be washed off land during intense rainfall. It can
reduce the clarity of water, blanket organisms that live in the bottom of water bodies, smother
plants, congest the gills of fish and smother the gravel-based spawning grounds of fish such as
salmon and trout. The Commission saw efforts in the River Glaven catchment sensitive farming
management project to reduce the sediment burden (Box 3D).
If agricultural practices change as a result of climate change, the types and amounts of fertilisers
or pesticides which find their way into water through run-off could also change, with implications
for river chemistry and biology.
Drought
2.58 The climate projections suggest drier summers, and so droughts may become more prevalent. The
implications for water supply for the natural environment and for domestic, agricultural, industrial
and recreational use will be significant.
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Chapter 2