Phosphorus and nitrogen in groundwaterThe very different chemistry of DIP and NO 3 - is illustrated by their behaviour in
groundwater. On the limestone island of Bermuda there is little surface water
because rainfall drains rapidly through the permeable rock to form groundwater.
Almost all of the sewage waste on Bermuda is discharged to porous walled pits
that allow effluent to gradually seep into the groundwater. The sewage has a N :
P ratio of about 16 : 1, and yet Bermudian groundwater is characterized by very
low DIP concentrations (average 3.5mmol l-^1 ) and very high NO 3 - concentrations
(average 750mmol l-^1 ). The N : P ratio of 215 : 1 for the groundwater implies
removal of more than 90% of the DIP, by natural precipitation of calcium phos-
phate minerals. This process is now used as a sewage treatment stage for DIP
removal from some waste waters.
High nitrate concentrations are characteristic of groundwater in areas of
intensive agriculture and can compromise its use as a drinking water supply
(Section 5.7). The main potential human health hazard is a condition called
methaemoglobinaemia, where NO 3 - combines with and oxidizes haemoglobin in
the blood, robbing the cells of oxygen. This condition affects some adults with
specific enzyme deficiencies, but also newborn babies, resulting in the name ‘blue-
baby syndrome’. To safeguard drinking water supplies in agricultural areas such
as southeast England, farmers are required to control fertilizer inputs in areas of
groundwater recharge.
SiliconOne other important nutrient, silicon, is used by diatoms (a group of phyto-
plankton) to build their exoskeletons. Diatoms are capable of rapid and prolific
growth in nutrient-rich conditions. In temperate rivers, diatom blooms occur
early in the year. For example, in the River Great Ouse in eastern England, silicon
levels fall in early spring as diatom growth begins and rise again in summer as
diatoms are displaced by other algal groups (Fig. 5.14). Since silicon is derived
entirely from weathering reactions, its naturally low concentrations may be dras-
tically reduced by diatom blooms, such that further diatom growth is limited, par-
ticularly where NO 3 - and DIP have been enriched by human activity. Thus silicon
availability limits species diversity but not total phytoplankton biomass.
The presence of lakes and dams in river catchments has an important effect
on nutrient transport. Increased water residence times and improved light
conditions promote algal (particularly diatom) blooms that are very effective at
removing dissolved silicon (DSi). In Scandinavia, for example, catchments
without lakes have DSi concentrations around 164mmol l-^1 , almost four times
higher than those in catchments where lakes and reservoirs cover more than 10%
of the area (DSi= 46 mmol l-^1 ). A particularly striking example of this effect is seen
in the Danube, the largest river draining to the Black Sea, which was dammed
by the ‘Iron Gates’ on the Yugoslavian/Romanian border in the early 1970s. DSi
concentrations fell three-fold as a result of the damming, and the resulting
decrease in Si : N ratios in water draining to the Black Sea (exacerbated by
The Chemistry of Continental Waters 169