The Chemistry of Continental Waters 165the existence of water. Under very oxidizing
conditions (Eh 0.6 to 1.2 V) water is broken
into oxygen and hydrogen ions and under
highly reducing conditions (Eh 0.0 to –0.6 V)
water is reduced to hydrogen. Eh-pH
diagrams are used to visualize the effects of
changing acidity and/or redox conditions. The
diagram for iron minerals is typical (Fig. 1).
The lines represent conditions under which
species on either side are present in equal
concentrations. The exact position of the
lines varies depending on the activities of the
various species.
From the diagram it is clear that the
mineral haematite (Fe 2 O 3 ) is usually the stable
iron species under oxidizing conditions with
pH above 4. Soluble Fe^3 +is only present
under very acidic conditions because of its
tendency to form insoluble hydroxides
(Section 5.2). This tendency is only overcome
under very acid conditions when hydroxide
ion (OH-) concentrations are low. Fe^2 +is less
prone to form insoluble hydroxides because
of its small z/rvalue (Section 5.2). Fe^2 +is thus
soluble at higher pH, but can only persist
under low Eh conditions, which prevent its
oxidation to Fe^3 +. The small stability field for
the common iron sulphide, pyrite (FeS 2 ),
shows that this mineral only forms under
reducing conditions, usually between pH 6
and 8. Iron carbonate, (siderite FeCO 3 ) is
typically stable at either slightly higher or
slightly lower Eh than pyrite.1.00–1.0–2.0–0.5 0 0.5 1.0PLog discharge (m^3 s–1)Log concentration (mmol l–1)NO– 3Na+HCO 3 –Fig. 5.10Relationship between dissolved ion concentration and river discharge in the River
Yare (Norfolk, UK). Direct discharges of phosphorus from sewage and products of weathering
(HCO 3 - and Na+) decline in concentration (are diluted) as discharge increases. By contrast,
heavy rainfall leaches NO 3 - from soil, causing NO 3 - concentrations to rise as discharge
increases. After Edwards (1973).5.5)), DIP can be returned to the water column in association with iron(III)
reduction to iron(II). This process of DIP release from sediments can confound
efforts to control eutrophication in lakes that are based on reducing the direct
DIP inputs.
Increased riverine NO 3 - concentrations due to human activity (see below) mean
that DIP is now the main limiting nutrient for plant growth in many freshwaters.
The consequent relationship between DIP and chlorophyll levels (a measure of
algal biomass) (Fig. 5.11) makes the management of phosphorus inputs to rivers
and lakes very important. In some areas DIP inputs are consequently stringently