they can, depending on the manganese levels contained in bottom sediments
and changes in oxidation-reduction potential near and in the sediments.
Normally manganese is contained in bottom sediments as insoluble particulate
oxides. During warm summer months, a number of things can happen to reduce
the dissolved oxygen (DO) content in the water near these bottom sediments.
A lack of rainfall with inadequate mixing of fresh and stagnant water,
increased algae growth, deterioration of organic matter as the water warms
up, and low wind conditions, can all contribute to depletion of DO levels.
If a reservoir becomes stratified as a function of temperature, the bottom
layer will be very low in DO.
leads to the development of anaerobic conditions in the deeper portions
of the lake bottom sediments. Manganese is converted from oxide forms
that are insoluble through bacterial action to manganese ions (Mn++) which
are very soluble and now leach out of the sediments. A manganese concentration
of just 0.5 mg/L is ten times the drinking water standard and can cause
significant color and staining problems. One of the cheapest methods of
manganese removal, chlorine oxidation, often cannot be used on such water
sources due to the formation of trihalomethanes (THMs) and haloacetic
acid (HAA) from the dissolved organic carbon in the water.
anoxic conditions (i.e. waters where oxygen is absent), increased release
of iron and manganese from bottom sediments may result in elevated concentrations
of these metals in the water. High concentrations of iron and manganese
can impair the use of water, as the metals are precipitated upon re-aeration
during the water treatment process. This is typically where the manganese
problems begin during stratification.
of phosphorus, ammonia, iron and manganese are greatly influenced by the
presence of oxygen in lake water and sediments. Ammonia is a breakdown
product of proteins. When little or no oxygen is present at the sediment-water
interface, concentrations of ammonia can be quite high. Ammonia is toxic
and represents a further threat to aquatic life. Sediments in lakes can
contain a lot of iron, manganese and phosphorus. These can be released
in large quantities from the bottom of the lake when oxygen levels are
Iron and manganese can cause treatment problems in water treatment plants.
Phosphorus released in the water fuels yet more plant growth. Bottom sediments
can provide a significant source of phosphorus to prairie lakes. Phosphorus
bound to the bottom sediments is released to the overlying waters when
the water overlying the bottom sediments in a lake become anoxic. During
the summer, most deep lakes experience a temperature-density gradient
in the lake profile from warmer surface (epilimnion) waters and cooler
bottom (hypolimnion) waters.
The process of establishing a density gradient in a lake, either in winter
or summer, can be beneficial in slowing down manganese is termed stratification.
Phosphorus-rich hypolimnetic water is isolated at the bottom of the lake
if the water is anoxic and a temperature-density gradient exists. However,
when the temperature gradient disappears during the spring and fall, the
entire water column in the lake mixes or 'turns over'. This allows the
phosphorus-rich bottom water to circulate to the lake surface where the
combination of available nutrients and sunlight facilitates algal growth.
An additional source of phosphorus to lake water is the resuspension of
sediments. Sediments can release phosphorus and manganese when resuspended
into the water column of a lake by wind action.