Criteria for the general water quality classification in Finland

Class

Class interpretation

Variables and their threshold values

I
Excellent

The watercourse is in a natural state, usually oligotrophic, clear or with some humus. Water use is not restricted by any special occurrence of algae.

Highly suitable for all modes of use.

• colour < 50 mg Pt/l
• transparency > 2.5 m
• turbidity < 1.5 FTU
• feacal coliforms or fecal streptococci < 10 CFU/100 ml
• total phosphorus < 12 µg/l
• mean chlorophyll-a in the growing season < 4 µg/l
  (in Baltic Sea < 2 µg/l)

II
Good

The watercourse is in a near-natural state, slightly eutrophic or clearly humic. Locally restricted algal blooms can occur occasionally. Water is still suitable for most modes of use.

• oxygen concentration in epilimnion 80–110%
• no oxygen defiency in the hypolimnion
• colour 50–100 mg Pt/l
  (< 200 in natural humic waters)
• transparency 1–2.5 m
• turbidity > 1.5 FTU
• feacal indicator bacteria
  < 50 CFU/100 ml
• total phosphorus < 30 µg/l
  (in Baltic Sea 12–20 µg/l
• mean chlorophyll-a in the growing season < 10 µg/l
  (in Baltic Sea < 2–4 µg/l)

III
Satisfactory

The watercourse is slightly affected by wastewaters, diffuse loading or other changing activity, or is appreciably eutrophic or humic due to natural causes. Algal blooms can occur repeatedly. Concentrations of harmful substances in water, sediment or biota can be slightly higher than in pristine conditions.

The watercourse is usually satisfactory for most modes of use.

• oxygen concentration in epilimnion 70–120%
• some oxygen defiency may occur in the hypolimnion
• colour < 150 mg Pt/l
• transparency < 1 m
• feacal indicator bacteria
  < 100 CFU/100 m
• total phosphorus < 50 µg/l
  (in Baltic Sea 20–40 µg/l)
• mean chlorophyll-a in the growing season < 20 µg/l
  (in Baltic Sea 4–12 µg/l)

IV
Passable

The watercourse is strongly affected by wastewaters, diffuse loading or some other changing activity. Algal blooms are common and may restrict water use for a long period. Concentrations of harmful substances in water, sediment or biota can be clearly higher than in pristine conditions. In catchments with Littorina Sea* clay deposits, the pH of water can be very low for short periods and die-offs of fish caused by the acidic conditions can sometimes occur.

Water is suitable only for modes of use having few water quality requirements.

• oxygen concentration in epilimnion 40–150%, frequent oxygen deficiency in the hypolimnion
• feacal indicator bacteria
  < 1 000 CFU/100 ml
• total phosphorus 50–100 µg/l
  (In Baltic Sea 40–80 µg/l)
• mean chlorophyll-a in the growing season 20–50 µg/l
  (In Baltic Sea 12–30 µg/l)
• algal blooms common
• concentrations of harmful substances:
  As < 50 µg/l, Hg < 2 µg/l, Cd < 5 µg/l, Cr < 50 µg/l, Pb < 50 µg/l, total cyanide < 50 µg/l
• off-flavours often found in fish

V
Poor

Wastewaters, diffuse loading or other changing activity extensively pollute the watercourse. Algal blooms occur frequently and are often abundant, restricting water use for a long period. Oxygen concentrations are clearly affected by eutrophication. Concentrations of harmful substances in water, sediment or biota can be at levels that cause a clear risk to the use of water or biota. In catchments with Littorina Sea* clay deposits, the pH of water can be very low for long periods and die-offs of fish caused by the acidic conditions occur repeatedly.

Poorly suited to any mode of use.

• major problems of oxygen balance, oxygen saturation in the epilimnion during summer may exceed 150%; on the other hand total oxygen depletion at the surface may occur; at the end of the stratification season the whole hypolimnion may be anaerobic
• feacal indicator bacteria
  >1 000 CFU/100 ml
• total phosphorus > 100 µg/l
  (In Baltic Sea > 80 µg/l)
• mean chlorophyll-a in the growing season > 50 µg/l
  (In Baltic Sea > 30 µg/l)
• one or more of the following exceeds the threshold-limit specific for class IV: As, Hg, Cd, Cr, Pb or total cyanide
• mercury concentration in predatory fish species > 1 mg/kg
• oil film on the water surface often observed