Monitoring, sizing and removal efficiency in stormwater ponds

Retention ponds and wetlands are frequently used in stormwater management to remove pollutants, reduce flow peaks and improve scenic views in parks and along roads. This study analyzes the correlation between long-term removal efficiency of pollutants (total suspended solids and heavy metals) and specific pond area (ratio between effective drainage area and surface area). For this purpose, all data on ponds in Sweden that have been monitored were collected and evaluated. The results show that of 27 measured ponds only nine had monitoring programs that were correctly designed to reveal anything about pollutant removal; this is because grab samples had been used to a large extent. There was no possibility to establish a correlation between specific pond area and long-term removal efficiency, even when the pond area was adjusted with regard to the amount of dead zones. This constitutes a "negative" research finding, but an important one since it indicates that other factors ought to be included when retention ponds are to be sized and designed, such as concentration of pollutants and vegetation.