Factors affecting retention of nutrients and organic matter in stormwater ponds

Stormwater ponds are a common way to handle urban runoff. Different pond designs have been tested for decades to retain as much water as possible. Lately also, retention of nutrients and organic matter has become increasingly important, to reduce the eutrophication of downstream aquatic systems and thereby, e.g. fulfil the European Water Framework Directive. We have examined the load of particulate and dissolved fractions of organic matter, phosphorus, nitrogen and iron in 66 Danish ponds to determine the importance of catchment type (66 ponds) and the retention efficiency of the ponds (39 ponds) dependent on their type, age, size and design. The results showed that discharge from nutrient enriched and industrial areas is the most polluted while urban and developing areas are the least polluted. Wet ponds combined with vegetated sand filters have higher retentions of the particulate fractions (40-80%) compared with traditional wet ponds (10-20%). Generally, optimized retention requires a ratio between pond volume and impermeable catchment area of > 250m(3) red. ha (reduced or impermeable hectare). Young ponds have higher retention than older ones, especially regarding the particulate fractions of organic matter, phosphorus and nitrogen. Here, 40-50% is retained in ponds < 5 years, 0-30% in 5-10-year-old ponds and almost nothing in ponds > 10 years. For the dissolved fractions, the trend is the same, but with lower retentions. Therefore, management and maintenance should be considered for all ponds, to avoid problems of internal loading, filling and resuspension. Copyright (C) 2015 John Wiley & Sons, Ltd.