Phosphorus release from the stream bed sediment in the management area of waterboard “Hoogheemraadschap De Stichtse Rijnlanden”
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Several waters in the management area of waterboard “Hoogheemraadschap De Stichtse Rijnlanden” should meet the requirements for the Water Framework Directive but do not fulfil due to high phosphorus concentrations in the water column. These phosphorus concentrations were expected to be (partly) caused by high phosphorus release rates from the stream bed sediment to the water column, especially in non-urban ditches. Hence, phosphorus release rates were quantified in 57 non-urban ditches, divided over 11 sub-areas. This quantification was based on prior observed correlations of phosphorus release rates with interstitial phosphorus, the interstitial iron:phosphorus ratio and the sediment iron:sulphur ratio. Also relations of phosphorus release with soil type, land use type, seepage/infiltration rate, nature management and chemical and non-chemical characteristics of the ditches were studied for more insight on phosphorus release (and mechanism behind it) and to define rules of thumbs about phosphorus release. These rules were based on easy determinable ditch characteristics (relative to interstitial phosphorus, interstitial iron:phosphorus and sediment iron:sulphur) and aimed to easily indicate ditches where phosphorus release is probably high or low. Mean phosphorus release rates in the sub-areas vary between 0.0 and 2.0 mg P/m2/day and the high release rates in some sub-areas might indeed explain high phosphorus concentrations in the water column. Phosphorus release largely differs per soil type; the release is highest in peat areas and lowest in (sabulous) clay areas. Phosphorus release also seems to differ per land use and be lower by fruit cultivation than by crops/grasslands. However, this was not statistically verified. No effect of infiltration is found. Results, however, revealed that iron-rich seepage leads to lower release rates. Also nature management (e.g. decline of fertilizer use) can lead to lower phosphorus release rates. Phosphorus release correlates with variables that indicate organic decomposition, which emphasizes the important role of organic decomposition for phosphorus release. Furthermore, most locations with highly organic stream bed sediment also had low sediment phosphorus binding capacity, partly due to low iron concentrations. The “Rules of thumbs” form a tool that can easily indicate the possibility of phosphorus release but not the height of phosphorus release. At locations with a possibility of phosphorus release, determined with this tool, water managers could decide to measure interstitial phosphorus concentrations to quantify the height of phosphorus release.