Phosphorus retention in a mature constructed wetland under base flow and storm flow conditions

Abstract

The main objective was to monitor a mature constructed wetland, Leikvollbekken, with respect to phosphorus retention, both over long term and during short storm events. The hypothesis was that the wetland would have a positive retention over time, but that short periods with intense precipitation would cause wash-out of accumulated phosphorus due to high water flow. To evaluate this, weekly flow proportional composite samples were analysed for phosphorus, iron and suspended solids, as well as turbidity measurements. Two storm events, Storm A and Storm B, were monitored with time dependent composite samples and analysed for TP, PO43- and TSS. Flow in and out of the wetland was monitored with a 15-minute interval during the whole thesis period.

Weekly measurements showed that Leikvollbekken retained TP and pTP (11.9 kg and 15.4 kg), while washing out sTP and PO43- (-3.5 kg and -3.0 kg). No clear connection between average weekly flow and weekly retention efficiency was observed. The monitoring showed a great fluctuation in measured turbidity, TSS, phosphorus and iron concentrations. A distinct connection between flow and phosphorus concentrations was observed during the storm events. Storm A washed out phosphorus and solids (-5.7 kg TP, -17.3 kg PO43-, 1.2 kg TSS), thus supporting the theory that heavy rain would produce great wash-out situations. Storm B retained phosphorus and solids (70.6 g TP, 9.9 g PO43-, 183.7 kg TSS), showing that not all storms cause washout situations.

In general, empirical data supporting the theory for wash-out situations of phosphors during high flow, and data supporting the assumed dependency of flow and concentrations, were obtained.