Assessment of Internal Phosphorus Release and Treatment with Iron Filings in five RPBCWD Ponds
Stormwater ponds are common stormwater runoff quantity and quality control measures in suburban and urban areas. Solids and associated pollutants such as phosphorus in the inflow runoff typically settle to the bottom of the pond, and the settled phosphorus is expected to be permanently buried in the pond. However, there are recent indications that ponds may not provide the expected phosphorus treatment, especially for dissolved phosphorus. Export of dissolved phosphorus in the pond outflows and high total phosphorus concentrations in the pond water suggest that many ponds are performing below their potential in terms of phosphorus retention, and may be a source of phosphorus to receiving water bodies.
Studies on phosphorus release from pond sediments and monitoring of runoff inflows and outflows at ponds have shown that internal phosphorus loading is contributing to the phosphorus loads in those ponds. Significant internal phosphorus release from pond sediments affects not only the pond water quality but also the downstream lakes and streams that receive the pond outflows, thereby increasing risks of harmful algal bloom occurrence in all water bodies.
Pond 849_W in Minnetonka receives iron filings treatment over ice to allow the iron particles to deposit to the pond sediments after ice melt.
The primary purpose of this project was to assess internal phosphorus release in five stormwater ponds in the RPBCWD and the treatment of sediments to reduce internal phosphorus release in three of those ponds. The project consisted of two phases - (i) a pretreatment study to evaluate the sediment phosphorus release potential through a laboratory column study and concurrent field monitoring, and (ii) a post-treatment study consisting of iron filings treatment to chemically inactivate the sediment phosphorus in select ponds and impact evaluation of treatment on the phosphorus water quality and pond sediments. For the pretreatment study, a column study with pond sediment cores was performed, along with in situ water quality monitoring at the five ponds for one field season (2019). Based on the results of the first-year study, iron filings application was implemented in three out of the five ponds and all ponds were monitored for two more field seasons (2020 and 2021). The results from the three-year study were used to interpret the risks of internal phosphorus loading in the ponds studied and to evaluate the effects of iron filings treatment on internal phosphorus loading and pond phosphorus concentration. Additionally, the data collected were combined with information from our other pond research projects to develop recommendations on pond maintenance measures to reduce phosphorus loading in ponds and improve their water quality.
Excerpt modified from Introduction and Purpose.