Assessment of Internal Phosphorus Release and Treatment with Iron Filings in five RPBCWD Ponds

Abstract

Five ponds, Aquila Pond (in Bloomington), Pond BC-P4.10C (in Chanhassen), Bren Pond (in Eden Prairie), Pond 849_W (in Minnetonka) and Pond 42 (in Shorewood), were evaluated in this two-part study. a) In the first part of the study, the potential anoxic sediment phosphorus release was evaluated using laboratory sediment cores. A moderately-high flux of phosphate was measured under anoxic conditions, which was supported by high sediment oxygen demand and high organic matter content in the sediments. A low oxic flux was observed only for Pond BC-P4.10C and Bren Pond sediments, indicating mobilization of organic P by bacteria. Detailed sediment phosphorus characterization revealed low to moderate concentrations of mobile P (redox-P + labile organic P) mass, which is releasable under low oxygen conditions and by microbacterial degradation under both oxic and anoxic conditions. The relative mobile P mass (as % of the total sediment phosphorus mass) was 53% in Aquila Pond, 43% in Pond BC-P4.10 C, 47% in Bren Pond, 41% in Pond 42, and 63% in Pond 849_W, highlighting the importance of mobile phosphorus in driving internal phosphorus loading during anoxia in the ponds. b) In situ monitoring of surface to bottom DO and temperature profiles in the ponds were indicative of a stratified water column that was anoxic from top to bottom during much of the summer period. The observation of pervasive anoxia was common in Pond BC-P4.10C, Bren Pond, Pond 849_W, and Pond 42 during all three field seasons, as indicated by the relatively high summer anoxic factor (AF) for these ponds. Aquila Pond appeared to partially mix intermittently although bottom DO was still low during certain periods. c) All five pond sites had floating vegetation (duckweed and watermeal) that had a dense surface coverage (nearly 100%) from June to September. We have found strong evidence of duckweed cover influencing the DO dynamics in several ponds and have observed a strong pattern between summer anoxic factor and duckweed cover in our pond research projects. It is possible that the effect of duckweed may be exacerbated in dry years (like 2021) when stormwater inputs to provide direct mixing are less frequent. d) The application of iron filings was utilized to reduce phosphate release from the pond sediments. Ponds BC-P4.10C and 849_W were treated with iron filings in February 2020 and Bren Pond was treated in February 2020. Aquila Pond can be used as a control for the RPBCWD region, where surface water TP was seen to increase greatly from 2019 to 2020, and then stayed about the same in 2021. In Pond BC-P4.10C, the average TP went up after treatment with iron filings in February 2020, but not as substantially as the Aquila Pond. In Bren Pond, the average TP had a slight reduction in all three years. In Pond 849_W, the average TP went up in 2020 but then reduced in 2021. A similar reduction can be seen in comparing average TP for Shoreview Commons Pond (a fourth iron-treated pond located in the Ramsey Washington Metro Watershed District) to the Alameda Pond (located in v Roseville), where Shoreview Commons had a reduced average TP in 2021 after iron filings addition and the Alameda Pond, with no iron filings addition, did not. e) The analysis of the iron-treated sediments from Pond BC-P4.10C, Pond 849_W, and Bren Pond showed an increase in the iron-bound P mass and a concomitant decrease in the mass of labile organic P and loosely-bound P after iron filings application to the sediments, suggesting the partial or full movement of phosphate from the organic P form and loosely- bound P to iron-phosphate minerals in the sediments. The iron-treated sediment cores from Bren Pond exhibited an anoxic phosphate flux that was significantly lower than the phosphate flux from untreated sediments. f) While the column studies confirmed that sediment phosphate flux was controlled after iron addition, the reduction in internal phosphorus loading in the ponds was not directly assessed. The post-treatment water quality data showed reductions in SRP levels (surface and epilimnion) at the three iron-treated ponds but did not conclusively show reductions in TP levels, specifically in ponds BC-P4.10C and 849_W. The interpretation and assessment of treatment effectiveness is complicated by the year-to-year variation in pond water quality driven by rainfall patterns and runoff inputs among other factors, especially in ponds BC- P4.10C and 849_W, which have pretreatment data for only one year before iron filings were applied. Treatment of the ponds will likely require a combination of remediation techniques such as sealing the sediments from phosphate flux, aeration to enhance mixing and watershed-based phosphorus control actions to reduce the inflow of TP. Aeration may work well in Pond 849_W, which has a small amount of inflow or outflow.