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Objective 3: Phosphorus Release from Stormwater Ponds

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Objective 3: Phosphorus Release from Stormwater Ponds 1 Stormwater Research Summary Banner 2 Objective 3: Phosphorus Release from Stormwater Ponds 3 Authors: Vinicius Taguchi, Tyler Olsen, Ben Janke, Heinz G. Stefan, Jacques Finlay, John S. 4 Gulliver 5 6 Background 7 There is growing concern that aging stormwater ponds may become net sources of phosphorus (P) to 8 receiving waters during high flow events. Decades of research in lentic ecosystems have demonstrated 9 potential for sustained release of P previously deposited in sediments (i.e. internal loading) in lakes and 10 wetlands. This body of work demonstrates that P is a major contributor to eutrophication that is difficult 11 to effectively manage (Gächter and Wehrli 1998, Søndergaard et al. 2007, Song and Burgin 2017). 12 Stormwater ponds often have high external P loading, and other characteristics that may increase the 13 likelihood of internal loading, such as low availability of metals that immobilize P. However, such ponds 14 have received comparatively little research attention. Objective 3 research examined controls on P 15 cycling in stormwater ponds toward predictive understanding of P retention behavior. Our 16 investigations focused on the examination of pond P using a data synthesis combined with detailed 17 characterization of sedimentary P release in five ponds (Obj. 3a), and annual P mass balance 18 measurements in three ponds coupled with analyses of factors associated with P retention or release 19 within these ponds (Obj. 3b). 20 21 Data Synthesis 22 To investigate the prevalence of excess P in stormwater ponds, we examined data previously collected 23 by the Riley Purgatory Bluff Creek Watershed District (RPBCWD) in Minnesota. Between 2010 and 2013, 24 RPBCWD surveyed 98 ponds and observed high total phosphorus (TP) concentrations (i.e. >0.5mg/L) in a 25 number of them (RPBCWD 2014). Surface grab samples were collected from the downstream third of 26 each pond between July and September. Some ponds were added during the study, so that ponds were 27 samples between 3 to 18 times (median of 5 samples), except for two that had single samples. We 28 compared RPCWD pond TP to characteristic stormwater runoff from the Twin Cities Metro Area (TCMA) 29 and the U.S. The expectation was that TP concentration in stormwater ponds would be less than the 30 typical inflow TP concentration due to loss of P from the water column from settling of particulates and 31 assuming other non-runoff sources of P loading (e.g., from local inputs of leaf litter, or grass clippings,) is 32 negligible by comparison in most ponds; however, the presence of internal loading from recycling of 33 sediment P within the ponds could cause elevated pond TP. TCMA stormwater runoff was characterized 34 using stormwater monitoring data (19 sites, 2362 storm events, n=2505 TP samples) from several TCMA 35 watershed management agencies (Janke et al. 2017). These data were fit to a log-normal distribution 36 (Van Buren et al. 1997), and the upper 95% confidence interval of expected values (centered about the 37 mean rather than the median as in the case of a percentile), which represents the maximum value for 38 approximately 97.5% of the distribution, was then calculated to be 0.38 mg/L. Thus, 97.5% of the inflow 39 data are estimated to be below 0.38 mg/L. For comparison, the process was repeated with a national 40 stormwater runoff characterization dataset (104 municipalities, 8602 storm events, n=7407 TP samples) 41 from the US EPA NPDES Stormwater Quality Database (Pitt et al. 2008), yielding a 95% CI of 0.42 mg/L 42 TP. We compared sampling data from RPBCWD against the Twin Cities Metro 95% CI threshold value of 43 0.38 mg/L in Figure 1, which shows box plots for each of the 39 ponds with median TP concentration 44 above the 95% CI and condenses the remaining 59 ponds into a single box plot. 1 45 46 Our analysis shows that 39% of the 98 ponds sampled had median TP concentrations greater than the 47 95% CI (Figure 2). For each dataset, measurements below reported limits of detection were 48 approximated as equal to half the detection limit (Kayhanian et al. 2002). We hypothesized that these -3 49 ponds have high internal P loading from the sediments, released as orthophosphorus (ortho-P or PO4 ), 50 and are therefore more likely to experience high P concentrations. We address this hypothesis through 51 the laboratory column studies and field measurements described below. 52 53 Figure 1. Box plot of RPBCWD TP grab sample data. The 59 ponds with a median below 0.38 mg/L (95% CI for TCMA 54 runoff data compiled in Janke et al. 2017) are combined into the far-right box plot. 55 56 57 Figure 2. Distribution of total phosphorus concentrations from the RPBCWD pond (grab sample) data presented in 58 Figure 1, with median epilimnion grab sample values from the five intensively studied ponds (A-E) from this study 59 also shown. 60 61 Sediment Core Methods 2 62 To address our hypothesis that P release from the sediments drives high P concentrations in stormwater 63 ponds, we sought to identify and better understand the mechanisms controlling P release from 64 stormwater pond sediments. We collected intact sediment cores and overlying water, using core tubes 65 with an approximately 7 cm outer diameter and approximately 1.5 m in height, from each of 5 66 stormwater ponds (Figure 3, Table 1). These ponds covered a range of characteristics (Table 1), including 67 water column P concentrations (Fig. 2). Six cores were collected from pond A, and five from ponds B-E. 68 69 Table 1. Characteristics of the five studied ponds. All ponds were included in the core study; ponds C, D, and E were 70 studied in the monitoring task. TIA = Total Impervious Area; n/a = not available. Grab Sample TP (n = 10 - 20 71 samples; hypolimnion and water column samples excluded) and influent event mean concentration (EMC) of 72 stormwater TP (n = 20 – 22 samples; snowmelt excluded) come from the monitoring task of the study for ponds C, 73 D, and E (see SI). Grab sample values for ponds A and B come from previous studies (Pond A: Noah Czech, City of St. 74 Cloud; Pond B: RPBCWD 2014). Water Drainage Max Median Grab Influent TP Site ID Site Name Area Area Depth Age Land Use TIA Sample TP EMC ha ha m yr % mg/L mg/L Residential/ A St. Cloud 52 0.21 22.1 1.2 18 60 0.57 n/a School B Minnetonka 849_w 0.66 2.8 1.7 >50 Residential 30 2.70 n/a C William Street Pond 0.23 15.4 1.5 >30 Residential 19 0.36 0.38 Parking Lot, D Church Pond 0.04 1.8 0.8 ~20 >75 0.24 0.12 Residential E Alameda Pond 1.05 115.3 1.8 >30 Residential 20 0.22 0.30 75 76 77 78 Figure 3. Stormwater pond field sites. Pond A is located in St. Cloud, MN, Pond B in Minnetonka, MN, and Ponds C, 79 D, and E in Roseville, MN. 80 81 3 82 Each core was incubated at the St. Anthony Falls Laboratory with 30-40 cm of sediment with 83 approximately one meter of overlying pond water in clear PVC tubes (Figure 4). Cores were incubated 84 under oxygenated and anoxic conditions, during which ortho-P and total phosphorus (TP), dissolved 85 oxygen (DO), temperature, and pH were monitored. Initially, the cores were drained and the overlying 86 water column was filtered through a 1.2 μm glass fiber filter to remove suspended particulates. The 87 filtered water was then added back to the respective core with care taken to avoid sediment 88 resuspension. The water columns were subsequently bubbled with air to simulate the mixing and 89 aeration that takes place in spring. Afterward, air bubbling was halted and cores were left unmixed for 90 one month, the first week of which was used to calculate an overall sediment P flux rate from the 91 measured water column P concentrations. This was done by fitting a linear regression through the P 92 concentration time series data and dividing the slope (P mass per time) by sediment surface area of the 93 cores. Discrete sampling events occurred every one to seven days, depending on the observed rate of 94 change, and are identified by points in Figure 5. Samples were taken from the center of the water 95 column during the mixed oxic phase and from both the center of the water column and 8 cm above the 96 sediment surface during the unmixed anoxic phase. DO, temperature, and pH readings were taken from 97 the center of the water column during the mixed oxic phase and in 15-cm increments, beginning 3 cm 98 above the sediment surface during the unmixed anoxic phase. In order to minimize variables and isolate 99 the sediment P flux for measurement, core incubations were not provided with particulate P additions 100 as would occur in-situ via atmospheric deposition and stormwater runoff. At the conclusion of the 101 month-long unmixed monitoring period for each core, a sequential chemical extraction of the sediments 102 was completed to illustrate differences in P speciation and identify how P was bound in the sediments 103 (Engstrom 2010). This information on P speciation and related P sediment binding characteristics in 104 important to understand mechanisms that control P release and internal loading across ponds with 105 varied P load and morphology.
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