Abstract
Recent studies have shown that stormwater control measures (SCMs) are less effective at retaining phosphorus (P) than nitrogen. We compared P retention between two urban/suburban SCMs and their adjacent free-flowing stream reaches at the Baltimore Long-Term Ecological Study (LTER) site, and examined changes in P retention in SCMs across flow conditions. Results show that, when compared with free-flowing stream reaches, the SCMs had significantly lower dissolved oxygen (%DO) and higher P concentrations, as well as lower mean areal retention rates and retention efficiencies of particulate P (PP). In all the SCMs, concentrations of total dissolved phosphorus (TDP) consistently exhibited inverse correlations with %DO that was lower during summer base flows. Particulate phosphorus (PP) concentrations peaked during spring high flow period in both streams and in-line pond/SCMs, but they were also higher during summer base flows in suburban/urban SCMs. Meanwhile, PP areal retention rates and retention efficiencies of the SCMs changed from positive (indicating retention) during high flows to negative (indicating release) during low flows, while such changes across flow were not observed in free-flowing stream reaches. We attribute the changing roles of SCMs from a PP sink to a PP source to changes in SCM hydrologic mass balances, physical sedimentation and biogeochemical mobilization across flows. This study demonstrates that in suburban/urban SCMs, P retained during high flow events can be released during low flows. Cultivation of macrophytes and/or frequent sediment dredging may provide potential solutions to retaining both P and nitrogen in urban SCMs.
Highlights
Excessive phosphorus (P) inputs from urban and agricultural land use are a major cause of freshwater and coastal eutrophication [1,2,3]
Water temperatures were slightly higher in stream than in the stormwater control measures (SCMs) at the suburban site Spring Branch (SPBR) but higher in oxbow SCMs than in the stream at the urban site GFGR (Table 2), but the differences were not significant
%DO in the SCMs was significantly lower than that in the free-flowing restored streams at both watersheds (One-Way ANOVA, F = 19.1 and 54.9, p < 0.05), suburban SPBR (81% ± 3% vs. 106% ± 2%) and urban
Summary
Excessive phosphorus (P) inputs from urban and agricultural land use are a major cause of freshwater and coastal eutrophication [1,2,3]. Reducing nonpoint sources of P-rich sediments from urban and suburban watersheds via best management practices (BMPs) including various stormwater control measures (SCMs), and stream restoration designs [4,5,6,7,8], are among the top choices for restoration strategies in the Chesapeake Bay and other coastal watersheds [9,10,11]. Ignoring land costs, constructed wetlands and ponds were the least expensive to construct and maintain among the six stormwater. The retained SRP may be remobilized with particulate P (PP) during storm events via erosive streamflow, or desorbed from particles under anaerobic conditions when Fe/Mn-oxides are reduced to soluble Fe2+
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