A two-year study evaluated the nitrogen (N) fluxes, processing, and treatment efficiency (TE) of a 14.85 ha stormwater detention area (SDA) receiving drainage from a vegetable farm in subtropical Florida. The TE was 62% and 89% in years 1 and 2, respectively. Seepage N losses are often ignored in estimating stormwater treatment. Approximately 11% and 20% of the incoming N left the SDA through seepage, reducing the TE to 51% and 67% in years 1 and 2, respectively, indicating the importance of subsurface N losses for downstream water quality. Rainfall variability controlled the timing and volume of the inflow drainage and surface water levels inside the SDA. Variable water levels controlled the aerobic and anoxic conditions inside the SDA, thus controlling the N processing and treatment. Coupled nitrification-denitrification, as a result of frequent wetting-drying cycles, was the main N treatment pathway during year 1. Drought conditions in year 2 led to 89% less surface outflow compared to year 1, resulting in water volume retention being the main process for N retention. The N TE could be increased from 68% to 86% if about two-thirds (63%) of aboveground biomass in the SDA area is harvested annually during the dry season. A payment for environmental services (PES) framework, with the state as buyer and the SDA owner as seller of N treatment services, was evaluated with a 20-year net present worth (NPW) of biomass harvesting for enhanced N treatment. The economic analysis included the benefit from composting the harvested biomass for on-farm use. A positive NPW ($835,000) indicated the economic feasibility of the project, predicting an annual benefit of $42,000 year -1 for the 112 ha farm. Scale-up of the PES approach can offer additional N treatment and C sequestration services as well as increased farm productivity.

Abstract:A simple method, modified from White's method, was developed and verified for estimating seepage from two stormwater detention areas (SDAs) for 2 years, using night-time changes in surface water levels. The SDAs were located in warm sub-tropical Florida where the assumption of negligible night-time evaporation for White's method does not hold true. Daily seepage was estimated using the nocturnal water level fluctuations on no flow days during winter when evaporation losses were insignificant. Specific yield, rather than the composite specific yield, provided accurate seepage estimates. The average annual seepage from the two SDAs was 2.03 m/year. At almost 70% of surface flows, seepage is a significant contributor to regional surface and sub-surface flows. Comparison of seepage estimates from the night-time method (NM) and the water balance (WB) method showed that the NM-based estimates were within the range of the estimates from the WB method. At SDA1, the differences between the NM and WB estimates were 1% and 11%, for the 2 years. The discrepancy between the two estimates became higher (27% and 23%) at SDA2. Larger differences at SDA2 were because of higher error in quantifying pumped inflows for the WB method. Successful performance of NM combined with its low resource (single well monitoring) requirements will help quantify seepage from detention areas and other similar features (e.g. ponds, constructed wetlands) in warmer climates. A scale-up for the Caloosahatchee River basin showed that seepage from SDA's accounted for 15% of annual river flows indicating the importance of seepage in evaluating water and chemical balances.

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