Groundwater and Base Flow BMPs BMPs for Nitrogen Removal in Marion County

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1 Groundwater and Base Flow BMPs BMPs for Nitrogen Removal in Marion County September 9, 2016 Evan Shane Williams, Ph.D., P.E. Marion County Office of the County Engineer

2 Marion County is home to three First Magnitude Springs. Springs in Marion County Two, Rainbow and Silver, are impaired due to nitrate concentration and have BMAPs in effect. Nitrogen is the pollutant of primary concern.

3 Marion County is characterized by internally draining (closed) basins. Typical Karst Terrain Runoff flows to the lowest point locally. At these low lying areas runoff percolates into the aquifer. Stormwater BMPs are built in or around these low lying areas. Wet detention ponds are not feasible in most cases. Retention is used.

4 Typical Stormwater Management Most common stormwater BMP: dry retention. Over 2,400 County maintained dry retention ponds.

5 Dry Retention Provides storage for flood control, effective groundwater recharge, and filtration of certain pollutants But also usually rapid movement of nitrate from root zone, and usually located on aerated soil with potential for nitrification.

6 The Nitrogen Cycle

7 Dry Retention and Springs Protection Dry retention ponds are a necessary stormwater management BMP in Marion County and provide much needed groundwater recharge. Marion County wants to do its part to reduce Nitrogen entering groundwater. Should the County be considering changing the design of dry retention ponds when Nitrogen is the pollutant of concern? What would that change be?

8 Nitrogen Transport and Transformation Study Performed by UCF Stormwater Academy: Funding from Marion County, SWFWMD, SJRWMD and FDEP Two Marion County DRAs studied: Hunters Trace DRA: high infiltration rate, sandier soils, water table >5 ft. South Oak DRA: lower infiltration rate, more clay, water table at/near surface. What soil conditions are needed to promote denitrification? Can these conditions be recreated with biosorption activated media (BAM)?

9 Nitrogen Transport and Transformation Study Two retention ponds instrumented and monitored by UCF. South Oak Subdivision: Clayey soils with low infiltration rate. Water at or above surface. Hunters Trace Subdivision: Sandy soils with high infiltration rate. Water table > 5 feet below surface.

10 Key Findings South Oak exhibited Nitrogen removal (denitrification) while Hunters Trace did not. Soil conditions that a retention pond is constructed in matter. The best sites for retention may provide adequate Nitrogen removal when this is the pollutant of concern.

11 Nitrogen Transport and Transformation Study (Part 2) Hunters Trace retention pond was altered by placing a BAM in a portion of the pond bottom. BAM was a 12 mixture of sand, expanded clay, and tire crumb with a native top soil on top. There are specific design requirements for each component. BAM used was developed by UCF and now sold as a proprietary product in/hr (0.5 ft/day) limiting infiltration rate.

12 Nitrogen Transport and Transformation Study (Part 2)

13 Nitrogen Transport and Transformation Study (Part 2) UCF performance monitoring at Hunters Trace found Nitrogen removal efficiency increased to 50% after installation of the BAM. Multiple lines of evidence indicated primary removal was due to denitrification and possibly dissimilatory nitrate reduction. Later Marion County monitoring at a newly completed retention pond (SW 85 th St & SW 40 th Ave Stormwater Retrofit Project) used 24 of the same BAM and found 70% removal efficiency. More on monitoring results later this afternoon.

14 Key Findings It is possible to improve Nitrogen removal efficiency in dry retention through the use of a BAM. In areas where Nitrogen is the pollutant of concern in groundwater, similar designs would help demonstrate reduced Nitrogen load.

15 SW 85 th St and SW 40 th Ave

16 SW 85 th St and SW 40 th Ave New construction with allowance for future connections. 24 of BAM in primary storage. Smaller cell for flood control (never gets runoff) and emergency overflow weir.

17 Villages of Rainbow Springs

18 Villages of Rainbow Springs Retrofit of an existing retention pond. 24 of BAM in central infiltration area. Lined pre-treatment cells to (hopefully) encourage nitrification.

19 Lake Weir Heights New construction for water quality treatment. 24 of BAM on entire pond bottom. Spillways to lake for conveyance of flood flows

20 Water quality project in existing retention ponds with consideration for flood control storage. Silver Springs Shores Unit of BAM on entire pond bottom in one pond but two cell design in the larger detention pond to keep flood stage within pond.

21 Water quality project in existing retention ponds with consideration for flood control storage. Rainbow Springs 5 th Re-Plat 24 of BAM. Both retention ponds use a two cell design but one pond includes pretreatment in swales.

22 General BAM Design Considerations Flood control storage and recovery. High water table depth and hydraulic conductivity. Suitability of on-site material for BAM mix (if mixing onsite). Consistency of mix when done on-site. Filter sand layer percent fines. Sod versus seed and mulch.

23 Other BMPs We Have Used Dry retention enhanced with BAM constitute the majority of projects Marion County has/will construct. Wet retention can be a viable option in the right situations. Rainbow Springs Country Club Estates Conventional stormwater management BMPs can also be useful in the right situation. Historical discharge to a sinkhole: SE-SW 31 st Street Stormwater Retrofit Discharge to a spring run: Silver Springs/River Stormwater Retrofit

24 Rainbow Springs Country Club Estates Existing dry retention pond. Water table at the pond bottom. Pond excavated to create a permanent pool with recovery through side slopes. Wetland vegetation planted.

25 Silver Springs/River Stormwater Retrofit Project 60 Monster Pipe outfall to tributary of Silver River. Pump station designed to pump runoff back to retention pond. First section of pond has developed wetland characteristics. Second section dry retention.

26 SE-SW 31 st Street Stormwater Retrofit Project Historical FDOT Discharge to a sinkhole. Stormwater wetland designed to provide treatment. Fore bay-emergent marsh design. Required extensive EPMD Liner.

27 Questions?