ALTERNATIVE DRY RETENTION POND DESIGN FOR ENHANCED NITROGEN REMOVAL Evan Shane Williams, Ph.D., P.E.
DRY RETENTION IN MARION COUNTY Over 2,400 drainage retention areas (DRAs) maintained by the County. DRAs provide important flood control functions and recharge the aquifer but also are a potential pathway for pollutants (Nitrogen) to enter aquifer. The aquifer is source of our drinking water and spring flow.
NITROGEN AND GROUNDWATER Rainbow and Silver Springs listed as impaired due to Nitrogen (Nitrate concentration). Urban sources are not the largest source but do dry retention ponds provide adequate treatment of nitrogen? Can we do better?
NITROGEN TRANSPORT AND TRANSFORMATION STUDY Performed by UCF Stormwater Academy with funding from Marion County, SWFWMD, SJRWMD and FDEP What soil conditions are needed to promote denitrification? Two Marion County retention ponds with differing soil characteristics studied. Can these conditions be recreated with biosorption activated media (BAM)?
Soil conditions promoting denitrification: Lower hydraulic conductivity Moisture retention in pore space leading to anoxic conditions and denitrification UCF STUDY RESULTS Recreating these conditions using BAM: Hunters Trace DRA modified with 12 of tire crumb, clay and sand by volume in portion of DRA bottom Limiting 0.25 in/hr infiltration rate 50% TN Reduction in soil pore water
NEXT STEPS: COUNTY PILOT PROJECTS SW 85 th Str. & SW 40 th Ave: New retention area Rural residential & agriculture land use Construction complete 2012 Monitoring complete through 9 of 10 events Village of Rainbow Springs: Retrofit of existing retention area Medium density residential land use Construction complete 2014 (modifications planned for 2015) Both projects intended to build upon field study and determine optimal use of BAM.
SW 85 TH ST & SW 40 TH AVE SITE PLAN Design called for 24 inches of Bold and Gold versus 12 inches at Hunters Trace. Increased depth intended to provide greater residence time in treatment zone. Typical section: 4 of coarse sand and 4 of top soil/sod over Bold and Gold.
SW 85 TH ST: BEFORE AND AFTER
SAMPLING POINTS
SAMPLING EQUIPMENT
BUCKET TYPE PAN LYSIMETER
MONITORING GOALS: During rain events is there an difference in TN concentration between water infiltrating from the surface and water infiltrating out of BAM? Does the vertical depth of the BAM improve performance?
Influent concentration lower than sub-surface? No water collected - Treated as no sample. 5 outliers out of 15 data points! Composite sample covered only a small portion of flow.
There is a statistically significant difference (p < 0.05) between influent and sub-surface concentrations. 2.90 mg/l 1.36 mg/l 0.73 mg/l
Filtration Present Filtration Not Present Filtration did contribute to the initial decrease in TN concentration. Further reduction in TN must either be dilution or removal. Decline in TN without increase in Nitrate.
Comparing changes in chloride and Nitrogen concentration will indicate whether the changes are dilution or other process. Chloride is conservative in the sub-surface. Chloride increase versus nitrogen Decline in Nitrogen versus chloride
CONCLUSIONS If the change in mean chloride concentration was due to volumetric changes in water, then Dissolved N should show similar change. Removal of Nitrogen is occurring in the BAM. The additional depth of BAM was beneficial. Treatment efficiency: 50% at 12 versus 73% at 24
PRELIMINARY CONCLUSIONS Smaller events add complexity to the assessment of performance. July 2013 No data from bottom of BAM October 2014 No Dissolved N or chloride data from bottom of BAM. Both events appear to have been less than 1 of rainfall. The BAM changes the soil characteristics by retaining some (or all) infiltrated water compared to native soils. Treatment efficiency estimates may underestimate the impact of the BAM
VILLAGE OF RAINBOW SPRINGS SITE PLAN Mineralization of TKN appears to be an important step. This would occur in aerobic soils in close contact with the atmosphere. Village of Rainbow Springs incorporates lined pre-treatment areas with underdrains and coarse sand that are expected to be aerobic.
VILLAGE OF RAINBOW SPRINGS: BEFORE AND AFTER
VILLAGE OF RAINBOW SPRINGS MONITORING GOALS Runoff entering at least one of the cells will be sampled. Runoff exiting underdrain will be sampled. Two lysimeter points at top and bottom of BAM
SOME FINAL THOUGHTS The results of each pilot project will help guide future implementations. For now potential projects will use the SW 85 th design.