R.A. Chavez | G.O. Brown LRGV Stormwater– South Padre Island May 18, 2016 The Impact of Variable Hydraulic Conductivity on Bioretention Cell Performance and the Implications for Design Innovations and Practices PARTNERS FOR A BETTER QUALITY OF LIFE

10 full-scale bioretention cells were constructed in Oklahoma to demonstrate P reduction efficiency. PROJECT

3% to 5% of area. Sized for runoff: —½” in pool —½” in filter 1’ topsoil. Bottom drain to atmosphere. Filter media a blend of sand and 5% fly ash. Sand plugs on 25% of surface for infiltration. GENERAL DESIGN

SAND PLUGS MINIMIZE STANDING WATER Designed to only pond water for 24 hr. Addition of sand plugs on surface compensate for lower conductivity of top soil. 25% of surface layer are sand plugs with a specification that none touch. Proved to be easy to construct and effective.

MaterialK d (ml/g) Sand2 Fly Ash2200 Sand & 5% fly ash 300 FLY ASH REDUCES P IN EFFLUENT AND EXPONENTIALLY REDUCES HYDRAULIC CONDUCTIVITY

MIXING FLY ASH ON SITE PROVED DIFFICULT

WIDE DISTRIBUTION IN FLY ASH

QUESTIONS (after the fact) How do sand plugs interact with the filter media? How does the variability in fly ash content impact the flow and phosphorous transport through the filter? How does mixing effort impact performance? Degree and scale of variation? How does the construction design impact P transport? How does variability of fly ash impact the expected BRC life span? How can we use this information to improve cell design and construction practices? What does this mean in terms of long term planning?

6 plug model FLOW MODEL Bioretention Cell modeled in COMSOL Multiphysics, Earth Science Module, with saturated conditions. Finite element model, 7.5 x 7.5 x 1.5 m, with over 200,000 elements. 75,088 randomly varied hydraulic conductivities. 3 configurations modeled. 20 realizations for each configuration for each design option = 180 simulations.

6 plug model K distribution 3 CONFIGURATIONS, 3 OPTIONS Only filter material. 6 and 14 sand plugs. Measured and double variation in fly ash. 1 and 27 liter scale of variation.

Mean (cm/hr) Standard Deviation MinimumMedianMaximum DEFINING VARIABILITY IN K Transform X to K for 162 field samples, where X = fly ash content (%) Zhang (2006)

DEFINE K DISTRIBUTION K distribution may be fit with Johnson Transform

RESULTS OF FLOW STUDY Variability in fly ash content creates complex flow through filter, but no significant preferential flow. Sand plugs do create some flow concentration, but not dominant. Number of plugs not significant, provided area is sufficient for desired infiltration. Variation or uniformity of fly ash mixing not too critical

TRANSPORT MODEL Same geometry, scenarios, and hydraulic conductivity files as in the previous section on flow 2 configurations modeled – Filter Only and 6 Sand Plug 20 realizations for each configuration for each design option = 120 simulations Sorption modeled as a linear isotherm Distribution coefficient, K d, calculated to correspond with random K values Assumed 1.0 mg/L phosphorus influent into cell 8 m of water per nominal year

SIMULATION

RESULTS P concentration in soil solution after 20 and 144 nominal years Concentration of P in soil solution higher under sand plugs

Estimating Expected Lifespan Assumed mean concentration of P in solution at the bottom plane of the model to be effluent concentration Mean effluent concentrations calculated from model results at approximately 5 year increments from nominal years Time to Exceed mg/L Oklahoma Scenic Rivers Criteria (equivalent years) Effluent Concentration after 144 Equivalent Years

COMPARISON OF EXPECTED LIFESPAN CALCULATIONS Mean Effluent P Concentration Exceeds Expected Lifespan, yr OriginalIncreased VarianceIncreased Scale Filter Only 6 Sand Plug Filter Only 6 Sand Plug Filter Only 6 Sand Plug mg/L mg/L mg/L132>144133>144136>144

Evaluating performance through phosphorus removal efficiencies

P REMOVAL EFFICIENCY OVER TIME FILTER ONLY CONFIGURATION

P REMOVAL EFFICIENCY OVER TIME 6 SAND PLUG CONFIGURATION

CONCLUSIONS years before C mean exceeds Oklahoma criteria for scenic rivers for phosphorus in cell effluent Top soil layer increases design life of cell by years More than 144 years of some P removal Concentration P in soil solution higher under sand plugs The three distributions/ mixing efforts show similar sorption patterns/ removal efficiencies for each configuration

RELATED PUBLICATIONS AND REFERENCES Chavez, R.A., G.O. Brown, R.R.Coffman, and D.E. Storm Design, Construction and Lessons Learned from Oklahoma Bioretention Cell Demonstration Project. Applied Engineering in Agriculture. 31(1): Chavez, R.A., G.O. Brown, and D.E. Storm. July Impact of Variable Hydraulic Conductivity on Bioretention Cell Performance and Implications for Construction Standards. Journal of Hydraulic Engineering. 139(7): Zhang, W Improvement of Phosphorus and Heavy Metal Retention in Stormwater Treatment. MS thesis. Stillwater, Oklahoma: Oklahoma State University. Department of Biosystems and Agricultural Engineering.

Rebecca Chavez For more information, visit our website and click on our water resources page THANK YOU PARTNERS FOR A BETTER QUALITY OF LIFE Let’s Socialize