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Mark S. Dortch, PhD, PE, D.WRE

Modeling Water and Sediment Contamination of Lake Pontchartrain following Pump-out of Hurricane Katrina Floodwater. Mark S. Dortch, PhD, PE, D.WRE Retired Research Civil Engineer, Environmental Laboratory, ERDC, USACE Senior Water Resources Engineer, Moffatt & Nichol.

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Mark S. Dortch, PhD, PE, D.WRE

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  1. Modeling Water and Sediment Contamination of Lake Pontchartrain following Pump-out of Hurricane Katrina Floodwater Mark S. Dortch, PhD, PE, D.WRE Retired Research Civil Engineer, Environmental Laboratory, ERDC, USACE Senior Water Resources Engineer, Moffatt & Nichol Dortch, M.S., Zakikhani, M., Kim, S.C., and Steevens, J.A. 2008, in J. Env. Mgt, 87, 429-442.

  2. Background • Interagency Performance Evaluation Task force (IPET) was commissioned by Congress following Hurricane Katrina to evaluate the reasons and consequences of failed protection of New Orleans • IPET Task 9 focused on socio-economic and environmental consequences • A major environmental questions was, what were the environmental impacts to Lake Pontchartrain as a result of pumping out contaminated flood water from New Orleans following Hurricanes Katrina and Rita.

  3. Study Objective As part of IPET Task 9, model the fate and transport of contaminants pumped into Lake Pontchartrain following the actual events to evaluate the environmental impacts as contrasted against conditions that would have occurred without levee failures and overtopping Constraint: study had to be done in a few months Funding: USACE/IWR to USACE/ERDC-EL

  4. Approach • A 3D numerical hydrodynamic (CH3D-WES/Z) and water quality model (CE-QUAL-ICM) were applied • Modeled constituents included: water column and bottom sediment concentrations of: • Arsenic (As) • Lead (Pb) • Benzo(a)pyrene (BaP) • DDE • Fecal Coliform bacteria (FCB), water concentration only • Compared actual versus base • Actual – as occurred with levee failures and overtopping • Base – without levee failures and overtopping • Compared water and sediment concentrations with protective eco and human health criteria

  5. CH3D-WES • 3D, time-varying, baroclinic, free surface Hydro model, sigma and Z-plane versions (used Z-plane version) • Transports salinity and temperature for density coupling and baroclinic forcing (not used in this study due to minor salinity differences resulting from the storm) • Structured, curvilinear, non-orthogonal, boundary-fitted coordinates, finite difference method • Block grid structure for sub-grid resolution and parallelization (not used in this study)

  6. CE-QUAL-ICM • Multi-dimensional, unstructured, finite volume, surface water quality, eutrophication, and contaminant model • Must provide Hydro to ICM • Developed on Chesapeake Bay (circa 1990), but has been applied to many other systems • Over 35 state variables ranging from temperature, salinity, and nutrients to sea grass and lower food chain, plus fate/transport of trace toxic substances • Benthic diagenesis sub-model for predicting bed-water column nutrient and DO fluxes • Domain decomposition with MPI for parallelization (not used for this study)

  7. Lake Pontchartrain Computational Grid • 6,038 surface cells • Max of 6 layers, 1.52 m thick except surface • 21,018 total cells Rigolets Inlet

  8. Pump Stations Included in Model Orleans Metro Orleans East

  9. Pump Hydrographs for Actual Conditions Started 9/11 and stopped Oct 17, 37 days Flows estimated from another IPET task 5-10% of lake volume pumped out (2005)

  10. Pumped Flow for Base Condition • Assumed no levee failures or overtopping • Used measured rainfalls at Slidell (Katrina had 20 cm) and NO A/P (Rita had 5.84 cm) and NO metro & east basin area (2.8E8 m2), yielding 5.7E7 and 1.7E7 m3 (Katrina and Rita, respectively) • Base pumped volume (collected rainfall)≈ 10% Actual pumped volume for Katrina • These volumes can be pumped out in a day or less (0.6 days for Katrina at total pump capacities) • Flow per pump = 0.6 * pump capacity • Duration = 1 day due to model requirement of daily flows

  11. Other Hydro Model Inputs • 90 day simulation starting on Sep 1, 2005 • Winds from NO Int. Airport (no data from 9/1 to 9/7, so linearly ramped up from 0, “spin-up”) • Seaward water level boundary at Rigolets (Waveland, MS gage was blown out) – used sum of: • Astronomical based on NOAA predictions at Waveland, MS • Meteorological based on Norco, Bayou LaBranche, LA gage with 48 hour filter and 24 hour shift forward • Tributary flows were not included

  12. Hydrodynamic Model Calibration At the Norco tide gage Hurricane Rita Katrina hit on August 29 Model spin-up

  13. Hydrodynamic Animation Sep 9-13, before and during 2 days of pump-out

  14. Contaminant Model Fate Processes • Equilibrium sorption to suspended solids • Settling of adsorbed particulate contam. • Volatilization of dissolved organic contam. • Surficial benthic sediment mass balance due to settling, resuspension, and burial • FCB die off

  15. Contaminant Model Inputs • Lake total suspended solids, TSS = 19.2 mg/L (based on NTU regression and observed lake-wide median NTU following Katrina) • TSS settling rate = 1 m/d based on Corps IHNC DMMU study • Sediment fraction TOC for water column and bed, foc = 0.02 based on DMMU study • Surficial benthic sediment porosity = 0.9 • Sediment burial rate = 0.026 m/yr based on steady-state solids balance with 0 resuspension • FCB die off rate = 1 per day (conservative value) • Degradation rate of metals and organics = 0.0

  16. Model Inputs (cont.)Kd Input Values for Model, L/kg • Pb: 4,000 • As: 500 • BaP: 0.3 E6 • DDE: 0.6 E6 Values are close to literature (metals) and Kow computed values (organics), but they were adjusted slightly using observed benthic sediment and water column data from NO flood waters to back out Kd

  17. Model Inputs (cont.)Volatilization Rates • BaP: 0.005 m/d • DDE: 0.19 m/d Computed using wind speed of 5 mph, Henry’s constants, and molecular weights

  18. Contaminant Model Loadings (Q*C) • Actual conditions • Based on measurements in NO floodwaters for total concentrations (EPA database of values monitored by EPA, USGS, LSU, LaDEQ) • Computed median and 95UCL of concentrations for NO East and NO Metro • Base conditions • Assumed to be same concentrations as Actual conditions; based on literature – concentrations in Katrina flood waters were typical for storm water

  19. Contaminant Loading Concentrations * MPN/100 ml

  20. Contaminant Model Validation • There were some observations for FCB and lake benthic sediment contamination following Katrina • Computed and observed FCB were same order of magnitude, but observations were spotty • Sediment contamination values not really comparable since there is a long-term sediment memory and model only had a brief loading history compared with many years in prototype • There was qualitative confirmation in that events had very little impact on lake sediment metal concentrations for predicted and observed

  21. Arsenic Animation, Water Surface 90 days starting Sep 1, 2005

  22. Maximum Incremental Arsenic Water Surface Concentrations Max of about 0.014 mg/L in both cases Actual Base

  23. Maximum Incremental Arsenic Sediment Concentrations Max of about 0.05 mg/kg Max of about 0.005 mg/kg Actual Base

  24. Computed maximum incremental water (ug/L) and sediment (mg/kg) concentrations (total) for Lake Pontchartrain for Actual and Baseline conditions and median and 95UCL loading concentrations Note: water concentrations about the same for both conditions, but sediment about 10X for Actual compared to Base *Units for FCB are MPN/100ml

  25. Lake Contaminant Incremental Concentration Conclusions • Water column – increases were about the same or less for actual versus base conditions • Surficial sediment – increases for actual conditions were about 10 times those for base • Increases in sediment for metals were small (<1%) compared with pre-Katrina concentrations • Increase in sediment BaP concentrations were about the same as post-Katrina measurements • Sediment increase in DDE was much lower than other contaminants

  26. Exceedence of Protective Water and Sediment Screening Benchmarks • Water Pb for Actual95 and Base95 • Sediment BaP for Actual and Actual95 • Sediment DDE for Actual and Actual95 • Water FCB for both conditions and both loadings (the norm for NO storm water removal)

  27. Overall Conclusions • Flood water contamination is thought to be due primarily to pre-Katrina-contaminated urban soils (typical urban storm-water), but a much greater water volume was pumped with levee failures, thus greater loadings • Lake water column concentrations about the same with/without levee failures • Sediment concentrations for BaP and DDE under actual conditions with levee failures exceeded eco screening guidelines close to south shore, but both degrade over time • Water Pb and FCB are a concern with/without levee failures

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