Modelling of PFOS fate and transport

Transcription

1 Modelling of PFOS fate and transport Thomas Franz, Adam Dawe, Lauren McDonald, Franz Environmental Inc. Chris>ne Levicki, Health Canada John Miller, Environment Canada

2 Conceptual model Par>>oning from soil to pore water (to soil gas) Transport of soil leachate through unsaturated zone Mixing of soil leachate into groundwater Transport in aquifer Leachate Groundwater flow PFOS source area (e.g. FFTA) Dissolved phase Drinking Water Surface Water

3 Model capabilities The model was designed to back- calculate soil concentra>on at PFOS source loca>on for a given concentra>on in groundwater or surface water Now also does forward calcula>ons by simula>ng PFOS transport from soil to groundwater to receptor within groundwater to receptor

4 CCME, 2006 SQG = DF1 x DF2 x DF3 x DF4 x DF5 x C MAC New: dilu%on at receptor (e.g. in wellbore or at groundwater - surface water interface) C MAC- DW C MAC- AW 10 m

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9 CCME, 2006 SQG PW = DF1 x DF2 x DF3 x DF4 x DF5 x C MAC C MAC- DW C MAC- AW 10 m

10 CCME, 2006 ~7.3 ~2.4 to ~ SQG PW = DF1 x DF2 x DF3 x DF4 x DF5 x C MAC C MAC- DW C MAC- AW 10 m

11 Sorp%on PFOS- surface electrosta>c PFOS- PFOS electrosta>c repulsion Hydrophobic sorp>on Tang et al., 2010 Cs = Kd Cw Higgins & Luthy, 2006)

12 What affects sorption? Hydrophobic sorp%on Hydrophobic interac>on is the dominant mechanism of PFOS sorp>on to organic carbon, Strong hydrophobic nature of its perfluoroalkyl chain (Higgins and Luthy, 2006). Hydrophobic interac>on can also arise between the hydrophobic chains of different PFOS molecules. K oc and f oc (frac>on of organic carbon) are used to calculate adsorp>on coefficient (K d ) foc increases à Kd increases Higgins & Luthy, 2006)

13 What affects sorption? Hydrophobic sorp%on PFOS sorp>on increases in oil contaminated soil Oil contaminated soil à Kd increases Chen et al., 2009

14 What affects sorption? PFOS- surface electrosta%c interac%on due to ph. PFOS is nega>vely charged under all environmentally relevant ph values. A mineral surface becomes more posi>vely charged (or less nega>vely charged) at lower ph. Enhanced electrosta>c afrac>on force (or reduced electrosta>c repulsion force) results between the nega>vely charged PFOS molecules and the more posi>vely charged mineral surface at lower ph. ph decreases à Kd increases Higgins & Luthy, 2006)

15 What affects sorption? PFOS- PFOS electrosta%c interac%on. Two adjacent PFOS molecules on a surface will repel each other due to their nega>vely charged sulfonate head groups. A strong PFOS- PFOS repulsion tends to prevent these molecules gehng close to each other. Thus, a solu>on with high ionic strength has a tendency to promote PFOS adsorp>on as a result of the suppressed electrosta>c repulsive force.

16 What affects sorption? PFOS- surface electrosta%c interac%on due to ph. PFOS is nega>vely charged under all environmentally relevant ph values. A mineral surface becomes more posi>vely charged (or less nega>vely charged) at lower ph. Enhanced electrosta>c afrac>on force (or reduced electrosta>c repulsion force) results between the nega>vely charged PFOS molecules and the more posi>vely charged mineral surface at lower ph. ph decreases à Kd increases Higgins & Luthy, 2006)

17 What affects sorption? PFOS- surface electrosta%c interac%on due to ionic strength. Electrosta>c interac>on can be significantly weakened at higher ionic strength due to the double layer compression effect. For a posi>vely charged mineral surface, the amount of adsorbed PFOS tends to be reduced due to the weaker electrosta>c afrac>on. However, for a nega>vely charged surface, adsorp>on tends to increase as a result of weaker electrosta>c repulsion. e.g. Ca 2+ decreases à Kd increases (Higgins & Luthy, 2006; Chen et al., 2009; Pan and You, 2010; You et al., 2010)

18 Reviewed partitioning studies Method Selected studies with low dissolved PFOS concentra>ons Went to original papers Eliminated duplica>ons and copies Problem: many studies are for marine sediments (not soil) Results PFOS Koc ranges from 229 to 6310 (Kd from 0.08 to 250 L/kg) Median Koc 1441 L/kg (log Koc = 3.16 L/kg) Programmed spreadsheet to determine median Koc Can add data to this spreadsheet and/or manually override Koc Programmed pseudo- func>on to simulate ph dependency Typical Koc s (L/kg) Benzene = Toluene = 268 B(a)P = Naphthalene = 1837 TCE = 67.7 Acenaphthene = 6123 PFOS = > 6310

19 Authors Year Kd units Koc units Soil type / soil source Enevoldsen & Juhler L/kg 1500 L/kg Jyndevad (Denmark) soil, agricultural topsoil, A horizon, sandy soil Enevoldsen & Juhler L/kg 4048 L/kg Sj. Odde (Denmark) soil, agricultural topsoil, A horizon, clayey soil Ferry et al L/kg 3514 L/kg Minnesota aquifer material from landfill Ferry et al L/kg 229 L/kg same microcosm, but at end of 740 d study 3M L/kg 704 L/kg clay 3M L/kg 374 L/kg clay loam 3M L/kg 1260 L/kg sandy loam 3M L/kg 571 L/kg river sediment Chen et al., L/kg 2659 L/kg marine sediment, S1, from Dalian coastal area, China Chen et al., L/kg 2596 L/kg marine sediment, S2, from Dalian coastal area, China Chen et al., L/kg 3101 L/kg marine sediment, S3, from Dalian coastal area, China Chen et al., L/kg 2660 L/kg marine sediment, S4, from Dalian coastal area, China Chen et al., L/kg 3774 L/kg marine sediment, S5, from Dalian coastal area, China Chen et al L/kg 1349 L/kg soil from paddyfield in Panjin, China Higgins and Luthy L/kg 372 L/kg freshwater sediments (rivers and lakes) from USA Ahrens et al L/kg 5012 L/kg sandy river sediment from Kogaigawa, Japan Ahrens et al L/kg 3162 L/kg muddy river sediment from Sakuragawa, Japan Ahrens et al L/kg 2512 L/kg muddy marine sediment from Tokyo Bay Ahrens et al L/kg 6310 L/kg marine sediment cores from Tokyo Bay, Japan Kwadijk et al L/kg 1445 L/kg 19 sediment samples from rivers, lakes, canals in Netherlands Labadie & Chevreuil L/kg 5012 L/kg sediment from Orge River, France (near Paris) Johnson et al L/kg Ofawa sand Johnson et al L/kg L/kg kaolinite Johnson et al L/kg 376 L/kg Lake Michigan sediment Johnson et al L/kg goethite Johnson et al L/kg high iron sand Johnson et al L/kg 610 L/kg clay Johnson et al L/kg 324 L/kg clay loam Johnson et al L/kg 1177 L/kg sandy loam Johnson et al L/kg river sediment

20 Model limitations Equilibrium par>>oning between soil, water, vapour phases Assumes rela>vely low concentra>ons in soil / groundwater PFOS concentra>ons should be less than 10 mg/l Solubility = 500 to 600 mg/l (in freshwater), 12 mg/l (in seawater) Homogeneous geology However, can be different in unsaturated / saturated zones Uniform, unidirec>onal groundwater flow Constant source concentra>on

21 Primary data requirements Coordinates: x, y, z Geometry of PFOS source Frac>on of organic carbon (foc) Unsat zone thickness Average linear groundwater velocity Aquifer thickness Dispersivity

22 Model comparison of predicted vs measured ground water concentrations Four sites evaluated for transport modeling and empirical Kd calcula>ons: All at civilian or military airports 3 FFTAs 1 disposal site Koc, foc- unsat, m C=? Koc, foc- sat, b, v, n, alfa Dissolved phase

23 Disposal Site Soil and Ground Water Concentrations = Ground Water Concentra>on, units of ug/l = Soil Concentra>on, units of mg/kg Oil Disposal Pit

24 FFTA 1 Shallow Ground Water Contours Fire Training Area

25 FFTA 1 Soil and Ground Water Concentrations <0.02 = Ground Water Concentra>on, units of ug/l = Soil Concentra>on, units of mg/kg Fire Training Area

26 Model result with literature Koc (Koc = 1445 L/kg; Kd,sat = 2.17 L/kg) 5 Predicted vs Measured PFOS Concentra>ons in Ground Water Concentra>on (mg/l) Predicted Measured Distance from source (m)

27 Model calibration Match between observed and modelled PFOS concentra>ons was not very good using literature Koc (Kd) values Approach to improve model results: 1) Derived site- specific Kd values based on co- located soil and groundwater samples from the site(s) - > re- run model 2) Adjust Kd to obtain best fit - > brute force

28 Empirical Koc s Calculated empirical Kd by comparing co- located soil and groundwater sample concentra>ons Calculated Koc from Kd = Koc foc foc based on site- specific data Empirical Koc values range from 85 to 7619 (L/kg) Literature Koc values range from 229 to 6310 (L/Kg)

29 Model result with median empirical Kd (Kd = Koc foc = 1.6 L/kg ; Koc = 424 L/kg) 5 Predicted vs Measured PFOS Concentra>ons in Ground Water Concentra>on (mg/l) Predicted Measured Distance from source (m)

30 Brute Force best fit Excel SOLVER used to adjust Kd - > obtain best fit between observed and modelled PFOS concentra>ons

31 Model result with best fit Kd (Kd sat = 0.1 Kd unsat) (Kd,sat = L/kg; Koc = 342 L/kg) 5 Predicted vs Measured PFOS Concentra>ons in Ground Water Concentra>on (mg/l) Predicted Measured Distance from source (m)

32 Adjust other model parameters Koc alone is not the problem Modified transverse (horizontal) dispersivity to achieve befer match Likely unrealis>c dispersivity value

33 Adjust transverse dispersivity (transverse = longitudinal dispersivity) (Kd,sat = 1.2 L/kg) 5 Predicted vs Measured PFOS Concentra>ons in Ground Water Concentra>on (mg/l) Predicted Measured Distance from source (m)

34 Conclusions Simple mathema>cal model for PFOS fate & transport has been developed. Runs in Excel, easy to use, has a series of limita>ons. Limita>ons are similar to other models used for guideline development and risk assessment Use with cau>on for site specific applica>ons Primary afenua>on process for PFOS is par>>oning (sorp>on). Greatest uncertainty for modelling of PFOS stems from par>>oning coefficient Review of Koc s Empirical Koc s range from 85 to 7619 L/kg Literature Koc s range from 229 to 6310 L/kg Model results match field data befer for Koc s at the low end of the range (i.e. less sorp>on / more mobile)

35 Recommenda>ons For assessment of PFOS fate & transport, we need to collect foc data in unsaturated soil zone foc data in aquifer (groundwater transport) zone Collect data pairs of co- located soil and groundwater directly below source to determine site- specific Kd ph

36 Acknowledgments Sanya Petrovic, Health Canada Brian Asher, Health Canada Luigi Lorusso, Health Canada Jo- Ann Aldridge, Environment Canada Philippa Coureton, Environment Canada

37 Disposal Site Soil and Ground Water Concentrations = Ground Water Concentra>on, units of ug/l = Soil Concentra>on, units of mg/kg Oil Disposal Pit

38 Cold lake best solution, solve for unsat and sat Kd (0 <= unsat kd <= sat Kd Predicted vs Measured PFOS Concentra>ons in Ground Water Concentra>on (mg/l) Predicted Measured Distance from source (m)