Measuring and Modeling Organochlorine Pesticide Response to

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Measuring and Modeling Organochlorine Pesticide Response to Activated Carbon Amendment in Tidal Sediment Mesocosms Jay M Thompson, Ching-Hong Hsieh, Thomas P Hoelen, Donald P. Weston, and Richard G Luthy Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05669 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 5, 2016

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Environmental Science & Technology

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Measuring and Modeling Organochlorine Pesticide Response to Activated Carbon

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Amendment in Tidal Sediment Mesocosms

3 4 5

JAY M. THOMPSON†, CHING-HONG HSIEH†, THOMAS P. HOELEN‡, DONALD P.

6

WESTON§, RICHARD G. LUTHY*,†

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†

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94305-5080, USA

Department of Civil and Environmental Engineering, Stanford University, Stanford, CA

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‡

Chevron Energy Technology Company, San Ramon, CA 94583, USA

11

§

Department of Integrative Biology, University of California, Berkeley, CA 94720-3140,

12

USA

13 14

Corresponding author:

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Richard G. Luthy

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Stanford University Room 191, Yang & Yamazaki Environment & Energy Building,

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473 Via Ortega, Stanford, California 94305-4020

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Email: [email protected]

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Telephone: 650-721-2615

20

Fax: 650-725-9720

21

ACS Paragon Plus Environment


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ABSTRACT

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Activated carbon (AC) sediment amendment for hydrophobic organic contaminants

24

(HOCs) is attracting increasing regulatory and industrial interest. However, mechanistic

25

and well-vetted models are needed. Here, we conduct an 18-month field mesocosm trial

26

at a site containing dichlorodiphenyltrichloroethane (DDT) and chlordane. Different AC

27

applications were applied and, for the first time, a recently-published mass transfer model

28

was field tested under varying experimental conditions. AC treatment was effective in

29

reducing DDT and chlordane concentration in polyethylene (PE) samplers, and

30

contaminant extractability by Arenicola brasiliensis digestive fluids. A substantial AC

31

particle size effect was observed. For example, chlordane concentration in PE was

32

reduced by 91% 6-months post treatment in the powdered AC (PAC) mesocosm,

33

compared with 68% in the GAC mesocosm. Extractability of sediment-associated DDT

34

and chlordane by A. brasiliensis digestive fluids was reduced by at least a factor of ten in

35

all AC treatments. The model reproduced the relative effects of varying experimental

36

conditions (particle size, dose, mixing time) on concentrations in polyethylene passive

37

samplers well – in most cases within 25% of experimental observations. While

38

uncertainties such as the effect of long-term AC fouling by organic matter remain, the

39

study findings support the use of the model to assess long-term implications of AC

40

amendment.

41


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Introduction

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As activated carbon (AC) amendment of sediments contaminated by hydrophobic organic

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contaminants (HOCs) matures as a remedial technology1-6, there has been increasing

45

emphasis7 on collecting field data8-11 and on developing mechanistic models describing

46

mass transfer5, 12-15 in AC-amended sediments. The former allows researchers to assess

47

the feasibility of AC amendment at scale and effectiveness in a natural environment. The

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latter is necessary to understand long-term AC performance in the field. Such an

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understanding is needed to create effective remedial designs, as there can be significant

50

differences between field and laboratory performance. For example, Zimmerman et al.

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reported 3.4% (m/m dw) AC reduced polychlorinated biphenyl (PCB) pore water

52

concentration in Hunters Point Shipyard (HPS), CA, USA sediment by 92% after 6

53

months of continuous mixing under laboratory conditions1. However, Cho et al.,

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observed only a 50% reduction in sediment pore water PCB concentration 6 months post-

55

treatment at the pilot-scale 8, 9. In contrast Beckingham and Ghosh reported reductions of

56

69-99% in Lumbriculus variegatus tissue PCB concentration 3 years post-treatment in a

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pilot study at the Grasse River, NY, USA10. Beckingham and Ghosh attribute the

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difference in kinetics between the two pilot studies to differences in sediment

59

characteristics and PCB congener mix, manifested, for example, in the different sediment

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desorption rates10. These cases underscore the need for models to understand site-specific

61

effects and treatment conditions on AC performance.

62 63

Choi et al., presented a mass transfer model (itself an extension of published AC models12,

64

13, 16

) describing transfer processes governing AC amendment: advection, dispersion, and



65

intra-particle diffusion14, 17. The model predicted pore water concentrations of PCBs and

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alkylated PAHs to within a factor of two compared to column tests. However, model

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field-testing is needed for two reasons18. Firstly, column studies only approximate field

68

conditions and cannot encompass all field variables. Secondly, field validation will

69

encourage the model’s widespread application.

70 71

One challenge with field validation is scale. Effective model validation requires that

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several experimental conditions be varied. If treatment plots are sized in the tens to

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hundreds of m2 range that is typical in literature,

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needed. For example, Cho et al. applied the aforementioned mass transfer model to

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several 34 m2 field plots, but did not systematically vary AC application conditions15.

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Here, field mesocosms were used to achieve the study goals without a large treatment

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area and its concomitant resource requirement.

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This work has three major outcomes. First, we observed several treatment condition

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effects on AC performance with respect to organochlorine pesticide concentration in pore

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water (as estimated by passive sampler observations) and bioavailability. Second, we

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validated the Choi et al. mass transfer model under field conditions. Third, we assessed

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the long-term site implications after AC amendment. This study is the first field test of

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the Choi et al. model14 under varying experimental conditions.

9-11

a large and complex field effort is

84 85

Materials and Methods

86

Materials.

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Pesticide-grade solvents were purchased from Fischer Scientific (Fair Lawn, NJ). Ten


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organochlorine pesticides were examined, cis- and trans-chlordane, trans-nonachlor, 2,4′-

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and

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dichlorodiphenyldichloroethylene, and dichlorodiphenyldichloroethane, respectively),

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and 4,4′-DDMU (dichlorodiphenylmonochloroethylene). Pesticide standards were

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purchased from Ultra Scientific (North Kingstown, RI). Activated carbon (Type-TOG,

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Calgon Carbon, Pittsburgh, PA) was used as received (median diameter, D50 = 219 µm,

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measured by laser diffraction, Particle Technology Labs, Downers Grove, IL) for

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mesocosms amended with granular activated carbon (GAC). The same material was

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ground to a powder (D50 = 42 µm) using a stone mill for powdered activated carbon

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(PAC) application.

4,4′-DDT,

DDE,

and

DDD

(dichlorodiphenyltrichloroethane,

98 99

Field Site and Study Design.

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The field site for this study was a tidal channel within an industrial property in Northern

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California, USA, with sediments containing the organochlorine pesticides DDT and

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chlordane. Partially enclosed mesocosms were used to study the effect of varying AC

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treatment conditions. Mesocosms were fabricated from 61-cm (24-inch nominal)

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diameter high-density polyethylene (HDPE) pipe cut to 0.90 m lengths. Twelve 5-cm

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diameter holes were drilled into each mesocosm as shown in Figure 1 to allow water

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exchange with channel waters. Photographs are available in the Supporting Information,

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Figure S1.

108 109

The study employed six mesocosms: one mixed control, one unmixed control, and four

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mixed AC treatments. The treatment conditions within the four AC-amended mesocosms



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were 5% (m/m dw) GAC and PAC mixed for 10 minutes, 10% GAC mixed for 10

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minutes, and 5% GAC mixed for 20 minutes. An additional 50% of AC over nominal

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values were used to account for potential overmixing into deeper sediment strata than

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intended and ensure that the target AC dose was delivered to the mixing zone (i.e., 7.5%

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AC was added to ensure that a 5% dose was achieved in the target depth interval). Note

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these levels were selected for mass transport model validation and were not intended to

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explore the whole range of typical deployment scenarios. For example, AC doses above

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5% are rarely used in the field and particle size much larger than studied here have been

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employed in field studies7, 19. A schematic diagram of the experiment is available in the

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Supporting Information, Figure S2. The mixed control mesocosm was mixed for 10

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minutes without AC. The unmixed control mesocosm was not mixed or amended.

122 123

Mesocosms were deployed in November 2012. Prior to placing the mesocosms,

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vegetation lining the creek bank was scraped away with an excavator (Figure S3). The

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mesocosms were then positioned adjacent to one another and pushed into the sediment by

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the excavator. AC was then deposited into the appropriate mesocosms and mixed into the

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upper 15 cm of the sediment for the prescribed duration (10 or 20 minutes), with a dual

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head electric mortar mixer (Collomix CX-44 DUO, Gaimersheim, Germany).

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Photographs of the preparation of the area and placement of the mesocosms are available

130

in the Supporting Information, Figure S3.

131 132

Mesocosm Deployment and Field Sampling.

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Three observations were made: one baseline, one 6-months post-treatment, and one 18-


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months post-treatment, commencing February 2013, May 2013, and May 2014,

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respectively. The duration between AC deployment and sampling events was made on

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the basis of a short proof-of-concept field study. A timeline of field events is presented in

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the Supporting Information, Figure S4. The baseline measurements, consisting of six

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sediment cores and ten low density polyethylene (PE) samplers, were taken immediately

139

outside of the mesocosms. At each post-treatment observation, two PE samplers were

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deployed inside each mesocosm, at least 15 cm from the mesocosm wall and 30 cm from

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each other. Mass transport modeling indicated that the distance between the HDPE pipe

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and PE samplers was sufficient to avoid interference (see Supporting Information). As no

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attempt was made to convert PE concentrations to pore water concentration, performance

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reference compounds were not added to the PE prior to deployment. Rather, PE

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concentrations are reported relative to measurements in untreated control mesocosms and

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baseline measurements. Further details on polyethylene passive samplers are available

147

elsewhere20, 21. Upon retrieval of each PE sampler, a sediment core sample was collected

148

at the same location.

149 150

The six mesocosms and three sampling events were used to test the effect of AC particle

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size, AC particle dose, AC mixing time, and deployment duration on amendment

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performance. This experimental design and allocation of sampling allowed for a number

153

of comparisons to be made: inside the mesocosms with outside the mesocosms, control

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mesocosms with treatment mesocosm, treated zone with untreated zone within a

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mesocosm, and temporal trends. However, the spatial density of sampling precluded

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replicate mesocosms or numerous temporal observations. Instead, spatial variability



157

within a mesocosm was addressed by placing two samplers (each sectioned to 12

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segments) in each mesocosm for each sampling event. Spatial variability within the entire

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study area was addressed with the baseline measurements and modeling (Supporting

160

Information Figure S5-S7). While greater certainty could be obtained with baseline

161

measurements in tandem with duplicate or triplicate mesocosms, such a study design

162

would result in an unmanageable number of samples if the same sample depth resolution

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was retained.

164 165

For passive sampler measurements, a polyethylene strip (52 µm, 0.92 g/cm3, Brentwood

166

Plastics, St. Louis, MO) was wrapped around a slotted stainless steel frame. The

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approximate strip dimensions were 90 × 2 cm, which resulted in sampling over a 45-cm

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depth. Samplers were embedded in the sediment for 28 days. Upon retrieval, samplers

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were cleaned with a lint-free tissue, sectioned to 2.5 and 5.0 cm depth intervals (within

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the 0-15 and 15-45 cm intervals, respectively), wrapped in aluminum foil and stored in a

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glass jar at 4 ºC. Samples were held for 15 cm) strata, were extracted using gut fluid from the polychaete Arenicola

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brasiliensis. Details are in the SI, but briefly, the approach is an in vitro assay that

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quantifies how much of the sediment-associated contaminant may be desorbable from the

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particles during passage through a deposit-feeder’s gut, and would thus be available for

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uptake23-27. Specific extraction procedures (e.g., dose, extraction time) are as described by

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Weston et al. with some minor modifications28. Gut fluid from A. brasiliensis (0.9 ml)

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was mixed with wet sediment equivalent to 0.3 g dry weight. After a 2.5 h extraction, the

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gut fluid was recovered and analyzed for the organochlorine pesticides of interest. In

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parallel with the gut fluid extractions, the sediments were also extracted with a sodium

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taurocholate solution, a synthetic cocktail designed to mimic the contaminant

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solubilization properties of actual gut fluid29 and with artificial seawater as a control. The

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purpose of the sodium taurocholate extractions was to test it as a potential compliment to

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gut fluid, given the difficulty in collecting the latter. The contaminant extraction

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efficiency of all fluids was expressed as percent solubilization (mass of contaminant in

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the extractant as a proportion of that initially in the dry sediment extracted). Initial

200

concentrations in each sediment stratum were determined from the mean of the five to six

201

baseline core samples (Table S1).

202



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Model Description.

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The mass transfer model employed in this study is described elsewhere14. Briefly, HOC

205

transport in AC amended sediment is modeled as the sum of three processes: intraparticle

206

diffusion in sediment and AC, dispersion in sediment pore water, and advection. This is

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described by Equation 1.

208 = + + −

− −

# 3 + " $% ! !

(1)

&' 3 () *+ 2-! *+ &' % − . − 0 &' ! *+ /*+

209 210

where CW is the aqueous concentration; Sf, Ss, SAC, and SPE are the volumetric

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concentrations (g cm-3) in fast- and slow-releasing sediment fractions, in AC, and in PE,

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respectively; Rf, Rs, and RAC particle diameters (cm) of fast- and slow-releasing sediment

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particles (collectively, Rsed) and of AC, respectively; VW, Vsed, VAC and VPE are the

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volumes (cm3) of water, sediment, AC and PE, respectively; Dx, Dy, Dz are the dispersion

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coefficients (cm2 s-1) in each dimension; Rsed and RAC are the radii (cm) of sediment and

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AC, respectively; k0 (cm s-1) is the overall mass transfer coefficient for the PE-water

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boundary layer; xPE is the PE thickness (cm); vsz is the porewater seepage velocity (cm s-

218

1

) and KPE is the PE-water partitioning coefficient (cm3 g-1).

219


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The system is modeled over two periods: an initial well-mixed period and a stagnant

221

period. During the well-mixed phase that simulates brief initial mechanical mixing, mass

222

transfer is a function of intraparticle diffusion12. The results of the well-mixed period

223

become initial conditions to the stagnant period in which advection-dispersion,

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intraparticle diffusion, and particle distributions are considered.

225 226

Model Parameterization.

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Site-specific modeling parameters were determined in laboratory experiments as

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indicated in Table S2, with literature values used when appropriate. Aqueous equilibrium

229

pesticide concentrations in the unamended sediment were obtained by agitating triplicate

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sediment slurries with PE samplers on an orbital shaker at 80 rpm for 28 days. The

231

aqueous pesticide concentration can be estimated as CW = CPE/KPE, where KPE is the PE-

232

water partition coefficient.

233 234

It was assumed that AC was homogenously distributed and that AC-water partitioning

235

coefficients derived in clean water apply. Although fouling by dissolved organic mater

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(potentially due to pore blocking at the AC external surface30, 31) has been shown to

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reduce the activated carbon partitioning coefficient (KAC) of hydrophobic compounds by

238

a factor of 100 or more6, most of the literature on similar systems report attenuation by a

239

factor of approximately ten or less14,

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attenuation at a similar magnitude would not affect the response in pore water

241

concentration for the duration of the experiment – see Supporting Information.

31-34

. A sensitivity analysis indicated that KAC

242



243

Sediment desorption rates were determined in triplicate well-mixed sediment slurries

244

exposed to Tenax TA (60/80 mesh, Sigma) as described by Werner et al12. After

245

predetermined periods from 6 h to 160 d, the Tenax was collected and replaced. The

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Tenax was extracted in hexane and the sediment desorption rate determined by fitting the

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pesticide mass in Tenax to a known radial diffusion model35. Full details of desorption

248

and aqueous equilibrium procedures are available in the Supporting Information. Pore

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water velocity was conservatively assumed to be zero. Literature reports suggest that pore

250

water advection-dispersion does not significantly affect HOC mass transport in AC-

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amended sediments in tidally-influenced environments16, 17.

252 253

Results and Discussion

254

Baseline Assessment.

255

The average DDT concentration in sediment (reported as the sum of 2,4′- and 4,4′-DDT,

256

DDE, and DDD and 4,4′-DDMU) was 296 ± 35 µg/kg (mean ± standard deviation, n =

257

18) in composite samples from a depth interval of 0-15 cm and 403 ± 67 µg/kg (n = 15)

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in the 15-30 cm interval. DDTs were dominated by DDT metabolites, with DDE and

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DDD comprising 92% of the overall DDT mass in sediment. Chlordane (reported as the

260

sum of cis- and trans-chlordane and trans-nonachlor) sediment concentrations were 399 ±

261

40 µg/kg (n = 18) and 546 ± 113 µg/kg (n = 15) in the 0-15 cm and 15-30 cm interval,

262

respectively. Cis- and trans-chlordanes made up 77% of the chlordane mass in sediment.

263 264

Baseline PE measurements suggested increasing concentration with depth, consistent

265

with the coring data. At 5 cm depth, the average DDT concentration was 124 ± 17 ng/g (n


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= 15) and the average chlordane concentration in PE was 235 ± 28 ng/g. At 35 cm depth,

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the PE concentration increased to 173 ± 14 ng/g and 397 ± 40 ng/g for DDT and

268

chlordane (n = 15), respectively. The 150 PE observations were used to construct a three-

269

dimensional site model of the relative concentrations within the mesocosm area. This site

270

model was then used to interpret post-treatment observations. A discussion of the

271

baseline assessment and site model is available in the Supporting Information.

272 273

Activated Carbon Distribution in Mesocosms.

274

Measurements of TOC indicated that the AC dose delivered to the nominal treatment

275

depth interval of 0-15 cm was equal to or greater than the design dose (Figure S8).

276

Sediment TOC was related to AC dose with Equation 29: 123&' = 45 − 45! /86.1 − 45100%

(2)

277 278

where TOC (%) is for an amended core section, 86.1 is the TOC of the type TOG AC (as

279

measured by Cho et al.8), DoseAC is the AC dose (% dw) and TOC0 is the average TOC at

280

the corresponding position within the mixed control mesocosm. Measured AC doses were

281

in line with the nominal applied dose (Figure S8). Overall, there was little spatial

282

variability in AC dose within the mixed zone in a given mesocosm, with the standard

283

deviation of the observed AC dose on the order of 10-25%. This homogeneity can be

284

attributed to the intense mixing imparted to the sediment by the impeller.

285 286

AC recoveries of 118%, 98%, 110%, and 117% were observed for the 5% GAC, 5%

287

PAC, 10% GAC, and 5% GAC 2× mix mesocosms, respectively. The recovery



288

calculations here account for the entire core, not just the nominal treatment zone.

289

Significant AC was recovered below the nominal treatment zone, indicating overmixing

290

with respect to depth.

291 292

BC measurements closely agreed with AC dose estimates by TOC (see Figure S9),

293

confirming the suitability of TOC measurements for AC dose estimation. To confirm

294

visual observations that untreated sediments were deposited into the mesocosms, the

295

black carbon content (BC) of the thin (~2-5 mm), topmost layer of soft sediment was

296

measured and compared to the BC content of the overall AC amendment zone (0-15 cm).

297

If sediment from outside the mesocosms settled into the mesocosms, the BC content of

298

the surficial sediment would be less than the overall AC amendment zone, as the settled

299

sediment would be relatively free of black carbon while the mixed zone would have

300

elevated BC content due to AC addition. BC, rather than TOC, was used in this

301

comparison to limit any bias due to settlement of organic-rich material. The surficial

302

sediment BC content was found to be significantly (four-way ANOVA, p < 0.05) lower

303

than the composite mixed zone– up to a factor of 7. Along with visual observations, this

304

provides an additional line of evidence that untreated sediment settled into the

305

mesocosms.

306 307

Response in Pesticide PE Concentration.

308

For all AC treatments, the PE pesticide concentrations within the treatment zone were

309

significantly reduced compared to the mixed control and to the deeper PE samples. PE

310

pesticide concentration depth profiles were compiled 6 and 18 months after treatment for


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chlordane and DDT (Figure 2 and Figure 3). An example depth profile with the

312

corresponding TOC profile is presented in Figure 2 for the 10% GAC mesocosm, 18-

313

month observation. Omitting the most surficial sample, the variability of the duplicate PE

314

samples was low, typically ? C>?,IJKELJC,! C>?,ABCDEFGB = H M H M C>?,IJKELJC C>?,!

(3)

336 337

where CPE,Relative is the relative pesticide PE concentration normalized to the mixed

338

control mesocosm, CPE is the pesticide PE concentration within a treatment mesocosm,

339

CPE, Control is the PE pesticide concentration in the mixed control mesocosm at the depth

340

corresponding to the original sample, and CPE,

341

baseline concentrations. Data from the surficial zone, as shown in Figure 2, are neglected

342

from these composite measurements. Six months post-treatment, PE samplers contained

343

71 and 76 % less chlordane and DDT, respectively, in the 5% GAC mesocosm compared

344

to the mixed control. Similarly, chlordane and DDT PE concentrations were both 71%

345

lower in the 10% GAC mesocosms compared to the mixed control. Chlordane and DDT

346

concentrations were 65 and 75% lower, respectively, in the 5% GAC mesocosm with 2×

347

mixing. Reductions were more significant in the 5% PAC mesocosm, with chlordane and

348

DDT concentrations 93 and 94% lower, respectively, in the 5% PAC mesocosms

349

compared to the mixed control.

Control,0

and CPE,

0

are the pesticide PE

350 351

PE pesticide concentration within the treatment zone significantly (4-way ANOVA, p
0.50), as


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400

indicated in Figure 3. For example, 6 months post-treatment, PE chlordane concentration

401

was reduced by 71% in the 5% GAC mesocosm and 71% in the 10% GAC mesocosm.

402

Chlordane PE concentration was reduced by 88% and 91% in the 5% and 10% GAC

403

mesocosm at 18 months post-treatment, respectively.

404 405

There was no significant mixing duration effect on PE chlordane concentration within the

406

nominal treatment zone (4-way ANOVA, p > .10), as shown in Figure 3. PE chlordane

407

concentration within the 5% GAC mesocosm with 20 minutes mixing time was reduced

408

by 65 and 85% 6 months and 18 months post-treatment, respectively, compared with

409

71% and 88% in the 10-minute mixed GAC mesocosm. The response with respect to

410

DDT was similar, with DDT reductions of 75 and 84% 6 months and 18 moths post-

411

treatment, compared with 76 and 83% in the 10-minute mixed GAC mesocosm. Mixing

412

can enhance the AC performance by uniformly distributing AC particles which results in

413

shorter diffusion distances between particles14, but these results imply that 10-minute

414

mixing was sufficient to homogenize the sediment. Neglecting the effect of mixing on

415

sediment homogenization, the long-term mass-transport effect of mixing is minimal at

416

these mixing durations (see Figure S15). However, note that field deployment at scale

417

will likely not achieve the degree of homogenization observed here9.

418 419

Gut Fluid Extraction.

420

In all treatment mesocosms, AC amendment reduced the pesticide mass solubilized by

421

the A. brasiliensis digestive fluids or the taurocholate synthetic analog. In control

422

treatments lacking AC, quantifiable gut fluid extraction efficiencies typically ranged from



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423

0.2-2% for most analytes, and never exceeded 3.1% (Figures S16, S17). Such values are

424

typical for other hydrophobic substances in field-collected sediments, for example, high

425

molecular weight PAH extraction efficiencies are typically 0.1-10%24-26,

426

organochlorines of the present study are insecticides that have not been used locally in at

427

least 30 years, and the gut fluid results indicate approximately 99% of the sediment-

428

bound residues cannot be extracted under digestive conditions even without AC addition.

27

. The

429 430

Within the 0-15 cm composite sediment sample, addition of AC reduced gut fluid

431

extractability even further. At the 6-month sampling point, extractability for DDT and

432

chlordane declined from approximately 0.3-0.7% in the unmixed control treatment to

433

near 0 in all AC treatments. Note that while there is some difference in extraction

434

efficiency between the mixed and unmixed controls, these differences also appear in PE

435

measurements from outside the mesocosm (Figure S7), suggesting the difference stems

436

from a localized area of high concentration near the unmixed control is not an effect of

437

the mixing. At the 18-month time point, extractability declined from approximately 1.0-

438

1.5% in the unmixed control to approximately 0.1% in all AC treatments. It appeared to

439

make little difference whether 5 or 10% AC was used, whether PAC or GAC, or whether

440

mixed for 10 or 20 min. There was a high degree of correlation in the amount of

441

contaminant solubilization by gut fluid and the taurocholate mimic (Figure S18). At both

442

the 6-month and 18-month sampling point, the extraction efficiency of the two fluids was

443

highly correlated (p

Measuring and Modeling Organochlorine Pesticide Response to

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