Viability of Using DGT Passive Samplers to Measure Dissolved Trace Elements in Subtropical Freshwater

and Estuarine Environments

Master of ScienceThesis Defense by

Michael S. Tomlinson

Acknowledgments (mahalo nui loa!)➟ Thesis Committee:

➟ Eric De Carlo (Chair)➟ Fred Mackenzie➟ David Karl➟ Khalil Spencer

➟ The Team:➟ Scott Narod➟ Vincent Todd➟ Norine Yeung➟ Sam Saylor➟ Vincent Beltran

➟ Others:➟ Dan Hoover➟ Hao Zhang (Lancaster Univ.)➟ Nancy Koike➟ Kathy Kozuma

➟ Funding/Other Support:➟ NOAA Sea Grant➟ USEPA➟ Hawai�i DOH & DLNR➟ NSF (ICP-MS)➟ USGS

Outline➟ Motivation➟ Study Location➟ Methods➟ Results &

Discussion➟ Conclusions

➟ To quantify dissolved trace element inputs to aquatic habitats:➟ Methods time consuming and expensive➟ Ambiguous, definitions of dissolved vary➟ Discrete water samples are snapshots in time➟ Relation between sediment/tissue

concentrations difficult to relate to water column concentrations

➟ Often trace element concentrations <MDL

Motivation for the study

Nonpoint source pollution (after NPDES)➟ �Nonpoint source [NPS] pollution . . .

a significant factor in coastal water degradation� (U.S. Congress, 1990)

➟ �Stormwater linked to major coastal problems� (EPA, 1993)

➟ �May be greatest threat to marine ecosystems� (Clark, 1995)

➟ bioavailability can ultimately threaten human health through consumption of aquatic organisms

Why dissolved trace elements?

➟ Dissolved phases considered bioavailable

➟ �Bioavailability�the fraction of total contaminant in surrounding medium which is correlated with a quantitative biological response such as biomagnification� (EPA, 1992)

➟ Definition of �dissolved� is operational & varies with filter pore size (typically 0.2 to 1 µm)

What is �dissolved�?

Measuring NPS pollution➟ Water column sampling

➟ Sediment sampling

➟ Bioaccumulation in resident & caged species (e.g., NS&T Mussel Watch)

➟ Passive samplers

Water column sampling:➟ Concentrations may be <MDL➟ Snapshot in time➟ Sampling, containment, &

preservation can alter chemistry

➟ Filtering can alter chemistry➟ Ambiguity between dissolved

& particulate phases

Sediment sampling:➟ Sediments tend to be patchy

requiring numerous replicates➟ Bioturbation & other disturbances

can confound results➟ Difficult to obtain undisturbed

sediment sample➟ Sampling, containment, &

preservation can alter chemistry➟ Concentration relation

[sediment] ≠ [water column]

Organism bioaccumulation:➟ Difficult to locate suitable type/quantity of animals➟ May accumulate dissolved & particulate pollutants➟ Inter- & intra-specific comparisons difficult➟ Animals can metabolize or depurate pollutants➟ Non-sessile organisms can move in & out of area➟ Concentration relation [organism] ≠ [water column]

NOAA NS&T Mussel Watch Program

Ostrea sandvicensis (Hawai�i)Mytilus edulis (Maine to Delaware Bay &

US West Coast)

Interspecies differences, an example

Passive samplers:➟ Relatively recent development➟ Time integrating device➟ Accumulate bioavailable pollutants

(exclusive of ingestable particulate matter & larger colloids)

➟ Measure pollutants <MDL➟ Relatively inexpensive➟ SPMDs sample dissolved nonpolar

organics (e.g., PCBs, pesticides)➟ DGTs sample dissolved trace elements

DGT

SPMD

DGT (Diffusive Gradients in Thin-films)➟ Developed by Davison and Zhang (1994) of

Lancaster University➟ Measures dissolved Cd, Cr, Cu, Pb, Zn, Co, Ni,

Ag, Mn, Fe, Al➟ Work in saltwater, freshwater, sediments & soils➟ Consists of membrane filter, diffusive hydrogel,

resin gel, and housing (see diagram)➟ Effective pore size generally 0.002-0.005 µm

& no >0.020 µm (�standard� DGT)➟ Inexpensive (£10 or ~$17, March 2002)

Components of a DGT sampler➟ outer sleeve & piston➟ 0.45-µm, polysulfone membrane filter➟ polyacrylamide hydrogel (~95% water)➟ layer of Chelex-100® resin in hydrogel

Cb = bulk solution concentrationDBL = diffusive boundary layerδ = DBL thickness∆g = diffusive gel thickness (ideally ≥10 × δ)

Howthe DGT works

DGT facts➟ Generally only labile trace elements measured➟ Temperature-related effects are predictable➟ Diffusion coefficient independent of ionic

strength of receiving water (must be >1 mM)➟ Operating pH range of 5-10 for most elements➟ Not affected by hydrodynamic conditions➟ MDL for DGT after 1 day deployment is <4 pM

(concentration factor of ~300 times)➟ Analysis involves batch leaching (typically with

80 % recovery) followed by AAS or ICP-MS

Flow effects on Cd accumulation

C�concentrationDGT�diffusive gradient in thin-filmsASV�anodic stripping voltammetry

Cd accumulation with time &different gel thicknesses

Time (hours) 1/∆g (1/mm)

∆g = diffusive gel thickness

Mas

s C

d (n

g)

(Zhang & Davison, 1995)

Effects of ionic strength & pH on Cd accumulation in DGTs

(Zhang & Davison, 1995)

pH effects on accumu-lation of different

trace elements in DGTs

Field tests of DGTs

Field tests of DGTs & Cu speciation

Ala Wai Canal Watershed,

O�ahu, Hawai�i

Environmental Setting

The study area then (ca. 1865) . . .Mānoa Valley from Waikīkī, Painting by Enoch Wood Perry, 1865

. . . and now!

Dual personality of theAla Wai Canal

Dredging the Ala Wai Canal

Sampling & DGT stations

Discrete sampling program➟ Manual quarterly sampling, typically near base

flow conditions (4 years)

➟ Automated storm sampling (4 years)

➟ Streamflow & water quality (T, C, pH, DO & turbidity) at 5-minute intervals (4 years)

➟ Estuarine grab samples collected & water quality measured in situ concurrently with DGT (8 months)

Station WK (WaiakeakuaStream, upper

watershed)

Lower Ala Wai Canal watershed

Station KHS(Mānoa-Pālolo Stream, lower

watershed)

Station YC(Yacht Club),

estuary

Manual water quality sampling

Dipping

In situmeasurements

Pole sampler

Streamflowmeasurements

Automatedstormwatersampling

Grab sample processing & analysisFiltration

FIA-ICP-MS

DGT study design

➟ Compare stream DGT results with data from discrete base-& storm-flow samples collected over 4 years

➟ Compare estuarine DGT results with discrete samples collected concurrently with DGT retrievals over 8 months

DGT sampling locations

Shallow water deployment schemesStreams Estuary

TidbiT®

temperature logger

DGT processing matériel

DGT processingStep 1 - DGT disassembly

Step 2 - Removal of resin gel

Step 3 - Resin gel leaching

Step 4 -ICP-MSanalysisof DGT leachate

Calculating mean concentration

where:Cw = mean metal concentration in waterM = mass diffused into DGT)g = diffusive hydrogel thickness +

membrane filter thicknessDT = diffusion coefficient at any temperaturet = deployment (exposure) timeA = area of DGT window

Watershed (comparing long-term data & DGTs)

Rainfall & mean flow during DGT deployment periods in upper (WK) & lower (KHS) watershed

Rain in upper watershed couldaffectDGT operation, but infrequently

Special 3-month deployment at WK

WK cumulative 3-month flow

Stream results and why➟ Results of various methods for determining means

from discrete samples differed considerably➟ Rating curves were appropriate for upper but not

lower watershed (except for Pb)➟ DGT results generally comparable to, but less

than, grab sample means➟ DGTs measure the aquo ion, inorganic complexes,

and possibly small organic complexes & colloids➟ Grabs include larger colloids & organic complexes➟ No clear relation between flow & dissolved trace

element concentration

Estuary (comparing concurrent grabs & DGTs)

YC�fouling after1 weekand4 weeks

YC�little foulingon DGT membrane, before& after cleaning

YC DGT & sample comparison

Estuary results & why➟ DGT deployments >2 weeks not recommended➟ Grab samples collected at different stages of tide &

under different streamflow & weather conditions➟ DGT results were significantly different (α = 0.05)

from concurrent grab results except for Co➟ DGT results were not consistently higher or lower

than results from concurrent grab samples➟ CuDGT > Cugrab & > chronic & acute HAR 11-54

standard (2.9 µg/L)➟ Need many more grab samples to accurately

characterize estuary

Conclusions:➟ To date this study is the longest deployment

of DGTs in diverse aquatic environments➟ DGTs preconcentrate dissolved trace

elements & remove matrix interference for ICP-MS

➟ DGTs are a simpler, faster, economical way to measure dissolved trace elements

➟ DGTs provide mean concentrations but they also can show long-term variability

Conclusions (continued):➟ Watershed DGT & sample mean trace

element concentrations were similar➟ DGT means, however, often were less than

means from discrete samples➟ DGTs measure aquo ions, inorganic

complexes, small organic complexes, & very small colloids

➟ DGTs do not measure trace elements in larger colloids or organic complexes & small particulates

DGT vs. 0.2-µm filter

Conclusions (continued):➟ Except for Co, DGT & concurrent estuary

samples were significantly different➟ Estuarine DGT results were not consistently

less or greater than discrete sample results➟ Dynamics & complexity of estuary requires

far more samples to characterize chemistry➟ DGTs can be deployed for up to 3 months

in relatively clean, freshwater systems

Conclusions (continued):➟ Biofouling limits DGT deployments

1-4 weeks in subtropical estuaries➟ Operational pH range for DGTs (5-10) is

normally not a problem➟ Ionic strength rarely < 1 mM (~0.2 % of the

time in the upper watershed during storms)➟ DGTs are viable method for measuring

dissolved trace elements in subtropical freshwater & estuarine environments

Eric, a man who loves his work . . .

. . . maybe a little too much?

Let it never be said that Eric . . .

. . . hides from his students!

Examplerating curve

(flow vs. Cu)

DGT and grab sample blanks

Upper watershed trace elements

Lower watershed trace elements

Estuarine trace elements

YC DGT & sample comparison

Open water mooring scheme

Revised Sampling Scheme➟ Multiple (>5) blank checks before deployment➟ Three replicate DGTs deployed at each site➟ Dilute leachate by no more than 4 times➟ Continue temperature recording with TidbiTs➟ Locate inexpensive conductivity recorder➟ Deploy mid-depth in deeper stream waters➟ Collect or locate OC & speciation data➟ Multiple depths & locations in estuary➟ Deploy short- and long-term DGTs in freshwater