23 rd annual narpm training program methods for measuring and assessing dissolved metals in sediment...
TRANSCRIPT
23rd Annual NARPM Training Program
Methods for Measuring
and Assessing Dissolved Metals in Sediment Porewaters
Mark Cantwell, US EPA
23rd Annual NARPM Training Program 2
Outline
Introduction and rationale for measuring porewater metals in sediments
How metals behave in sediment porewater and why
General considerations for porewater sampling, matching methods with sites and desired results
Presentation of common sampling methods, in situ and ex situ, designs, features
Porewater extraction and preservation techniques
23rd Annual NARPM Training Program 3
Introduction
At many sites, sediments are a primary source of contaminants after discharges/releases cease
Metals do not degrade or decay, posing unique challenges for remediation
A sediment’s characteristics control changes in dissolved metal concentration and speciation
Physical, biological and chemical variables can influence the state and speciation of metals
Sediment toxicity from metals can change over short periods of time
23rd Annual NARPM Training Program 4
Introduction
Porewater metal concentrations provide important information on overall behavior and risk from metals at a specific site
Sediment porewater metal concentrations are useful to predict bioavailability and sediment toxicity i.e., better than bulk metal measurements
For this discussion, dissolved metals are defined as those passing through a 0.45µ filter*
*Colloidal metals are often present in this operationally defined fraction
23rd Annual NARPM Training Program 5
Physical Factors Affecting Dissolved Metals Behavior in Sediments
Many sites may present considerable challenges to measuring porewater metals
Tides, currents and water bodies with high flow such as streams and rivers may erode, transport or deposit surficial sediments
Sediment resuspension can potentially affect porewater concentrations to considerable depths affecting RPD boundaries
23rd Annual NARPM Training Program 6
Biology Matters!
Bioturbation leads to increase in sediment-water interface; relocation of contaminated sediment
Species assemblage and abundance can affect this process
Wayne Davis
23rd Annual NARPM Training Program 7
Major Chemical Interactions of Sediment Porewaters
Modified from Di Toro et al., 2001
Sediment organic carbon
Fe oxyhydroxides
Clays
Acid volatile sulfides
Mz+
Potential metal complexes in porewater
HO- HCO3- CO3
2-
SO42- Cl- DOC
Sediment-bound metals Porewater metals
Metal adsorption;metal substitution
in amorphous sulfides
23rd Annual NARPM Training Program 8
Sediment RPD Boundary
Vertical zone with transition from oxic to anoxic conditions, varies from site to site
Visual changes in sediment color Major changes in sediment chemistry Declines in O2, redox support a reduced
environment e.g., from CO2, SO4, NO3 to CH4, H2S, NH3
This results in change to the sediment phases binding metals
If ∑metals exceed sediment binding capacity, metals may be present in porewater
23rd Annual NARPM Training Program 9
Metals and Sediment Parameters
Commonly Measured Porewater Metals Ag, Cd, Cr, Cu, Ni, Pb, Zn behavior well documented
in scientific literature and USEPA Documents Fe and Mn
Relevant Sediment Measurements Acid volatile sulfide (AVS) Sediment organic carbon (TOC) Oxidation reduction potential (ORP) pH DOC Grain size
23rd Annual NARPM Training Program 10
Non-metals: NH3, H2S
Ammonia and hydrogen sulfide are natural and commonly encountered constituents in sediments
Both can be toxic to aquatic and benthic dwelling organisms
The presence of NH3, H2S can cause complications in interpreting porewater toxicity results
Both can be measured in extracted porewaters and identified as potential sources of toxicity using ion specific electrodes (ISE)
23rd Annual NARPM Training Program 11
Porewater Sampling Approach
Determine project needs, goals, budget
Site characteristics - area, depth, currents, sediment type(s), location, other factors
Volume of sample required (e.g., chemical-toxicity tests), number and type of analytes measured
Selection of an in situ or ex situ sampling method
Field sampling design (number of sample sites)
23rd Annual NARPM Training Program 12
In situ Samplers
Peepers (dialysis)
Rhizon samplers
Tube samplers (Henry)
Lysimeters
23rd Annual NARPM Training Program 13
Peepers
Passive sampling devices inserted into sediment at a range of depths, porewater enters chamber(s) through a filter membrane (dialysis)
Equilibration period (~14d) provides an average concentration over deployment time
High vertical resolution, low potential for artifacts
Small sample volumes, membrane diffusion rates
Samplers must be deoxygenated prior to insertion
Sample immediately following removal and preserve
23rd Annual NARPM Training Program 14
Peepers for Vertical Profiling
Johnston et al., 2009 Modified Hesslein
23rd Annual NARPM Training Program 15
Rhizon Sampler
Microfiltration membrane with a nominal pore size of 0.12 - 0.17 µm
Hydrophilic membrane composed of a blend of polyvinylpyrrolidine and polyethersulfone structure
Porewater collected by applying vacuum from a syringe or vacuum pump
High-resolution sampling is possible because they may be closely spaced, limited sampling depth
Minimal disturbance of soils/sediment
23rd Annual NARPM Training Program 16
Henry Samplers
Collects porewater by inserting S/S tube with sampling slots into sediment
A vacuum hose attached to a syringe or vacuum pump collects porewater
Minimal disturbance of soils/sediment
Capable of limited vertical profiling
Reusable between sites
Inexpensive/fast
Sampling volume limited
23rd Annual NARPM Training Program 17
Henry Sampler
From Pitz 2009
23rd Annual NARPM Training Program 18
Suction Lysimeters
Plastic cylinder with porous ceramic end cup admits porewater to tube interior
Samplers deployed and sealed in ground using clay seal
Used for soil nutrients, agricultural chemicals and other dissolved ions
Collection period typically 2-3 days
Samples exposed to air during collection period
23rd Annual NARPM Training Program 19
Soil Lysimeters
Soil Moisture, Inc.
23rd Annual NARPM Training Program 20
Ex situ Samplers
Bulk sediments are collected from sites using corers or grab samplers
Cores are preferred as they limit potential artifacts (oxidation), can be taken on land or water, and are easy to handle
Large volumes of porewater can be isolated
Sample handling can affect data quality
Temperature, time and oxidation are critical handling factors to consider
23rd Annual NARPM Training Program 21
Push Corer
Can be used on land or shallow water, or by scuba diver
Quick and inexpensive
Provides samples with a vertical profile
Works best on fine grained, well-sorted sediments
Lower potential for artifacts than other methods
23rd Annual NARPM Training Program 22
Gravity Corer
Used from boats to sample submerged sediments
Rapid deployment-recovery
Provides samples with a vertical profile
Plastic core sleeve isolates sediment from contamination
Low potential for artifacts
Performs best on fine grained sediments
23rd Annual NARPM Training Program 23
Grab Samplers
Van Veen Ponar
23rd Annual NARPM Training Program 24
Porewater Extraction Methods
Squeezing
Sediment is placed in a cylinder between teflon or Ti discs and filter membranes
Moving piston compresses sediment, displacing porewater from sediment
Mechanical or pneumatically operated systems to compress sediment
Limited volume of porewater produced
Preparation can be time consuming
23rd Annual NARPM Training Program 25
Squeezers
Bufflap and Allen, 1995
23rd Annual NARPM Training Program 26
Reeburgh Squeezer
Reeburgh 1967
23rd Annual NARPM Training Program 27
Porewater Extraction Methods
Centrifugation
Can generate large volumes for TIEs and toxicity tests
Sediments are loaded in centrifuge tubes (PE, PTFE) spun, and porewater supernatant is produced
Tubes should be sealed with inert gas and kept at constant temperature (4°C) during extraction
Syringes and in-line filtering recommended
Handling of samples in inert gas, glove box or bag is necessary to prevent artifacts
Minimize holding time (24h), inert atmosphere, 4°C storage
23rd Annual NARPM Training Program 28
Sample Preservation
Post-extraction porewater should be filtered immediately
Plastic disposable syringe with in-line filter best for most applications Most common filter size is 0.45 µ, size depends on
study requirements
Different membranes (e.g. polycarbonate, nylon) available
Porewater should be preserved with acid ~1 µl/ml HCl or HNO3 depending on elements being
measured
23rd Annual NARPM Training Program 29
Summary
Different methods exist for isolating porewater from sediments
Passive sampling (e.g. peepers) provide good information on actual in situ metal concentrations
Direct sampling methods (Henry sampler) in many cases allow for rapid collection and greater spatial coverage
Ex situ methods such as centrifugation allow greater volumes of porewater to be generated
23rd Annual NARPM Training Program 30
Summary
Select methods carefully based on project needs, site location, budget and data quality objectives
Sample collection, handling, extraction and processing techniques need to be carefully monitored to avoid generating artifacts
Temperature, time and oxidation are critical factors with regard to sample integrity
23rd Annual NARPM Training Program 31
Parting Thoughts-Questions
All porewater is not the same! Extraction methods functionally define porewater and
change its character and thus affect the speciation and bioavailability of any chemical contaminants it
may contain.
Chapman et al., 2002