Mark Cantwell, US EPA

1. Methods for Measuring and Assessing Dissolved Metals in Sediment Porewaters Mark Cantwell, US EPA

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

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

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

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

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

7. Major Chemical Interactions of Sediment Porewaters Porewater metals Sediment-bound metals Mz+ Sediment organic carbon Fe oxyhydroxides Clays Acid volatile sulfides Potential metal complexes in porewater HO- HCO3- CO32- SO42- Cl- DOC Metal adsorption; metal substitution in amorphous sulfides Modified from Di Toro et al., 2001

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

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

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)

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)

12. In situ Samplers • Peepers (dialysis) • Rhizon samplers • Tube samplers (Henry) • Lysimeters

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

14. Peepers for Vertical Profiling Johnston et al., 2009 Modified Hesslein

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

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

17. Henry Sampler From Pitz 2009

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

19. Soil Lysimeters Soil Moisture, Inc.

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

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

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

23. Grab Samplers Van Veen Ponar

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

25. Squeezers Bufflap and Allen, 1995

26. Reeburgh Squeezer Reeburgh 1967

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

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

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

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

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