23 rd annual narpm training program methods for measuring and assessing dissolved metals in sediment...

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


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


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


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


Biology Matters!

Bioturbation leads to increase in sediment-water interface; relocation of contaminated sediment

Species assemblage and abundance can affect this process

Wayne Davis


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


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


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


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)


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)


In situ Samplers

Peepers (dialysis)

Rhizon samplers

Tube samplers (Henry)

Lysimeters


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


Peepers for Vertical Profiling

Johnston et al., 2009 Modified Hesslein


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


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


Henry Sampler

From Pitz 2009


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


Soil Lysimeters

Soil Moisture, Inc.


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


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


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


Grab Samplers

Van Veen Ponar


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


Squeezers

Bufflap and Allen, 1995


Reeburgh Squeezer

Reeburgh 1967


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


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


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


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


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

23 rd annual narpm training program methods for measuring and assessing dissolved metals in sediment...

Documents

porewater slide

porewater metals tides

sediment phases binding

colloidal metals

sediment parameters

dissolved metals behavior

measured porewater metals

porewater concentrations