Natural Attenuation of Groundwater Contaminants: New Paradigms, Technologies, and Applications(Rice University Coursera)

Matrix difussion principles

Key points

Matrix diffusion is important attenuation process because of storage effect

Acts like a capacitor

Even small amounts of biodegradation in the matrix is important for long term managment of groundwater plumes

Diffusion Vs Dispersion

Mechanical dispersion

Variations in flow velocity

Different solute flow paths

Cause the plume to spread

Difussion

The spread of particles through random motion from regions of higher concentration to regions of lower concentration

Matrix diffusion in the lab and field

New conceptualization of contaminant transport that

Abandons dispersion as basis for accounting for local heterogeneities in aquifers and as an explanation for dilute concentration in wells

Embraces diffusion and slow advection as fundamental governing processes at contaminated sites

Diffusion vs Dispersion is important for understanding how natural attenuation works

MAtrix diffusion is important in many common hydrogeogic settings

Lab and field studies have demonstrated importance of matrix diffusion

Samppling for matrix diffusion requires different strategies, different tools

Matrix diffusion sampling

First generation approaches to site characterization

Groundwater samples

Soil Samples

Plume delineation

Focus on unsaturated zone

1.Real-time profiling to identify intervarls of interest

Soil subsampling

Hunt for interfaces between transmissive and low-k zones

Then do high resolution sampling of low -k zones to determine presence and concentration of contaminants in low-k zones

Groundwater Models for matrix diffusion

Type site simulation

Parallel Fractures type site

Fracture Network

Two layer Sand/Clay

Multi-layer San/Clay Type site

Random Clay Layer Type Site

Models

Dandy-Sale Analytical matrix Diffusion model

Numerical modeling requires much higher resolution than commonly practiced

SERD Project ER-1740 Type Sites provide key insights on behavior of plumes affected by matrix diffusion

Analytical models can model matrix difussion, but require simplification of the model domain

ESTCP Matrix Difussion Toolkit is key tool for matrix diffusion modeling

Degradation processes within Low-k Zones

Do contaminants in low permeability unit attenuate?

Sometimes

Some evidence based on lab/field studies

Not investigated much yet

Slow attenuation could be significant for chlorinateds

Caracteristics that favor attenuation

Long retention times

Reducing conditions are common, favorable for biological and biogeochemical reductive dechlorination

Potentially large reservoir of organic carbon

Potentially large reservoir of reactive mineral species

Characteristics that hinder attenuation

Smaller pore throat size restricts migration of microbes, influx of nutrients/ carbon sources, and growth density

Salinity can be high and may limit microbial activity

Limited bioavailability of organic carbon

Reactivity of mineral species may be limited due to dependen on microbial activity

Potential lines of evidence for low k zone attenuation to support MNA

Molecular biological data confirming presence and/or activity of degraders within low k zone

Daugther product distribution suggests greater extent of degradation in low k zone relative to adjacent transmissive zone

Favorable geochemical conditions within low k zones

CSIA data showing higher fractionation within low k zone

Mineralogical analysis of low k zone soil samples that show minerals capble of abiotic degradation of contaminants

This is a critical, relatively new MNA process for chlorinated solvent MNA; Perceived as not very important for fuels

Ways to demonstrate low-k attenuation: look for bugs, analyze isotopes, look at daughter products

But only a few studies completed to date, much to learn

click to edit

Growndwater flow review

Darcy's Law

Hydraulic gradient

i: dH/dL

Hydraulic conductivity

Hydraulic head

Key points

Groundwater velocity not correlated to hydrocarbon plume length: Biodegradation so strong it overpowers velocity

Groundwater velocity is correlated to chlorinated solvent plume length

Biodegradation no so strong

Attenuation due to mixing and dispersion

Texas risk reduction rules

Soil to GW pathway

GW soil: Leachate dilution factor (LDF)

GW darcy velocity

GW mixing zone thickness

Net infiltration

Width of affected soil in direction of GW flow

Key factors

Area of source

Aquifer thickness

Distance to receptor

Screened interval

Infiltration rate

Ground water velocity

First order decay

Human Health PCLs: Groundwater

Groundwater to surface water pathway

Risk-Based Exposure Level

Dilultion factor for affected GW entering SW

Three SURFACE water specific rules RCRA

Plume must already have reached the stream

No statistically significant increase over background in the surface water

No receptors at unsafe level before discharging

Mixing used in many risk assesment studies

Key parameters

Infiltration

Groundwater mixing zone thickness

Dispersions

Surface water mixing allowed in some cases.

Low flow surface water techniques come into play such as 7-day, 10 year low flow and base flow stimation

Mass Flux/Mass Discharge concepts for estimating dilution

Concentration

Mass discharge

Plume capture by a supply well

CSw : Md/ Qsw

Csw is the maximum concentration of contaminant in water extracted from the supply well

Md is the mass discharge

Q sw is the pumping rate from a supply well

Solute concentration nomogram

Mass discharge transect

Concentration paradigma vs Mass discharge paradigm

Mass discharge (Md) can be used to stimate potential impacts to pumping wells and surface

Concentration in well/stream: Md/ flowrate

Good for first order estimate of impacts and prioritization

Mass Flux/ Mass Discharge Measurement Methods

Estimating well impacts

Use mass discharge of plume to predict constituent of concern concentration in downgradient water supply well

Combine flow, size, concentration

Mass flux:J

Mass per area per time

Mass discharge, Md

Mass per time

Methods

Transect method

Step-by-step approach assuming uniform groundwater velocity

Well capture/ Pumping methods

Passive flux meters

Using existing data (Isocontours)

Solute transport models

Draw transect: with polygons for each well

Determine area

Multiply and sum together

Md: Sum( CnAnVd)

Md: Mass discharge

Cn: Concentration in polygon n

An: Area de segment n

Vd: Darcy velocity

Measure Q, Cwell from well

Md: Q*Cwell

Md: Mass discharge (grams per day)

Cwell: Concentration in recovery well (grams per liter)

Q: Well pumping rate (liters per day)

Permeable sorbent

Accumulates contaminant based on flow and concentration

Soluble tracers

Loses tracer based on groundwater velocity and flux convergence calculations

Contaminant adsorbed onto passive flux meter over time to get concentration

Tracer desorb from passive flux meter over time to get flow

Uses plume map

Combine with flow data

Two dimensional transect based on isocontour data

Enter flow, concentration data and calibrate model

If you know the mass discharge somewhere upgradient of a water supply well or a receiving stream, you can estimate the potential impact

The ITRC Mass Flux/Mass discharge guide provides 5 different methods to get mass flux/ mass discharge measurements

Plume magnitude classification system/ Mass Discharge wrap up

Mass Flux/ Mass discharge

Combine flow/size, concentration

OoM: Order of magnitude

Concentration, hydraulic conductivity have a log normal distribution

Site Prioritization using mass descharge

9 OoM

Plume Magnitude classification system

What Mag Plume Does it Take for impact?

Mag 2 plume

Domestic well

Mag 5 plume

Municipal well

Mag 8 Plume

Stream

Managing surface water Quality with mass Discharge: Total Daily Loads (TMDL)

Mass discharge can be used as prioritization system, and as "Trip wire" to estimate if your plume is relatively strong or relatively week

Dilution can be used as an MNA process, but only under certain circumstances

Types of isotope analysis and sampling techniques/ Lab Analysis

Isotopes

Stable

Non stable

Radioactive

Compound-specific stable isotopes

Fractionation

Ligther isotopes are degraded preferentially

More rapidly

Degradation causes remaining TCE to become enriched in heavier isotopes

The isotopic ratio is changing due to degradation

Ratio: R: ("Heavy")/ ("Light")

"del": (Rtce-Rstd)/(Rstd)*1000

Unit are "per mil" or%o

Rayleigh Equation : Classic form

Rt/Ro: f(alfa-1)

Rt: R of compound at time t

R0: R of compound at time 0

f: Fraction of compound remaining

alfa: Fractionation factor

Rayleigh Equation : more useful form for environmental data

dt:E Ln f + d0

d of compound at time t

From analysis of site samples by lab

Provides LOE for degradation

Enrichment factor (E): 1000*(alfa-1)

Literature or lab value

Compound and pathway specific

Ln f : Fraction of compound remaining

d0: d of compound at time 0

How do you collect samples for isotopic analysis?

Groundwater is typical matrix

40-ml VOAs w/ preservative (Chech with lab)

Soil samples also an option

How do you analyze samples for isotopes?

More specialized analysus (GC/IRMS)

Fairly limited number of commercial labs (relative to standar analytes)

100s of $ per isotope per analysis (Plus need concentration too)

May require lots of sample to get detectable signal

Key points:

Measurements of ratios of stable isotopes in contaminants can provide information about wheter degradation has occurred

Compounds become enriched in heavier isotopes as degradation occurso because the lighter isotopes are preferentially used- this process is known as fractionation

Interpreting CSIA data

Evidence of parent compound degradation

Increased fractionation *("Heavier") over time

Need > 2 %o for to confirm C fractionation

The hight in y reflects extent of paren degradation

Pattern can be used to estimate fraction degrades and rate degraded

Maybe better than estimates based on standard C vs t data?

Evidence of daughter compound degradation

No degradation of daugther product

Daugther approaches d parent s as C parent approaches 0

Degradation of daughter product

d daughter exceeds d0 parent as C parent approaches 0

Evidence of degradation rate

Derivation assumes first-order degradation (pretty standard)

Can be calculated with time or distance

kt: (d0-dt)/(E*t)

kt: 1st-order rate coefficient

d of compound at time 0 and at time t

Enrichment factor (Look up value)

The change in the isotopic ratio can be used to estimate the fraction of compound that has been degraded and the rate of degradation

The isotopic of a daughter product is initially lighter(more negative) than the paretn but gets h eavier (less negative) as it degrades

Literature values of enrichment factors can aid in the interpretation of CSIA data

Emerging technologies with isotopes: Pathways and modeling

CSIA Challenges

Data can be difficult to interpret

Might get different interpretations

Recognition that fractionation occurs even with non destructive processes

MNA processes showing evidence of isotope effects:

Biotic degradation

Abiotic degradation

Dispersion

Diffusion

Sorption

Volatilization

Dual Isotope Analyses Can Help Interpretation

C12-C13

O16-O18

N14-N15

H1-H2

Cl35-Cl37

Primary Benefit:

Increased Centainty that fractionation is ocurring

Isotope ratios in benzene in groundwater samples from multiple wells

Shifts in one isotope may be more apparent than other isotope

Secondary Benefit: Improved estimation of enrichment factors

Isotope ratios in benzene in groundwater samples from multiple wells

Provides better basis for degradation extent and rate

Possible Benefit:

Identify relevant reaction pathways

Lab test suggest pathways have characteristic and distiguishable fractiation paaterns

Identify Relevant reaction Pathways

Lots of uncertainty about whether this is really possible

Complicated by contribution of other non-destructive processes

Conceptual diagram of isotope ration in 1.1.1 TCA samples along plume transect

Identify Relevant reaction Pathways

Very powerful for source apportionment

Less of a priority for MNA applications

Distinguishing two sources of perchlorate

Generation 3 MNA analysis

CSIA data combined with reactive transport modeling

ESTCP-funded project (ER-201029) rovides model and extensive guidance

PHAST 2 D Model cross-section assuming complete reductive dechlorination to ethene

Analyzing and plotting data from two or more isotopes can provide additional forensic information

One potential benefit of this type of dual isotope analysis is to identify or differentiate relevant attenuation pathways

Reactive transport modeling is emerging as a powerful method for interpreting CSIA data

Demonstrating In-Situ Biodegradation

How does one prove that biodegradation is occuring in situ?

Document loss of contaminants at the field scale

Graphical ans statistical analyses of contaminant concentration trends

Plume contour vs time

Concentration vs distance

Well transects

Can be used to determine contaminant flux at transects

Fi: Suma( ViCiAi)

Vi: Contaminant velocity perpendicular to the line of wells

Ci: Contaminant concentration at the well screen

Ai: Tributary area for a given well screen

A decrease in flux over travel distance indicates natural attenuation

The first-order decay coefficient (Lambda) can be obtained as the slope of Ln (Flux) versus travel time (x/v)

The mass balance approach

Needs a dense monitoring well cluster (good for non-steady plumes)

Useful to assess whether a plume is shrinking, stable, or expanding

Total mass of dissolved contaminant monitored over time by interpolation and integration of monitoring well data

Lambda is determined as the percent of contaminant deplected per time, usually normalized to a unit of per day

Lambda: (-dM/dt)/M:[(Mo-M)/Mo]/dt

Geochemical indicators

To demonstrate indirectly the type of degradation processes active at the site

For spills of oxidizable pollutants (e.g hydrocarbons) look for O2, NO3 and SO4 levels below background in the core of the plume, and Fe2 and Ch4 levels above background.

Also, Higher CO2 and alkalinity

For chlorinated solvents, look for higher Cl- levels than backgorund as well as for appearance of dechlorinated byproducts

Laboratory or in situ microcosm showing biodegradtion in biologically active incubation but not in sterile controls

Stainless steel cylinders that isolate about 2 L of the acuifer.

Valves allow for adding test contaminants and for sampling

ISM is open to the ambient aquifer at the bottom

Thus, one needs to correct for dilution effects using tracers

Single well push-pull test

During the injection phase, a prepared test solution containig a tracer and a subtrate is injected into the saturated zone using an existing monitoring well

During the extraction phase, flow is reversed and samples are collected and anlyzed to determine mass recovered for tracer, substrate, and byproducts

Molecular Evidence

Stable Isotope Enrichment

Converging lines of evience are usually needed to demonstrate that in situ biodegradation is a significant attenuation process

Strong evidence can be provided by showing that the plume is shrinking, or a decrease in contaminant flux and/or dissolved mass, with microcosm studies as supporting evidence

Geochemical indicators such as stoichiometric consumption of electron acceptros and accumulation of their reduced byproducts are also useful

Molecular evidence of biodegradation

Genetic Biomarkers

Target

Ribosomal (r) RNA

Genes for r RNA on DNA

Catabolic genes on DNA

Messenger (m) RNA

Protein

Information gained

Phylogenetic identity

Phenotypic potential

Question answered

Who is there?

What can they degrade?

What genes are being expressed?

Who is active and what are they doing?

DGGE band preferentially enriched after anaerobic benzene degradation

Corresponds to desulfobacterium sp

Highest biomarker concentration coincided with point of highest anaerobic benzene degradation activity

Benzylsuccinate Synthase (Coded by bssAgene)

Initiates anaerobic Toluene or Xylenes biodegredation

bssA biomarker concentration predicted anaerobic toluene degradation rates in microcosms

1, 4 Dioxane biodegradation by dioxane mooxygenase

20 microcosm sets mimicking dioxane natural attenuation

Degradation activity predicted by dxmA Probe but not by a generic total bacteria (16S)Probe

DNA probes can provide strong evidence of the presence of specific degraders and their enrichment as a result of biodegradation

Some genetic biomarkes hold potential lfor use to predict biodegradation rates and assess whether degradation is proceeding faster than migration

Stable isotope probing

It's not the same thing as compound stable isotope analysis

CSIA

Measure naturally occurring isotopic ratio in contaminant and track over time and/ or distance to show degradation

Add radiolabeled contaminant and see if it accumulates in biomarkers over time

If it accumulates, then it means that organisms are growing on the contaminant

If it's growing, then that means degradation is ocurring

Different types of biomarkers can be targeted and anlyzed

PFLA (Phospholipid fatty acid)

DNA/ RNA

Proteins

Relevant isotopes for SIP

C 13

N 15

O 18

Very high isotopic enrichment is needed to see signal

10-100% heavy isotope

Beacuse microbes would prefer to use ligther isotope

Contaminants tested using SIP

Benzene

Toluene

Phenol

PAHs

RDX

TNT

MTBE

1,4 Dioxane

All are carbon/energy sources for microbes

Contaminants that cannot be tested using SIP

PCE

TCE

cDCE

Anaerobic pathway

1,1,1, TCA

Aerobic pathway

Other chlorinated ethanes

Chlorinated methanes

Perchlorate

Compounds that serve as electron acceptors- SIP won't work for processes that aren't assimilatory

Typical application of SIP for MNA

Passive microbial sampling devices

Bio traps are installed in monitoring well for 30 days or more

Strengths of SIP for MNA

Strong line of evidence that attenuation can occur/is occuring

Establishes how attenuation is ocurring and who is responsable

Mimics in situ conditions

Can be used to support rate calculations

Culture independent

Complmentary and adaptable

Can use existing monitoring wells

Limitation of SIP for MNA

Not all compounds can be tested

False positives

Cross feeding

In situ test are conducted in preferential conditions

Cost

Availability

Few labs

Requires expertise

Protocols not always standarized

Stakeholder unfamiliarity with results

Stable isotope probing (SIP) involves elements of both CSIA and MBTs

Radiolabeled compound is added to a lab microcosm or passive microbial sampling devices accumulation of the radiolabeled compound in biomarkers over time seves as indicator that degradation is occuring

SIP works for compounds that serves as carbon sources but does not work for compounds that serve as electron acceptor

High resolution Characterization for MNA

First generation site characterization technology

Key elements of first/generation approaches to site characterization

Groundwater samples

Collected as primary characterization data

Plume delineation and to monitor trends

Long/screened well provides florw/weigthed average concentration

Domitade by high K zones with little info on low K zones

Soil samples

Collected for sources delination

Soil coring often focuses on unusaturated zone * not extended to saturated low Kzones( with limited vertical sampling density

Variations in mass flux across a transect

Simultaneous measurements of groundwater flow and concentration are made at representative grid points.

Mass flux is calculated using those estimates in eq. 2-1

Summing the segments of all mass flux values across the entire plume cross section yields the contaminant mass discharge

Plume structure and mass flux distribution in a hypothetical contaminant plume developing from a DNAPL source Zone

A series of transects is ahown, with the leading edge of contaminant arrival on the right

Note the changes in the mass storage in less transmissive zones with distance from the source

Changes in mass flux distribuyion ina an expanding plume over time

The mass flux at any location along a plume represents the combined effects of contaminant transport.

destructive attenuation (if any), and storage processes (sorption and difussion into low-K zones)

Losses of contaminat mass temporarily lower mass flux relative to the flux that is later observed at plume maturity.

Second generation site characterization technology

All compartments and alla processes should be considered during site characterization

Heterogeneity is an important governing factor for contaminant fate and transport

High-resolution characterization is key to understanding heterogeneity-

The scale must be appopriate for the site conditions

Several rapid data acquisitation tools and methods can provide valuable data as part of an integrated characterization approach

Site characterization must be dynamic and adaptive

MIP with constituent-specific detection capabilities

Better optical screening tools

More emphasis on direct-push

Refined geophysical methods

Rapid field extraction and analysis

New core collection techniques

Better tools to measure/ understand attnuation rates

Tools for contaminant/amendment accessibility

Mass flux techniques

Passive flux meters

Simple modeling tools to aid data interpretation

visualization tools

High resolution sampling vs long term minutoring for MNA

Site Characterization

Must use high resolution sampling for this

Long term monitoring (LTM)

But Ok to use monitoring wells for this?

Key points

Second generation (High resolution) Site characterization is collection of key themes and key tools

They allow you to characterize the scale where you can see high flor zones vs storage zones

Mass fluz/ mass discharge transects key example of hi res characterization

High resolution techniques good for characterization, but monitoring wells may be efficeient for long term monitoring

Designing an MNA Monitoring program

Characterization/ Remedy selection

Line of evidence

Decreasing historical trends in concentration/mass

Favorable geochemical and daughter product data

Microcosm or field data showing degradation is ocurring (and rate)

Long-term performance monitoring

Objectives for MNA Monitoring

Demonstrate that natural attenuation is ocurring

Detect changes in conditions that reduce attenuation efficiency

Identigy toxic/mobile by-products

Verify that plume is not expanding

Verify no impact to downgradient receptor

Detect new releases

Confirm institucional controls are working

Verify attainment of medical objectives

For site specific

Remedial action objectives (RAOs)

Preliminary Remediation Goals (PRGs)

Source area

Source zone attenuation, identify new seleases, monitor institucional controls, assess performance

High concentration plume core

Monitor conditions in highly transmissive zones, transects for mass discharge measurements

Low concentration plume fringe

Potential for earlier attainmen of remediation goals due to more rapid concentration reductions

Plume boundaries

Monitor plume expansion or shrinkage at sidegradient or downgradient locations or transects, confimr no risk receptor

Recalcitrant zones

Areas where progress towards remedial goals is slow due to matrix diffusion or poor characterization, may require higher resolution monitoring

Upgradient areas

Establish backgorund conditions, identify changes in conditions

Analytes for long term MNA Monitoring

Constituents of concern

Transformation products

Geochemical indicators

Others

Daughters products, metals

Oxidation- reduction potential, pH, temperature, methane, sulfate, iron, nitrate

Water level, isotopes, biomarkers, minerals

Key points

MNA monitoring program can involve characterization to support the selection of MNA as a remedy, or long-term performance monitoring to document that MNA is working

MNA monitoring programs are designed to address specific remedial action objectives

MNA monitoring locations and analytes are selected to ensure that objectives like performance and plume stability can be assesed

Monitoring frequency

Long-term attenuation rate vs Short term variability

Short-term variability makes it harder to determine trend and increases the amount of monitoring needed to evaluate progress in remediation

Long term trend apparent over longer monitoring period

Increasing the time between monitoring events will increase the confidence and accurancy of your long-term attenuation rate...

Accuracy/Confidence Cost

Medium Confidence

Statistically- significant

Decreasing concentration trend (p<1) for 80% of monitoring wells

Medium Accuracy

Determine the long term attenuation rate with an accuracy of +- 50 % or 0.1 yr-1 for 80% of monitoring wells

It commonly takes seven years or more of quarterly monitoring data to characterize the attenuation rate with even a medium level accuracy

Making decisions based on insufficient data can result in incorrect decisions

Short term variability makes it harder to determine trend and increases the amount of monitoring needed to evaluate progress in remediation

It comonly takes seven years or more of quarterly monitoring data to characterize the attenuation rate with even a medium level of accuracy

Less frequent monitoring over longer periods of time may be more cost appopriate for detemining trands during MNA

Trend Analysis

Why do we need trend analysis?

Short term variability can make this challenging

Need statistical methods

Avoid " Recency bias"

Are contaminant concentrations decreasing over time?

When will concentration fall below the clean up goal?

Mann- Kendall trend analysis

Linear regression

Parametric test

Isn't always applicable

Non parametric test

Requires no assumptions about distribution

Only cares about relative magnitudes of concentrations

Easier to establish trend

Non-detects more easily handled

Simple method

How to perform Mann-Kendall?

Calculate three Different Metrics

S statistic (S)

Indicates if trendis increasing or decreasing

Confidence Factor (CF)

Reflects degree of confidence in results

Confidence of variation (COV)

Variability in concentration vs. t data

Used to distinguish between stable and no trend

Statistical methods like Mann- Kendall provide a way to establish long term concentration trends in data that exhibits significant short term variability

Mann- Kendall is a non parametric test, and that makes it suitable for MNA data that can range over several orders of magnitude and may not be collected at regular frequencies

Trend analysys can be performed easily using free software tools

Monitoring well Dynamics

Why can it be hard to stablish a concentration trend?

Variability is inherent to monitoring data

Implications for manitoring frequency and data requiriments

Sources of variability not well understood

Potential sources of variability

source attenuation

Not really short-term variability

Laboratory analysis

Minor source

Well and sampling dynamics

Minor source

Between sampling events

During sampling event

Ambient flow

Groundwater flows through screened interval

Less resistance to hydrostatic pressure

Continuous flushing of well with water from aquifer

Greater contribution from zones of higher permeability

Ambient mixing

Groundwater mixesvertically within the well

Temperature, pressure, salinity

Redistribution of contaminants

Closer to flow weighted average

Seasonal when driven by temperutre

Mostly affects shallow wells

< 10 m deep

Well Purging

Sampled water representative of formation water

Flow weighted average concentration

Obtained if well has been "Sufficiently" purged

Concentration result would be similar to no- purge sample

If ambient flow and in-well mixing occur

Cost vs variability

Effect of sampling method

Sample method has little or no effect on concentration or variamility

No apparent benefit to monitoring purge parameter comáred to fixed volume purge

Select sample method based on cost, ease of implementation, and sample colume requirements

Signal variability

During monitoring, purging is typically used to obtain a flow weighted average concentration

Between monitoring events, ambient flow and mixing of water can occur within the monitoring weel and caouse water to be fairly representative of formation

Little difference in data obtained using different sampling methods

Our ability to reduce short term variabilit is generally limited

Key MNA Calculations and graphics

Lines of evidence

LOE Historical contaminant mass reduction monitoring data vs time

I shrink therefore I am

Direct method to demonstrate decreasing trend

Always

Mass Loss and plume stability

Define groundwater plume status as stable, shrinking, or expanding

Evaluate historical concentration measurements in gorundwater

Always apply based on sufficient historical data

Demonstrate mass loss, plume stability

Two common graphical methods

Plume outer contour vs time

Rate calculations

Concentration vs distance at different times

Some key graphics: plume size vs time, concentration plots

Source well concentration vs time : shows attenuation in source

Concetration vs. distance: shows attenuation in plume once contaminants leave the source

Plots of geochemical parameters often performed

Chapter 8

How long, How far? Key Question for modeling tools

How far will plume Go?

Gorundwater transport Modeling

Advective -dispersive -degradation equation

Concentration at Downgradient LocationX

Source concentration

Longitudinal dispersivity

Groundwater seepage velocity

First order Decay Constant

Retardation Coefficient

Error Function (erf)

Transverse dispersivity

Gorundwater source width and depth

Vertical Dispersivity

How long will be there?

Source term mass balance

Assume source zone is a Box

Flow rate through source zone

Concentration in source zone at time

Total mass of contaminant in source zone

Concentration declines with tail

How to use a Model to evaluate if MNA Can/Will stabilize a plume

Calibrate model to existing monitoring data

Increase time to some time in the future

See if plume gets larger or smaller or becomes stable

Why use models?

Systems to organize site data

Tool to help understand site processes

Additional line of evidence

Screen for applicability of MNA

Key points

Computer models can help answer two types of questions:

How far will the plume travel?

How long will the source and plume persist?

Models are good for organizing site data, help understand key processes

But models don't provide detailed, precise answers

Fate and transport equations for MNA

Transport Processes of contaminants in groundwater

Source zone

Groundwater flow direction

Advection alone

Dispersion/Difussion

Sorption/ retardation

Degradation

Advection- Dispersion Sorption Equation

Key Points

Analytical models are useful tool to assess if natural attenuation is occurring

Compare plume with "No MNA" simulation, using Lamda =0)

Limited predictive capability (Order of magnitud capacity) because groundwater flow and microbial behavior rarely follow the simplyfying model assumptions

Analitycal Groundwater models for MNA

Commonly used analytical MNA Models

Bioscreen

Biochlor

RemChlor

REMFuel

Matrix Diffusion Toolkit

Source

Analytical model for source behavior

Mass balance model on source zone predicts discharge including effects of remediation

Flow

Couple models at the edge of the source zone to provide contaminant discharge to plume model

Plume

Plume model simulates mass balance based on advection, dispersion, retardation, and degradation reactions

plus plume remediation

Developing the mass balance

Source Mass

Source depletion % Mass Removed

Source Decay Rate Constant

Mass discharge

Plume mass

Mass Discharge

Plume remediation Lamda

MNA

Source power function

Menor 1

NAPL is mostly in high conductiviy zones, or as pools in homogenous media

Mayor 1

NAPL is mostly in low permeability zones in heterogeneus system

Source

Zone 1

High anaerobic decay rates

Zone 2

Possible enhanced aerobic decay

Zone 3

Low or Background decay rates

Plume Remediation model

Divide space and time into "Reaction zones"

Solve the coupled parent daugther reaction for chlorinated solvent degradation in each zone

Each of these space time zones can have a different decay rate for each chemical species

REMChlor-MD

Semi-analytical method adapted to the perform of chemical diffusion with first order decay

4 types

Aquifer/ Aquitard system

Transmissive zone

Low permeability confining

Layered system

Low permeability layers

Transmissive zones

Low permeability confinig layer

Heterogeneous system

Transmissive zone

Low permeability confinig layer

3D Fractured Media

Key points

Quick overview of a powerful but relatively simple gorundwater model that can be used to evaluate MNA

REMChlor

It has an innovative source term so you control when and how contaminants leave the source (Gamma)

The plume has Space- Time zones to model different times and places that biodegradation occurs

REMChlor- MD will be important new tool- includes matrix diffusion in plume

Estimating Remediation timeframes

Commonly used MNA timeframe Models

Logarithmic extrapolation

SourceDK-Tier 1

Box Model (First order decay)

SourceDK Tier 2

BIOSCREEN

BIOCHLOR

Power Model (Any Type of decay)

REMChlor

REMFuel

Diffusion Models

Matrix diffusion toolkit

Mass Balance

NAs Toolkit

Enter concentration vs time data

Enter clean up level

Assuming constant concentration leaving source (Step function)

Flow Rate Through source

Total Mass in source

Concentration in source zone at time

Remediation timeframe equations- step function

Remaining fraction of source mass source deleption

RTF sd: Remediation timeframe with source depletion

RTF mna: Remediation timeframe MNA (untreatd source zone)

Deacay function

Source power function

Gamma: 0

Step function

Gamma: 0.5

Linear

Gamma: 1.0

First order

Gamma> 1

Matrix difussion

Key points

Several timeframe models available

Time reduction is not linearly related to mass removal

Consideable uncertainty in remediation timeframe estimates

MNA Lessons from BIG DATA Studies

The fourth paradigm

Experimental Science

Theoretical Science

Computational modeling

Application of massive databases

Geotracker database

Data mangment system for sites in california with affected gorundwater

Key points

Big data studies give insightsa on how big plumes are and how fast they change

Numerous studies for variety of contaminant classes

Geotracker important resource

Numerical gorundwater computer models and MNA

Continuum of groundwater tools

Limited

Hand calculations

Basic

Site data

TAxonomic Screening

Binning/ Screening

Site data: Simplifying assumptions

Simple analytical models

Exploratory or desision level

Complex incorporates more site data

Numerical models

Detailed site-specific simulations

Strengths

See how macro scale heteogeneities affect you

Simulates recharge, discharge to streams

More complex reactions for sorption, biodegradation

More realistic

Weaknesses

More time and expenses

Have to be careful with matrix diffusion

Typically you have to enter your source concentrationvs time

RT3D

MT3DMS

Can be used to model the complexities of flow, geology, and reaction for natural attenuation or accelerated in situ bioremediation scenarios.

Modular 3D multispecies transport model for sminulation of advection, dispersion and chemical reactions of contaminants in groundwater systems

Type site simulations

Parallel fractures type site

Fracture Network

Two layer sand/Clay

Multi layer sand/CAly type site

Random Clay layer type site

Key points

Numerical models can model thing analytical models cannot

Some limitations: Can be very powerful but with some limitations

Don't often have " Modeled Source term"

May need very fine grid to model matrix diffusion

Type site can be middle ground

9 Week

1.BioPIC: Bio Pathway identification criteria screening tool for MNA

historical foundwater data that demonstrate a clear and meaningful trend of decreasing contaminant... concentration over time at appropiate monitoring locations

Hydrogeologic and geochemical data that can be usedd to demonstrate indirectly the tupes of natural attenuation processes and the rate at wich such processes will reduce to required levels

Quantitative framework

A systematic approach to evaluate wheter MNA is an appropiate remedy based on site specific conditions

Use GW fate and transport model to extrac rate constant fro field data to determine the necessary rate of degradation to achieve goal

Use BioPIc to confirm if that rate is consistent with rates that have been observed in other studies for any potentially-applicable pathways

How work

Attenuation pathways that are included

Complete anaerobic reductive dechlorination

Partial anaerobic reductive dechlorination

Aerobic biological oxidation

Abiotic degradation

Parameters found to have direct correlation on attenuation rate

Dehalococcoides density (for TCE, cDCE, and VC)

Magnetic susceptibility

Iron sulfide

Methane

Ferrous iron

Key points

Free on lin e that helps decide if MNA is right for your site

If MNA is not appropiate, it helps decide between biostimulation and bioaugmentation

Uses correlations between paramete concentrations and known attenuation rates to determine wich processes can explain field rates

Applicability of MNA for different contaminant classes

USEPA

Plume is not expanding and sorption is occurring

ID the attenuation mechanism and stimate rate

Determine capacity and sustainability

Develop monitoring and contingency measures

Key points

Primary attenuation pathway for many inorganic is transformation to less mobile forms through co precipitation or sorption

REactions ares generally more complex and highly influenced by geochemical conditions

Oxygenates

Not promising in early protocols

Lots of research and field work in the following 5-10 years and we ended up with a completely different story

MTBE/TBA

MTBE exhibits similar characteristics to BTEX in terms of median plume size and attenuation rate

TBA also similar to BTEX but fewer are stable/shrinking

Which emerging contaminants are MNA candidates?

Dioxiane

TCP

NDMA

Phthalates