LNAPL Tracer Dyes - 2019 International CleanUp...

Advances in the Use of LNAPL Tracer Dyes for Remediation Characterisation in Fractured Basalt Keith Maxfield

INTERNATIONAL CLEANUP CONFERENCE ADELAIDE 2019

LNAPL Tracer Dyes What are they and how do they work


What are tracer dyes used for?

Used to appraise LNAPL transport, mobility and recoverability

Dye data is utilised to:

• Measure LNAPL mobility - and calculate LNAPL seepage velocity and transmissivity

• Measure route of LNAPL migration - under natural gradient or forced gradients

• Utilised on low-mobility LNAPL, to demonstrate it is effectively immobile / impracticable to recover

Expands on research by Colorado State University (under Patent and license-use agreement)

Introduction to LNAPL Tracer Dyes


What are they?

LNAPL tracer dyes are oil-soluble fluorescent dyes added to LNAPL – analogous to groundwater dyes

Fluorescent dye is added to LNAPL in a well - the decrease in LNAPL fluorescence is a direct measure of LNAPL flux out of the well into the surrounding aquifer

LNAPL

Visible Light UV Light

Mobile and residual LNAPL

Groundwater

LNAPL Tracer Dye Research

Groundwater

Mobile LNAPL

Mobile and residual LNAPL

Dyed LNAPL in the well

Approx. Water Table

← LNAPL Flow Direction LNAPL is being extracted from a ‘well’ on the left side of the tank

Undyed LNAPL added to the right side of the tank

Sand-tank study from CSU

LNAPL is extracted from left side of tank; undyed LNAPL added to right

CSU used this research to correlate rate of dye decrease to the LNAPL pumping rate, and followed up with field tests which confirmed technique worked at the field scale


Tracer dyes only dissolve in oil, and change the LNAPL fluorescence profile

Fluorescence measured using an optical spectrometer and fibre-optic cable to transmit UV light to the LNAPL, and transmit fluorescence to the spectrometer

The fluorescence peak of each dye is utilised as the unique tracer, and the fluorescence intensity as the concentration range.

Methodology – generally similar to groundwater tracer tests:

• Bench tests are used to establish baseline and target dye concentrations

• Dye is dosed at a measured rate into the LNAPL in the well

• The fluorescence intensity provides a direct measure of the rate of dyed-LNAPL transport out of the well and replenishment with undyed-LNAPL

• The fluorescence profile is a tracer to verify dyed-LNAPL breakthrough in surrounding wells and extraction wells

How do LNAPL Tracer Dye Tests Work?


Baseline LNAPL fluorescence (no dye), comprising a broad asymmetric 450-650nm hump, peak at 508nm of 3200 counts

Dyed-LNAPL fluorescence, showing a distinctly different spectral profile, comprising two clear peaks at 545nm (18,500 counts) and 580nm (15,000 counts)

AECOM study site required measurement of LNAPL connectivity and mobility within a fractured basalt aquifer

Application advanced the state of the science via:

1. First commercial real-world application of dye moving from well-to-well due to pumping

2. Identification of new LNAPL tracer dyes with unique fluorescence

3. Use of multiple dyes concurrently to trace LNAPL flow in multiple wells

4. Verifying pumping-induced LNAPL flow via the rate of dye dilution in the dose well and dye breakthrough at each extraction well

Advances in LNAPL Tracer Dye Technology


LNAPL Tracer Dye Application Case study of tracer dye application, results and key findings


Client site comprised a 8.5ha LNAPL plume, with variable LNAPL recovery rates

• Potentially unpredictable flow pathways in fractured basalt

• Uncertainty in characterising LNAPL capture zones surrounding recovery wells, and optimising placement of recovery wells

• Auditor indicating potential need for high-intensity remediation techniques and/or high well density

In order to verify to the Regulator the LNAPL flow pathways and recoverability in the fractured basalt, characterisation of the LNAPL flow pathways in the aquifer was necessary

The Challenge:



Aim: characterise the LNAPL connectivity and flow in the fractured basalt

• Using LNAPL tracers dyes to generate empirical evidence of pumping-induced flow between wells

• Multiple datasets to correlate tracer dye findings with aquifer data: − Core log and fracture analysis - lava sequence, fracture abundance over historic smear zone − LNAPL analysis - via Tn, DvD plots and DGPs − LNAPL historic yields - to plot LNAPL yield zones across plume

The Solution:

Objectives of the LNAPL tracer dye study were:

1. Verify that pumping-induced LNAPL flow was occurring through the fractured basalt

2. Characterise the heterogeneity in fracture connectivity and LNAPL flow

3. Provide quantification of the LNAPL capture zone surrounding recovery wells

To reduce the test duration, multiple dyes were used concurrently.

The absence of similar works in the literature necessitated undertaking a research effort to identify additional dyes and verify their suitability for dye tracer testing

• Nine LNAPL dyes were subject to a ‘Dose Conditioning’ program to test their suitability and fluorescence response for the LNAPL conditions

• Identified three dyes which demonstrated a suitable detection range, unique fluorescent properties, and were discernible when mixed together

Dye Research – Dye Dose Conditioning and Response Testing

• Study undertaken at four LNAPL extraction wells located throughout plume, each surrounded by multiple wells at differing orientations

• Testing comprised four sequential dose stages per well, 3-months each • Dye dosing undertaken in separate surrounding wells in sequence, • Monitoring then undertaken to assess rate of dye dilution (i.e. LNAPL flow out of

the well) and breakthrough of dyed-LNAPL at the extraction well.

• Preparatory works: • Additional wells installed in close proximity to extraction wells • LNAPL tanks installed at each to monitor recovered LNAPL

Tracer Dye – Test Scope and Set-Up


LNAPL tracer dye key findings:

• Showed dyed-LNAPL flow at all test wells • Demonstrated flow between multiple dose wells to the extraction wells,

through the fractured basalt - in response to pumping

• Dye dilution rates: • Varied from complete dilution in a few days, to partial dilution over 200 days

• Dye breakthrough • Correlating with dilution, profiles of a few days to 140 days

• Characterised variability in LNAPL flow between different wells

• Different dyes were clearly distinguishable • Allowed flow rates from separate wells to be measured

Tracer Dye Findings


• Dye breakthrough curves at the extraction well demonstrated several profiles: • Single-peak breakthrough – indicative of preferential fracture flow channelling. • Multi-peak breakthrough – indicative of interrupted flow • Long tailed breakthrough – indicating secondary flow/drainage and/or diffusion

• Dyed-LNAPL demonstrated a stratification profile within the LNAPL • Zones of increased dilution correlated to individual fractures in the core log • Provided direct evidence of lateral fracture interconnectivity at the near-water-table elevation

• LNAPL recovery rates: • LNAPL recovery rates were variable and intermittent occurring in ‘pulses’ through the aquifer • Recovery rates varied from 1 to 500 L/week, and experienced episodes of low recovery for

up to six weeks before returning to high yields • Suggests LNAPL flow was dynamic, and appeared to follow different flow pathways resulting

in variable rates of recovery

LNAPL Tracer Dye – Data Analysis


Single-peak dye breakthrough curve at the extraction well

Dye fluorescence stratification in the LNAPL in the well against the basalt core –

illustrating the specific fracture sets where LNAPL is flowing through the well

LNAPL recovery rates were variable and intermittent, indicating LNAPL flow through

the aquifer is dynamic

Multi-peak breakthrough curve

Long-tailed breakthrough curve

• LNAPL laterally continuous at a local- to meso-scale (i.e. 10m) • Via a combination of high-flow fracture sets and/or low-flow / secondary fracture sets • LNAPL flow was not exclusive to discrete and unpredictable fracture pathways

• Flow velocities varied widely, generally aligned with Tn values • i.e. high-flow in high-Tn wells

• LNAPL recovery - occurring in ‘pulses’ through the aquifer: • Variable and intermittent, indicating LNAPL drainage is dynamic • Likely due to occlusion of LNAPL fracture-flow pathways

− In response to changing pressure dynamic between water, LNAPL and air, i.e. within the finite porosity and pore-pressures of the local fracture set

• Correlation of dye findings with secondary datasets: • Tn and DvD profile - good indicator of local continuity of LNAPL mobility • Core fracture logging - highlighted fracture abundance at the near-water-table elevation

− Correlated with Tn and LNAPL continuity (locally)

LNAPL Tracer Dye in Fractured Basalt – Key Findings

Visualisation of Fracture Flow Pathway Channelling

Potentiometric Surface

LNAPL within fractures

Dye-concentration data from single-well tests can be used with a geometric solution or first-order decay model to estimate LNAPL flux from the well. Used to calculate LNAPL transmissivity and LNAPL velocity.

Requires estimates for LNAPL gradient, LNAPL saturation, well convergence factor and aquifer porosity.

Not undertaken for this study as dyed-LNAPL velocity measured directly between wells.

Further reading:

• Pennington, Smith, Koons, Divine, 2016. “Comparative Evaluation of Single-Well LNAPL Tracer Testing at Five Sites”. Groundwater Monitoring & Remediation.

• Smith, T., 2008. “Direct Measurement of LNAPL Flow Using Single Well Periodic Mixing Reactor Tracer Tests”. M.S. Thesis, Department of Civil Engineering, Colorado State University, Fort Collins, Colorado.

• Sale, T. et al, 2007. “Measurement of LNAPL Flow Using Single-Well Tracer Dilution Techniques”. Ground Water 45(5): 569-578.

Further Analysis of LNAPL Tracer Dye Data


LNAPL tracer dyes were essential in generating direct evidence of pumping-induced LNAPL flow and connectivity through the fractured basalt aquifer

Dye data formed basis for characterising LNAPL capture zones around recovery wells, and rationale for siting further wells to optimise LNAPL plume recovery

The dye data, together with the accompanying LNAPL and aquifer characterisation data, generated a high-resolution dataset to characterise the basalt connectivity and LNAPL flow

The data analysis also generated a basis for using indicator metrics and aquifer testing to appraise LNAPL connectivity beyond the test wells

Conclusions



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