new api guide on petroleum natural source zone depletion

© 2017 CH2M

Fourth International Symposium on Bioremediation and Sustainable Environmental Technologies

New API Guide on Petroleum Natural Source Zone Depletion (NSZD)

Tom Palaia

Chief Technologist

Julio Zimbron

Founder

Eric Nichols

Principal

2Delivering Sustainable Solutions to Complex Local Challenges, Worldwide© 2017 CH2M

New API NSZD Guide

• Applicability: Measurement of mass depletion rates in LNAPL source zones

• Goal: Practice consistency

• What’s new:

– Emphasis on vadose zone mass losses as biogas (CH4 and CO2)

– Strategies for planning NSZD evaluations

– Field procedures and QA/QC

– Data evaluation guidance

From T.Palaia, 2016. Applied NAPL Science Review, Vol.

6, Issue 1. May.


NSZD Concepts and Usage

• NSZD quantification based on

stoichiometric conversion of

chemical fluxes in:

a) Groundwater

b) Vadose zone

• Aqueous-based losses <<

vapor-based losses

(at most sites)

• API NSZD guide focuses on

vapor phase-related

processes


NSZD Evaluation ConsiderationsSite conditions of concern

• No evidence of LNAPL

• Permanently saturated/frozen soils

• Thin vadose zone

• Large [CH4] near ground surface

• Lack of LNAPL delineation

• Intermittent flooding

• High natural organic matter in soils

• Impermeable ground cover

• Active on-going remediation

• Regional sources of CH4 and/or CO2

from deep reservoirs

• Large depth to LNAPL

• Cold climate

• Saturated silt/clay geology overlying hydrocarbon impacted soils

• Natural CO2 from calcareous sands/rock

NOT RECOMMENDED

RECOMMENDED WITH CARE


NSZD Evaluation ConsiderationsKey LCSM elements

• Lateral and vertical extent of LNAPL

• LNAPL type and fluid density

• Depth to groundwater and fluctuation

• Ambient temperature clime

• Depth to top of hydrocarbon impacts

• Soil type and moisture content

• Methane in shallow soil gas

• LNAPL distribution and hydrostratigraphy


NSZD Evaluation ConsiderationsNomograms to screen rates


NSZD Evaluation ConsiderationsBackground correction

• Background gaseous influence varies

widely

– Affects all methods

• Requires planning and special data

handling

– Background gas flux monitoring outside

the LNAPL footprint

– Supplementing field analysis with a

second method

• Measuring 14C in soil vapor to estimate

fossil fuel fraction of CO2

Higher

Background

Low

Background

Too high

Background?


NSZD Estimation – Field Methods

• Gradient

• Passive Flux Trap

• Dynamic Closed Chamber

Soil Flux Chamber

Gradient Method

CO2 Trap


Gradient MethodOverview

• Based on Fick’s first law of

diffusion:

– Assumes diffusion is dominant

and homogeneous, isotropic

geology

• Requires depth-discrete vapor

monitoring data for O2 and CO2

– Fixed gas concentration profiles

– Effective vapor diffusion

coefficients (tracer test)


Gradient MethodApplication

Accuracy is

affected by choice

of lower boundary

control point

Above the methane oxidation zone, flux of

reactants should be relatively constant with

depth


Gradient MethodConsiderations and caveats

• Simple in concept, but difficult to obtain robust results

• Challenges:

– Variations in soil moisture affect gas profiles

– Heterogeneous soils will affect soil gas profiles

– Single “snap-shot” in time

– Selection of lower boundary control point

– Diffusion only, advection of gases will confound the results

Site without vadose zone impacts


Dynamic Closed Chamber (DCC) MethodOverview

• Developed as a short-term

measurement of total CO2 efflux

– Industry standard method in

ecosystem studies

• Field measurement of CO2

concentration over time in closed

chamber with pressure equalization

• Can collect short- or long-term

measurements

• Diffusion + advection


DCC MethodApplication

• Site survey based on total CO2

fluxes

• Requires background correction

– Measure total CO2 flux at

unimpacted locations

– Take into account differing

ground cover and vegetation


DCC MethodConsiderations and caveats

• Real-time results, can measure

20-30 locations per day

– High data density achievable

• Relatively inexpensive

– Moderately labor intensive

• Total CO2 fluxes vary diurnally

• Background correction can be

difficult in diverse conditions

– Not carbon-isotope friendly With permission from Ma et al, 2013


Passive Flux Trap MethodOverview

• Two sorbent layers separate soil

gas efflux from ambient CO2

• Deployment for multiple

consecutive days

– 2 weeks typical to avoid short-

term effects

• Diffusion + advection

– Sorbent is porous and allows

free air flow

• Field method with lab analysis


Passive Flux Trap MethodApplication

• Field efforts are minimal

• No power or moving parts

• 14C analysis (also known as carbon dating) allows splitting between

modern and fossil fuel CO2 contributions (Method DASTM 6866-12)


Passive Flux Trap MethodConsiderations and caveats

• Long-term, time-integrated flux

• Fossil fuel analysis (14C) is standard

• Higher quality data, but also higher

cost

• Relatively long turn-around time

(4 weeks)

• As other methods, continuous gas

transport is needed

– Carefully consider soil water

saturation


Method Comparison Caveats

• Each method has specific limitations

• Sand-tank studies have validated

each method

• Order-of-magnitude accuracy is

expected


Methods Summary

Gradient Passive Flux TrapDynamic Closed

Chamber

Best for sites with:

Vadose zones >5 feet

with existing vapor

probes

Variable effects of soil

respiration on O2

and CO2 flux

Uniform background

gas flux

Intrusiveness High, for new probesLow, shallow

installation

Low, shallow

installation

Transport processes Diffusion Diffusion, advection Diffusion, advection

Instantaneous or time-

averaged

measurement?

Instantaneous Time-averaged Both

Method of background

correction

Background O2 and/or

CO2 flux monitoring14C

Background CO2 efflux

monitoring


Methods Summary (continued)

Gradient Passive Flux TrapDynamic Closed

Chamber

Spatial coverage/data

densityLow Moderate High

Real-time dataYes, using field gas

analyzerNo Yes

Laboratory analysis Optional Yes No

Field labor intensity High Low Moderate

Relative one-time

implementation cost

$$$

($-$$ existing probes &

field analyzer)

$$ $


Emerging Method

• Biogenic heat


Closing

• New API NSZD guide will be available

online soon

• Written to be useful for regulators, owners,

and practitioners alike

• Many options to consider and results to

anticipate

• Practical guidance for:

– NSZD screening

– Method and procedure decision support

– Field work design and implementation

– Data analysis

Thank you!

© 2017 CH2M

Fourth International Symposium on Bioremediation and Sustainable Environmental Technologies

Tom Palaia

Chief Technologist

Julio Zimbron

Founder

Eric Nichols

Principal


• Tom Palaia, CH2M

[email protected]

(303) 679-2510

• Julio Zimbron, E-Flux

[email protected]

(970) 492-4343

• Eric Nichols, Substrata

[email protected]

(603) 770-6577

mailto:[email protected]



new api guide on petroleum natural source zone depletion

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