Innovative Design and Implementation of Air Sparge System for the Treatment of

VOCs, SVOCs, and Arsenic Authors: Omer J. Uppal, Annie Lee, Matthew J. Ambrusch, Nadira Najib, Steven A. Ciambruschini, Stewart H. Abrams

Langan Engineering & Environmental Services, Inc., 619 River Drive Center 1, Elmwood Park, NJ 07407

ABSTRACT

Background: An air sparging alternative was compared to a biosparging remedy for groundwater impacted with

volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), and arsenic constituents of

concern (COC) in a complex geologic setting at a state-mandated cleanup program site in northern New Jersey.

The operation of the biosparging system was started at the site in 2002 to address the COC impacts within an

approximate one-half acre area designated as the Former Lagoon Area (FLA). The system was decommissioned

in August 2012 due to its lack of effectiveness. Due to the sustained presence of Non-Aqueous Phase Liquid

(NAPL) within the FLA, an excavation was performed in October 2012 consisting of removal and off-site disposal

of approximately 2,200 tons of NAPL-impacted soil and 52,000 gallons of groundwater. A modified remedial

strategy consisting of an air sparge system was developed and implemented in 2013 to address the residual

groundwater impacts at the FLA.

Approach: The air sparge system was innovatively designed to provide in-situ flow-through treatment of VOCs

(primarily benzene at concentrations up to 24,000 micrograms per liter [µg/L]) by a combination of volatilization

and aerobic biodegradation, and arsenic by precipitation and sorption. SVOCs (i.e., phenol) that are not

expected to be completely volatilized were projected to degrade by aerobic microorganisms in the oxygenated

groundwater within and downgradient of the FLA. Various laboratory and field-scale pilot tests, in-situ

stripping analysis, and subsurface pneumatic modeling were performed to evaluate the effectiveness of air

sparging technology in removing COCs.

Results: The air sparge system design consists of 53 nested sparge wells (127 well screens) to target up to three

aquifer zones, 11 nested chimney wells to relieve pressure buildup and facilitate air flow in the subsurface,

above ground piping, and air compressors capable of providing an air flow rate of 1,000 standard cubic feet per

minute (scfm) at 27 pounds per square inch (psi) pressure. The system design also consists of a vapor collection

component for the capture, treatment, and discharge of organic vapors generated from sparging operation. The

vapor collection system consists of 41 horizontal wells, an impermeable surface cap, 17 vent wells, below grade

piping, and vacuum blowers capable of providing an air flow rate of 1,600 scfm at 8 inches of mercury (“Hg)

vacuum. The results of the pilot testing, assessment, and the remedial strategy and design of the modified

system are presented.

GW Flow Direction DIAGNOSTIC TESTING

FOCUSED AREAS

SITE LAYOUT MAP

SITE GEOLOGY

Geology

• Fill layer

• Alluvium layer

• Glacial Till layer

Hydrogeology

• Groundwater table 1.5 to 6.5 feet bgs

TARGET GROUNDWATER IMPACTS

Primary COCs

• Benzene up to 20,900 ug/L

• Phenol up to 12,800 ug/L

• Arsenic up to 31.2 ug/L

REMEDIAL DIAGNOSTIC TEST AREAS

Testing Methods

• SVE/Point Permeability

• Air Sparge/Helium Tracer

• Biorespiration

Parameters of Interest

• Air flow rate

• Pressure

• Vacuum

• Test results indicated favorable conditions (i.e., air flow rates and pressure distribution in the subsurface)

for air stripping

• Analytical model used for design and performance assessment of air sparging system

o Air Stripping (Treatment) Capacity Analysis for VOCs

o Sparge Air Flow Models

• Mathematical Description:

o Model based on well established Henry’s law constant and mass transfer coefficient

(aqueous to gaseous mass transfer) relationships

• Conventional sparging multi-level vertical wells modeled

• Mass transfer & VOC removal prediction

Sparging Trench Dimensions

5 to 10 ft

Where:

C L,e = COC concentration in reactor/trench effluent (ug/L), 20 ft

C L,i = COC concentration in reactor/trench influent (ug/L),

Qg = Gas or air flow rate (ft3/day),

QL = Liquid or groundwater flow rate per unit length (ft3/day),

Hc = Henry’s law constant (unitless), and Groundwater Flow

φ = Saturation parameter

Where:

K(La)COC = Mass transfer coefficient for COCs (1/day), and

V = Volume of reactor per unit length/porosity (ft3).

PNEUMATIC MODELING

• Approach – MDFIT™ Pneumatic Software

o Simulated air flow field in subsurface

o Determine design parameters

• Outputs

o Ki, ROI, FD , PV exchanges

o Horizontal SVE Well Design with an Engineered Surface Seal / Upper

Confining Layer for effective vapor capture

• Benefits

o More cost-effective design

o Valuable tool for SVE, VI Mitigation & Air Sparging design

Contaminant’s Henry’s Law Constant Required for

Effective Stripping > 1x 10-5 atm.m3/mol

TEST RESULTS AND ASSESSMENT

Integrated Remedial Strategy

• Phase I – Air Sparging and Vapor Capture

• Phase II – In-Situ Chemical Oxidation (Select Areas, Potential Residual NAPL)

• Phase III – Chemical Reduction (Contingency for Potential Dissolved Chromium)

Phase I – Conventional Air Sparging Approach Air sparging to treat impacted groundwater and vapor capture via SVE

• VOCs / BTEX

o Volatilization / Stripping o Aerobic biodegradation

• PHARMACEUTICALS (Phenol) and ALCOHOLS (Ethanol)

o Aerobic biodegradation

• METALS (Arsenic) o Arsenic - Precipitation and sorption via sparging

Air Sparging Component

• 53 multilevel AS wells spaced ~25 ft. on center, ROI = 10-15 ft., 10 to 20 SCFM design air flow rate per

sparge well screen, on average

• 17 passive venting wells / 11 chimney wells

• Air compressors capable of 350 SCFM air flow rate at 14 PSI pressure and 475 SCFM air flow rate at 21 PSI

• Pulsing strategy

Vapor Capture SVE Component

• 41 shallow horizontal SVE well screens spaced 30 ft. on center, ROI = 15 ft., screen lengths up to 15 ft., 30

SCFM design air flow rate per well, on average

• Vacuum blowers capable of 1,300 SCFM air flow rate at 4 inches of mercury (“Hg) vacuum – blowers in

parallel

• Engineered surface seal / upper confining layer (Geomembrane Liner)

• Vapor phase activated carbon for off-gas treatment

• Liquid phase activated carbon and dry wells for condensate water recirculation

• Expedited Remedial Timeframe

• System Construction scheduled to begin in Spring/Summer 2014

• System Start-up anticipated in Fall 2014

FULL-SCALE REMEDIAL DESIGN

Design Considerations

• Leaky Confining Layer

• Low Permeable Vadose Zone

• Shallow Water Table

Design Considerations

• Mass Removal Rate of COC’s

• Target Treatment Interval