Non-Aqueous Phase Liquids - Michigan · 2017. 6. 2. · Non -Petroleum NAPL can affect the risk assessment assumptions, but the likelihood of finding NAPL (many types) is lower. p

Non-Aqueous Phase Liquids

What are they and how do we manage them?

Nick Swiger

Michigan Department of Environmental Quality (231) 876-4458 / [email protected]

Definitions for NAPL p NAPL – liquids that are immiscible or sparingly

soluble in water. n Typically characterized by their density and/or specific

gravity – dense (D) or light (L)

Definitions for NAPL

From ITRC, 2009

Definitions for NAPL p Residual NAPL – NAPL present in the soil/aquifer

however not at sufficient saturation to become mobile. NAPL that will not move under natural environmental gradients.

p Mobile NAPL – NAPL present in the soil/aquifer that is of sufficient saturation to have the potential to move. The NAPL has the ability to “flow” (e.g., in a well), but is not expanding.

p Migrating NAPL – Mobile NAPL that has a gradient and the NAPL body is expanding (e.g., footprint vertically, horizontally, and/or vertically is increasing)

Brief Overview p When NAPL is released you now have a 4 phase

system with a solid and three fluids (air/vapor, water, NAPL). n Pore spaces the fluids occupy depend on the system and

if the fluid is the wetting fluid or non wetting fluid.

Brief Overview Cont. p For NAPL to be mobile, it must be of sufficient

saturation to become interconnected and must displace fluid out of an adjacent soil pore.

Brief Overview Cont. p For NAPL to migrate, it must first be mobile and

must have sufficient head/gradient to “push” the edges; normally within a few years after a release is stopped. n Once a release is stopped, this is a self limiting process.

Brief Overview cont. - LNAPL p Even with buoyancy (less dense than water),

LNAPL vertical migration will not stop at water saturation. n NAPL will displace some water; therefore to adequately

characterize, you need to sample below the groundwater surface.

n Not a fine line.

LNAPL Saturation (% Pore Space)

Brief Overview cont. - DNAPL p DNAPL migration

n With sufficient mass there will be density driven vertical migration.

n Vertical migration will continue until there is a capillary barrier to prevent further migration.

Building the Conceptual Site Model – Characterization of the Site / NAPL p It is best to use multiple lines of evidence to

build the CSM. n Greater risk possibility = more detail

p For NAPL releases you have two “potential risks” to deal with – saturational and compositional

p Will need to have the compositional plumes and potential receptors / pathways delineated and depicted Thorough Characterization is the Key!

Types of Lines of Evidence p Two Main Types:

n Direct n Indirect

Direct Lines of Evidence p Provide definitive proof that NAPL is present. p Reasonably sure NAPL is absent, but all have

limitations.

Indirect Lines of Evidence p Generally will have to be used with other lines of evidence

(which can also be indirect).

p Will have to utilize professional judgment to infer the presence / absence of NAPL.

p We have calculated “screening values” that can be used with indirect lines of evidence.

n Csat for Solvents (PCE, TCE) n TPH for hydrocarbons (GRO, DRO)

p All have limitations

p Examples include: n Analytical data (Including TPH data) n PID n Olfactory

Conceptual Changes for NAPL Management p If a NAPL was released and not physically

removed it is likely still present.

p If sustained plumes are present at a site, NAPL is present.

p If observed – borings, sheens, in wells, etc., NAPL is present.

p CAUTION: n Just because you have a detection in subsurface does

not automatically mean NAPL is present. n Higher concentrations or the presence of NAPL does not

automatically mean an unacceptable risk is present.

NAPL Management p Big Picture – managing NAPL is about managing

RISKS. n Mobility Reductions n Toxicity Reductions n Compositional Reductions n Mass Removal n Exposure Control/Elimination

Risk Assessment and Management p Petroleum NAPL can affect the risk assessment

assumptions, but having some residual (or even mobile) NAPL at a site does not equate to unacceptable risk.

p Must evaluate and manage all risks: n Saturational risks n Compositional risks

Risk Assessment p Non -Petroleum NAPL can affect the risk

assessment assumptions, but the likelihood of finding NAPL (many types) is lower.

p Should be treated more like a single “system” with source and plume: n Saturational risks – less important n Compositional risks – more important n Back diffusion - an important

source for compositional plumes

NAPL Recovery p Need for recovery of all NAPL is directly tied to risks

p For non-petroleum, it is better to treat the NAPL and plumes as a system n Source and Plume

p For petroleum, the saturational risks can be separated and recovery evaluated with multiple factors.

NAPL Recovery (Petroleum) p Some misconceptions about recovery and recovery

analysis – it should not just be hydraulic recovery of mobile NAPL

p Risks will dictate the need for recovery and/or remediation. n Encourage recovery if easily removed. n Using NAPL transmissivity (Tn) of 0.5 ft2/day as a

metric.

Summary p If NAPL was released and not physically removed,

it is likely present.

p Lines of evidence will be used to determine the presence/absence of NAPL and build the CSM

p Assess and manage the risk – if NAPL presence doesn’t really change the risk assessment, the “precision” can be less.

p Use the risk assessment to guide the need for NAPL treatment and recovery.

p Can close sites with NAPL left in place.

Questions or Comments?

Nick Swiger [email protected]

(231) 876-4458

Michigan Department of Environmental Quality

800-662-9278 www.michigan.gov/deq

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