management of napl contaminated sediments with engineered caps – how cap layers may or may not...
DESCRIPTION
TRC engineer Katherine Vater describes the function and effectiveness of layered sediment caps. Her poster presentation was part of the Fifth International Symposium & Exhibition on the Redevelopment of Manufactured Gas Plant Sites (MGP 2014).TRANSCRIPT
GasVent
Erosion ControlLayer (Rip Rap)
ImpermeableContainment Layer
TransmissionLayer
NATIVE SOIL
Water Body
Accumulation Zone
SEDIMENT
VentedGas
NAPL Deposit
LNAPL
GasBubbles
NAPL Collection Sump
GENERALIZED NAPL TRAPPING CAP(U.S. Patents #8,419,314 and #8,651,768)
Engineered caps can be an effective and economical permanent remedy for non-aqueous phase liquid (NAPL)-contaminated sediments. On the other hand, incorrect selection of construction materials and/or under design for relevant transport mechanisms can lead to shortened cap life expectancy and even full-scale failure. Current science informs us that NAPL can move from sediments both in the dissolved phase, and attached to bubbles generated from biodegradation of organic matter in the sediment. Transport in the dissolved phase is generally at low concentrations and at relatively low velocities. Transport via ebullition occurs at high concentrations and at high velocities, relative to advective migration.
Engineered treatment caps can be designed to address both types of contaminant transport, but often are not. Sediment caps containing adsorbent media (e.g. organoclay) are effective at stopping transport of dissolved phase contaminants for a finite time. That same adsorbent media is also effective at stopping ebullition-driven NAPL transport, but for shorter periods of finite time. One possible reason for this reduced lifespan is observable.
Ebullition-driven NAPL transport occurs in vents, or focused bubble streams, transporting globules of NAPL, which eventually overwhelms the adsorbing capacity of a treatment media at a vent location. An engineered cap can be designed to control the ebullition-driven transport mechanism by redirecting gas flow. When designed for, any NAPL transported by gas is permanently separated from the overlying water because gas vents from distinct points of the cap where NAPL cannot pass into the surface water.
Sediment cap design features can be combined to address both dissolved and ebullition-driven transport as demonstrated in the full-scale applications constructed to date, and illustrated here. These designs are covered by U.S. Patent # 8,419,314 and # 8,651,768.
Installation of NAPLcollection sump, sump detail
Above: Post-restoration shoreline on Huron River
Above: Capping in progress - Transmission layer is being laid down inside a double turbidity curtain
Center: Placement (top) and precision
grading (bottom) of the AquablokTM
containment layer
Sheen seepage from river bank
Excavation of uplandcoal tar source, and sediment/bank soil
Management of NAPL Contaminated Sediments with Engineered Caps How Cap Layers May or May Not Prevent NAPL Releases
Katherine Vater, P.E.; Tom Stolzenburg, Ph.D.; Theodore O’Connell, P.G.; and John Rice, P.E., P.H.TRC Environmental Corporation: 708 Heartland Trail, Madison, WI 53717 - [email protected]; (608)826-3663