Modeling Efficienza GCW

Ground Water Circulation Wells (GCWs) can be used in conjunction with other in situ remediation technologies to treat halogenated VOCs, semi-VOCs (SVOCs), pesticides, and petroleum products and their constituents such as benzene, toluene, ethyl benzene, and xylene (BTEX).

They have been applied to a wide range of soil types, from fine silty clay to coarse sandy gravel. With at least two screen sections, GCWs are universally applicable remediation tools. They can be employed in several configurations, such as in well stripping (IWS), bioaugmentation, enhanced natural attenuation adding nutrients and/or electron acceptors for stimulating bioremediation processes, bioventing, soil vapour extraction, reactive nanoparticle dehalogenation, in situ denitrification and chemical oxidation (ISCO) or reduction (ISCR). They may also be combined with a LNAPL/DNAPL recovery system in the aquifer.

Different well configuration and/or composition allows wide and versatile solutions, e.g. the vPRB (in situ Virtual Permeable Reactive Barrier) where several vertical circulation wells are arranged in one line perpendicular to the natural groundwater flow to obtain a curtain of overlapped circulation cells; such configuration could be promising to treat huge plumes generated by contaminated mega-sites.

Groundwater Circulation Well systems are designed to create in situ vertical groundwater circulation cells by drawing groundwater from an aquifer through one screen section of a double-screened well, and discharging it through the second screen section. The pressure gradient between two hydraulically separated screen sections in the well induces a circulation flow in the aquifer. The groundwater moves through the treatment zone both horizontally and vertically before entering the influent screen.

Groundwater circulation commonly occurs from the top of the formation to the bottom (herein termed "standard flow"). Under standard flow conditions, groundwater is pumped upward inside the remediation well as it enters a lower screen section and exits via an upper screen section. Groundwater flow upward through the GCW can be achieved via an airlift effect, or it can be induced via a submersible, in-well groundwater circulation pump (use of a pump offers control on the air/water ratio which can be important in regulating efficiency of stripping). The circulation cell flow path thus encompasses groundwater flowing from the upper part of the treatment zone into the lower part.

General Flow Schematic of a Reverse-Flow GCW-Type System (Mohrlok 2003)

In a reverse circulation mode, the flow of groundwater within the GCW well is downward via the aid of an in-well groundwater pump (i.e., water flows from the bottom of the aquifer formation in a torroidal upward pattern). In the reverse circulation mode, water in the lower half of the aquifer moves away from the well while water in the upper half of the aquifer moves toward the well.

In both the standard and reverse flow modes of operation, groundwater is circulated around the central GCW, but none is removed from the aquifer without any draw down. Induced differences in potentiometric head establish and maintain the 3-dimensional circulation cell in an ellipsoidal area around the circulation well. The majority of the groundwater captured by the circulation cell circulates a number of times through the GCW before being released down-gradient. As such, water serves as the in situ carrier bringing components of interest from throughout the capture zone to the GCW system where it is treated and then discharged back into the formation. The vertical and horizontal circulation flow patterns force water to move through the entire aquifer portion within the circulation cell by forcing flow through less permeable formation lenses. Remediation time is faster because the vertical flow forces the groundwater to flow perpenticular to low permeable zones, thereby enhancing the mobilization of contaminants

These flow dynamics and dimensions of the capture zone, circulation cell, and release zone can be calculated for a specific site and used as design aids based on numerical simulations of the groundwater hydraulics . However, site-specific calibration of the modelled value should be conducted during a pilot test prior to the final design and engineering of a remedial system composed of multiple, overlapping GCW systems.

The treatment component of the GCW combines a series of biotic and abiotic processes to affect removal or destruction of site specific COI (Components of Interest):

• In situ soil flushing: Movement of groundwater in a circular mode is realized through vertical groundwater circulation. The circulating groundwater constantly transports the newly dissolved COI to the well, wherein they are removed. Clean groundwater is released back into the aquifer to be recycled or to exit the cell downstream. The radius of influence of the groundwater circulation cell is very site specific, but has been shown to range between 2- and 5-times the thickness of the saturated zone being treated.

• Physical stripping: In-well aeration yields in situ stripping of VOCs from groundwater. Physical stripping is achieved under vacuum and can be done in situ or in on-site reactors. GCW systems are offer very effective negative pressure stripping efficiency for even those COI that are relatively difficult to strip.

• Accelerated biodegradation: In-well aeration also results in the addition of oxygen to the groundwater that is returned to the aquifer and circulated throughout the formation. Combined with the overall mixing effect, this serves to enhance the rate and extent of in situ, aerobic biodegradation of susceptible organic COI.

• In situ precipitation: Dissolved oxygen constantly re-infiltrated back into the aquifer create an aeration zone. Subsequently iron will be precipitated in the sphere of influence of the GCW.