Depending on subsoil conditions (geology and hydrogeology) and contaminant type and concentration, remediation of sites  can take several decades. Consequently, "Pump&Treat" technique is considered as being suitable and effective only for containment to prevent further spreading of contaminants, but is no longer considered as a remediation method. With the "Pump & Treat" method groundwater is radially extracted out of the aquifer. Due to a change in the hydraulic gradient, both contaminated and clean groundwaterflows through the contaminated subsoil and is constantly treated above ground with considerable technical effort. GCW systems are designed to create in-situ vertical groundwater circulation cells by drawing groundwater from an aquifer through one screened section of a multi-screened well and discharging it through another screened section. If a circulation flow is generated in the aquifer, the treated groundwater is circulated several times in the aquifer before it flows downstream. This guarantees a considerably more efficient course of remediation compared with the "Pump&Treat" method.
Fine-grained, silty or clayey layers and lenticular intercalations are typical for many quaternary sedimentations, where horizontal flow can circumvent at the borders, but since there is a clearly defined vertical hydraulic gradient, a forcing flow through less permeable formation lenses develops. Thus, contaminants can be mobilized effectively and consequently the remediation period can be shortened. Even in low yielding aquifers the employment of GCW can be advantageous. The scientific calculation methods for the hydraulic functioning of GCW-Systems with pump and packer are developed by the Institute for Hydromechanics, University of Karlsruhe, under a research program funded by IEG. Under university oversight, an objective evaluation of the system was executed within the scope of large-scale model experiments in the research institution VEGAS and at the research site Knielingen by the Universities of Stuttgart and Karlsruhe. Research projects, comprehensive scientific modellings, hydraulic tests and tracer experiments prove the effectiveness of GCW at numerous remediation sites. Practical experience with GCW systems has been present for more than 20 years.
The GCW technology was developed in a way that it can be continuously operated and requires little maintenance only. Infiltration wells, drain pipes into discharge systems or sewer and discharge fees are not necessary. The investment costs of the systems are within the limits of conventional remediation techniques. Compared with conventional remediation methods, comparatively low operating costs and simultaneous reduction of the remediation period finally allow for considerable cost savings with the employment of circulation techniques. Corresponding with the remediation conception, one or several vertical wells with at least two screen sections are installed in an aquifer. In case of two aquifers, separate GCW-systems are installed. The direction of rotation of the circulation flow can be changed by using a two-pump system The circulation flow can be adapted to both the distribution of contaminants and the remediation progress. Groundwater circulation commonly occurs from the top of the formation to the bottom (herein termed “standard flowâ€). Under standard flow conditions, groundwater is pumped upward inside the remediation well as it enters a lower screen section and exits 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. Using a pump offers the advantage that the water/air ratio can be controlled and the stripping performance can thus be adjusted and an internal well short circuit can be avoided at the same time. The circulation cell flow path thus encompasses groundwater flowing from the upper part of the treatment zone into the lower part. Also areas with low penetrability are intensively penetrated horizontally and vertically. 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. 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 downgradient.
As such, water serves as the in-situ carrier bringing constituents of interest (COI) 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 thus improving COI mobilization by forcing flow through less permeable formation lenses. With natural groundwater flow, the total amount of water circulating around a GCW will consist of upgradient groundwater being captured, groundwater being recirculated and groundwater of downgradient release zone following treatment.Â
 General flow model of a GCW ("Reverse Circulation") modified after Mohrlok et al 2003
The flow dynamics and the dimensions of the upgradient areas ("Capture Zone"), of the circulation cell (("Circulation Cell(") as well as of the downgradient area ("Release Zone") are used for remediation planning. These dimensions and the circulation time depending on the distance from the GCW can be calculated for a specific site and used as a design tool based on numerical simulations of the groundwater hydraulics.
Capture zone, Release zone and Circulation cell of a Standard GCW-System and a Reverse GCW-System
An important precondition for reaching the determined official cleanup concentrations in the aquifer is sufficient penetration of the higher contaminated fine-grained zones, like sandy, silty or clayey layers and lenticular intercalations.
For a reliable prognosis of the remediation periods with the Groundwater Circulation System, apart from a precise exploration of the geological conditions, the spatial extension of the contamination area, and an approximate evaluation of the mass of contaminants is required. A decrease of up to 90 % of the original concentration of contaminants in the groundwater is reached by means of several complete circulation passages. Reaching official cleanup con-centrations in the µg/l range can take several years.
Pathlines in a Vertical Cross Section through a GCW in an aquifer (HERRLING & STAMM 1992) a. Without natural groundwater flow
b. Groundwater flow v=0,1m/d
c. Layer aquifer without natural groundwater flow
d. Layer aquifer with groundwater flow v=0,1m/d
The achievable sphere of influence of a GCW-System depends on the distance of the two active screen sections, the anisotropy of the aquifer, the groundwater flow velocity and the selected pumping rate. The bigger the circulation cell is, the longer a water molecule needs to pass through the cell. With huge aquifers it turns out to be advantageous to arrange several circulation cells (multiple circuits, "stacked circulation") on top of each other. Thus, smaller circulation cells with shorter flow periods are formed.
The application of such multiple-screened GCWs can also be advantageous if the contamination is restricted to certain aquifer sections or if the hydro-chemical characteristics of the groundwater change with the depth of the aquifer, thus requiring dedicated units vertically and horizontally.
POTENTIAL ADVANTAGES OF GCW TECHNOLOGY:
• Immediate containment and remediation of dissolved phase plume constituents via in-well stripping;
• Effective in-situ treatment without groundwater removal from the subsurface;
• Only GCW technology that can operate effectively under confined aquifer conditions;
• Only GCW technology that can operate in a reverse-flow and stacked cell mode ;
• Uniquely able to regulate the water/air ratio to yield efficient COI removal;
• Accelerated mass removal to reduce treatment time and accelerate site closure;
• Simultaneous in-situ treatment of vadose zone, capillary fringe and groundwater;
• Minimally invasive and non-disruptive to site conditions;Â
• Enhanced groundwater treatment due to the ability of the GCW system to create vertical and horizontal components of groundwater flow;
• Stimulation of natural attenuation processes (aerobic or anaerobic) for more rapid treatment of downgradient plume COI;
• Effective mechanism for adding nutrients and oxygen for stimulating bioremediation Â
• Very low energy requirements;
• Low operation and monitoring requirements
• Demonstrated effectiveness at over 500 related sites;
• Systems can be modified such that there is no aeration and the in-situ groundwater circulation serves to mix and distribute amendments to enhance reductive dechlorination processes.
• 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 the case where anaerobic conditions are desired (e.g. reductive dechlorination of solvents), the stripping can be done under closed-loop conditions so that no air is introduced (except in the early stage of start up).