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CHEMICAL ENGINEERINGTRANSACTIONS

VOL. 49, 2016

A publication of

The Italian Association
of Chemical Engineering
Online at www.aidic.it/cet

Guest Editors:Enrico Bardone, Marco Bravi, Tajalli Keshavarz
Copyright © 2016, AIDIC Servizi S.r.l.,
ISBN978-88-95608-40-2; ISSN 2283-9216

First Pilot Test on the Integration of GCW (Groundwater
Circulation Well) with ENA (Enhanced Natural Attenuation) for

Chlorinated Solvents Source Remediation.

Marco Petrangeli Papini*a, Mauro Majonea, Firoozeh Arjmanda, Daniele Silvestria,
Marco Sagliaschib, Salvatore Sucatob, Eduard Alesic, Ernst Barstchc, Lucia Pierroa
aDepartement of Chemistry,University of Rome “La Sapienza”, Rome, Italy
bEDF-Fenice SpA, Torino, Italy
cIEG Technologie GmbH, Gruibingen, Germany
marco.petrangelipapini@uniroma1.it

The remediation of aged source zone affected by residual chlorinated aliphatic hydrocarbons (CAHs)
represents one of the main challenges in contaminated aquifers. Groundwater Circulation Wells (GCWs) could
be considered a strategy for the progressive source zone remediation; this in situ remediation technology is
designed to create an 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.
The pressure gradient between the two hydraulically separated screen sections in the well induces a
circulation flow in the aquifer forcing water through less permeable layer where usually CAHs residual source
are located. The groundwater moves through the treatment zone both horizontally and vertically and as a
consequence the low permeable layer is constantly penetrated by the vertical flow of the GCWs. We tested
the possibility to use GCW to enhance in-situ bioremediation (ISB) in an operative industrial site heavily
contaminated by different chlorinated solvents (at concentration up to 100 mg/L) in a complex hydrogeological
saturated zone. A 30 mts deep GCW, with three screen sections, was designed and installed at the site for a
pilot testing. Groundwater is pumped towards two screen sections of the GCW and is reinjected into the
aquifer by another screen section after passing through an external unit treatment. External treatment unit is
composed of a sand filter tank and two reactors: one reactor was filled with a biodegradable polymer (poly-
hydroxy-butyrrate, PHB) and the other one with a mixture of zero-valent iron (ZVI) and PHB. Results from the
first eight months of operation clearly demonstrated how groundwater recirculation through the PHB reactor
allowed delivering continuously electron donors in the contaminated aquifer enhancing the mobilization of
CAHs and stimulating Natural Attenuation biological processes.

1. Background/Objectives
Chlorinated aliphatic hydrocarbons (CAHs) are the most common source of DNAPLs (Dense Non-Aqueous
Phase Liquids) contamination [McCarty, 2010: Pankow and Cherry, 1994]. Due to their higher density respect
to water DNAPLs tend to migrate downward through the unsaturated zone and often accumulate in low
permeable layers creating DNAPLs source zones/pools [Schwille, 1988; Anderson et al., 1992]. Traditional
technologies, such as Pump and Treat (P&T), are usually adopted to control/manage the contaminated
persistent plumes that develop from DNAPL pools, which act as a long-term source of groundwater
contamination. On the other hand, source zone treatment is advantageous over plume management or
containment strategies in that it effectively reduces the mass of contaminant. In Situ Bioremediation (ISB) is
one of the effective removal approaches on active secondary sources [ITRC 2005]. However, ISB for a source
zone removal should face several limitations [ITRC, 2008]. Effective delivery and distribution of electron
donors other than bioavailability of contaminants in heterogeneous aquifers are some of the primary limitations
in most hydrogeological settings. By this regards, Groundwater Circulation Well (GCW) was chosen as a more

DOI: 10.3303/CET1649016

Please cite this article as: Papini Petrangeli M., Majone M., Arjmand F., Silvestri D. , Sagliaschi M., Sucato S., Alesi E., 2016, First pilot test on
integration of gcw (groundwater circulation well) with ena (enhanced natural attenuation) for chlorinated solvents source remediation,
Chemical Engineering Transactions, 49, 91-96 DOI: 10.3303/CET1649016

appropriate remediation strategy to enhance ISB in an operative industrial site heavily contaminated by a wide
spectrum of CAHs located in the northern part of Italy.
A large source zone, probably characterized by a significant amount of residual DNAPL, has been identified
below operative industrial warehouses in a quite complex hydrogeological setting, from high permeable
transmissive zone to low permeable zone. As a consequence, even though an intensive pumping is active
since 9 years to avoid contaminant spreading outside the site, dissolved CAHs concentration still remain
significantly high. A typical specific site hydrogeological setting is reported in Figure 1; it shows a multiple-
layered heterogeneous aquifer which consists of materials that vary in their water-transmitting properties
laterally and vertically (fine to middle sands with intercalation of less permeable sandy silts to clayey silts
layers with permeability in range of 10-7-10-4 m/s). In this situation, trapped DNAPL in low k zones act as
continuous persistent source releasing contaminants in the more permeable layers by slow back diffusion;
contamination present at the site consists of quantities of pooled or trapped DNAPL-phase (hotspots) and
various dissolved plumes are generated from the residual phase.

Figure 1: A representative geological cross section across the main contamination area

Site characterization surveys also indicated that natural attenuation is already ongoing in the investigated site:
the presence of cis-DCE (cis-dichloroethene) and VC (vinyl chloride) indicates an intense but incomplete
microbial dechlorination activity, who had leaded the formation of more toxic compounds such as VC. Due to
the decreasing of the reductive dechlorination rate, under anaerobic conditions, an accumulation of cis-DCE or
VC is often observed at CAHs contaminated sites [Tiehm et al 2011]. Therefore, laboratory microcosm
experiments were set up to assess the possibility to significantly enhance the biodegradative potential of
natural microbiota with the addition of electron donors as well as the interactions of dechlorinating populations
with other microbial populations which can compete for the supplied electron donor [Fennel et al. 1997;
Aulenta et al.2002, Heimann et al 2007] and their effect on the dechlorination process in the subsurface.
Microcosms were also used to evaluate the applicability of PHB as carbon releasing source at the studied site.
PHB is a biopolymer produced (synthesized) by numerous microorganisms as an intracellular energy-storage
material under unbalanced growth conditions [Brennan et al 2006], such as limited essential nutrients or
excess carbon sources. This polymer was chosen as electron donors based on previous studies [Aulenta et
al, 2008; Baric et al 2012, 2014] conducted at the lab scale which clearly demonstrated the ability of PHB
fermentation products (volatile fatty acids, VFA and molecular hydrogen) to stimulate/sustain CAHs biological
reduction dechlorination. Based on the geophysical characterization of the site and the distribution of the
contamination, two locations were selected for saturated soil sampling for the microcosm investigation: one
close to piezometer (Pz 6) downgradient the hydraulic barrier at the outer boundary of the site and the other
one close to piezometer (Pz 14) inside an industrial warehouse close to the source area (metal industrial
washing machines). Results of microcosm study indicated how an approach based on the delivery of electron
donor in the area characterized by the presence of very high concentration of chlorinated solvents (source
zone) could be successful in quantitatively degrade the contaminants in harmless compounds such as ethene
and ethane (Figure2).

Figure2: Reductive dehalogenation of chlorinated solvents and acetic acid in anaerobic reactors amended with
PHB. Error bars indicate the difference between two duplicate microcosms.

Based on the satisfactory results of the laboratory investigations, indicating that reductive dechlorination might
be enhanced successfully at the considered site, the design of a pilot test was performed in order to select the
best remediation strategy. As well known, one of requirements for successful in situ bioremediation
implementation for chlorinated DNAPLs is the homogenous electron donor distribution. In this case, because
of geochemistry and hydrogeology of site, the conventional addition methods are not suitable; traditional
injection approaches are often limited by preferential migration of injected fluids through better permeable
zones, while delivery through less permeable and contaminated layers is usually limited. On the other hand,
Groundwater Circulation Well remediation technology allows creating in-situ vertical groundwater circulation
cells by drawing groundwater from an aquifer through one screened section of a multi-screened well and
discharging. The typical GCW configuration consists of installation of a single operating well with at least two
isolated screen section. Groundwater is extracted from one screen and after treatment (or amendment) it is
injected back into the aquifer through the second screen, creating a three-dimensional flow pattern (circulation
cell) in the aquifer. The pressure gradient between two hydraulically separated screen sections in the well
induces a circulation flow in the aquifer [Xiang and Kabala 1997, US EPA1998]. The groundwater moves
through the treatment zone both horizontally and vertically and as a consequence the low permeable layer is
constantly penetrated by the vertical flow of the GCW (Figure 3).

Vertical
Flow

Figure3: Schematic overview of an operating Groundwater Circulation Well

Source Zone (Pz 14) Site boundary (Pz 6)

P
H

B
: e

le
ct

ro
n
d

o
n

o
r

Time (d)

0 20 40 60 80 100

μM

120

150

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cis-DCE
VC

Eth

Time (d)

0 20 40 60 80 100

μM

30
cis-DCE
VC

Eth

Time (d)
0 20 40 60 80

A
ce

tic
A

ci
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[ μ
M

]

0,0

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1,0

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2,0

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3,0

3,5

Time (d)
0 20 40 60 80 100

A
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ci
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[ μ
M

]

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2. Pilot (Field) Test
On the basis of the geological and hydrogeological characteristics of the site, a 30 mts deep GCW was
designed in order to obtain an in-situ vertical groundwater circulation by drawing groundwater from two lower
screened sections (-24mts and -14mts) of a multi-screened well separated by packers and discharging it
through an upper screened section (-7mts). Thus, GCW worked under standard flow conditions: water flow
comes in through the lower and intermediate screen and out to the upper screen. As consequence, two
pumps were installed on the surface to extract groundwater from deep permeable layer and low permeability
intermediate layer, hereafter called pump 1 and pump 2, respectively. Following that, groundwater is pumped
upward inside an above-ground installed external treatment unit. Figure 4 illustrates the P&ID of the above-
ground set-up plant connected to the GCW. According to P&ID, nine different possibilities are made available
to change process and work under favorable condition.

Process n. 1: IEG-GCW® + Sand Vessel

Process n. 2: IEG-GCW® + Sand Vessel+ PHB Reactor

Process n. 3: IEG-GCW® + Sand Vessel+ ZVI Reactor

Process n. 4: IEG-GCW® + Sand Vessel+ PHB and ZVI Reactors

Process n. 5: Internal Recirculation PHB Reactor

Process n. 6: Internal Recirculation ZVI Reactor

Process n. 7: Internal Recirculation PHB and ZVI Reactors

Process n. 8: Wash in an up-flow mode Sand Vessel

Process n. 9: IEG-GCW® + Sand Vessel+ partial flow in PHB and ZVI Reactors

Figure 4: Piping and Instrumentation Diagram - P&ID, of the remediation plant with applicable processesand
schematic diagram of groundwater flow pattern due to use of GCW.

As shown in Figure 5, the external treatment unit is composed of a sand filter tank, a PHB reactor (where
fermentation takes place and dissolved VFA and H2 are produced) and a ZVI reactor (where CAHs are
partially removed before reinjection).

Figure 5: The external treatment unit.

3. Results
3.1 Enrichment of groundwater with PHB fermentation products
Figure 6 depicts volatile fatty acids (VFAs) effluent concentrations as function of operating time as measured
in the outlet of the PHB reactor and even in the outlet of the vessel where the groundwater that will be
reinjected into the aquifer is collected.

Time (d)

0 50 100 150 200

m
g

L-
1

500

1000

1500

2000

2500

3000 B

Time (d)

0 50 100 150 200

m
g

L-
1

500

1000

1500

2000

2500

3000

Acetic Butyric VFAtot

Figure6: VFA effluent concentrations of PHB reactor (A) and vessel (B)

Groundwater pumped from deep (-25m) and intermediate screen (-14m) was enriched with products from the
continuous fermentation of PHB. Tuning the flow rate groundwater through PHB reactor allowed optimizing the
concentration of the produced dissolved electron donors and the spreading of biostimulants in the subsurface
can be varied accordingly.

3.2 Enhancing CAHs mobilization
Groundwater recirculation through PHB reactor allowed a continuous delivering of the PHB fermentation
products in the contaminated aquifer also in the low permeable layer due to the vertical flow induced by the
GCW installation. The field results clearly demonstrated that mobilization of chlorinated
aliphatic hydrocarbons was strongly enhanced (Figure 7).

Time (d)

0 50 100 150 200 250

m
g

L-
1

cis-Dichloroethene
Vinyl chloride

Time (d)

0 50 100 150 200 250

m
g

L-
1

Figure7: CAHs concentration in the groundwater extracted from the lower (A) and intermediate (B) screen

The CAHs mobilization has been proven by analysing the mass of 1,2-DCE and VC extracted during the
operation time (Table1).

Table 1: CAH mass extracted during the field test

deep permeable layer low permeability intermediate layer

Extracted volume [m3] 4200 25

cis-DCE (g) 210 200
VC (g) 84 87.5

It is worth of noting that the calculated extracted amounts for both contaminants are very similar, but the
relative volume at the two examined depths is strongly greater for the deep screen (-24 mts) than the
intermediate screen (-14 mts).

4. Conclusions
The installed GCW combined with the external treatment unit allowed the effective amendment of groundwater
with the PHB fermentation products. The combined effect of groundwater recirculation with VFA production
(probably by a co-solvent effect) clearly enhanced the mobilization of pollutants from less accessible low
permeable zone where a significant mass of contaminants is strongly retained. By this regard the traditional
Pump & Treat approach could allow the removal of contaminants mostly from the more transmissive zones of
an aquifer but it has a negligible effect on the contaminant mass stored in low permeability media which are
released out slowly by back-diffusion. Moreover, coupling GCW with the continuous production of electron
donors should allow the enhancement of the biological reductive dechlorination inside the less permeable
layer thus potentially reducing the remediation time.

Reference

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