First full-scale test for reductive dechlorination of residual DNAPLs in a 30 m heterogeneous aquifer via Groundwater Circulation Well (GCW)

M.Petrangeli Papini (1)

, L. Pierro (1)

, M. Majone (1)

, M. Sagliaschi (2)

, S. Sucato (2)

, E. Alesi (3)

, E. Barstch (3)

, B.Matturro(4)

, S. Rossetti4)

Introduction Pilot plant design and installation

(1) University of Rome “La Sapienza”,

(2) FENICE - EDF Spa,

(3) IEG GmbH Technologie,

(4) IRSA-CNR

A new approach for the remediation of Dense Non Aquoeus Phase Liquid (DNAPL) aged source zone has

been verified through a pilot test carried out at an operative industrial site located at north Italy, heavily con-

taminated by Chlorinated Aliphatic Hydrocarbons (CAHs).

Lab Scale Treatability: microcosm studies

Laboratory microcosm experiments were set up

to assess the possibility to enhance the biodegra-

dative potential of natural microbiota until com-

plete dechlorination with the addition of electron

donors. Among the different tested electron do-

nors, poly-hydroxy-butyrrate (PHB), a biode-

gradable polymer easily fermented to volatile fat-

ty acids and molecular hydrogen, have been ex-

perimentally verified as effective in stimulating bi-

ological RD at the investigated site.

Groundwater Circulation Well

A 30 mt GCW equipped with an external treatment unit was designed and installed at the site

GCW was screened at three different depths separated by packers (first permeable layer, low permea-

bility intermediate layer, and deep permeable layer). Groundwater is drawn out from the lower and inter-

mediate screen and, after passing through the “external treatment unit”, is reinjected by the upper

screen

The “external treatment unit” is composed of a sand filter tank, a PHB/sand reactor (where fermenta-

tion takes place and dissolved VFA and H2 are produced into the flux of groundwater), a ZVI/PHB reac-

tor (where CAH are partially removed before reinjection) and a collecting tank (where the treated

groundwater is collected prior to discharge back into the aquifer

2 Multilevel well systems (MLWS) were installed inside the radius of influence of the GCW. Each well al-

lows to collect groundwater samples at 5 different depths.

Field test results

1. The vertical flow induced by the GCW installation through the less permeable layer strongly enhanced the mobilization of CAHs thereby shortening the source zone remediation time.

2. The external treatment unit allowed to effectively reduce contaminant concentration before reinjection (reductive dechlorination by zero valent iron) without generation of external streams.

3. PHB acts as a effective slow releasing source of electron donors and carbon source for bio-logical reductive dechlorination process,

4. GCW allowed the effective distribution of electron donors in the extremely complex hydroge-ological setting. The distribution of the electron donors in the contaminated aquifer clearly stimulated the dechlorination biological activity as indicated by the significant increase in the abundance of dechlorinating native microorganisms.

5. The enrichment of Dehalococcoides strains involved in the metabolic dechlorination of cis-DCE and VC to ethene was observed overtime.

High Residual CAH

Slow back diffusion in the

more permeable layers

PNS2

PNS1

reinjection

External treatment

unit

Extraction

injection

Site characterization confirmed a complex hydroge-

ological situation (fine to middle sands with interca-

lation of less permeable sandy silts to clayey silts

layers with permeabilities in range of 10-7

-10-4

m s-1

)

with the occurrence of residual CAHs in low perme-

ability layers which act as persistent slow-releasing

contamination secondary source, kinetically con-

trolled by slow back diffusion mechanisms.

Natural attenuation by biological reductive dechlorination (RD) has been recognized as active mechanism

by an extensive microbiological characterization and microcosm study, and electron donor availability has

been identified as limiting factor. The objective of the remediation is to enhance in situ bioremediation by

coupling Groundwater Circulation Wells (GCWs) with an electron donor continuous production system.

Time (d)

0 20 40 60 80 100

mo

l L

-1

0

30

60

90

120

150

180

cis - DCE

VC

Eth

ca. 13 mg L-1

ca. 2.5 mg L-1

Due to the complex stratigraphy of the aquifer, Groundwater Circulation Well (GCW) has been identi-

fied as a strategy to allow effective delivery and distribution of electron donors in the less permeable

layer and to mobilize the contaminants thus increasing their bioavailability.

GCW technology creates a three-dimensional

circulation cell in the aquifer. This circulation is

established by installing a multiple screened well

where a packer is inserted to hydraulically isola-

te the screen intervals. Groundwater is extracted

from one screen and after an external treatment

is circulated back into the aquifer through

another screen, thus creating the circulation cell.

The pressure gradient between the hydraulically

separated screen sections induces a circulation

flow in the aquifer forcing water through less

permeable layer. MLWS2 7.6 m

MLWS2 11.6 m

MLWS2 16.6 m

MLWS2 20.6 m

MLWS2 24.6 m0,0

5,0e+7

1,0e+8

1,5e+8

2,0e+8

2,5e+8

MLWS1 7.6 m

MLWS1 11.6 m

MLWS1 16.6 m

MLWS1 20.6 m

MLWS1 24.6 m16S

rR

NA

Dh

c m

cc

art

y g

en

e c

op

ies

L-1

0,0

2,0e+8

4,0e+8

6,0e+8

8,0e+8

1,0e+9

1,2e+92 weeks after plant start-up

4 months after palnt operation

16 months after palnt operation

2 weeks after plant start-up

MLWS2 7.6 m

MLWS2 11.6 m

MLWS2 16.6 m

MLWS2 20.6 m

MLWS2 24.6 m

ge

ne

co

pie

s L

-1

0,0

5,0e+6

1,0e+7

1,5e+7

2,0e+7

2,5e+7

3,0e+716 months after plant operation

MLWS2 7.6 m

MLWS2 11.6 m

MLWS2 16.6 m

MLWS2 20.6 m

MLWS2 24.6 m

tceA

vcrA

bvcrA

tceA

vcrA

bvcrA

4 months after plant operation

MLWS2 7.6 m

MLWS2 11.6 m

MLWS2 16.6 m

MLWS2 20.6 m

MLWS2 24.6 m

PHB fermentation pathway