First results of operating and monitoring an innovative design of a permeable reactive barrier for the remediation of chromate contaminated groundwater

Applied Geochemistry 24 (2009) 687–696

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Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem

First results of operating and monitoring an innovative design of a permeable reactive barrier for the remediation of chromate contaminated groundwater Bettina Flury *, Urs Eggenberger, Urs Mäder Rock–Water-Interaction Group, Institute of Geological Sciences, University of Bern, Baltzerstrasse 3, 3007 Bern, Switzerland

a r t i c l e

i n f o

Article history: Available online 24 December 2008

a b s t r a c t An innovative setup of a permeable reactive barrier (PRB) was installed in Willisau, Switzerland to remediate chromate contaminated groundwater. Instead of a conventional continuous barrier, this PRB consists of cylinders installed in rows: a single row for lower expected CrVI-concentrations and an offset double row for higher expected CrVI-concentrations. The cylinders are filled with reactive grey cast-Fe shavings mixed with gravel to prevent extensive precipitation of secondary phases in the pore space. The treatment of the contaminants takes place both within the cylinders and in the dissolved FeII plume generated downstream of the barrier. Monitoring of the contamination situation over a period of 3 a provided evidence of the mobilization, transport and behavior of the contaminants in the aquifer. Groundwater and reactive material were sampled upstream, within and downstream of the barrier by a Multi-Port Sampling System (MPSS) that revealed the geochemical processes as a function of time and space. Comprehensive chemical analyses included sensitive parameters such as CrVI, FeII/FeIII, redox potential, dissolved O2 and pH. Several campaigns using multiple optical tracers revealed a rather complex hydrological regime at different scales, thereby complicating the barrier performance. Results from the large 3D hydrogeochemical dataset show that the double row of cylinders successfully treated the chromate contamination. Remediation by the single row was not effective enough due to insufficient lateral overlap of the cylinders and their FeII-plumes. The low amount of precipitated secondary phases observed in the pore space of the reactive material reduced the risk of clogging the system and suggested a favorable longevity of the barrier. Limiting factors for the long-term operation are inferred to be the availability and accessibility of FeII within the cylinders and the concentration within the generated FeII-plume. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Chromium compounds have been used in dyes and paints, for the tanning of leather, in chemical processing, galvanic industries and wood impregnation (EPA, 1997). They are often found in soil and groundwater of former industrial sites, which now require remediation measures according to environmental regulations. Chromium exists in various oxidation states, but only the trivalent and hexavalent oxidation states are of major environmental concern due to their stability in the natural environment. The dominant species of hexavalent Cr are chromate (CrVI O2 4 ) at pH 6.2–10.7 and HCrVI O 4 at pH <6.2 (Palmer and Wittbrodt, 1991). Hexavalent Cr is highly soluble and mobile and is very toxic with mutagenic and carcinogenic effects. In contrast, Cr in its trivalent oxidation state is far less soluble and nearly immobile and is required in trace amounts for human metabolism. Remediation of CrO2 4 contamination in groundwater is realized with different approaches that are mainly based on active methods * Corresponding author. Fax: +41 31 631 48 43. E-mail address: bettina.fl[email protected] (B. Flury). 0883-2927/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.12.020

such as pump-and-treat techniques (treatment of contaminated water pumped to the surface). Recently, passive in situ methods have become more widely accepted as cost-effective and sustainable solutions (EPA, 2002a; Naftz et al., 2002; Roehl et al., 2005). Permeable reactive barriers (PRBs) represent one of the new passive remediation methods that have great potential to replace conventional treatment procedures. In a PRB, reactive material might be gradually consumed to immobilize the dissolved contaminant or convert it into a non-toxic form (Naftz et al., 2002). contamination at pilot or full Some PRBs are treating CrO2 4 installations in the USA and Europe. These sites provide valuable information regarding the applied materials and the setup of the barrier (EPA, 2002b, 2003; Wilkin et al., 2005). First results regarding effectiveness and longevity of the treatment zones are now available after some years of operation (Burmeier et al., 2006; ESTCP, 2003; ITRC, 2005; Simon et al., 2003). Common problems relate to the reduction of the pore space of the reactive material by various precipitates followed by a blocking and eventual hydraulic failure of the barrier. Barriers having such difficulties are often implemented as continuous barriers or as a funnel-and-gate design.

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The pilot installation in Willisau, Switzerland represents a different type of PRB (IGT et al., 2003a,b; Köhler, 2003). The innovative design of a row of cylinders combined with the use of a mixture of gravel and grey-cast Fe shavings as reactive material reduces hydraulic interference of the aquifer and reduces installation costs (Köhler, 2003). This study provides first results on the effectiveness of the barrier and discusses advantages and disadvantages of the design. The findings are based on tracer experiments and a comprehensive geochemical dataset of analyses of groundwater and reactive material obtained during an extensive 3 a-monitoring of the barrier in operation.

2. Site characterization and barrier setup 2.1. Location and hydro-geological setting The surface geology at the Willisau site is composed of two different lithologic units: a fluviatile deposited gravel in the top 6–8 m consists of layers of clayey and silty sand with small to moderate permeability. These top layers overlie highly permeable sand and gravel layers which were preloaded by glacial ice during the last ice age stage (IGT et al., 2003a). Groundwater velocity was shown to be quite high (several meters per day, Köhler, 2003). The groundwater table is located at a depth of around 14 m below land surface and varies considerably within a range of approximately 4 m (from here on referred to as the ‘transition zone’). The aquifer (saturated zone) reaches the aquitard that consists of a highly consolidated moraine deposit at a depth of about 42 m below land surface. The groundwater chemistry is dominated by the carbonate system (Table 1) with a total alkalinity of 7 meq/L, nearneutral pH conditions (7.0–7.3), a redox potential of approximately 100–200 mV (relative to the standard hydrogen electrode (SHE)), and an electric conductivity in the range of 450–750 lS/cm. The dissolved O2 content is medium to high at the site (3.5–6 mg/L) and varies with aquifer depths (5–6 mg/L in the transition zone and 3.5–5 mg/L at lower depth). The remediation site is located in the water protection zone ‘Au’ according to the Swiss water protection ordinance (Swiss Confederation, 1991). Objectives and urgency of the remediation are dependent on the groundwater protection zone. Protection zone Au comprises exploitable groundwater bodies and is therefore highly vulnerable.

2.2. Contamination type and source The contamination originates from a wood impregnation factory that used a chromate-solution to preserve the timber from deterioration. The contamination input lasted for an estimated time span of 20 a and presumably ended in 1987 (IGT et al., 2001) when surface sealing and waste water treatment were implemented. Although the production sites are well known, localities of storage and potential disposal are most likely not all documented. The largest infiltration of contaminants occurred at one particular site where the impregnated wood was temporarily stacked to drain the chemicals and dry directly after pressure impregnation (from here on referred to as ‘hotspot’). Solutions containing CrVI, Cu and B seeped into the subsoil and were adsorbed in the unsaturated zone. Analyses of the subsoil suggest that a large amount of Cr compounds still remain in the unsaturated zone beneath the hotspot. Estimates of total Cr accumulated at a depth of 3–12 m are near 2000 kg (IGT et al., 2001). The contaminants are not equally vertically distributed in the unsaturated zone (Fig. 1). Contaminant loads of total Cr measured in the solid material show highest concentrations near the surface due to the input of immobile CrIII formed in the impregnation process. In contrast, analyses of CrVI-concentrations of the eluate of the solid material (determination by distilled water extraction using solid–liquid ratio 1:10, Swiss Confederation, 1990) show that values are highest between 3.3 and 11 m and decrease in the transition zone at a depth of 12–16 m. This distribution is explained by a constant mobilization of CrVI and transport by the groundwater percolating through the contaminated subsoil (IGT et al., 2003b). Groundwater downstream of the hotspot is contaminated with CrVI at concentrations in the range of 0–10 mg/L thereby often exceeding the Swiss critical limit of 0.01 mg/L for CrVI in groundwater (Swiss Confederation, 1998). 2.3. Construction, design and remediation principle of the PRB The PRB that was implemented in November 2003 (Fig. 2) was designed as a row of permeable reactive cylinders instead of a continuous barrier or a funnel-and-gate setup. This kind of ‘fence post’ setup was previously installed as a small-scale field test at the US Coast Guard air base near Elizabeth City, North Carolina (Puls et al., 1999) but has never been implemented as a full-scale remediation installation. The PRB Willisau consists of two different components

Concentration in the eluate [mg/L]

Table 1 Representative groundwater analysis of the remediation site. Values are compiled from analyses sampled in KB02/02 downgradient of the hotspot at a depth of 19 m below land surface. Groundwater chemistry does not vary up- and down-stream of the hotspot apart from CrVI-concentrations.

0

Parameter

Unit

Value

6

Electrical conductivity pH Redox potential (Eh) Dissolved oxygen (O2) HCO 3 (as total alkalinity) Ca2+ 2+ Mg Na+ K+ Cl NO 3 SO2 4 DOC

lS/cm

450–750 7.0–7.3 100–200 3–6 425 120–130 15–18 7.8–10.1 1.4–2.2 12–18 10.4–12.7 19–40 0.5

The parameters are measured by the following methods: electrical conductivity (conductometric), Eh (after DIN 38404), related to Ag/AgCl, O2 (oxygen electrode), HCO 3 (titrimetric), cations (ICP-OES), anions (IC) and DOC (wet chemical).

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

2 4

depth [m]

mV mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

0.0

8

Cr(VI) in eluate - photom. determination [mg/L] [mg/l] Cr(VI) in eluate - determination with AAS AAS [mg/l] [mg/L] Cr total - pulping w.nitrohydrochloric acid - det. w. AAS [ppm]

10 12 14 16 18 0

50

100

150

200

250

300

Total concentration (solid material) [ppm] Fig. 1. Distribution of Crtot and CrVI in the unsaturated zone (IGT, 2003a) from samples taken during installation of the sampling well KB02/02 4 m downgradient of the hotspot. Analyses of Crtot in the solid material were performed by AAS after pulping with nitrohydrochloric acid. CrVI-concentrations in the distilled-water eluate were determined either photometrically or with AAS.

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(Eqs. (1) and (2)) and possible production of H2-gas (Eq. (3)). The resulting FeII either appears in solution or precipitates as di- or tri-valent Fe oxides or hydroxides (Eary and Rai, 1987; Ebert, 1997) depending on the amount of available dissolved O2. According to (Noubactep, 2006) direct reduction of CrO2 4 with zerovalent Fe is not possible due to the instability of Fe0 and immediate corrosion on the surface of the Fe shavings. The reduction therefore takes place either in the oxide film on the surface of the shavings (indirect reduction) or with the dissolved FeII

Fe0 þ 0:5O2 þ 2Hþ $ Fe2þ þ H2 O 2þ

2Fe

3þ

þ

þ 0:5O2 þ 2H $ 2Fe

ð1Þ

þ H2 O

ð2Þ

Fe0 þ 2Hþ $ Fe2þ þ H2 3þ

Fe

þ Cr

III

þ

þ H2 O $ Fe Cr ðOHÞ3 ðsÞ þ 3H

ð4Þ ð5Þ

3Fe

(Fig. 2): a single row of cylinders for lower expected CrO2 4 concentrations and an offset double row of cylinders for higher expected CrO2 4 concentrations. This configuration provides further information regarding the optimal design of a PRB. The PRB is constructed as a so-called ‘hanging reactive’ barrier where the cylinders are not embedded in the aquitard but are terminated at a distance above the aquitard (Fig. 3). This construction is advantageous for setting up a reactive zone at a depth of 12–23 m where the construction of a continuous barrier would be difficult and very expensive because of the compacted gravels and the presence of boulders. Furthermore, the construction of a continuous barrier may require bentonitic slurries, reducing the permeability after installation. contamination is achieved by a The remediation of the CrO2 4 series of redox reactions summarized in Eqs. (1)–(6) (Blowes et al., 1997; Burmeier et al., 2006; Naftz et al., 2002) in which CrVI reacts with zerovalent Fe or dissolved Fe2+ according to Eqs. (5) and (6). Hexavalent Cr is reduced to CrIII and presumably precipitates as mixed CrIII/FeIII-hydroxides (Eq. (4)). Zerovalent Fe is not stable in water, which leads to corrosion of the Fe shavings and to subsequent formation of FeII accompanied by consumption of O2 and H+

ð3Þ III

þ 3þ Fe0 þ CrVI O2 þ Cr3þ þ 4H2 O 4 þ 8H $ Fe 2þ

Fig. 2. Map of the remediation site showing the hotspot, the location of the PRB and sampling localities. KB, cored drilling; RB, percussion drilling; ML, A, and B, multiport sampling system (MPSS). All sampling localities of the MPSS upstream, within and downstream of the barrier are shown in the insert. Line perpendicular to the barrier indicates the transect shown in Fig. 3. Equipotential lines of hydraulic head show flow direction to NW.

3þ

VI

þ Cr

O2 4

þ

þ 8H $ 3Fe

3þ

þ Cr

3þ

þ 4H2 O

ð6Þ

The following two principles are thought to be involved (Köhler, 2003): (1) chemical release of Fe2+-ions and reduction of CrO2 4 within the cylinders; (2) advective–dispersive transport of Fe2+ions, generation of a FeII-plume downstream of the barrier and reduction of CrO2 4 within the plume. The principle of generating a FeII-plume downstream of the barrier is also described at Elizabeth City (Wilkin et al., 2003) where FeII is released from the aquifer due to the decreased redox potentials in this region. The reactive material in the Willisau PRB is composed of a mixture of zerovalent Fe shavings and gravel in the ratio of 1:3 (by weight). This ratio ensures an initial permeability of the reactive material approximately three times larger than the surrounding sub-soil. The selected mixture was tested in laboratory column experiments before it was implemented in the barrier to investigate its CrO2 4 reduction capacity and clogging behavior (Köhler, 2003). 2.4. Installation of the cylinders The cylinders are 1.3 m in diameter and the gap between cylinders within a row is approximately 1 m. They were installed using large-scale cased drilling whereby the aquifer material was subsequently excavated by scooping. The cylinders were refilled with the reactive material. This was accomplished by a so-called contractor method to avoid a separation of the different components during subsidence of the material. The filling level of the reactive material within the cylinders is from a depth of 23–12 m below land surface. The uppermost section was refilled with excavation

Fig. 3. Profile across contamination source and double row of cylinders. The PRB is constructed as a ‘‘hanging” barrier. The MPSS (see Section 2.5) allows sampling of groundwater upgradient (ML1) and downgradient (ML4) of the barrier. Within the cylinders, groundwater and reactive material is sampled at four levels (numbers along cylinders and piezometers) in five localities (A1–A4, B1). To keep the illustration simple, the localities within the cylinders are only marked in the profile as one single element in the direct inflow of the cylinders. For a plan view with exact positions of A and B within the cylinders see Fig. 2.

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material after the casings were completely extracted. A total of 233 metric tons of reactive material with a volume of 150 m3 was emplaced with a packing density of 1.55 t/m3 (Köhler, 2003). 2.5. Monitoring of the PRB The monitoring of the PRB Willisau was performed using a sophisticated Multi-Port Sampling System (MPSS) that was developed by the Swiss company SOLEXPERTS. The individual sampling of groundwater and reactive material at different depths at several localities provides a 3D-resolution of the hydro-geochemical processes in the barrier (Figs. 2 and 3). Upgradient of the barrier, the multi-level sampling wells ML1 (in front of double row) and ML2 (single row) were installed at a distance of 2 m to the barrier. Downgradient of the barrier, the multi-level sampling wells ML3 (single row) and ML4 (double row) were installed at a distance of 4 m to the barrier. ML1 and ML2 reach only as deep as the cylinders to exclude vertical down-flow of contaminated water inside the piezometers and subsequent contaminant transport underneath the barrier. Individual 2.5 cm (1 in.)-diameter piezometers equipped with filter sections access three sampling and measuring depths (numbered in Fig. 3). ML3 and ML4 reach the aquitard and are equipped with five piezometers with filter sections at different depths. Within the barrier, two cylinders of the double row are equipped with piezometer bundles to sample four levels of depth at different positions within the cylinders. In the cylinder of the front row, A1 is installed in the frontal and A4 in the lateral inflow region, A2 in the centre, and A3 in the outflow. In the cylinder of the back row, B1 is installed in the frontal inflow region. Each of the MPSS piezometer bundles also contains a large-diameter piezometer for sampling the reactive material (details in Section 3.2). The system was developed for multiple sampling and therefore is a low-cost method to sample reactive material within a PRB. Supplementary to the MPSS, several sampling localities with standard piezometers are distributed around the remediation site (Fig. 2). They recorded the existing contamination situation upstream of the barrier, monitored the groundwater conditions downstream of the barrier, and controlled a potential bypass of the barrier.

3. Methodology 3.1. Groundwater sampling and analyses Sampling of groundwater was carried out in various localities upstream, downstream and within the barrier and at different levels (14–45 m below land surface). Sampling points are indicated in Figs. 2 and 3 and sampling levels are shown in Table 2. The time span of monitoring discussed here is 3 a from the installation of the barrier in November 2003–December 2006. Frequency of sampling depended both on the groundwater level and the locality; most relevant sampling points were monitored almost monthly. Multi-level sampling wells (ML, A and B wells) equipped with 2.5 cm (1 in.)-piezometers were sampled using a SOLINST gas driven double valve pump. Sampling of the ordinary single-level 5 cm (2 in.)-piezometer wells was performed with a GRUNDFOS MP1 pump. Several tube volumes of groundwater were pumped beforehand to guarantee uncontaminated and fresh samples. Samples for chemical analyses were filtered at 0.45 lm and either immediately analyzed or acidified (cations), refrigerated and transported to the laboratory. Complete chemical analyses included the following parameters: alkalinity, major cations and anions, relevant redox elements (listed in Table 1) and potentially harmful metals. Concentrations of CrVI and dissolved Fe (FeII/FeIII) were

Table 2 List of sampling wells with the respective sampling depths. Sampling well

Depths (m)

RB1/83 KB02/01 KB04/01 KB02/02 KB03/02 KB04/02 KB05/02 KB07/02 Sampling well

Level 4 (m)

20 20 45 19 19 19 35–37 24–26 Level 5 (m)

30.255 30.255

41.36 41.36

ML1 ML2 ML3 ML4 A1 A2 A3 A4 B1

Level 0 (m)

Level 1 (m)

Level 2 (m)

Level 3 (m)

13.67 13.67 13.67 13.67 13.67

14.21 14.21 14.21 14.21 15.88 15.88 15.88 15.88 15.88

18.252 18.252 18.252 18.252 18.69 18.69 18.69 18.69 18.69

213 213 21.24 21.24 21.210 21.210 21.210 21.210 21.210

Sampling depths in table are average values and refer to the following intervals (m): 1: 12.8–15.6, 2: 18.0–18.5, 3: 20.7–21.2, 4: 21.1–21.6, 5: 29.7–30.8, 6: 40.8–41.8, 7: 13.0–14.1, 8: 15.6–16.0, 9: 18.1–19.1, 10: 20.7–21.7. Multi-level sampling wells (ML, A, B) are equipped with filter sections at the indicated depths; RB (percussion drilling) and KB (cored drilling) wells have perforation sections of several meters.

measured either colorimetrically with on-site rapid diagnostic tests (Cr-test Merck or Aquaquant and Merck, Fe-test Aquamerck) or by ICP-MS and ICP-OES in an accredited laboratory (Institute Bachema, Switzerland). Laboratory analytical results on duplicate samples enabled a check of the on-site measurements. Sensitive parameters such as redox potential (Eh), dissolved O2, pH and electrical conductivity were measured on-site with Knick portable meters (Portamess) and Hamilton electrodes. 3.2. Sampling of reactive material Some reactive material was placed in 5 cm (2 in.)-piezometers as part of the MPSS piezometer bundles emplaced in the cylinders. The material was enclosed in wire-mesh cages that were connected by a flexible packer tube and held in position and separated by inflatable packers. This system guaranteed no vertical exchange of groundwater. For sampling, the wire-mesh cages were cut open to take out some reactive material. Analyses of the reactive material were only performed macroscopically for this study. Microscopic analyses of the reactive material and interpretations are described elsewhere (Flury et al., in press). Reactive material was sampled at different localities and depths: frontal and lateral inflow (A1, A4), centre (A2) and outflow (A3) of the cylinder of the front row, and frontal inflow (B1) of the cylinder of the back row. All depths (13.5, 16, 18.5 and 21 m) were sampled. Sampling was performed four times (September 2004, May and September 2005 and June 2006). 3.3. Tracer experiments Two optical-tracer experiments were performed in summer 2005, 1.5 a after installation of the barrier. The first experiment used Na-fluorescein (Na2C20H10O5) and Na-naphthionate (C10H8O3NSNa) in the proximal region of the single and double row of cylinders. Well ML1 served for the input of Na-fluorescein and well ML2 for the input of Na-naptionate. Input depth was level 2. The quantity of input was calculated according to Eq. (7) (Wernli, 1994) where M is the amount of tracer (g), a a correction factor (g/m) dependent on the tracer type, L is the test distance (m) and A

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ffiffiffiffiffiffiffiffiffiffiffiffi s 3    tm 1  tm =t i tm  ti C i ¼ cm   exp  t0  ti 4  D=ðmxÞ  tm t0 K f ¼ mav erage  g=i

ð8Þ ð9Þ

4. Results and interpretation 4.1. Effect of the groundwater level on chromate concentrations found in the groundwater was closely The amount of CrO2 4 linked to the level of the groundwater table. Fig. 4 shows that a rising groundwater table resulted in an increase of CrO2 4 concentrain the groundwater depended tions. The concentrations of CrO2 4 both on the rate of the rise and on the time period elapsed since the last rise of the groundwater table. The data further show that a continuous rise of the groundwater table did not cause a continuous rise of CrO2 4 concentration. Chromate is assumed to migrate faster in layers of coarse-grained material and higher permeability and accumulates by means of adsorption preferentially on clay minerals in layers of fine grained material (Fritzen et al., 2006). At the Willisau site, the amount of CrO2 4 being washed out by a rising groundwater table therefore also depends on the type of layer

4.5

540

Groundwater level (m.a.s.l)

539.5

CrVI concentration (mg/L) (mg/l)

4 E2

3.5

539

3

538.5

2.5 installation of PRB

538

2

537.5

1.5

537

1

536.5

0.5

536 06/Sep

06/May

05/Dec

05/Aug

05/May

04/Dec

04/Aug

04/May

04/Jan

03/Sep

0 02/Dec

The second experiment with eosin (C20H6Br4Na2O5) aimed at the extended region of the barrier. The input well was KB02/02 at a depth of 19 m. The quantity of input was 250 g eosin based on Eq. (7). The tracer was dissolved in water in a ratio of 1:10 and the piezometer flushed with 100 L of water. Sampling of the tracer was performed in the multilevel wells ML1, ML2, A1, A2, A3, A4, B1, ML3, ML4 at the depths of levels 2 and 3, and in KB03/02, KB02/01, KB07/02, KB04/02, KB05/02, KB06/02. In both runs, sampling occurred either with gas driven double valve pumps or disposable polyethylene bailers for sampling in 2.5 cm (1 in.)-piezometers, and with a GRUNDFOS MP1 for sampling in 5 cm (2 in.)-piezometers. To minimize cross-contamination, each well was sampled with individual sampling equipment. The tracers were analyzed on-site with a GGUN-FL30 fluorometer (Schnegg and Flynn, 2002) and in the laboratory with a Perkin Elmer LS5B fluorescence-spectrophotometer. Detection limits of the latter are 0.001 lg L1 for Na-fluorescein, 0.005 lg L1 for eosin and 0.2 lg L1 for Na-naphtionate; and the spectra are located at 490/515, 323/418 and 512/537 nm EX/EM, respectively. Groundwater samples of the second campaign still featured Nafluorescein from the first campaign. The resulting interferences were analytically avoided by lowering the pH to 4–5 in order to suppress the signal of Na-fluorescein. Break-through curves were evaluated by the best-fit-method. The average time and the dispersivity were calculated according to Eq. (8) with ci being the concentration at time i, cm the measured concentration of standardization at standardization time tm, x the distance, D/(vx) the dispersivity parameter with m ¼ x=t 0 and t0 the average flow time. The average flow velocity was calculated from t0 and the distance, and the dispersivity was obtained by curve-fitting. The hydraulic conductivity (Kf) was determined after Darcy’s Law (Eq. (9)) from the average flow velocity, the porosity of the media (n) and the hydraulic gradient (i); where n and i were taken from a previous site study (Köhler, 2003).

5

E3

03/May

ð7Þ

E1

540.5

Groundwater level (m.a.s.l)

M aLA

10mg/L

541

CrVI concentration (mg/L)

is a matching coefficient for the aquifer conditions. The calculation resulted in 25 g of Na-flurorescein and 375 g of Na-napthionate. For the input, the tracers were separately dissolved in water at a ratio of 1:10 and the respective piezometers were flushed with 20 L of water. Sampling wells for tracer analysis were A1, A2, A3, A4, B1, ML3, ML4, KB02/04 at the depths of levels 2 and 3 (Fig. 3).

Fig. 4. CrVI-concentrations measured in well KB02/02 (downgradient of the hotspot) plotted against the fluctuation of the groundwater table (m.b.s.l.: meters below surface level) over time. The diagram shows the direct connection between rising groundwater tables and increasing CrVI-concentrations. E1–E3: Events of exceptionally high groundwater levels.

and the accumulated contaminants since the last wash-out event. The unsaturated zone in the area of the hotspot consists of layers of clayey and silty sand with variable amount of gravel (IGT et al., concentration increased with every rise 2001). Moreover, CrO2 4 of the groundwater table. This is evidence that CrVI is not only mobilized by washing out, but that it was also continuously transported downwards in the unsaturated zone by infiltrating meteoric water. 4.2. Behavior of contaminants within the aquifer The contaminant plume downgradient of the hotspot remained in the uppermost meters of the saturated zone, showing little verconcentrations were highest in tical dispersion. Thus, CrO2 4 groundwater sampled in the transition zone and decreased an order of magnitude 3–5 m below the groundwater table. Based on groundwater analyses and results of tracer experiments, the lateral extent of the contaminant plume is clearly limited and well documented during normal groundwater level fluctuations (referred to as normal conditions in the following). Events of exceptionally high groundwater levels (E1–E3 in Fig. 4) caused expansion of the contaminant plume. The lateral expansion can be seen in a comparison of CrVI-concentrations in wells ML1 and ML2 (Fig. 5). Concentrations in ML1 (upgradient of double row) were generally higher than in ML2 (upgradient of single row). Under normal conditions the contaminant plume therefore preferentially flows through the double row (Fig. 5a), a factor considered in the PRB design. In exceptionally high groundwater events (e.g. E2 and E3), CrVI-concentrations increase in well ML2. In fact, similar CrVI-concentrations were measured in wells ML1 and ML2 when groundwater level reached the uppermost sampling level (Fig. 5b). Therefore, the contaminant plume laterally extended during these conditions and flowed through the double and single row in equal concentrations. Fig. 5b also shows that CrVI-concentrations in ML1 and ML2 downgradient of the barrier exceeded CrVI-concentrations in KB02/02 during times of exceptionally high groundwater levels. Therefore, CrVI-concentrations analyzed in KB02/02 did not necessarily portray the contaminant conditions immediately downgradient of the contaminant source as well as was hoped. The full heterogeneity of the system probably cannot be accurately captured with the installed wells and their filter sections.

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a

0.25

CrVI concentrations (mg/L)

0.225

E2

E3

ML1-2 (mg/L) (mg/l) ML2-2 (mg/L) (mg/l)

0.2 0.175 0.15 0.125 0.1 0.075 0.05 0.025

06/Oct

06/Aug

06/Jun

06/Feb

06/Apr

05/Dec

05/Jul

05/Sep

05/May

05/Jan

05/Mar

10 mg/L

E2 CrVI KB02/02 (mg/L) (mg/l) CrVI ML1-1 (mg/l) (mg/L) CrVI ML2-1 (mg/l) (mg/L)

06/Oct

06/Aug

06/Jul

06/May

06/Apr

06/Mar

06/Jan

05/Dec

E3

05/Oct

7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

05/Sep

CrVI concentrations (mg/L)

b

04/Nov

04/Jun

04/Aug

0

Fig. 5. CrVI-concentrations at varying groundwater levels: (a) at normal conditions, CrVI-concentrations were higher in ML1 (upgradient of double row) than in ML2 (upgradient of single row). Samples were taken in level 2 and (b) during times of exceptionally high groundwater levels (e.g. E2, E3) and enhanced mobilization (note change in CrVI-concentrations compared to Fig. 5a) similar CrVI-concentrations were determined in ML1 and in ML2. Samples were taken in level 1. A comparison between these values with CrVI-analyses of KB02/02 resulted in higher CrVI-concentrations in samples of ML1 and ML2. See text for further explanation.

4.3. Characteristics of the hydrological regime and groundwater velocities The tracer experiments gave further insight into the complexity of the hydrological regime with heterogeneities at different scales. Results of the tracer tests revealed average groundwater velocities of 5–6 m/d, hydraulic conductivities of 3  103 m/s with a dispersivity of 3–4 m derived from a test length of 60 m. A more detailed analysis distinguishing different areas resulted in higher groundwater velocities upstream of the barrier (10–15 m/d) and lower average groundwater velocities within the barrier (0.5–3 m/d). A decrease in velocity in the area of the barrier might be the result of two mechanisms: (1) buildup of corrosion products that partially block the pore space of the reactive material (discussed in Section 4.5) and (2) smearing and accumulation of fine-grained aquifer material during installation of the cylinders causing a reduced conductivity at the cylinder/aquifer interface. These heterogeneities resulted in a local decrease of groundwater flow and favored alternative groundwater flow paths. Fig. 6 shows break-through curves of the tracer experiments in the area of the barrier that exemplify this process. The break-through curve showed a clear single peak implying a uniform flow (Fig. 6a) upgradient of the bar-

Fig. 6. Break-through curves of a tracer experiment using eosin injected into KB02/ 02: (a) break-through curve in ML1 upgradient of the barrier with a single peak and (b) multiple break-through curves observed within the cylinders (e.g. well A4, lateral inflow).

rier in well ML1. Within the cylinder (e.g. in well A4, lateral inflow) several breakthrough curves were distinguished and peak concentrations decreased (Fig. 6b). This implies a breakup of the homogeneous flow into several individual flows guided by channels in the subsoil and associated heterogeneities of permeability. Flow velocities of the second break-through (10 m/d) in well A4 were similar to average velocities upstream of the barrier (10–15 m/d) and flow velocities of the first break-through were even higher (22 m/d). Downgradient of the barrier (ML3, ML4 and KB04/02), the breakthrough curves showed one single but distinctively broader peak. Downgradient groundwater flow was thus undisturbed. Results from modeling during the planning stage of the barrier predicted a preferential bypass of the cylinders in case of a reduced permeability at the frontal cylinder/aquifer interface (Köhler, 2003). The results from the study revealed an inflow into the cylinders with similar flow velocities as upstream of the barrier, and thus the modeled bypass scenario appears to be a minor risk. The tracer experiments further showed that a potential bypass of the entire barrier can be excluded during times of normal groundwater levels. The tracers were not detected in sampling wells laterally outside of the barrier. During times of exceptionally high groundwater levels, no data were recorded. However, based on the monitoring of the contaminant situation at exceptionally high groundwater levels, a bypass of the barrier was inferred. 4.4. Effectiveness of the remediation method To assess the effectiveness of the PRB, normal conditions and extraordinary conditions (events of exceptionally high groundwa-

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0.5 Groundwater level (m.a.s.l) b ML1-2 upgrad.well ML4-2 downgrad.well

E3

E2

0.4

14.25

0.3

15.25 16.25

0.2

17.25

0.1

CrVI concentration (mg/L)

Groundwater level (m.b.s.l)

13.25

18.25

0

07/Jan

06/Oct

06/Jul

06/Apr

06/Jan

05/Oct

05/Jul

05/Apr

05/Jan

04/Oct

4.5

7 mg/L

E2

13.25

4

E3

3.5 14.25 15.25

Groundwater level (m.a.s.l) b

3

ML1-1 upgrad.well ML4-1 downgrad.well

2.5 2

16.25

1.5

17.25

1 18.25

0.5

19.25

06/Dec

06/Oct

06/Aug

06/Jun

06/Apr

06/Feb

05/Oct

6

12.25 12.75

5.5

b Groundwater level (m.a.s.l.)

13.25

E3

5

ML2 (mg/L) (mg/l) ML3 (mg/L) (mg/l)

E2

13.75

4.5 4

14.25

3.5

14.75

3 2

15.75

1.5 16.25

1

06/Oct

06/Aug

06/Jul

06/Jun

06/May

06/Apr

06/Mar

06/Feb

06/Jan

05/Dec

0

05/Nov

17.25

05/Oct

0.5

05/Sep

16.75

Fig. 7. CrVI-concentrations (mg/L) plotted against the groundwater level (m.b.s.l.: meter below surface level) over time. E1–E3: events of exceptionally high groundwater level: (a) CrVI-concentrations in ML1 (upgradient of the double row) vs. CrVI-concentrations in ML4 (downstream of the double row). Samples from level 2. No CrVIwas detected in the downgradient well; (b) CrVI-concentrations in ML1 (upgradient of the double row) vs. CrVI-concentrations in ML4 (downstream of the double row). Samples from level 1. Increased CrVI-concentrations were measured in the downgradient well during event 3 and (c) CrVI-concentrations analyzed in samples of ML2 (upgradient of single row) and ML3 (downgradient of single row) showed similar values thereby indicating the insufficient effectiveness of the single row. Samples were taken in level 1.

CrVI KB02/02 (mg/L) (mg/l) CrVI KB04/02 (mg/L) (mg/l)

12.25 13.25 14.25 15.25

Groundwater level (m.a.s.l.) b 10 mg/L

E1

E2

E3

30 Aug 05 KB02/02: 10mg/L KB04/02: 0.17mg/L

4. May 06 KB02/02: 2.55mg/L KB04/02: 0.44mg/L

14. Sept 05 KB02/02: 2.67mg/L KB04/02: 0.77mg/L

31. May 06 KB02/02: 0.19mg/L KB04/02: 0.46mg/L

2.6 mg/L1.4 mg/L

1

E3 E2

0.9 0.8 0.7 0.6 0.5

16.25

0.4 17.25 18.25

0.3 installation of PRB

0.2

CrVI concentration (mg/L)

2.5

15.25

CrVI concentrations (mg/L)

Groundwater level (m.b.s.l.)

05/Dec

0

05/Aug

c

CrVI concentration (mg/L)

Groundwater level (m.b.s.l)

b 12.25

04/Jul

04/May

19.25

0.1

19.25 02/Dec 03/Feb 03/Apr 03/Jul 03/Sep 03/Nov 04/Feb 04/Apr 04/Jun 04/Aug 04/Nov 05/Jan 05/Mar 05/Jun 05/Aug 05/Oct 06/Jan 06/Mar 06/May 06/Jul

a 12.25

ination at normal conditions (Fig. 7a). Analyses of well ML4 downgradient of the barrier at depth level 2 showed no detectable CrVI over the entire monitoring period. At extraordinary conditions, the groundwater table reached sampling level 1. Fig. 7b shows that during the event E3, CrVI-concentrations downgradient of the double row in well ML4 increased (maximum: 0.15 mg/L). Compared to the upgradient CrVI-concentrations of 4 mg/L in well ML1 the contaminants were still reduced by 96%. Two possible reasons for the reduced efficiency of the double row during event E3 may be inferred: (1) different flow paths of the contaminant plume with a local bypass of some cylinders (event E3 featured the highest groundwater table within the monitoring period); (2) an insufficient capacity of the FeII-plume to reduce the contaminants. The latter hypothesis is supported by the following calculation: the reduction of 1 mole CrVI requires 3 moles FeII according to Eq. (6). The maximum FeII-concentration measured in the FeII-plume was 5 mg/L, and thus CrVI-concentrations exceeding 1.5 mg/L could exceed the reducing capacity of the FeII-plume. Moreover, FeII might also be additionally consumed by other redox-sensitive species II 2 (e.g. O2, NO 3 , m4 ) as reducing agents in competing reactions or Fe might have precipitated as secondary phases (e.g. siderite). In contrast to the double row, the remediation capacity of the single row was not efficient enough to reduce the CrVI-concentrations below the critical limit of 0.01 mg/L at any sampling depth. Fig. 7c shows that CrVI occurred in similar concentrations upand down-gradient of the barrier at the depth of level 1. At the depth of level 2 (data not shown), CrVI-concentrations in ML3 (downgradient of the barrier) often exceeded concentrations measured in ML2 (upgradient of the barrier). This apparent contradiction again reflects the inferred heterogeneity of the aquifer at small-scales resulting in varying flow paths. Reasons for failure of the single row might be an insufficient lateral overlap of cylinders and their resultant dissolved Fe plumes ending up in insufficient FeII-concentrations within the ‘gaps’. Chromium(VI)-concentrations determined in samples of KB04/ 02 (further downgradient of the barrier) provided more information about the efficiency of the entire remediation method. Fig. 8 shows a comparison of CrVI-concentrations measured in KB02/02 and KB04/02. The remediation objectives were not achieved in KB04/02, apparently due to the failure of the single row combined with a potential bypass of the barrier during exceptionally high

Groundwater level (m.b.s.l)

ter levels with large contaminant mobilization) are distinguished. The double row of cylinders successfully treated the CrO2 4 contam-

Fig. 8. CrVI-concentrations analyzed in samples from KB02/02 (downgradient of the hotspot) and KB04/02 (further downgradient of the barrier). With the present design of the PRB, CrVI-concentrations often exceed the permissible values of 0.01 mg/L. Comparing peak concentrations revealed a time lag of two weeks (data are given for events E2 and E3). This corresponds to the time needed to cover the distance between KB02/02 and KB04/02.

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groundwater level conditions since the double row was effective. The CrVI-concentrations in KB04/02 were considerably lower than in KB02/02 (mainly seen during events 1–3). A correlation of peak CrVI-concentrations measured in KB02/02 and KB04/02 yielded a significant time lag. An increase or decrease of CrVI-concentrations determined in KB02/02 showed up in KB04/ 02 roughly two weeks later (data given in Fig. 8 for events E2 and E3), suggesting an average groundwater velocity of 4 m/d between the two wells. A comparison of these data with average velocities derived from the tracer test performed at normal conditions reveals no significant dependency of the flow velocities on groundwater level. Groundwater flow velocities were therefore not higher during times of exceptionally high groundwater levels, and a reduced capacity of the barrier in these conditions is not associated with kinetic problems. 4.5. First results on the current state and the long-term operation of the barrier Macroscopic analyses showed that the reactive material was still unconsolidated in all wire-mesh cages 3 a after installation of the barrier (Fig. 9). Thus, clogging of the barrier due to precipitates in the pore space of the reactive material did not occur. This observation suggests that the applied mixture of Fe-shavings and gravel has a lower risk of pore space clogging compared to reactive material consisting of pure Fe. Detailed mineralogical investigations are discussed elsewhere (Flury et al., in press). Ongoing geochemical processes within the barrier were observed as different discolorations of the wire-mesh cages and reactive material depending on locality, depth, and time. In the frontal inflow of the cylinders (A1, B1) oxidizing conditions predominated at the depth of the transition zone (level 1). In the centre (A2) and outflow (A3) of the cylinders reducing conditions prevailed. With time, conditions in A2 (centre) at the depth of level 1 changed from reducing to oxidizing. After 2 a of operation, organic matter appeared in the direct inflow of the cylinders. The Fe-oxidizing bacteria gallionella ferruginea was identified. The results of the macroscopic investigations of the reactive material are in concordance with the results of the groundwater analyses. Within the barrier, FeII-concentrations were low in the inflow (A1, B1), modest in the centre (A2) and high in the outflow (A3) of the cylinder (selected analyses in Table 3). This state was

Fig. 9. Reactive material sampled from the wire-mesh cages of the MPSS. The flexible tube connects the cages and feeds the packer system. The reactive material was still unconsolidated 3 a after installation of the barrier at all localities and depth levels. Sample derived from the A1, level 1.

Table 3 FeII- and CrVI-concentrations at different localities within the cylinders at different levels of depths. Highest consumption is observed in the inflow (A1, B1) at the depth of level 1, where FeII decreased and CrVI appeared with time. Depths of level 1 (transition zone): 15.6–16.0 m, level 2: 18.1–19.1 m, level 3: 20.7–21.7 m; b.d.: below detection limit (field tests: CrVI:0.005 mg/L, FeII: 0.1 mg/L) and n.m.: not measured. Level

A1 FeII (mg/ L)

A2

A3

A4

B1

CrVI (mg/ L)

FeII (mg/ L)

CrVI (mg/ L)

FeII (mg/ L)

CrVI (mg/ L)

FeII (mg/ L)

CrVI (mg/ L)

FeII (mg/ L)

CrVI (mg/ L)

July 2004 1 1 2 2.5 3 4.3

b.d. b.d. b.d.

9.9 14.1 14.9

b.d. b.d. b.d.

n.m. 19.9 17

n.m. b.d. b.d.

11.5 3.5 12.2

b.d. b.d. b.d.

4 2.9 2

b.d. b.d. b.d.

May 2005 1 n.m. 2 1 3 5

n.m. b.d. b.d.

n.m. 12 10

n.m. b.d. b.d.

n.m. 16 15

n.m. b.d. b.d.

n.m. 1.9 14.1

n.m. b.d. b.d.

n.m. 0.4 4

n.m. b.d. b.d.

September 2005 1 b.d. 0.16 2 1.5 b.d. 3 8.2 b.d.

0.9 14.4 13.1

b.d. b.d. b.d.

28.8 14.2 15.4

b.d. b.d. b.d.

9.3 0.8 12.3

b.d. b.d. b.d.

b.d. b.d. 2.3

1.75 0.06 b.d.

April 2006 1 b.d. 2 1 3 25

0.5 16 12.5

b.d. b.d. b.d.

15 12 n.m.

b.d. b.d. n.m.

n.m. n.m. n.m.

n.m. n.m. n.m.

b.d. b.d. 0.5

0.2 0.1 b.d.

0.5 15 15

b.d. b.d. b.d.

22.5 12.5 12.5

b.d. b.d. b.d.

0.1 0.1 n.m.

b.d. b.d. b.d.

b.d. 0.04 0.3

0.08 b.d. b.d.

0.27 b.d. b.d.

September 2006 1 b.d. 0.1 2 0.6 b.d. 3 3.6 b.d.

reached 7 months after installation of the barrier but – as a positive indicator – did not change significantly after that. The redox potential decreased from +150 to +200 mV upgradient of the barrier to 50 to 100 mV in the inflow of the cylinders (A1 and B1) and to 150 to 300 mV in the centre of the cylinder (A2). Dissolved O2 was not detected within the barrier. The pH-values generally increased within the barrier from 7.2 to 7.4 but did not vary within the cylinders. Corrosion reactions within Fe walls generally raise the pH (Gu et al., 2002; Wilkin et al., 2005) according to Eqs. (1) and (2). The small pH increase observed at the PRB Willisau is explained by the buffering capacity of the carbonate system and the dilution of the reactive material compared to barriers composed of solely Fe0. Groundwater analyses at the depth of the transition zone (level 1) featured less reducing conditions compared to analyses from lower depths (levels 2 and 3). This is observed for all localities. The differences were observed in lower FeII-concentrations and in less negative values of the redox-potential in the upper level. Two reasons are assumed to be responsible. First, the reactive material at the depth of level 1 is constantly exposed to groundwater table variations and therefore also to larger amounts of O2. Second, CrO2 4 is preferentially transported in the uppermost part of the groundwater body and its reduction reaction contributes to the Fe consumption. The direct consequence of this additional consumption was visible in A1 at level 1: CrO2 4 was found here for the first time 1.5 a after installation of the barrier. At this locality, a change was also observed in chemical conditions whereby the reducing conditions (O2: 0.1 mg/L, Eh: 200 to 250 mV) changed to oxidizing conditions (O2: 4 mg/L, Eh: +100 mV). The FeII-plume generated downstream of the barrier was clearly derived from the reactive material, because FeII was measured within the barrier. The FeII-plumes developed completely different shapes downstream of the single and the double row (Fig. 10). The extent of the plume depended on the oxidation capacity introduced by both O2 and CrO2 4 . In well ML3 (downgradient of the sinwas constantly present at the depth of the gle row), CrO2 4

B. Flury et al. / Applied Geochemistry 24 (2009) 687–696

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 cies other than CrVI (O2, SO2 4 , NO3 ) contribute to the Fe consumption and decrease the amount of FeII being released by the barrier. In addition, microorganisms might affect the Fe oxidation mechanisms. These negative effects were not fully considered in the above-mentioned estimate. Moreover, although precipitates of secondary phases did not fill the pore space of the reactive material in this test, they might passivate the Fe surface and decrease the Fe reactivity. However, the results of the geochemical analyses (including analyses of FeII and CrVI, Table 3) after 3 a of operation revealed an insufficient reduction capacity only in the inflow of the cylinders at the depths of level 1. In the centre and outflow part of the cylinder the contaminants were completely reduced which suggests that the barrier is still fully functional (with some short comings as discussed above). More information about the geochemical processes and the state of the reactive material is needed to give a serious estimation of the long-term performance of the barrier.

5. Discussion

Fig. 10. Sketch of FeII-plumes generated downstream of the barrier: (a) downgradient of the single row, the FeII-plume was only generated at the depth of level 3 due to consumption of FeII by CrVI and O2 in the upper levels and (b) the FeII-plume downgradient of the double row showed a different shape with highest FeIIconcentrations at the depths of level 2. In the transition zone (level 1) the generation of a FeII-plume depends on the amount of CrVI present. Numbers (in mg/ L) given are the ranges of FeII (normal font) and CrVI-concentrations (italic) measured at different levels (L1–L3) over time.

transition zone due to insufficient remediation capacity, while the FeII-plume was only developed at greater depth with varying concentrations of 1–8 mg/L. In well ML4 (downgradient of the double row), the FeII-plume was most developed at a depth of level 2 with constant FeII-concentrations of 2.5–5 mg/L. Below level 2, the FeIIplume was less developed and CrVI-concentrations varied, which was not fully understood. In the transition zone (level 1), concentrations in the Fe-plume strongly depended on the presence of CrVI but were always distinctly lower (max. 2 mg/L) than at the depth of level 2. The less developed FeII-plume in the transition zone is explained by the initially larger amount of O2 in groundwater at this depth combined with the greater abundance of CrO2 4 and its consuming effect on the FeII-plume. Reducing conditions within the barrier also affect other redoxsensitive species in the groundwater. The reduction reactions of these species compete with the CrO2 4 reduction reaction. Oxygen,  SO2 4 and NO3 were completely reduced by the double row of cylinders. They were not reduced by the single row thereby showing  the same behavior as CrO2 4 . Observed concentrations of NO2 and þ NH4 were very low and fall below the Swiss critical limits (Swiss Confederation, 1998). The long-term performance of a PRB is limited by various factors. A first estimation of the lifetime of the reactive material resulted in 9–12 a (Köhler, 2003). This calculation considered the mass of Fe shavings, the water volume and only the concentrations of FeII, CrVI and dissolved O2. The estimation was based on higher flow velocities (20 m/d) than those found by this study (5–6 m/d) that would yield a longer estimated lifespan of the reactive material. The limiting factors for the longevity of the barrier are most likely the availability and accessibility of FeII. Redox-sensitive spe-

The innovative design of the PRB at Willisau as a double row of hanging cylinders represents a good geotechnical solution for large installation depths in heterogeneous subsoil. The cylinders offer a reduced risk of disturbing the entire hydrological regime in case the filling material becomes partially blocked by ferric hydroxides. The construction as a double row is effective at the site and only limited in the case of exceptionally high groundwater level events. Even under such extreme conditions, very high remediation effectiveness was maintained. In contrast, the single row of cylinders did not produce satisfactory remediation results. The choice of the reactive mixture material proved very promising regarding the longevity of the installation. Groundwater chemistry dominated by the carbonate system might present a risk of causing a large amount of carbonate and Fe-carbonate precipitates in the system. Despite these concerns, the reactive material in Willisau was still unconsolidated after 3 a of operation – assuming that samples taken from the MPSS were also representative for the reactive material within the cylinders. This observation indicates minimal precipitation of secondary phases within the pore space and suggests a low risk of failure of the barrier due to clogging of the system by precipitation. The applied mixture of zerovalent Fe shavings and gravel generates a smaller amount of FeII/FeIIIprecipitates, which is viewed to be beneficial compared to the use of 100% Fe(0). This issue is especially significant because the remediation site contains a large amount of dissolved O2, which renders the system even more susceptible to large precipitation rates. The risk of H2 gas production is also diminished when reactive surfaces are reduced. Hydrogen and CH4 might form as a result of the corrosion of metallic Fe followed by methanogenesis (Naftz et al., 2007), but this was not observed at the Willisau site. The limiting factor for long-term operation is most likely the availability and accessibility of Fe within the cylinders. A larger proportion of Fe shavings to gravel would increase the reactive surfaces to generate a FeII-plume, but would likely also increase the risk of clogging. The large amount of dissolved O2 at the Willisau site and the large groundwater table variations stress the system,  and the competing reduction reactions of SO2 4 and NO3 also contribute to the Fe consumption. Finding the appropriate mixture of Fe and gravel therefore presents one of the challenging tasks using this type of barrier and would be best addressed in laboratory column experiments. At the Willisau site, the average groundwater velocity is considerably higher than at other PRB sites (cm–dm/day as e.g. at Elizabeth City, Denver Federal Centre, Rheine, Hardkroom (Ebert et al.,

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1999; EPA, 2002a, 2003)). This fact makes the remediation considerably more difficult. Heterogeneities – also at small-scale – play an important role regarding the flow paths. Groundwater flow velocities within the cylinders were reduced compared to values upstream of the barrier. These reduced flow velocities most probably derive from large heterogeneities at the cylinder/aquifer interface due to smearing and accumulation of fine-grained aquifer material during installation of the barrier. Therefore special attention and careful handling during installation of the cylinders are very important. Appropriate placement and dimensioning of the barrier and the positioning of monitoring wells are also essential. 6. Conclusions After 3 a of operating and monitoring the PRB at Willisau, some important conclusions can be drawn: Chromate that accumulated in the unsaturated zone of the polluted site is constantly being washed out and subsequently transported by groundwater through the contaminated subsoil. The in the groundwater is strongly correlated with amount of CrO2 4 the groundwater level, the rate of level variation, and the period of time elapsed since the last major level change. The continuous supply of contaminants in the transition zone implies that CrVI is continuously transported downwards in the unsaturated zone by means of meteoric water. The offset double row of cylinders is only limited by extraordinary high water level events that result in a substantial mobilization of CrO2 4 (up to 10 mg/L). Nevertheless, the remediation effectiveness is 96% even under these conditions. In contrast, the single row does not sufficiently remediate the contamination. The cause might be an insufficient overlap of the cylinders resulting in insufficient concentrations and mixing of dissolved Fe in the FeII-plume. The contaminants are transported in the uppermost few meters of the aquifer and do not migrate vertically in the vicinity of the barrier. The length of the cylinders used in Willisau turned out to be sufficient, and an underflow of the barrier can be ruled out based on the data collected. The reactive material is unconsolidated after 3 a, and therefore the risk of premature clogging of the system by precipitation remains low. The mixture of Fe shavings and gravel is suitable and highly effective for a PRB in a carbonate dominated hydrologic environment with locally large heterogeneities and varying permeability. All of these findings strongly support the design of the PRB Willisau at a very challenging remediation site (heterogeneous subsoil, fast groundwater flow velocities, high concentrations of  dissolved O2, presence of SO2 4 and NO3 ). At present, the limiting factor for long-term operation is assumed to be the sustained availability and reactivity of Fe within the cylinders and at sufficiently high concentration within the FeII-plume. Acknowledgments The authors wish to acknowledge the Swiss Federal Office for the Environment FOEN, division for the promotion of environmental technology, the Geologische Beratungen Schenker, Korner & Partner GmbH, Luzern, Switzerland and the Imprägnierwerk Willisau, Wilisau, Switzerland. R. Wilkin, G. Lee, T. Bullen and B. Evans are gratefully acknowledged for providing useful suggestions for the improvement of the final manuscript. References Blowes, D.W., Ptacek, C.J., Jambor, J.L., 1997. In-situ remediation of Cr(VI)contaminated groundwater using permeable reactive walls: laboratory studies. Environ. Sci. Technol. 31, 3348–3357.

Burmeier, H., Birke, V., Ebert, M., Finkel, M., Rosenau, D., Schad, H., 2006. Anwendung von durchströmten Reinigungswänden zur Sanierung von Altlasten. Universität Lüneburg, Fakultät III (Umwelt und Technik), Suderburg. Eary, L.E., Rai, D., 1987. Chromate removal from aqueous waters by reduction with ferrous iron. Environ. Sci. Technol. 22, 972–977. Ebert, M., 1997. Der Einfluss des Redoxmilieus auf die Mobilität von Chrom im durchströmten Aquifer. Universität Bremen. Ebert, M., Möller, W., Wegner, M., 1999. R&D project: permeable reactive barrier (PRB) in Rheine – latest results. Altl. Spektr. 2, 105–112. EPA, 1997. Permeable reactive subsurface barrier for the interception and remediation of chlorinated hydrocarbon and chromium(VI) plumes in ground water. US Environmental Protection Agency, Office of Research and Development, Ada, OK. EPA, 2002a. Cost and performance report/permeable reactive barriers interim summary report: permeable reactive barriers using continuous walls to treat metals. US Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, DC. EPA, 2002b. Field applications of in-situ remediation technologies: permeable reactive barriers. US Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, DC. EPA, 2003. Capstone report on the application monitoring and performance of permeable reactive barriers for ground-water remediation: vol. 1 performance evaluations at two sites. US Environmental Protection Agency, Office of Research and Development, Ada, OK. ESTCP, 2003. Cost and performance report: evaluating the longevity and hydraulic performance of permeable reactive barriers at department of defence sites. Environmental Security Technology Certification Program. 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