AN INNOVATIVE TECHNIQUE FOR REMEDIATION OF ...

AN INNOVATIVE TECHNIQUE FOR REMEDIATION OF GROUNDWATERS: DISCONTINUOUS PERMEABLE REACTIVE BARRIERS I. BORTONE*, A. DI NARDO*, M. DI NATALE*, D. MUSMARRA* AND G. F. SANTONASTASO* *

CIRIAM, Centro Interdipartimentale di Ricerca in Ingegneria Ambientale – Dipartimento di Ingegneria Civile, Seconda Università di Napoli, via Roma, 29, 81031 Aversa (CE), Italy

SUMMARY: In the recent years the aquifer contamination has become a widespread environmental concern. The development of innovative technologies, capable of ensuring the efficiency, reliability of remediation at a sustainable cost is consequently of considerable interest. Numerous solutions are available, each one with its limitations in terms of times, costs and environmental impact. In this work an innovative solution for a Permeable Reactive Barrier (PRB) is put forward. The methodology proposed is based on a discontinuous PRB (PRB-D), using absorbing materials, formed by a grid of deep passive wells. This solution allows to apply the PRB technology to deep aquifers, and makes its realization more flexible and cheaper. The case study deals with a Tetrachlorethylene (PCE) contaminated aquifer close to Naples (Italy), and the results are compared with those previously obtained by a more traditional continuous barrier (PRB-C) on the same site.

1.INTRODUCTION The design and optimization of an effective groundwater depuration technology is a rather complex process and has to take into account both technical assessments (i.e. the hydrological and geotechnical properties of the entire polluted aquifer, the pollutant type and the contaminated area extension, etc.), and economic valuations. In last years, several ‘in situ’ remediation technologies have been proposed for the treatment of polluted groundwaters, such as in situ soil flushing by surfactant solutions (Lee et al., 2005; Qin et al., 2007), in situ biodegradation (Pour et al., 2005) and Permeable Reactive Barriers (PRB) (Vogan et al., 1999; Komnitsas et al, 2006; Li et al., 2006; Di Natale et al., 2008). In

particular, PRBs seem to be very effective alternatives. They exploit the natural hydraulic gradient of the groundwater plume to move the contaminants through the reactive zone thus being more cost-effective than the traditional pump and treat technologies because of the lower long term maintenance costs (Roehl et al., 2005). A variety of reactive materials and sorbents have been successfully used in remediating contaminated groundwater for PRBs. These materials, such as Fe0 fillings (Beck et al., 2001), peat (Roehl et al., 2005), limestone (EPA, 1998), zeolite (Ouellet-Plamondon et al., 2011), and granular activated carbon (GAC) (Plagentz et al., 2006, Erto et al., 2011) are easily available, and some are moderately inexpensive. In the literature PRB design is mainly done by means of empirical approaches (such as trial and error) based on chemical-hydraulic simulation (Guerin et al. 2002, Higgins and Olson, 2009). Furthermore, modelling of groundwater flow through a PRB is commonly performed by using numerical simulators, such as PMWIN by U.S. Geological Survey (Chiang and Kinzelbach, 1996), ChemFlux by SoilVision (Fredlund, 2006), or RT3D (Clement, 1997), that allow the mono, bi- and tri-dimensional modelling of groundwater flow, and the simulation of contaminant and/or reactive multi-species transport. In any case barrier dimensions (length and height) have to be comparable with pollutant plume extension, therefore a systematic design approach that allows the complete pollutant plume capture with the minimum volume of reactive material should be applied. This paper deals with an innovative technology, defined as Discontinuous PRB (PRB-D). A PRB-D is characterized by one or several lines of passive reactive wells, whose hydraulic conductivity is higher than the that of the surrounding soils, thus deviating the natural flow of polluted groundwater to the well, where the reactive media capture the contaminants. A Tetrachlorethylene (PCE) contaminated aquifer near Naples, where several solid and hazardous waste landfills are located, was examined as a case study. The PRB-D was designed by using a commercial calculation code to analyze the PRB water outflow and a dedicated code to simulate the adsorption processes. The results show that PRB-D is effective in capturing the contaminant plume; the results are also compared with those obtained in a previous study (Erto et al., 2011) where a continuous PRB had been designed.

2. PRB-D DIMENSIONING The preliminary step in the design of any continuous and discontinuous PRB for the remediation of polluted aquifers is a detailed site characterization aimed at identifying the site hydraulic and geotechnical properties and the typology and extension of the contaminant plume (i.e. vertical and lateral distribution of the contaminants). This allows to determine the barrier characteristics, such as the geometric dimensions to totally intercept the contaminant plume, and the reactive barrier material. To design a PRB-C it is usually necessary to determine the position, orientation, length, height, thickness and characteristics of the reactive material, the functions of the aquifer and the contamination properties (Erto et al., 2011). For a PRB-D also the following variables must be considered: well diameter, D; well-to-well distance, I; CRETE 2012

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well column number in the grid, nf; column-to-column distance, d. Figure 1 shows an example of PRB-D, represented as a two-column grid of wells (represented as a square to simplify the PRB-D computation); in the figure the meaning of the above defined PRB-D parameters is clearly outlined; the figure shows that the calculation domain is divided in two domains: D and N, that are the contaminated domain and the not contaminated domain, respectively. The latter is defined by the last well column of the PRB-D and its projections on the coordinate axis (A and F). The purpose of PRB design is, then, to separate the contaminated domain (D ) from the not contaminated domain (N). The design process cannot be approached by direct calculation, but an iterative procedure is required (Erto et al., 2011). In particular, a preliminary grid of wells has to be assumed, by choosing the above mentioned design parameters, and, consequently it is essential to check whether these choices allow the complete pollutant capture during the whole PRB-D working time or should be modified. The procedure must be repeated until the best results both in terms of site treatment efficacy and of realization costs are obtained. In order to simplify the preliminary choice of PRB-D properties, the following general criteria can be taken into account: position: barrier has to be the closest possible to the pollutant plume; orientation: each line of wells has to be orthogonal to groundwater flow lines; length: each line of wells should have the same extension of pollutant plume; height: equal to the corresponding pollutant plume dimensions; number of columns: to be chosen considering that the residence time of contaminated flow through the barrier should be long enough for adsorption processes to take place.

Figure 1. PRB-D representation with the contaminated (D) and not contaminated (N) domains.

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3. CASE STUDY The case study examines a large area (2.25km2) in Giugliano in Campania, in the North metropolitan area of Naples (Italy), where many solid waste landfills are located. Over the past 20 years, about eight million tons of urban and special wastes were deposited, legally and illegally, in these landfills. The groundwater aquifer, situated at a depth of 35-40m from the land surface and confined by an aquitard (50m), is contaminated by a large number of pollutants, both inorganic and organic (Di Nardo et al., 2010). Soil composition (Neapolitan yellow tuff) may be approximated to a single mineral whose hydraulic conductivity is 5*10-5 m/s (Di Nardo et al., 2010). Because of the very low adsorption capacity for organic compounds of this material (Erto et al., 2009), the initial solid concentration can realistically be assumed to be zero throughout the entire flow domain. The groundwater flow lines are east-west oriented, with piezometric heights ranging between 5 and 12.5m a.s.l, under a piezometric gradient (J) of 0.01m/m. In Figure 2, PCE isoconcentrations in the actual conditions, and the position of the PRB-D wells are reported. Figure 2 shows that PCE concentration values in the area span over a wide range with a peak that is more than 20 times higher than the Italian PCE regulatory limit for groundwater quality (Clim), set at 1.1μg/l. The solid used for the barrier set-up is a commercially available non-impregnated granular activated carbon (GAC), Aquacarb 207EATM (Sutcliffe Carbon). This material has a BET surface area (Sbet) of 950m2/g and an average pore diameter (dpore) of around 26Å. Dry bulk density (ρb) is 500kg/m3, porosity (nb) is 0.4m3/m3 and hydraulic conductivity of about 0.001m/s (Erto et al., 2009).

Figure 2. PCE iso-concentration at the initial condition, with a focus on A, B, C, D, E and F zones.

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Figure 3. PCE adsorption isotherm onto Aquacarb 207EA™ GAC at a temperature of 10°C. The adsorption isotherm of PCE-Aquacarb 207EATM at the temperature of 10°C, typical of groundwater, is reported in Figure 3. The Langmuir model resulted to be the best for PCE adsorption description (Erto et al., 2009), and is described by the following equation: KC 1 KC max

(1)

Specifically, the Langmuir parameters obtained, at the temperature of 10°C, typical of groundwater, are: ωmax=913.9mg/g and K=19.830l/mol.

4. RESULTS AND DISCUSSION Several simulations have been required in order to identify the optimal PRB-D dimensions. The PRB-D obtained is composed of a 2m diameter, D, deep adsorbing well grid, and, as in the PRBC case (Erto et al., 2011), it is 6m from the PCE plume, with a north direction orientation. Furthermore, in order to reduce the PCE concentrations within the normative limit, a total number of PRB-D adsorbing wells, nT, equal to 299, positioned by following the well grid schematization in Figure 1, were required. The number of the PRB-D well columns, nf, depends on the PCE concentration values and, then, according to the PCE concentration value the PRB-D has been divided into six zones called A, B, C, D, E, and F (Figure 2), respectively. In Table 1, for each zone represented in Figure 2, the number of columns, nf, the column-to-column distance, d, and the well-to-well distance in the same column, I, are reported. Table 1 highlights that in the A, B, C, E, and F zones, that are the less polluted areas of the domain, there are 2, 3, 4, 4 and 3 columns of wells (nf), respectively, while, the distance I is 6 or 8 times the diameter (6D and 8D) in the A zone, and 6D in the others. In the D zone, instead, where contaminant concentrations are the highest, 6 columns of wells with a distance I of 6D are required. CRETE 2012

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Table 1. Characteristics of PRB-D adsorbing wells. Zone A B C D E F

nf 2 3 4 6 4 3

d [m] 2 2 2 2 2 2

I [m] 6D;8D 6D 6D 6D 6D 6D

D [m] 2 2 2 2 2 2

Figure 4 shows numerical results of PCE concentration in the form of contour plot. In Figure 4, the evolution of the PCE pollutant plume during the simulation time, respectively after 10, 30, 50 and 70 years is shown. The figure shows that the PRB-D allows to treat the whole pollutant plume over the considered simulation period of 70 years, with PCE outlet concentrations lower than the PCE regulatory limit value, set at 1.1 µg/l. Table 2 shows the PRB-D adsorbent material volume, Vad, compared with the adsorbent material of the PRB-C (Erto et al., 2011); in the table the dimensions H, L and S, i.e. the depth, the length and thickness of both PRBs, are also reported. The table clearly shows that the PRB-D is more efficient because the remediation of the site is obtained with about 50% less adsorbent material than in a continuous barrier (PRB-C). The comparison between the two design solutions shows that the PRB-D is more economical and simpler to realize that PRB-C.

Figure 4. PCE iso-concentration into the aquifer during the whole simulation time. CRETE 2012

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Table 2. Absorbent material amounts. nT H [m] L [m] PRB-C 1 12.5 900 PRB-D 299 12.5 2

S [m] 3 2

Vad [m] 33,750 14,950

4. CONCLUSIONS This paper deals with the evaluation of the capability of discontinuous reactive permeable barriers (PRB-D), composed by deep reactive passive wells for remediating contaminated groundwater. The case study examines a Tetrachlorethylene (PCE) polluted aquifer in proximity of the city of Naples. After a proper site characterization and the assessment of pollutant distribution, the optimal barrier properties (location, orientation, dimensions and adsorbing material) have been defined by using an iterative simulation and by maintaining outlet PRB-D concentrations right below regulatory limits. A dedicated computer code has been developed to describe pollutant transport within groundwater and pollutant adsorption of the absorbing wells and numerical simulations have been carried out for the determination of optimal parameters. The results show that the PRB-D proposed is an effective solution for the case study and, furthermore the adsorbing material volume required for the PRB-D is significantly lower that that of the PRB-C designed for the same site.

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