A field trial for in-situ bioremediation of 1,2-DCA

doi:10.1016/S0013-7952(03)00099-1

Engineering Geology

Volume 70, Issues 3–4, November 2003, Pages 315-320

https://doi.org/10.1016/S0013-7952(03)00099-1 Get rights and content

Abstract

Historic spillages of chlorinated hydrocarbons at a vinyl chloride plant in the Rotterdam Botlek area in The Netherlands have lead to contamination of the underlying aquifer. The principal contaminant is 1,2-dichloroethane (1,2-DCA). The contamination is temporarily contained by a pump-and-treat system. A field trial was carried out to investigate the feasibility of treating the dissolved phase of 1,2-DCA via reductive dechlorination by injection of an aqueous solution of methanol, ammonium chloride and sodium chloride into the confined aquifer using an array of eight boreholes. Biodegradation of 1,2-DCA was localised. This was attributed to limited mixing of the carbon substrate within the test zone. In addition, clogging of recharge wells complicated groundwater circulation.

Introduction

The in-situ bioremediation of organic pollutants is critically dependent on the application of complex microbiological processes in a heterogeneous geological environment. In many cases, the main challenge is the engineering of environmental biotechnology using traditional geotechnical techniques. In the case of chlorinated solvents and other dense non-aqueous phase liquids (DNAPLs), the situation may be further complicated by the deep and intrusive nature of the pollution due to the chemical's high density, low viscosity and low aqueous solubility (Pankow and Cherry, 1996). Consequently, the selection and delivery of nutrients and substrates for the bioremediation of chlorinated solvents can pose a major problem.

As part of an ongoing study into the selection and delivery of carbon substrates for the bioremediation of DNAPLs, the paper describes the results from field tests into the bioremediation of 1,2-dichloroethane (1,2-DCA). The site is located at a vinyl chloride plant in the Botlek area of Rotterdam harbour. It is reclaimed land overlying an extensive thickness of Quartenary deposits greater than 35 m in depth. The Quartenary deposits principally comprise silty clay between +1 to −21 m NAP (Normaal Amsterdams Peil) with interbedded strata of silty clayey sand. A confined aquifer of sand is located between −21 and −29 m NAP, which is further underlain by extensive deposits of silty clay. Overlying the whole site is a 4-m thickness of man-made ground (sand) from earlier land reclamation. Sand columns extending down to the confined sand aquifer from previous ground improvements further complicated the ground conditions. Accidental leaks of chlorinated hydrocarbons have polluted the site. In particular, seepage of 1,2-DCA through the sand columns has polluted the underlying confined sand aquifer between 23 and 29 m depth NAP.

Section snippets

Groundwater chemistry

Data on groundwater chemistry from pumping wells at the site is shown in Table 1. The groundwater samples were taken using a peristaltic pump at the ground surface with a Viton pump tubing and polyethylene tubing extending down to the base of the wells at between 26 and 29 m NAP. Samples were collected in 40 ml glass vials sealed with Teflon lined butylrubber septa. Samples were stored at 4 °C until analyses. The data with indicate largely pH neutral conditions with a relatively low redox

Field tests

An array of eight boreholes were sunk at the site in 1996 using cable percussion techniques. The layout of the wells is shown in Fig. 1. The location is downstream of the vinyl chloride production plant and abstraction well 2507B, which is used as part of a temporary pump-and-treat operation. The wells were arranged in a 12×14 m plan area with four wells (E, F, G and H) used for water abstraction as shown in Fig. 2. Two other wells (A and B) were used as recharge wells for injection of water.

Results

In-situ bioremediation was monitored within the test zone by sampling from the filter screens in borehole D at three different levels. Groundwater samples were taken approximately every 2 weeks using a peristaltic pump and stored in 40 ml vials sealed with Teflon lined butyl rubber septa. The results of laboratory analyses from groundwater samples are presented in Fig. 4, Fig. 5, Fig. 6. The graphs show aqueous concentrations of 1,2-DCA and degradative by-products by reductive dechlorination.

Groundwater modelling

In support of the field tests, Delft Geotechnics were commissioned to carry out a limited groundwater modelling exercise to investigate the transportation of the methanol solution within the test zone (Taat et al., 2000). The 3-D groundwater model was developed for the site using the two finite difference programmes MODFLOW and MODPATH McDonald and Harbaugh, 1998, Pollock, 1989. Longitudinal dispersivity was taken as 0.25 m and transverse dispersivity as 0.0125 m. The result indicated a very

Conclusions

The case study was a rare opportunity to investigate insitu bioremediation of a chlorinated hydrocarbon by reductive dehalogenation. The study had mixed success. Initially clogging of the recharged wells hampered groundwater circulation by salt precipitation and biofouling of the filter screens. These difficulties were overcome by using distilled water (with a low oxygen content of <0.1 mg/l) from the chemical works for preparation of an aqueous solution of methanol. The resulting injection of

Acknowledgments

The field tests formed part of a project funded by the Dutch NOBIS programme for research into the application of in-situ bioremediation of contaminated land. The project consortium comprised Arcadis, Akzo Nobel, Delft Geotechnics, and TNO Institute of Environmental Biotechnology. Dr Dyer was supported by an Engineering Foresight Award from the Royal Academy of Engineering in the UK.

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