REHAMOS. LAYMAN s REPORT

Transcription

1 REHAMOS LAYMAN s REPORT Rehabilitation of groundwater containing combined hexavalent chromium and CAHs: large scale remediation with injection of molasses LIFE Project Number LIFE03 ENV/B/

2 OBJECTIVES AND METHODS Surface treatment and production of (metal) parts, related to automotive industry, has often led to severe groundwater pollution. They are mostly of a complex nature, i.e. consisting of a mixture of heavy metals, chlorinated aliphatic hydrocarbons and frequently also of petroleum hydrocarbons. Typical for the automotive industry is the combined occurrence of hexavalent chromium, used for chromation of metal surfaces, and TCE, used for degreasing purposes. The groundwater at the Tenneco Automotive Europe site in Sint-Truiden (Flanders/Belgium) has been polluted with these compounds, involving a total area of 24 ha, including a neighboring residential area. Figure 1 Overview of the contaminate area Cicindria creek Residential area Residential area Tenneco In the REHAMOS project, an alternative remediation technique injection of molasses was tested for the large scale remediation of the CAHs and hexavalent chromium contamination at the Tenneco site. The goal of the REHAMOS project was the following: Demonstration of the economic, social and environmental benefits of the remediation technique injection of molasses on a large scale and for total removal of the contaminants Obtaining reliable data that can be extrapolated to other soil remediation project in the European union Dissemination of knowledge concerning the use and benefits of this alternative remediation technology. The REHAMOS project is unique because the technique of injection of molasses is applied on a very large scale, tackling a combination of different pollutants in high 2

3 concentrations. It was the first time that this technique was used on a very large geographical scale in Europe TECHNOLOGY The technology comprises of injection wells that extend into the contaminated groundwater. A conduit located within the injection well conveys carbohydrates to the contaminated groundwater. Microbes digest the carbohydrates to produce reducing conditions within the reactive zone that include a dissolved oxygen level less than about 0.5 mg/l, an ORP (oxido reduction potential) less than -200 mv, and a dissolved organic carbon to contaminant ratio greater than 50:1. These biogeochemical conditions lead to the reduction of CAHs and to the reduction of Chromium VI to Chromium III which will precipitate as Cr(OH) 3 (harmless). CAHs The reduction of CAHs is known as the Reductive Dechlorination Process. This is a form of Enhanced Natural Attenuation because the reduction of the CAHs is done by a specific group of bacteria. By injecting the right amount of substrate (carbohydrates), the ideal microbiological conditions (i.e. reduced conditions) are created for bacteria which can digest CAHs. Reductive dechlorination involves the sequential removal of a chlorine atom from the CAH, while substituting with a hydrogen atom. The degradation sequence for PCE is the following: PCE TCE DCE VC ethene ethane carbon dioxide & water. The degradation process for CAHs is shown in figure 2. Figure 2 Degradation process CAHs The later steps of this process, such as the degradation of c12dce to VC, and the degradation of VC to ethene, generally require much more strong reduced conditions in groundwater (typically in the range of -200 to -400 mv) than the initial degradation steps. Higher 3

4 chlorinated compounds are most susceptible to reductive dechlorination because of their higher state of oxidation. Substrate injection is based on the concept of enhancing the natural conditions in a groundwater system in order to drive the conditions to a state that is more conducive to degradation of the CAHs. Often, natural degradation is limited or stalled at a site due to one or more limiting conditions at the site (aerobic or oxidizing conditions in ground water, deficiency of organic carbon in ground water, deficiency of nutrients in ground water, ). The most common rate-limiting factors that result in slow, or little, degradation in ground water are a lack of organic carbon in the site groundwater and relatively mild ORPs (often slightly aerobic). Degradation is often slowed due to the depletion of natural organic carbon in the ground water and less than optimal reducing conditions present in the aquifer (less than mv). A possible solution in these situations relies on enhancing the reductive dechlorination reactions by supplying additional organic carbon as an energy substrate to the ground water system and driving the ORP conditions to lower, more strong reduced conditions. This is accomplished by supplying the groundwater system with a sucrose and carbohydrate source in the form of a mixture of molasses and water. Molasses is a costeffective, innocuous, and edible amendment for groundwater that has been accepted by the regulatory agencies to enhance bioremediation. This technology has been applied at a number of sites throughout the United States. Hexavalent chromium Reduction of chromium VI to chromium III and the subsequent precipitation of chromium III occurs at an ORP of less than 0 mv. This process is known as In Situ Precipitation and is independent of the type of bacteria. The in situ precipitation of the chromium (and heavy metals in general) is a less complicated process than the decomposition of the CAHs. METHODOLOGY The diluted molasses solution is periodically injected through a network of existing wells and other injection points perpendicular to the natural groundwater flow direction. The effect of the electron donor injection is monitored by sampling observation wells downstream of the molasses injection wells. The injection started in 2002 with pilot tests at 4 locations. When these pilot tests indicated that chromium VI was reduced rapidly and TCE was degraded, the remediation was scaled up in several phases to a total of about 300 injection wells (see figure 3). The system is full scale operated since

5 Figure 3 Overview on the location, groundwater pollution contours and molasses injection Molasses solution was off site manually injected using a small trainer-mounted container, pump and garden hosing (see figure 4). Injection wells onsite are equipped with an automated (PLC) injection system (see figure 5). Manual injections were carried out weekly, every two weeks or every three weeks. Automated injections were carried out several times a week in small volumes. Figure 4 Trailer with diluted molasses container, power group and pump (left), molasses injection well with manifold (right) 5

6 Figure 5 Automated injection system (from left to right: automated injection, pump house & injection line) Ground water samples were regulary taken from monitoring wells to measure main parameters as ORP, ph, chromium VI, chromium total, TOC, CAHs, ethane, ethane and methane. Hexavalent chromium The chromium VI and chromium total concentrations off site are systematically decreasing in a similar way. The decrease of chromium VI is very clear in monitoring wells EPB172 and EPB36 (see figure 6). Approximaly 52 % of the off-site area is already been fully remediated. For the time being the levels of contamination in the central zones of the plume are still elevated but decreasing. It is clear that the injections of substrate successfully remove chromium VI and that within a couple of years the active remediation for the off site contamination can be ended. The remediation technique is considered as the outmost obvious remediation technique for groundwater remediation of chromium VI for this site is and this from: Technical point of view: remediation targets easily met, no nuisance for the plant or the neighbourhood and very low rest concentrations (mostly < detection limit) Financial point of view: much cheaper than classic remediation techniques 6

7 Figure 6 Results of chromium VI removal in 4 selected wells EPB74 EPB36 EPB172 PB214 CAHs The remediation of chloroethenes is also successful but the biodegradation of CAHs takes more time and much stronger reducing conditions than the reduction of chromium VI does. Nevertheless remediation targets are already nearly met on some locations, mostly the locations where the injections started in the first place and injections as a consequence are running the longest (3 to 4 years) i.c.; the pilot test locations. The influence sphere seems to be sufficiently large. Increased TOC values have being measured at distances of 100 to 200 metres starting from the injection lines. With the present injection lines, the complete groundwater contamination will be treated as expected. The maximum influence sphere will be reached in probably about 6 month time. The follow-up of the influence sphere is part of the monitoring program. In most of the monitoring wells, where an increased TOC value is measured, a full start-up of the degradation of chloroethene (including the formation of vinylchloride and the degradation of vinyl- chloride to ethene) can be observed after 3 to 6 months. The complete degradation CAHs, including the breakdown from vinylchloride to ethene and ethane has been confirmed repeatedly during monitoring. Temporarily rather high values of vinylchloride can be formed during the breakdown process (This is presented with the yellow triangles on figure 7). On the one hand this is a good indication for a smooth degradation of dichloroethene to vinylchloride, on the other hand care should be taken that VC concentrations don t increase to unacceptable levels. Therefore VC is frequently measured in key wells followed by an evaluation of the results by risk assessment. Although elevated concentrations of VC have been observed downstream from the injection lines, concentrations never reached levels which necessitated adjustment of the injection regimes. Apparently the formed vinylchlorides are quickly broken down to ethene/ethane. Ethene/ethane measurements support this finding. Besides, the value of vinylchloride is 7

8 stabilizing in the several monitoring wells which also is an indication for a good degradation (otherwise the value of vinylchloride should accumulate). Figure 7 shows the results of CAH degradation for 5 selected monitoring wells in the source area of the contamination. Figure 7 Results of CAH degradation in 5 selected monitoring wells CONCLUSIONS Full-scale molasses injection into the aquifer at the Tenneco Automotive Europe site in Sint- Truiden, Belgium, has induced a significant reduction of the chromium(vi) and TCE mixed pollution present in the groundwater, by anaerobic bioremediation processes (bioprecipitation of chromium and reductive dechlorination of TCE). The areas that have been treated for the longest periods show almost complete dechlorination to ethane and ethane and chromium levels below the remediation goal of 40 µg/l. However, many monitoring wells within the plume remediation area still show high levels of DCE and VC. Continued remediation by electron donor supplementation, focusing on an optimal coverage of the total plume area, is expected to eventually lead to complete dechlorination. The large-scale groundwater remediation reported in this paper is an example of a risk-based approach of a complex groundwater pollution (heavy metals and organics), combining a source and plume treatment leading to a reduction of estimated remediation costs and time compared to conventional technology.a cost assessment for different possible remediation techniques was carried out in the Soil Remediation Project dd. 07/03/2003, prior to the start of the project. The applied technique is about factor 3 cheaper than conventional remediation techniques and in addition little or no impediment for the neighbouring area (remediation of groundwater contamination beneath 100 to 150 homes) 8

9 CONTACT INFORMATION Tenneco Automotive Europe Remediation consultants 9