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Land Contamination & Reclamation, 9 (4), 2001 DOI 10.2462/09670513.600

2001 EPP Publications

In situ bioremediation of groundwater contaminated with phenols, BTEX and PAHs using nitrate as electron acceptorRichard J.F. Bewley and Graham Webb

Abstract This paper describes the implementation of an in situ bioremediation scheme to address historical groundwater contamination at the site of a former oil works, now occupied by an operating facility manufacturing bitumen-related products. Site investigations had indicated that groundwater beneath the facility was contaminated with up to 20 600 g/L phenols, 169 g/L polyaromatic hydrocarbons (PAHs) and 440 g/L benzene, toluene, ethylbenzene and xylenes (BTEX). The site was mostly covered by hardstanding so that the key risks posed by the contamination were to an adjacent river located 50 m from the site boundary. The remedial scheme involved the installation of four abstraction wells located towards the site boundary with the river from which groundwater was abstracted at a typical rate of 4050 m3/day and conveyed via reticulation pipework to two sets of three reinjection wells located hydraulically upgradient of the source area. Prior to reinjection, the groundwater was supplemented with a continuous source of sodium nitrate as an oxygen source, together with nutrients, these being supplied in the form of a commercial mixture of urea and diammonium phosphate. A commercial mixture of phenol-degrading bacteria, PHENOBAC, was also applied periodically in batch mode as point source injections to the most contaminated area, and to the reinjected groundwater. Over the 2.5 years that the scheme was operating, mean concentrations of phenols were reduced from 1100 g/L at the start to 12 g/L, PAHs from 11 g/L to 0.9 g/L and BTEX from 58 g/L to 19 g/L. Following termination of the scheme, ongoing monitoring of the groundwater indicated an initial increase in concentrations assumed to be due to bounce-back, followed by a further decline. Mean concentrations of contaminants two years after termination were 93 g/L phenols, 0.7 g/L PAHs and 11 g/L BTEX. Monitoring of groundwater indicated a general increase in viable counts of bacteria, declining after termination, and a transient increase in nitrite, indicative of biological dentifrication. Key words: BTEX, in situ bioremediation, nitrate, PAHs, phenols

INTRODUCTION AND SITE DESCRIPTION This paper presents the results of a groundwater remediation programme undertaken at an operating facility

Received March 2001; accepted June 2001 Authors Richard J.F. Bewley1 and Graham Webb2 1. Dames & Moore, 5th Floor, Blackfriars House, St. Marys Parsonage, Manchester M3 2JA, UK (author for correspondence). 2. Dames & Moore, Iveagh Court, 4th Floor, 6-8 Harcourt Road, Dublin 2, Ireland

in the UK which currently produces a range of flooring, damp proofing and roofing materials based on bitumen and synthetic resins. The site is located on an industrial estate in the north-west Midlands between a railway line to the south and a major river approximately 50 m to the north. At the beginning of the 20th century, oil shale extraction took place in the area. According to historical maps an oil works formerly occupied the present site and on a 1: 2600 Ordnance Survey map dated 1934, the presence of a gasometer indicated potential coal gasification processes. Prior to its current use part of the site had also been occupied by a rail wagon repair yard.

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Land Contamination & Reclamation / Volume 9 / Number 4 / 2001

A series of investigations concerning soil and groundwater quality within the site boundary were followed by some localised remediation of soil contaminated by volatile organic compounds in an area adjacent to a Low Flash products building. This involved excavation and disposal of contaminated soil to a licensed landfill, reinstatement with clean fill and construction of protective measures for underlying soil. An additional area of excavation and disposal involved a former railway sleeper pit and adjacent contaminated soil which was also removed to a licensed landfill, and reinstated with clean granular fill. Whilst these areas were both localised in nature and accessible for soil remediation, much of the remainder of the site was covered by hardstanding and buildings. A series of boreholes were excavated over the site area, to evaluate soil and groundwater quality, as shown in Figure 1. These indicated that the geological succession beneath the site consisted of granular fill materials overlying a thin natural clay, underlain by a succession

of fine to medium-grained sands. These were further underlain by glacial clays as proved in borehole 401, on the northern boundary. In summary, the following sequence was inferred: (1) Fill: predominantly granular but with some clay and silt content up to 1.8 m thick; (2) Clay: soft, firm and stiff, silty and occasionally described as silt up to 1.2 m thick with a base at 2.0 2.8 m depth; (3) Sand: loose to medium dense, black, very silty and fine-grained becoming light grey, silty and fine medium and occasionally coarse-grained below 7 m and red/brown below 11 m, having a thickness of approximately 10 m and a depth to base of 12.3 m (in BH401 on the northern boundary (Figure 1); (4) Clay: stiff red brown, silty, slightly sandy and gravelly, thickness or depth not proved but the latter is in excess of 13.5 m.

R I V E RPHENOLS PAHs BTEX SEP 93 815g/l 37g/l 440g/l NOV 93 7090g/l 45g/l 137g/l PHENOLS PAHs BTEX 105 304 PHENOLS PAHs BTEX SEP 93 502g/l 146g/l 88g/l 401 NOV 93 5930g/l 169g/l 55g/l 202 SEP 93 2.3g/l 6.4g/l 0.3g/l NOV 92 NA NA 4.5g/l DEC 93 3.5-7.0m 9.2-12.0m PHENOLS 20600g/l 1660g/l PAHs 24g/l 26g/l BTEX 305g/l 60g/l DEC 92 5g/l 68g/l NA

305

PHENOLS PAHs BTEX

203 DEC 92 10300g/l 140g/l NA

PHENOLS PAHs BTEX

PHENOLS PAHs BTEX

301

204 303

302

PHENOLS PAHs BTEXKEY

SEP 93 3.3g/l 1.8g/l 1.9g/l

NOV 93 0.08g/l 4.7g/l 1.5g/l

PHASE I SAMPLING LOCATIONS (GRAB SAMPLES) 250mm GROUNDWATER MONITORING WELL

PHENOLS PAHs BTEX

DEC 92 0.2g/l 0.6g/l 2.3g/l

PHENOLS PAHs BTEX

DEC 92 1357g/l 3.9g/l NAPAHs BTEX

150mm GROUNDWATER MONITORING WELL 75mm GROUNDWATER MONITORING WELL 34mm GROUNDWATER MONITORING WELL POLYAROMATIC HYDROCARBONS (16 USEPA LISTED) BENZENE, TOLUENE, ETHYLBENZENE, XYLENES USEPA TARGET COMPOUNDS NA NOT ANALYSED

0

10

20

30

40

50m

APPROXIMATE SCALE

Figure 1. Site plan showing concentrations of key contaminants detected in groundwater

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In situ bioremediation of groundwater contaminated with phenols, BTEX and PAHs using nitrate as electron acceptor

Generally, the fill materials were found to be dry and groundwater was encountered at shallow (2 3 metres) depth within the superficial sands. The sands beneath the site formed a shallow aquifer, believed to be in hydraulic continuity with the river, the bed level of which was reportedly at 2 m AOD. The river was tidal adjacent to the site with a tidal range of approximately 5 m. Groundwater levels at the site were marginally above mean water levels in the river and inferred a net flow of groundwater towards the river. A hydraulic gradient of 1:260 was estimated. The soil and groundwater investigations identified the occurrence of significant contamination in the groundwater, specifically arising from benzene, toluene, ethylbenzene and o-, m-, and p-xylenes (BTEX), phenols, particularly phenol itself, cresols (methylphenols) and xylenols (dimethylphenols), and polyaromatic hydrocarbons (PAHs). Concentrations of the above determinands identified in the groundwater during the various phases of investigation are depicted on Figure 1. On the basis that off-site migration of such contamination could potentially impact the adjacent river, the site owners retained Dames & Moore to design and implement a volunteered remedial strategy with the objective of minimising the potential risk to this receptor.

culatory approach was therefore selected as representing the most appropriate method of introducing nutrients and an electron acceptor into the groundwater to enhance in situ microbial activity. However, whereas hydrogen peroxide had previously been used in treatment of a sandy gravel aquifer (Bewley et al. 2001), the prolonged travel time and assumed half-life of this reagent were considered to represent serious limitations at this site. Nitrate, on the other hand, represented a soluble electron acceptor (when present as the sodium or potassium salt for example) that could be injected in potentially high quantities into the aquifer. Under anaerobic conditions, nitrate will undergo denitrification, with reduction to gaseous oxides of nitrogen and nitrogen gas. Additionally, it has been shown (primarily through laboratory microcosms or studies of intrinsic biodegradation) to be suitable as an electron acceptor during the degradation of aromatics such as phenols (e.g. Bossert and Young 1986; Lerner et al. 2000), naphthalene (Al-Bashir et al. 1990) and BTEX compounds, although in the case of the latter, the evidence for benzene degradation has often been negative (Hner et al. 1995; Krumholz et al. 1996; Bewley 1996). The nitrate reduction process for phenol can be summarised thus:28 28 + 14 29 C 6 H 6 O + ----- NO 3 + ----- H 6CO 2 + ----- N 2 + ----- H 2 O 5 5 5 5

SELECTION OF REMEDIAL APPROACH Whereas potentially historic soil contamination existed under the operational areas and other covered parts of the site (as confirmed by the soil investigations), it would not have been possible to have addressed this without significant demolition of above ground structures and disruption of site activities. However, because th

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