Download - Phenols PAHs BTEX

Land Contamination & Reclamation, 9 (4), 2001 © 2001 EPP PublicationsDOI 10.2462/09670513.600

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 investi-gations had indicated that groundwater beneath the facility was contaminated with up to 20 600 µg/L phenols, 169 µg/L polyaromatic hydrocarbons (PAHs) and 440 µg/L ben-zene, 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 40–50 m3/day and conveyed via reticu-lation pipework to two sets of three reinjection wells located hydraulically upgradient of the source area. Prior to reinjection, the groundwater was supplemented with a continu-ous 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 peri-odically 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 µg/L at the start to 12 µg/L, PAHs from 11 µg/L to 0.9 µg/L and BTEX from 58 µg/L to 19 µg/L. Following termination of the scheme, ongoing monitoring of the groundwater indicated an initial increase in con-centrations assumed to be due to ‘bounce-back’, followed by a further decline. Mean concentrations of contaminants two years after termination were 93 µg/L phenols, 0.7 µg/L PAHs and 11 µg/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 reme-diation programme undertaken at an operating facility

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 histori-cal 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.

Received March 2001; accepted June 2001

AuthorsRichard J.F. Bewley1 and Graham Webb2

1. Dames & Moore, 5th Floor, Blackfriars House, St. Mary’s Parsonage, Manchester M3 2JA, UK (author for correspond-ence).2. Dames & Moore, Iveagh Court, 4th Floor, 6-8 Harcourt Road, Dublin 2, Ireland

335

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

A series of investigations concerning soil and groundwater quality within the site boundary were fol-lowed by some localised remediation of soil contami-nated 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 (Fig-ure 1);

(4) Clay: stiff red brown, silty, slightly sandy and gravelly, thickness or depth not proved but the lat-ter is in excess of 13.5 m.

336

PAHs

BTEX

NA

R I V E R

KEY

PHASE I SAMPLING LOCATIONS (GRAB SAMPLES)

250mm GROUNDWATER MONITORING WELL




POLYAROMATIC HYDROCARBONS (16 USEPA LISTED)

BENZENE, TOLUENE, ETHYLBENZENE, XYLENES

USEPA TARGET COMPOUNDS

NOT ANALYSED0 3010 4020 50m

APPROXIMATE SCALE

302

303204

304

105

203

305

401

202

301

SEP 93 NOV 93PHENOLS 5930µg/lPAHs 169µg/lBTEX 55g/l

502µg/l146µg/l88µg/l

SEP 93 NOV 93PHENOLS 0.08µg/lPAHs 4.7µg/lBTEX 1.5 g/lµ

3.3µg/l1.8µg/l1.9 g/lµ

DEC 92PHENOLS 1357µg/lPAHs 3.9µg/lBTEX NA

DEC 92PHENOLS 0.2µg/lPAHs 0.6µg/lBTEX 2.3µg/l

SEP 93PHENOLS 2.3µg/lPAHs 6.4µg/lBTEX 0.3µg/l

DEC 92PHENOLS 10300µg/lPAHs 140µg/lBTEX NA

SEP 93 NOV 93PHENOLS 7090µg/lPAHs 45µg/lBTEX 137 g/lµ


NOV 92PHENOLS NAPAHsBTEX 4.5 g/lµ

NA

3.5-7.0m 9.2-12.0mPHENOLS 1660µg/lPAHs 26µg/lBTEX 60g/l


DEC 93DEC 92

PHENOLS 5µg/lPAHs 68µg/lBTEX NA

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

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 approxi-mately 5 m. Groundwater levels at the site were mar-ginally 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, tolu-ene, ethylbenzene and o-, m-, and p-xylenes (BTEX), phenols, particularly phenol itself, cresols (methylphe-nols) and xylenols (dimethylphenols), and polyaro-matic hydrocarbons (PAHs).

Concentrations of the above determinands identi-fied in the groundwater during the various phases of investigation are depicted on Figure 1.

On the basis that off-site migration of such contami-nation 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 recep-tor.

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 struc-tures and disruption of site activities. However, because the majority of the site area was covered by buildings and hardstanding then the amount of infiltra-tion over such an area (with the potential to leach con-tamination into the groundwater) was likely to be very limited. The focus of the remedial scheme was there-fore to address contamination in the saturated zone.

A review of potential remedial options was under-taken and it was considered that the best practicable approach involved in situ bioremediation. This was based on the fact that the contamination was present as dissolved rather than free phase. Also, given the poten-tial for sorbed phase to be present it was considered that an in situ approach offered significant advantages over pump-and-treat, especially given the relatively fine-grained nature of the aquifer. The fine-grained nature of the aquifer also indicated that a passive in situapproach would be of very limited effectiveness due to the reduced radius of influence of dosing wells. A recir-

culatory approach was therefore selected as represent-ing 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 treat-ment of a sandy gravel aquifer (Bewley et al. 2001), the prolonged travel time and assumed half-life of this rea-gent 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 potas-sium salt for example) that could be injected in poten-tially 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 biodegra-dation) 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 ben-zene degradation has often been negative (Häner et al. 1995; Krumholz et al. 1996; Bewley 1996).

The nitrate reduction process for phenol can be sum-marised thus:

C6H6O 285------NO3

285

------H++ + 6CO2145------N2

295

------H2O+ +→

Reports of nitrate having been used in active in situ bioremediation projects have largely been confined to clean-up of BTEX and petroleum hydrocarbons (Bat-termann et al. 1993; Hutchins et al. 1991; Werner 1985) although there is also a report of its use in treat-ment of pentachlorophenol (Campbell et al. 1989).

The proposed scheme therefore involved abstract-ing groundwater from an area hydraulically down-gra-dient of the site, supplementing with nutrients and nitrate as an oxygen source and re-injecting the amended groundwater up-gradient of the contaminated area. The nitrate as oxygenating agent and nutrients would then accelerate the natural biodegradation proc-esses taking place within the saturated zone (normally limited by sub-optimal levels of these factors). Dia-grammatic representation of the scheme is provided in Figure 2.

The feasibility of using bioremediation was con-firmed by a bench-scale laboratory test. This demon-strated that in a sample of groundwater from the most contaminated area, concentrations of the key contami-nant groupings (BTEX, phenols and PAHs) could be reduced to below 50 µg/L, albeit using an inoculum and under aerobic conditions. A constant discharge (pump) test was also undertaken to assess the hydraulic

337


characteristics of the fine sand aquifer beneath the site. Groundwater computer modelling was then carried out to optimise the number and locations of abstraction and re-injection wells, together with appropriate abstrac-tion and re-injection rates.

The target concentrations for switch-off of the scheme were:

(i) for phenols (as the sum of phenol, cresols and xylenols on the US EPA Target List, i.e. phe-nol, 2-methylphenol (o-cresol), 4-methylphe-nol (p-cresol) and 2,4-dimethylphenol (2,4-xylenol)) not to exceed 100 µg/L;

(ii) for BTEX, with benzenes, toluene, ethylben-zene and each xylene isomer not to exceed 20, 50, 60 and 60 µg/L respectively and not to exceed 100 µg/L;

(iii) for polyaromatic hydrocarbons (PAHs) as the sum of 16 US EPA target list priority pollut-ant compounds not to exceed 75 µg/L.

These targets were based on what was considered practicably achievable from the bench-scale testing and were broadly consistent with the Dutch guidelines at the time, where applicable.

338

0 4020

APPROXIMATE SCALE

60

DIRECTION OFGROUNDWATERFLOW

RETICULATIONPIPEWORK

NUTRIENT/NO3INJECTION POINT

KEY

ABSTRACTION BOREHOLE (AW)

INJECTION BOREHOLE (IW)

INOCULUM INJECTION WELLS

MONITORING WELLS (BH)

R I V E R

100m80

12m DEPTHABSTRACTION

WELLS (AW)SECTION

RIVER

NORTH SITEBOUNDARY

CLAY

FINESAND

SAND?

MEDIUMSAND

DYNAMICGROUNDWATER

LEVEL

INOCULUM

RETICULATION PIPEWORK

NUTRIENTS9m DEPTHINJECTIONWELL (IW)

MEANWATERLEVEL

ZONE OF IN-SITUBIOREMEDIATION

SAND BECOMINGCOARSER

CONCRETE

OXYGENSOURCE:NITRATE

CLACIAL CLAY

FILL

?? SANDY CLAYWITH FILL

CLAY CLAY

PLAN

SUBMERSIBLEPUMP

AW403AW401 AW404

BH303

IW405

IW408IW409

IW407

AW402

IW406

IW410

BH302

Figure 2. Diagrammatic representation of remedial scheme


Following termination of the active recirculatory scheme, a monitoring period of up to two years was to be carried out to take account of potential ‘bounce-back’ and intrinsic biodegradation.

METHODOLOGY

Installation of remediation systemA schematic of the remediation system is shown in Fig-ure 2.

A series of four abstraction wells (AW401 – AW404) were located along the down-gradient bound-ary of the site. Each well was installed to the base of the aquifer, a depth of approximately 12 m below ground level and comprised 150 mm diameter well casing screened from 2 m to 11 m below ground level, installed within a 300 mm diameter borehole. The annulus between the borehole wall and well casing was filled with a sand filter pack across the screened sec-tions of well casing, with a bentonite grout seal installed above the filter pack. A submersible borehole pump was installed within each of the four wells. For each well location, a brickwork chamber was con-structed to a depth of l m to house a flow control valve and a sampling tap to enable collection of groundwater samples directly from the rising main. A rising main network comprising 75 mm diameter PVC pipe com-bined the flows from the four abstraction wells into a single discharge pipeline. Connected to the latter was a nutrient dosing facility that enabled addition of nutri-ents from a 500-litre mixing tank and pump (in batch form), and continuous dosing of sodium nitrate and Purisol 100 from 1000-litre IBCs.

The nutrient dosing facility also housed the electri-cal supply and control panel for each of the abstraction well pumps and dosing pumps. Discharge pipework led from the facility to a series of six re-injection wells, located as shown on Figure 2. These injection wells were installed to depths varying from 9 to 11 m bgl. Each hole was drilled at 460 mm nominal diameter and single wall, screened PVC well casing of 250 mm nom-inal diameter was installed from the base of each well to 2 m bgl. Blank casing was installed above the screened section, from ground level to 2 m bgl. A filter pack was installed from the base of each well across the screened section with a bentonite seal from 0.5 m to 2 m bgl.

The installation of three of the abstraction wells (AW402, AW403 and AW404) and the six injection wells (IW405 to IW410) was undertaken between 15 August 1994 and 16 September 1994. A fourth abstrac-tion well, AW401, had been installed in November 1993 during a previous phase of work at the site. The remaining components of the remediation system were

installed between 5 September 1994 and 30 September 1994.

Following initial pump testing of the abstraction wells commencing during the week of 6 October 1994 (week 1) it was found that unacceptable amounts of silt were migrating from the aquifer, through the well con-struction and into the system pipework. To mitigate this problem, each abstraction well was lined with a 100 mm nominal diameter well screen wrapped with an 80 micron nominal pore size geotextile. This work was undertaken during week 3. Siltation problems were also encountered within the injection wells, due to materials in the aquifer being more fine-grained and looser than expected. As a result of the siltation, it was found during initial testing of the remediation system that a number of the injection wells were unable to accept the design flows from the abstraction wells. To reduce this problem, and hence to increase the flow capacity of the injection wells, each well was lined with 150 mm casing and the resulting annulus was filled with a finer grained filter pack. This work was under-taken during weeks 8 to 9.

In addition to the wells utilised in the abstrac-tion/re-injection system, four inoculum injection wells were installed in the vicinity of well AW401, using ‘window sampling’ equipment. The wells were drilled at 75 mm nominal diameter to a depth of 4 m bgl. A piezometer comprising 38 mm PVC screen, a sur-rounding filter pack and a bentonite seal was installed at each location. Stop-cock covers were installed at each location to protect the installations. Monitoring boreholes (BH302 and BH303) had previously been installed on site in September 1993 as part of the inves-tigation works. The boreholes had been advanced to between 3 and 4 m depth with the installation of 150 mm and 75 mm ID piezometers in BH302 and BH303 respectively.

Commissioning and operationCommissioning trials of the remediation system included stepped discharge pump tests and a constant discharge pump test which were performed during weeks 9 to 10. The pumping trials indicated the follow-ing operational characteristics of the system:

(i) the maximum potential abstraction rate from any one abstraction well was of the order of 4 – 5 m3/hr;

(ii) the maximum potential injection rate into any one of the injection wells was of the order of 0.6 m3/hr (wells 408, 409, 410) to 0.8 m3/hr (wells 405, 406, 407);

(iii) the results of the test indicated a steady-state draw-down of the water table at the abstrac-tion wells of the order of 1.0 – 1.5 m at the

339


Key(a) Method of addition as indicated.(b) Batch addition made on date shown.(c) Continuous flow commencing on date shown (Purisol 100 added on alternate weeks from April 1995).(d) On this occasion constituents added to inoculum injection wells, not reinjected groundwater.(e) Commercial mixture consisting of urea and diammonium phosphate supplied by ICI Chance & Hunt.

Table 1. Nutrient and nitrate addition to groundwater prior to re-injection

Date Week Methoda Urea(kg)

Dipotassium phosphate

(kg)

Magnesium sulphate

(kg)

Sodium nitrate

(36%) (T)

Purisol100e (T)

19/12/94 11 B b 100 25 10 – –21/12/94 11 B 200 50 20 – –04/01/95 13 B 200 50 20 – –06/01/95 13 B d 200 50 20 – –19/12/94 11 C c – – – 24 –09/01/95 13 C – – – – 5

final flow rate, and an increase in water levels at the injection wells of approximately 0.5 m for the southern three injection wells 408, 409 and 410. The increase in water table lev-els in the vicinity of the northern three wells 405, 406 and 407 was estimated to be of the order of 0.8 m;

(iv) hydraulic conductivity of the aquifer was of the order of 1 m/day and total abstraction (and re-injection) rate was of the order of 100 m3/day.

Initial testing of the groundwater re-circulation sys-tem took place in October 1994. Groundwater abstrac-tion and re-injection commenced in December 1994 and continued to be operated until August 1997 (week 148). Following completion in August 1997, periodic monitoring of the groundwater continued through 1999.

The groundwater re-circulation system was mostly run for 24 hours per day with abstraction taking place from up to four wells at a time, although this was later reduced to three wells from early 1995. Groundwater abstraction was subsequently concentrated at wells AW403 and AW401, with groundwater abstraction from well AW403 only, during 1997. Fouling of the re-injection wells significantly reduced their efficiency so that the typical groundwater abstraction rate was approximately 40–50m3/day over the course of the treatment, significantly below that during the pumping trials.

Due to this build-up of slime within the screen of the re-injection wells, it was necessary to shut down the system temporarily for a period of three weeks during March 1995 (weeks 22 to 25) whilst the wells were treated with concentrated hydrogen peroxide. The

slime was considered to have originated from rapid build-up of microbial cells within the screen owing to the favourable conditions for microbial activity (i.e. presence of nutrients in a relatively aerobic environ-ment).

The process was repeated during July 1995 (weeks 41 to 42) and subsequently at two to three month inter-vals.

Nutrient and inoculum applicationApplication of nutrients (as batch additions of urea, dipotassium phosphate and magnesium sulphate) together with sodium nitrate (as oxygen source) com-menced on 19 December 1994 following completion of the pump tests.

The purpose of the initial nutrient addition was to provide a readily available source of nitrogen, phos-phorus and potassium at the same time as the inoculum was added. This would primarily provide the inocu-lated bacteria with a source of nutrients as they were transported through the zone of contamination by the re-circulated groundwater. To avoid too rapid micro-bial growth (which would produce a high oxygen demand and exacerbate pore blockage from slime for-mation) the nutrients were added in four batches during weeks 11 to 13, as shown in Table 1.

For ongoing supply of nitrogen and phosphorus, Purisol 100, a commercial mixture of urea and ammo-nium phosphates, supplied by ICI Chance & Hunt, was added following completion of the batch additions. The Purisol 100 and the 36% sodium nitrate solution were each injected as a continuous feed into the discharge pipeline using peristaltic pumps. Both these supple-ments were stored in separate 1 m3 IBC containers. As from April 1995, the Purisol 100 was added every alter-nate week, rather than continuously, to reduce accumu-

340


lation of slime. Later, the supply of Purisol 100 was terminated, once contaminant concentrations appeared to have diminished substantially, and it was judged that the groundwater was sufficiently well-charged with nutrients. The nitrate application was continued how-ever, through 1995 and 1997 as it was considered that oxygen would continue to be a limiting factor.

The inoculum used was PHENOBAC, a proprietary mixture of naturally occurring bacteria and growth stimulants supplied by Microbac Ltd., Durham. The product had received previous US EPA authorisation for release into navigable waters on a case-by-case basis. Following rehydration and filtering, a slurry of the inoculum was added in batch form to the ground-water re-injection and inoculum injection wells. This was performed initially during week 10 and was repeated three times during weeks 13 and 14. Addi-tional inoculation was performed during weeks 44 and 81.

MonitoringSamples of groundwater from the abstraction and mon-itoring wells were taken periodically for chemical anal-ysis. The initial sampling (week 0) took place at the time of the initial pump tests in October 1994 (other samplings being reported to the nearest week). The final sampling during the re-circulation of the ground-water took place in May 1997 (week 137). Following switch-off in August 1997 (week 148), sampling was carried out through May 1999 (week 240). Sampling of the abstraction wells was carried out from taps allow-ing water to be withdrawn during the course of the treatment. The remaining boreholes were sampled using either a Teflon bailer or an MPI pump, having purged each one of at least three borehole volumes to ensure representative samples. Each groundwater sam-ple was temporarily stored in a cool box refrigerated at 3 to 4°C prior to despatch to the laboratory. All chemi-cal and microbiological analysis was performed by AES, Wallsend and included benzene, toluene, ethyl-benzene and xylenes by purge and trap, GC/MS, pol-yaromatic hydrocarbons by GC/MS following Soxhlet extraction, phenols by HPLC/electrochemical detec-tion (British Gas Method) and other determinands by standard ‘MEWAM’ methods or similar. Colony counts were carried out at 22°C using HMSO pub-lished methods (Anon 1983). More detailed speciation of phenols was carried out using GC/MS on the final sampling occasion in May 1999 and undertaken by Chemex, Cambridge.

RESULTS

Microbial populationsThe total viable counts of microbial populations (expressed as colony forming units, cfu/mL) following incubation at 22°C are shown in Figure 3. Within the abstraction wells there was a significant increase from before the pump test to the start of the remediation pro-gramme. The effect of the pump test in drawing groundwater through the aquifer may have resulted in a stimulation of microbial activity through disturbance and localised oxygenation of the groundwater.

Microbial population levels then fluctuated consid-erably. The numbers following switch-off in week 169 were particularly elevated in all wells, but especially in monitoring well BH303 (2.9 x 107 cfu/mL) and abstraction well AW404 (1.8 x 107 cfu/mL). By week 199 microbial counts had also increased to >107

cfu/mL in all wells apart from AW403 and AW404 where there was a decline to 105 cfu/mL since January. However, by the final sampling during week 240 there had been a significant decline in all wells sampled.

The reasons for such fluctuations cannot always be readily explained but are probably due to variations in the chemical (particularly indigenous nutrient) compo-sition of the abstracted groundwater, or in other cases localised enhancement of microbial populations through inoculated microorganisms, either into the inoculum or groundwater, injection wells. It should be stressed that the population levels in the groundwater do not in themselves necessarily indicate the overall level of microbial activity. Microorganisms responsible for contaminant degradation will also be present sorbed onto the solid phase, and it is these, which may be responsible for the bulk of the contaminant degrada-tion. Also, the plate count method is selective for bacte-ria, which can be readily cultured under laboratory conditions and is likely to underestimate total popula-tions present. The main value of the viable count data was to provide a relatively rapid index to assess if there was any gross inhibition of microbial activity which appears not to be the case, even on the occasions where high concentrations of contaminants had been identi-fied.

Concentrations of ammonia and oxidised nitrogenVariations in concentrations of ammoniacal and oxi-dised nitrogen are shown in Figure 3 and summarised in Table 2. Relatively low levels of ammoniacal, nitrite and nitrate nitrogen were present prior to the pump tests in all wells. The high level of ammoniacal nitro-gen in the abstraction wells (particularly AW403) at the 56-week sampling may represent some localised breakthrough of nutrients added to the inoculum injec-tion wells during August. Ammoniacal nitrogen con-

341


Key(a) Mean of four abstraction wells and two monitoring wells.(b) The week 0 data follow pump testing of abstraction wells which may have resulted in localised transient charges in ground-

water quality. Week 11 data were taken at the start of the groundwater recirculation.(c) As sum of the 16 US EPA ‘priority pollutant’ polyaromatic hydrocarbons.(d) As sum of phenol, cresols, dimethyl and ethyl phenols except where stated.(e) As sum of phenol, cresols and 2,4-dimethylphenol (target compounds).(f) – not sampled for nitrogen.

Table 2. Concentrations of target compounds, ammoniacal and oxidised nitrogen at the start of the remediation programme, on termination of the recirculation system and two years afterwards

Determinand Unit

Mean concentration ± S.E.M.(a)

At start of remediation(b)Prior to

termination of active

remediation

Two years following

termination

Week 0 Week 11 Week 137 Week 240

PAHs(c) µg/L 11 ± 6.5 4.0 ± 2.1 0.90 ± 0.34 0.67 ± 0.67

Target phenols(d) µg/L 88 ± 65 1100 ± 1100 12 ± 4.6 93 ± 87(e)

Phenol µg/L 6.8 ± 3.2 75 ± 75 3.8 ± 1.2 <1

Cresols µg/L <1 450 ± 450 0.83 ± 0.54 32 ± 31

Benzene µg/L 13 ± 6.1 <10 <10 0.67 ± 0.49

Toluene µg/L 14 ± 8.4 12 ± 3.8 <10 2.5 ± 2.3

Ethylbenzene µg/L 3.8 ± 3.8 <10 <10 0.83 ± 0.4

Total xylenes µg/L 27 ± 15 16 ± 16 19 ± 15 6.7 ± 5.3

BTEX µg/L 58 ± 31 28 ± 18 19 ± 15 11 ± 7.4

NH4-N mg/L 0.97 ± 0.37 – (f) 2.4 ± 0.7 0.98 ± 0.26

N02-N mg/L 0.01 ± 0.005 – 0.04 ± 0.03 0.09 ± 0.08

N03-N mg/L 0.02 ± 0.022 – 10 ± 9.0 0.97 ± 0.61

centrations in the monitoring wells however, reflect the input of Purisol 100 until this was discontinued.

Apart from the 56-week result for ammoniacal nitrogen, consistently elevated concentrations of ammonia, nitrite and nitrate in the monitoring wells were observed over the course of the groundwater recirculation, both compared to previous concentra-tions and to concentrations in the abstraction wells. Nitrite nitrogen, an intermediate in biological nitrate reduction, was detected in groundwater within the monitoring wells throughout the treatment period and two peaks were observed at 29 and 92 weeks. Much smaller peaks were observed in the abstraction wells at 47 and 121 weeks, potentially representing some breakthrough. In the case of nitrate, peaks were observed in the monitoring wells at 39 and 92 weeks. In the abstraction wells however, only a single peak was observed at 56 weeks, following which nitrate concen-trations remained at mostly non-detectable levels for the remainder of the treatment and post-treatment peri-ods. Following termination of the recirculation system concentrations of ammoniacal nitrogen appears to have

stabilised in both the abstraction and monitoring wells. Concentrations of both nitrite and nitrate, whilst con-siderably lower than during the early phase of the re-injection, remained elevated in the monitoring wells compared to the abstraction wells following termina-tion.

The continued presence of nitrite provided evidence of continuing microbial denitrification of the added nitrate.

Polyaromatic hydrocarbons (PAHs)The clean-up criteria proposed for PAHs for the termi-nation of the active remedial scheme were that the sum of the 16 US EPA Target List priority pollutant PAHs should not exceed 75 µg/L. Mean and maximum con-centrations of the sum of the 16 US EPA Target List pri-ority pollutants over the course of treatment are shown in Figure 4 and the concentrations before and after treatment are summarised in Table 1.

Concentrations of PAHs were somewhat lower at the commencement of remediation than in 1993 in

342


NH4-N

0

5

10

15

20

0 50 100 150 200 250Weeks

NH

4-N

(mg

/l )

NO2-N

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 50 100 150 200 250Weeks

NO

2-N

(mg/

l)

NO3-N

0

2

4

6

8

10

12

14

16

0 50 100 150 200 250Weeks

NO

3-N

(mg/

l)

CFU's

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250Weeks

CFU

's/m

l(G

eom

etric

mea

n)

10

10

10

10

10

10

10

10

10

10

Switch off

Switch off

Switch off Switch off

AW WellsBH Wells

AW WellsBH Wells

In excess ofthese values

AW WellsBH Wells

AW WellsBH Wells

Figure 3. Viable counts of microorganisms in groundwater samples (geometric means) of colony forming units (CFUs) expressed logarithmically and concentrations of ammoniacal nitrogen, nitrite nitrogen and nitrate nitrogen (arithmetic means) in abstraction and monitoring wells during treatment. Asterisked results are approximate and indicate CFUs in excess of indicated value.

AW401 and in AW402 (compared with the adjacent BH304) as illustrated in Figure 1.

Whilst concentrations were below the clean-up cri-teria, both in the abstraction and monitoring wells, the results indicate a general decrease over the treatment period, with the mean concentration falling to 0.9 µg/L (maximum 2.1 µg/L) at week 92. Because of the con-centration being significantly below the criteria on each sampling occasion, and the consistent decrease observed, no further analyses were undertaken during the operation of the active scheme from week 92 to switch-off at week 148.

On re-sampling during week 169, an increase in PAH concentrations was observed compared with the previous monitoring in July 1996, with a maximum concentration of 5.3 µg/L. Total concentrations of the 16 priority pollutant PAHs then subsequently decreased in all wells (both abstraction and monitoring wells) and by week 199 were the lowest since monitor-ing began. In both of the monitoring wells, PAH con-

centrations were below the limits of detection. The maximum detectable concentration of PAHs in AW403 had then fallen to 1.2 µg/L. However, when this well was re-sampled three months later, in week 212, none of the 16 PAHs were present above their detection lim-its.

At the final sampling during week 240, concentra-tions of PAHs were below detectable concentrations in all wells except for AW404, where a total concentration of 4 µg/L of naphthalene was identified. None of the other PAHs (including the more toxic 4, 5 or 6-ringed compounds) were present above detection limits.

PhenolsThe clean-up criteria proposed for phenols for the active remedial scheme were that the sum of phenol, cresols and xylenols on the US EPA Target List should not exceed 100 µg/L; these compounds being phenol, 2-methylphenol, 4-methylphenol and 2,4-dimethylphe-nol.

343


START OFREMEDIATION SWITCH OFF

Mean Target Phenols

1200

1000

800

600

400

200

00 50 100 150 200 250 300

WEEKS

µg/l


Maximum Target Phenols

100,000

10,000

1000

100

10

1-93 0 50 100 150 200 250

WEEKS

µg/l

(LO

G S

CA

LE)


Mean Total (USEPA) PAHs

12

10

8

6

4

2

00 50 100 150 200 250 300

WEEKS

µg/l

SWITCH OFF

100

10

1-93 0 50 100 150 200 250

WEEKS

µg/l

(LO

G S

CA

LE)

Maximum (USEPA16) PAHs

0 50 100 150 200 250 300

WEEKS

Mean Total BTEX

100

80

60

40

20

0

µg/l

SWITCH OFF

Maximum Total BTEX

SWITCH OFF

100

1,000

10

1-93 0 50 100 150 200 250

WEEKS

µg/l

(LO

G S

CAL

E)

START OFREMEDIATION

START OFREMEDIATION

START OFREMEDIATION

Figure 4. Mean and maximum concentrations of key contaminants detected in abstraction and monitoring wells before and after remediation. Maximum concentrations detected are expressed logarithmically and also show concentrations two years prior to remediation. (Target phenols may also include ethylphenols apart from final sampling indicated by *.)

For monitoring purposes during the active remedial scheme however, phenols were determined by the ‘British Gas’ (HPLC) method (HPLC being the method recommended for analysis of phenols arising from gas-works sites (Department of the Environment 1987)). Because this method provides a quantification of phe-nol, total cresols (methyl phenols) and dimethyl plus ethyl phenols (as reported in these categories), other phenols may have been included besides the specific target compounds listed above. The concentrations of total ‘target phenols’ (reported as the sum of phenol, cresols, dimethyl plus ethyl phenols) over the remedial

programme are provided in Table 1 and are depicted graphically in Figure 4.

In summary, both the mean and maximum concen-trations of phenols demonstrated a substantial decrease over the treatment period, albeit with some fluctua-tions. Following switch-off there was an initial increase followed by a decrease in concentration.

As with PAHs and BTEX, total target phenols were also generally lower than comparative samplings in 1993 (Figures 1 and 4). The mean total concentration of target phenols in the groundwater being abstracted from AW403 where the highest concentrations were

344


detected, increased to a relatively high level (6550 µg/L) between the start of the pump tests and the start of the groundwater recirculation in week 11. However this subsequently fell by almost an order of magnitude during weeks 26 and 27. Conversely, concentrations in groundwater from AW402 increased from 243 µg/L in week 11 to 1430 µg/L in week 26 before declining. Two further increases (though to a lesser degree) were observed in the abstraction wells, one in week 47, the second in week 92 (the latter occurring mostly in AW403). Both of these were followed by decreases in concentration and are probably indicative of heteroge-neities in contaminant distribution beneath the site, i.e. concentrations fluctuated as localised pockets of con-tamination were drawn towards the abstraction wells. In the case of the week 47 sample this was taken after the system had been temporarily shut down for five weeks, so that the increase here could have been due to desorption.

Loss of phenols was most pronounced in the moni-toring wells. In BH303 concentrations reached a maxi-mum of 206 µg/L in week 29 but then decreased to below 10 µg/L. In BH302, located further down-gradi-ent from the nearest injection well than BH303, con-centrations were at a maximum in week 47 (64 µg/L), then decreased to 21 µg/L. Since week 78, concentra-tions of target phenols in this well remained below 10 µg/L.

In all wells, total concentrations of target phenols were below the clean-up criterion of 100 µg/L for the final two samplings in 1997 prior to switch-off (weeks 121 and 137). Following switch-off, an increase in phe-nols was observed in most of the abstraction wells when re-sampled in week 170. In two of these, AW402 and AW403, concentrations exceeded 100 µg/L (133 µg/L and 320 µg/L respectively). In monitoring well BH302, 15 µg/L total phenols was detected whilst in BH303 concentrations had decreased to below the limit of detection.

By week 199, concentrations of target phenols were in excess of 100 µg/L only in well AW403, where a concentration of 764 µg/L was obtained, in the remain-der, concentrations were of a similar order to week 170, some slightly higher, others lower. An additional sam-ple from this well only (not shown on Figure 4) was then taken for more detailed speciation of phenols by GC/MS in week 212 to examine the four specific target phenols, these being phenol, 2-methylphenol, 4-meth-ylphenol and 2,4-dimethylphenol. None of these target phenols, nor any of the other semi-volatile organics on the US EPA Target List were present above their detec-tion limits.

At the final sampling in week 240, GC/MS was also used to speciate the listed phenols. Concentrations were again below detection limits in AW403 as was

also the case in wells BH302 and BH303. Apart from a single value of 529 µg/L in AW402, concentrations in the remaining wells were well below 100 µg/L, namely 24 µg/L in AW401 and 4 µg/L in AW404. As noted in Table 1, the ‘target phenols’ results for this final sam-pling are not entirely comparable with the previous dataset given the fact that these were performed using GC/MS rather than the HPLC method (which includes the two other xylenol isomers plus any ethylphenols which may be present). However, phenol itself was below its detection limits (<1 µg/L) in all wells (Table 1) with cresols present only in well AW402 at 189 µg/L (Table 1). In all other wells, cresols were below 10 µg/L.

Concentrations of BTEXThe clean-up criteria for BTEX compounds, for the active remedial scheme were for benzene not to exceed 20 µg/L, toluene not to exceed 50 µg/L, ethylbenzene not to exceed 60 µg/L, xylene (each isomer) not to exceed 60 µg/L, sum of above (BTEX) not to exceed 100 µg/L. The reductions in concentrations of each of the individual BTEX compounds are summarised in Table 1, whilst total BTEX concentrations are shown in Figure 4.

As with the other contaminants, concentrations of benzene, toluene, ethylbenzene and xylenes at the start of the remediation were generally lower than in December 1993, in AW401 and also in AW402, com-pared with the concentrations detected in adjacent BH304 in September and November 1993 (Figure 1). The groundwater abstracted from AW403 and AW404 was most contaminated with respect to BTEX com-pounds. Concentrations in these, as with the other abstraction wells, fluctuated considerably with time, as the groundwater was abstracted (Figure 4). By week 137 concentrations of all individual BTEX compounds had fallen below their target concentrations in all abstraction wells, with total BTEX below 100 µg/L (Table 1, Figure 4). On resampling during week 170 following switch-off, concentrations of total BTEX in AW404 were still below 100 µg/L, with only xylenes being detectable (56 µg/L total isomers). BTEX con-centrations in AW403 had increased slightly above 100 µg/L (105 µg/L), with benzene at 40 µg/L. Other indi-vidual BTEX compounds remained below their respec-tive targets. By the final sampling however, there had been subsequent decreases in both individual and total BTEX compounds (Table 1, Figure 4), none of the indi-vidual BTEX compounds being present above 20 µg/L in any abstraction well or total BTEX above 50 µg/L.

Concentrations of each of the individual BTEX compounds in the two monitoring wells BH302 and BH303 remained consistently less than 10 µg/L from week 21 onwards, including all of the monitoring

345


rounds, following switch-off, through week 240 in 1999.

GENERAL DISCUSSION

Comparison of the concentrations of the target contam-inants one to two years before the start of the remedia-tion (Figures 1 and 4) indicates that there had already been a degree of reduction in contamination levels prior to active remediation. Some of this may have been attributable to localised source removal, and improvements in surface integrity including extending concrete hardstanding over previously unprotected areas and thereby minimising leaching from the unsaturated zone. Natural attenuation may also have been a contributory factor. In studies at a former coal tar distillery, Lerner et al. (2000) demonstrated that whilst concentrations of (mostly) phenol contaminants (in excess of 1000 mg/L TOC), were inhibitory within the central core of a plume within the Sherwood Sand-stone, natural attenuation within the more dilute areas at the fringe of the plume readily took place.

Bench-scale testing had established that microbial activity was not significantly inhibited by the elevated phenol concentrations, which was also supported by the plate counts (notwithstanding the limitations of this technique). The addition of an inoculum to this area of contamination may only have had a marginal effect on degradation and was considered appropriate in this example as a precautionary measure. Addition of inoc-ula in bioremediation (bioaugmentation) has often been the subject of controversy (Bewley 1996), although there have been instances of controlled condi-tions (i.e. with or without bioaugmentation) where a significant difference was observed, especially with the more recalcitrant contaminants (e.g. Bewley et al. 1989).

Although definitive verification of bioremediation full-scale is not possible, the results of the monitoring post switch-off have indicated that the treatment car-ried out has achieved its remedial objectives and that the degree of microbial activity within the aquifer has been sufficient to address desorption of any residual contamination present. The mean concentrations of tar-get contaminants were all below the target concentra-tions both at termination of the recirculation scheme and after two years of switch-off. This was also the case for concentrations within the individual wells except for one single concentration of total phenols within well AW402 (529 µg/L). Where there had been some evidence of ‘bounce-back’ in concentrations of con-taminations following completion of the active recircu-lation system (Figure 4), this appeared to be transient in nature as concentrations of all determinands subse-

quently declined over the following two years. Where the bounce-back occurred, this took place in the most contaminated area: in well AW403 for example where concentrations of phenols were 34 µg/L at termination rising to 320 µg/L 33 weeks later, and 764 µg/L after a further 29 weeks. Thirteen weeks later however, con-centrations had fallen below detectable levels, which was the case after a further 28 weeks at the final sam-pling. The phenol concentrations in AW402 were therefore assumed to follow a similar cycle. Concentra-tions of PAHs and BTEX also showed similar trends as the phenols; in each case however, all such determi-nands were below their respective target concentrations on both the penultimate and final sampling occasions.

Supporting indicators of biological degradation were the occurrence of nitrite as an intermediate in den-itrification during the nitrate reduction process, the depletion of nitrate and the occurrence of a high bacte-rial count in the aqueous phase. As discussed, given the selectivity of isolation methods and the likelihood of microbial activity being concentrated on the solid phase the results of the microbial counts in the free phase need to be interpreted with caution. However, it is of interest that there is a general decline at the end of the monitoring phase, potentially indicative of an over-all decline in degradable carbonaceous material.

This case study therefore illustrates that in situbioremediation using nitrate as an electron acceptor represents a viable remedial solution for groundwater contaminated with low molecular weight monoaromat-ics, PAHs and phenols. It also demonstrates that enhanced passive remediation may be of significance following completion of active remediation in main-taining residual concentrations at acceptable levels.

ACKNOWLEDGEMENTS

We wish to thank the owners of the site for permission to publish this paper and acknowledge the support of our colleagues, Mr John Alexander during detailed design, Mr Richard Andrews for hydrogeological mod-elling and Mr Mark Jones for field engineering support. The support of the Environment Agency during the implementation of this project is also appreciated. The views expressed in this publication are those of the authors and do not necessarily reflect the views of URS Dames & Moore.

REFERENCES

Al-Bashir, B., Cseh, T., Leduc, R. and Samson, R. (1990) Effect of soil/contaminant interactions on the biodegradation

346


of naphthalene in flooded soils under denitrifying condi-tions. Appl. Microbiol. Biotechnol., 34, 414-419.

Anon (1983) The bacteriological examination of drinking water supplies. Reports on Public Health and Medical Sub-jects, 71. HMSO, London.

Battermann, G., Fried, R., Meier-Lohr, M. and Werner, P. (1993) Pilot and large-scale experiences in the in-situ biore-mediation of a refinery site polluted with hydrocarbons, pp. 1135-1144. In F. Arendt, G. J. Annokkee, R. Bosman and W. J. van den Brink (ed.) Contaminated Soil '93. Kluwer Aca-demic Publishers, Dordrecht.

Bewley, R.J.F. (1996) Field implementation of in-situ biore-mediation: key physicochemical and biological factors. In Soil Biochemistry, Volume 9, G. Stotzky and J.M. Bollag (eds.), pp. 473-541. Marcel Dekker Inc., New York.

Bewley, R., Ellis, B., Theile, P., Viney, I., Rees, J. (1989) Microbial clean-up of contaminated soil. Chemistry and Industry, 23, 778-783.

Bewley, R.J.F., Alexander, J.G. and Webb, G.H. (2001) Ex situ and in situ bioremediation of former oil distribution ter-minals. Land Contamination and Reclamation, 9 (3), 269-277.

Bossert, I.D. and Young, L.Y. (1986) Anaerobic oxidation of p-cresol by a denitrifying bacterium. Appl. Environ. Micro-biol., 52, 1117-1122.

Campbell, J.R., Fu, J.K. and O’Toole, R. (1989) Biodegrada-tion of PCP-contaminated soils using in-situ subsurface bioreclamation, pp. 17-27. In Biotreatment – The Use of

Micro-organisms in the Treatment of Hazardous Materials and Hazardous Wastes. Hazardous Materials Control Research Institute, Silver Spring, MD.

Department of the Environment (1987) Problems Arising from the Redevelopment of Gasworks and Similar Sites (2nd ed.), prepared by Environmental Resources Limited. HMSO, London.

Häner, A., Höhener, P. and Zeyer, J. (1995) Degradation of p-xylene by a denitrifying enrichment culture. Appl. Envi-ron. Microbiol., 61, 3185-3188.

Hutchins, S.R., Sewell, G.W., Kovacs, D.A. and Smith, G.A. (1991) Biodegradation of aromatic hydrocarbons by aquifer microorganisms under denitrifying conditions. Environ. Sci. Technol., 25, 68-76.

Krumholz, L.R., Caldwell, M.E. and Suflita, J.M. (1996) Biodegradation of ‘BTEX’ hydrocarbons under anaerobic conditions. In Bioremediation: Principles and Applications, ed. R.L. Crawford and D.L. Crawford, pp. 61-99. Cambridge University Press, Cambridge, UK.

Lerner, D.N., Thornton, S.F., Spence, M.J., Banwart, S.A., Bottrell, S.H., Higgo, J.J., Mallinson, H.E.H., Pickup, R.W. and Williams, G.M. (2000) Ineffective natural attenuation of degradable organic compounds in a phenol-contaminated aquifer. Groundwater, 38, 922-928.

Werner, P. (1985) A new way for the decontamination of polluted aquifers by biodegradation. Wat. Supply, 3, 41-47.

347

Apart from fair dealing for the purposes of research or private study, or criticism or review, this publication may not be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photographic or otherwise, without the prior permission in writing of the publisher.

The views expressed in this and in all articles in the journal Land Contamination & Reclamation are those of the authors alone and do not necessarily reflect those of the editor, editorial board or publisher, or of the authors’ employers or organizations with which they are associated. The information in this article is intended as general guidance only; it is not comprehensive and does not constitute professional advice. Readers are advised to verify any information obtained from this article, and to seek professional advice as appropriate. The publisher does not endorse claims made for processes and products, and does not, to the extent permitted by law, make any warranty, express or implied, in relation to this article, including but not limited to completeness, accuracy, quality and fitness for a particular purpose, or assume any responsibility for damage or loss caused to persons or property as a result of the use of information in this article.