Dilute and disperse landfills: Dilute and disperse landfills: evidence for natural attenuationevidence for natural attenuation

Brian Bone

Environment Agency Science Group

G.M. Williams, J.K. Trick, D.J. Noy, R.D. Ogilvy (all British Geological Survey) and

T.H.E. Heaton (NERC Isotope Geology Laboratory)

Study of dilute and disperse sitesStudy of dilute and disperse sites• 1974 IGS/DoE survey of landfill sites

– 56 of 3,000+ sites with potential to seriously pollute surface or ground water

• 1973-1978 DoE “Brown Book” study– The Behaviour of Hazardous Wastes in Landfill Sites

• 1980s continued investigation of some Brown Book sites

• 1990s NRA (now EA) desk study to identify potentially polluting landfills– Thriplow selected for study 1996-2002

Phase Ioperated 1957 - 1977household and some commercial/industrial waste7.2ha, filled to 3 - 9 m bglthin soil cover

Phase IIoperated 1981 - 1987household, commercial & industrial waste4ha, filled to 1.8 - 9 m bglcontoured “clay” cap

Thriplow landfill site

Study objectivesStudy objectives

• Characterise plume/s of contaminated groundwater from the landfill

• Identify controls on leachate migration and attenuation

• Develop a well-constrained model for groundwater flow and mass transport

• Provide facilities for long-term monitoring to determine NA processes and test model predictions

Desk studyDesk study

• Previous investigations

• Groundwater monitoring data from existing boreholes

• Environment Agency/local authority archives (disposal history)

• Aerial photographs

• Meteorological data

Preliminary conceptual modelPreliminary conceptual model

Landfills previously gravel pits, up to 9 m deep

Leaching may vary seasonally

Contamination identified in BHs to west

Regional gw flow to NW

Vertical flow possibly limited by Plenus Marls

UncertaintiesUncertainties

• Borehole construction and quality of previous monitoring data

• Borehole coverage

• Waste characteristics and leachate quality

• Hydraulic continuity between Middle and Lower Chalk

• Direction, depth and extent of leachate plume

Site investigations 1996 - 97Site investigations 1996 - 97

• CCTV and sampling from 7 of the 11 previously drilled boreholes (1976 - 1993)

• Landfill characterisation (5 new BHs in waste)

– waste, leachate, porewater, BMP, gas, infiltration

• Surface resistivity imaging (2D)

• Characterisation of Chalk aquifer (1 new BH)

• Groundwater sampling

• Preliminary modelling

Waste characterisationWaste characterisation

Significant findingsSignificant findings• Some BH depths at variance with those recorded

• Some BH completions in both Middle and Lower Chalk - distortion of groundwater flow & dilution of leachate?

• Waste in Phase I was more degraded than waste in Phase II (function of age and cover thickness)

• Landfill resistivity appears to reflect leachate front rather than waste - Chalk interface, artefacts below ~ 20 m

• Elevated TOC, Cl and NH4-N in groundwater downgradient of landfills

• Perimeter resistivity surveys indicate distinct plumes (but one anomaly drilled did not intercept leachate - poor resolution of formation resistivity below 30 m bgl)

UncertaintiesUncertainties

• Drift thickness - geometry of buried channel along western perimeter

• Aquifer properties

• Seasonal variation in groundwater flow

• Calibration of resistivity surveys

• Leachate release from landfill

• Evidence for natural attenuation

Site investigations 1998 - 2002Site investigations 1998 - 2002

• 3D resistivity survey of landfill phases

• 6 (No.) BHs in waste/Chalk

• 3 (No.) BHs north of landfill

• Downhole resistivity arrays in new BHs

• Sampling– waste, porewater, leachate, groundwater, gas

• Trace organics analyses

• Stable isotope analyses

3D resistivity tomography3D resistivity tomography

Porewater chemistry below wastePorewater chemistry below waste

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TP10 Ammonium (mg/l)

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Soil

Waste

Chalk

Saturated Zone (10/11/99)

Soil Fill

Waste

Phase I Phase II

Time-lapse Monitoring of landfill boreholes to Time-lapse Monitoring of landfill boreholes to detect transient leachate plumesdetect transient leachate plumes

Electrode sensors have been permanently installed in landfill boreholes.Monthly monitoring helps to detecttransient plumes and seasonal variations in leachate distribution within landfill

Electrodes on outsideof PVC casing ensures electrical contact withsurrounding materialirrespective of watertable

2D inversion results for prism model2D inversion results for prism model

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Z(m

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N separat ion

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ApparentResistivity(ohm-m)

GROUND SURFACE AT Z=0m

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(a) (b) (c)

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Fig. 8

ERT inversion images for Borehole TP09ERT inversion images for Borehole TP09

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Dec. 1999

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W.T

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W.TW.T W.T

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W.T= 11.91m

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W.TMissing Data

W.T

Jan. 2000 Feb. 2000 Mar. 2 000 Apr. 2000 May 2000

Jul. 2000Jun. 2000 Aug. 2000 Sept. 2000 Oct. 2000 Nov. 2000

W.T

Cl = 41.9 mg/l

Cl = 35.8 mg/l Cl = 36.9 mg/l

Plume?

Plume ?

Plume?

Plume?

Plume?

Chalk

TP09

Fig. 10

Plume?

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So, with a better (although not perfect) idea of the waste

characteristics, groundwater flow, leachate distribution and flow, is

there any evidence for natural attenuation?

Conventional chemistryConventional chemistry

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TOCCl

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TOCCl

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Stable isotope findings ( S & N)Stable isotope findings ( S & N)

100 metres

Methanogenesis

Sulphate reduction

Nitrificaton

Denitrification

Enantiomeric forms of MecopropEnantiomeric forms of Mecoprop

H3CO

Cl

HOOC

CH3H

H3CO

Cl

COOH

H C3 H

2-(4-chloro-2-methyl phenoxy) - propionic acid2-(4-chloro-2-methyl phenoxy) - propionic acid

(R) - Rectus(R) - Rectus (S) - Sinister(S) - Sinister

Biodegradation of mecopropBiodegradation of mecoprop

Redox R-mecoprop S-mecoprop

Methanogenicsulphate red.

NO NO

Nitrate red. YES NO

Aerobic YES YES (faster)

Redox zonesRedox zones

MMeetthhaannooggeenniicc zzoonnee

NNiittrraattee rreedduucciinngg zzoonnee

225500 mm

SSuullpphhaattee rreedduucciinngg zzoonnee

Conceptual model (latest)Conceptual model (latest)

Uncertainties/challengesUncertainties/challenges

• Access

• Spatial data

• Source term evolution with time

– a decade of missing data!

• Significance and frequency of pulsed release of leachate

• Any more plumes?

SuccessesSuccesses

We found a plume!

Development and refinement of tools

Greater awareness of leachate release

Established lines of evidence to support NA