Groundwater Sampling Methodology Groundwater Sampling Methodology Guidance, Good Practice and the Importance of Screen Guidance, Good Practice and the Importance of Screen

Length in the Assessment of Contaminated LandLength in the Assessment of Contaminated Land

Advances in Land Contamination Assessment & Remediation

18 September 2012

Peter DumblePeter DumbleHydrogeologist / Technical Director

Waterra-In-Situ

Lindsay MLindsay MccMillanMillanNERC Doctoral Researcher

University of Birmingham

Mike RivettMike RivettSenior Lecturer in Contaminant Hydrogeology

University of Birmingham

John TellamJohn TellamProfessor of HydrogeologyUniversity of Birmingham

Lindsay MLindsay MccMillanMillanNERC Doctoral Researcher

University of Birmingham

UK Groundwater Sampling GuidanceUK Groundwater Sampling GuidanceEnvironment Agency / SEPA Guidance

LFTGN 02 2003 Guidance on monitoring of landfill leachate, groundwater and surface water (NB SEPA Guidance is an earlier version of this)

ISO / British Standards - Methodology

BS ISO 21413 2005 Manual methods for the measurement of a groundwater level in a well.

BS ISO 5667-11 2009 Guidance on sampling of groundwaters.

BS ISO 5667-22 2010 Guidance on the design and installation of groundwater monitoring points.

ISO / British Standards - Specific Applications

BS ISO 22475-1 2006Geotechnical investigation and testing - sampling methods and groundwater measurements. Part 1: Technical principles for execution.

BS 10175 2011 Investigation of potentially contaminated sites - Code of Practice.

BS 10175 BS 10175 –– GuidanceGuidanceInvestigation of Potentially Contaminated Ground CoPInvestigation of Potentially Contaminated Ground CoP

• Defers to BS ISO Standards– BS ISO 5667-11 (Groundwater Sampling)– BS ISO 5667-22 (Design of Monitoring Points)

• Requires– Collection of a representative sample– Purging before sampling

• Where LNAPLS and/or DNAPL are present– Consider micro-purging (low flow sampling) to avoid

redistribution of contaminants

LowLow FlowFlow SamplingSampling GuidanceGuidance

BS ISO 5667-11

“Micro-purging …. is most suited to open boreholes or

piezometers with long screen lengths where the formation has significant

permeability.”

(Section 6.1.2 Micro-purging)

USEPA Guidance

“For high resolution sampling needs, screens less than 1 m should be

used.”

(Puls & Barcelona, 1996, Page 5)

Be wary of BS ISO 5667-11 guidance

MethodsMethods forfor samplingsampling groundwater groundwater -- OptionsOptions

• Disposal of possibly large volume of water

• Sample larger volume of aquifer

• Long‐standing “conventional”sampling method (late‘70s)

• Pump rate < 0.5L/min• Pump until determinants stabilise

• Since 1980s

• Sample without purging• Alternative to Low Flow• Since 2000’s

• Mainly US‐lead R&D focused on short‐screened wells• Ongoing debate to suitability in long‐screen wells (>3m)

Volume Purge Low‐Flow/Micro‐Purge Passive Sample

Where does the sample come from?Where does the sample come from?

Flow-Weighted Average (FWA) samples

Short screens / no vertical flow in borehole

Contaminated Site Contaminated Site –– Short ScreensShort ScreensExample of flowExample of flow--weighted averaging weighted averaging

Single 3m screened monitoring wells compared to multilevel samplers

See: Sabre bulletins, available

from www.claire.co.uk

Figure modified from presentation by Wealthall et al. (2010) to ATV Jord Og Grundvand (http://www.atv-jord-grundvand.dk/)

Establishing clean-up criteria at the Source Area Bioremediation (SABRE) Site.

Monitoring well concentration scale  (log uM)

Mulitlevel sampler contoured data

Permeable layer

Sampling in LongSampling in Long--Screened WellsScreened WellsFlow in BoreholesFlow in Boreholes -- Vertical Hydraulic GradientsVertical Hydraulic Gradients

44.7

44.6

38.8

42.8

44.8

44.9

Multilevel borehole

Long screened borehole

~15

met

res

Water quality in upper part of borehole dominated by inflows near top of screen

Water quality in lower part of borehole dominated by inflows near base of screen

Alan Mayo, 2010. Hydrogeology Journal 18, 823-827

Only need a head difference of as little as 1 cm/m for flow to occur

“Flow-Biased” samples

Environment Agency Contaminated Land Site (Unpublished data)

• Similar results - samples could be in same vertical flow stream.• BUT samples could be biased by vertical flows arising from

different depths in the aquifer – more information needed.

Contaminated Site Contaminated Site –– Long ScreensLong ScreensExample of flowExample of flow--bias?bias?

Samples taken at mid-point between water-table and bottom of borehole

NERC PhD Industry Case supported by:

Groundwater Quality: Rigorous sampling and

interpretation of long-screen wellsLindsay McMillan (NERC PhD Case award student)

Michael Rivett, John Tellam

Lindsay McMillan  LAM906@bham.ac.uk

Peter Dumble

Helen Sharp

What does a sample tell us about groundwater quality?

• Uncertainty exists due to vertical intra-borehole flows

• Uncertainty greatly increases with screen length

A literature perspective on long-screen wells...

“…. practice of installing long well

screens in contaminant

monitoring wells should be

abandoned.”[6]

“…use of long-screened monitoring

wells should be phased out”[2]

“Passive and micro-purge sampling should not be

used...”[4]“ ...cross-

contamination...persisted until 1600 well-bore volumes

purged”[3]

“...ambient flows are ubiquitous in both

confined and unconfined aquifers”[2]

Applicability of existing sampling methods?

Likelihood of vertical

flows?

Use in groundwater monitoring?

14

Four 50-70m deep boreholes (Chalk):

Field Case Study 2

3

1 3

4

Groundwater Flow

2

• Historical data indicatesrising trend in NO3

• EU Drinking Water Standards exceeded

Nitrate Concentration in BH 1

EU DWS

Single Borehole Tracer Test

Conductivity

Late Time

Conductivity

Dep

th

Early Tim

eMid Time

Late Time

• Saline water injected into borehole increasing conductivity above ambient levels [5]

• Conductivity (EC) Logger measured conductivity decay with time

• Conductivity trends reproduced by simple Excel flow and transport model

• Agency permission required before tracer tests can be undertaken[2].

• Approach limited by:

• density effects (low‐flow conditions)

• maximum logger pass speed (high‐flow conditions)

Specific Conductivity (mS/cm)

Dep

th below

 casing bo

ttom

 (m)

Flow Measurement Results

21

19.6

20.6

20

Nitrate As N (mg/L)

Borehole 4 flushed in 24 minutes (1000L)

Specific Conductivity (mS/cm)

Dep

th below

 casing bo

ttom

 (m)

Borehole 3 flushed in 60 minutes (950L)

17

15

18

18

Nitrate As N (mg/L)

Specific Conductivity (mS/cm)

Dep

th below

 water (m

)

Flow Measurement Results

19.6

19.5

20.1

19.9

Nitrate As N (mg/L)

Borehole 2 after 48 hours(no significant through‐flow)

Specific Conductivity (mS/cm)

Dep

th below

 water (m

) 11

0

12

Nitrate As N (mg/L)

11

Borehole 1 flushed in 10 minutes (1300L)

Predicting Aquifer Nitrate ConcentrationsUsing a simple mass balance approach and assuming full mixing, the calculated flows and hydrasleeve results can be used to constrain estimates of inflowing Nitrate concentration:

17

15

18

18

15

24

Borehole 3

20

21 21

19.6

20.6

20

Borehole 4

Hydrasleeve samples

Modelled values

• Vertical flow delineation is vital– Field Trials

• Flow delineation using in-borehole tracer test (salt)• Discrete depth samples using hydrasleeves (passive samplers)

– Modelling• Modelling of flow• Modelling of aquifer water quality

• Individual borehole flows are highly variable– can’t assume vertical flow paths are similar in adjacent boreholes

(i.e. field measurement is necessary)• Can begin to estimate water quality in adjacent aquifer

– Modelling can be used to calculate quality in the aquifer (representative sample?)

– Method could possibly be evolved to provide an indicator of “uncertainty”

Long Screen Wells - Early Conclusions

• Volume Purging– Should in time overcome vertical gradients and

produce a flow-weighted average?

• Low Flow and Passive Methods– Samples collected within a vertical flow stream are

likely to yield similar results using passive or low flow methods

– Sample position alone is not indicative of the origin of the sample.

– May need more than one sample (from different depths)

• Number is dependent on complexity of flow

Sampling Methods - Early Conclusions

• Short screens – “flow-weighted average” samples– Less uncertainty on origin of samples - but vertical flows can further bias

samples

• Long screens – “flow-biased” samples– High uncertainty on origin - flow dominated by presence of vertical

gradients

• Practical considerations– Low Flow, Passive and Volume Purge methods all have their place– May need more than 1 sample in long-screened wells

• Interpretation of Water Quality Data– Main objective of research – can a single sample / historical data from

long-screened wells be interpreted retrospectively?

Conclusions

References1. Ashworth, A. J., Barnes, B. C., et al. (2005) Indicators for Land Contamination.

Environment Agency Science Report SC030039/SR

2. Elci, A., Molz, F. J. I., et al. (2001). Implications of Observed and Simulated Ambient Flow in Monitoring Wells. Ground Water 39(6): 853-862.

3. Mayo, A. L. (2010). Ambient well-bore mixing, aquifer cross-contamination, pumping stress, and water quality from long-screened wells; What is sampled and what is not? Hydrogeology Journal 18(4): 823-837.

4. McDonald, J. P. & Smith, R. M. (2009). Concentration Profiles in Screened Wells under Static and Pumped Conditions. Ground Water Monitoring & Remediation 29(2): 78-86.

5. Maurice, L., Barker, J.A., et al. (2011). A Tracer Methodology for Identifying Ambient Flows in Boreholes. Groundwater 46(2): 227-238.

6. Reilly, T. E., Franke, O. L., et al. (1989). Bias in Groundwater Samples Caused by Wellbore Flow. Journal of Hydraulic Engineering 115(2): 270-276.

CL:AIRE • www.claire.co.uk

• This bulletin outlines the principles and practice governing the collection of representative groundwater samples, by understanding the source and mitigating the effects of, potential errors occurring during the sampling process.

Monitoring References www.waterra-in-situ.com

1. Fixed Volume Purge & Sample– Higher pumping rates– Remove 3 or more well volumes– Stability monitoring of chemistry - optional

2. Low Flow Purging & Sample– No prior disturbance of water column (use dedicated pumps)– Low pumping rate (<0.5 litre/min)– Stability monitoring of chemistry– No drawdown

• Max of 10 cm or • 25% of distance from pump intake to top of well screen

– Short screen (<<6m)

3. No Purge (Passive Sampling)– No prior disturbance of water column– Short screen

Summary of Sampling MethodsSummary of Sampling Methodsfor Short Screened Wellsfor Short Screened Wells

Low Flow Sampling Set UpLow Flow Sampling Set Up

Pump Control Unit(e.g. for Bladder

Pump)

Pumped discharge

from borehole

Water Level Dip Meter Discharge measured

and collected into bucket

Water Quality Probe in Flow

Cell

Air Line