Advances in Land Contamination Assessment & Remediation · Groundwater Sampling Methodology...
Transcript of Advances in Land Contamination Assessment & Remediation · Groundwater Sampling Methodology...
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 [email protected]
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