Obtaining Representative Groundwater Samples

Transcription

1 Obtaining Representative Groundwater Samples Two field studies demonstrating the effect of sampling method and device selection Water Technologies Symposium April 2016

2 Acknowledgments Clients Maxxam and ALS SNC-Lavalin Audience

3 Introduction Why do we collect geochemical data from aquifers and aquitards? General Characterization Trend Analysis and Water Quality Objective Comparison Obtaining Representative Groundwater Samples Water Technologies Symposium April

4 Introduction Why do we collect geochemical data from aquifers and aquitards? Source Characterization Speciation / Risk Assessment Obtaining Representative Groundwater Samples Water Technologies Symposium April

5 Introduction Why do we collect geochemical data from aquifers and aquitards? Contaminant Fate and Transport Numerical (left) or Analytical (right) Obtaining Representative Groundwater Samples Water Technologies Symposium April

6 Introduction Why do we collect geochemical data from aquifers and aquitards? Site Remediation and Receptor Protection Obtaining Representative Groundwater Samples Water Technologies Symposium April

7 Introduction Given the high value outputs, what assessment can we draw from compromised data? Compromised integrity 10 m screen 300 NTU Obtaining Representative Groundwater Samples Water Technologies Symposium April

8 The Challenge/Objective Accurate and Precise Analytical Results Well design: Flow paths Screen and filter pack design Drilling method Intended use Development Maintenance Sample collection: Consistent methodology Pressure and temp change Artificial entrainment Agitation/aeration Obtaining Representative Groundwater Samples Water Technologies Symposium April

9 The Challenge/Objective Bias Free Sample bias is the systematic or persistent distortion of a measurement process that causes errors in one direction Common sources of bias due to sampling equipment and methodology Turbidity Positive bias due to artificial entrainment of immobile particles (e.g. clays, metal hydroxides, organic materials), negative bias due to artificial sorption and subsequent field filtration Agitation Volatilization negative bias of volatile and semi volatile species Dissolution of dissolved gases Oxidation and reduction of metals Interval Systematic sampling from select intervals Cross contamination Sampling sequence, decontamination procedures, leaching from materials Obtaining Representative Groundwater Samples Water Technologies Symposium April

10 The Reality Every Well is Unique Obtaining Representative Groundwater Samples Water Technologies Symposium April

11 The Methods Well-Volume Purging and Sampling Gibb et al 1981 demonstrated sensitivity of sampling results to Q, V and agitation during purging Water quality parameters stabilize within purging of 6 well volumes Accepted and widely used in industry Significant issues continued to arise with the method: Adherence to purging 3 well volumes regardless of water quality parameters variation Inconsistent results when used in conjunction with inappropriate equipment (e.g., bailers and high rate pumps) Generates large volume of waste water for disposal Obtaining Representative Groundwater Samples Water Technologies Symposium April

12 The Methods Low-Flow Purging and Sampling Late 1980 s and early 1990 s, it was realized that monitoring field parameters to stabilization during purging would minimize purge volumes Pumping equipment required to be clean and inert, intake set in screen interval Screen entrance velocity < 0.03 m/s (minimize well disturbance) Purge until drawdown rate and water quality parameters stabilize Barcelona et al 2005 Obtaining Representative Groundwater Samples Water Technologies Symposium April

13 The Methods No-Purge (Passive Sampling) Based on the concept that the well screen and filter pack act as a high K zone Formation water constantly renews across the screen Self purging allows use of no-purge sampling equipment and chemically selective PDB samplers Minimizes time requirements and waste generation, and logistically simple Significant considerations include: Project objective PCOCs MW integrity MW design and development Stratified water quality potential Stagnant zones Lost opportunities K form < K filter 10 m redox buffer Obtaining Representative Groundwater Samples Water Technologies Symposium April

14 The Equipment Device Method Depth Limit (m) Field Parameters Bailer WV - ph, SC and temp Inertia Lift WV 70 ph, SC and temp Peristaltic Environmental pump (e.g., DVP, Bladder, piston) Equilibrated grab sampler (e.g. HydraSleeve) WV or LF WV or LF 8 ph, SC and temp 300 (model dependant) ph, SC, temp, DO, ORP, alkalinity NP - ph, SC and temp Major Ions Metals VOCs Non-volatile Organics Dissolved Gas Microbiology Other considerations: Scientific, Industry and Regulatory acceptance Ambient geochemistry and temperature Equipment material, energy source, diameter, weight, portability, head lift, discharge rates, ease of cleaning, and maintenance Sample volume limitations Obtaining Representative Groundwater Samples Water Technologies Symposium April

15 Consistency is Key Develop a Sampling Analysis Plan MW ID Well depth Screen interval Casing diameter Expected water level Concerns Purge/Sample method Purge/Sampling device Sample depth Purge/sample discharge rate (if applicable) Analyses required Order of analysis priority Sample volume requirements Filtration and preservation required Shortest hold time Obtaining Representative Groundwater Samples Water Technologies Symposium April

16 Case Studies Case Study #1 Low-Flow versus Well-Volume (Risk Driver) Case Study #2 No-Purge versus Well-Volume (Optimization Driver)

17 The Unfortunate Reality 30 70% samples above 100 NTU 95% samples above 5 NTU (US EPA DWQG) % Samples Sample turbidity (NTU) Obtaining Representative Groundwater Samples Water Technologies Symposium April

18 Field Study #1 Low-Flow Equipment: Fultz pump (gear drive) used for low-flow sample Head lift ~ 30 m Stainless steel housing, teflon gears, PVC wrapped power line, polyethylene sample tubing 1000 W generator Aquaread multiparameter water quality (temp, ph, EC, DO, Eh and turbidity) meter Water level E-tape Equipment cleaning solutions: Phosphate free Liquinox (detergent) Citranox (acid cleaner/detergent) Well-Volume Equipment: Disposable bailer (PVC) Obtaining Representative Groundwater Samples Water Technologies Symposium April

19 Field Study #1 Methodology: Set pump intake at top of the screen interval or 2/3 saturated thickness Purge at 500 ml/min or less until drawdown rate, ph, EC temp, DO and Eh stabilize Collect sample Measure sample turbidity Remove equipment and clean Remove three additional casing volumes using a bailer Allow water level to recover Collect sample from screen interval Measure sample turbidity Samples analysed for Total As, Cr and Cu Obtaining Representative Groundwater Samples Water Technologies Symposium April

20 Field Study #1 Obtaining Representative Groundwater Samples Water Technologies Symposium April

21 Field Study #1 73 method duplicates collected to date 8/73 (10%) of low-flow samples had elevated turbidity (>50 NTU) 24/78 (33%) of well-volume samples had elevated turbidity Samples with turbidity values less than 50 NTU are considered good Well-volume samples are poor for 1/3 of samples, and when they are the metals concentrations are biased high Obtaining Representative Groundwater Samples Water Technologies Symposium April

22 Field Study #1 Where: A = result from low-flow sampling method B = result from well-volume sampling method Positive RPD indicates method A (low-flow) provides higher concentrations than method B Cl As Cr CrVI Cu Turbidity +1% -28% +27% +2% -3% -64% Overall, well-volume samples may bias As high, and Cr low relative to low-flow sample results in this study Obtaining Representative Groundwater Samples Water Technologies Symposium April

23 Field Study #1 Conclusions: Portable environmental pumps allow consistent methods to be employed, and reduce purge water disposal Bailers are logistically simple Time requirements to sample are similar for both methods Bailer samples are more likely to contain artificially entrained sediment When sample turbidity is elevated the bailer sample metal concentrations are significantly (1.7 to 2.5 times) elevated relative to the pump sample Average turbidity using a bailer was 62 NTU, using the pump it was 22 NTU Obtaining Representative Groundwater Samples Water Technologies Symposium April

24 Field Study #2 Screening Process: Ensure No-Purge is suitable (7 step process) No-Purge Equipment: HydraSleeve (polyethylene) Stainless steel suspension cable and weight Multiparameter water quality sonde (ph, EC temp) Well-Volume Equipment: Disposable bailer (PVC) Multiparameter water quality sonde (ph, EC temp) Obtaining Representative Groundwater Samples Water Technologies Symposium April

25 Field Study #2 Methodology: Attach bottom weight and lower HydraSleeve to sit suspended within the screen interval Allow to equilibrate Collect sample of water column core by pulling up smoothly 0.3 to 0.6 m/s Remove excess water Pierce sleeve using pointed discharge tube and collect sample Remove three additional casing volumes using a bailer Allow water level to recover Collect sample from screen interval Samples analysed for BTEX, PHC F1 and F2 Obtaining Representative Groundwater Samples Water Technologies Symposium April

26 Field Study #2 Obtaining Representative Groundwater Samples Water Technologies Symposium April

27 Field Study #2 31 method duplicates collected to date BTEX concentrations up to 1.2 mg/l F1 concentrations up to 5 mg/l F2 concentrations up to 0.7 mg/l HydraSleeve concentrations for BTEX and F1 consistently greater than bailer sample Bailer samples typically elevated in F2 concentration relative to HydraSleeve Positive RPD indicates method A (no-purge) provides higher concentrations than method B Benzene Toluene Ethylbenzene Xylenes F1-BTEX F2 +10% +14% +8% +1% +5% -6% Overall, HydraSleeve concentrations are greater likely due to less sample agitation and volatilization Bailer samples elevated due to increased agitation and turbidity. Labs extract entire contents of the bottle (dissolved + sorped F2) Obtaining Representative Groundwater Samples Water Technologies Symposium April

28 Field Study #2 Conclusions: HydraSleeve samples are logistically simple, minimize purge water and minimize site time Bailers are logistically simple Bailer samples are more likely to contain artificially entrained sediment (F2 positive bias) Bailer samples are more likely to have lost volatile species (BTEX and F1 negative bias) No-purge sampling requires specific site conditions for it to be appropriate No-purge sampling represents a lost opportunity Limited sample volume often restrict laboratory QA/QC processes Limited sample volume may restrict field and laboratory analysis Field measured DO and Eh Obtaining Representative Groundwater Samples Water Technologies Symposium April

29 Summary % Samples Initial evidence points toward inadequate monitoring well design/development, and/or sampling processes There is an over reliance on tools such as filters to clean up samples prior to analysis These methods will create positive and negative biases in sensitive parameters Such biases make managing contaminated or potentially contaminated sites exceptionally difficult Sample turbidity (NTU) Obtaining Representative Groundwater Samples Water Technologies Symposium April

30 Summary % Samples Field Study #1 Environmental pumps used in conjunction with low-flow methods produce repeatable and consistent groundwater samples Bailers used in conjunction with well-volume methods produce more inconsistent results, with a greater potential for elevated turbidity and metal concentrations Reliance on biased data from bailers results in expensive mismanagement of sites when the PCOC has a high sorption coefficient or are hydrophobic Sample turbidity (NTU) Obtaining Representative Groundwater Samples Water Technologies Symposium April

31 Summary % Samples Field Study #2 HydraSleeve samples (no-purge) produced repeatable and consistent groundwater samples Bailers used in conjunction with well-volume methods produced results with negative bias for volatile parameters and positive bias in sorped species No-purge sampling is very economical but its applicability should be demonstrated prior to use Reliance on biased data from bailers may give a false confidence of site clean-up when the PCOC is volatile Sample turbidity (NTU) Obtaining Representative Groundwater Samples Water Technologies Symposium April

32 Questions

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