Is Purging Necessary? A Comparison of Groundwater Data from Pre and Post- Purge Samples

Transcription

1 Is Purging Necessary? A Comparison of Groundwater Data from Pre and Post- Purge Samples 2-Jul-8 Alexander Oiffer a, James Armstrong b, Brent Moore c, Trevor Butterfield a a WorleyParsons Komex b WorleyParsons Komex c Devon Canada Corporation

2 Objective To evaluate the ability of No-Purge sampling to yield representative groundwater samples within a long-term groundwater quality monitoring program. Potential Advantages of No-Purge Sampling include: Cost/Time savings Decreased groundwater disposal costs

3 Outline 1. Overview of purging (Downfalls, Alternatives) 2. Review of previous no-purge sampling studies 3. Evaluation of no-purge sampling based on conceptual understanding 4. Evaluation of no purge sampling based on field data 5. Conclusions and Future Plans

4 Traditional Purging A means to remove stagnant, non-representative water Purge volumes commonly (and somewhat arbitrarily) designated as 3 well volumes. Can generate excessive turbidity Can induce aeration of the water column and volatilization of organic compounds. Similar effect from turbulent recharge. Time consuming and disposal can be problematic.

5 Alternatives to Purging Passive Sampling Devices Low Flow Sampling Devices No-Purge Sampling

6 Some Previous Studies Komex (1999) VOCs, DOC, Major Ions comparable in Pre/Post Purge Robin and Gillham (1987) Flushing of water column less rapid above well screen. USGS (26) Water quality comparable when purging 1 or 3 wellbore volumes. San Mateo Health Services Agency (1997) No purge sampling recommended for unconfined aquifers and well screened across the water table. No-Purge sampling has not been broadly accepted (e.g. Barcelona, 2).

7 No-Purge Sampling Evaluate No-Purge sampling based on: Conceptual understanding of well hydraulics and mass transport processes Comparison of field samples collected prior to and following purging

8 After Wilson et al Well Hydraulics Conceptual Model (Where Does the Water Come From) Plan view of flowline convergence to a monitoring well (with no filter pack) under natural gradients 2d d

9 Well Hydraulics Conceptual Model (Where Does the Water Come From) Plan view of the radius of influence of a monitoring well (with no filter pack) after purging three casing volumes. -Assumes water level in screen 1.2d d Assume n=3%

10 After Robin et al., 1987 Well Hydraulics Conceptual Model Zone of less Active Mixing Zone of Active Mixing

11 Potential Pitfalls of No-Purge Sampling Volatile Loss from Water Column Calculated % Benzene Loss from a 1 m Thick Water Column C 6 H 6 Benzene loss calculation after Schwarzenbach et al., 23

12 Study - Setting 5-1 m Shallow monitoring wells (approx 5 to 1 m deep) Dataset generated from up to 18 wells for each parameter Water table within or immediately below the screen. Till, sand or sedimentary bedrock. Inertial pumps and bailers

13 Evaluation of Field Data What is a significant difference? In this case assessed based on: If sample concentrations differ by more than the field duplicate pair If sample concentrations differ by more than five times the method detection limit (Zeiner, 1994) or a RPD of 2% (which ever is greater)

14 Results Conservative and Semi-Conservative Parameters 15 Chloride 15 5 Sodium 5 Post-Purge (mg/l) Post-Purge (mg/l) Key: 25 Calcium 25 Pre and Post Purge Samples Field Duplicates Pre and Post Purge Samples (water level occurs above well screen) Post-Purge (mg/l) to 1 line 1 Relative Percent Difference Relative Percent Difference

15 Results Redox Indicators 6. Iron Manganese 1. Post-Purge (mg/l) Key: 4 Sulphate 4 Pre and Post Purge Samples Field Duplicates Pre and Post Purge Samples (water level occurs above well screen) 1 to 1 line 1 Relative Percent Difference 2 Relative Percent Difference Post-Purge (mg/l)

16 Results VOCs.16 Benzene Benzene Ethylbenzene y.14 Post-Purge (mg/l) Post-Purge (mg/l) Key: Pre and Post Purge Samples Field Duplicates Pre and Post Purge Samples (water level occurs above well screen) 1 to 1 line 1 Relative Percent Difference Post-Purge (mg/l) Xylenes Relative Percent Difference

17 Results Polycyclic Aromatic Hydrocarbons 1.E+ 1.E+ 1.E-1 1.E-1 Post-Purge (mg/l) 1.E-2 1.E-3 1.E-2 1.E-3 1.E-4 1.E-4 1.E-5 1.E-5 1.E-5 1.E-4 1.E-3 1.E-2 1.E-1 1.E+

18 Effect of Purging Relative to Spatial and Temporal Heterogeneity Differences in concentration between Purged and No-Purge samples are minimal relative to spatial and temporal heterogeneity. SO 4 concentrations at DP2 and DP3 differed by 2-3 orders of magnitude. SO 4 concentrations at MW1, in samples collected during consecutive sampling events were observed to vary from 4 to 1 mg/l (and vice versa). Depth DP1 DP2 DP3 P34 MW1 MW

19 Conclusions 1. Conceptual model suggests No-Purge sampling most applicable in monitoring wells screened across the water table, completed in reasonably permeable formations. 2. Initial data shows good reproducibility, and promising for the application of No-Purge Sampling for compliance monitoring. 3. Comparability of Pre/Post Purge data consistent with previous field studies (e.g., Komex 1999) 4. Effect of purging on analyte concentrations less than effects of screen length, and spatial/temporal variability.

20 Future Plans Study is Ongoing As the dataset grows, it will be possible to more fully evaluate the effect of physical variables (e.g., well screen position and porous medium material) on Pre and Post-Purge reproducibility

21 Questions?

22 Extra Slide(s).25 Toluene.25 Post-Purge (mg/l)