CHLORINATED SOLVENTS. The Value of Modified Active Gas Sampling in Assessing Remedial Options and Risk

Transcription

1 CHLORINATED SOLVENTS The Value of Modified Active Gas Sampling in Assessing Remedial Options and Risk

2 Introduction Issues with chlorinated solvent impacted sites Review traditional approaches to assessment of chlorinated solvents Site investigation and development of CSM Modified Active Gas Sampling overview and principles Case studies October 26,

3 Project Life Cycle Cost vs. Level of ESA Effort Consultant costs % of LCC Skip PSI, CSM Rely on NES SCS and Guideline values 3 Don t use SQEP October 26, 2012

4 Chlorinated Impacted Sites? October 26,

5 Chlorinated Solvent Release CSM October 26,

6 DNAPL Injection Experiment Pouring dyed PCE into the ground October 26,

7 Soil Data Challenges Dr. John Cherry, Federal Contaminated Sites National Workshop, May 2010 PCE release experiment Borden site - Vadose zone excavation October 26,

8 Investigating Chlorinated Solvents Phase 1 But what if only limited information available on the facility? Typical intrusive investigation Discrete soil sampling site coverage or target potential hotspots Don t often locate hotspots Passive soil gas sampling Sub-slab where building present Soil vapour nested vapour probes October 26,

9 Chlorinated Solvent Release October 26,

10 Case Study Industrial Cleaners October 26,

11 Chlorinated Solvent Release CSM October 26,

12 Chlorinated Solvents Evaluating Risk Identifying the source is there one and do we need to find it? What do soil impacts above acceptance criteria mean? Variable criteria for PCE Auckland Council Regional Plan 0.5 mg/kg (CCME 2007) US EPA Tier mg/kg (Residential) and 1,100 mg/kg (C&I) Groundwater impacts identified Is soil remediation required? Would remediation be required for a site with: A maximum PCE soil concentration of 3.1 mg/kg at 0.5 m bgl PCE soil vapour concentration of 104 mg/m 3 at 1.0 m bgl Trace PCE in groundwater (at drinking-water standard) Compare to site with PCE soil concentration of 4,000 mg/kg at 4 m bgl with PCE soil vapour of 22,000 mg/m 3 October 26,

13 Limitations with Traditional Approach Heterogeneity of soil samples over small scales Complex release patterns Exact location of release may not be known Multiple release points both spatially and temporally Transport patterns Large variations in concentrations over small intervals where contaminant exists in pure phase Vapour transport can create false positives Source(s) may not be detected Don t often see degradation of chlorinated solvents in aerobic zone (unlike petroleum hydrocarbons) October 26,

14 Limitations with Traditional Approach Laboratory typically analyses 5 g of sample (300 g jar) Is this statistically representative of the total area assessed? 10 x 10 x 5 deep area (20 Samples) 100 x 100 x 5 deep area (20 Samples) Discrete Soil Sampling 5 grams of soil analysed per sample % (sampled mass) % (sampled mass) Passive Gas Sampling (w/pid) Screen 100 grams sample of soil 0.008% (sampled mass) % (sampled mass) Active Gas Sampling Air removed from boring at ~1 L/min, with ROI = 1 foot 63% (sampled mass) 0.63% (sampled mass) October 26,

15 Modified Active Gas Sampling (MAGS) Technique developed by HSA (2000) Adopted by Florida DEP as part of Drycleaning Solvent Cleanup Program Except where depth to groundwater <1 m or stiff clay Involves fitting a regenerative blower to a temporary soil vapour extraction well. Vacuum measurements are recorded at regular intervals at the wellhead, the blower, and at surrounding monitoring points. As soil vapour is extracted, it is analysed using a PID. Selected samples analysed using field screening (colourmetric tubes, potable GC) and laboratory analysis (tedlar bags, canisters). Provides anticipated mass recovery rates, flow rates, and site-specific radius of influence (ROI) information needed to evaluate the feasibility of soil vapour extraction/dual phase extraction as a remediation option. October 26,

16 Schematic Set-Up of MAGS October 26,

17 Case Study Industrial Cleaners October 26,

18 Schematic Set-Up of MAGS October 26,

19 Vacuum (in H 2 O) MAGS Radius of Influence 7 Wells Pressure (in. H 2 O) Extraction 20 0 Distance (m) 6 5 y = ln(x) R² = MW MW MW MW MW MW MW Distance from Extraction Well (m) October 26,

20 MAGS Vapour Profiles October 26,

21 Applied Vacuum Pa MAGS NZ Case Study 100,000 Vacuum in Silt y = e x R² = ,000 18,000 1,000 ROI = 300 Pa Distance from Vac Well (m) October 26,

22 PID Vapour Concentration ppm MAGS NZ Case Study 350 Abstracted Vapour Concentrations vs Time Time (mins) October 26,

23 Benefits of MAGS Another line of evidence to understand CSM Minimise risk of failing to identify an area of elevated concentrations between sparsely spaced discrete sample locations Minimise effect of small-scale spatial variability by providing a sample volume consistent with the concept of a representative elemental volume Additional information regarding sub-slab permeability, slab leakage etc Concept of purging or collecting high volumes is not consistent with many regulatory guidance documents. October 26,

24 Benefits of MAGS Ability to clear areas based on overlapping ROIs Assisting with characterisation of potential off-site impacts Can assist with identification of source area or multiple sources Assist with design of in-situ remedial system Cost savings against remedial excavation October 26,