New Insights Into Exposure Through Preferential Pathway Vapor Migration

Size: px

Start display at page:

Download "New Insights Into Exposure Through Preferential Pathway Vapor Migration"

Vivien Porter
5 years ago
Views:

1 New Insights Into Exposure Through Preferential Pathway Vapor Migration Megan Hamilton Director of Vapor Intrusion and Risk Assessment Casey McFall, CHMM Vapor Intrusion Project Manager EnviroForensics March 21, 2017 AEHS West Coast Conference San Diego, California

2 The EPA Final Vapor Intrusion (VI) Guidance defines a preferential pathway as: a naturally occurring subsurface feature or anthropogenic subsurface conduit that is expected to exhibit little resistance to vapor flow in the vadose zone or groundwater flow depending upon its location and orientation relative to the water table and ground surface, thereby facilitating the migration of vapor-forming chemicals in the subsurface and/or into buildings. Naturally occurring examples include fractures and macropores. Anthropogenic examples include sewer lines and manholes, utility vaults and corridors, elevator shafts, subsurface drains, permeable fill, and underground mine workings that intersect subsurface vapor sources or vapor migration routes. In highly developed residential areas, extensive networks of subsurface utility corridors may be present, which can significantly influence the migration of contaminants. A preferential migration route can be a significant influence on vapor intrusion when it is of sufficient volume and proximity to a building that it may be reasonably anticipated to influence vapor migration towards or vapor intrusion into the building.

3 The Indiana Department of Environmental Management (IDEM) defines a VI preferential pathway as: subsurface routes of least resistance for vapors. Vapors typically move from areas of high concentration to low concentration (diffusion), and areas of high pressure to low pressure (advection). Preferential pathways serve as a conduit that facilitates both of these processes. Examples of preferential pathways include: underground conduits and utility corridors (e.g., sewer lines, tile drains), fractured bedrock, karst geology, sump pumps, etc.

4 IDEM defines a significant VI preferential pathway as: a preferential pathway that extends through both a soil source area or an area where ground water exceeds VI ground water screening levels (GWSLs) and a building footprint. This may include utility conduits overlying ground water that exceeds the VIGWSLs. Significant preferential pathways merit further investigation. Courtesy of the Indiana Department of Environmental Management

5 Objective: Identify preferential pathways most likely to allow contaminated vapors to reach the indoor air of a structure at concentrations with the potential to adversely affect human health.

6 Preferential Pathway Database This study evaluates a portfolio of sites in Indiana contaminated by subsurface releases of chlorinated solvents where utility corridor backfill has been documented as a vapor migration pathway. Data Set Attribute Full Database Number of Sites 14 Number of Structures 26 Identified Contaminants tdce, PCE, TCE, and VC

7 Preferential Pathway Database Building Use Full Database Residential 11 Small Commercial <5000 square feet Large Commercial >5000 square feet 12 2 Mixed Use 1

8 Building Construction Slab 10 Structures Basement 12 Structures Crawl Space 3 Structures Basement/Slab 1 Structure Soil Gas Concentrations PCE: 120 ug/m 3 1, 440,000 ug/m 3 TCE: 22 ug/m 3 231,000 ug/m 3 Preferential Pathway Database Distance from Soil Gas to Structure: 5 feet 80 feet Number of Paired IA and SS Events per Structure: 1-5

9 Summary of Vapor Intrusion Assessment Results at 26 Properties with Potential Preferential Pathway via Sanitary Sewer 3 23 Complete Incomplete

10 Indoor Air Screening Results By Soil Gas Concentration In Pathway 8 7 Number of Properties IA = ND IA <SL IA >SL 1 0 < >10 Soil Gas Concentration as Proportion of Screening Level

11 Indoor Air Screening Results By Distance From Soil Gas Sample 8 7 Number of Properties IA = ND IA < SL IA > SL Distance from Sample (ft)

12 Indoor Air Screening Results By Construction Type Number of Properties IA = ND IA <SL IA >SL Basement Crawl Slab Foundation Type

13 Conclusions Only 2 of 26 structures (8%) identified as being directly connected to a vapor migration preferential pathway had indoor air detections at concentrations exceeding the health protective levels set by IDEM. The utility corridor backfill around the sanitary sewer was the identified preferential pathway in each case. Both structures were slab on grade construction and located within 10 feet of the elevated soil gas concentration. Two identified structures with elevated indoor air concentrations were also: o Located above the source area and, o The sanitary sewer pipe itself was determined to be impacted

14 Conclusions (cont) The findings of this study indicate that the relationship between the presence of elevated soil gas within the utility corridor backfill leading to a completed exposure pathway within the indoor air, at concentrations with the potential to adversely affect human health may be overstated. Placing emphasis on the subsurface utility corridor backfill material as the main indicator of preferential pathways may not be the most reliable vapor intrusion screening device.

16 A Look at Atypical Preferential Pathways

17 Case Study #1 The building is a 2-story brick structure that is 8,000ft 2 in size. The building houses 2 residential suites and 6 commercial suites. The western portion of the building is constructed of slab-on-grade foundation and the eastern portion contains a basement and crawl space. A 1-story slab-on-grade structure was constructed onto the southern perimeter of the building and is attached to the exterior of the north portion of an active dry cleaner that utilizes PCE in its operations. Soil and groundwater impacted with PCE and TCE.

18 Initial VI Assessment

19 Post Vapor Mitigation System (VMS) Installation

20 MITIGATION MEASURES TIMELINE SITE STOPS ROOF VENTING SSDS NOT INFLUENCING ENTIRE SLAB IMMEDIATE ACTION DEMAND VI SAMPLING IMMEDIATE ACTION DEMAND PFE TESTS CONFIRM NEG INFLUENCE VI SAMPLING - ALL COMPLIANT Feb-14 Mar-14 Apr-14 May-14 Jun-14 Jul-14 Aug-14 Sep-14 Oct-14 Nov-14 Dec-14 Jan-15 Feb-15 Mar-15 Apr-15 May-15 Jun-15 Jul-15 Aug-15 Sep-15 Oct-15 Nov-15 Dec-15 Jan-16 Feb-16 Mar-16 Apr-16 May-16 Jun-16 Jul-16 Aug-16 Sep-16 PASSIVE SAMPLING - SUITE F SEALED CHIMNEY; EXPAND VMS SITE RESTARTS ROOF VENTING MODIFIED SSDS

21 Building Cross Section

22 Final Confirmatory Sampling

23 Case Study #2 Former Dry Cleaner in large strip mall building Built in mid-1970 s, slab on grade, single-story; 14 tenant spaces, ~23,000 square feet One suite operated as a dry cleaner using chlorinated solvents from mid-1970 s through 2011 Same suite operated as a drop shop for PCE-cleaned clothes from 2011 through 2014 Then as a green dry cleaner from 2014 through early Stated they use no chlorinated solvents in their operations. Release limited to soils directly underneath dry cleaner suite and adjacent suites; no groundwater impacts due to cohesive soil and lack of true aquifer (interbedded, discontinuous sand lenses)

25 INITIAL VI ASSESSMENT

26 SECOND VI ASSESSMENT

27 PREFERENTIAL PATHWAY INVESTIGATION 1

28 THIRD VI ASSESSMENT

29 Post-SSDS IA Sampling & Preferential Pathway Investigation 2 Installed traditional sub-slab depressurization system (SSDS) within dry cleaning suite Confirmed negative pressure across entire dry cleaning slab 45-days post install IA sample: TCE > 50x SL Use HAPSITE instrument to identify unidentified preferential pathways and potential indoor air sources. No additional sources identified with HAPSITE. Heavy rains several days before the HAPSITE investigation filled floor drains with water. Standing water under building may have impeded vapor migration through PP.

30 PREFERENTIAL PATHWAY INVESTIGATION 3

in dry cleaning suite and Suite A Sealed unused floor drains with

31 Preferential Pathway Investigation 3 (cont d) Corrective / Mitigative Measures Taken Installed or repaired P-traps within sinks in dry cleaning suite and Suite A Sealed unused floor drains with concrete Installed one-way drain plugs to block sewer vapor in remaining Suites

32 FIFTH VI ASSESSMENT Indoor air and sub-slab vapor concentrations collected from all suites were below their applicable screening levels

33 Lessons Learned Expect the unexpected with preferential pathways Don t take for granted plumbing or other building fixtures are installed correctly or to code When possible, attempt to verify the information provided by the tenants regarding indoor air sources Get a plumber involved sooner Be aware of older building construction, including historic features and numerous additions.

34 Thank You! Megan Hamilton Director of Vapor Intrusion and Risk Assessment

A Rational Approach to Vapor Intrusion Preferential Pathways

A Rational Approach to Vapor Intrusion Preferential Pathways PRESENTED BY: David J. Folkes, PE Geosyntec Consultants, Inc. AEHS West Coast Conference EPA Vapor Intrusion Technical Workshop March 22, 2016