KARST GEOLOGY, VAPOR INTRUSION, AND HUMAN HEALTH RISK ASSESSMENT. Fundamental Issues to Consider

Transcription

1 KARST GEOLOGY, VAPOR INTRUSION, AND HUMAN HEALTH RISK ASSESSMENT Fundamental Issues to Consider

2 Introduction 2 Development of property with karst features Standard geologic investigation methods to determine structural complexity of karst geology Risk Assessment on site with contamination in karst geology How do we differentiate the SOW when the property is contaminated with VOCs? How might we interpret the study results if contamination exists? How will the Conceptual Site Model (CSM) assist with process?

3 Karst Geology vs. Contamination 3 Typical Karst Assessment Structural Condition Groundwater Use Risk Assessment with Contamination in Karst Geology Structural Condition Groundwater Use Potential Chemical Vapor Migration Exposure/Human Health Risk Assessment

4 Outline 4 Human Health Risk Assessment Preliminary Conceptual Site Model Development of Scope of Work Elements of Karst for a Hydrogeologic Model Investigative Methods Evaluation of Karst Refining the Conceptual Site Model CSM Figure

5 Human Health Risk Assessment 5 What is a Human Health Risk Assessment? It s a tool used to estimate whether exposure to chemicals in the environment (e.g. soil, groundwater, air) will cause adverse health effects

6 Basic Components of a RA 6 Site Characterization (data collection) Measure the chemical concentration in source media Exposure assessment (conceptual site model) Evaluate potential fate and transport, receptors and exposure pathways Toxicity assessment Assess the health effects the chemicals may have on a receptor (carcinogenic and noncarcinogenic effects) Risk characterization Combines the amount of chemical intake with the toxicity assessment to establish a numeric risk estimate

7 7 Basic Components of a RA

8 Basic Components of CSM 8 There are several components to a conceptual site model (CSM): Hydrogeologic Model Human Health Model

9 Conceptual Site Model 9 Different forms Picture Flowchart Table

10 Preliminary CSM 10 Start by gathering information: Regional geology Future development plans Historic use of property Well survey (local water usage) Helps to define Geologic/Hydro SOW

11 Development of Scope of Work 11 Develop characterization SOW to define vertical and horizontal extent of contamination Extent of pinnacles and voids (solution cavities) Bedrock topography GW in fractures vs. unconsolidated or both Extent of unconsolidated zone and type of material Level of fracturing DNAPL/LNAPL Groundwater flow

12 Important Elements of Karst for Hydrogeologic CSM Development 12 Variable Depth to Bedrock Sinkholes and Closed Depressions Fractures and Voids Disappearing Streams Complex Groundwater Flow

13 Variable Depth to Bedrock 13 Soil cover ranges from thick to absent Exposed rock at surface Contact of rock with buildings/foundations SIGNIFICANCE: Direct entry of contaminants to fractured bedrock and/or groundwater through exposed fractures No attenuation capacity for soil gas emanating from groundwater or from subsurface impacts

14 Variable Depth to Bedrock 14 Heating oil on top of weathered limestone bedrock at 2 ft-bgs

15 Sinkholes and Closed Depressions 15 Significance of Closed Depressions Zones of groundwater recharge Future sinkhole development Flooding Sinkholes Direct pathway to groundwater system Instability Flooding Seasonal flooding may be result of backing-up of conduit system Transport mechanism delivering contamination to surface

16 Sinkholes and Closed Depressions 16 Sinkhole development in stormwater retention basin

17 Fractures and Voids 17 Solution enhanced conduits for contaminant accumulation and migration Conduit network is generally unpredictable and complex Provides significant pockets for accumulation of residuallycontaminated sediments and water Seasonal activity Seasonal or episodic activity sediments or contaminated subsurface pool may only be in contact with rest of karst system when flooded during heavy precipitation events Timing of monitoring is critical can miss initial flush of impacts from system Provides additional element of variability to analytical data

18 Fractures and Voids 18 Groundwater discharge from conduit flow

19 Disappearing Streams 19 Unpredictable Surface Water Intermittent occurrence of streams due to majority of water transport occurring below ground Seasonality of quantity and quality Surface water may be present during wetter periods of year when conduit network is full or near capacity Surface water quality is variable due to flushing from conduit network

20 Disappearing Streams 20 Losing stream in a limestone valley, central PA

21 Disappearing Streams 21 Swallow-hole within losing stream bed, central PA

22 Groundwater 22 Complex Groundwater Flow Direction of decreasing potentiometric head versus actual groundwater flow path Conduits/voids provide seasonal contaminant loading to groundwater Groundwater occurrence overburden, bedrock or both? Multiple flow regimes possible Groundwater follows different conduits based on overall stage of groundwater system Diffuse versus conduit flow Diffuse flow saturated groundwater flow through the rock matrix Conduit flow flow within the fractures, solution-enhanced conduits Which is representative of groundwater system?

23 Groundwater and Surface Water 23 Heating oil seep along bedrock in drainage ditch

24 Groundwater 24 Groundwater flow pre- and post-hurricane Ivan, Sept August 31, 2004 September 30, 2004

25 Investigative Methods 25 The complexities with karst require multiple investigative techniques to minimize unknowns Geophysics electrical resistivity, GPR, etc. Soil Borings Confirmation of geophysical results depth to bedrock Assessment of soil impacts Monitoring Wells assess groundwater flow and impacts Tracer Testing identify groundwater flow and discharges that your monitoring network has missed

26 Investigative Methods 26 Use of geophysics to map bedrock and sinkholes Terrain Conductivity Investigation

27 Investigative Methods 27 Tracer testing of Karst System

28 Evaluation of Karst Data 28 Evaluation of karst data relative to development of Hydrogeologic Conceptual Site Model How does karst setting effect contaminant migration pathways? How does karst setting effect potential exposure pathways?

29 Karst and Contaminant Migration 29 Contaminant migration Vapor migration from shallow bedrock Soils as source Fractures/conduits as source Sediments within fractures/conduits as source Standing pools within conduits as source Surface water Identify how the impacts migrate to surface water Identify where impacts discharge to surface water Groundwater Potential for significant off-site migration given conduit network Groundwater to surface water discharge Inferred groundwater flow direction versus actual flow direction and path Bedrock groundwater to overburden and overburden groundwater contaminant transport Periodic back-up of conduit system can transport contaminants to overburden

30 Karst and Exposure Pathways 30 Identify potential exposure pathways relating to the occurrence and migration of contamination Soils capability to provide attenuation for soil gas Soil thickness Soil type Separation from groundwater

31 Karst and Exposure Pathways 31 Groundwater Potential for significant off-site migration given conduit network = potentially large number of receptors Seasonality Diffuse versus conduit flow Inferred groundwater flow direction versus actual flow direction and path Frequency of conduit backup Flooding of overburden with bedrock groundwater thereby decreasing the maximum depth of exposure to contaminated groundwater Significant conduit flooding resulting in formation of surface water containing contaminated bedrock groundwater

32 Refining the CSM 32 So far Vertical/horizontal extent accomplished by monitoring well and soil boring placement Fate and Transport of constituents Use geologic/hydro model to determine receptors/exposure pathways Now we can refine the CSM

33 Refining the CSM: Things to Consider 33 Volatiles within solution cavities? Preferential pathway Do bedrock pinnacles reach near the surface? Do fractures lead to building? How much attenuation? How far do karst fractures extend? Does contamination on-site migrate through karst fractures to off-site buildings or potable water source?

34 34 CSM Figure

35 Contacts 35 The Mahfood Group LLC 1061 Waterdam Plaza Drive Suite 201 McMurray, PA (724) DMS Environmental Services LLC 103 South Spring Street Bellefonte, PA (814)