Aquitard Characterization The Legend of Indiana s Magic Clay Layer. Juliet Port, LPG #2214 July 2014

Transcription

1 Aquitard Characterization The Legend of Indiana s Magic Clay Layer Juliet Port, LPG #2214 July 2014

2 Topics What is an Aquitard? Why do we care? Review of Indiana glacial geology Conceptual Framework Investigation techniques

3 Definition Aquitard: A geological unit of low permeability that can store ground water and contamination and also transmit them slowly from one aquifer to another. Confining Layer Aquitard Source: Cherry and Parker, 2004

4 Why do we care? Indiana s water resources are threatened by releases of chlorinated volatile organic compounds (VOCs). There are municipal water supplies with deep wells (>250 feet below grade) that have chlorinated VOCs or other contaminants (e.g., ions) in raw water.

5 Why do we care? Aquitards are common throughout Indiana. Fine grained glacial deposits and sedimentary rocks can be aquitards. Aquitards can be crucial to the fate and transport of contaminants (i.e. plume behavior assessments). Hint: Confining units are not common in Indiana.

6 Indiana Glacial Geology 1970 Glacial Geology Map Source: Kingsbury, 1970

7 Current Surficial Geology Map Source: IGS Wisconsin Lobes

8 Glacial Deposits Source: USGS, 1999

9 Glacial Landscapes Source: USGS, 1999

10 Things are not as simple as we initially assumed Allen County updated cross-section Source: Prentice and Letsinger, 2012 Lagro overlying Huntertown Lake mud cracks

11 Deformed till Jointed till (vertical fractures) Source: Fleming and Rupp, 1994 Trafalgar basal till (typical diamicton) Lagro till with 1 gravel layer Magic Clay?

12 Aquitard Characterization

13 Conceptual Framework Ground water flows through aquitards. Aquitards can have sufficient areal extent, thickness, and geometry to impede or deflect ground water flow from or into aquifers. Aquitards can determine flow paths and serve as storage units for both water and contaminants, and changes in ground water flow.

14 Conceptual Framework (cont.) Ground water often spends more time in aquitards than in aquifers. Hydraulic properties can cause very long response times to water levels. Source: Cherry and Parker, 2004

15 Conceptual Framework: flow paths Contaminants, especially DNAPL, can flow along the surface of an aquitard. Ground water flows vertically through aquitards. DNAPL = dense non aqueous phase liquid Source: Cherry and Parker, 2004

16 Conceptual Framework: extent Natural Preferential Pathways: fractures, macropores, or plant roots. Shallow Aquitards and Man made Preferential Pathways: typically breached due to engineering/public works, wells, etc.

17 The k is why look further? Geotechnical analysis of soil types is often useful for conceptual site model (CSM) refinement, evaluation of remedy options, etc. However, many of Indiana s tills have dual porosities (due to fracturing, etc.) that cannot be measured by Shelby tubes, etc. k = measure of hydraulic permeability

18 Water seeping from an open fracture in the Lagro till showing iron staining. Source: Fleming and Rupp, 1994 Filled drainage channel Hint: Always define the data gaps you intend to address in your proposed work plan. What are the data for? What are the data quality objectives (DQOs)?

19 Extent: to be an effective barrier to vertical flow, an aquitard has to have sufficient known thickness and extent. <20 feet thick cannot be considered effective barriers. Well A Well B Well C This OR That? Well A Well B Well C

20 Aquitard geometry can vary Source: Cherry and Parker, 2004

21 Aquitards can be sources Over time, contaminants in contact with an aquitard can become entrained in the pore spaces of the material. The primary mechanism of contaminant transport becomes diffusion into aquifers rather than advective flow.

22 Source: Cherry and Parker, 2004

23 Components of Aquitard Integrity The entire conceptual site model (CSM) is important. Source: Cherry and Parker, 2004

24 Investigative Strategies Map the aquitard surface with geophysical techniques and identify any low spots. Electromagnetics and electric imaging Spotaneous potential (SP) Microgravity Seismic refraction, reflection, and surface wave analysis Ground penetrating radar (GPR)

25 Sampling Sample Soil: Base of permeable unit/aquifer Top of aquitard Sample Water: Across the interface (set well screens at base of the permeable unit and top of aquitard) Include wells at any low spots Hint: Define the DQOs, plan properly, and involve all stakeholders.

26 Do I have to drill deeper? If the top of an aquitard is contaminated, the collection of deeper water samples is typically needed. Consider the risks: e.g., what is the ground water use in the area? If there are existing deep wells, a pump test may be very useful to assess connectivity. So yes, you probably have to drill deeper.

27 References Bradbury, K.R., et al. (2006), Contaminant transport through aquitards: A State of the Science Report, 144 p. AWWA Research Foundation, Denver, CO. Source: J.A. Cherry and B.L. Parker, (2004) Role of Aquitards in the Protection of Aquifers from Contamination: A State of the Science Review, University of Waterloo, Waterloo, Ontario, Canada, 144 p. Fleming, Anthony and Robin Rupp, Surficial Geology, Indiana Geological Survey, Source: Kingsbury, Robert C. An Atlas of Indiana, Source: gazetteer

28 References (cont.) Prentice, M. L. and S.L. Letsinger, New Understanding of Lagro Formation Aids Evaluation of Aquifer Sensitivity Source: IDEM Technical Guidance Document, Aquitard Characterization, March 3, 2014: Indiana Geological Survey, Surficial Geology webpage: USGS, Sustainable Growth in America s Heartland 3 D Geologic Maps As The Foundation, Circular 1190, Source: 72.pdf Further Study: Midwest GeoSciences webinar: HYDROGEOLOGY OF AQUITARDS AND LOW PERMEABILITY MATERIALS. (Parts 1 and 2) php

29 Questions? Juliet Port, LPG OLQ Geological Services (317)