Carbonates in Ontario - Porous medium, double … IAH 2013Nov12.pdfCarbonates in Ontario - Porous...

transcript

Carbonates in Ontario - Porous medium, double porosity,

or karst aquifers?

Steve Worthington Worthington Groundwater

Presentation to IAH, Toronto

November 12, 2013

Topics

1) Case study: source area for the pathogenic bacteria at Walkerton

2) Preferential flow in other carbonate aquifers in southern Ontario. Is the Walkerton aquifer unusual?

3) How, where, and when dissolution occurs

4) Conclusions

1) Case study: Source area for the pathogenic

bacteria at Walkerton

Michigan basin carbonate rocks

Worthington et al. (2012)

MississippianDevonianSilurianOrdovician

0 100 km

LakeHuron

LakeMichigan

Lake Superior

LakeErie

LakeOntario

Ontario

Locations in Ontario

W WalkertonC CambridgeS Smithville

Carbonates are widespread in S Ontario

Water supply from 3 wells

Rainfall at Walkerton - 130 mm

“The Walkerton Tragedy”

Worthington et al. (2002)

Chair of Walkerton Inquiry

- Justice O'Connor

Walkerton Tragedy - basic facts

P Municipal water supply contaminated by pathogenic bacteria. P Town of 5000

P Seven deaths

P 2300 illnesses (some chronic)

P chlorination inadequate

Walkerton - drawdown from pumping test at two wells

16.14.0

0 200 m

Well 7

Well 6Spring

Well 9

Drawdown (m)RoadsStreams

Looks like porous medium response

Worthington et al., 2012

Walkerton - response to pumping

1.0E-4 0.001 0.01 0.10.01

Time, t/r2 (min/m2)

Response at Well 7 to pumping at Well 9 Fits Theis curve Looks like porous medium response

Gamma/ flow meter logs at Walkerton

Gamma log for

depth correlation.

Flow meter show inflows to wells are concentrated on a few

bedding planes

9 horizons in Well 6 5 horizons in Well 7

average spacing ~10 m

Inflow to wells in sand and carbonates

Interpretation of flow in carbonates

Hydraulic conductivity of the matrix

• No data from Walkerton • Good packer test data from dolostone at Smithville

(Novakowski et al., 1999) • Unfractured intervals • Geometric mean K = 10-8 m/s • 4 orders of magnitude less than K from pumping tests • Almost all flow is through fractures / channels

Similar to other carbonates aquifer

0.01 0.1 1

Porosity

matrix

channeland

fracture

0.0001 0.001

S = Silurian at Smithville Worthington and Ford (2009)

Solutionally-enlarged fractures

are continuous

Positive feedback process - most dissolution occurs where the fractures are largest and there is most flow

Note: not to scale fracture spacing 5m maximum aperture 5cm

Worthington and Ford (2009) after Dreybrodt et al. (2005)

recharge

Interpretation of flow in unconfined carbonate aquifers

Travel time calculation for Walkerton (and for wellhead protection areas)

T = L ne K i T time L length ne effective porosity K hydraulic conductivity i hydraulic gradient

Travel time calculation for Walkerton (and for wellhead protection areas)

T = L ne two unknowns K i T time L length ne effective porosity K hydraulic conductivity i hydraulic gradient

How to estimate effective porosity

A) K and orthogonal fracture sets B) Storage from pumping tests C) Fracture apertures from downhole video D) MODFLOW and tracer test velocity

Method A - Model to calculate effective porosity

Hydraulic gradient andgroundwater flow

Fracturespacing

Aperture(b)

cubic law K = ρgNb3 / 12 b = (6Kµ / ρgN)1/3 horizontal flow for 2 fracture sets ne = 3bN for 3 fracture sets

Walkerton - fracture aperture/spacing

modified from Worthington et al., 2012

Gamma/ flow meter logs at Walkerton

Gamma log for

depth correlation.

Flow meter show inflows to wells are concentrated on a few

bedding planes

9 horizons in Well 6 5 horizons in Well 7

average spacing ~10 m

Walkerton - fracture aperture/spacing

Velocities from cubic law 100

Channel aperture (mm)0.01 0.1 1 10

Rapid velocities from very modest apertures

Method B - early time storage for double porosity aquifer

Kruseman and de Ridder, 1994 s=drawdown, t=time

Walkerton - K and S from pumping test

Method C - downhole video

Golder Associates, 2000

Where 50% of the flow enters Well 6

Well 7 test well - Walkerton

•Almost all flow from just a few bedding planes

• These are channels

Natural gamma (cps)0 200

25%15%

Figure- Worthington et al., 2012; Photos - Golder Associates, 2000

Method C - effective porosity from video

Travel time calculation for Walkerton

T = L ne two unknowns K i estimates 2.5% or 0.1% T time L length ne effective porosity K hydraulic conductivity i hydraulic gradient

Method D - tracer testing (Well 9 eosin injection)

photo by Steve Worthington 95 m from Well 7 (pumping well)

Well 6 - tracer injection (sodium fluorescein)

photo by Steve Worthington 354 m from Well 7 (pumping well)

Determination of mass of tracer to inject

Equations based on results of 272 tracer tests

Worthington and Smart, 2013

Tracer recoveries at Well 7

Distance: 95 m

Distance: 354 m

modified from Worthington et al., 2001

Walkerton 3-day travel times from MODFLOW

0 200 m

3-day particle trackwith n =2.5%tracer test vectorhead (m asl)roadcreek

Well 9

Well 6

Well 7

0 200 m

Well 9

Well 6

Well 7

3-day particle trackwith n =0.05%tracer test vectorhead (m asl)roadcreek

Effective porosity 2.5% - original model

Effective porosity 0.05% - model calibrated to tracer test from Well 6

Method D - MODFLOW and tracer tests

Predictions at Walkerton Inquiry

2) Preferential flow in other carbonate aquifers in southern Ontario

- Is the Walkerton aquifer unusual?

Small channels in dolostone (horizontal view) clearly solutionally-enlarged bedding planes

video (apertures 2-5 mm) televiewer (aperture 5 cm) Worthington (2008) and McFarland (2010)

Groundwater velocities in carbonates in S. Ontario

Tracer tests at Walkerton are in centre of distribution So they are typical of Ontario carbonate aquifers

3) How, where, and when dissolution occurs

How dissolution occurs

limestone + carbon dioxide + water ↔ calcium + bicarbonate (dissolved) (dissolved)

CaCO3 + CO2 + H2O ↔ Ca + 2HCO3

Limestone dissolved if equation proceeds to the right

Precipitation (e.g. stalactites) if equation moves to left

CO2 in atmosphere 394 ppm

CO2 in soil air 1000 - 100,000 ppm

Saturation with respect to calcite

Lab studies starting with Berner and

Morse (1974)

- most dissolution close to bedrock surface, which produces a weathered zone

- dissolution deeper in bedrock created network of solutionally-enlarged fractures

- results in high K aquifer

0.0001

0 0.2 0.4 0.6 0.8 1Ca / Ca (equilibrium)

Berner

Herman

Plummer

Svensson

Eisenlohr

Fracture dissolution - time to onset of turbulent flow

where:

T - time

a - initial fracture aperture

L - distance

i - hydraulic gradient

Dreybrodt, 1990

T = 0.033 L1.25 a-2.8 i-1.3 (years)

Importance of recharge type

• Percolation recharge

• Sinking stream recharge

Where percolation recharge then many small channels

c =0.98c ; t=100,000 yin eqB

Small channels in dolostone (horizontal view) - clearly solutionally-enlarged fractures

video (apertures 2-5 mm) televiewer (aperture 5 cm) Worthington (2008) and McFarland (2010)

Where sinking streams recharge then large channels + small channels

c =0; t=16,700 yinA

Sinking stream at Nexus Cave, Hamilton

Photo: Steve Worthington

Substantial dissolution - Large flux of water - Water is undersaturated wrt calcite

Nexus Cave (Hamilton)

WindowEntrance

Exploration ends at this point, butwater-filled passage (sump) continues.

SecondEntrance

Main Entrance

First Entrance(swallet)

Nexus Creek

speleothemsamples

shaft(inside cave)

agnetic

N e x u s

V a l l e y

Shallow quarryOutline of blind valleyStream channel (surface)

Cave passage (inferred)Cave passage (surveyed)

Doline, depressionIsolated soil pipe

Thin overburden (< 1.0 m thick)Cave entrance, grike

LEGEND

Surveyed from 1999 to 2001 toBCRA Grade 5D by M. Buck,G. Warchol and N. Pietroiusti.Drafted by M. Buck, March 2001.Revised January 2002. Magneticdeclination is 11.8°W of grid north.Surveyed length: 324 metresEstimated length: 344 metres

Nexus CaveStoney Creek, Ontario

0 10 20 30 40 50

metres

Photo by Marcus Buck

Solutionally-enlarged fractures

are continuous

Positive feedback process - most dissolution occurs where the fractures are largest and there is most flow

Timescale for channel enlargement

- fast – 16,000 years

- slow - 100,000 years

- flow through Ontario carbonates for millions of years so plenty of time

General relationship between K and TDS

F = fresh water (<500 mg/L TDS, high K = substantial groundwater flow and dissolution B = brackish (500 - 5000 mg/L TDS, lower K = moderate flow and dissolution S = saline (>5000 mg/L TDS, even lower K = little flow or dissolution

modified from Worthington, 2011

Water-yielding capabilities

based on specific capacity data from Ontario water well database

Singer et al., 2003

4) Conclusions

Do carbonates behave as porous media? Yes, for flow (but not for transport)

1.0E-4 0.001 0.01 0.10.01

Time, t/r2 (min/m2)C

16.14.0

0 200 m

Well 7

Well 6Spring

Well 9

Drawdown (m)RoadsStreams

Do carbonates behave as double porosity aquifers? Yes, needed to understand transport.

Hydraulic gradient andgroundwater flow

Fracturespacing

Aperture(b)

In practice, use MODFLOW with low effective porosity

Are Ontario carbonate aquifers karstic?

It depends on one's definition

There is no consensus

Definition of "karst aquifer" in Freeze and Cherry (1979) works well

References

Dreybrodt, W., 1990. The role of dissolution kinetics in the development of karst aquifers in limestone: a model simulation of karst evolution. Journal of Geology, 98, no. 5, 639-655.

Dreybrodt, W., Gabrovšek, F., and Romanov, D., 2005, Processes of speleogenesis: a modeling approach. Karst Research Institute at ZRC SAZU, Postojna – Ljubljana, 376 p.

Freeze, R.A. and J.A. Cherry, 1979. Groundwater. Prentice-Hall, Englewood Cliffs, NJ, 604 p. Golder Associates, 2000. Interim report on hydrogeological assessment, well integrity testing, geophysical surveys and land use

inventory, bacteriological impacts, Walkerton town wells, Municipality of Brockton, County of Bruce, Ontario, 351 p. (Walkerton Inquiry Exhibit 258).

Kruseman, G.P., and de Ridder, N.A., 1994. Analysis and evaluation of pumping test data. International Institute for Land Reclamation and Improvement, Wageningen, Netherlands, Publication 47, 377 p.

McFarland, S., 2010. Witness statement, Proposed Nelson Aggregate Co. Quarry Extension, Burlington Associates. Golder Associates report 021-1238.

Novakowski, N., P. Lapcevic, G. Bickerton, J. Voralek, L. Zanini and C. Talbot, 1999, The development of a conceptual model for contaminant transport in the dolostone underlying Smithville, Ontario. National Water Research Institute, Burlingon, Ontario, 98 p.

Singer, S.N., Cheng, C.K., and Scafe, M.G., 2003. The hydrogeology of southern Ontario. Ontario Ministry of the Environment, 200 p.

Worthington, S.R.H., 2008. Karst investigations at the proposed St Marys Flamborough Quarry. In: AECOM Canada Ltd., 2009, Hydrogeological Level 2 report, St Marys Flamborough Quarry.

Worthington, S.R.H., 2011. Karst assessment. OPG's Deep Geologic Repository for low and intermediate level waste. Nuclear Waste Management Organization report DGR-TR-2011-22.

Worthington, S.R.H., and D.C. Ford, 2009, Self-organized permeability in carbonate aquifers. Ground Water, 47, no. 3, 326-336. Worthington, S.R.H., Smart, C.C., Ruland, W., 2001, Karst Hydrogeological Investigations at Walkerton. Addendum report, 27 p.

Submitted to the Walkerton Inquiry, November 2001. Worthington, S.R.H., Smart, C.C., and Ruland, W.W., 2002, Assessment of groundwater velocities to the municipal wells at

Walkerton, Proceedings of the 2002 Joint annual conference of the Canadian Geotechnical Society and the Canadian Chapter of the International Association of Hydrogeologists, Niagara Falls, Ontario, p. 1081-1086.

Worthington, S.R.H., Smart, C.C., Ruland, W., 2012, Effective porosity of a carbonate aquifer with bacterial contamination: Walkerton, Ontario, Canada. Journal of Hydrology, 464-465, 517-527.

Worthington, S.R.H., and Smart, C.C., 2013, Determination of tracer mass for effective groundwater tracer tests. Carbonates and Evaporites. In press.

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