Transcript Slide 1

Assessing Mining Impacts
Groundwater Modeling in Karst Aquifers
Todd R. Kincaid, Ph.D.
Hazlett-Kincaid, Inc.
H2H Associates, LLC
www.h2hmodeling.com
Timothy J. Hazlett, Ph.D.
Ardaman Associates
www.ardaman.com
Problem / Purpose
• “Limestone areas are bad places for limerock quarries”
Environmental Consultant Pennsylvania
• If our society needs quarry products, then the challenge is to
locate them and manage them such that environmental
impacts are minimized and repaired to every extent possible
after the quarry closes.
• Impact assessments are often obscure and not accessible
• Describe how modeling contributes to impact assessments
• Describe how I think it SHOULD be used to assess impacts
What is a Groundwater Model?
Groundwater models are tools most often used to make predictions
about future impacts to groundwater resources associated with
proposed activities or existing problems.
They can be:
• analytical (meaning mathematically solvable),
• stochastic (meaning statistically solvable),
• numerical (meaning solvable through iteration),
• or some combination of the three.
They are often required by State and Federal regulatory agencies for
impact and contamination assessments specifically because of their
ability to predict the future.
The problem is that most of these models are based on simplifying
assumptions that render the resulting predictions dangerously
inaccurate particularly when applied to karst aquifers.
Basic Conceptualizations
Most commonly assumed
Most commonly true
Describing an Aquifer
Describing an Aquifer
Flow Through A Porous Media Aquifer
Recharge
Isotropic & Homogeneous
Discharge
Describing an Aquifer
Flow Through A Karst Aquifer
Recharge
Heterogeneous - Anisotropic
Discharge
Types of Models
Types of Models
Why Does it Matter?
Wakulla Springs Basin – North Florida
Simple World
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Simple models produce simple simulations
Produce very generalized estimations of
groundwater flow directions
Under-estimate velocities
Misrepresent connections
Real World
Example: Santa Fe River Basin
Water Budget / Flow Chart
• Models produce predictions
of groundwater levels from
which all other things are
calculated
• Calibration is the process of
matching model predicted
conditions to observed
conditions
• Typically only done for
groundwater levels
• In Karst it is critical that models are also calibrated to
• spring flows (regional)
• observable conduit velocities (tracer tests, observed
responses to storms or collapse, etc)
Sinkhole Overview
•19 sinkholes in current model
•15 known to exist
•4 assumed
•Sinkhole systems can be grouped by three dominate
sinks
•Santa Fe River Sink (largest)
•Capacity estimated at 1000’s of cfs
•Large volume conduit network connects to Santa Fe Rise
•Mill and Rose Creek sinks
•Capacities estimated up to 100 cfs
•Large volume conduits extend downstream from sinks
•All other sinks estimated to have capacities > 30 cfs
•Alligator and Waters Lakes known to drain through low
capacity sinks during drought years
•Watermelon Pond, Burnetts Lake, Lake Wilson, and Lake
Lona assumed to have similar sinks
Springs Overview
•18 Springs in current model
•Santa Fe River Rise is Largest Spring
•Discharge in 1000’s of cfs
•Direct flow from Santa Fe Sink
•Hornsby second largest spring
•Discharge up to 250 cfs
•Majority of discharged believed to be
sinking surface water
•Can go dry during droughts (fall, 2002)
•Other springs part of three groups
•Ichnetuckenee
•Ginnie
•Devil’s Ear
Defining Conduits
Incorporating Cave and Karst Pathways
• Hand-contoured potentiometric surface
• Composite data - May 2005
• lines colored by elevation
• depressions assumed to reflect presence
of conduits/caves
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• Projected & known caves & pot surface
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• Old Bellamy only cave shown
• Traced pathways based on KES data
1) Rose Sink cave – Ichetucknee group
2) Mill Creek cave – Hornsby spring
• Projected caves
- follow potentiometric lows
- connect swallets to springs or to
established pathways
• Connections
• sinking streams to Hornsby spring,
Ichetucknee springs, and Santa Fe rise
• Ginnie, Twin, Dogwood, Gilchrist Blue,
Lilly, Poe not connected to sinks
• July & Devil’s Eye not yet in model
Cave vs No Cave Results
Evaluation Approach – Preliminary Comparisons
• Develop independent models
– One with no caves
– One with caves
• Compare results using calibration as the
measuring stick
• No-cave model similar to standard approach
• Cave model significantly more thorough
Cave vs No Cave Results
Cave Model
No-Cave Model
Model Comparison: Southern Area
Cave Model
No-Cave Model
Quarry Area
• models not very comparable
• no-cave model shows large area of anomalously
elevated head (+ 3-5 meters) / okay in confined areas
• cave model is well calibrated everywhere
• cave model shows quarries in springshed
Example: Corkscrew Mine
Lee County, Florida – 2003 Quarry Permit Application
• Define impacts to the water
table near the mine due to
the removal of mined
materials – operational stage
• How will they change under
proposed dewatering?
• What is the magnitude of
these impacts and how long
might they last?
• How might karst features
impact the groundwater flow
field?
Dewatered Quarry
Schmidt Property
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Copyright Hazlett-Kincaid, Inc. 2003. All rights reserved.
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Results: Dewatering Scenario
• Drawdown indicated
from unperturbed
condition to 36
months of dewatering
at 25’ b.l.s.
• Recharge trench
included
– Perimeter of mine
– Flux limited
• Model was designed
such that effect of
trenches could be
assessed
• Larger scale with less
assumptions yields
different predictions
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Copyright Hazlett-Kincaid, Inc. 2003. All rights reserved.
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Potential Impact of Karst
• Hypothetical conduit
extending from wetland
discharges to quarry
• Conduit assumed to be in the
limestone beneath the
quarry floor
• Flow concentrated along
conduit
• Drains the water table in a
localized fashion
• Lower heads in conduits
• Higher flow rate in hotter
colors
• Same calibration but
different prediction of where
the impacts will be felt
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Copyright Hazlett-Kincaid, Inc. 2003. All rights reserved.
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Corkscrew Modeling Results
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The modeled excavation of the pit demonstrates a drawdown in the surficial
aquifer.
The maximum drawdown is about 10 cm (~1 ft) under the Schmidt property, at
the northeast corner.
The water table rebounds to near equilibrium conditions in about 150 days.
The water table will not reach full equilibrium over the long term, due to
increased loss from the aquifer from evaporative flux from the pit.
Substantially greater groundwater drawdown will occur offsite of the mine
property if mechanical dewatering is implemented as described in the permit
application (recently withdrawn)
The recharge scenario as described in the permit application will not
significantly mitigate the drawdown effect
• Modeling results are dependent on the initial conceptualization and
model design
• Should be designed to match real-world as closely as possible
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Copyright Hazlett-Kincaid, Inc.
2003. All rights reserved.
Example: Hercules Quarry
Preliminary No Karst Model
Karst Model
• Localized Cone of Depression
• No account for exchange with nearby creek
• Larger Cone of Depression
• Conduit exchange with creek included
Summary / Conclusions
• The quality of an impact assessment is heavily determined
by the accuracy and scale of the conceptual model on
which they’re based
• Florida limestones are predominantly karst
• Conceptual models therefore need to be large enough to
account for local and regional hydrologic features
(recharge & discharge)
• Modeling can be an effective tool for predicting impacts
IF they’re based on reasonably accurate conceptualizations
Summary / Conclusions
• Need for basin-wide characterization models
– Continuously updated with new data
– Goal: provide ongoing background levels – ie what is the pre-mining
condition from which impacts can be addressed
– Incorporate high frequency data
– Publically available & transparent
– Required foundation for all local-scale investigations & assessments
• Need for comprehensive look at water use throughout the life of mine
– Consumptive use, recharge, discharge, reclaimed use
(Water Balance Evolution)
– What is the post-operation mine going to be used for?
Summary / Conclusions
• Quarries are (most likely) an unwanted given
– Must look at how we manage them to ensure the best possible
outcome for the environment
– Where they go, how big they are, how long they operate
All these factors can and probably will impact groundwater
resources and should therefore be at least partly determined by
comprehensive and consistent regional characterizations
• How can we get these comprehensive characterization models done?
– Public private partnerships
– Partially publically funded / partially funded by quarries