Assessing Mining ImpactsGroundwater Modeling in Karst Aquifers

Todd R. Kincaid, Ph.D.Hazlett-Kincaid, Inc.H2H Associates, LLC

www.h2hmodeling.com

Timothy J. Hazlett, Ph.D.Ardaman Associateswww.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 AquiferFlow Through A Porous Media Aquifer Recharge DischargeIsotropic & Homogeneous

Describing an AquiferFlow Through A Karst Aquifer

Heterogeneous - AnisotropicRecharge Discharge

Types of Models

Types of Models

Why Does it Matter?

• Simple models produce simple simulations• Produce very generalized estimations of

groundwater flow directions• Under-estimate velocities• Misrepresent connections

Wakulla Springs Basin – North Florida

Simple World

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

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

• Projected & known caves & pot surface• 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

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Defining Conduits

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

No-Cave Model

Cave vs No Cave Results

Cave Model

No-Cave Model Cave Model

• 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

Model Comparison: Southern Area

Quarry Area

1911/13/03 Copyright Hazlett-Kincaid, Inc. 2003. All rights reserved.

• 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?

N

Example: Corkscrew Mine

Dewatered Quarry

Schmidt Property

Lee County, Florida – 2003 Quarry Permit Application

2011/13/03 Copyright Hazlett-Kincaid, Inc. 2003. All rights reserved.

• 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

Results: Dewatering Scenario

2111/13/03 Copyright Hazlett-Kincaid, Inc. 2003. All rights reserved.

• 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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Potential Impact of Karst

22 11/13/03 Copyright Hazlett-Kincaid, Inc. 2003. All rights reserved.

Corkscrew Modeling Results• 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

Example: Hercules QuarryPreliminary No Karst Model

• Localized Cone of Depression• No account for exchange with nearby creek

Karst Model

• 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