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Assessing Mining Impacts Groundwater Modeling in Karst Aquifers

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Presentation on theme: "Assessing Mining Impacts Groundwater Modeling in Karst Aquifers"— Presentation transcript:

1 Assessing Mining Impacts Groundwater Modeling in Karst Aquifers
Todd R. Kincaid, Ph.D. Hazlett-Kincaid, Inc. H2H Associates, LLC Timothy J. Hazlett, Ph.D. Ardaman Associates

2 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

3 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.

4 Basic Conceptualizations
Most commonly assumed Most commonly true

5 Describing an Aquifer

6 Describing an Aquifer Flow Through A Porous Media Aquifer Recharge
Isotropic & Homogeneous Discharge

7 Describing an Aquifer Flow Through A Karst Aquifer Recharge
Heterogeneous - Anisotropic Discharge

8 Types of Models

9 Types of Models

10 Why Does it Matter? Wakulla Springs Basin – North Florida Simple World
Simple models produce simple simulations Produce very generalized estimations of groundwater flow directions Under-estimate velocities Misrepresent connections Real World

11 Example: Santa Fe River Basin

12 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)

13 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

14 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

15 Defining Conduits Incorporating Cave and Karst Pathways 1 2
Hand-contoured potentiometric surface Composite data - May 2005 lines colored by elevation depressions assumed to reflect presence of conduits/caves 1 2 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 1. Data_potmap.jpg - shows hand drawn pot-map based on May 2005 static well data talk: lines colored by elevation talk: areas of head depression assumed to be due to presence of condiut 2. Data_potmap_caves.jpg - shows projected and known caves over pot map talk: Old Bellemy cave only surveyed cave shown (not shown; Rose Sink, mill creek, Ginnie, Hornsby) talk: Tracer caves are drawn based on pot-map connecting Pete Butts sinks and springs talk: projected caves are based only on pot-map 3. Data_potmap_caves_streams.jpg - shows relationship between caves and sinking streams talk: pot lines greyed out talk: caves connect sinking creeks to Hornsby spring, Ichetucknee springs, and Santa Fe rise Ginnie, Twin, Dogwood, Gilchrist Blue, Lilly, Poe not connected to sinks Talk: July and Devil's Eye springs not yet in model 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

16 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

17 Cave vs No Cave Results Cave Model No-Cave Model
1. Flordan_nocave_head_all.jpg - shows modeled flordan head with no caves added talk: developed for calibration comparison purposes. Could a good flordan model be built without caves? talk: in general heads are higher across the whole model talk: heads are esecially high in south central part of model, suggests that condiuts are needed

18 Model Comparison: Southern Area
Cave Model No-Cave Model Quarry Area 1. Flordan_head_calsouth.jpg - shows flordan head in southern part of model with calibration points overlaid talk: this is model with caves talk: calibration is good everywhere (within +- 1 meter). Model needs minor calibration adjustments 2. Flordan_nocave_head_calsouth.jpg - shows flordan head in southern part of model with calibration points overlaid talk: this is model without caves talk: calibration is good closest to Bell Ridge and Sanchez Prarie talk: calibration is poor in central part of model (heads off by 3 to 5 meters) talk: model head in general too high over entire southern half of model due to lack of condiuts 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

19 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 N 11/13/03 Copyright Hazlett-Kincaid, Inc All rights reserved.

20 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 Mine Model Model shows no offsite impacts Most water infiltrated through holding ponds, but not shown in model Leakage between holding ponds Trench is built up with berms (horiz. flow) Our Model Trench is strictly 10 ft wide Head is strictly 2 ft deep Recharge flux is limited to ~72 mgd (from permit) Trench is part of top layer of sand Sand and limestone layers use thickness from GFM Clay is bottom of model Heads are steady state flow from north to south In pit, heads are set at 25’ b.l.s., as specified in permit application All hydraulic parameters taken from permit app. Pit modeled as a hole in the ground 11/13/03 Copyright Hazlett-Kincaid, Inc All rights reserved.

21 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 N 11/13/03 Copyright Hazlett-Kincaid, Inc All rights reserved.

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

23 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

24 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

25 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?

26 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

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