Ground%20Water%20Flow%20in%20Aquifer%20Systems:%20Floridan%20Aquifer%20Case%20Study

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• Ground Water Flow in Aquifer Systems Floridan
  Aquifer Case Study

Envi 518 September 10, 2002
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Global Water Supply

• Approximately 29 of the worlds fresh water
  resources exists in aquifers

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Aquifers

• Definition A geological unit which can store
  and supply significant quantities of water.
• Principal aquifers by rock type
• Unconsolidated
• Sandstone
• Sandstone and Carbonate
• Semiconsolidated
• Carbonate-rock
• Volcanic
• Other rocks

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Ground Water
Ground water occurs when water penetrates the
subsurface through cracks and pores in soil and
rock
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Recharge

• Natural
• Precipitation
• melting snow
• Infiltration by streams and lakes
• Transpiration by plants

• Artificial
• Recharge wells
• Spread water over land in pits, furrows, ditches,
  or erect small dams in stream channels to detain
  and deflect water

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Hydrologic Cycle Rainfall in becomes Recharge
to the water table
Evapotranspiration
Precipitation
Runoff
Infiltration
Soil zone
Unsaturated zone
Recharge to water table
Water table

Saturated zone below the water table
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Over Pumping
Pumping Well
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Section 21 Wells
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Northern Tampa Bay (NTB)
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NTB Overpumping Issue
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NTB Overpumping Impacts

• Excessive Groundwater Pumping has Caused
• Decline in aquifer water levels
• Lowered water levels in lakes,wetlands springs
• Formation of sinkholes
• Reduced flow in river systems
• Seawater intrusion

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Surface Water Issues
Dock on Florida Lake in 1970s
Same Dock in 1990
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Negative Impacts Sinkholes
Over 50,000 homeowners in South Pasco and North
Hillsborough counties have been hit with massive
land subsidence, as a result of over pumping.
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Sinkhole Formation

• dissolution of soluble carbonate rocks by weakly
  acidic water
• the process starts in the atmosphere, where rain
  falls on the ground and percolates through the
  soil
• dissolves carbon dioxide gas from the air and
  soil, forming carbonic acid (H2CO3), a weak acid
• carbonic acid percolates through the ground cover
  down to the bedrock
• carbonic acid reacts with limestone and dolomite
  and dissolves these carbonate rocks into
  component ions of calcium (Ca2), magnesium
  (Mg2) and bicarbonate (HCO3-).

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Negative Impacts Sinkholes
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Wetlands and Lakes in the NTB Area
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Negative Impacts Wetlands
Overpumping has negative effects on surface
waters as well wetlands in the area continue to
dry out
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Radius of Influence South Pasco Wellfield
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Lake Levels vs. Pumping
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• Thirsty Tampa Bay ponders huge desalination
  plant April 20, 2000
• Want to build the largest desalination plant in
• the Western Hemisphere
• Projected cost of 100 million
• Could supply about 25 MGD
• (about 1/10 of the region's needs)
• Critics are concerned that the high salinity
  wastewater pumped back into the bay will hurt the
  environment.

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Impact of Pumping on Heads
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Rainfall/Recharge
53 rainfall observation points Monthly Readings
from January 1989-January 2000
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Rainfall to Recharge
Assigned rain gages to basins used recharge
equation from previous studies Rech (node)
Radj ((Rechss/P(b,ss))P(b,m) Radj
Runoff adjustment per basin per month
Rechss Recharge of node in May 1989 Steady
State Model P(b,ss) May 1989 Rainfall per
Basin P(b,m) Rainfall per basin per
month
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Uses of Modeling

• A model is designed to represent reality in such
  a way that the modeler can do one of several
  things
• Quickly estimate certain aspects of a system
  (screening models, analytical solutions, back of
  the envelope calculations)
• Determine the causes of an observed condition
  (contamination, subsidence, flooding)
• Predict the effects of changes to the system
  (remediation, development, waste disposal)

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Types of Ground Water Models

• Analytical Models
• 1-D solution, Ogata and Banks (1961)
• 2-D solution, Wilson and Miller (1978)
• 3-D solutions, Domenico Schwartz (1990)
• Numerical Models
• Flow-only models (MODFLOW)
• Transport-only models (MT3D, RT3D, MODPATH, etc.)
• Require a coordinated flow model, such as MODFLOW
• Combined flow and transport models (BIOPLUME,
  FEMWATER, FLOTRAN)

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NTB Model MODFLOW

• Three dimensional finite difference groundwater
  flow model (McDonald Harbaugh, 1988)
• Simulates horizontal flow based on following
  inputs
• Aquifer properties - Pumpage
• Recharge -
  Evapotranspiration
• River/spring flow - General Head
  Boundaries

Allows for vertical interchange between layers
Surficial Aquifer Upper Floridan (1) Upper
Floridan (2)
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NTB MODFLOW Model Description

• GMS
• Encompasses all/or part of five counties
• 1500 mile2 area
• Variable grid spacing (0.25 - 1.0 miles2)
• 62 Rows 69 Columns
• Three layers

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MODFLOW

• Cell-centered, 3D, finite difference groundwater
  flow model
• Iterative solver
• Initial values of heads are provided
• Heads are gradually changed through time steps
  until governing equation is satisfied
• Divided into a series of packages
• Each package forms a specific task
• Each package stored in a separate input file

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MODFLOW

• MODFLOW based on the following partial
  differential equation for three-dimensional
  movement of groundwater of constant density
  through porous earth material
• Kxx, Kyy, and Kzz hydraulic conductivity (x,
  y, and z axis)
• h potentiometric head
• W volumetric flux per unit volume pumped
• Ss specific storage of the porous material
• t time

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Recharge Equation

• Assigned rain gages to basins used recharge
  equation from previous studies
• Rech (node) Radj ((Rechss/P(b,ss))P(b,m)
• Radj Runoff adjustment per basin per
  month
• Rechss Recharge parameter in May 1989
  Steady State Model
• P(b,ss) May 1989 Rainfall per Basin
• P(b,m) Rainfall per basin per month

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Runoff Parameter
Runoff Adjustments Runoff Adjustments Runoff Adjustments
Watersheds Watersheds
1, 3, 7, 8, 9, 10 2, 4, 5, 6
Jun 0.8 0.7
Jul 0.5 0.4
Aug 0.4 0.3
Sep 0.4 0.3
Oct 0.4 0.3
Nov 0.5 0.4
Dec 0.5 0.5
Jan 0.6 0.5
Feb 0.7 0.6
Mar 0.8 0.7
Apr 1.0 0.9
May 1.0 1.0
(5)
(10)
(1)
(4)
(8)
(6)
(2)
(7)
(9)
(3)
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Calibration Quotient
Recharge Parameter for Calibrated Model
Thiessen Polygons for May 1989 precipitation
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MODFLOW Inputs

• Recharge/Rainfall
• Variable parameter, dependent on type of rainfall
  used in recharge calculation
• Pumping Well Data
• Over 1500 wells used for pumping information
• Starting Heads
• Starting heads interpolated from May 2000 data,
  Inverse Distance Weighted Method
• Observation Coverage544 Observation Points
• Created per layer in the grid for each month
  data were obtained from District monitoring wells

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MODFLOW Inputs
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Qualitative Analysis
Ending Heads for NxrdRaw run for Layer 2
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Qualitative Analysis NxrdRaw Layer 1
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Quantitative Analysis
where hc computed head, ho observed head and
n number of observations
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Quantitative Analysis
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The Major Aquifers of Texas
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The Minor Aquifers of Texas
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The Edwards Aquifer
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The Edwards Aquifer
Pumpage to Date 33,035.30 mg (million
gallons) Average Daily Pumpage 144.26 mg
Minimum Edwards Level for 2000 649.7 Historic
Minimum (8/17/56) 612.5 Maximum Edwards Level
for 2002 690.5 Historic Maximum (6/14/92)
703.3
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The Edwards Aquifer

• When the limestone was exposed, it was
  extensively eroded creating cavities and conduits
  making it capable of holding and transmitting
  water
• Then it was covered over with relatively
  impermeable sediments forming a confining unit

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Geology of Edwards Aquifer

• Primary geologic unit is Edwards Limestone
• one of the most permeable and productive aquifers
  in the U.S.
• The aquifer occurs in 3 distinct segments
• -The drainage, recharge, and artesian zones

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Drainage Zone of Edwards Aquifer

• located north and west of the aquifer in the
  region referred to as the Edwards Plateau or
  Texas Hill Country
• largest part of the aquifer spanning 4400 sq.
  miles
• water in this region travels to recharge zone

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Recharge Zone of Edwards Aquifer

• Geologically known as the Balcones fault zone
• It consists of an abundance of Edwards Limestone
  that is exposed at the surface
• -provides path for water to reach the artesian
  zone

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Artesian Zone of Edwards Aquifer

• The artesian zone is a complex system of
  interconnected voids varying from microscopic
  pores to open caverns
• located between 2 relatively less permeable
  layers that confine and pressure the system
• underlies 2100 square miles of land

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Artesian Wells

• A well whose source of water is a confined
  aquifer
• The water level in artesian wells is at some
  height above the water table due to the pressure
  of the aquifer

• This level is the potentiometric surface and if
  it is above the land surface, it is considered a
  flowing artesian well

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The Edwards Group
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The Edwards Group

• The Edwards limestone is between 300-700 ft.
  thick
• Outcrops at the surface is tilted downward to the
  south and east and is overlain by younger
  limestone layers and thousands of feet of
  sediment
• The immense weight of this sediment layer caused
  faulting in the region

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Typical Dip Section
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Regional Dip Section
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Flowpaths of the Edwards Aquifer