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Technical Meeting 10/19/2013 – Slide 1
1
Groundwater 101:Overview of the Essential Science
What Lies Beneath: Reasons to Care (and be Excited) AboutGroundwater Use and Management in the Southwest
University of Colorado School of LawWolf Law Building, Wittemyer Courtroom
Thursday, June 7th, 2018
Overview
• Groundwater hydrology• The hydrologic cycle• Flow• Pumping• Surface-water connection
• Equations of flow• Hydraulic gradient• Resistance• Equations• Uncertainty
• Pivotal management concepts
2
Groundwater Hydrology
3
Hydrologic Cycle
• Continuum through time and space
• Groundwater is one component of the continuum
• Groundwater budget• Flows in• Flows out
4
Groundwater Flows in Aquifers
• Aquifer: water-bearing permeable rock, rock fractures or unconsolidated materials (gravel, sand, or silt)• Unconfined aquifer• Confined aquifer
• Hydraulic Conductivity: Resistance to flow
• About 30% of all fresh water is in groundwater
• Groundwater residence time: 2 weeks – 10,000 years
5
https://water.usgs.gov/edu/watercyclegwstorage.html
How Do We Typically Interact with Groundwater?
• Springs
• River seepage (gains/losses)
• Pumping
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Springs
• Groundwater forced to the surface
• Combination of topographic and geologic conditions
• Favorites• Black hills (SD)• Bottomless lakes
(NM)• Montezuma
Well (AZ)
7
https://water.usgs.gov/edu/watercyclesprings.html
Springs
• Between Flagstaff and Phoenix
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Springs
• Montezuma Well
• Roughly 1,000 gpm
• Sinaguahistoric site
• Water diverted for agriculture
9
Flow Between Surface Water and Groundwater
10after Winters et al., 1998
• Surface water behavior is, in many cases, a manifestation of the groundwater system
• Interaction in three basic ways…
Groundwater Pumping andDrawdown
• Unconfined
• Dewatering pores (slow)
• Porosity ~ 0.01 – 0.35
11after W.M. Alley, T.E. Reilly, and O.L. Franke
• Confined
• Expansion of water and compressibility of matrix (fast)
• Storativity ~ 5×10-5 to 5×10-3 (Todd 1980)
Groundwater Pumping
• The source of water for pumping is supplied by• (1) more water entering the
ground-water system(increased recharge),
• (2) less water leaving the system (decreased discharge),
• (3) removal of water that was stored in the system or,
• some combination of these three.
• Capture = increased recharge + decreased discharge
(Lohman et al. 1972; Bredehoeft and Durbin 2009; Leake 2011).
12after Winters et al., 1998
Equations of Flow
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Darcy’s Experiments
• Flow of water through a column of soil
• Darcy Velocity (q) • Hydraulic conductivity (K)
• Potential, or head, difference (Dh)
• Distance (Dx)
• Simple, proportional relationship
14After Slichter
𝑞 = 𝐾∆ℎ
∆𝑥
Flow Changes with Gradient
• Flow rate adjusts to gradient• Increasing path, decreases the gradient
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𝑞 = 𝐾∆ℎ
∆𝑥
Confined Flow DirectionDictated by Gradient
• Flow is not always “downhill”
• Gravity plus confining layers direct flow
• Flow updip due to gradient
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from Barson et al. , 2001
What Can We Represent?
• Three Dimensions, Transport, Reactions, Sorption…
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𝑞 = 𝐾∆ℎ
∆𝑥
Complexity
• Heterogeneity in the subsurface
• Uncertainty of budget components
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Pivotal Concepts
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Three Pivotal Concepts
• Aquifer size alone is not informative
• Groundwater provides a line of credit, not a savings account
• Depletion can be a long term commitment
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• Deep, thick aquifer
Demonstrating Concepts with an Aquifer Pumping Scenario
R
• Deep, thick aquifer
• Start pumping groundwater
Demonstrating Concepts with an Aquifer Pumping Scenario
Minor changes in water levels impact:
• Plant access to groundwater
• River gains/losses
• Dry-season base flows
• Discharge from springs
Capture happens with virtually any level of pumping
Pumping is like a credit-card purchase…not a savings withdrawal
Q
R
• Deep, thick aquifer
• Take water out (withdrawal)
• More effort to pump deeper
Demonstrating Concepts with an Aquifer Pumping Scenario
Capture increases
• Existing affects increase
• Subsidence potential
4Q
R
• Impacts persist after pumping stops
• Changes continue at distance
• e.g., spring discharge threshold
• Deep, thick aquifer
• Take water out (withdrawal)
• More effort to pump deeper
Demonstrating Concepts with an Aquifer Pumping Scenario
e.g., Bredehoeft and Durbin
(2009)
4Q
R
Carrying debt, long afterpurchase is completed
• Example based on Shomaker(2007)
• Middle Rio Grande Basin: deep, thick aquifer with lots of water in storage
• Pumping that water has immediate impacts
• Offset impacts:• Set up fund to finance pumping
perpetually• Pump 100,000 af/yr• Use a portion …. Decreasing to zero• Supplement river flow to offset
seepage changes …. Supplement persists in perpetuity
• Establish and maintain a new equilibrium through perpetual pumping
Demonstrating Concepts:Offsetting the Cost of Water From Storage
QR
Demonstrating Concepts:Transition from Surface to Groundwater• Historic surface-water supply
• Surface flows through canals• Canal seepage recharged
aquifer (~30% of diversion)• Diversions and irrigation only
when flows permitted
• Proposed GW supply• Pumping pulls water from
aquifer• Less potential for recharge• Pumping allows:
• Not limited to runoff timing• Not limited to runoff
amounts• Season extension• Increase acreage
• Need to consider changes due to pumping
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SW DeliveryTranspiration
Evaporation
Percolation
GW Pumping
Summary
• Groundwater is part of the hydrologic cycle
• Groundwater physics• Water moves down gradient
• Quantify with Darcy’s law and governing equations
• Limited by uncertainty of subsurface
• Aquifer size alone is not informative• Pumping affects capture, with relatively small drawdown
• Groundwater as a credit card• There are costs to using groundwater
• Depletion can be a long term commitment• With separation costs are deferred…but costs persist longer
Questions?
Gilbert Barth, Ph.D.Senior HydrologistS.S. Papadopulos & Associates, Inc.3100 Arapahoe Ave, Suite 203Boulder, CO 80303-1050Office: 303-939-8880 x106Fax: 844-273-8297Direct line: 720-572-4670
