Sustainable Groundwater Management

Applications for Central Gulf Coast Aquifer

Venkatesh Uddameri

Texas A&M University-Kingsville; MSC 213 Kingsville, TX 78363Texas A&M University-Kingsville; MSC 213 Kingsville, TX 78363vuddameri@tamuk.edu

Ph: 361-593-2742; Fax: 361-593-2069Ph: 361-593-2742; Fax: 361-593-2069

Background

Effective groundwater management is necessary for sustainable use of aquifer resources Groundwater resource consists of two components

Natural capital – Groundwater in stock Physical capital – Groundwater utility

Benefits obtained from groundwater use Often quantified in monetary terms

Sustainability current generation does not threaten future generations access to this resource Weak Sustainability – Future generations have same access to total

groundwater capital as the current Does not differentiate between physical and natural capital Availability of alternative water resources

Strong Sustainability – Future generation have same access to each individual components Differentiates between physical and natural capital May curtain economic growth and utilities

Groundwater Planning and Management

Aquifer management requires two interconnected pieces of information How much water is in the aquifer?

Hydrogeology aspect How much do we want to extract?

Policy making aspect

Q

Groundwater Management

Groundwater Planning can be mathematically expressed as follows:

H2 = F * H1

HydrogeologyPolicy

F = 0 Mining F = 1 Conservation

Risk-Attitudes

H1

H2

Groundwater Management

The hydrogeology aspect is derived using a groundwater flow model Provide estimates for groundwater head under different scenarios

Hydraulic head defines amount of water in the aquifer Use water balance and Darcy’s law

Groundwater planning requires a crystal ball The flow model provides one

Flow model can characterize the head variability due to: Hydroclimatology

Precipitation, evapotranspiration Anthropogenic factors

Abstractions for various uses Alterations to land use and land cover

Hydrogeologic variability Changes in geologic features in space / time

Surface water-Groundwater Interactions Exchange between rivers/creeks and aquifers

Hydrogeology

Questions related to flow modeling

What is the right model to use? Depends upon the application context Models are like maps

Need a different map for different purposes

“Scale” of the model is an important factor Large scale models capture regional interactions

Usually better for screening purposes Does Refugio has more groundwater than Goliad

The resolution of the model has to be fairly coarse Refined models capture local interactions

Required for contaminant transport Are persons in House A exposed to chemical A more than those in House B

Data requirements magnify significantly

Selection of an appropriate model is often an iterative process

Hydrogeology

Case-Study

Groundwater Flow Models for use in Central Gulf Coast Aquifer (Refugio Groundwater District)

A County-scale Model A two-layer county scale model was calibrated for Refugio county

GHB and horizontal hydraulic conductivity as calibration parameters 0.3 mi x 0.3 mi grid size

Computed Vs Observed

-10

0

10

20

30

40

50

60

70

80

-10 0 10 20 30 40 50 60 70 80

Observed Values in ft

Com

pu

ted

val

ues

in f

t

Layer -II

Hydrogeology

County-scale Model – Post-audit

The county scale model post-audit revealed the following: Model captured current trends fairly reasonably

Parsimonious model Model domain consistent with GW district boundary

Need for arbitrary boundary conditions on the western side Checked for reasonableness and conservatism

Recharge areas of the aquifer outside the district boundary Actions of other districts can affect GW in Refugio and vice-versa

Goliad and Bee counties Need a model to incorporate these effects

Satisfactory for preliminary planning & provided insights on data limitations and future modeling efforts

Hydrogeology

Basin-scale modeling

TWDB recently released a basin-scale model for the Central Gulf Coast aquifer 1 mile x 1 mile grid Model domain is large

Wharton to Kenedy counties Model was reasonably calibrated

Focus was on high GW use areas Jackson county & Kleberg county

State GAM

Hydrogeology

Basin-scale modeling - Post Audit

Model was qualitatively evaluated for use in the Central section of the central gulf coast aquifer Refugio, Bee, Goliad, DeWitt, Victoria and Live Oak

Provides useful information on geology, hydrology Estimates for recharge, geologic cross-sections, etc.

Limited data from the six county area was used in the model development / calibration

Model underpredicted baseflows in several streams within the area of interest

Not suited when wells screen entire (or lower sections) of the Evangeline aquifer

Useful starting point for a more-refined modeling effort

Hydrogeology

Multi-County scale flow model

A Multi-county groundwater flow model is currently being developed Phase-I is the development of a steady-state model Incorporate lessons learnt from previous efforts Use TWDB GAM information as basis

Refine some of its limitations Close interactions with several groundwater districts

Update water use data; identify new pumping wells Technical Stakeholder Group - provide guidance and data

Representatives from a wide section of stakeholders with differing interests

Hydrogeology

San Diego, San Fernando Creek

Lavaca River

Model Domain

State GAM

Hydrogeology

Model Conceptualization - Geology

Model follows TWDB GAM conceptualization Model has 4 aquifer formations

Chicot Evangeline

Goliad sands Burkeville

Aquitard Jasper

Each water-bearing formation is represented by two layers Better distribution of well pumping

Hydrogeology

Model Conceptualization - Geology

Stratigraphy from Baker (1979) and Ryder and Carr (1985); Solis (1981) Drillers records 35 e-logs used to construct the cross-sections

~ 10 used for QA/QC

Chicot 2 Layers

Evangeline 2 Layers

Burkeville 1 Layer

Jasper 2 Layers

Cross-SectionLavaca river

Coast

Carrizo

-Wilc

ox

TOP VIEW

Sanjose,Sandiego, Sanfernando creeks

Hydrogeology

Model Conceptualization - Discretization Use 0.5 x 0.5 mile grid (0.25 sq. miles)

216 x 170 grids (36720 cells) Orientation parallel to the regional groundwater flow

Hydrogeology

Model Conceptualization – Boundary Conditions No-Flow boundary on the west

Carrizo-Wilcox – Gulf Coast Interface General Head / No flow Boundary on the east

Coastal region Lavaca River on the north San Jose/San Diego/San Fernando creeks in south

CHICOT

Evangeline

Burkeville

Jasper

General Head boundary

No Flow Boundary

Anthropogenic demand

Precipitation

Evapotranspiration

Groundwater flow

Discharge

Cross formational flow

Hydrogeology

Current efforts

Finalize the water use information Start calibration

Hydraulic conductivity and conductance as calibration parameters

Use TWDB groundwater database to identify calibration and evaluation wells

Use PEST for automatic calibration

Hydrogeology

Policy aspects of Groundwater availability

Groundwater Management – Policy Instruments Groundwater Districts need to:

Assess existing total usable amount of groundwater in the district Develop production rules for permitting wells Develop well-spacing guidelines Protect against negative externalities

Drying up of shallow wells due to large-scale pumping Reductions in baseflows to streams and creeks

Fish kills and other surface water impairments Intrusion of salt-water Subsidence Enhanced vulnerability to water supplies

Groundwater models can be used to address these issues Will help make a more informed decision

Decision-support tools

Optimization Model Coupled the groundwater model with a management model to

estimate available groundwater under pre-specified constraints Preserve baseflows, prevent salt-water intrusion, etc. Use optimization technique

Simulation Model Optimization Model

•Roughly 40,000 acre-feet of water is available for the conditions specified•Saltwater intrusion constraint is very significant•Most of the groundwater recharge occurs in adjoining counties

•Cooperation amongst adjoining districts is vital

What about Uncertainties?

Does geologic variability or diverse preferences of decision-makers have a bigger impact on water availability estimates?

Problem Statement

Geologic variability is to be expected Lack of data Calibration is non-unique

Stakeholders have different viewpoints Place all the pumping wells in recharge areas

Economically-inclined Restrict pumping to discharge areas

Conservation-oriented

Should be spend our resources collecting better geologic data OR reconcile differences between stakeholders?

Interval Optimization

Used interval optimization technique to assess water availability under uncertainty All geologic parameters were assumed to be accurate only within two orders of

magnitude Typical variability

Pumping was mostly restricted to either recharge areas or discharge areas

Interval Optimization Both had almost equal impacts

Collect more data AND reconcile stakeholder differences Consensus necessary on salt-water constraint

8000

18000

28000

38000

48000

58000

68000

78000

88000

98000

108000

0 -5 - 0 -5-1 -5-2 -5-3 -5-4 -5 -6 -7 -8 -9 -10 -11 -12 -15 -20

Salt water Intrusion constraints

Tot

al a

vaila

bilit

y ac

ft/a

nnum

Parameters

DMP

Stakeholder Technical Group

Field Sampling Efforts

Impacts of Model Discretization

Do model development decisions affect groundwater availability estimates? Do we need to specify “safety factors” to induce conservatism

Factorial Sensitivity Testing to study discretization effects Discretization can have some effect, but not as much as geology

or management choices Coarser grids yield conservative estimates when permeability is

low (all other things being equal) Finer grids yield conservative estimates in highly permeable sand

and gravel (all other things being equal)

Developing Well Spacing Guidelines

How can we use models to obtain well spacing guidelines

Well Spacing Guidelines

A nonlinear optimization approach was used to determine Well spacing guidelines for Refugio GCD using Theis equation Applicable to confined aquifers Conservative in unconfined aquifers

Unconfined aquifers have greater storage

Q r

t=0

t=t t=t

Drawdown

Observation well Pumping

well

h=0

h=t

T S Confined Aquifer

Well Spacing Guidelines

Well spacing based on identifying the radius of influence of the well Drawdown at the perimeter should be negligible

< 0.05 ft No negative externalities

Well spacing determined as a function of pumpage Using multiple optimization runs

Well spacing ranges from 0.63 ft/gpm – 4.2 ft/gpm Average well spacing ~ 2 ft/gpm Well spacing is not an issue when pumping is less than 20 gpm

Not an issue for exempt domestic wells ( < 25000 gpd)

Baseflow Externalities

Reconcile Competing Economic and Ecological Objectives to estimate water availability

Problem Statement

Pumping groundwater can lead to economic benefits Costs associated with pumping

Marginal costs increase with increased pumping Benefits associated with pumping

Marginal benefits decrease with increased pumping

Pumping groundwater can reduce baseflows Impairment and lost recreational opportunties

Maximize economic gains over the planning horizon while avoiding baseflow externality

Q

Critical Limit

hH

e

cs

Tt

QtxHFt

HS

TS

dttQHU

,,,

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,,0

Baseflow Externality

Hydroeconomic analysis to identify allowable pumping rates during the planning period Optimal control theory

Model results indicated that explicit policy statements are necessary to avoid baseflow externality If pumping is not curtailed to some critical limit, there will be

significant reductions to baseflows

Comparison with a purely economic-driven policy was helpful Shadow price of maintaining a particular baseflow Compare shadow price with willingness to pay

Closing Remarks

Models are useful tools to guide policy planning endeavors Will never provide exact answers

Models need to be understood and accepted by stakeholders Modeling is a process not an event

Models should be evaluated in the context of the application Keep the “big picture” in mind

Easy to lose sight of forest for the trees Evaluating models => Assessing its assumptions

All models have assumptions!!

Closing Remarks

Emphasize simplicity – Goal is to find the simplest model that will fit your needs Not any simpler or complex Avoid “cosmetic complexity” traps

Evaluate cost-benefits

Be skeptical but not cynical about models Carryout post-audit analysis Proceed with caution

Coupling flow models with other tools is beneficial Optimization; data collection; pump tests

Sponsors

Refugio Groundwater Conservation District South Texas Alliance of Groundwater Districts

Goliad Groundwater Conservation district (lead)

National Science Foundation – Center for Research Excellence in Science and Technology – CREST-RESSACA

National Oceanic and Atmospheric Administration (NOAA)

Texas Water Resources Institute (TWRI/USGS)