panola county gcd dfc evaluations

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Panola County GCD

DFC Evaluations

May 19, 2016

By:

Wade Oliver, P.G.

To:

Texas Alliance of

Groundwater Districts

May 19, 2016

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Issuing Permits

Summary

• The MAG is a consideration, not a permitting or

pumping cap

• The requirement is to manage on a long-term basis to

achieve the DFC

• MAG appears 6 times Ch. 36 Texas Water Code

• DFC appears 56 times in Ch. 36 Texas Water Code

The primary purpose of an alternative

approach to DFC development is to ensure

that the DFC is meaningful, achievable and

results in a realistic MAG.

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GMA 11 Aquifers

Panola County

A

A’

• Cross-sections show a

vertical slice of the aquifer

• The middle Wilcox is the

uppermost aquifer unit

throughout most of Panola

County

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GMA 11 Aquifers

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GMA 11 Aquifers

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Analytical Well Impacts Tool

Purpose

• Enable the District to quickly evaluate the local-

scale lateral and vertical impacts on water

levels of new and existing wells

Highlights

• Based on an analytic element model (TTIM) that

solves for water level in the well bore

• High vertical resolution: 20-ft thick aquifer layers

• Automatic layering – don’t need to specify layer,

just surface elevation and screened interval

• Accepts one or more pumping wells for analysis

• Evaluated impacts of existing wells

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Existing Well #1 Existing Well #2 Existing Well #3

50 gpm each

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50 gpm each

6.7 feet 0.9 feet 3.2 feet

Drawdown after 1 year

11.3feet

12.0feet

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Evaluating Impacts of Existing Wells

• Pumping assigned to each well based on yield (if

available) and well type.

• Category pumping from most recent 3 years in

TWDB Water Use Survey Groundwater Pumping

Estimates

• Rural domestic assumed as 400 gallons per day per

householdWell Type

Estimated Current

Pumping (ac-ft/yr) Well Count

Domestic and Livestock 719 1,365

Irrigation 281 46

Manufacturing 1 9

Mining 1,629 1,273

Municipal 3,185 65

Unused 0 588

Total 5,815 3,346

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0

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Mo

re

Freq

uen

cy

Number of Wells within Given Radius

Histogram of Well Proximity

Radius: 1 mile

Radius: 3 miles

Interpretation:Approximately 1,200 wells have between 20 and 40 wells within a 1 mile radius

Interpretation:Apprximately 270 wells have between 200 and 220 wells within a 3 mile radius

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PCGCD Alternative DFC Approach

Purpose

• Enable adoption of a DFC that is meaningful,

achievable and results in a realistic MAG

Primary Issue

• Aquifer tests indicate confined responses to

pumping, compared to unconfined in GAM

Alternative Methods Considered

• Water budget calculation

• Analytic element model (TTIM)

• Simple numerical groundwater model for

Panola County with link to GAM (MODFLOW)

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Results Summary

Sce

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io 1

Sce

nar

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Sce

nar

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Sce

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Sce

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Sce

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Pumping

Factor 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Pumping (ac-ft/yr) 6,953 7,946 8,940 9,933 10,926 11,920 12,913

Panola County Carrizo-Wilcox Average Drawdowns (feet)

GAM Average Drawdown* -4.0 -3.6 -3.3 -3.0 -2.8 -2.5 -2.3

Analytical Tool 4.2 4.8 5.4 6.0 6.6 7.2 7.8

Combined Average Drawdown 0.2 1.2 2.1 3.0 3.8 4.6 5.5* Scenarios modified to remove pumping in Panola County from GAM

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Results Summary

• Unconfined conditions in Panola County in

the GAM, while aquifer tests show confined

conditions

• Evaluated response of aquifer using

alternative analytical tool with confined

properties overlaying GAM results

• Incorporation of vertical hydraulic

conductivity and confined storage

• Increase drawdown impacts horizontally

• Decrease drawdown impacts vertically

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Groundwater Management

Area 11

panola county gcd dfc evaluations

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