Pred7-GH2O

Water Resource Management, Groundwater Capture Zones (optional)

Water Resource Management

Roko Andričević, University of Split, Croatia

Igor Janković, State University of New York at Buffalo, USA


Outline

• Definition• Various approaches• Effect of parameters on capture zone

geometry• Inherent assumptions• Surface water capture zones


Capture zones / Capture Areas• Defined as the volume of the aquifer from

which an extracting well or surface water body obtains it’s water for a given period of time

• May be used for – Source water protection– Pollution impact assessment– Designing pump and treat systems– Identifying interacting surface water bodies


Source Water Protection

• EPA-mandated delineation of “Well head protection areas” (WHPAs) for wells that may be used as a source of drinking water– 1994 Amendment to the safe drinking water

act required WHPAs for all drinking water sources by May 2003

• These zones are later used for policy decisions regarding land use


Capture Zone Delineation: Modeling Approaches

• There are multiple approaches for capture zone delineation of varying accuracy and complexity– The “dixie cup” approach (very simple analytic

method)– Analytic solutions– Numerical particle tracking methods


The “dixie cup” approach• A.K.A. the “Calculated fixed radius approach”• Assume that the capture zone of the well is

circular• Based upon assumption that regional flow is

negligible• Capture zone radius given by

• Can be grossly incorrect

θπ h

Qtr = Where Q=pumping rate t =time

h=avg. aquifer thickness θ=porosity


The “dixie cup” approach

5 years

10 years


More advanced approaches

• Use analytical models of capture zones in uniform flow– Todd, 1980

• More accurate in general• Limited by assumption of regional flow

effects only


Well in Uniform Flow

• Superimposed solution for well and uniform flow:

• Can easily calculate capture zone dimensions from this expression

CyxQ

xQ w +++−=Φ 220 ln

2π


Qo

Qw

0QQw

04QQw

02 QQw

π

Stagnation point

Capture zone for specific time period


More Advanced Approaches

• Use numerical models to calculate capture zones by modeling local system and backtracking particles from the well– Steady state models often used – Finite difference-based solutions for complex

layered 3D Aquifer– Visual Bluebird…


Capture zones: Effects of Parameters

• Given the same system, capture zones– Widen with increased pumping rate,

decreased uniform flow– Lengthen with increased conductivity

• In systems with recharge, capture zones have finite extent– All captured water comes from recharge


Effect of pumping rate

Q / 2 Q


Effect of decreased conductivity

K=0.1 m/d K=0.05 m/d


Effect of increased conductivity

K=0.1 m/d K=0.15 m/d


Effect of heterogeneity in K

K=0.1 m/d K=0.15 m/d


Effect of recharge

• Causes CZ to taper to a point- all water captured is from recharge


Considerations & Assumptions• Capture zones may not be completely

reflective of actual system– Pathlines are 2D only– Recharge / Leakage has an unseen effect in

two-dimensional capture zones– Transient flow not accounted for– Heterogeneity often not accounted for

• An intelligent “factor of safety” may be required


Conceptual model: 3D effectsWith recharge, backtracked particles “start”outside of aquifer

Depending upon starting location of backtracked particle, source location is different

This results in a truncated capture zone


Conceptual model- Steady-state

Qo Qw

General flow direction is assumed known and constant, pumping rate is constant over time


Conceptual model-Transient Effects

Qo?

Qw

Effective transient wellhead protection zone may be defined as the union of all realistic possible capture zones

This approach may also be used to incorporate unknown values of conductivity

For pump and treat, the effective capture zone should be the intersection of possible CZs


Surface water capture zones• In addition to well capture zones, surface water

capture zones may be identified with models• Similar approach- backtrack particles from

surface water boundary• Helpful in determining

– “Groundwatershed” boundaries– Impacts of pollution on surface water bodies– Surface water protection areas– Areas of high susceptibility to surface pollution– Connections between surface water bodies


Surface Water Capture Zones

Backtracked particles may be used to identify the portion of the aquifer (and other rivers) feeding a given stream reach


Surface water capture zones

• Same precautions must be taken (as with wellhead protection areas)– Consider possible 3D effects– Incorporate possible effects of heterogeneity– Consider possible transient effects

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