Groundwater Models for Wellhead Protection Areas WHPA

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Groundwater ModelsforWellhead Protection Areas
(WHPA)
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Groundwater ModelingContact Information

• Brant Fisher, DWRP, Environmental Engineer,
• [phone], fisherb_at_state.mi.us
• Rick Mandle, LWMD, Groundwater Modeling Program
  Manager, [phone], mandler_at_state.mi.us
• Dave Fongers, LWMD, Environmental Engineer,
• [phone], fongersd_at_state.mi.us
• Internet, groundwater modeling program
• www.deq.state.mi.us/lwm/water_mgmt/gwater/gwater.
  html

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WHPA Delineation Guidance

• This presentation is based on MDEQ-DWRP Guidance
  Document WHP 1-101, August 1998. Additional
  detail is contained in this document.

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WHPA Delineation Modeling Goals

• Delineation of a WHPA that
• Safeguards public health.
• Is based on site-specific data and technically-
  sound interpretation of site hydrogeological
  conditions.

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Delineation Process
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Overview of Delineation Process

• Assemble available information
• Conceptualization meeting with MDEQ
• Develop appropriate delineation approach for
  water supply
• Field work to obtain better understanding of
  hydrogeologic system
• Modeling to delineate WHPA
• Submit delineation report

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Available Information Sources

• Aquifer test or water supply reports
• Water well records
• Investigations of sites of environmental
  contamination
• Geologic and topographic maps
• Nearby WHPA delineations
• Regional hydrogeological investigations

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Pre-Meeting Work Products

• Preliminary potentiometric surface
• Geologic cross-sections
• Preliminary WHPA delineation and time-of-travel
  estimates

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Field Work

• One aquifer test is required
• 24 hours for confined aquifer
• 72 hours for unconfined aquifer
• Determination of groundwater flow directions and
  hydraulic gradients
• Possible well installation

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Delineation Approaches
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Delineation Approaches

• Confirmation of Existing Information
• Direct Determination of Groundwater Flow
  Direction and Hydraulic Gradient
• Extensive Hydrogeological Investigations

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Delineation Approaches (cont.)

• The additional uncertainty associated with a
  simpler approach generally results in a more
  conservatively sized WHPA compared to a more
  elaborate approach.
• A simple approach will be inadequate for a
  complex flow system.

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Delineation Approach Confirmation of Existing
Information
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Confirmation of Existing Information

• A potentiometric surface map is prepared from
  reported static water levels.
• Lake levels or river stages may be used if
  appropriate.
• Static water levels are measured in a sufficient
  number of wells to confirm the potentiometric
  surface.

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Confirmation of Existing Information (cont.)

• A simple groundwater flow model is constructed
  and reverse particle tracking is used to identify
  the WHPA.

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Delineation Approach Direct Determination of
Groundwater Flow Direction and Hydraulic Gradient
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Direct Determination

• This approach places a greater emphasis on field
  work to provide a greater number of actual static
  water elevation measurements.
• Selected wells (12 or more) are surveyed and a
  potentiometric surface generated from the
  observed data.
• The wells must be generally upgradient of the
  water supply well and suitably distributed.

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Direct Determination (cont.)

• Potentiometric surface may be supplemented by
  reported static water levels from water well
  logs.
• Lake elevations or river stages from topographic
  map may also be used to supplement potentiometric
  surface map.
• A simple groundwater flow model is constructed
  and reverse particle tracking is used to identify
  the WHPA.

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Delineation Approach Extensive Hydrogeologic
Investigations
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Extensive Hydrogeologic Investigations

• Necessary when groundwater flow system is
  complex.
• Incorporates features such as rivers, lakes,
  variable aquifer thickness, variable hydraulic
  conductivity, 3-D groundwater flow, or multiple
  aquifers.

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Extensive Hydrogeologic Investigations (cont.)

• Extensive dataset is required to define
  groundwater flow system.
• Because the data requirements exceed the data
  available in most WHPA studies, this approach may
  not be recommended except where the site
  hydrogeology requires this level of effort.

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Extensive Hydrogeologic Investigations (cont.)

• Hydrogeologic characterization is incorporated
  into a sophisticated analytical model or
  numerical model program.
• Properly constructed and calibrated, these models
  can provide an accurate WHPA delineation.
• Model may also be useful for evaluating future
  groundwater development or well-field management.

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Modeling Considerations
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Choice of Model

• Model selection is up to consultant.
• Model must be documented, tested and accepted by
  hydrogeological community.
• Model must be appropriate for site
  hydrogeological conditions.
• We would prefer a Windows-based software package.

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Submitting Digital Files

• You are required to submit numerical model
  datasets in digital format.
• Model datasets can be sent via e-mail, ftp site,
  or disk (floppy, jaz, zip, or CD).
• MODFLOW datasets should include both the generic
  MODFLOW format and the pre-processor format (e.g.
  Visual MODFLOW, .vmf).

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Submitting Digital Files (cont.)

• We use the datasets to evaluate model simulations
  included in the delineation report.
• Models are re-run to assess the impact of
  parameter uncertainty on WHPA delineation.

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Professional Judgement

• The accuracy of computer modeling is no better
  than the accuracy of the data used in the
  analysis.
• Always question modeling results.
• Do the model results agree with your
  understanding of the site hydrogeology and
  sound hydrogeological principles?
• Is your understanding of the site hydrogeology
  reasonable?

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Professional Judgement

• Interpolated potentiometric surfaces
• A single hydraulic head measurement may result
  in an unrealistic contour.

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Professional Judgement

• Interpolated potentiometric surfaces
• Removing suspect data point smooths
  potentiometric surface contours.

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Interpolated Potentiometric Surfaces

• Interpolation schemes may result in WHPA width
  which narrows upgradient.
• Judgement should be used to maintain a more
  uniform WHPA width.

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Data Variability

• Data variation is caused by uncertainty in the
  appropriate data value or by physical reality.
• Where there is significant variation in a model
  parameter value, do not use an average value. Use
  the range of values, which reflect data
  variability, to bracket delineation results.

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Data Variability
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Data Variability
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Data Variability
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Use of textbook valuesfor model parameters

• Transmissivity, hydraulic conductivity, aquifer
  thicknesses, and the hydraulic gradient must be
  based on site-specific measurements.
• Textbook values for other parameters, such as
  porosity, may be acceptable.
• Where textbook values are used, they should be
  used conservatively.

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Modeled Pumping Rate

• The modeled pumping rate must represent the
  average day of the peak month.
• A higher rate may used if increased demand is
  anticipated.
• The annual average or the maximum pump capacity
  should not be used.

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Use 10-year Particle Tracking

• WHPA delineations are based on 10-year reverse
  particle tracking.
• Study area boundaries should extend well beyond
  the 10-year time of travel.

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Use 10-year Particle Tracking
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Accounting for Uncertainty

• In spite of our efforts, there is a lot we dont
  know about the hydrogeological conditions at a
  site.
• Account for uncertainty when delineating WHPA.
• We may include a buffer surrounding the
  delineated WHPA to account for uncertainty in the
  hydrogeological data.

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A Word About Particle Tracking

• WHPA is delineated using particle tracking.
• Particle Tracking is advective transport only -
  does not account for contaminant dispersion or
  diffusion.
• Because of dispersion and diffusion,
  contamination outside a WHPA defined by particle
  tracking can impact a well.

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Particle Tracking and Dispersion (cont.)
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Particle Tracking and Dispersion (cont.)
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Particle Tracking and Dispersion (cont.)
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Some Friendly Advice

• Dont oversimplify a complicated hydrogeologic
  system.
• Reported static water levels are probably wrong.
  Use them with caution.
• Question the reasonableness of potentiometric
  surface - especially an interpolated surface.

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More Friendly Advice

• Interpreted conditions from aquifer test must
  match hydrogeology and delineation method.
• Use the right pumping rate. The pumping rate
  should be the average daily rate of the peak
  month.

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Still More Friendly Advice

• Questions or concerns which arise during the WHPA
  delineation process can be addressed at any time
  by contacting DEQ staff. Dont wait until after
  the first report submittal.
• Working together during the delineation process
  will minimize resubmittals.

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Finally

• Consider upgrading delineation capabilities
• Windows-based software
• Numerical Modeling
• Evaluate impact of dispersion on WHPA delineation.

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• MDEQ, LWM, 1999