Wake County Comprehensive Groundwater Investigation

1
Wake County Comprehensive Groundwater
Investigation Groundwater Study Advisory
Committee Meeting May 14, 2003
2
Project Schedule
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Agenda

• Review Conceptual Long-Term Monitoring Well
  Network
• Receive Comments on Draft Conclusions
• Review Draft Recommendations
• Agree upon a Schedule for the Final Report

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Long Term Monitoring Well Network Objectives

• Provide a long term period of record to assess
  the impact of sustained groundwater withdrawals,
  especially in the NUA/WSWs.
• Provide data to monitor water level declines due
  to groundwater withdrawals, drought, and/or
  changing land use.
• Provide information to understand better the
  impacts of urbanization on groundwater quality
  and quantity.

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Long Term Monitoring Well Network Objectives

• Provide a mechanism to monitor changes in raw
  groundwater quality over time.
• Provide defensible data to support potential
  groundwater resource management decisions.

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Conceptual GroundwaterMonitoring Well Network
WC-4
WC-3
WC-2
WC-6
WC-1
WC-15
WC-16
WC-5
WC-11
WC-7
Lake Wheeler Wells
WC-14
WC-9
WC-8
Land Use
WC-12
Commercial Industrial Residential Vacant Agricultu
ral Golf Course Other County-Owned
WC-10
DWR Well
WC-13
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General Classifications
Legend
Corp. Limit ETJ SRUSA LRUSA NUA WSW Critical Area
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Hydrogeologic Units
Source NCGS, 2003
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Community Water System Wells
Legend
CWS Well
Source NCDENR PWS Section
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Watershed Classifications
Legend
Degraded Impacted Healthy
Source Wake County Comprehensive Watershed
Management Plan (CH2M Hill, 2003)
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Water Quality
Legend
Rolesville Granite Ethylene Dibromide Detected in
CWS well 1,2-Dichloropropane Detected at CWS well
Sources NCGS and NCDENR PWS Section
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Potential Cooperating Agencies

• USGS
• NCDENR DWQ Groundwater Section
• NCDENR DWR Groundwater Branch
• State Climate Office
• Well Drilling Community
• NC State Laboratory
• Universities
• Municipalities
• NCGS
• Land Owners

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Draft Report Conclusions Questions or Comments
?
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Recommendations

• Prepare a Groundwater Management Plan
• Define water resource sustainability
• Develop a formal approach to review water
  resource impacts of new developments that use
  groundwater
• Further consider requirements for groundwater
  quality testing of domestic wells
• Evaluate surface water/groundwater conjunctive
  use scenarios (integrated water resource planning)

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Example Approach to Assess the Water Resource
Impacts of a Proposed Development

• Step 1 Determine if the total groundwater
  withdrawal is supported by the estimated recharge
  rate.

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RechargeRates(in/yr)NCDENRModifiedLandscape
Model
H.E. Mew, Jr., D. Hirth, D. Van Lewis, R.B.
Daniels and Keyworth. Methodology for Compiling
Ground Water Recharge Maps in the Piedmont and
Coastal Plain Provinces of North Carolina. Ground
Water Bulletin No. 25, NCDENR Div. of Water
Quality
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Distribution of Supported Residential
Densities Based on Estimated Recharge Rates

• Assumptions
• 300 gal/day/RU
• 3,400 sq. ft. of
• impervious surface
• for each RU
• - No return through
• septic tank

Recharge Supported Residential
Development Density Contours (acres/residential
unit)
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Example Approach to Assess the Water Resource
Impacts of a Proposed Development

• Step 1 Determine if the total groundwater
  withdrawal is supported by the estimated recharge
  rate.
• Step 2 Assess the potential impact of
  withdrawals to area streams during a drought.

No unacceptable impacts Move to Step 3 Potential
unacceptable impacts Trigger Hydrogeologic Assess
ment
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Variations in Maximum Residential Density
Based on Drought Condition Recharge Rates
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Example Approach to Assess the Water Resource
Impacts of a Proposed Development

• Step 1 Determine if the total groundwater
  withdrawal is supported by the estimated recharge
  rate.
• Step 2 Assess the potential impact of
  withdrawals to area streams during a drought.
• Step 3 Estimate the amount of groundwater
  storage available in the regolith.

Ample Storage Move to Step 3 Limited
Storage Trigger Hydrogeologic Assessment
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Regolith Thickness Map

• Also need to estimate
• saturated thickness
• porosity

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Example Approach to Assess the Water Resource
Impacts of a Proposed Development

• Step 1 Determine if the total groundwater
  withdrawal is supported by the estimated recharge
  rate.
• Step 2 Assess the potential impact of
  withdrawals to area streams during a drought.
• Step 3 Estimate the amount of groundwater
  storage available in the regolith.
• Step 4 Determine if there are any known water
  quality concerns.

YES Require Sampling Analysis
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Areas of Known Water Quality Concerns
Legend
Rolesville Granite Ethylene Dibromide Detected in
CWS well 1,2-Dichloropropane Detected at CWS well
Sources NCGS and NCDENR PWS Section
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Recommendations

• Perform a study to assess the impacts to
  groundwater and surface water from dense, rural
  development (NCDENR recommendation).
• Record and track failed attempts to install new
  wells due to low yield.
• Further investigate radionuclides in groundwater
  and radon in indoor air, especially in the
  northeast portion of the County.

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Recommendations

• Perform sampling and analysis of select domestic
  wells to determine if 1,2-dichloropropane, EDB,
  and arsenic are present in areas of eastern Wake
  County where the constituents were detected in
  CWS wells.
• Include a public education program to accompany
  any additional investigations of groundwater
  quality.
• Implement a cooperative long-term monitoring well
  network.

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Schedule

• May 15th to May 31th Receive Comments
• June 1st to June 13th Incorporate Comments
  into Final Report
• June/July ?? Present to Board of
  Commissioners