Wake County Comprehensive Groundwater Investigation

1
Wake County Comprehensive Groundwater
Investigation Groundwater Study Advisory
Committee Meeting December 11, 2002
2
Project Schedule
3
Water Budget
Review of November GSAC Meeting

• P WW/IND Rech EDR SW Disch
• ET RO SWW GWW EDW BF

P Average precipitation WW/IND Rech
Wastewater and industrial discharge to GW EDR
Estimated domestic recharge from private septic
systems SW Disch Discharge to streams (NPDES
discharges) ET Evapotranspiration RO Runoff
component of precipitation SWW Surface water
withdrawal from creeks GWW Groundwater
withdrawal from PWSs and Industrial wells EDW
Estimated withdrawal from domestic wells BF
Median Baseflow of streams
4
Water Budget
Review of November GSAC Meeting
All values in inches per year
5
Baseflow and Recharge
Review of November GSAC Meeting

• Baseflow accounts for approximately 45 of total
  streamflow in Wake County Streams
• Middle Creek 55
• Crabtree Creek 45
• Walnut Creek 47
• Swift Creek 34
• On average, 15 of precipitation recharges the
  groundwater system in Wake County
• Jordan Lake 6
• Harris Lake 8
• Black Creek, Beaver Dam Lower Neuse 18
• Little River 19

6
Completing the Water Budget

• Runoff
• Wastewater and Industrial Recharge (Spray
  Irrigation)
• Estimated Domestic Withdrawal
• Estimated Domestic Recharge

7
Estimating Runoff

• Pervious Area Runoff
• Identify the hydrologic soil groups
• A High infiltration rates (gt.3 in/hr)
• B Moderate infiltration rates (.15 to .30
  in/hr)
• C Slow infiltration rates (.5 to .15 in/hr)
• D very slow infiltration rates (0 to 0.5 in/hr)
• Assign a runoff factor to each soil group
• Runoff factors based on previous studies of
  runoff from Piedmont watersheds (NVPDC, 1979)

8
Estimating Runoff

• Jordan Lake
• Soil Types
• A 0
• B 13
• C 46
• D 40
• Water 1
• Pervious Area
• Runoff 5.61 in/yr

9
Estimating Runoff

• Lower Neuse
• Soil Types
• A 2
• B 80
• C 8
• D 8
• Water 1
• Pervious Area
• Runoff 4.01 in/yr

10
Estimating Runoff

• Impervious Area Runoff
• Wake GIS Department GIS coverage
• Area of rooftops (footprint), streets
  sidewalks, and parking lots were estimated
• 95 runoff coefficient

11
Estimating Runoff
12
Water Budget
Basins where RO gt BF
All values in inches per year
13
Water Budget
Sandling Beach and Falls Lake Recreation Area
Raleigh, Neuse River WWTP Reuse
Wrenn Road Spray Irrigation Facility
All values in inches per year
14
EstimatedDomesticWithdrawal
Public water line
PWS well
Parcel with a domestic well
Subdivision supplied by Heater Utilities or
Carolina Water Service
15
Estimated Domestic Withdrawal
16
EstimatedDomesticRecharge
Public sewer line
Parcel with an onsite wastewater system
17
Estimated Domestic Recharge
18
Reality Check...

• Persons served by domestic wells
• 1997 Water/Sewer Plan estimate - 97,500
• Current estimate (2002) - 92,850
• Persons served by onsite wastewater systems
• 1997 Water/Sewer Plan estimate - 143,000
• Current estimate (2002) - 142,825

19
Reality Check...

• Approximate number of new wells and onsite
  wastewater systems approved per year (1990s)
• Wells - 660
• Onsite wastewater systems 1,600
• Number of parcels annexed per year
• 1997 - 5,385
• 1998 - 5,083
• 1999 - 5,238
• 2000 - 4,916
• 2001 - 6,842

20
Water Budget
Not Yet Estimated

• Commercial withdrawals and recharge
• from onsite systems (630 parcels )
• Dewatering at nine quarries (1.6 mgd, DWR)
• Irrigation (crop and golf) demand
• (3.6 mgd, USGS)

All values in inches per year
21
Net Groundwater ConsumptionGWW EDW WW/IND
Rech - EDR
22
Net Groundwater Consumption vs. Average Annual
Baseflow
23
A More Conservative ApproachLow Flow Data

• 7Q10 The minimum average discharge for a 7-day
  consecutive period occurring, on average, once in
  10 years.
• 30Q2 The minimum average discharge for a 30-day
  consecutive period occurring, on average, once in
  2 years.

24
Low Flow Data (USGS)
25
Low Flow Data (USGS)
26
Comparison of Average Annual Baseflow, 7Q10, and
30Q2
27
Net Groundwater Consumption vs. 7Q10 and 30Q2
Flows
28
Next Meeting January 8, 2003
29
Questions / Comments?
30
Up. Falls Lake Use average of Harris Lake
Determining EDW
52
Upper Falls Lake
Beaver Dam Use average of Little River
Usage Rate by Basin (gals/person/day)
Lower Falls Lake
99
Beaver Dam
Upper Neuse
Crabtree Creek
71
Jordan Lake
68
68
77
Jordan Lake Use average of Harris Lake
Little River
64
Walnut Creek
52
Lower Neuse
55
Swift Creek
60
Harris Lake
65
Middle Creek
52
59
Kenneth Creek Use average of Black Creek
59
Black Creek
Kenneth Creek