HomeMy WebLinkAboutAgenda - 12-02-1996 - 15a 1
ORANGE COUNTY
BOARD OF COMMISSIONERS
ACTION AGENDA ITEM ABSTRACT
Meeting Date: December 2, 1996
Action Agenda
Item# 15-4
SUBJECT: REPORT: GROUND WATER RECHARGE RATES IN ORANGE COUNTY
ACTION BY:
DEPARTMENT: PLANNING PUBLIC HEARING: Yes__2L_No
BUDGET AMENDMENT REQUIRED: Yes___X_No
ATTACHMENT(S): INFORMATION CONTACT: Dave Stancil
• Staff Report Summary Analysis (Ext 2590)
• USGS Report"Ground Water Recharge to the
Regolith-Fractured Crystalline Rock
Aquifer System,Orange County,NC"
(provided under separate cover)
TELEPHONE NUMBERS:
Hillsborough - 732-8181
Durham - 688-7331
Mebane - 227-2031
Chapel Hill - 967-9251
PURPOSE: To consider the findings of a U.S. Geological Survey(USGS)report on ground
water recharge rates in Orange County. This project was jointly funded by Orange
County and the USGS.
BACKGROUND: On March 21, 1995,the Board of Commissioners authorized a jointly-funded
study of ground water recharge in Orange County to be performed by the U.S.
Geological Survey. This project was proposed by the Water Resources Committee
as the first step toward the Committee's charge to investigate ground water
resources in the County.
The project was overseen by Dr. Charles Daniel, Research Hydrologist with the
USGS and an Orange County resident. Dr. Daniel is among the pre-eminent
researchers of ground water in the Piedmont region from New York to Alabama,
and has published a number of reports on Piedmont ground water as well as co-
authored a computer technique for ground water analysis.
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Stream flow within a waterway generally consists of two component water
sources - stormwater runoff and ground water discharge. As much as 50%of total
annual streamflow is known-to result from ground water discharge. The ground
water component of total stream flow is known as "baseflow."For the purposes of
hydrological evaluation,the volume of this base flow is considered to be equal to
the volume of the portion of precipitation which infiltrates into the land surface.
The infiltration component of precipitation"recharges"the ground water
storage/aquifer system. In this report,the determination of volume and rate of
baseflow-and ground water recharge- for the County's river and stream basins
are derived by applying an analysis technique known as "hydrograph separation"
to stream flow records. Long-term historical records of streamflow data from 17
USGS gaging stations cover all but one (the 5.5 square mile South Hyco Creek
basin)of the drainage basins in Orange County, were analyzed to develop
baseflow and ground water recharge estimates for each basin.
The purpose of the report is to provide a broad overview on a basin-by-basin
approach toward a better understanding of ground water availability. The report
does tip provide data that would allow for site-specific estimates of ground water
quantity. Two examples are provided in the report that illustrate ways of using
historical streamflow data and assumed site conditions for planning purposes,but
neither example is designed for use in site-specific analysis.
A copy of the report is being provided under separate cover, and a staff summary
of the report is attached. The principal findings of the report are:
• Most of the ground water in Orange County is found in a geologic zone of
weathered bedrock and soil,termed the"regolith"
• The recharge to the ground water system is highest along a north-south zone
in the center of the County, and lowest in the western and southeastern
(Triassic basin)portions of the County. The Haw River basin in western
Orange has the lowest mean recharge with 311 gallons/day/acre, while the
Morgan Creek basin upstream of Chapel Hill has the highest rate at 477
gallons/day/acre.
• A significant portion of streamflow in Orange County streams is ground water
"baseflow", ranging from 48%in Morgan Creek upstream from Chapel Hill to
32%in New Hope River(Bolin Creek and the areas draining to Jordan Lake).
• Mean recharge rates and recharge duration statistics(the percentage of time
during which ground water flow meets or exceeds a specified amount)can be
used for ground water management planning. Two examples are provided that
illustrate ways data could be used to estimate a required recharge area.
• There is considerable ground water available in Orange County,to varying
degrees in the different basins. These resources can be relied upon to provide
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potable water to a significant part of the County's population, through careful
planning and application of sound hydrogeologic principles.
Next steps for the Water Resources project will find the Water Resources
Committee considering a scope of work for phase two of the ground water
resource investigation on November 26. This recommendation is to be presented
at the December 7 Board of Commissioners retreat.
RECOMMENDATION: Receive as information.
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M E M O R A N D U M
Orange County Planning & Inspections Department
306F Revere Road Hillsborough,NC 27278
Telephone: (919)732-8181 FAX(919)644-3002
To: John Link,County Manager
From: Paul Thames,County Engineer
Dave Stancil,Planner II/Land Use and Environment
Date: November 26, 1996
Subject: Summary of USGS Ground Water Recharge Report
Copies: Marvin Collins,Planning Director
Ron Holdway,Environmental Health Director
Gene Bell,Planner II-Comprehensive
On December 2,the findings of a report on ground water recharge rates in Orange County will
be presented to the Board of Commissioners.This report is the result of a jointly-funded project
with the U.S.Geological Survey(USGS)to investigate,on a general level,the rate of recharge to
ground water supplies in the County.The project was recommended to the Board of
Commissioners by the Water Resources Committee in early-1995,and funding was authorized
for the 18-month project in May 1995.This project is designed as the first phase of the Water
Resources Committee's charge to investigate the quantity and quality of ground water within
Orange County.
It should be noted that the purpose of the report is to provide a broad overview on a basin-by-
basin approach toward a better understanding of ground water availability. The report does
not and was not intended to provide the level of data that would allow for precise site-specific
estimates of ground water quantity. Two examples are provided in the report that illustrate
ways of using historical streamflow data and assumed site conditions for planning purposes,
but neither example is designed for use in site-specific analyse
This document provides a basic summary of the main findings of the USGS report.The report
was overseen by Dr.Charles Daniel,Research Hydrologist with USGS.Dr.Daniel,an Orange
County resident,has published a number of studies on ground water in the Piedmont province
and is among the foremost authorities on ground water in this area.He is a member of the Water
Resources Committee and is currently preparing a similar report for Guilford County,NC.
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Report Abstract
Approximately one in three Orange County citizens rely on ground water for their potable water
supply. While study of surface water supplies and needs have been undertaken,relatively little
has been done to date to study ground water supplies in this area. As the population of Orange
County grows,there will be an increased demand on the ground water resource that very little is
known about. In response to this population growth, the USGS and Orange County began this
study in 1995 to assess ground water recharge rates to the county's ground water aquifer system,
and to examine ways of determining ground water storage capacities throughout the county. The
goal was to produce accurate quantitative data that would help water resource management
efforts.
In this study,a hydrograph separation technique was used along with streamflow data to estimate
seasonal and long term recharge rates in 12 drainage basins in Orange County.Methods for
estimating ground water storage rates are also discussed in the report.
It is estimated that the Morgan Creek Basin(upstream from White Cross and Chapel Hill)has
the highest recharge rates in the county,and that ground water constitutes a higher percentage of
streamflow in Morgan Creek than in any of the other study areas.Conversely,the Haw River
basin in southwestern Orange has the lowest recharge rate on all basins in the County.In general,
recharge rates are highest along a north-south zone in the center of the county,and lowest in the
west and southeast. Overall,the range of net annual average recharge varies from 311
gallons/day/acre to 477 gallons/day/acre.
Ground water storage capacities can be expected to vary throughout the county. Most ground
water storage in Orange County is found in the regolith,a zone of soil and weathered bedrock.
Storage capacity in the Piedmont is typically a function of the saturated thickness of the regolith
and of the specific yield of the regolith.
Study Objectives
The objectives for the study as determined by the Orange County Water Resources Committee
and the USGS were to:
• Evaluate long-term ground water recharge rates throughout Orange County based on
available data
• Refine those estimates by comparing differences in recharge rates between basins
• Further refine estimates by determining the effect of seasonal climatic changes on recharge
rates
• Produce a report describing recharge rates, including tables and hydrographs which can be
used to accompany GIS watershed maps
Orange County Hydrogeology
Orange County's hydrogeology is defined by an intricate relationship between streams and rivers
that convey runoff from the county,and the regolith-fractured crystalline rock aquifer system
that stores ground water and functions as a conduit for ground water from recharge to discharge
areas. Ground water discharge to streams and rivers is an important component of total
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streamflow in the county. Recharge rates vary from basin to basin depending on precipitation,
topography, soil, land use,etc. Ground water storage is a function of recharge, and of the
characteristics of the aquifer.
The majority of Orange County is underlain by extensively folded and fractured bedrock which
is almost entirely covered by a layer of unconsolidated material called regolith.The
characteristics of these geologic layers and their relationship determines the water-supply
potential of the ground-water system in the county.The aquifer system in Orange County is
composed of four tiers: an unsaturated zone in the regolith(organic soil layers),underlain by a
saturated zone; followed by the lower regolith(which contains the transition zone),and finally
the fractured crystalline bedrock.The regolith serves as a conduit that slowly channels water
down into bedrock fractures.
Recharge and Storage
Ground water is a part of the hydrologic cycle,which is often quantified as a"water budget." A
water budget can be expressed as the following equation:
precipitation =evaporation+transpiration+streamflow±change in storage
Streamflow and storage are the two areas from which ground water may be found.The two
important components of the ground water system are ground water recharge and ground water
storage.
Ground water recharge occurs when water percolates down through the unsaturated regolith
layer to either the saturated layer of the regolith,or further down to the fractures in the bedrock
where the water begins to move laterally. The rate at which water does this is expressed as a
volume per unit of area per period of time(for example,gallons/day/acre).Recharge rates vary
from season to season and year to year,and are affected by changes in precipitation,
evapotranspiration, land use and other factors.Most recharge to the ground water system occurs
during late fall,winter and early spring.
One important distinction about ground water in the Piedmont physiological province is that
nearly all ground water storage is in the regolith zone-with very little storage in the bedrock.As
previously noted,the regolith serves as a"reservoir"conducting water into the fractured
bedrock.The level of storage is affected by the seasonal variations in recharge,and the water
level can change by as much as 12 feet between seasons.There is very little annual variation,
however,so ground water storage can be assumed to be relatively stable over time.The water
table usually reaches its highest point in May or June.
The amount of water in storage can be estimated by examining the saturated thickness of the
regolith from wells.A previous report on wells in the Piedmont found that the average depth of
well casing is 52 feet,although this can vary depending on topography.In general,depth to
ground water is greatest beneath hills and ridges and least under valleys and draws. Work in the
Piedmont areas of Georgia contains similarity of hydrogeology,such that equations to determine
the amount of ground water in storage can be used with confidence.
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The H3&mgraph Separation Methodology
The method used to estimate recharge rates in Orange County is termed hydrograph separation.
It is not possible to measure the direct ground water recharge from precipitation.However,the
amount of precipitation that is discharged from the ground into streams can be measured by
splitting the years of historical streamflow data(hydrographs)from the 17 USGS gaging stations
that cover Orange County.The hydrographs are separated into two components: ground water
discharge, and overland runoff. Once this is done,the amount of flow determined to be ground
water discharge is added up over the hydrograph period.If(as research indicates)there has been
no long-term change in ground water storage, ground water discharge will be equal to ground
water recharge.
USGS examined several hydrograph separation methods in this study. The Pettyjohn-Henning
"local-minimum"method,which estimates values of daily mean base flows,was selected.This
method was selected because it provides the lowest(most conservative)estimate of daily mean
baseflow. Baseflow estimates account for the loss of ground water to riparian evapotranspiration
(i.e.,ground water lost to vegetation and evaporative losses on floodplains). Using this method
of estimating recharge will help ensure that sufficient ground water is available for riparian
vegetation.
Ground Water Recharge in Orange o un j's 12 Drainage Basins
Using the methodology described above,the USGS prepared recharge hydrographs and duration
statistics for each of the 12 basins.The hydrographs illustrate the monthly levels of recharge
along with median and mean recharge rates,over the years in which the gaging station was
reporting data.The duration statistics show the percentage of time certain flows were met or
equaled.These statistics allow for some comparison of base-flow in the different basins.
The results of the recharge rate analysis,using 17 gaging stations over 12 basins, are summarized
in the following chart:
BASIN' Years a
Haw River 67 98.4 sq.mi. 311 gal/day/acre
Cane Creek 20 33:7 sq.mi. 361 gal/day/acre
Morgan Creek-Upstream of CH 20 `37.6 sq.mi. 477 gal/day/acre
Morgan Creek-Upstream of WC 6 8.35-sq.mi. 427 gal/day acre
New Hope Creek 13 34.6 sq mi 339 gal/day/acre
New Hope River 24 24.5 sq mi 324 gal/day/acre
Upper Eno River 54 66 .0 sq mi 399 gal/day/acre
Eno River-lower 18 56.3 sq mi 341 gal/day/acre
7-Mile Creek 8 14.1 sq mi 367 gal/day/acre
Little River 34 63.6 sq mi 352 gal/day/acre
Flat River 70 10.5 sq mi 347 gal/day/acre
North Hyco Cr 31 4.1 sq mi 353 gat/day/acre
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3a•,s = ._ _ _;a _ •;_ I r 1. Haw River
^• 2. Cane Creek
C �F (1 1 7r If GRANVILLE
y�� f I t �l 3. Morgan Creek-
4( f- ? \ Upstream of Chapel Hill
ALAMANCE, - 1 14l%15 I 4. Morgan Creek-
*F I " . Upstream of White Cross
,,__ + 0 s �. i 1� � 5. New Hope Creek
}�� ORANGE ?�
�h�.' ,4 �„ jo URHAM 6. New Hope River
ry' - f"`" "•
j t: 7. Eno-Upstream of
Hillsborough(Upper Eno)
' . s 8. Eno River Sub-basin
2� �� ., c (Lower Eno)
J ► ,�„�,,,, •. / 9. Seven-Mile Creek
1 7 10. Little River
m.,. _ ,.� ° y i 11. Flat River
12. Hyco Creek
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33vs t .w.CHATHAM'.•/ e}� ' .,)c.�.•�. L.
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Comparison of Basins
The mean ground water recharge in the 12 basins ranges from a low of 311 gallons/day/acre in
the Haw River basin,to a high of 477 gallons/day/acre in the Morgan Creek basin upstream of
Chapel Hill.Considered together,the 12 basins have an annual mean recharge of 365
gallons/day/acre.The two Morgan Creek basins are substantially on the top end of the recharge
rate spectrum.If they are excluded from the analysis,the range of mean recharge rates in the 12
basins drops by almost one-half.
The median(mid-point)recharge varies substantially,from 80.7 gallons/day/acre in the New
Hope River basin to 370 gallons/day/acre in the Morgan Creek basin upstream of Chapel Hill.
The overall median is 228 gallons/day/acre.The amount of streamflow that is ground water
baseflow also varies across the different basins.Ground water constitutes the highest proportion
of total streamflow in the Morgan Creek basin,upstream of Chapel Hill,where 48%of
streamflow is baseflow.The New Hope River basin,at 32.2%,has the lowest percentage of
baseflow to streamflow.
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A correlation between recharge rates and the underlying hydrogeology is not immediately
apparent,except in the Triassic basin in southeastern Orange. Among the reasons for this is the
fact that all of the basins contain multiple hydrogeologic units. Infiltration capacity of the soil,
topographic relief, and depth of channel incision also play an important role. For example,the
streams in the Morgan Creek basin are deeply incised, and significant topographic relief occurs
in the basin. The mean recharge rate generally increases as one moves downstream in basins
because of these factors.
Use of Recharge/Storage Data for Plannino
Information on the amount of ground water recharge and the amount of ground water in storage
can be used to help in long-term ground water management planning. The ultimate limit to
ground water extraction is the rate of recharge to the system. Ground water storage can sustain
wells during dry periods, but long-term pumping at rates in excess of recharge will eventually
cause well yields to decline.To ensure that this"carryover"between recharge and storage works,
the location of wells becomes very important.Locating wells in areas with thick saturated
regolith is likely to produce the best results.
Examples are provided in the report that illustrate how recharge rate data can be used with
projected demand to estimate needed recharge areas.Both use the data generated in the recharge
rate analysis of the 12 basins in Orange County. However,the report notes that using the basin-
wide average recharge rates may not work in every case,especially for small tracts.
The first example assumes that no site-specific data are available for the creation of single-
family lots on wells and septic tanks.This example uses the calculated mean recharge rate in
combination with assumptions about the anticipated number of household occupants,per capita
water demand,amount of impervious cover and consumptive loss of wastewater on site to
generate an estimate of the area needed for recharge in the Cane Creek basin.
It should be noted that some of the assumptions in this example related to typical household
occupancy,per capita water demand,change in infiltration capacity and consumptive loss
of wastewater in on-site treatment systems are conservative and thus may be subject to
challenge and re-interpretation.Others factors may call for an additional adjustment that
could increase the recharge area.These issues are summarized as follows:
• 1990 Census data indicates that the typical rural Orange County household contains
approximately three persons per family,which represents a potential twenty-five percent
reduction in water demand from the example.
• Water consumption data can be produced to show that typical per capital domestic water use
may be nearer to seventy gallons per day,which represents a potential thirty per cent
reduction in water demand.
• It can also be shown that when on-site waste treatment systems-particularly ground
absorption systems-are utilized,the waste water itself is a source of ground water recharge.
That is,total water use does not represent a complete or consumptive loss of water from the
ground water system. Some unknown percentage of water used,perhaps as much as fifty to
eighty percent, is returned to the ground water system through the nitrification field.At the
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same time, however,most wastewater systems are not designed to provide ground water
recharge,and a significant portion of wastewater return may be lost to evapo-transpiration.
• Additional adjustments for changes in infiltration capacity of the land may increase the
needed recharge area.For example,conversion of land from permanent seldom-grazed
pasture or woodland to maintained lawns may reduce infiltration capacity by 50-60%and
require a subsequent increase in recharge area.
The above issues are subject to challenge and interpretation. Further research and study will
be required on these issues before the approach outlined in this example could be put into use.
Findings that ground water demand is lower than assumed(lower per capita use rates, lower
person/dwelling rates, inclusion of wastewater return)could result in an average recharge
area that is smaller than that shown in the example. On the other hand,conservative
estimation of ground water demand and use can produce a more reliable water supply and
help preserve stream base flow rates
The second example assumes that there is site-specific data available for a proposed community
water system.Here,test wells have been drilled and recharge duration statistics are used to
estimate the amount of land needed for recharge of the community wells.
Further research will be needed in this area to determine the implications for developments
that utilize community wells..
Again, it should be noted that conditions on a particular tract may not be typical of an entire
basin, so these examples should be considered as part of a basin-wide analysis only.
Next
As phase one of the Water Resources Committee's ground water resource investigation,this
report provides new information about the rate of recharge to ground water in Orange County.
The examples shown illustrate ways that ground water data may be used for management of the
resource.However,this study is only an overview of average recharge rates in 12 basins.Use of
these approaches will require additional information not currently known about the County's
ground water resources.
To further the knowledge of ground water resources,the Water Resources Committee will be
proposing a scope of work to take the next step toward investigating the availability of ground
water.Although a site-specific level of data about ground water availability will not be cost-
effective,the second phase of the ground water investigation will work toward the creation of a
"contour map"of ground water availability by analysis of test wells and other hydrogeologic
research.This phase of the study will be presented to the Board of Commissioners at the
December 7, 1996 Goal-Setting Retreat.
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Conclusions
Although a great deal of data and information is generated and used in the creation of this report,
several basic findings can be highlighted from the report:
• Approximately 41%of County residents depend on ground water for their
drinking water supply.
• Most of the ground water in Orange County is found in a geologic zone of
weathered bedrock and soil,termed the"regolith"
• The recharge to the ground water system is highest along a north-south zone in
the center of the County,and lowest in the western and southeastern(Triassic
basin)portions of the County. The Haw River basin in western Orange has the
lowest mean recharge with 311 gallons/day/acre,while the Morgan Creek basin
upstream of Chapel Hill has the highest rate at 477 gallons/day/acre.
• A significant portion of streamflow in Orange County streams is ground water
"baseflow",ranging from 48%in Morgan Creek upstream from Chapel Hill to
32% in New Hope River(Bolin Creek and the areas draining to Jordan Lake).
• Mean recharge rates and recharge duration statistics(the percentage of time
during which ground water flow meets or exceeds a specified amount)can be
used for ground water management planning.Two examples are provided that
illustrate ways data could be used to estimate a required recharge area.However,
a number of assumptions are utilized in this approach that may be open to
interpretation.The examples are provided as illustrations of ways that data can
be used for planning purposes,rather than a standard template for application in
review of individual sites.
• Overall,there is considerable ground water available in Orange County,to
varying degrees in the different basins.These resources can be relied upon to
provide potable water to a significant part of the County's population,through
careful planning and application of sound hydrogeologic principles.
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