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"..r-~D'elineating the Drinlr"-a Water Protectian Area far Graundwc' 'r Systems
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Delineating the Drinking Water Protection Area for
IGroui1awjlte_rJ~yst~ms 1_ ,_ _ _ ". _ _"_
(includes Springs andrConjunctive_D_elllJ.ei!.t!ons)
Delineating the drinking water protectian area (DWPA) far a graundwater
system is perfarmed by identifying the surface area that directly .overlies that
portion' of the aquifer that supplies the well, wellfieJd .or springs used far drinking
water purpases by a public water system, Far aquifers that are being used by a
public water system far aquifer starage and recavery (ASR), the delineatian will
identify the surface area that .overlies the injectian "bubble" praduced during the
starage phase, The geametry .of the bubble will be determined through a
cambinatian' .of madeling and manitar well water level and water quality analysis,
The determinatian is a natural autcame of Oregan's existing ASR permitting
pracess,
Oregan's "Wellhead Pratectian Guidance Manual" .outlines in detail the minimum
requirements far state-certificatian .of a drinking water protectian area (DWPA),
These requirements are in Administrative Rule (OAR 333-061-057) and are
clasely linked with a patential manitaring reductian far public water systems as
well as land use regulatians in the state,
The methads far delineating public water systems using wells are based
primarily an papulatian and are summarized in the text belaw, Springs are
delineated using hydrogealagic methads as they are cansidered ta be
groundwater saurces,
Wells
The size and shape .of the d~lineated areas are determined by substituting
specific parameters inta a mathematical expressian the salution .of which gives
information abaut the area in questian, The impartant parameters that are
camman ta all methads are the factars that cantrol the geametry .of the
delineated area, including pump rate. time-af-travel, and, where apprapriate,
aquifer characteristics and the distribution .of hydraulic head,
Adjusted Pump Rate. The pump rate (ie" gallans/minute) used in the madels
is based an the average manthly rate determined aver the three manths .of
highest use, As an example. can sider a cammunity whase water usage far the
three highest manths is 1,200,000 gallons, 2,500,000 gallans and 770,000
gallans, representing July, August and September, respectively, The average
manthly use then. is 1,490.000 gallans, which equals -48,586 gallans/day,
which in turn equals -34 gpm, If actual data is nat available, the pump rate is
estimated based an camparable cammunities, pump capacity .or papulatian
served,
Time-at-Travel. Although the capture zane far a given well extends ta an
upgradient boundary, this distance may be measured in tens .of miles and be
impractical with respect ta management strategies, Cansequently, the
http://www.deq.state.or.us/wq/dwp/DWPAreaDelineationsGW.htm
1/3/02
Delineating the DrinV"',? Water Pratectian Area far Groundw"'~T Systems
. . . .1
upgradient extent .of the capture zane is terminated at a specific time-af-travel
(TOT) far groundwater through the aquifer, For systems with papulatians >500,
the TOT used is 10 years, Far systems serving < 500 the TOT is 15 years
because the methad .of delineatian is much less precise, Within each delineated
area, subzanes are delineated at 2- and 5-year TOTs, The sharter time-af-travel
delineatians are canstructed ta provide the cammunity with a better
understanding af,the relatian between an indentified patential cantaminant
saurce and the well. Patential saurces cauld be appraached from differing
management strategies as a functian .of their TOT distance from the well. The
area within the 2-year time-af-travel represents the "hat zane" far the area:
potential contaminant sources within this area will prabably need extra attentian
because .of their praximity ta the well. The 2-year TOT alsa pravides the .outer
limit fram which micrabial saurces are likely ta impact the well. The gaal .of the
subzanes is ta provide the cammunity with better data fram which ta build
pratection strategies,
Systems Serving <500 Population: Far systems of this size, the calculated
fixed radius (CFR) methad is utilized, In this methad, the valume .of water
needed aver the next 15 years is estimated, Using this value, the valume .of the
aquifer, visualized as a vertical cylinder, is calculated using a porasity cansistent
with well lag infarmatian and a length equal ta that .of the water-bearing zane, .or
perforated interval, whichever is smaller. The drinking water protectian area,
then is a circle directly above the cylinder, with the same radius as the cylinder,
Similar calculatians are perfarmed using time values .of 2- and 5-years, The final
delineation, therefare, cansists .of three concentric circles centered an the
productian well having radii that refiect 2-,5- and 15-years .of water usage,
A TOT .of 15 years is required far CFR delineatians because .of the very general
nature .of this methad, Na infarmatian regarding groundwater fiaw .or aquifer
permeability is cansidered Small systems with multipie wells, the CFRs of
which averiap, are madeled in a mare saphisticated methad Cthe 500-3299
methad),
~ (~y_sJems_Serving.>500-Populiit~Farall systems serving >500 papulatian,
-the develapment .of a canceptual madel .of the graundwater system is required,
r The canceptual madel is a three-dimensianal pi~ture .of the hydragealagy .of the
area, I.e" the aquifers. aquitards, areas .of recharge and discharge, and head
distributian, The canceptual madel pravides far a mare representative
determinatian .of the DWPA and is accomplished by the process .of
hydrogealagic mapping, the process .of using available data ta .obtain
infarmatian regarding the geametry and characteristics .of the relevant
hydrogealagic units,
The canceptual madel is a regianal picture .of the hydrogealagy, using regianal
studies, well reparts, cross sectians and fence diagrams, estimates .of recharge
and discharge. hydragealagic characteristics and hydraulic head distributian,
Far systems in this class serving <3300, the abave parameters can be
estimated fram regianal studies, specific capacity tests, or from infarmatian an
available well reparts, Far systems serving 3300 ta 50,000, the hydrogealagic
characteristics .of the aquifer will be derived wherever passiblefram canstant
rate aquifer tests and the distributian .of hydraulic head will be determined by
field measurement. The canceptuai madel is the basis far estimating the
parameters used in the analytical madels that create the delineatians,
Parameters include pump rate, parosity, degree .of canfinement .of the aquifer,
hydraulic canductivity, aquifer thickness, gradient directian and magnitude,
recharge if appropriate, and any aquifer baundaries,
" - --------
~ Sys!el!)_sServing,>.50,000,P-opulation,.:r;he analytical models discussed abave
assume that the aquifer is hamageneaus and unifarm in its praperties, In
http://www.deq.state.ar.us/wq/dwp/DWPAreaDelineatiansGW.htm
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Delineating the DrinV~,? Water Protectian Area far Groundwr' 'r Systems
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actuality. this is rarely the case; aquifers may change permeability, thickness,
etc, alang their extent. Far systems serving larger papulatians, a mare
saphisticated madel appraach, capable .of accammadating heterogeneaus
aquifers, is required, Far these systems, a numerical appraach is cansidered
apprapriate, In the numerical approach, the area is divided inta cells (palygans),
each .of which can vary in their characteristics, Models can be twa- .or three-
dimensianal. A further benefit .of the numerical madel approach is that they are
capable .of being calibrated and validated sa that there is even greater
canfidence in the madel results,
NOTE: Far systems serving more than 50,000, OHD will work with the staff from
the public water system in developing and/or refining existing models that
predict the Drinking Water Protection Area, Most of these larger public water
systems have already perfarmed some level of aquifer analysis and modeling,
Model Sensitivity Analysis. In many cases, a range .of hydraulic characteristics
is passible far a given aquifer .or hydragealagic setting, The sensitivity analysis
pravides far the evaluatian .of haw sensitive the madel results are ta the variatian
in any given parameter. This pracess allaws the madeler ta determine which
parameters are mastimpartant in cantrblling the geametry .of the delineatian and
therefare which shauld be determined ta the greatest degree .of accuracy, In
thase cases where data refinement is nat possible, the delineatian will be
campleted by .overlaying the individual delineatians representing the range(s) .of
parameter(s) .of cancern, e,g,. transmissivity .or porasity,
Porous Media Assumption. As indicated in the accampanying flaw chart, an
evaluatian .of the paraus media assumptian relative ta the aquifer must be made,
In a granular .or paraus medium, groundwater will mave directly dawngradient,
assuming na significant preferential flow paths exist. Mast analytical models
assume such a canditian, In fractured rock, hawever, graundwater will flaw
preferentially alang fractures, which frequently are nat oriented perpendicular ta
hydraulic head cantaurs, In areas where graundwater flaw is cantrolled by
discrete fractures, the delineation will be .oriented similarly with the length being
cantralled by estimated time-af-travel .or hydragealagic baundaries,
Variations in Hydraulic Gradient. In many hydragealagic settings, the gradient
may change significantly in directian and magnitude an a seasanal basis. awing
ta changes in pumping levels and recharge, If the magnitude .of that variatian is
knawn, delineatians are perfarmed under wet and dry canditians and .overlaid ta
praduce the final delineatian, If the magnitude is nat knawn, the gradient
directian is rotated through an angle .of 25 or 45 degrees, depending an whether
the gradient is based on field measurements or regional data, respectively, The
rotatian is accamplished fram bath sides .of the .observed gradient with
independent delineatians perfarmed at the extreme pasitians and .overlaid on
the .original delineatian, '
Multiple Water-Bearing Zones. If multiple water-bearing zanes occur within the
hydrogealagic aquifer unit that are persistently separated by lawer-permeability
units, the cambined thicknesses .of thase units is used as the aquifer thickness,
If the permeability .of these multiple zanes vary significantly, the pump rate is
partitianed between them based an their hydraulic canductivities and
thicknesses and individual DWPAs are calculated far each and .overlaid ta
praduce the final delineatian, '
If the water system is cantemplating adding new wells ta their inventories and
nave patential sites selected, preliminary delineatians will be performed at thase
sites using regianal aquifer data and estimated pump rates provided by the
water system, This data may prave useful in selecting the ultimate site .of the
new well.
http://www.deq.state.ar.us/wq/dwp/DWPAreaDelineatiansGW.htm
1/3/02
Delineating the DrinJr'~16 Water Protectian Area for Groundwc' 'r Systems
Springs
The delineatian of spring saurces is accamplished salely through hydrogeologic
mapping, This field pracess determines the gealagic structure that controls the
lacatian .of the spring, its geametry and haw it is .oriented in the subsurface, This
infarmatian allaws the hydragealagist ta determine the land surface that .overlies
this structure and therefare accamplish the delineatian .of the spring's recharge
area, The baundaries .of the spring's recharge area (the DWPA) is .often
cantralled by gealagic features, In areas where the hydragealagic baundary is
nat evident .or is same distance fram the spring. the size .of the DWPA will be
canstrainedusing estimates .of the vela city .of graundwater in the subsurface,
This will allaw the determinatian .of time-related zanes within the DWPA, .or
determine the area .of spring recharge by dividing the spring discharge rate at
law flaw (ft3/year) by an estimate .of the recharge rate ta the aquifer (ftiyear),
The delineated areas will be placed in a GIS caverage that will be available an a
statewide basis through the state's GIS Service Center's website, Individual
water systems will receive a tapagraphic map with a preliminary map of the
delineatian shawn as a functian .of the time-af-travel as saan as it has'been
campleted, The purpase .of this is ta facilitate the cammunity's .or water system's
invalvement in DEQ's inventary process, Final maps will be provided in a SWAP
repart that will cantain the delineation, aquifer sensitivity analysis, patential
cantaminant saurce inventary and susceptibility analysis,
f1:onjuncti~e D!!]neationsl
Graundwater and surface water are related campanents of the hydrolagic
system that cantrols the distribution .of water an the surface and in the
subsurface, Within this system, generally speaking, surface water receives
"base flaw" fram graundwater and groundwater is .often in hydraulic cannectian
with surface water. At a specific site, however, this may .or may nat be the case,
Far example, alang a given stretch of stream flaw, the stream may be
discharging ta graundwater, I.e" a lasing stream. rather than a gaining stream,
Likewise, in a given drinking water pratectian area, the aquifer may nat'be
deriving water from a surface water saurce,
The Surface Water Treatment Rule .of the Safe Drinking Water Act directs states
ta determine thase public water systems whase groundwater saurces are under
the "Direct Influence .of Surface Wate~' because .of the patential risk .of Giardia in
their drinking water. The Health Divisian develaped a pratacal ta evaluate this
by first recagnizing graundwater systems that were in hydraulic connectian and
then asking them ta canduct micraarganism analysis to evaluate the risk .of
Giardia. Hydraulic cannectian can .occur either naturally, along lasing stretches
.of streams .or at high-river stage, .or can be induced through high pumping rates,
Through evaluatian afca-variatians .of temperature, canductivity, pH, etc" OHD
has recagnized 57 (as .of 12/98) community water systems that were
hydraulically cannected ta surface water. Same .of these systems had wells as
deep as 200 feet and are as far fram a surface water saurce as 400 feet. Same
cammunity water systems and mast nancammunity water systems are still in the
pracess .of evaluatian, OHD will use the protacal established under the Surface
Water Treatment Rule to identify graundwater systems drawing from surface
water saurces far purpases .of the SWAP,
It is envisianed that far thase public water systems that have groundwater
saurces that are in hydraulic cannectian with a surface water saurce, the
delineatian .of the groundwater saurce will .occur as described abave,
Delineatian .of the surface water cantributian ta the graundwater system will be
accamplished an a case-by:case basis, depending an the extent .of the
connectian, distance from the surface water saurce, etc, In all cases. hawever, a
http://www.deq.state.ar.us/wq/dwpIDWPAreaDelineatiansGW.htm
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. Delineating the DrinV'<:; Water Protectian Area far Groundw~'-" Systems
partian .of the watershed area will be identified as a saurce .of drinking water far
the cammunity in graundwater systems with hydraulic cannectians ta surface
water. . '
DEQ and OHD will use data fram existing hydragealagic studies ta identify
those surface-water saurces that receive significant cantributian fram
graundwater, In same basins, there will be existing hydragealagic studies that
will identify hydraulic head distributian fram which it can be inferred whether .or
nat groundwater is discharging ta the stream .or other surface water bady .of
concern, In ather cases, we will rely an data assaciated with the surficial soils
and graundwater fiaw regime that discharge ta the surface as evidenced by
streams exhibiting significant gains with little cantributians fram surface water
runaff,
DEQ Online is the official web site for the Oregon Department of Environmental Quality.
If you have questions or comments about the web site contact DEQ's.vebmaster.
http://www.deq.state.ar.us/wq/dwpIDWPAreaDelineatiansGW.htm
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