HomeMy WebLinkAboutNassau Pt Devel-Impacts on Groundwater Quantity 1982,DEVELOPIiENT OF NASSAU POINT:
,IM?,ACTS ON GROUNDWATE'R (~UANTIT~
SUFFOLK COUNTY DEFAR'k~.~T OF ,~.,ALTH SERVICES
DAVID HARRIS, M.D., M.P.H.
COMMiSSiONER
H. W. DAVIDS, P.E.
DIRECTOR, DIVISION OF ENVIRONMENTAL HEALTH
PREPAKED BY:
BUREAI~ OF WATER RESOURCES
JOSEPH H. BAIER, P.E., CHIEF
SY F. ROBBINS, HYDROGEOLOGIST
DECEHBER 1982
TABLE OF CONT~N'I'S
I. S,---m ry ............................................................... 1
II. In=ro~ucKiou ......................................................... 2
III. Exis~in~ Freshwater ~eourcee ........................................ 4
IV. Impacts of Dcou~h~ ................................................... 8
V. I~pacts of Full Develops~at .......................................... 8
VI. Impacts o~ D~ou~h~ and Full Develop~en~ ............................. 10
Bibliosrnphy ........................................................ ! 1
Appendix A: Calcula=ious for Region I .............................. A-1
Appendix B: Calculations for Res!on II ............................. B-1
LIST OF F'rGUltES
1. Location Map: Nassau Point ........................................... 3
2. ~egious Used for Groundwater Calculations ............................ 5
3. Freshwater L~nses in Cross Section: Nassau Po£nt ..................... 9
LIST OF TABLES
1. $,~.~*ry of R~sults: Analysis of Nassau'Point .......................
LIST OF ABBREVIATIONS
gpd - gallons per day
gpd/cap - gallons per day per capita
m~d - m/ilion gallons per day
mil gal - million gallons
m.a.1. - m~an sea level
USGS - United States Geological Survey
The residents of Lon~ Island's Nassau Point rely soley on the suall
lenses o6 fresh water below the peninsula for their source of drinking
water. The exiatim~ n,--ber of single family houses (312) is expected to
increase to between t85 and 585 units at full development, thus raising con-
cat-ns about the potent/al depletion of the resource, particu/arly if the
nsTiu,~ number of --its are constructed and drought conditions are exper-
fenced. Such depletion o6 the resource could cause private wells to become
contaminated by salt as the freshwater/saltwater interface that delimits the
freshwater lenses moves upward.
This study utilized an analytical model of .Nassau Point*s groundwater
system to determine the potential impacts of development and drought on the
size and shape of the freshwater lenses below the peninsula. Based on con-
se~-vative, 'vorst case' est'--res, the study found that:
1)
The volume of fresh water normally available below the pen/nsula as a
whole is 609 nillion gallons; under drought conditions, tMs volume
could be reduced by about 30I to 431 million gallons.
2)
The rate of recharge to the lenses under normal rainfall conditions is
0.6 mgd; in comparison, the upper ltntt of ~onsumptive use at full de-
velopment (485-585 dwelling units) is only 0.029-0.035 msd (or
5.81 of recharge), which would reduce the volume o6 available ~acer in
storage within the lenses by only 2.~1-3.0I (15-18 ~illion gallons).
3)
The mo~m,,m upward movement of the in=erface due to consumptive use at
full development would be only 1.3 feet and would occur at the center
of the peninsula, where the lens is normally over 41 feet thick; there
would be no appreciable changes near the shoreline, where the lens is
thinnest and private wells are most vulnerable.
The effects of drought on Lens thickness would overshadow chose caused
by development; ~Tt~m reductions could reach lA feet near the center
of the peninsula, of which only about 2 feet would be due to consump-
tive use.
Thus, the potential impacts of development alone on the overall quanti-
ty of fresh water available below Nassau Point are smell compared to those
of drought, and the difference between ~85 and 585 units ts insiEnificant.
These findings, however, do not imply that adequate water will be available
on every building got, or that future development will not affect water
quantity below isolated spits of land; the problem of groundwater quality is
also not addressed. Further study and planning are required.
I~. Ih'i~.ODU CT[ ON
Nass&u Point (also known as I~ttle ~og Neck) is a
exte~s south~rd f=~ ~n~ ~land~s Horth Fork into
Cutcho~e, To~ of ~u~o~d (Fi~re 1). ~e subsurface
h~g~y pebble ~ds and Erave~s of ~c~ or[~n (pr~r~ly ouC~sh,
~th s~ t~l). Fresh ~cer ~low the pe~a is stored ~thin a lens
~c f~Cs ~bove de. er, s~ gro~d~cer ~ht ~8 ~r~ced the aqua-
far fr~ surro~d~S ~da~ ~Cers. ~e f~esh~ter lens
above by precipitation; a~u~ an eq~ ~c of f~esh water Is d~sc~sed
fr~ c~ ~8~ at the s~re~ne, ~hus ~nta~n~ an overa~ bahnce
lc equ~br~).
The lens of fresh water is the sole source of dr~-~4-S water for the
residents of the peninsula, where all 312 single family houses utilize pri-
vets wells.! There are 173 vacant single f~m~ly residential tax parcels
on the peninsula, bringing the total number of resident/al.tax parcels (both
developed and undeveloped) to '&8~. About 15 of these csx parcsls could be
further subdivided into smaller plots that would conform to the I acre
(40,000 square foot) area requirement of the Town zonin~ code, thus yielding
another 37 building lots.2 In addition, about &8 other tax parcels con-
tain two or ~ore entire subdivision lots from the original "old file" maps
for uhe area; if developed, an additional 63 uuits could be built.3 ..~hus,
the max/mumuumber of single family houses at full development wuuld be 585,
or about 21Z more than the 485 units based on existing residential tax par-
cels alone.'
1. Host of the existing houses are taxed as year-round residences; less
than 10Z are taxed as seasonal residences, although a much larEer per-
centage is probably used seasonally.
2. Hany of the undeveloped parcels are "undersized" according to the Town
zoning code and the Suffolk County health code (see footnote 3); both
codes, however, provide for variances for undersized tax parcels that
are in single-and-separate ownersh/p.
3. The 48 tax parcels (some of which already have houses on them) contain
111 subdivision lots, only 2~ of which neet the Town zonin~ code area
requirement (40,000 square feet) and the Suffolk .County Department of
Health Services* area requirement for private wells (also.&O,O00 square
feet). The "old-file' subdivision maps, however, were exempted from
Town zon/nS requirements; thus, all ~! lots would probably receive
Town building per~L=s.
-2-
NA?SAU POINT
FIG. 1 LOCATIOH MAP: NASSAU POINT
-3-
The purpose of this report is to assess the impacts of residential de-
velopment on the ~uantit~ of Nassau Point's overall groundwater resource..
?otential changes in the size and shape of the ~alor freshwater lenses below
the peninsula caused by the ,,~tt~ate develop~emt of 485-585 single family
houses are calculated, and the overall availability of fresh ground water on
the peninsula is assessed. This report does not, however, attempt to deter-
mine the avai~abLtlty of fresh water under individual but.tding lots, or tbs
~pacts of deve-tope~nt on groundwater quality, which ~ay be significant.
The only quality i~pacts discussed i~ this report are potential increases in
chloride concentrations that ~ay result fro~ the upward movement of the
free.eater/seaLmaster interface that deli~Lts the freshwater lenses.
III. k~ISTINC FRESI~ATER RB.~OURCB$
The shap~ of the fresh.aCer lenses below ~assau Point are influenced by
the numerous tidal coves, ponds, and lagoons that ~rder and cut ~nto
pen~a. ~e freshwater resource, ~here~ore, ~s ~de up o[ n~erous,
re~larly s~d Lenses. ~ order ~o ~impltfy ~he a~lyses ~de in ~his
report, the cwo uJor lenses below the pen~ns~a ~ere represented (appro~-
~ced) by the ~o regions sho~ in Figure 2 -- one elltpCically s~ped (~-
lion I), ad onl recta~larly s~ped (~tion II). ~ coabined area o[ the
t~o reeions is 0.58 sq~re ~les (see Table 1), which is about 13I less than
~he total la~ area o[ ~he pe~ns~a (0.7~ sq~re ~les). ~ereiore,
for ~hese c~ re,ions of, such ~ramecers as ~he rate o~ preclpita~iou
c~rge co ~he aquifer, and the vol~e of available fresh ~a~ar iu storage
~ch~n the aqutitI, represent ~inim~ (couse~aCive) esti~tes for the pen-
Ins~a as a whole. ~e use of chess conservative fibres, however,
'~rs~ case' i~ac~s of deveio~ent co b~ calc~ated.
The volumes of water recharged by precipitation to the Lenses under
Regions I and II are sho~m in Table 1. The conservative estimate of total
recharge under normal (average) rainfall conditions (44 inches/year or 0.01
feet/day) is 0.6 ~Lllton tellons per day (~gd); this estt~te is based on
~he ass~pCiou ~ 50I of =he rainfall is lost co evapocransp[ra=lon, and
50I percolates ~o ~he aq~fer.
The volu~es of ~ater avat_tabls for pu~aping from Regions [ and Il under
natural conditions (normal rainfall, no consumptive usa) are 4?5 and 13&
million gallons, respectively; thus, a- conservative estimate of the total
available raaou£ce under the entire peninsula is 609 ~il gal (Table i).
CaLculations for R~gious I and Il are presented in Append~ce A and B.
-4-
N
I
FIG. 2 REGIONS USED FOR GROUtiDWATER CALCULATIONS
TABLE 1
SUMMARY OF. RESULTS:
ANALYSIS OF NASSAIJ POINT
P~ameter
· e~Ion I
1. A~ea (sq miles)
0.376
0.202
0.392 (1001)
0.196 (50.01)
0.371 (94.6%)
0.367 (93.5I)
0.175 (44.5%)
0.171 (43.51)
0.211 (1001)
0.I05 (50.0%)
0.203 (96.4I)
0.201 (95.41)
0.098 (46.4%)
0.095 (45.2%)
3. Avail, Ri~er (mil gal)~)
a. na~ura.l conditions(~)
b. drought couditiousU)
c., full developm~n~a)
(1)..t.~.,. (485)~)
(2). maximm (585)(o
d. dro~h~ i dtvelop}~
(1). ~ (185)~'~'
(~). ~ (585)~°
475 (100z)
336 (70.7%)
462 (97.31)
460 (96.81)
317 (66.71)
313
134 (100%)
94.7 (70.7%)
132 (98.2%)
131 (97.7%)
91.4 (68.2%)
90.0 (67.2%)
4. Max. Lens Thick. (lc)
a. natural conditionsCt~
b. drousht conditionsU>
c. full develop~u~c~
(1). uininum (485)
(2). ~aximm (585)
d. drou~h~ & develop~~
(1). mi~t~ (485)~'~
(2). ~ (585)~
41.~ (1001)
29.3 (70.7%)
40.3 (97.3%)
40.1 (96.81)
27.6 (66.7%)
27.3 (66.0%)
t8.4 (1001)
13.0 (70.7%)
18.1 (98.2%)
18.0 (97.7%)
12.5 (68.2%)
12.4 (67.2%)
0.578
0.603 (100%)
0.301 (50.0%)
0.574 (95.2%)
0.568 (94.2%)
0.273 (45.3%)
0.266 (44.11)
609 (1001)
431 (70.7%)
594 (97.5%)
591 (97.0%)
408 (67.7%)
403 (66.21)
== ,
.TABLE 1 continued
Parameter Re~ion ~ Region II
Total
5. Conmmptive Uee ~atec~)
a. ~imimum (485 units)
(1). vol. rate (gpd) 2.14 x 104 7.68 x 103
(2). I nat. recharge 5.5~ 3.61
b. max.t-,u~ (585 u~)
(~). voZ. r~e (gpd) 2.5~ x ~0~ 9.66 x ~03
(2). ~ us~. ~ec~ge 6.5~ ~.6~
2.91 x 104
4.8%
3.51 x 104
5.8%
(a). Volume of water recharged to lens under each region (W x Area).
(b). Natural conditions -- periods of normal rainfall (44 inch/year).
(c). Drought conditions -- periods of reduced rainfall (22 inch/year).
(d). Natural reeh=rge m/nus consumptive use (2.a - 5.a or 5.b).
(e). Based on tax parcels (Region I 357, Region II 128, Total 485).
(f). Tax parcels + subdivision lots (Region I 424, Region II 161, Total 585).
(g). Drought recharge minus cousumptive use (2.b - 5.a or 5.b).
(h). Vol. of water available for pumping (lens vol. x specific yield (0.22)).
(i). Periods of normal (average) rainfall and no consumptive use.
(j). Periods of reduced rain, all and no consumptive use.
(k). M1 development -- natural rainfall conditions and consumptive use.
(1). Drought & develop. -- drought rainfall conditions and consumptive use.
(m). Region I at the center axis; Region II at the §rouadwater divide.
(n). Upper limit est. based ou 60 gpd/household; may be over 4 times too high.
-7-
The calculated shapes of cbc freshwater lenses below P~glons
under natural conditions are shown in cross section in Figure 3. The maxi-
m~n thickness of ~ch lens occurs at its center (furthest i~tand);
has a maximu~ thickness of 41.4 fee= at its center axis, while Region II has
a ma=4~,- thickness of 18.4 feet at the groundwater divide (Table
~V. IMPACTS OF DROUth1'
The impacts of drought were determined by reducln$ the recharge rate by
50~ (to 11 inchas/yenr or 0.0025 feet/day). The result was to reduce the
volume of available water in storage below the peninsula and the depth of
the interface by about 30~ (Table 1).5 Under drought conditions, the Co-
cai volume of available water would be reduced by 178 mil gal, to 431 ~11
gal (Table 1). Hex/mum reductions in aquifer thickness would occur at the
center s~ts of Region I (12.1 feet) and along the groundwater divide tn Re-
gion II (5.4 feet). These changes are sho~m graphically in Figure 3, Such
changes in the position of the freshwater/saltwater interface could cause a
significant imcrease in chloride concentrations in private wells uow screen-
ed in the lower portions of =he aqulfer, especially chose wells located fur-
these inland.
V. D/PACTS OF FULL DEVELOPMENT
The imPacts of the cons,~ptlve use of water at full development on ~he
sizes and shapes of the freshwater lenses were determined by reducing the re-
charge rate Co the aquifer (under natural conditions) by the rate at which
water would be lost ~o the system (consumed).6 The 'upper limit' esti-
mates of consumptive 'use at minimum full development (485 units) and maximum
full developomenc (585 un/ts) are 29,100 gpd and 35~100 gpd, respectively
(Table 1). These upper limit values are based on a 60 gpd/household con-
sumptive use rate, which may be more than four times the actual rate for the
Nassau Poin~ peninsula.? Nevertheless~ the use of these upper limit val-
ues, along wi~h ~he conservative estimates of the size and shape of the
freshwater resource~ allowed 'worst case' impacts to be assessed.
5. Lens volume and thi~kness are proportional to the square root of the
recharge rate; (0.50)}-0.707, see Appendices A and B.
6. The rate of consumptive use is equal to the difference between the rate
of groundwater w~.thdrawal (pumping) and the rate at which used water ts.
returned to the aquifer via cesspools or lawns (irrrigation).
7. This assumes a 100 gpd/cap water use rate, a 20~ consumptive use pe~-
centage~ and an average~ yeah-round household size of 3.0 people per
household (see Appendix A, Section A.VI).
-8-
WATER TABLE
NORMAL
INTERFACE
REGION
AXIS
3OO'
4O.0'
I I I I I I
REGIOi~ Tr
WATER TABLE
12.S°
15.0'
2O.0'
I I I
FIG. 3 FRESHWATER LENSES IN CROSS SECTION: NASSAU POINT
-9-
The upper limit consumptive use rates represent 4.8I-5.8~ of the total
natural recharge rate to the aquifer (Table 1). Since the vol,~e of avail-
able water in storage is proportional to the square root of the recharge
rate, reducing the recharge rate by the consumptive use rates would reduce
the total volume of available water by 15-18 million gallons (2.5Z-3.0~).
Aquifer thickness, which is also proportional to the square root of the re-
charge rate, would be similarly affected. The ~s~tmum thickness of Ragion
I, which occurs at its center axis, would be reduced by 1.1-1.3 feet (2.7~-
3.2~)1 the max/mum thickness of Region SI, which occurs along the ground-
water divide, would be reduced by 0.3-0.4 feet (1.8Z-2.3~).
The changes in interface position due to development are shown in cross
section in Figure 3. Since aquifer thickness would be reduced by a percent-
age, the largest changes would occur furthest i-land; reductions would de-
crease toward the shoreline, and would be smallest where the lens is thin-
nest and private wells are most vulnerable. It is unlikely, therefore, that
private wells would be impacted '.by the effects on groundwater quantity of
development alone.
VI. II4PACTS OF DROUGHT AND FULL DEVELOPMENT
The impacts of the consumptive use of water at full development under
drought conditions were determined by reducing the drought recharge rate by
the consumptive use rate. The total volume of available water in storage
would be reduced by 201-206 mil gal (32.3Z-33.8~) when compared to natural
conditions, and by 23-28 mil gal (5.3~-6.5~) when compared to drought condi-
tions alone (Table 1). Max/mum aquifer thicknesses would be reduced by 1.7-
2.0 feet (5.8~-6.8Z) in Region I, and by 0.5-0.6 feet (3.8Z-4.6~) in Region
SI, when compared to drought conditions alone (Table 1 and Figure 3). In
terms of natural aquifer thickness, the largest decrease (14.1 feet) would
occur at the center of Region I, of which 12.1 feet would be due to drought,
and 2.0 feet would be due to development (585 units). Clearly, the impacts
of drought on groundwater quantity would overshadow those of development,
even if the mayimum number of houses is built.
-LO-
BI~Z. IOG~,.PH~
Crandell, H.C. 1963. Geology and Ground-Water Resources of the Town of
Southold, Suffolk County, New York. USGS Water-Supply Paper 1619-GG.
l. Petter, C.W. Jr.
Oceanic Islands.
1972. Position of the Saline Water Interface beneath
Water ~asources Research $(5):1307. Oct. 1972.
3. McClymonds, N.E. and O.L. Franke. 1972. Water-Transmitting Proper=les
of Aquifers on Long Island, New York. USGS Prof. Paper 627-E.
-11-
Region I is elliptical (Figure 3), with dimensions 2900 feet (east-
wast) and 4600 feet (north-south). ~he area of an ellipse is equal to
where a and b are the stag-axes.1 ~he area of Region I is therefore:
~ab = v(1450)(2300) = 1.05 x 107ft2 = 0.376 sq u dj. es
ThEe area is equivalent to the area of a circle with a radius of about 1830
feet.2
A.II VOLUMETRIC P~ECHA~GE PATES: NATURAL AND DP. OUGHT CONDITIONS
~he volume of water recharged daily by precipitation to the aquifer
below Region I is equal to the area multiplied by the recharge rate per un/t
area. For Nassau Point, the esr~-~ted natural recharge rate per unit area
is 50% of the 44 inch/year precipitation rate -- 22 inches/year (0.005
it/day). ~he volt~etric recharge rate is ~h~s:
(1.05 x 107ft2)(0.005 ft/day) - 5.24 x 104ft3/day - 0.392 mgd
Under drought conditions, ~hen the recharge rate per unit area is assumed ~o
be reduced by 50% to 0.0025 fi/day, the volumetric recharge rate would also
be halved to 0.196 m~d.
A.III VOLUME OF AVAIEARLE WATER: NATURAL CONDITIONS
The volume of the freshwater lens below a titular oceanic
given by:
Volume of lens
where W - recharge rate per unit area (ft/day);
K - hydra-lit conductivity of aquifer (It/day); and,
R - radius of island
island-is
1. The area of a circle, which is a special form of au ellipse in which
a = b = r, where r is the radius, is ~r2.
2. See Section l. III.
A-1
The volume of the lens contains both solid aquifer material and water.
The amount of water available for withdrawal is equal to the specific yield
(S) of the aquifer thnes the volume of the lens, thus:
Volume of available water - S 9.483
For Nasnsu Point, Region I, the following input values were used:
recharge rate - 22 inches/year - 0.005 fi/day
hydraulic conductivity3 = 200 fi/day
island radius - 1830 fee:
specific yield - 0.22 (Crandell, 1963)
The volume of fresh water available from the lens under natural conditions
is therefore:
(0.22) 9.483 (0.005/200)}(1830)3 - 6.35 x lOift3 - 475 mil gal
A.IV THICKNESS OF ~ AQUIFER: NATURAL CONDITIONS
The height above mean sea level (m.s.1.) of the water table on a circu-
Lar oceanic island is given by: .._
h(r) ' [~ (R2- r~)]~
L 82 K.
where ~, K, and R are defined as in Section A. IlI, and
h(r) ' height of wa:er table above m.s.1, at position r (feet); and,
r ' distance from center axis of island to position r (feet).
According to the Ghyben/Herzberg theory, :he depth of the freshwater/salt-
water interface below ~.s.1. at location r is 4Oh(r), and the to,al thi~k-
ness of the freshwater lens is 41h(r).
Using the above formula, and the input values listed in Section A.IlI,
the following results were calculated:
3. Upper glacial aquifer, north central Suffolk (~cCly~onds and Franks,
1972, Table 6).
A-2
dist. from dist. from height of depth of thick, of
axis shoreline water table interface aquifer
r R-r h 40h 41h
0 1830 L.O1 40.4 41.4
830 1000 0.90 36.0 36.9
1330 500 0.69 27.8 28.5
1580 250 0.51 20.4 20.9
1780 50 0.23 9.4 9.6
1830 0 0.00 0.0 0.0
These values ware plotted to give the curve shown in Figure 3(a). The cal-
culations show that there is about 10 feet of aquifer thickness 50 feet from
the shoreline, and a maximum thickness of 41.4 feet at the center of the
island, where the water table elevation is at its max/mum (1 foot); these
values are consistent with data from private wells on Nassau Point.
A.V IMPACTS OF DROUGHT CONDITIONS
The impacts of drought conditions on the size and shape of the lens
were assessed by reducing the recharge rate by 50% to 11 ~nches/year (0.0025
ft/day). Since both the volume of available water in storage and the thick-
ness of the lens are proportional to the square root of the recharge
each would be reduced to (50%)} - 70.7% of the natural value, a reduction
of about 29%, thus:
(0.707) (475 mil gal) = 336 mil gal
(0.707) (41.4 feet) - 29.3 feet
A.VI CONSUHPTIVE USE RATES AT FULL DEVELOPMENT
The ultimate number of houses within Region I is estimated co be be-
tween 357 and 424. If the average number of people per household is as-
s,,m-d to be 3.0, and ~he consumptive ~se rate is assumed to be 20 gpd/cap
(20% of 100 gpd/cap water usage), then the per household consumpticn rate
is 60 gpd. The consumptive use rates for ~gion I at full development are
thus:
(357) (60) ~ 2.14 x ~0& gpd
(424) (60) - 2.54 x 104 gpd
A-3
which are 5.5~ aud 6.§Z of the natural recharge race to Region
tively.
T~ese figures represent "upper limit' ssttm-tes of consumptive use.
The average household size used in their com~utatiou (3.0) is somewhat
er than the average for the Cutchogue-New Suffolk area (2.6), according to
the 1980 Census. Many of the houses on the penins,,]- ara seasonal, and
would not draw on the resource year-round. The per capita usage race (100
spd) is a standard sever design criteria that asa,~es mixed land uae (com-
mercial, industrial, residential) and is probably too large for an area like
Nassau Point :hat has ouly single-family residential development; the Suf-
folk County Water Authority est/mates that the average usage rate in such
resi~eutial e£eaa of Suffolk County is about 60 gpd/cap. In addition, the
20I consumptive uae percentage is also a general criteria and is probably
too large for an area with sull residential lawns, where watering require-
ments and resultant evapotranspiration losses are small; a 102 figure is
probably more accurate. Using the lower household size and per capita con-
sumption rates, the per household consumption rate would be 15.6 gpd, or
only 26Z of the upper limit value, and would be even smaller if seasonal
occupation of some of the houses is taken into account.
In subsequent calculations, the upper limit estimates of consumptive
uae rates at full development are used to evaluate worst case impacts.._ It
should be kept in mind that these values may be over four times as much as
the actual ultimate consumptive use rates.
A.VII IMPACTS OF CONSUMPTIV.E USE AT FULL DEyELOPMENT: NATUBAL CONDITIONS
Since private wells are fairly evenly distributed over the area of Re-
gion l, the effects of consumptive use at full development on the size and
shape of the freshwater lens could be determined by reducing the recharge
rata (under natural conditions) by the rata of consumptive use. For minimum
full development (357 units) this gives:
(3.92 x 105gpd) - (2.14 x 104gpd) - 3.71 x 105gpd - 0.371 mgd
This rate is equal to 94.62 of the natural recharge rate, i.e., is 5.&Z less
than the natural recharge rate. Since both the volume of available water in
See Sections A.II and A.VI. Dividing the volume=tic recharge rate by'
the area of Regiou I gives the recharge rate per unit area (W).
storage and the max/mum thickness of the lens are proportional to the square
root of the recharge rate, both would be reduced by 2.7~, thus:5
(0.973) (475 mil gal) ' 462 mil gal
(0.973) (41.4 feet) - 40.3 feet
For maximum full development (424 units), the reduced recharge rate is:
(3.92 x 105gpd) - (2.54 x 104gpd) - 3.67 x 105Tpd - 0.367 mgd
This rate is equal to 93~62 of the natural recharge rate, i.e., is 6.4~ less
than the natural recharge rate. The volume of available water in storage
and the .~.4,~,m lens thickness, therefore, would be reduced by 3.22 to:6
(0.968) (475 mil gal) - 460 mil gal
(0.968) (41.4 feet) - 40.1 feet
A.VIII COMBINED IMPACTS OF DROUG~tT AND FULL DEVELOPMENT
The combined effects of drought conditions and consumptive use at full
development on the size and shape of the freshwater lens were determined by
reducing the recharge rate (under drought conditions) by the rate of. con-
sumptive use.? For minimum full development (357 units), the reduced re-
charge rate is:
(1.96 x 105gpd) - (2.14 x 104gpd) - 1.75 x lOSgpd = 0.175 mgd
This rate is equal to 44.5% of the natural recharge rate, i.e., is 55.5%
less than the natural rate. Since both the volume of available ~a~er in
storage and the m--~mum thickness of the freshwater lens are proportional to
the square root of the recharge rate, both would be reduced 33.32, thus:8
(0.667) (475 mil gal) - 311 mil gal
(0.667) (41.4 feet) = 27.6 feet
5. (0.946)* - 0.973
6. (0.936)} - 0.968
7. See Sections A.II and A.VI.
8. (0.445)} - 0.667
A-5
For max~num fu~l development (424 units), :he reduced recharge rate £s:
(1.96 x 105gpd) - (2.5~ x lO~spd) - 1.~1 x lO~spd - 0.1~I
~is rate ~s eq~l to 43.5% of the ~tural rec~rge rate, i.e., is
~ess ~hau the ~ur~ ra~e. ~e vol~e of available ~te~ in storage and
the ~-~,~ [e~ thic~ess, therefore, ~d be reduced by 3A.O~, thus:9
(0.660) (475 mil gal) - 313 ~ ~a~
(0.660) (41.4 fe~) - 27.3 feet
9. (0.435)-4 = 0.660
A-6
APPENDIX B
CALCULATIONS FOR REGION
B.I AREA
P~gion II is rectangular (F~-~ure 3),
~ith dimensions 1150 feet (east-
west) and 4900 feet (north-south). Its area, therefore, is equal to:
(1150)(4900) = 5.64 x 106ft2 - 0.202 sq miles
B.I1 VOLUMETRIC RECtAl. GE RATES: NATUI{AL AND DROUGHT CONDITIONS
The vol,~e of water recharged daily by precipitation to the aquifer
below P~giou II is equal to the area multiplied by the recharge rate per
unit area. For Nassau Point, the estimated natural recharge rate per unit
area is 50% of the 44 inch/year preci~tation rate -- 22 inches/year (0.005
f=/day). The volumetric recharge race is thus:
(5.64 x 106ft2)(0.'005 ft/day) - 2.82 x 104ft3/day - 0.211 mgd
Under drought conditions, when the recharge rate per unit area is assumed to
be reduced by 50% to 0.0025 f~/day, the volumetric recharge rate would also
be halved to 0.105 m~d.
B.III VOLUI~ OF AVAILABLE WATER: NATURAL CONDITIONS
The volume of the freshwater lens below a rectangular oceanic island is
given by:
Volume of lens - 10.06 (W/K)} h a2
where W - recharge rate per unit area (ft/day);
K - hydraulic conductivity of aquifer (fi/day);
L - length of island (feet); and,
a - distance from groundwater divide to shoreline (feet).
The volt~e of the lens contains both solid aquifer material and water.
The amount of water available for withdrawal is equal to the specific yield
(S) of the aquifer.times the volume of the lens, thus:
Volume of available water - S 10.06 (W/K)} L a2
B-1
For Nassau Point, Region II, the following input values were used:
recharge rate - 22 inches/year ' 0.005 it/day
hydraulic conductivityI - 200 it/day
island length - t900 feet
island half-width - 575 feet
specific yield - 0.22 (Crandell, 1963)
The volume of fresh ~ater available from the lens under natural conditions
is therefore:
(0.22) 10.06 (0.005/200)~(&900)(575)2 = 1.79 x IO7ft3 = 13& mil gal
B.IV THICKNESS OF THE AQUIFER: NATURAL CONDITIONS
The height above mean sea level (m.s.1.) of the water table on a rec-
tangular oceanic island is given by:
L41K
where W, K, and a are defined as in Sectiou B.III, and ...
h(x) ' height of water table above m.s.1, at position x (feet); and,
x - distance from groundwater divide (center line) to x (feet).
According to the Ghyben/Herzberg theory, the depth of the freshwater/salt-
water interface below m.s.1, at location x is 40h(x), and the total thick-
ness of the freshwater lens is 41h(x).
Using the above formula, and the input values listed in Section B.III,
the following results were calculated:
dist. from dist. from height of depth of thick, of
divide shoreline water table interface aquifer
x a-x h 40h 4lb
0 575 0.45 18.0 18.4
275 300 0.39 15.6 16.0
375 200 0.34 13.6 13.9
475 100 0.25 10.0 10.2
525 50 0.18 7.2 7.4
575 .0 0.00 0.0 0.0
B-2
These values were plotted to give the curve showa in Fisurs 3(b). The cal-
culations show that there is about 10 feet of aquifer thickness 100 feet
from the shoreline, and a max/mum thickness of 18.4 feet at the center line
of the island, where the ~ater table elevation is at its max/mum (0.45
feet); these values are consistent with data from private walls on Nassau
Point.
B.V L~PACTS OF DP. OUGHT CONDITIONS
The hapacts of drought conditions on the size and shape of the lens
were assessed by reducin~ the recharge rate by 501 to 11 inches/year (0.0025
ft/day). Since both the volume of available water in storage and the thick-
ness of the lens are proportional to the square root of the recharge rate,
each would be reduced to (501)} = 70.7~ of the natural value, a reduction
of about 29I, thus:
(0.707) (134 mil gal) = 94.7 mil gal
(0.707) (18.4 feet) - 13.0 feet
B.VI CONSUI~PTIVE USE RATES AT' FULL DEVELOPMENT
The u.l. tinmte uumber of houses within Region II is est/mated to .be be-
tween 128 and 161. If the average number of people per household is as-
sumed to be 3.0, and the consumptive use rate is assu~ed to be 20 gpd/cap
(i.e., 20l of 100 gpd water usage), then the per household consumption rate
is 60 gpd. The consumptive use rates for Region II at full development are
thus:
(125) (60) - 7.68 x 103 gpd
(161) (60) - 9.66 x 103 gpd
wh/ch are 3.6I and 4.61 of the natural recharge rate to Region II, respec-
tively. A~ain, these values are upper limit set,m-tee, and may be over four
t/mss the ac:ual ultimate consumptive use rates (see Section A.VI).
1. Upper glacial aquifer, north central Suffolk (McClymonds and Franke,
1972, Table 6).
B-3
B.VII II, PACTS OF CONSII~P~¥~. USE &T FULL DEVELOPNEI~: NATURAL CONDITIONS
Since private wells are fairly evenly distributed over the area of Re-
gion Il, the effects of constnptive use ac f~LL1 development on the size and
shape of the freshwater lens coy.rd be de~e~ned by reduc~n~ ~he ~ec~e
ra~e ~under ~Curai condi~io~) by ~he ra~e of coua~p~lve use. Yo~ ~n~
f~ deve~o~en~ (128 ~i~s) C~s gives:
(2.11 x 105gpd) - (7.68 x 103gpd) - 2.03 x 105gpd ~ 0.203 mgd
This rate is equal to 96.4% of the natural recharge rate, i.e., is 3.6% less
than the natural recharge rate. Since both the volume of available water in
storage and the ~aximum thickness of the lens are proportions1 to the square
root of the recharge rate, both would be reduced by 1.81, thus:2
(0.982) (134 mt1 gal) - 132 mil gal
(0.982) (18.4 feet) - 18.1 feet
For ~aximum full development (161 units), the reduced recharge rate is:
(2.11 x 105gpd) - (9.66 x 103gpd) - 2.01 x 105gpd - 0.201 mgd
This race is equal to 95.4% of the natural recharge rate, £.e., is 4.6%..less
than the natural recharge rate. The volume of available water in storage
and the aaxlmum lens thickness, therefore, would be reduced by 2.3% to:3
(0.977) (134 mil gal) = 131 mil gal
(0.977) (18.4 feet) = 18.0 feet
B.VIII COHBINED LMPACTS OF DROUGHT AND FULL DETELOPI~ENT
The combined effects of drought conditions and consumptive use at f~ll
development on the size and shape of the freshwater lens were determined by
reducing the recharge rate (under drought conditions) by the rate of con-
sumptive use. For minimum full development (128 units), the reduced re-
charge rate is:
(1.05 x 105gpd) - (7.68 x 103gpd) - 9.78 x 104gpd - 0.098 mgd
2. (0.964)~ - 0.982
3. (0.954)} - 0.977
4. See Sections B.II.and B.VI..
8-4
,/
This rate is equal to 46.4Z of the natural recharge rate, i.e., is 53.6~
less than the natural ra~e. Since both the volume of available
storage and the ~ =h~c~ess of =he freshwater lena are propor=~o~l
the sq~re root of =he recharEe rate, both ~d be reduced 31.8~, thus:~
(0.~82) (13~ ~1 gal) - 91.4 ~1 scl (0.682) (18.~ ~ee=) ' 12.5
For ~-~ f~l develo~en~ (161 ~i=s), =he reduced rec~rSe race is:
(~.05 x.lOSgpd) - (9.66 x 103Spd) - 9.53 x 104~pd - 0.095
~is ra:e is eq~l :o 45.2~ of the ~tural recharge rate, i.e.,
less :h~ the ~tural ra:e. ~e vol~e of available water in s:orage and
:he ~ lens :hic~ess, :herefore, ~uld be reduced by 32.8~,
(0.672) (13~ mil gal) - 90.0 m~l gal (0.672) (18.4 fee:) - 12.4 fee:
5. (0.464)} - 0.682
6. (0.4.52)} - 0.672