DET-NING AHD MAPPING MINOS TOPOGRAPBY by C . K. Wentworth
I n t h e absence of surveying instruments and without precise methods, u s e f u l mapping of s m a l l d e t a i l s can be achieved and made more u s e f u l by a t t e n t i o n t o
c e r t a i n p r i n c i p l e s and p r a c t i c e s . I f a .kmd lefol. (with s i g h t i n g mirror) i s a v a i l a b l e it should be checked against t h e ocean horizon from t h e beach f o r gross e r r o r s . The observer's height t o t h e eyes should be measured, and t h i s value used. I f horizontal d i s t a n c e s per shot a r e not over 50 f e e t , successive hand l e v e l l i n g w i l l give f a i r l y accurate d i f f e r e n c e s i f care i s used t o mark o r i d e n t i f y successive p i n t s . Even without a );and I.ovel,fair estimates can be made i n places very near sm 1 m l i . f t h e ocean horizon can be seen and projected.
Wen climbing a f a i r l y s t e e p b l u f f , d i f f e r e n c e s of l e v e l can be measured within 2 o r 3 f e e t i n a hundred witin a hand l e v e l , and f o r more gentle slopes and lower f e a t u r e s r e s u l t s within 1 5 o r 1 0 p e r cent by hand l e v e l w i l l have g r e a t e r v a l i d i t y t h a n an off-hmd guess, and should be s o described in notes.
Level l i n e s of more than
50
f e e t per shot can be run by hand l e v e l but w i l l be open t o question unless checkea and unless t h e observer has unusual s k i l l and describes t h erec cautions
taken. T.wo short pieces of g l a s s tube with 25 o r50
f e e t of rubber tube wil; make a more r e l i a b l e l e v e l l i n g apparatus f o r p r o j e c t i n g long m o t s ( s e e t h a t a i r bubbles a r e eliminated).
A s i g h t h g compass such a s t h e Brunton, commonly used by geologists and which includes a clinometer, i s useful i n many ways and can provide a p a r t of t h e c o n t r o l needed in making a f a i r l y r e l i a b l e sketch map. Nearly t h e same
r e s u l t s can be obtained with any compass having s i g h t i n g points o r a square box and with a two- o r three-inch needle t h a t swings f r e e l y . Distances can be paced on smooth ground with f a i r consistency i f t h e observer makes a sustained e f f o r t t o c a l i b r a t e h i s pace. For somewhat more accurate y e t rough-and-ready quadrats, o r t r a v e r s e s , a cord o r rope with 10-foot knots o r paint marks i s convenient and o f t e n f a s t e r than a metal. tape, even i f t h e l a t t e r be available. C a l i b r a t e t h e markings occasionally i f t h e cord g e t s wet.
I n using t h e compass, whether using t h e m i r r o r a t eye l e v e l , o r viewing t h e needle from above, keep t h e box l e v e l and t h e needle f r e e . Check t h e readings with yourself o r o t h e r observers; form your own 2udgment a s t o whether your readings a r e r e l i a b l e t o 1 degree, o r a r e i n doubt up t o
4
o r5
degrees. The l a t t e r may need some remedy i n instrument o r procedure. I f t h e Brunton compass i s used f o r d i p or slope measurements, it should be so held t h a t t h e axis of t h e pivot i s normal t o t h e d i r e c t i o n of steepest slope and t h e s i g h t i n g l i n e should be e i t h e r d i r e c t l y up o r down t h a t slope, o r j u s t a t right-angles t o it.'egardless of what instruments o r devices a r e used, t h e accuracy and u t i l i t y of a map i s based on use of lengths and d i r e c t i o n s of l i n e s and of
angles and t r i a n g l e s arranged t o make t h e map a firm, small-scale r e p l i c a of t h e f e a t u r e s on t h e ground. The l a r g e r t h e map, t h e more it needs reference t o a s i n g l e long l i n e , t o one o r more strong t r i a n g l e s , o r t o a well-defined g r i d of some s o r t . Any of t h e s e i s preferable t o a non-planned l o c a t i n g of minor
f e a t u r e s successively t o each other. Any such sketch maps o r sections should be made c l e a r a s notes by giving scale, compass d i r e c t i o n s and a legend o r code of
symbols; even t h e observer h m s e l f may f o r g e t .
by C. K. Wentworth
Lacking special equipinent o r t h e s e r v i c e s of a s p e c i a l i s t , t h e range of coarseness of sedimentary accumulations can be described f o r t h e record by systematic use of simple devices. The following s c a l e of grade terms conforms t o most generdl usage;
.ze i n inches
---
1.8 2.5 1/12 1/25 l/5C 1/1OO 1/200
l/4Oo
1/6400
Size in mm. Name of m a t e r i a l gravel f irie grave11 very coarse: sand
wwrse sand mrdj.m s a d
Pine sand vezy fine sand
si1.t clay
~f g r a i n s 'mulders
cobbles pehbles grains
11
For extensive m r k s i e v e s a r e necessary; it w i l l , however, be very useful i f casual observations a r e made s ~ e c i f i c and consistent with t h e above t e r n s . For l a r g e r s i z e s , one can s e l e c t and. l a y out samples showing approximately t h e above l i r r i t s f o r more ready v i s u a l i z a t i o n i n making d e t a i l e d notes. I n t e r - mediate and smaller s i z e s can be l a i d on a g r i d background of q u a d r i l l e paper f o r rough estimation megascopically o r under a magnifier. Any essay of t h i s s o r t i s g r e a t l y superior t o snap guesses which tend t o become stereotyped without v a l i d b a s i s . Proportions a r e b e s t s t a t e d by weights o r volumes reduced t o percentages (even when only estimated).
Chapter 4
--
XydrologykIIG &iD TECHNIQUES FOR. GROUND;-WATER 'IN'&?STIGATION
. .
- by Dbak C . ' C O X .. . 2 r i t i c a l Factors:
. .
hi he
aim of a g r o A d ~ t e r i n ~ e s t i g a & n on t h e i s l a n d s o f an a t o l l i s t o i n d i c a t e t h e q u a n t i t y and q u a l i t y o f water available, and t h e v a r i a t i o n i n both i n s p a c e . and. the, a s a b a s i s f o r determining t h e e f f e c t i v e n e s s of t h e ground water a s an ecological control. Generallr, t h e f r e s h ground water w i l l be found t o r e s t on sea water. On krno t h e permeability of t h e r e e f platform was so high t h a t t h e f r e s h water body probably behaved according t o t h e Ghyben- Herzberg p r i n c i p l e t h a t t h e r a t i o of t h e depth of f r e s h water below s e a l e v e l t o . t h e head, o r height of t h e water-table above s e a l e v e l , i s t h e difference between t h e s p e c i f i c g r a v i t i e s of t h e f r e s h and s a l t water. Ascertaining t h e g e n e r a l i t y of t h i s p r i n c i p l e should be one of t h e p u q o s e s of ground-water i n v e s t i g a t i o n . The rreari head is t h e most convenient index, though an imperfect one, t o t h e q u a n t i t y of ground-water. Chlorinity, hardness and density a r e convenientindice's t o t h e q u a l i t y . it i s t h e job o f . t h e h y d r o l o g i s t ~ t o r e l a t e t h e magnitudes and v a r i a b i l i t y of t h e s e indices t o t h e magnitude and v a r i a t i o n of t h e causative f a c t o r s . On Arno t h e r e l a t i o n s were worked out withconsiderab3e i n t e n s i t y but almost e n t i r e l y f o r one i s l a n d only. A more e x t e n s i v e t r e a t m e n t seems:desirable now, but ' w i l l be a t t a i n e d with d i f f i c u l t y so f a r as t h e head is. concerned because t h e head can be u s e f u l l y approximated only by r a t h e r intensive i n v e s t i g a t i o n . S a l i n i t y i s ' e a s , i i y investigated extensively,. however,. and on a d r y 'island w i l l have r.ore i n t e r e s t i n g v a r i a b i l i t y than on Arno. It i s t h e s a l i n i t y , moreover, t h a t i s most important a s an ecological f a c t o r . Xore enphasis on t h e s a l i n i t y than was made on Arno thus seems j u s t i f i a b l e , . . . .
Measwine: Points:
Ground-water may be measwed and sampled a t any,.lana surface depression i n t h e land surface t h a t penetrates t h e water tabLe; i n n a t u r a l ponds, o r i n a r t i - f i c i a l r e t t i n g p i t s , t a r o p i t s , o r wells. On Arno a r t i f i c i a l ' p i t s were conpara- t i v e l y p l e n t i f u l but on d r i e r i s l a n d s where t h e ground-water w i l l be of poorer q u a l i t y t h e y w i l l probably be s c a r c e r . hhe@e t h e l a n d surface i s l e s s t h a n 3 o r
4
f e e t above sea l e v e l , a s it i s i n much of t h e i n t e r i o r p a r t of a t o l l i s l e t s , it i s simple and quick t o d i g p i t s t o t h e water-table. Such p i t s should be s m a l l , a foot o r s o i n diameter, t o avoid ca-=city e f f e c t s , and it would be. b.est t o excavate such s m a l l p i t s on t h e s i d e Of t h e l a r g e excavations aLso, r a t h e r than using t n e 'largesits
themselves f o r water-level readings. m e r e t h e groundsurface i s higher it w i l l be more expeditious t o d r i v e o r d r i l l small diameter wells. Cheap d r i v e pipe and well p o i n t s w i l l penetrate sand but w i l l break at
j o i n t s when d r i v e n i n coral. limestone o r beach rock. Drive pipe with s p e c i a l j o i n t s might be s a t i s f a c t o r y , b u t simple w e l l s can be driven t o hard rock by excavating with a s o i l auger i n s i d e a 1$" pipe, using h a d d r i l l i n g vdth a rock b i t and a s m a l l sledge i n s i d e t h e pipe f o r breaking up boulders. The c o r a l con- glomerate and harder rock w i l l not r e q u i r e casing so t h a t when t h e p i p e . . i s driven through t h e sand l a y e r t h e hole can be extended a s needed with t h e hand d r i l l and s o i l aug& alone. cgater l e v e l s f n such holes may be measured by lowering a s t i c k o r t a p e a measured distance i n t o t h e hole and noting t h e l e v e l t o which it
i s wetted. Sanples can be withdrawn by suction through a rubber tube o r by lowering an elongate cup.
-24-
- -
Measurements :
-.--
The head i s d i f f i c u l t t o o b t a i n with s u f f i c i e n t accuracy because of low water-table g r a d i e n t s and because of v a r i a t i o n i n sea l e v e l and well water l e v e l . Useable estimates of mean head r e q u i r e averaging of t i d a l . v a r i a t i o n and accurate l e v e l i n g by t e l e s c o p i c l e v e l , t r a n s i t , o r alidade. Srunton o r hand l e v e l i n g i s not good enough. Leveling could be probably done s a t i s f a c t o r i l y with a long (LOO ft.?) rubber t u b e with g l a s s tube ends, f i l l e d with water (and no bubbles) and a pocket t a p e o r r u l e o r two. Determination o f t i d a l v a r i a t i o n i n a well i s e a s i e s t done by a recording gage, but can be done very simply and well by
observing f r e q u e n t l y over at l e a s t a 24-hour period. Determination of t i d a l f l u c t u a t i o n i n ocean i s e a s i e s t and b e s t done by recording gage of Coast &
Geodetic Survey portable type. The use of t h e gage is described i n a Hanual of Tide Observations, U. S. Coast & Geodetic Survey Spec. Pub. 196, 19W. This gage f i t s a &-in. standpipe which must be at l e a s t 6 f e e t longer than expectable t i d a l f l u c t u a t i o n . P l a s t i c pipe, very iight-weight, now a v a i l a b l e , would prob- a b l y meet t h i s need admirably. Nithout a recording gage frequent observations over at l e a s t a 24-hour period w i l l s u f f i c e , but t h e water l e v e l s observed must have wind-waves damped out. Observat,ions can be made i n a p a r t l y submerged, v e r t i c a l 2" p l a s t i c t u b e plugged a t t h e bottom with a rubber stopper containing o r i f i c e s fmm 1/8" t o 1/21! de2ending on r a t i o of wave height t o t i d e range. O r observations can be made by mercury manometer of t h e suction i n a gallon jug p a r t l y f i l l e d with water and connected with t h e ocean by rubber tube f i l l e d with water. Suction recorded i n height of mercury can be reduced t o height o f water l e v e l i n jug above temporary s e a l e v e l . The rubber t u b e furnishes t h e damping.
(Wentworth: Wash. kcad. Sci. Jour., vol. 26, p. 3L7, 1936.) Piean head a s
measured above a s i n g l e day's ocean t i d e l e v e l w i l l be subject t o a considerable e r r o r owing t o long period t i d e s , only p a r t l y compensated by adjustment of
ground-water l e v e l .
S a l i n i t y measurement i s t r e a t e d i n a separate section.
I n space, t h e mean head w i l l increase with t h e width of t h e i s l a n d , with t h e distance f r o m t h e shores, with t h e permeability, and with t h e r a i n f d l i . The estimation of t h e mean r a i n f a l l i s t r e a t e d i n a s e p a r a t e section. It w i l l be n e a r l y constant over an a t o l l . Measurements of head should be combined with observations of t h e s i z e of t h e i s l a n d s , with t h e p o s i t i o n s of t h e measurements on t h e i s l a n d s , and with whatever observations a r e possible on t h e distri'oution of rocks of varying permeabilities in t h e i s l a n d , t o permit a n a l y s i s of t h e e f f e c t s of t h e s e independent v a r i a b l e s on t h e head. It should be noted t h a t t h e head measured at any time o r even over t h e longest period of observation
~ e r m i t t e d on an expedition w i l l not be t h e mean head. Estimation of t h e mean head w i l l depend on knowledge of t h e v a r i a b i l i t y of t h e head and correction f o r it. The v a r i a b i l i t y i n head with time i s caused by v a r i a t i o n of recharge of t h e ground water body caused by v a r i a t i o n i n r a i n f a l l , v a r i a t i o n i n t h e r a t e of with drawal (of minor importance on a t o l l i s l a n d s ) , v a r i a t i o n i n barometric pressure
( r e s u l t s n e g l i g i b l e on a t o l l i s l a n d s ) , and v a r i a t i o n i n t h e r a t e of discharge caused by v a r i a t i o n i n s e a l e v e l . R a i n f a l l measurement i s t r e a t e d i n t h e s e c t i o n on gathering weather data. R a i n f a l l c o r r e l a t i o n with head has not y e t been done f o r t h e Arno d a t a , s o no t e s t e d r u l e s of procedure can be l a i d down.
The c o r r e l a t i o n qade must include recognition t h a t t h e r a i n f a l l of a g i v e n i n t e r v a l of time has an e f f e c t on t h e head not only during t h a t o r t h e f i r s t succeeding i n t e r v a l , but a l s o f o r a number o f ' i n t e r v a l s , t h e e f f e c t presumably dying out i n according t o an exponential decay Saw. . . .
. , .
Before analyzing t h e raj-rifall e f i k c t s on heads, it w i l l generally be
e a s i e s t t o correct t h e observed heads f o r t i d a l e f f e c t s through a t i d a i ' &alysis o r a t l e a s t t o obtain a mean f o r a t l e a s t ' a day. Hethods of analyzing t h e t i d a l f l u c t u a t i o n s a r e given in a separate section. The parameters obtained from t h e t i d a l analys4s will again be found t o vary with t h e s i z e of t h e i s l a n d , t h e dis-lance from t h e shores, and t h e permeability.
It should be pointed out here t h a t t h e dainping of t i d a l f l u c t u a t i o n s i s a function of t h e i r period. Fluctuations w i t h periods of two weeks o r more pass across ground-water bodies i n small i s l a n d s almost undamped, and a s they
comnonly have amplitudes of a t l e a s t s e v e r a l t e n t h s of a f o o t , heads not c o r ~ e c t e d fo? them may d i f f e r very m a t e r i a l l y from t r u e mean heads. Without f a i r l y i n t e n s i v e t i d a l a n a l y s i s t h e b e s t head measurements a r e subJect t o e r r o r s l a r g e i n proportion t o t h e heads themselves.
Analysts of S a l i n i t y :
Tne s a l t s i n t h e ground-water a r e derived ( 1 ) from s o l u t i o n of s a l t
c r y s t a l s i n t h e a i r o r a t t h e . ground s u r f a c e . by t h e r a i n : ( 2 ) from t h e s o l u t i o n of minerals i n t h e ground by t h e ground-water, and ( 3 ) from t h e mixing of t h e f r e s h ground-water with t h e sea-water i n which it i s i n contact a t t h e bottom edge of t h e f r e s h water body.
The s a l t s i n -the a i r and on t h e ground surface a r e derived from t h e evaporation of t h e sea-water spray. They 'should, t h e r e f o r e , show t h e same balance of composition a s t h e sea-water. The chloride content of t h e water c o l l e c t e d i n n a t u r a l o r a r t i f i c i a l rain-catchment s t r u c t u r e s may be used a s an index of t h e t o t a l s a l t s so dissolved,. Dens'ity may be s i m i l a r l y used but with l e s s precision, p a r t i c u l a r l y i n t h e low s a l i n i t y range., The s a l t s derived from n i t u r e of s a l t water with t h e f r e s h water i n t h e . r o c k s ihbuld again show t h e
same balance. Tte l i f f e r e n c e between t h e chloride content of t h e ground-water and t h a t of t h e usual rain-catch water w i l l provide a good index t o t h e degree of t h i s mixture. The degree of mixture w i l l depend on t h e thickness of t h e iround-water body and on t h e amount of f l u c t u a t i o n o f , t h e head, .and t h e r e f o r e , l i k e t h e heaa. and i t s f l u c t u a t i o n , on. t h e amount of r a i d a l l -and i t s v a r i a b i l i t y , and t h e s i z e of t h e i s l a n d , t h e d i s t a n c e of t h e point o f observation from t h e c o a s t s , and t h e permeability. Thus, t h e s a l i n i t y alone w i & i i n d i c a t e t h e same s e t of conditions a s th e head, and t h e s a l i n i t y i s much e a s i e r t o measure. On i r n o t h e r a i n f a l l was so g r e a t t h k except on verynarrow i s l a n d s o r w i t h i p a
few hundred f e e t of t h e shore t h e chloride contents were so s m a l l t h a t d i f f e r - ences from t h e chloride conten.; i n r a i n water were n o t s i g n i f i c a n t . On d r i e r ' a t o l l s , l a r g e r i s l a n d s should show i n t e r e s t i n g v a r i a t i o n s i n chloride content t o t h e i r c e n t e r s . The technique f o r determining the ch1o:rj.de 'content of water i s described i n a separate s e c t i o n . "
The degree of s o l u t i o n of t h e rocks i s a l s o of very great i n t e r e s t from t h e standpoint of i t s importance in physiogaphic processss. The rocks of t h e
a t o l l s a r e almost e n t i r e l y limestone o r perhaps limestone and dolomite. The hardness of t h e water, i t s content of calcium and magnesium ions, i s an index t o t h e importance of t h i s solution, but t h e hardness a t t r i b u t a b l e t o sea water admi&ure must be subtracted. It seems a l o g i c a l assumption t h a t t h e hardness a t t r i b u t a b l e t o sea water admixture should be proportional t o t h e chloride content. The r a t i o of hardness t o chloride content should, therefore, be determined f o r t h e s e a water and f o r t h e ground water.
The hardness a t t r i b u t a b l e t o rock s o l u t i o n is t h e r e f o r e
Where t h e 3 ' s a r e hardness concentrations C l ' s a r e chloride concentrations and t h e s u b s c r i p t s
S i n d i c a t e s t h e concentration r e s u l t i n g from rock s o l u t i o n G i n d i c a t e s t o t a l concentration i : ~ t h e ground-water
0 i n d i c a t e s concentration i n ocean water HO/C10 w i l l be about 1/4 o r 1/3
A simple and accurate method of hardness determination i s given i n t h e s e c t i o n on s a l i n i t y determination, ~ i h i c h a l s o includes a discussion of calcium hardness determination ( a n a l y s i s of calcium i o n concentration alone) which
should be i n t e r e s t i n g i n studying t h e r e l a t i v e amounts of dolomite and limestone a v a i l a b l e f o r s o l u t i o n .
Ecologic Controls:
The study of a t o l l ground water i s i n t e r e s t i n g not only on i t s own account but because it provides c e r t a i n ecologic c o n t r o l s on vegetation and man. Taro c u l t u r e i s almost c e r t a i n l y controiled by t h e d i s t r i b u t i o n of, f r e s h groundwater.
Breadfruit i s apparently so controlled on Rrno. Influence of s a l i n i t y on o t h e r economic p l a n t s may be found. Mangroves and some o t h e r p l a n t s require ground- water i n a very brackish range. The extension of recognition of s a l i n i t y con- t r o l s , and t h e i n v e s t i g a t i o n of t h e i r e f f e c t on t h e p t t e r n of c u l t u r a l adjust- ment of man t o t h e various i s l a n d s and on individual i s l a n d s , c o n s t i t u t e t h e chief reasons f o r s u p ~ o r t of ground-water work on P a c i f i c Science Board projects.
TEGHNIQUB FOR SALINITY DETWMINATION by Doak C. Cox
Chlorides:
as discussed i n t h e s e c t i o n on ground-water i n v e s t i g a t i o n , t h e s a l i n e c o ~ s t i t u e n t of chief i n t e r e s t i s t h e cMoride ion. The determination of chloride ion concentration aepends on t i t r a t i o n with s i l v e r n i t r a t e s o l u t i o n using potassium chromate a s an i n d i c a t o r . Chloride concentrations encountered on an a t o l l may range from l e s s than 1 0 p a r t s per m i l l i o n t o n e a r l y 20,000 m r t s per ;nillion. I n analyzing t h i s very l a r g e range it w i l l be helpfui t o have two concentrations of s i l v e r n i t r a t e . The following reagents and equip- me& w i l l permit a n a l y s i s of a t Least 100 samples i n high and 100 sanples i n low C l - range:
1 p i n t AgX03 soin. ( I ml. = 1 mg. ~ 1 - ) (4.791 &.per l i t e r soln.) i n dark b o t t l e .
1 p i n t kgN03 s o h . (1 m l .
=
1 0 ng. ~ 1 - ) (47.91 g.per l i t e r s o l n . ) i n dark b o t t l e .2 oz. K2Cr04 s o h . ; p a r t ' i n 1 oz. dropper b o t t l e . 2 p i p e t t e s ,
5
m i . cap., graduated t o 1/10 ml.2 casseroles, porcelain, 100 ml. cap.
2 gla.ss s t i r r i n g rods.
T'ne reagents can be made up i n any chemical l a b o r a t o r y , and t h e equipment i s a v a i l a b l e at any chemical sul:ply house. Keep AgN03 s o h . i n dark t o i n h i b i t d e t e r i o r a t i o n . The p o c e d w e i s as foilows:
1. Determine racge of C1- content by hydro~neter {see discussion l a t e r ) o r by t a s t e , or b y t e s t i n g f i r s t a s i f high range.
Range Taste Use kgB03 soln.
0-200 None 1 m l .
=
1 mg. C1'200-2,003 &?i&t o r brackish 1 rd .
=
1 0 mg. C1-2000-20,COO salty d i l u t e sample ( s e e s t e p 2) and use AgNO3 1 nil.
=
1 0 mg. C1- 2. Kessure 25 ml. of sample L? graduated cylinder and t r a n s f e r t o casserole. To avoid excessive u s e of AgN03 s o h . d i l u t e samples containing more t h a n 2,000 ppm. C:- a s follows: Heasuse 2.5 m l . sample with p i p e t t e(do n o t use t h i s p i p e t t e f o r AgiU03 solns.). Transfer t o graduate. Dilute t o 22 ml. with d i s t i l l e d o r r a i n water. Use t h i s d i l u t e d sample f o r t e s t . If r a i n water i s used it must be t e s t e d f o r C1- and a correction made a s indicated i n s t e p 6.
3. Add 5 drops K$rO4, stir. S o h . w i l l t u r n b r i g h t yellow.
4. F i l l p i p e t t e above zero mark with appropriate AgNO s o h Drain t o
A
zero mark, c o n t r o l l i n g flow with f i n g e r at t o p of p i p e t t e . Ad AgN03 slowly from p i p e t t e t o sample i n casserole, with continual s t i r r i n g . Clear yellow s o h w i l l become t u r b i d , very t u r b i d if high chloride. Each drop of AgN03 w i l l make
a b r i c k red f l a s h in t h e s o h . in t h e casserole, which w i l l disappear on stirring.
AS t h e end point i s approached t h e r e d f l a s h e s w i l l become l a r g e r and more p e r s i s t e n t . The end point i s reached when t h e whole soln. in t h e casserole
acquires a f a i n t permanent red t i n g e . It i s sometimes h e l p f u l t o pour onlyabout 23 t o 24 ml. of t h e sample i n t o t h e casserole at t h e s t a r t , t i t r a t e quickly p a s t t h e endpoint, then add t h e remainder of t h e sample, reversing t h e endpoint, and t i t r a t e - t o t h e endpoint i i t h care.
. . . .
5 - Read t h e amount of AgN03 s o l n . used from t h e p i p e t t e . 6. Compute t h e chloride content as follows:
With d i l u t e AsN03 With concentrated AnN03
( 1 ml. AgN03 soln.
=
1 mg. el-) ( 1 ml. AgN03 s o h .=
10 mg. ~ 1 - )... . ,
f o r 25 ml. sample
ppm. C1-
=
ml. AgN03 so1n:x 40 ppm. C 1 - = ml. AgN03 s o h . x 400 l i m i t of accuracy, equivalent of 1 drop AgNo3 soln.Approx. 1.3 ppm. , . Approx.
13
ppm.,, .With 1 to 10 d i l u t i o n
Determine C1- ppm. i n t e s t samplewith concentrated AgN03 as above.
C1- ppm. of o r i g i n a l
=
10 (Cl- ppm. of t e s t sample)-
9 (Cl' ppm. of d i l u e n t water). Limit of accuracy approx. 130 ppm.Chloride may be reported a s N a C l as follows: Chlorides a s ppm.
NaCl
=
1.649 ppm. C1- Total hardness:A s discussed i n t h e s e c t i o n on ground-water, t h e hardness of t h e water i s a key t o t h e amount of limestone it has dissolved.. The following simple and
accurate method f o r t h e determination of hardness depends on t i t r a t i o n with an organic agent which sequesters calcium and magnesium ions, in an a l k a l i n e s o h . using an organic i n d i c a t o r . Total hardness i s u s u a l l y expressed a s parts of CaCO.3 o e r m i l l i o n . both Ma++ and Ca+., being t r e a t e d a s i f t h e y were Ca++ and comb&&d with
co3:-.
TO& hardness encounzered on an a t o l l may range from l e s s than 1 0 t o n e a r l y 7,000 ppm. CaCO A s i n chloride determination, two concentra- t i o n s of t h e t i t r a t i n g reagentwig
be conveniknt. The following reagents w i l l permit a n a l y s i s of a t l e a s t 100 samples in high and 100 samples i n low range.1 p i n t hardness t i t r a t i n g solution. ( I ml.
=
1 mg. CaC03) 1 p i n t hardness t i t r a t i n g solution. ( 1 n i l .=
10 mg. CaCO?)2 oz. hardness buffer reagent. ( 1 oz. i n drogper b o t t l e ) 1 oz. hardness i n d t c a t o r . ( i n dropper b o t t l e )
pWjidrion paper t r i l i be u s e f u l i n checking p11 i n buffering.
The buffer reagent, k d i c a t o r , and d i l u t e t i t r a t i n g s o l u t i o n may be obtained from 1.:. H. & L. D. Betz, Philadelphia 24, Pa. For i n s t r u c t i o n s f o r making a l l
--
reagents see Betz and 3011, (.Am. Mater Narks kssn. Jour., bol. 42, p. 49, 1950).
Equipment necessary i s same a s f o r chloride determination. IIse p i g e t t e used i n d i i u t i o n s f o r hardness t i t r a t i c g s o h . o r c a r r y a t h i r d p i p e t t e .
The procedure i s a s follows:
1. Determine range of hardness. I n general, hardness a s CaC03 w i l l be i n same range a s C l - content except f o r sea water, which :.sill have about 1/3 o r
114
a s much hardness a s 61- content.2. Eeasure 25 m l . saxiple a s i n chloride determination. Dilute i f necessary a s i n c.hloride determination.
3. Add 5 drops b u f f e r reagent and s t i r . Solution should have pH of 8 o r 9.
Add 2 o r 3 drops of hardness i n d i c a t o r and stir. Soln. w i l l t u r n red.
4 .
T i t r a t e with hardness t i t r a t i n g s o h . i n same manner a s in t i t r a t i n g with AgN03 s o h . f o r C1- determination. As endpoint i s approached s o l u t i o n w i l ls t a r t c h a g i n g from r e d t o blue. Endpoint i s reached w i t h f i n d discharge of red.
5. Read t h e amount of t i t r a t i n g soln. used. Add a drop more t o t h e evaporating d i s h t o a s c e r t a i n thak t h e r e i s no f u r t h e r coior change.
6.
Compute t o t a l hardness a s follows:Iz'ith d i l u t e hardness t i t r a t i n n s o h . With concentrated hardness t i t r a t i n g s o h .
( 1 m l .
=
1 mg. CaC03) (1 ml.=
1 0 mg. CaC03) Ilardness, a s ppn. CaC03=
ml. Hardness, a s ppm. CaC03 = ml.t i t r a t i n g s o h . x LO t i t r z t i n g soln. x 400
L i m i t of accuracy approx. 1.3 ppm. Limit accuracy approx. 13 p p . With 1 t o 1 0 d i l u t i o n
determine CaC03 ppm, i n t e s t s a ~ g l e with concentrated t i t r a t i n g s o h . a s above.
Hardness a s CaC03 ppm. of original.
=
1 0 (CaC03 ppn. i n t e s t sample) -9 (CaC9 ppm. i n d i l u e n t water) L i n i t of accuracy approx. 130 ppm.-31-
Calcium hardness:
I n t h e above method t h e r e i s no separation of Ca'+ and Mg++ ions. They a r e l m e d and t h e resriLts computed a s if t h e y were Ca+*. viith t h e use of a d i f f e r e n t i n d i c a t o r and sodium hydroxide f o r pII control, calcium alone may be determined with t h e same t i t r a t i n g s o l u t i o n s a s used i n t o t a l hardness deter- mination. The i n d i c a t o r , a s o l i d , i s a v a i l a b l e from Betz i n 50 gm. b o t t l e s with a measuring dipper. Equipment i s t h e same a s f o r t o t a l hardness deter- mination. About 8 oz. of NaOH 1.0 N a r e required.
The procedure i s a s follows:
1. Determine range. Probably l/h t o 1/2 t o t a l hardness.
2. Measure 25 ml. sample a s above. Dilute i f necessary.
3.
Add 1 ml. NaOH 1 . 0 N and stir. Add 1/2 dipper of calcium i n d i c a t o r and s t i r . Solution w i l l t u r n salmon-pink.4. T i t r a t e with hardness t i t r a t i n g s o l u t i o n a s above. Solution w i l l t u r n purple a s endpoint i s approached. E n d p i n t i s f i n a l change t o orchid.-purple.
5. Read t i t r a t i n g s o h . used a s above.
6 . Compute Ca hardness a s CaC03 i n same manner a s computing t o t a l hardness a s CaCO Compute calcium l o n il d e s i r e d a s follows:
3'
Ca++ .LO0 Ca hardness a s CaC03.
Compute magnesium i o n i f d e s i r e d a s follows:
. .
I4g hardness a s CaC03
=
( t o t a l hardness a s CaC03)-
(Ca++ a s caC03) Mg++=
.243 (PIg hardness a s CaC03)Calcium and magnesium hardness were not d i f f e r e n t i a t e d on Arno because t h e m a t e r i a l s f o r t h i s method could not be procured i n time.
Total s a l i n i t x :
I n ocean water t h e r a t i o of t h e chloride content t o t h e t o t a l s d l i n i t y i s n e a r l y constant.
C1-
=
.55 (Svirdrup, Johnson, Fleming, The Oceans,t o t a l s a l i n i t y p. 166, 19h2)
The c h l o r i n i t y i s t h u s a convenient index t o t o t a l s a l i n i t y i n any water d i l u t e d from t h e sea water i f d i l u e n t has r o u g h l y t h e same balance of salt a s sea water. This w i l l not be t h e case i f t h e r e i s much CaC03 dissolved from the rocks i n t h e water. I n s e a water o r i t s d i l u t i o n s , d e n s i t y a l s o i s a very convenient index t o t o t a l s a l i n i t y . Densities may be measured by hydrometers.
A s e t of t h r e e , a s manufactured by G. K. Manufacturing Co. f o r t 3 e Coast m d Geodetic Survey, with ranges 0.996 t o 1.011, 1.010 t o 1,021., and i.020 t o 1.030, covers t h e range of sea water t o d i s t i l l e d water i n most temperature ranges.
A convenient cup f o r use with t h e hydrometers i s a v a i l a b l e from Mercer
'lassworks, 725 Broadway, New Pork
3 ,
N. Y. The temperature of t h e water must be measured t o lo C., a s a temperature correction i s c r i t i c a l . Tables f o r t h e temperature correction t o d e n s i t i e s and d e n s i t y t o s a l i n i t y reduction a r e given i n t h e Manual of Tide Observations,U.
S. Coast and Geodetic Survey Spec. Pub.196,
1941.
Density measwements were not made on Arno because t h e one hydrometer c a r r i e d broke.b i Doak C
.
Cox ., 1..4s i s f u r t h e r discussed in t h e s e c t i o n on ground-water investigations, t h e study of t h e head of a ground-water body i n an a t o l l i s l a n d must include some s o r t of analysis of t h e t i d a l f l u c t u a t i o n s of both t h e water-table and sea level.
Simple averaging of a day's water l e v e l s w i l l s u f f i c e f o r some purposes. For o t h e r s , a more i n t e n s i v e a n a l y s i s i s required. The t i d e f l u c t u a t i o n i n a ground- water body i s i t s e l f a key t o conditions i n t h e rocks. A study of t h i s
f l u c t u a t i o n depends u$on separation and i n d i v i d u a l a n a l y s i s of t i d a l components of d i f f e r e n t periods, because t h e periods a f f e c t t h e changes i n t h e t i d e s a s t h e y progress from t h e ocean through t h e rocks. The techniques discussed here have not been published y e t , b ~ t have been applied t o a number of problems (Cox and lIunk, Hawn. Acad. Sci. Proc. 1951, i n preparation).
Analysis of t i d a l c o m p o m :
Many t i d a l components of d i f f e r e n t periods go t o make up t h e t i d e . Several have periods near 24 hours o r near 1 2 hours, t h e d i u r n a l and semidiurnal
components respectively. The separation of t h e i n d i v i d u a l components i n e i t h e r d i u r n a l o r semidiurnal group r e q u i r e s a long t i d e record, but t h e separation of d i u r n a l from semidiurnal components can be made f a i r l y simply from a short t i d e record, a s short a s a day. These components a r e analyzed and expressed a s i f t h e i r periods were e x a c t l y 2L, and 1 2 hours respectively.
The procedure i s a s follows:
1. From t i d e records, choose a period of an i n t e g r a l multiple of 24 hours f o r which records a r e a v a i l a b l e f o r ocean and w e l l s t o be compared.
2. L i s t water l e v e l s f o r each s t a t i o n , a s measured from any datum, f o r h a l f hour i n t e r v a l s s t a r t i n g with t h e time of t h e period of t h e a n a l y s i s ( t E oh).
I f t h e o r i g i n a l records a r e discontinuous, p l o t them up so t h a t h a l f hour readings may be i n t e r p o l a t e d .
3. Kake two t a b l e s of 24 columns each, f o r d i u r n a l and semidiurnal
components r e s p e c t i v e l y of each record. Enter readings f o r hourly periods ( f o r t
=
0.0, t=
1.0", t = 2 . 0 ~ e t c . ) i n t h e d i u r n a l t a b l e and readings f o r half- hourly periods ( f o r t=
0.&, t=
O.gh, t= l.oh,
e t c . ) i n t h e semidiurnal table.S e t f i r s t 24 readings i n a row a c r o s s t h e columns i n each t a b l e . I f t h e r e a r e more t h a n 24 readings put t h e next 24 i n a second row beneath t h e f i r s t , e t c . Readings should complete t h e l a s t row.
4. Add a l g e b r a i c a l l y each column and obtain a mean. This will give 24 mean v d u e s f o r each t a b l e .
5. Add t h e s e 24 values and d i v i d e by 24 t o o b t a i n a general mean f o r each t a b l e . Subtract t h i s general mean from t h e individual means t o obtaindepar$urea.
For example t h e 24 departures f o r t h e d i u r n a l t i d e i n Hilo Bay, Hawaii f o r a day beginning a t 1/24/%, 0:CO are:
+.450, -..251, -.951, -1.501, -1.751, -1.751, -1.b51, -.851, -.251, +.24?, c.549, +.649, +.599, +.399, +.1?9, +.049, -.051, -.001, c.249, +.649, +.949, c1.213, +1.24?, +1.149. Those values a r e analyzed on t h e attached sample f o m .
6.
Prepare a form l i k e t h e sample f o r each component f o r each st&ion.K u l t i p l y t h e f i r s t twelve of t h e above departures by 100 with t h e s i g n s shown i n t h e twelve corresponding ~03.~1en.s of row d of t h e form, and e n t e r t h e products i n row 1. Enter i n t h e twelve columns of row 2 t h e products of t h e second
twelve of t h e departures multiplied by 100 with t h e s i g n s shown i n row e.
Example: t h e 1 3 t h departure i s +.599. Column 0, row e i s
(-1.
+.599 X(-100)
=
-59.9. Enter. -59.9 i n column 0, row 2.7 . Add a ~ ~ e b r a i c a i l y row 1 and row 2, e n t e r i n g sums i n row
3.
8. Transfer numbers i n columns
7
t o 11 i n c l u s i v e of row 3 t o columns 1 t o5
i n c l u s i v e of row4,
but i n reverse order (column 7, row3
goes t o c ~ ~ u m n5,
row
4)
entering o r i g i n a l s i g n s ( t o p ) and a l s o reversed s i g n s (bottom).9. Add columns 1 to
5
i n c l u s i v e of rows 3 and4
using o r i g i n a i ( t o p ) signs and e n t e r sums i n row5.
~ d d coiumns 1 t o5
i n c l u s i v e of rows 3 and4
using new (bottom) s i g n s i n row4
and e n t e r i n row 6 .10. X u l t i p l y f i g u r e s i n row
5
by corresponding f i g u r e s i n row b and e n t e r products v e r t i c a l l y i n column 0, rows4
t o 8 i n c l u s i v e (Column 1, row5
times column 1, row b goes t o column 0, row4 ) .
k u l t i p l y f i g u r e s i n row 6, c o l m s 1 t o 5 , by corresponaing f i g u r e s i n row c, and e n t e r products v e r t i c a l l y i n column 6, rows4
t o 8 i n c l u s i v e .11. Add rows 3 t o 8 i n column 0 and e n t e r sum i n row
9.
Add rows 3 t o 8 i n column6
and e n t e r sum i n row 9.12. Divide f i g u r e i n column 0 row
9
by 1200 t o obtain "p".Divide f i g u r e i n column -6 row 9 by 1200 t o obtain "qU.
13. Compute t h e amplitude of t h e component, A
)-.
A will. be i n f e e t i f t h e o r i g i n a l water l e v e l s were i n f e e t .
14.
Compute t h e cotangent of t h e phase angle of t h e conponent, cot QC.2 p/q.Use trigonometric t a b l e s t o determine & i n degrees. The cotangent 1.611 not discriminate between two values of W, 180° a p a r t . Actual d i s c r s n a t i o n may be made by inspection of t h e record o r by t r i z l when both components have been analyzed f o r a s t a t i o n a s described below. I f t h
=
A i s t h e time when t h e height of t h e component i n question reaches a maximum, measured i n hours a f t e r zero time of t h e observations, 6 i n degrees=
3 6 0 t / ~ ,where T i s t h e ~eeriod of t h e component i n hours.
1 5 . When both d i u r n a l and semidiurnal components have been analyzed f o r a s t a t i o n , compute t h e t i d e and conpare with t h e a c t u a l t i d e f o r a check.