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Anion exclusion during transport through the unsaturated zone
Journal of Hydrology, 87 (1986) 267-283 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
267
[2] ANION EXCLUSION DURING UNSATURATED ZONE
TRANSPORT
THROUGH
THE
HAIM GVIRTZMAN1, DANIEL RONEN 1'2 and MORDECKAI MAGARITZ1 1Isotope Department, The Weizmann Institute of Science, 76100 Rehovot (Israel) 2Research Department, Hydrological Service, Water Commission, P.O.Box 6381, 91063 Jerusalem (Israel)
(Received December 28, 1985; revised and accepted May 10, 1986)
ABSTRACT Gvirtzman, H., Ronen, D. and Magaritz, M., 1986. Anion exclusion during transport through the unsaturated zone. J. Hydrol., 87:267 283. A 20-year chronological record of the flow of water and anions, along 27 m, in the unsaturated zone of a phreatic aquifer was reconstructed. Water was traced according to its tritium content, using the difference between the environmental tritium content of rain and irrigation water. Anions were traced using data about their sequential input to the overlying cultivated field and their concentration in the profile. Evidences of anion exlcusion were found along a 10-m thick clay loam layer. The vertical velocities of water molecules and anions (SO2- and C1- ) were calculated to be 0.7 + 0.05 and 1.35 _+ 0.05myr 1, respectively. The average thickness of the water films and the equivalent distance of exclusion, at a 15% gravimetric water content, were 25 and 12A, respectively. These field data fit and support the theoretical relationship between the water film thickness and the relative exclusion concentration proposed by Bresler (1973). INTRODUCTION The transport of solutes through the unsaturated zone to the water table of a p h r e a t i c a q u i f e r is a c o m p l e x p r o c e s s w h i c h d e t e r m i n e s g r o u n d w a t e r q u a l i t y . Most studies on this topic have been conducted in the laboratory. The process o f e x t r a p o l a t i o n o f r e s u l t s t o a c t u a l field s i t u a t i o n s is d o u b t f u l . O n t h e o t h e r hand large distances and travel times are required to simulate natural processes. T h e r e f o r e i t is d i f f i c u l t t o c o n d u c t i n - s i t u e x p e r i m e n t s . T h e o b j e c t i v e o f t h i s w o r k is t o p r e s e n t a field s t u d y w h i c h h a s a t t e m p t e d t o o v e r c o m e t h e a b o v e difficulties. In natural systems water molecules artificially tagged with tritium have been used to investigate water movement in the unsaturated zone (Zimmerm a n n e t al., 1966; A t h a v a l e e t al., 1980). T r i t i u m i n r a i n w a t e r w a s s u c c e s s f u l l y used for this purpose, mainly in the sixties. At that time, large amounts of tritium produced by thermonuclear tests were injected into the atmospheric m o i s t u r e ( B o l i n , 1958; G a t , 1980; I A E A , 1981). T r i t u m c a n b e u s e d a s a t e m p o r a l marker utilizing data on its atmospheric pulse and calculating its radioactive d e c a y ( h a l f - l i f e o f 12.43 yr). R e c h a r g e q u a n t i t i e s a n d v e r t i c a l flow r a t e s w e r e
268 determined by detecting the penetration depth of the 1963 tritium peak in soil moisture (Smith et al., 1970; Andersen and Sevel, 1974). The profile of soil moisture created by alternate sources of water that differ in their tritium content was also used as a long-term record of water transport (Gvirtzman and Magaritz, 1986). Tritium is a component of water molecules and therefore an ideal tracer of their movement. However, the transport of water and solutes is not necessarily synchronic. Specifically, the transport of anions in porous media is affected by "anionexclusion" (Bresler et al., 1982). Anions are repelled from negative clay surfaces by electrostatic forces and they are concentrated in the center of the pores where the velocity is relatively faster. Hence, in some cases anions move faster than the average velocity of the water molecules. Laboratory studies have demonstrated this phenomenon (Mokady et al., 1968; Thomas and Swoboda, 1970; Warrick et al., 1971; Krupp et al., 1972). In leaching experiments using 15 widely varying soils and 0.01 N CaC12, Smith (1972) observed that chloride moved 1.04 to 1.67 times faster than the average velocity of the water molecules. Dyer (1965) estimated that the equivalent anion-free volume for chloride and nitrate was about 50% of the moisture content near the wilting point. The phenomenon of anion exclusion is related to the nature of the clay minerals, clay content, saturation percentage and solute concentration (Appelt et al., 1975). Anion exclusion models have been developed for quantitative analysis. In these models the distribution of the anion concentration with distance from a negative colloid surface is calculated according to the diffuse double layer theory (De Haan, 1965; Bresler, 1970). The equivalent distance of exlcusion is estimated according to the surface charge density of the clay mineral, the equilibrium soil moisture concentration and the structure of the hydration envelope of the anion under consideration (Schofield, 1947). The volume of the anion-free solution is calculated based on the distance of exclusion, the specific surface area of the medium material and the volumetric water content (Bresler, 1973). The ratio between the volume of the anion-free solution to the total liquid volume determines the relationship between the average velocities of water and the anions. Under controlled laboratory conditions, the simultaneous transport of anions and water can be predicted quantitatively by linking the above models to dispersive convective equations (Biggar and Nielsen, 1967). Unfortunately, the high variability found in the field and the many approximations which are necessary make the use of these calculations of limited practical value. This study presents an in-situ field record from the unsaturated zone of a deep phreatic aquifer. By independent tracing of water and anions the different rates of movement of water, chloride and sulfate were determined and the anion exclusion effect was quantified. The anions were traced using data about their input; water was traced by variations in tritium levels.
269 MATERIALS AND METHODS
Site description A research well (WT-2) was drilled, down to the water table, in a cultivated field of Kibutz Glil-Yam, 10 km north of Tel-Aviv (Ronen and Magaritz, 1985). The field is located on the replenishment area of the sandy, phreatic Coastal Plain Aquifer of Israel. A production well (Glil-Yam B) is located 1200m northeast of the research well. Winter precipitation ranges from 350 to 750 mm, with an average of 600 mm. During some summer seasons (April to September) the field was irrigated with 450 mm of water applied by the sprinkler method. Between 1967 and 1982 municipal sewage effluents were utilized as a water source for irrigation. Before 1967 and after 1982 local groundwater was used. The sewage effluents derived from the supply of local groundwater to the nearby city of Hertzlia (Ronen and Magaritz, 1985). Sewage had been treated by passage through an oxidation pond before 1977, and subsequently by an extended aeration treatment plant. Potatoes, peanuts and cereals were alternately cultivated at the area under investigation. Fertilization mainly involved application of ammonium-sulfate. Prior to 1967, the field had been used only for growing cereals. Two sedimentary units characterize the profile at the site where the research well was drilled: (i) a 10-m-thick clay loam layer and (ii) underlying homogeneous sand and sandstone sediments that extended down to the water table at a depth of 27.5 m (Fig. 1). The top soil is composed of a 0.9-m layer of sandy loam. The clay loam unit contains 15% clay minerals which are essentially montmorillonite (Mercado et al., 1975). ........ O! F '"
[ '"
Soil
a
..... I
'~'1.... el''' i
0
Clay Loam
E
i[
i
x= c. a)
c B 20
¢" 2(
Sand
and
Sandstone
.... I .... I .... .... I,,, 5 I0 15 20 Moisture (%)
25
Lithology
.... io'"~O"iO ~'4o 'so3° Tritium (T.U.)
Fig. 1. Gravimetric water content vs. depth (a), and tritium concentration vs. depth (b). The peaks A, B and C are discussed in the text. The lithological profile is shown in the center.
270
Sampling The research well was drilled by a spiral-driller dry method without addition of water and samples were collected at 50-cm depth intervals. Two kg of each sample were preserved with dry ice for tritium analyses. An additional amount of each sample was introduced in pre-weighted glass vials for moisture content and anions concentration measurements. Samples were also collected from the nearby production well, Glil-Yam B, and from effluents of the sewage treatment plant.
Analytical methods Water for tritium analyses was extracted from each sample in a vacuum distillation system. The tritium concentration was enriched electrolytically (Cameron, 1967; Taylor, 1981), converted to ethane and analyzed with a lowlevel counting system (Ashkenazy and Carmi, 1970; Bowman and Hughes, 1981). Tritium concentration is expressed in tritium units (T.U.) where 1 T.U. corresponds to 1 atom of tritium per 1018 hydrogen atoms. The gravimetric water content was calculated by drying to a constant weight at 105°C. Anions were extracted with distilled water from 15 g sediment samples after shaking for 20min. The weight ratio between water and dry-soil in all the extractions was 1.33. The solutions were centrifuged for 10 min at 10,000 RPM and filtered through a 0.45/~m micropore filter. CI-, NO[ and SO 2- concentrations were analyzed by HPLC with a Wescan 261 Ion Analyzer with a precision of 4%. The results were recalculated on the basis of the field moisture content. RESULTS The gravimetric water content along the unsaturated zone reflects the two sedimentary units which form the geological column (Fig. la). The clay loam and the sandy sediments contained about 15 and 5~/o water, respectively. The water table in the well was found at a depth of 27.5 m and a capillary fringe was identified up to 1.5 m above it. The tritium content of water along the profile is shown in Fig. lb. The tritium concentration varies between 14 to 24 T.U. in the clay loam layer. Concentration levels between 20 to 45 T.U. were detected in the sandy sediments. At the capillary fringe, tritium decreases from 22 T.U. to 11 T.U., which is the tritium content of the saturated region at the top of the aquifer. The tritium content in groundwater of Glil-Yam B pumping well ranged between 3 to 11 T.U. The concentrations of Cl-, NO~ and SO~- along the unsaturated zone fluctuate between 4-18, 3-10 and 2-22 meq 1-1 respectively (Fig. 2). In order to analyze the results obtained from the unsaturated zone, historical data concerning the agricultural activities and the amounts of rain, irrigation and fertilization over the period 1967 to 1984 were collected (Table 1). The tritium content of the sewage effluents used for irrigation, was estimated according to the original tritium concentration of the pumped groundwater
271
% E 1~
lo
.6.
~
20
%
% x~
2
[
a
....
b
,,
s ....
C1- (meqxl -])
;
c
oi,,,,~.,I,,,,I,,,,I.,,/O,,O,,, , o
5'"
NO~ (rneqxl I)
SO, =
,10 ~,,,
,,,,,~/,,,2~o 1~
(meqX]
')
Fig. 2. Chloride (a), nitrate (b) and sulfate (c) along the unsaturated profile in research well WT-2.
and the e x c h a n g e w i t h tritium c o n t a i n e d in atmospheric moisture w h i c h w o u l d h a v e t a k e n place during aeration and storage. The tritium c o n c e n t r a t i o n of local g r o u n d w a t e r (Glil-Yam B well) varies b e t w e e n 3 to 11 T.U. The residence time of s e w a g e in the treatment plant (including aeration and storage before TABLE 1 Water and fertilizer inputs C~pl Ye~
Se~
2
Water Input "l~ypes
F~tiliserJ (k¢ ha-t}
Tritium (T.U,) s
(NH4)~SOt
161e
18
1S4~3
1984
W
Pot
,.
w ~
P...~
~
,o 14
S
_ Pot
R
_ 483
_ 13
1500
1981
SW
Pot
R
6O6
IS
1500
1980
S
Pea -
S
~4~
11 17
482
23
4.57
*4
1982
R
A m m m t (ram) 4
-
1973
W S
C~r
R
462
3-I
1971
~/
Pe._
RS
4~
6416
p:,
~
,k
;,
,9,o w s
400
CO(NH2)2
IO0
,~o
p o t = P o t a t o a , P e t = P e ~ m t s , C~C4u-el0a; Dat* from Kibuts GliI-Ym, = W=Wiater, S=Summ~. s R = R m , G ~ P a m p e d Gmnnclwate~. $=Sewale. Data on rai= ~ u u a f r ~ K f ~ Shmsryshu Meteorolotka] St&tioa. Irrigation amounts from Kibuts G l i l - y ~ . s D t t a from the W a t ~ Library of the Gmlaotop* G m v p at the W e l s m ~ n Institute. • Rain m o u n t till 10.1,84 ( 8 ~ p l i n g date).
s
272 300
........
I .... I .... Pa~h~
250
r ''1
= Tritium in rainwater
Solid
= Corrected
1970
i975
for decay
200
150
E i00
E E50
~P
i
1960
1965
1980
Date (Years) Fig. 3. Tritium in rainwater at the Coastal Plain of Israel (data from the Water Library of the Geoisotope Group at the Weizmann Institute of Science).
irrigation ) is about I week. The vapor exchange can be calculated according to the relative humidity, the amount of evaporation during the period available for exchange and the tritium content of atmospheric moisture (Gat, 1970). In 1984, this exchange enriched the tritium content of the effluents by about 1-2 T.U. In the sixties, when tritium in atmospheric moisture was higher (Fig. 3), this exchange could have enriched the effluents by up to 10 T.U. Exchange with tritium of atmospheric moisture during municipal water use is neglected because of the relatively short times involved. Fertilizers and sewage utilized for irrigation contributed the major part of the input of chemicals to the cultivated field. Additional amounts were supplemented by rain and groundwater. The average concentrations of chloride, nitrogen and sulfate in rainwater, groundwater and sewage are presented in Table 2. The nitrogen content of sewage effluents changed in 1977 when an extended aeration treatment plant replaced the old oxidation ponds (Water Commission of Israel, 1982). The chloride concentration of sewage changed with time as a result of the continuous salinization of the Coastal Plain aquifer of Israel (Ronen and Magaritz, 1985). The dry fallout (mainly from seawater spray) amounts about 10% of the salt content in rain and is not taken into consideration. DISCUSSION Tritium is a hydrogen isotope which comprises a part of every population of water molecules; its percentage representation among these molecules acts as an effective identification indicator for differing water sources. For each water source during a particular time span a distinctive bracket of representation concentrations may be noted. Its variation along the unsaturated profile represents the alternating input of water sources which penetrated the land
273 TABLE 2 Average chloride, nitrogen, sulfate and tritium content of water sources Input
C1- (meq
Rain 1 Groundwater (1980-1984) Sewage 2 (1978-1984) Sewage 3 (1970-1977) Sewage 4 (1965-1969)
0.54 4.11
0.02 0.72
0.55 0.62
See Fig. 3 3-11
6.90
1.50
0.80
10-13
6.76
4.50
0.80
14-15
5.92
4.50
0.80
16-20
1Data 2Data a Data 4Data
x
1-1)
N (meq
x
1-z)
SO2- (meq
x
1-1)
Tritium (T.U.)
from Mercado et al. (1975). supplied by the treatment plant laboratory, Hertzlia. from the record of Israel Ministry of Health. from Raveh et al. (1972).
-
R = Rain S = Sewage
--
R 7a R ~a
~
I
l o ~ R 710 i
!
S?I ~ ~ s aa
:i: "°°
20 0
R 64
s 6a
R ~:'. R e~
S e2
' ~210
?u
z~ 165 TU
7
20 Tritium
40
60
80
100
(T.U.)
Fig. 4. Tritium input function - - Accumulated input amount of irrigation and rainwater according to their sequence during the last 23 yr vs. their tritium content. The volumetric datum line (zero accumulation) is based upon 1984 rainwater. Each segment represents the seasonal input and is characterized by the corresponding water amount (ordinate) and by its tritium content (abscissa), s u r f a c e ( G v i r t z m a n a n d M a g a r i t z , 1986). T h e l a y e r s w i t h h i g h a n d l o w t r i t i u m content are related to winter rains and irrigation water, respectively. Two c h a r a c t e r i s t i c s d e t e r m i n e the shape of the t r i t i u m peaks on Fig. lb: (1) T h e w i d t h w h i c h is d e t e r m i n e d b y t h e n e t a m o u n t o f r a i n w h i c h percolated after evapotranspiration. (2) T h e h e i g h t w h i c h is d e t e r m i n e d b y t h e c o n c e n t r a t i o n o f t r i t i u m i n t h e
274 corresponding precipitation, and by some other processes which change this concentration (e.g., radioactive decay and mixing). Since samples were collected at 50-cm depth intervals, they were not necessarily in phase with the summer and winter water layers. Each sample represents an unknown mixture of irrigation and rainwater and the measured concentrations are average values of the sampling intervals. Thus, the tritium amplitude is smaller t h a n what would be obtained by a more detailed sampling scheme. Fig. 4 summarizes the chronological sequence of water applied at the research area. We define this figure as the "tritium input function".
Identification of water layers Dating of water layers in the unsaturated zone is possible by comparing the peaks of tritium along the profile with the tritium input function (Figs. 1 and 4, respectively). Our interpretation is presented in Fig. 5. It is based on the following two principles: (1) Tritium content of water in the sediments can not be higher than the tritium content of the rainwater of the suggested year. Radioactive decay and mixing could only reduce the tritium content of the rainwater while percolating downward. The increase of tritium content by isotopic fractionation during evapotranspiration is negligible (Zimmermann et al., 1966). (2) Water amount of a layer in the profile should fit the input water amount of the suggested year (or years) according to the evapotranspiration fraction. Based on the first principle, an initial rough dating of water layers can be achieved. The two peaks of 45 T.U. at 24 and 21 m depth, in the sandy unit (A and B - - Fig. lb) can be related only to winter rains which fell between 1962 and 1966. This rough identification supplies a good approximation about the ,
0
10
I ~
.¢
^ ~ @
I
~.~
R 69 or R 70/71 ~65 or R 89 67 or R 56 R66oR 66 or R 67
~ 1 ~
20
20
Reoor~6s
R 64 or R 65
3o
,
"' '
20
~~
I 40
,
I , 60
30
Tritium (T.U.) Fig. 5. Identification of rainwater peaks and the calculated water velocities (V) for the two lithological units.
275 time scale involved and the amount of yearly replenishment. The water contained in the profile down to 25 m depth is thus related to the water input of the last 19 to 23 yr. The amount of water in this segment is 3100 mm (see calculation below), which means a yearly replenishment of about 150 mm. More than that, considering the water amount of the two sedimentary units (2050 and 1050 mm for the clay loam and the sand, respectively), we concluded that the water contained in the clay loam unit represents input of 14 _+ 1 yr and that of the sandy unit of 7 + 1 yr. A more accurate identification of the tritium peaks along the profile was achieved by dating the peak of 40 T.U. at 12 m depth (C - - Fig. lb). This peak is the upper one of the sandy unit and according to the above rough dating, is related to a winter rain which fell in the late sixties or the early seventies. Two alternative identifications may be reached according to the above two principles (Fig. 5): (1) Based on tritium level considerations, this peak is related to 1969 rainwater (Fig. 4). (2) Based on the water amount, this peak is related to 1970-1971 rainwater (there was no irrigation during the summer of 1970; Table 1). This peak extends between 10.5 and 13.5 m depth and contains a large quantity of water (240 ram). It may be more reasonable to assume t h a t such a large amount of water was left after the evapotranspiration of two winter seasons, 1970-1971, rather t h a n from a single winter of 1969. Note t h a t the difference between the two possible identifications is only 1 yr. In the sandy unit the average yearly recharge is contained in a sediment 2 m thick. Two samples, from 18.5 and 19.0m, were lost during the analytical procedure. We suggest t h a t the water contained between 17 to 21m depth included a winter peak which was not detected. No large peaks of tritium were detected in the upper 10 m of the profile. This is a result of : (i) the small difference between the tritium content of rains and of irrigation water between 1970 and 1984 (Fig. 4); and (ii) the sampling frequency. A 50-cm-depth sampling interval is sufficient for detecting water layers in the sandy unit where, as explained before, the water input of each year is contained in a sediment segment of a thickness of about 2 m. In the upper clay loam unit the same amount of water is contained in a sediment layer of only 70 cm thick. Therefore, due to the sampling resolution, even existing peaks would not have been clearly detected in this unit.
Determination of the vertical flow rate The tritium peak A (Fig. lb) represents the rain input of 1964 or 1965. Therefore, the mean vertical velocity is 1.2myr 1. Due to the lithological differences found, the velocities can not be similar along the whole profile. The upper 10 m of clayey sediments contain the input water of the last 13 to 15 yr. Therefore, the average vertical velocity is about 0.7 _+ 0.05myr i. The lower 15 m of the sandy unit contains 6 to 7 yr water input, and thus, the mean rate
276 TABLE 3 Average amount of applied and recharged water* Profile unit
Input time (yr)
Profile water (mm)
Applied water (mm)
Recharge fraction (%)
Yearly application (ram)
Yearly recharge (mm)
Clay Sand
13-15 6-7
2050 1050
9700 + 700 7200 + 700
21 + 2 15 + 2
700 1100
150 _+ 10 160 + 10
* Calculations of the range ( + ) depends on the two possible identifications of the peaks.
of water movement is 2.3 + 0 . 2 m y r -1 (Fig. 5). Indeed, when the percolating water molecules cross the border between the clay loam and the sandy units their velocity increases about 3 times. In consistency with the law of continuity, the threefold decrease in the sediment water content causes a threefold increase in the vertical water flow rate. The vertical water velocity of the clay loam unit is similar to t h a t found in the loess sediments in a semiarid zone near Be'er-Sheva (Gvirtzman and Magaritz, 1986).
Calculation of aquifer replenishment The amount of water contained in the unsaturated zone down to a depth of 25 m is 3.1 m. This was calculated, using the bulk densities of the sediments (1.35 and 1.4 g cm -3 for the clay loam and sand, respectively) and the gravimetric water content profile (Fig. la). The amount of input water, 16.5 + 0.5 m, was obtained by summarizing the amount of rain and irrigation water from 1964/5 to 1984 (Table 1 and Fig. 4). The 3.1m of water presently contained in the unsaturated zone is 19% of the applied water. The remainder, 81%, evaporated from the land surface, transpired through the plants leaves, or was removed with crops. The identification of the tritium peaks enables one to calculate separately the average yearly recharge amount for the two periods represented by the water found in the two lithological units of the profile (Table 3). The differences found in the pattern of irrigation are presented in Table 1 and Fig. 4. During the period 1964 to 1970 the field under study was irrigated annually, whereas during the subsequent 14 yr it was irrigated only during 6 summers. Evapotranspiration is significant mainly in the summers. Therefore, the amount of water t h a t evaporated yearly between 1964 and 1970 is relatively higher t h a n t h a t between 1971 and 1984. Consequently, the recharge fractions are significantly different while the yearly recharge is similar.
Anions mass balance Comparisons were made between the chloride and sulfate amounts applied to the research field during 1964 to 1984 and the amounts found along 25 m of
277 TABLE 4
Comparison between input and profile anions content Anion
Calculated input Profile content
C1- ( e q h a - ] )
SO 2- (eqha -1)
420,000 287,000
330,000 267,000
the unsaturated zone. The amounts of applied anions were estimated from the amounts of applied water (rain and irrigation), the concentration of the anions in these water sources and the fertilizer input (Tables 1 and 2). The amount of anions along the unsaturated profile were calculated by integrating their respective concentrations according to the volumetric water content. The results are presented in Table 4. Since the amounts of chloride and sulfate actually found in the unsaturated zone are 30% smaller than the calculated input we conclude that the amounts of anions found in the profile represents a shorter input sequence than estimated. This large discrepancy can not be explained by processes like plant uptake or sulfate reduction. The system is aerobic as shown by the large concentrations of nitrate (Fig. 2). Thus, the rate of transport of water molecules and anions must be different. In order to date the peaks of the anion profiles we calculated the chronological sequence of chloride and sulfate applied to the research area, which we define as the "anion input function" (Fig. 6). o . . . . . . i .... ir.... I " ~
i'"i
,,
N~+F
"i .... r'"L
c~ R 83 Ra2~F RSI+P S 80 R 80
~
1
I
-
]
R'rg* F
R 78 R77*F
__
1
i -~.~ 7 . 5 ~
s ?4 R 74 R?a* P R '72 F s?l
I
--r' 12.
N
76
~
]
N69 +
i
E-~ L v
F
s 68 R68+F
] .... I.... I,~, o
2
4
6
Input CI- (meqxl ~1)
0
2
4
6
8
Input N (meq×l-l) Input
2
4 SO 4"
6 (r~eqxl
8 t)
Fig. 6. Anion input ftmction - - Accumulated input amount of irrigation and rainwater according to their sequence during the last 1 7 y r vs. their chloride, nitrogen and sulfate content. Zero
accumulation refers to 1984 rainwater. Each segment represents the seasonal input according to sources (water type and fertilizers). The scale on the right hand side indicates the a n n u a l anion source (R = r a i n ; G = G r o u n d w a t e r ; S = s e w a g e ; F = Fertilizer).
278
°r'°
. . . . . .
I ....
! . . . . ~' 't' ' ~~
I,'
-~:~
"1
-
~!
....
--
S76
0
10
S 74
m
2C
--
.,
R ?~
ra
8 ?0
i
V
0
....
S?[
20 1
i
& 30;
~
J
s 69 b
I .... i0
I .... 20
S04= (meqx1-1)
.... ] .... ] .... I .... i .... 3o
I,,
30
0
5
I0
15
20
25
Cl- (meqX[ -t)
Fig. 7. Identificationof sulfate (a) and chloride(b) peaks and their vertical velocities(V) in the two lithological units. The sulfate concentration along the profile (Fig. 2c) is similar to its input function (Fig. 6). Since the main source of sulfate is the fertilizer, ammoniumsulfate, it is possible to correlate the winter periods (when fertilizers are applied) to peaks in the unsaturated zone. The large peak at 5 m depth is related to the winter of 1979 (Fig. 7a). The minimum at 20 m depth is related to the winter rains of 1972-1974. The amount of fertilizers applied at t h a t period was small as compared to the inputs before and after (Table 1). The minor peaks of 11, 15, and 25m depths are related to the winters of 1977, 1975 and 1970, respectively (Fig. 7a). Note t h a t the sulfate concentration in the pore water (Fig. 2) is higher t h a n the concentration in the applied water (Fig. 6) as a result of evapotranspiration. A fivefold increase in the salt concentration resulted from the fact t h a t 19% of the applied water percolated downward, as was calculated above. In a similar way the chloride profile may be compared to its input function. Since the main source of chloride is sewage, we may correlate the summer periods (when sewage is applied) with peaks along the profile. The minima between 15 and 18m depth are related to the 3-yr period 1972-1974 without sewage irrigation. The two peaks below at 21 and 26 m depth reflect the chloride content in sewage applied in the summers of 1971 and 1969, respectively. The peaks at 5, 7, 11 and 14 m depths are related to the summers of 1980, 1978, 1976 and 1974, respectively (Fig. 7b). It is possible to identify sulfate and chloride peaks in the clay loam unit (where the tritium peaks were not clearly identified) due to the large differences found in the concentrations of these anions between water sources, and the expanded segment which represents a yearly input (7-8yr for sulfate and chloride, respectively, as compared to 13-15 yr for the water molecules). A corrected comparison between the input and the profile content of anions based on the above peak identification (Fig. 7), is summarized in Table 5. The
279 TABLE 5 Corrected comparison between input and profile anions content Anion
Calculated input Profile content
CI- (eqha -1)
SO 2- (eqha -I)
241,000 287,000
235,000 267,000
increased agreement between the amounts of sulfate and chloride supports the time-scale suggested in Fig. 7.
Anion velocity The chronological identification of the sulfate peaks (Fig. 7a) was used for determining its vertical velocity in the unsaturated porous medium. The clay loam unit (10 m thick) containing sulfate inputs of 7 yr (1978 to 1984), thus, involves an average velocity of 1.4 m yr- 1. The sandy unit (between 10 and 27 m depth), containing the inputs of the years 1970 to 1977, involves an average velocity of 2.1 m yr- 1. Similar velocities were calculated for chloride (Fig. 7b). The slight difference found between chloride and sulfate in the clay loam unit may relate to difference in their ionic charge and radius.
Evidence for anion exclusion In the upper clay loam unit the velocity of water molecules is calculated to be 0.7 + 0.05myr -1 (Fig. 5), whereas the anion velocities are computed as 1.3-1.4 m yr -1 (Fig. 7), almost double. This difference is a result of the anion exclusion effect. In fact, the anions found towards the bottom of the clay loam unit are encountered in water t h a t infiltrated through the soil surface about 5 yr previously. On the other hand, in the sandy unit, water and anions have almost the same velocity ( > 2 m y r - ~). The ratio between the unexcluded volume and the total volume of water is calculated by taking the ratio between water and anion velocities. This ratio is 0.7/1.35 = 0.51, and thus the unexcluded volume is 51 + 5% of the total water volume. An uncertainty of + 5°/0 in this estimation results from the uncertainty involved in estimating velocities of anions and water. The volumetric water content of the clay loam layer is 20% involving about 10~o unexcluded water volume. The average thickness of the water film surrounding soil particles can be calculated by Kemper's (1961) formula if one knows the values of the hydraulic conductivity of the unsaturated medium, its tortuosity and water content. If these parameters are not known one can estimate the average thickness of the
280 water films (b) by dividing the gravimetric water content (0g) by the specific surface area (A): b = 0~ A
(1)
This approximation is based on the assumption t h a t a water film is distributed uniformly over all particle surfaces. It should be noted that this assumption does not apply in a saturated medium because the majority of the solution flows only through the larger pores (Bresler et al., 1982). However, in unsaturated media this assumption may be valid because the larger pores are partially filled with air and the water distribution over the particles surfaces is more uniform. The gravimetric water content in the clay loam layer is 0.15 (Fig. la). The clay loam layer contains 15°//o clay minerals which are essentially montmorillonite (Mercado et al., 1975). The specific surface area ofmontmorillonite is 400 m 2g- 1. Therefore, the specific surface area of this layer is 60 m s g 1and the average film thickness of the water is 25 A. Considering t h a t the excluded volume of the water content is 49 + 5°//0, then the average equivalent distance of exclusion is about 12 A (which means about four layers of water molecules). Since the total anions concentration (C) at the surface of a clay particle is practically zero, the amount of surface excluded anions F - (in meq per cm 2 of surface) can be calculated, by multiplying the average anion concentration by the equivalent distance of exclusion (d). F-
=
Cd
(2)
In this instance, the surface excluded amount which corresponds to the measured solute concentration is 4 × 10-gmeq cm -2. To the best of our knowledge, there are no measured field values in the literature about the effect of anion exclusion on flow in unsaturated media. The available experimental data are only from saturated flow. Therefore, comparison between our measurements and other existing data can only be done by using the empirical relationship proposed by Bresler (1973). Bresler plotted the relative exclusion concentration (f) as a function of b v ~ (where b is the water-film thickness in A and C is the total solute concentration in eq 1 1). Figure 8 shows Bresler's theoretical curve (1973) and the saturated-flow experimental data reported by: (i) Mokady et al. (1968) for kaolinitic and illitic clay soil; (ii) Thomas and Swoboda (1970) for montmorillonitic clay soil; and (iii) Krupp et al. (1972) for montmorillonitic clay soil. In our case f = 0.5, b = 25 A and C = 0.03 eq 1-1. The calculated data point (f = 0.5; bx/~ = 4.3) matches the theoretical line in a region where data were lacking previously. The obtained value demonstrates a case in an unsaturated medium where the water film thickness is relatively small and the exclusion effect is large. S U M M A R Y AND CONCLUSIONS A record of 20 yr flow of water and anions along 30 m of the unsaturated zone was reconstructed, providing evidences for the anion exclusion effect under
281 Z O
......
I
.........
L
........
L
........
~
........
J
F-Z Ld (D Z O
~,1(5 2
! ~ . ~ ~T.AL~S~ 163 rt-
~_
• KRUPP ET AL. ( 1 9 7 2 7 -w ~ • THOMAS AND SWOBODA (19"?O)
,.
Io
Io2
Io3
•
IO"
b,E
Fig. 8. Relative solute exclusion concentration (f) as a function of b~-Cwhere C is expressed in meq1-1 and b in A (based on Bresler, 1973). field conditions. Water molecules were traced by their tritium concentration. Identification was possible since waters of two types (rain and irrigation water), which are different in their tritium content, were applied to the field. Anions were traced by matching their concentration in the pore water to their input amounts in fertilizers, sewage and other salt sources. These tracing methods can serve to quantify the leaching velocity of water, accompanying anions, and the volume of exclusion. For the Mediterranean climate zone and for the clay loam and sandy profiles t h a t were investigated, the following values were found: (a) the average replenishment of the aquifer is 150mm yr -1 which is 19% of the total amount of applied water; the amount of infiltration of irrigation water is largely reduced by evapotranspiration; (b) the vertical water velocity is 0.7 + 0.05 m yr 1and 2.3 + 0.2 m yr -~ in the clay loam and the sandy sediments, respectively; (c) the vertical velocity of chloride and sulfate is 1.35 + 0.05myr -~ and 2.1 m yr -~ in the clay loam and the sandy sediments, respectively; (d) the velocity of chloride and sulfate almost doubled as compared to the average velocity of water molecules in the clay loam unit due to the anion exclusion phenomenon; (e) the unexcluded volume in the clay loam unit, at a 20% volumetric water content, is 51 + 5~/o; the average thickness of water film in the unsaturated zone is about 25 A; and the equivalent distance of exclusion is about 12 A. This study suggests t h a t the anion exclusion phenomenon can not be neglected in the study of groundwater pollution rates. Any negatively charged pollutant ought to percolate much faster than water molecules through clay-rich sediments. At present the time of degradation of a pollutant and the time of water transport through the unsaturated zone are the criteria for allowing fertilization and sewage irrigation above phreatic aquifers. This study indicates t h a t the anion exclusion effect should be considered as well.
282 ACKNOWLEDGEMENTS
This research was performed in partial fulfillment of the requirements for a Ph.D. degree of the senior author, at the Feinberg Graduate School at the Weizmann Institute of Science. We are grateful to Kibutz Glil-Yam for allowing the drilling of the research well and for supplying the background data. We also acknowledge the help of I. Carmi in the tritium laboratory and M. Collin for editing. This work was supported by grants from the Water Commission of the Israeli Ministry of Agriculture, and by the Seagram Center for Soil and Water Sciences (The Hebrew University of Jerusalem). REFERENCES Andersen, L.J. and Sevel, T., 1974. Six years environmental tritium profiles in the u n s a t u r a t e d and saturated zone, Gronhoj, Denmark. In: Isotope Techniques in Groundwater Hydrology, Vol. 1. IAEA, Vienna, pp. 3-20. Appelt, H., Holtzclaw, K. and Pratt, P.F., 1975. Effect of anion exclusion on the movement of chloride t h r o u g h soils. Soil Sci. Soc. Am. Proc., 39: 264-267. Athavale, R.N., Murti, C.S. and Chard, R., 1980. Estimation of recharge to the phreatic aquifers of the Lower M a n e r Basin, India, by using the tritium injection method. J. Hydrol., 45: 185-202. Ashkenazy, Y. and Carmi, I., 1970. On-line computer in a low-level counting laboratory. Nucl. Instrum. Methods, 89: 125-130. Biggar, J.W. and Nielsen, D.R., 1967. Miscible displacement and leaching phenomena. In: R.M. Hagen (Editor), Irrigation of Agricultural Lands. Agronomy, 11: 254-272. Bolin, B., 1958. On the use of tritium as a tracer for water in nature. Proc. 2nd U.N. Conf. of Peaceful Uses of Atomic Energy, Geneva, 18: 33~342. Bowman, W.W. and Hughes, B.M., 1981. Proportional counting techniques for routine tritium analysis at environmental levels. In: Methods of Low-Level Counting and Spectrometry. IAEA, Vienna, pp. 352-358. Bresler, E., 1970. Numerical solution of the equation for interacting diffuse layers in mixed ionic system with nonsymmetrical electrolytes. J. Colloid Interface Sci., 33: 278-283. Bresler, E., 1973. Anion exclusion and coupling effects in nonsteady t r a n s p o r t through u n s a t u r a t e d soils: 1. Theory. Soil Sci. Soc. Am. Proc., 37: 663~o69. Bresler, E., McNeal, B.L. and Carter, D.L., 1982. Saline and Sodic Soils. Springer-Verlag, Berlin, 236 pp. Carmi, I. and Gat, J.R., 1973. Tritium in precipitation and freshwater sources in Israel. Isr. J. E a r t h Sci., 22:71 92. Cameron, J.F., 1967. Survey of systems for concentration and background counting in water. In: Radioactive Dating and Methods of Low-Level Counting. IAEA, Vienna, pp. 543-573. De Haan, F.A.M., 1965. The interaction of certain inorganic anions with clays and soils. Centrum voor Landbouwpublikaties en Landbouwdocumentatie, Wageningen, 167 pp. Dyer, K.L., 1965. U n s a t u r a t e d flow phenomena in Panoche sandy clay loam as indicated by leaching of chloride and nitrate ions. Soil Sci. Soc. Am. Proc., 29: 121-126. Gat, J.R., 1970. Environmental isotope balance of Lake Tiberias. In: Isotope Hydrology. IAEA, Vienna, pp. 109-127. Gat, J.R., 1980. The isotopes of hydrogen and oxygen in precipitation. In: P. Fritz and J.Ch. Fontes (Editors), Handbook of Environmental Isotope Geochemistry, Vol. 1. Elsevier, Amsterdam, pp. 2147. Gvirtzman, H. and Magaritz, M., 1986. Investigation of water movement in the u n s a t u r a t e d zone under irrigated area using environmental tritium. Water Resour. Res., 22: 635~42. I n t e r n a t i o n a l Atomic Energy Agency (IAEA), 1981. Statistical treatment of environmental isotope data in precipitation, IAEA, Tech. Rep. Ser., No. 206, Vienna.
283 Kemper, W.D., 1961. Movement of water as effected by free energy and pressure gradients, 1. Application of classic equations for viscous and diffusive movements to the liquid phase in finely porous media. Soil Sci. Soc. Am. Proc., 25: 255-260. Krupp, H.K., Biggar, J.W. and Nielsen, D.R., 1972. Relative flow rates of salt and water in soil. Soil Sci. Soc. Am. Proc., 36: 412-417. Lapidus, L. and Amundson, N.R., 1952. Mathematics of adsorption in beds. J. Phys. Chem., 56: 984~988. Mercado, A., Avron, M. and Kahanovich, Y., 1975. Groundwater salinity of the Coastal Plain - - chloride inventory and assessment of future salinity trends. Tahal Rep., No. 01/75/22, 68 pp. (in Hebrew). Mokady, R.S., Ravina, J. and Zazlavsky, D., 1968. Movement of salt in saturated soil colums. Isr. J. Chem., 6: 159-165. Raveh, Y., Avnimelech, Y. and Saliternik, C., 1972. Nitrate concentration in the soil layer above the Coastal Plain Aquifer. Tahal Rep., No. HR/72/073, 149 pp. (in Hebrew). Ronen, D., Kanfi, Y. and Magaritz, M., 1984. Nitrogen presence in groundwater as affected by the u n s a t u r a t e d zone. In: B. Yaron, G. Dagan and J. Goldshmid (Editors), Pollutants in Porous Media. Springer-Verlag, Berlin, pp. 223-236. Ronen, D. and Magaritz, M., 1985. High concentration of solutes at the upper part of the saturated zone (water table ) of a deep aquifer u n d e r sewage irrigated land. J. Hydrol., 80: 311-323. Schofield, R.K., 1947. Calculation of surface areas from measurements of negative adsorption. Nature (London), 160: 408-410. Smith, D.B., Wearn, P.L., Richards, H.J. and Rowe, P.C., 1970. Water movement in the u n s a t u r a t e d zone of high and low permeability s t r a t a by measuring n a t u r a l tritium. In: Isotope Hydrology. IAEA, Vienna, pp. 73-87. Smith, S.J., 1972. Relative rate of chloride movement in leaching of surface soils. Soil Sci., 114: 259-263. Taylor, C.B., 1981. Present status and trends in electrolytic enrichment of low-level tritium in water. In: Methods of Low-Level Counting and Spectrometry. IAEA, Vienna, pp. 303-323. Thomas, G.W. and Swoboda, A.R., 1970. Anion exclusion effects on chloride movement in soils. Soil Sci., 110: 163-166. Water Commission of Israel, 1982. Sewage collection t r e a t m e n t and utilization survey (1982). Dep. of Sewage and Eit~luent Utilization for Agriculture, Water Commission, Ministry of Agriculture, Jerusalem, 149 pp. (in Hebrew). Warrick, A.W., Biggar, J.W. and Nielsen, D.R., 1971. Simultaneous solute and water transfer for an u n s a t u r a t e d soil. Water Resour. Res., 7: 1216-1225. Zimmermann, U., Miinnich, K.O. and Roether, W., 1966. Tracers determine movement of soil moisture and evapotranspiration. Science, 152: 346-347.
267
[2] ANION EXCLUSION DURING UNSATURATED ZONE
TRANSPORT
THROUGH
THE
HAIM GVIRTZMAN1, DANIEL RONEN 1'2 and MORDECKAI MAGARITZ1 1Isotope Department, The Weizmann Institute of Science, 76100 Rehovot (Israel) 2Research Department, Hydrological Service, Water Commission, P.O.Box 6381, 91063 Jerusalem (Israel)
(Received December 28, 1985; revised and accepted May 10, 1986)
ABSTRACT Gvirtzman, H., Ronen, D. and Magaritz, M., 1986. Anion exclusion during transport through the unsaturated zone. J. Hydrol., 87:267 283. A 20-year chronological record of the flow of water and anions, along 27 m, in the unsaturated zone of a phreatic aquifer was reconstructed. Water was traced according to its tritium content, using the difference between the environmental tritium content of rain and irrigation water. Anions were traced using data about their sequential input to the overlying cultivated field and their concentration in the profile. Evidences of anion exlcusion were found along a 10-m thick clay loam layer. The vertical velocities of water molecules and anions (SO2- and C1- ) were calculated to be 0.7 + 0.05 and 1.35 _+ 0.05myr 1, respectively. The average thickness of the water films and the equivalent distance of exclusion, at a 15% gravimetric water content, were 25 and 12A, respectively. These field data fit and support the theoretical relationship between the water film thickness and the relative exclusion concentration proposed by Bresler (1973). INTRODUCTION The transport of solutes through the unsaturated zone to the water table of a p h r e a t i c a q u i f e r is a c o m p l e x p r o c e s s w h i c h d e t e r m i n e s g r o u n d w a t e r q u a l i t y . Most studies on this topic have been conducted in the laboratory. The process o f e x t r a p o l a t i o n o f r e s u l t s t o a c t u a l field s i t u a t i o n s is d o u b t f u l . O n t h e o t h e r hand large distances and travel times are required to simulate natural processes. T h e r e f o r e i t is d i f f i c u l t t o c o n d u c t i n - s i t u e x p e r i m e n t s . T h e o b j e c t i v e o f t h i s w o r k is t o p r e s e n t a field s t u d y w h i c h h a s a t t e m p t e d t o o v e r c o m e t h e a b o v e difficulties. In natural systems water molecules artificially tagged with tritium have been used to investigate water movement in the unsaturated zone (Zimmerm a n n e t al., 1966; A t h a v a l e e t al., 1980). T r i t i u m i n r a i n w a t e r w a s s u c c e s s f u l l y used for this purpose, mainly in the sixties. At that time, large amounts of tritium produced by thermonuclear tests were injected into the atmospheric m o i s t u r e ( B o l i n , 1958; G a t , 1980; I A E A , 1981). T r i t u m c a n b e u s e d a s a t e m p o r a l marker utilizing data on its atmospheric pulse and calculating its radioactive d e c a y ( h a l f - l i f e o f 12.43 yr). R e c h a r g e q u a n t i t i e s a n d v e r t i c a l flow r a t e s w e r e
268 determined by detecting the penetration depth of the 1963 tritium peak in soil moisture (Smith et al., 1970; Andersen and Sevel, 1974). The profile of soil moisture created by alternate sources of water that differ in their tritium content was also used as a long-term record of water transport (Gvirtzman and Magaritz, 1986). Tritium is a component of water molecules and therefore an ideal tracer of their movement. However, the transport of water and solutes is not necessarily synchronic. Specifically, the transport of anions in porous media is affected by "anionexclusion" (Bresler et al., 1982). Anions are repelled from negative clay surfaces by electrostatic forces and they are concentrated in the center of the pores where the velocity is relatively faster. Hence, in some cases anions move faster than the average velocity of the water molecules. Laboratory studies have demonstrated this phenomenon (Mokady et al., 1968; Thomas and Swoboda, 1970; Warrick et al., 1971; Krupp et al., 1972). In leaching experiments using 15 widely varying soils and 0.01 N CaC12, Smith (1972) observed that chloride moved 1.04 to 1.67 times faster than the average velocity of the water molecules. Dyer (1965) estimated that the equivalent anion-free volume for chloride and nitrate was about 50% of the moisture content near the wilting point. The phenomenon of anion exclusion is related to the nature of the clay minerals, clay content, saturation percentage and solute concentration (Appelt et al., 1975). Anion exclusion models have been developed for quantitative analysis. In these models the distribution of the anion concentration with distance from a negative colloid surface is calculated according to the diffuse double layer theory (De Haan, 1965; Bresler, 1970). The equivalent distance of exlcusion is estimated according to the surface charge density of the clay mineral, the equilibrium soil moisture concentration and the structure of the hydration envelope of the anion under consideration (Schofield, 1947). The volume of the anion-free solution is calculated based on the distance of exclusion, the specific surface area of the medium material and the volumetric water content (Bresler, 1973). The ratio between the volume of the anion-free solution to the total liquid volume determines the relationship between the average velocities of water and the anions. Under controlled laboratory conditions, the simultaneous transport of anions and water can be predicted quantitatively by linking the above models to dispersive convective equations (Biggar and Nielsen, 1967). Unfortunately, the high variability found in the field and the many approximations which are necessary make the use of these calculations of limited practical value. This study presents an in-situ field record from the unsaturated zone of a deep phreatic aquifer. By independent tracing of water and anions the different rates of movement of water, chloride and sulfate were determined and the anion exclusion effect was quantified. The anions were traced using data about their input; water was traced by variations in tritium levels.
269 MATERIALS AND METHODS
Site description A research well (WT-2) was drilled, down to the water table, in a cultivated field of Kibutz Glil-Yam, 10 km north of Tel-Aviv (Ronen and Magaritz, 1985). The field is located on the replenishment area of the sandy, phreatic Coastal Plain Aquifer of Israel. A production well (Glil-Yam B) is located 1200m northeast of the research well. Winter precipitation ranges from 350 to 750 mm, with an average of 600 mm. During some summer seasons (April to September) the field was irrigated with 450 mm of water applied by the sprinkler method. Between 1967 and 1982 municipal sewage effluents were utilized as a water source for irrigation. Before 1967 and after 1982 local groundwater was used. The sewage effluents derived from the supply of local groundwater to the nearby city of Hertzlia (Ronen and Magaritz, 1985). Sewage had been treated by passage through an oxidation pond before 1977, and subsequently by an extended aeration treatment plant. Potatoes, peanuts and cereals were alternately cultivated at the area under investigation. Fertilization mainly involved application of ammonium-sulfate. Prior to 1967, the field had been used only for growing cereals. Two sedimentary units characterize the profile at the site where the research well was drilled: (i) a 10-m-thick clay loam layer and (ii) underlying homogeneous sand and sandstone sediments that extended down to the water table at a depth of 27.5 m (Fig. 1). The top soil is composed of a 0.9-m layer of sandy loam. The clay loam unit contains 15% clay minerals which are essentially montmorillonite (Mercado et al., 1975). ........ O! F '"
[ '"
Soil
a
..... I
'~'1.... el''' i
0
Clay Loam
E
i[
i
x= c. a)
c B 20
¢" 2(
Sand
and
Sandstone
.... I .... I .... .... I,,, 5 I0 15 20 Moisture (%)
25
Lithology
.... io'"~O"iO ~'4o 'so3° Tritium (T.U.)
Fig. 1. Gravimetric water content vs. depth (a), and tritium concentration vs. depth (b). The peaks A, B and C are discussed in the text. The lithological profile is shown in the center.
270
Sampling The research well was drilled by a spiral-driller dry method without addition of water and samples were collected at 50-cm depth intervals. Two kg of each sample were preserved with dry ice for tritium analyses. An additional amount of each sample was introduced in pre-weighted glass vials for moisture content and anions concentration measurements. Samples were also collected from the nearby production well, Glil-Yam B, and from effluents of the sewage treatment plant.
Analytical methods Water for tritium analyses was extracted from each sample in a vacuum distillation system. The tritium concentration was enriched electrolytically (Cameron, 1967; Taylor, 1981), converted to ethane and analyzed with a lowlevel counting system (Ashkenazy and Carmi, 1970; Bowman and Hughes, 1981). Tritium concentration is expressed in tritium units (T.U.) where 1 T.U. corresponds to 1 atom of tritium per 1018 hydrogen atoms. The gravimetric water content was calculated by drying to a constant weight at 105°C. Anions were extracted with distilled water from 15 g sediment samples after shaking for 20min. The weight ratio between water and dry-soil in all the extractions was 1.33. The solutions were centrifuged for 10 min at 10,000 RPM and filtered through a 0.45/~m micropore filter. CI-, NO[ and SO 2- concentrations were analyzed by HPLC with a Wescan 261 Ion Analyzer with a precision of 4%. The results were recalculated on the basis of the field moisture content. RESULTS The gravimetric water content along the unsaturated zone reflects the two sedimentary units which form the geological column (Fig. la). The clay loam and the sandy sediments contained about 15 and 5~/o water, respectively. The water table in the well was found at a depth of 27.5 m and a capillary fringe was identified up to 1.5 m above it. The tritium content of water along the profile is shown in Fig. lb. The tritium concentration varies between 14 to 24 T.U. in the clay loam layer. Concentration levels between 20 to 45 T.U. were detected in the sandy sediments. At the capillary fringe, tritium decreases from 22 T.U. to 11 T.U., which is the tritium content of the saturated region at the top of the aquifer. The tritium content in groundwater of Glil-Yam B pumping well ranged between 3 to 11 T.U. The concentrations of Cl-, NO~ and SO~- along the unsaturated zone fluctuate between 4-18, 3-10 and 2-22 meq 1-1 respectively (Fig. 2). In order to analyze the results obtained from the unsaturated zone, historical data concerning the agricultural activities and the amounts of rain, irrigation and fertilization over the period 1967 to 1984 were collected (Table 1). The tritium content of the sewage effluents used for irrigation, was estimated according to the original tritium concentration of the pumped groundwater
271
% E 1~
lo
.6.
~
20
%
% x~
2
[
a
....
b
,,
s ....
C1- (meqxl -])
;
c
oi,,,,~.,I,,,,I,,,,I.,,/O,,O,,, , o
5'"
NO~ (rneqxl I)
SO, =
,10 ~,,,
,,,,,~/,,,2~o 1~
(meqX]
')
Fig. 2. Chloride (a), nitrate (b) and sulfate (c) along the unsaturated profile in research well WT-2.
and the e x c h a n g e w i t h tritium c o n t a i n e d in atmospheric moisture w h i c h w o u l d h a v e t a k e n place during aeration and storage. The tritium c o n c e n t r a t i o n of local g r o u n d w a t e r (Glil-Yam B well) varies b e t w e e n 3 to 11 T.U. The residence time of s e w a g e in the treatment plant (including aeration and storage before TABLE 1 Water and fertilizer inputs C~pl Ye~
Se~
2
Water Input "l~ypes
F~tiliserJ (k¢ ha-t}
Tritium (T.U,) s
(NH4)~SOt
161e
18
1S4~3
1984
W
Pot
,.
w ~
P...~
~
,o 14
S
_ Pot
R
_ 483
_ 13
1500
1981
SW
Pot
R
6O6
IS
1500
1980
S
Pea -
S
~4~
11 17
482
23
4.57
*4
1982
R
A m m m t (ram) 4
-
1973
W S
C~r
R
462
3-I
1971
~/
Pe._
RS
4~
6416
p:,
~
,k
;,
,9,o w s
400
CO(NH2)2
IO0
,~o
p o t = P o t a t o a , P e t = P e ~ m t s , C~C4u-el0a; Dat* from Kibuts GliI-Ym, = W=Wiater, S=Summ~. s R = R m , G ~ P a m p e d Gmnnclwate~. $=Sewale. Data on rai= ~ u u a f r ~ K f ~ Shmsryshu Meteorolotka] St&tioa. Irrigation amounts from Kibuts G l i l - y ~ . s D t t a from the W a t ~ Library of the Gmlaotop* G m v p at the W e l s m ~ n Institute. • Rain m o u n t till 10.1,84 ( 8 ~ p l i n g date).
s
272 300
........
I .... I .... Pa~h~
250
r ''1
= Tritium in rainwater
Solid
= Corrected
1970
i975
for decay
200
150
E i00
E E50
~P
i
1960
1965
1980
Date (Years) Fig. 3. Tritium in rainwater at the Coastal Plain of Israel (data from the Water Library of the Geoisotope Group at the Weizmann Institute of Science).
irrigation ) is about I week. The vapor exchange can be calculated according to the relative humidity, the amount of evaporation during the period available for exchange and the tritium content of atmospheric moisture (Gat, 1970). In 1984, this exchange enriched the tritium content of the effluents by about 1-2 T.U. In the sixties, when tritium in atmospheric moisture was higher (Fig. 3), this exchange could have enriched the effluents by up to 10 T.U. Exchange with tritium of atmospheric moisture during municipal water use is neglected because of the relatively short times involved. Fertilizers and sewage utilized for irrigation contributed the major part of the input of chemicals to the cultivated field. Additional amounts were supplemented by rain and groundwater. The average concentrations of chloride, nitrogen and sulfate in rainwater, groundwater and sewage are presented in Table 2. The nitrogen content of sewage effluents changed in 1977 when an extended aeration treatment plant replaced the old oxidation ponds (Water Commission of Israel, 1982). The chloride concentration of sewage changed with time as a result of the continuous salinization of the Coastal Plain aquifer of Israel (Ronen and Magaritz, 1985). The dry fallout (mainly from seawater spray) amounts about 10% of the salt content in rain and is not taken into consideration. DISCUSSION Tritium is a hydrogen isotope which comprises a part of every population of water molecules; its percentage representation among these molecules acts as an effective identification indicator for differing water sources. For each water source during a particular time span a distinctive bracket of representation concentrations may be noted. Its variation along the unsaturated profile represents the alternating input of water sources which penetrated the land
273 TABLE 2 Average chloride, nitrogen, sulfate and tritium content of water sources Input
C1- (meq
Rain 1 Groundwater (1980-1984) Sewage 2 (1978-1984) Sewage 3 (1970-1977) Sewage 4 (1965-1969)
0.54 4.11
0.02 0.72
0.55 0.62
See Fig. 3 3-11
6.90
1.50
0.80
10-13
6.76
4.50
0.80
14-15
5.92
4.50
0.80
16-20
1Data 2Data a Data 4Data
x
1-1)
N (meq
x
1-z)
SO2- (meq
x
1-1)
Tritium (T.U.)
from Mercado et al. (1975). supplied by the treatment plant laboratory, Hertzlia. from the record of Israel Ministry of Health. from Raveh et al. (1972).
-
R = Rain S = Sewage
--
R 7a R ~a
~
I
l o ~ R 710 i
!
S?I ~ ~ s aa
:i: "°°
20 0
R 64
s 6a
R ~:'. R e~
S e2
' ~210
?u
z~ 165 TU
7
20 Tritium
40
60
80
100
(T.U.)
Fig. 4. Tritium input function - - Accumulated input amount of irrigation and rainwater according to their sequence during the last 23 yr vs. their tritium content. The volumetric datum line (zero accumulation) is based upon 1984 rainwater. Each segment represents the seasonal input and is characterized by the corresponding water amount (ordinate) and by its tritium content (abscissa), s u r f a c e ( G v i r t z m a n a n d M a g a r i t z , 1986). T h e l a y e r s w i t h h i g h a n d l o w t r i t i u m content are related to winter rains and irrigation water, respectively. Two c h a r a c t e r i s t i c s d e t e r m i n e the shape of the t r i t i u m peaks on Fig. lb: (1) T h e w i d t h w h i c h is d e t e r m i n e d b y t h e n e t a m o u n t o f r a i n w h i c h percolated after evapotranspiration. (2) T h e h e i g h t w h i c h is d e t e r m i n e d b y t h e c o n c e n t r a t i o n o f t r i t i u m i n t h e
274 corresponding precipitation, and by some other processes which change this concentration (e.g., radioactive decay and mixing). Since samples were collected at 50-cm depth intervals, they were not necessarily in phase with the summer and winter water layers. Each sample represents an unknown mixture of irrigation and rainwater and the measured concentrations are average values of the sampling intervals. Thus, the tritium amplitude is smaller t h a n what would be obtained by a more detailed sampling scheme. Fig. 4 summarizes the chronological sequence of water applied at the research area. We define this figure as the "tritium input function".
Identification of water layers Dating of water layers in the unsaturated zone is possible by comparing the peaks of tritium along the profile with the tritium input function (Figs. 1 and 4, respectively). Our interpretation is presented in Fig. 5. It is based on the following two principles: (1) Tritium content of water in the sediments can not be higher than the tritium content of the rainwater of the suggested year. Radioactive decay and mixing could only reduce the tritium content of the rainwater while percolating downward. The increase of tritium content by isotopic fractionation during evapotranspiration is negligible (Zimmermann et al., 1966). (2) Water amount of a layer in the profile should fit the input water amount of the suggested year (or years) according to the evapotranspiration fraction. Based on the first principle, an initial rough dating of water layers can be achieved. The two peaks of 45 T.U. at 24 and 21 m depth, in the sandy unit (A and B - - Fig. lb) can be related only to winter rains which fell between 1962 and 1966. This rough identification supplies a good approximation about the ,
0
10
I ~
.¢
^ ~ @
I
~.~
R 69 or R 70/71 ~65 or R 89 67 or R 56 R66oR 66 or R 67
~ 1 ~
20
20
Reoor~6s
R 64 or R 65
3o
,
"' '
20
~~
I 40
,
I , 60
30
Tritium (T.U.) Fig. 5. Identification of rainwater peaks and the calculated water velocities (V) for the two lithological units.
275 time scale involved and the amount of yearly replenishment. The water contained in the profile down to 25 m depth is thus related to the water input of the last 19 to 23 yr. The amount of water in this segment is 3100 mm (see calculation below), which means a yearly replenishment of about 150 mm. More than that, considering the water amount of the two sedimentary units (2050 and 1050 mm for the clay loam and the sand, respectively), we concluded that the water contained in the clay loam unit represents input of 14 _+ 1 yr and that of the sandy unit of 7 + 1 yr. A more accurate identification of the tritium peaks along the profile was achieved by dating the peak of 40 T.U. at 12 m depth (C - - Fig. lb). This peak is the upper one of the sandy unit and according to the above rough dating, is related to a winter rain which fell in the late sixties or the early seventies. Two alternative identifications may be reached according to the above two principles (Fig. 5): (1) Based on tritium level considerations, this peak is related to 1969 rainwater (Fig. 4). (2) Based on the water amount, this peak is related to 1970-1971 rainwater (there was no irrigation during the summer of 1970; Table 1). This peak extends between 10.5 and 13.5 m depth and contains a large quantity of water (240 ram). It may be more reasonable to assume t h a t such a large amount of water was left after the evapotranspiration of two winter seasons, 1970-1971, rather t h a n from a single winter of 1969. Note t h a t the difference between the two possible identifications is only 1 yr. In the sandy unit the average yearly recharge is contained in a sediment 2 m thick. Two samples, from 18.5 and 19.0m, were lost during the analytical procedure. We suggest t h a t the water contained between 17 to 21m depth included a winter peak which was not detected. No large peaks of tritium were detected in the upper 10 m of the profile. This is a result of : (i) the small difference between the tritium content of rains and of irrigation water between 1970 and 1984 (Fig. 4); and (ii) the sampling frequency. A 50-cm-depth sampling interval is sufficient for detecting water layers in the sandy unit where, as explained before, the water input of each year is contained in a sediment segment of a thickness of about 2 m. In the upper clay loam unit the same amount of water is contained in a sediment layer of only 70 cm thick. Therefore, due to the sampling resolution, even existing peaks would not have been clearly detected in this unit.
Determination of the vertical flow rate The tritium peak A (Fig. lb) represents the rain input of 1964 or 1965. Therefore, the mean vertical velocity is 1.2myr 1. Due to the lithological differences found, the velocities can not be similar along the whole profile. The upper 10 m of clayey sediments contain the input water of the last 13 to 15 yr. Therefore, the average vertical velocity is about 0.7 _+ 0.05myr i. The lower 15 m of the sandy unit contains 6 to 7 yr water input, and thus, the mean rate
276 TABLE 3 Average amount of applied and recharged water* Profile unit
Input time (yr)
Profile water (mm)
Applied water (mm)
Recharge fraction (%)
Yearly application (ram)
Yearly recharge (mm)
Clay Sand
13-15 6-7
2050 1050
9700 + 700 7200 + 700
21 + 2 15 + 2
700 1100
150 _+ 10 160 + 10
* Calculations of the range ( + ) depends on the two possible identifications of the peaks.
of water movement is 2.3 + 0 . 2 m y r -1 (Fig. 5). Indeed, when the percolating water molecules cross the border between the clay loam and the sandy units their velocity increases about 3 times. In consistency with the law of continuity, the threefold decrease in the sediment water content causes a threefold increase in the vertical water flow rate. The vertical water velocity of the clay loam unit is similar to t h a t found in the loess sediments in a semiarid zone near Be'er-Sheva (Gvirtzman and Magaritz, 1986).
Calculation of aquifer replenishment The amount of water contained in the unsaturated zone down to a depth of 25 m is 3.1 m. This was calculated, using the bulk densities of the sediments (1.35 and 1.4 g cm -3 for the clay loam and sand, respectively) and the gravimetric water content profile (Fig. la). The amount of input water, 16.5 + 0.5 m, was obtained by summarizing the amount of rain and irrigation water from 1964/5 to 1984 (Table 1 and Fig. 4). The 3.1m of water presently contained in the unsaturated zone is 19% of the applied water. The remainder, 81%, evaporated from the land surface, transpired through the plants leaves, or was removed with crops. The identification of the tritium peaks enables one to calculate separately the average yearly recharge amount for the two periods represented by the water found in the two lithological units of the profile (Table 3). The differences found in the pattern of irrigation are presented in Table 1 and Fig. 4. During the period 1964 to 1970 the field under study was irrigated annually, whereas during the subsequent 14 yr it was irrigated only during 6 summers. Evapotranspiration is significant mainly in the summers. Therefore, the amount of water t h a t evaporated yearly between 1964 and 1970 is relatively higher t h a n t h a t between 1971 and 1984. Consequently, the recharge fractions are significantly different while the yearly recharge is similar.
Anions mass balance Comparisons were made between the chloride and sulfate amounts applied to the research field during 1964 to 1984 and the amounts found along 25 m of
277 TABLE 4
Comparison between input and profile anions content Anion
Calculated input Profile content
C1- ( e q h a - ] )
SO 2- (eqha -1)
420,000 287,000
330,000 267,000
the unsaturated zone. The amounts of applied anions were estimated from the amounts of applied water (rain and irrigation), the concentration of the anions in these water sources and the fertilizer input (Tables 1 and 2). The amount of anions along the unsaturated profile were calculated by integrating their respective concentrations according to the volumetric water content. The results are presented in Table 4. Since the amounts of chloride and sulfate actually found in the unsaturated zone are 30% smaller than the calculated input we conclude that the amounts of anions found in the profile represents a shorter input sequence than estimated. This large discrepancy can not be explained by processes like plant uptake or sulfate reduction. The system is aerobic as shown by the large concentrations of nitrate (Fig. 2). Thus, the rate of transport of water molecules and anions must be different. In order to date the peaks of the anion profiles we calculated the chronological sequence of chloride and sulfate applied to the research area, which we define as the "anion input function" (Fig. 6). o . . . . . . i .... ir.... I " ~
i'"i
,,
N~+F
"i .... r'"L
c~ R 83 Ra2~F RSI+P S 80 R 80
~
1
I
-
]
R'rg* F
R 78 R77*F
__
1
i -~.~ 7 . 5 ~
s ?4 R 74 R?a* P R '72 F s?l
I
--r' 12.
N
76
~
]
N69 +
i
E-~ L v
F
s 68 R68+F
] .... I.... I,~, o
2
4
6
Input CI- (meqxl ~1)
0
2
4
6
8
Input N (meq×l-l) Input
2
4 SO 4"
6 (r~eqxl
8 t)
Fig. 6. Anion input ftmction - - Accumulated input amount of irrigation and rainwater according to their sequence during the last 1 7 y r vs. their chloride, nitrogen and sulfate content. Zero
accumulation refers to 1984 rainwater. Each segment represents the seasonal input according to sources (water type and fertilizers). The scale on the right hand side indicates the a n n u a l anion source (R = r a i n ; G = G r o u n d w a t e r ; S = s e w a g e ; F = Fertilizer).
278
°r'°
. . . . . .
I ....
! . . . . ~' 't' ' ~~
I,'
-~:~
"1
-
~!
....
--
S76
0
10
S 74
m
2C
--
.,
R ?~
ra
8 ?0
i
V
0
....
S?[
20 1
i
& 30;
~
J
s 69 b
I .... i0
I .... 20
S04= (meqx1-1)
.... ] .... ] .... I .... i .... 3o
I,,
30
0
5
I0
15
20
25
Cl- (meqX[ -t)
Fig. 7. Identificationof sulfate (a) and chloride(b) peaks and their vertical velocities(V) in the two lithological units. The sulfate concentration along the profile (Fig. 2c) is similar to its input function (Fig. 6). Since the main source of sulfate is the fertilizer, ammoniumsulfate, it is possible to correlate the winter periods (when fertilizers are applied) to peaks in the unsaturated zone. The large peak at 5 m depth is related to the winter of 1979 (Fig. 7a). The minimum at 20 m depth is related to the winter rains of 1972-1974. The amount of fertilizers applied at t h a t period was small as compared to the inputs before and after (Table 1). The minor peaks of 11, 15, and 25m depths are related to the winters of 1977, 1975 and 1970, respectively (Fig. 7a). Note t h a t the sulfate concentration in the pore water (Fig. 2) is higher t h a n the concentration in the applied water (Fig. 6) as a result of evapotranspiration. A fivefold increase in the salt concentration resulted from the fact t h a t 19% of the applied water percolated downward, as was calculated above. In a similar way the chloride profile may be compared to its input function. Since the main source of chloride is sewage, we may correlate the summer periods (when sewage is applied) with peaks along the profile. The minima between 15 and 18m depth are related to the 3-yr period 1972-1974 without sewage irrigation. The two peaks below at 21 and 26 m depth reflect the chloride content in sewage applied in the summers of 1971 and 1969, respectively. The peaks at 5, 7, 11 and 14 m depths are related to the summers of 1980, 1978, 1976 and 1974, respectively (Fig. 7b). It is possible to identify sulfate and chloride peaks in the clay loam unit (where the tritium peaks were not clearly identified) due to the large differences found in the concentrations of these anions between water sources, and the expanded segment which represents a yearly input (7-8yr for sulfate and chloride, respectively, as compared to 13-15 yr for the water molecules). A corrected comparison between the input and the profile content of anions based on the above peak identification (Fig. 7), is summarized in Table 5. The
279 TABLE 5 Corrected comparison between input and profile anions content Anion
Calculated input Profile content
CI- (eqha -1)
SO 2- (eqha -I)
241,000 287,000
235,000 267,000
increased agreement between the amounts of sulfate and chloride supports the time-scale suggested in Fig. 7.
Anion velocity The chronological identification of the sulfate peaks (Fig. 7a) was used for determining its vertical velocity in the unsaturated porous medium. The clay loam unit (10 m thick) containing sulfate inputs of 7 yr (1978 to 1984), thus, involves an average velocity of 1.4 m yr- 1. The sandy unit (between 10 and 27 m depth), containing the inputs of the years 1970 to 1977, involves an average velocity of 2.1 m yr- 1. Similar velocities were calculated for chloride (Fig. 7b). The slight difference found between chloride and sulfate in the clay loam unit may relate to difference in their ionic charge and radius.
Evidence for anion exclusion In the upper clay loam unit the velocity of water molecules is calculated to be 0.7 + 0.05myr -1 (Fig. 5), whereas the anion velocities are computed as 1.3-1.4 m yr -1 (Fig. 7), almost double. This difference is a result of the anion exclusion effect. In fact, the anions found towards the bottom of the clay loam unit are encountered in water t h a t infiltrated through the soil surface about 5 yr previously. On the other hand, in the sandy unit, water and anions have almost the same velocity ( > 2 m y r - ~). The ratio between the unexcluded volume and the total volume of water is calculated by taking the ratio between water and anion velocities. This ratio is 0.7/1.35 = 0.51, and thus the unexcluded volume is 51 + 5% of the total water volume. An uncertainty of + 5°/0 in this estimation results from the uncertainty involved in estimating velocities of anions and water. The volumetric water content of the clay loam layer is 20% involving about 10~o unexcluded water volume. The average thickness of the water film surrounding soil particles can be calculated by Kemper's (1961) formula if one knows the values of the hydraulic conductivity of the unsaturated medium, its tortuosity and water content. If these parameters are not known one can estimate the average thickness of the
280 water films (b) by dividing the gravimetric water content (0g) by the specific surface area (A): b = 0~ A
(1)
This approximation is based on the assumption t h a t a water film is distributed uniformly over all particle surfaces. It should be noted that this assumption does not apply in a saturated medium because the majority of the solution flows only through the larger pores (Bresler et al., 1982). However, in unsaturated media this assumption may be valid because the larger pores are partially filled with air and the water distribution over the particles surfaces is more uniform. The gravimetric water content in the clay loam layer is 0.15 (Fig. la). The clay loam layer contains 15°//o clay minerals which are essentially montmorillonite (Mercado et al., 1975). The specific surface area ofmontmorillonite is 400 m 2g- 1. Therefore, the specific surface area of this layer is 60 m s g 1and the average film thickness of the water is 25 A. Considering t h a t the excluded volume of the water content is 49 + 5°//0, then the average equivalent distance of exclusion is about 12 A (which means about four layers of water molecules). Since the total anions concentration (C) at the surface of a clay particle is practically zero, the amount of surface excluded anions F - (in meq per cm 2 of surface) can be calculated, by multiplying the average anion concentration by the equivalent distance of exclusion (d). F-
=
Cd
(2)
In this instance, the surface excluded amount which corresponds to the measured solute concentration is 4 × 10-gmeq cm -2. To the best of our knowledge, there are no measured field values in the literature about the effect of anion exclusion on flow in unsaturated media. The available experimental data are only from saturated flow. Therefore, comparison between our measurements and other existing data can only be done by using the empirical relationship proposed by Bresler (1973). Bresler plotted the relative exclusion concentration (f) as a function of b v ~ (where b is the water-film thickness in A and C is the total solute concentration in eq 1 1). Figure 8 shows Bresler's theoretical curve (1973) and the saturated-flow experimental data reported by: (i) Mokady et al. (1968) for kaolinitic and illitic clay soil; (ii) Thomas and Swoboda (1970) for montmorillonitic clay soil; and (iii) Krupp et al. (1972) for montmorillonitic clay soil. In our case f = 0.5, b = 25 A and C = 0.03 eq 1-1. The calculated data point (f = 0.5; bx/~ = 4.3) matches the theoretical line in a region where data were lacking previously. The obtained value demonstrates a case in an unsaturated medium where the water film thickness is relatively small and the exclusion effect is large. S U M M A R Y AND CONCLUSIONS A record of 20 yr flow of water and anions along 30 m of the unsaturated zone was reconstructed, providing evidences for the anion exclusion effect under
281 Z O
......
I
.........
L
........
L
........
~
........
J
F-Z Ld (D Z O
~,1(5 2
! ~ . ~ ~T.AL~S~ 163 rt-
~_
• KRUPP ET AL. ( 1 9 7 2 7 -w ~ • THOMAS AND SWOBODA (19"?O)
,.
Io
Io2
Io3
•
IO"
b,E
Fig. 8. Relative solute exclusion concentration (f) as a function of b~-Cwhere C is expressed in meq1-1 and b in A (based on Bresler, 1973). field conditions. Water molecules were traced by their tritium concentration. Identification was possible since waters of two types (rain and irrigation water), which are different in their tritium content, were applied to the field. Anions were traced by matching their concentration in the pore water to their input amounts in fertilizers, sewage and other salt sources. These tracing methods can serve to quantify the leaching velocity of water, accompanying anions, and the volume of exclusion. For the Mediterranean climate zone and for the clay loam and sandy profiles t h a t were investigated, the following values were found: (a) the average replenishment of the aquifer is 150mm yr -1 which is 19% of the total amount of applied water; the amount of infiltration of irrigation water is largely reduced by evapotranspiration; (b) the vertical water velocity is 0.7 + 0.05 m yr 1and 2.3 + 0.2 m yr -~ in the clay loam and the sandy sediments, respectively; (c) the vertical velocity of chloride and sulfate is 1.35 + 0.05myr -~ and 2.1 m yr -~ in the clay loam and the sandy sediments, respectively; (d) the velocity of chloride and sulfate almost doubled as compared to the average velocity of water molecules in the clay loam unit due to the anion exclusion phenomenon; (e) the unexcluded volume in the clay loam unit, at a 20% volumetric water content, is 51 + 5~/o; the average thickness of water film in the unsaturated zone is about 25 A; and the equivalent distance of exclusion is about 12 A. This study suggests t h a t the anion exclusion phenomenon can not be neglected in the study of groundwater pollution rates. Any negatively charged pollutant ought to percolate much faster than water molecules through clay-rich sediments. At present the time of degradation of a pollutant and the time of water transport through the unsaturated zone are the criteria for allowing fertilization and sewage irrigation above phreatic aquifers. This study indicates t h a t the anion exclusion effect should be considered as well.
282 ACKNOWLEDGEMENTS
This research was performed in partial fulfillment of the requirements for a Ph.D. degree of the senior author, at the Feinberg Graduate School at the Weizmann Institute of Science. We are grateful to Kibutz Glil-Yam for allowing the drilling of the research well and for supplying the background data. We also acknowledge the help of I. Carmi in the tritium laboratory and M. Collin for editing. This work was supported by grants from the Water Commission of the Israeli Ministry of Agriculture, and by the Seagram Center for Soil and Water Sciences (The Hebrew University of Jerusalem). REFERENCES Andersen, L.J. and Sevel, T., 1974. Six years environmental tritium profiles in the u n s a t u r a t e d and saturated zone, Gronhoj, Denmark. In: Isotope Techniques in Groundwater Hydrology, Vol. 1. IAEA, Vienna, pp. 3-20. Appelt, H., Holtzclaw, K. and Pratt, P.F., 1975. Effect of anion exclusion on the movement of chloride t h r o u g h soils. Soil Sci. Soc. Am. Proc., 39: 264-267. Athavale, R.N., Murti, C.S. and Chard, R., 1980. Estimation of recharge to the phreatic aquifers of the Lower M a n e r Basin, India, by using the tritium injection method. J. Hydrol., 45: 185-202. Ashkenazy, Y. and Carmi, I., 1970. On-line computer in a low-level counting laboratory. Nucl. Instrum. Methods, 89: 125-130. Biggar, J.W. and Nielsen, D.R., 1967. Miscible displacement and leaching phenomena. In: R.M. Hagen (Editor), Irrigation of Agricultural Lands. Agronomy, 11: 254-272. Bolin, B., 1958. On the use of tritium as a tracer for water in nature. Proc. 2nd U.N. Conf. of Peaceful Uses of Atomic Energy, Geneva, 18: 33~342. Bowman, W.W. and Hughes, B.M., 1981. Proportional counting techniques for routine tritium analysis at environmental levels. In: Methods of Low-Level Counting and Spectrometry. IAEA, Vienna, pp. 352-358. Bresler, E., 1970. Numerical solution of the equation for interacting diffuse layers in mixed ionic system with nonsymmetrical electrolytes. J. Colloid Interface Sci., 33: 278-283. Bresler, E., 1973. Anion exclusion and coupling effects in nonsteady t r a n s p o r t through u n s a t u r a t e d soils: 1. Theory. Soil Sci. Soc. Am. Proc., 37: 663~o69. Bresler, E., McNeal, B.L. and Carter, D.L., 1982. Saline and Sodic Soils. Springer-Verlag, Berlin, 236 pp. Carmi, I. and Gat, J.R., 1973. Tritium in precipitation and freshwater sources in Israel. Isr. J. E a r t h Sci., 22:71 92. Cameron, J.F., 1967. Survey of systems for concentration and background counting in water. In: Radioactive Dating and Methods of Low-Level Counting. IAEA, Vienna, pp. 543-573. De Haan, F.A.M., 1965. The interaction of certain inorganic anions with clays and soils. Centrum voor Landbouwpublikaties en Landbouwdocumentatie, Wageningen, 167 pp. Dyer, K.L., 1965. U n s a t u r a t e d flow phenomena in Panoche sandy clay loam as indicated by leaching of chloride and nitrate ions. Soil Sci. Soc. Am. Proc., 29: 121-126. Gat, J.R., 1970. Environmental isotope balance of Lake Tiberias. In: Isotope Hydrology. IAEA, Vienna, pp. 109-127. Gat, J.R., 1980. The isotopes of hydrogen and oxygen in precipitation. In: P. Fritz and J.Ch. Fontes (Editors), Handbook of Environmental Isotope Geochemistry, Vol. 1. Elsevier, Amsterdam, pp. 2147. Gvirtzman, H. and Magaritz, M., 1986. Investigation of water movement in the u n s a t u r a t e d zone under irrigated area using environmental tritium. Water Resour. Res., 22: 635~42. I n t e r n a t i o n a l Atomic Energy Agency (IAEA), 1981. Statistical treatment of environmental isotope data in precipitation, IAEA, Tech. Rep. Ser., No. 206, Vienna.
283 Kemper, W.D., 1961. Movement of water as effected by free energy and pressure gradients, 1. Application of classic equations for viscous and diffusive movements to the liquid phase in finely porous media. Soil Sci. Soc. Am. Proc., 25: 255-260. Krupp, H.K., Biggar, J.W. and Nielsen, D.R., 1972. Relative flow rates of salt and water in soil. Soil Sci. Soc. Am. Proc., 36: 412-417. Lapidus, L. and Amundson, N.R., 1952. Mathematics of adsorption in beds. J. Phys. Chem., 56: 984~988. Mercado, A., Avron, M. and Kahanovich, Y., 1975. Groundwater salinity of the Coastal Plain - - chloride inventory and assessment of future salinity trends. Tahal Rep., No. 01/75/22, 68 pp. (in Hebrew). Mokady, R.S., Ravina, J. and Zazlavsky, D., 1968. Movement of salt in saturated soil colums. Isr. J. Chem., 6: 159-165. Raveh, Y., Avnimelech, Y. and Saliternik, C., 1972. Nitrate concentration in the soil layer above the Coastal Plain Aquifer. Tahal Rep., No. HR/72/073, 149 pp. (in Hebrew). Ronen, D., Kanfi, Y. and Magaritz, M., 1984. Nitrogen presence in groundwater as affected by the u n s a t u r a t e d zone. In: B. Yaron, G. Dagan and J. Goldshmid (Editors), Pollutants in Porous Media. Springer-Verlag, Berlin, pp. 223-236. Ronen, D. and Magaritz, M., 1985. High concentration of solutes at the upper part of the saturated zone (water table ) of a deep aquifer u n d e r sewage irrigated land. J. Hydrol., 80: 311-323. Schofield, R.K., 1947. Calculation of surface areas from measurements of negative adsorption. Nature (London), 160: 408-410. Smith, D.B., Wearn, P.L., Richards, H.J. and Rowe, P.C., 1970. Water movement in the u n s a t u r a t e d zone of high and low permeability s t r a t a by measuring n a t u r a l tritium. In: Isotope Hydrology. IAEA, Vienna, pp. 73-87. Smith, S.J., 1972. Relative rate of chloride movement in leaching of surface soils. Soil Sci., 114: 259-263. Taylor, C.B., 1981. Present status and trends in electrolytic enrichment of low-level tritium in water. In: Methods of Low-Level Counting and Spectrometry. IAEA, Vienna, pp. 303-323. Thomas, G.W. and Swoboda, A.R., 1970. Anion exclusion effects on chloride movement in soils. Soil Sci., 110: 163-166. Water Commission of Israel, 1982. Sewage collection t r e a t m e n t and utilization survey (1982). Dep. of Sewage and Eit~luent Utilization for Agriculture, Water Commission, Ministry of Agriculture, Jerusalem, 149 pp. (in Hebrew). Warrick, A.W., Biggar, J.W. and Nielsen, D.R., 1971. Simultaneous solute and water transfer for an u n s a t u r a t e d soil. Water Resour. Res., 7: 1216-1225. Zimmermann, U., Miinnich, K.O. and Roether, W., 1966. Tracers determine movement of soil moisture and evapotranspiration. Science, 152: 346-347.