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The relationship between the nitrate concentration and hydrology of a small chalk spring; Israel
Journal of
Hydrology ELSEVIER
Journal of Hydrology 204 (1998) 68 82
The relationship between the nitrate concentration and hydrology of a small chalk spring; Israel Avi Burg a'b, Tim H.E.
Heaton
c
alnstitute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel bGeological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel CNERC Isotope Geosciences Laboratory, Keyworth, Nottingham NGI2 5GG, U.K
Received 2 April 1997; revised 28 August 1997; accepted 28 August 1997
Abstract
Discharge from a spring draining a small, perched, Cretaceous chalk aquifer in the Upper Galilee, Israel, was monitored over a period of two years. The water has elevated nitrate concentrations, with ~5N/14Nand chemical data suggesting that it is a mixture of low-nitrate and high-nitrate end-members; the latter derived from the sewage of a centuries-old village served by septic tanks. Hydrograph data allowed distinction between fissure flow during the period of winter rainfall, and matrix drainage during the dry summer months. These different flow types, however, did not have markedly different nitrate concentrations: a 50-fold increase in spring discharge due to fissure flow, compared with matrix drainage, was reflected in only a 35% decrease in nitrate concentrations. The relatively high nitrate concentrations in the fissure waters suggests that they have had close contact with, and are possibly displaced from the matrix. This should help to accelerate the decline in the spring's nitrate concentrations following the recent completion of the village's central sewage drainage system. © 1998 Elsevier Science B.V. Keywords: Nitrate; Pollution; Chalk; Spring; Ground water; Isotopes
1. Introduction
More than 60% o f the total water supply in Israel is provided by groundwater, but the continued exploitation o f this resource is being compromised by a deterioration in its quality (Sehwarz, 1992). A large part o f the country is covered by Upper Cretaceous carbonates (Fig. 1). O f these, the limestones and dolomites o f the Judea Group form significant aquifers, and springs issuing from them have been important supplies o f water throughout history. The aquifers, however, are vulnerable to pollution by infiltration of surface contaminants, including nitrate derived from sewage - - many o f the villages, especially in the Galilee region o f northern Israel, having been served by old sewer systems. Immediately above the 0022-1694/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved Pll S0022-1694(97)00105-4
Judea Group, the chalks and marls of the Mount Scopus Group are generally considered to be aquitards or aquicludes (Vengosh and Rosenthal, 1994). Under ideal conditions such rocks might afford some protection from the infiltration o f surface contaminants into underlying aquifers. There is particular interest in the manner by which pollutants such as nitrate pass through chalk (Foster, 1993), and the effectiveness o f chalk as a hydrologic barrier to pollutants in Israel (Nativ et al., 1995). As part o f a range o f studies on the hydrology of carbonate aquifers (Burg, 1997), the chemistry of water from a spring issuing from chalks of the Mount Scopus Group in the Upper Galilee region of Israel (Figs. 1 and 2) was monitored over a period of 2 years. The spring, 'En Bardi, drains a small perched
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
69
STRATIGRAPHY
I .=-
FOR.ATIO.
r,, ~ ~ z ~ . < ~ ~ E i] -~ ~-
P_.
Timrat Taqiye f---~
z
~ ~
~]
|
'En Bardi ~
~
'En Zeti
Limonite ~
Z <
.
-'.,'-;,, y ,
B'ina
~
~
3g----------------~;-: s
Sakhnin
~
w _< < "" ~:< =
"
u~ o = z ~ ~- ~
1 . ~ 1 Marl ~
Chalk
~
Chert nodules
I'Ll
~
V
:~ ; - l c ~~ Deir Hanna
'En Kaukabe ',En Retet ,
-~" ~
i;~
Karst phenomena
~' .i,p ........
Yanuh
Z
Quartz geodes
~ ,,,,
., ,~, ,,,,,v,,,
O e.~
"<
Megafossils
~
7-
o
Bituminousrock Phosphatic rock
o ~ tad
Legend
Ghareb I
__~: ~° ~
LIT.OLOOY
~
En Neria ~
Yagur
I
~Vf'l Dolomite
~
Limestone
i
...... / . - / / 3. / /
/
='
." /
.;/ .:;F:
Rama
(Schematic and not to scale) Fig. 1. Compositecolumnarsectionof the Judea and MountScopusGroupsin the Galilee,withthe fourspringsmarkedin their stratigraphic positions. aquifer with a well-defined recharge area. Its hydrochemistry therefore largely reflects processes in the unsaturated zone, which is of particular importance in relation to controls on the migration of pollution through chalk. The nitrate concentrations displayed regular seasonal variations associated with changes in discharge, with a weighted mean nitrate concentration above the limit recommended for drinking water. With chemical and isotopic data we discuss the origin of the nitrate at 'En Bardi, and consider
how temporal changes in its chemistry can be related to the hydrology.
2. Study area and methodology 2.1. Spring sites
Four springs in the Galilee were studied: 'En Kaukabe, 'En Neria and 'En Retet, which drain
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
70
190
1711
D
m recharge
ti "talk te .d area age of Jish)
A b )
A'
- SW - Jish 19179/27000 / m/ /
'En Bardi
- NE 19251/27088 /m
I 750
750
/a.
o I
50o 18 81 Bituminous
z5o I
I
I
rock
I m
~"~
Marl
-~
Chalk
Fig. 2. (a) Locations of the four springs and the rainfall station at Neria field school, in the Upper Galilee, Israel. (b) Diagrammatic cros: section of the perched aquifer drained by the spring '"En Bardi'.
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
71
Table 1 Properties of chalks at 'En Bardi - - physical properties of hand specimens from outcrops at different heights above 'En Bardi. (Maximum stratigraphic thickness above 'En Bardi = ca. 101 m; analyses by Laboratory of the Geological Survey of Israel) Lithology
Height (m)
Porosity (%)
Permeability (millidarcy)
Density (g c m 1)
Bituminous chalk Chalk Chalk Chalk
5 52 71 86
17 18 31 36
0.03 1.13 0.03 0.03
2.10 1.82 1.70 1.58
southern part is occupied by the village of Jish (current population ca. 2200) which has been populated since at least the 12 th century and, up to the 1970s, discharged its sewage to septic tanks dug into the chalk. Since that time a central system draining sewage to the Wadi Gush Halav has been gradually introduced, with the last area of the village being completed in 1995 - - after the collection of samples reported here.
dolomites of the Judea Group; and 'En Bardi, situated in the Upper Galilee, draining chalks of the overlying Mount Scopus Group (Fig. 1). This report concentrates on the latter spring. The Upper Galilee experiences a Mediterranean climate characterised by precipitation in the winter (November-March), with a warm dry summer (April-September). Total precipitation amounts to ca. 850-900 mm year -~, and the mean annual air temperature is 15-17°C. 'En Bardi is a small perennial spring having the highest discharge of a group of five springs emerging on the western slope of the Wadi Gush Halav (Fig. 2(a)). The spring is located at a horizon where bituminous chalks lie on a marly aquiclude (Fig. 2(b)), and its perennial flow is from a perched water table. The topography and structural geology allow the recharge area to be fairly well defined (Fig. 2(a)). It is estimated to be no more than 1.5 km 2, and is composed of bituminous chalks, chalks and clayey chalks of the 'En Zetim and overlying Ghareb and Taqiye Formations (Fig. 1). Porosities of hand specimens representing the main rock types in the stratigraphic section above 'En Bardi range from 17 to 36%, and their permeability is low (Table 1). Numerous fractures, however, are visible where these rocks are exposed above the spring. The recharge area is partly cultivated (mainly olives), but not irrigated. Its
2.2. Sampling and analysis
The four springs were monitored and sampled once per week during the winter, and once per fortnight during the dry summer season, over a 2 year period from winter 1991/2 to spring 1994. Discharge was measured with pail and stopwatch. Water samples were collected directly after emergence from the rock, and stored refrigerated until filtering and analysis. Samples of 'En Zetim bituminous chalk were crushed and stirred for 60 h with deionised water to release water-soluble components. Cations and SO4-S were determined by atomic absorption and inductively coupled plasma spectrometry, NO3 and CI by colorimetry using Flow Injection Analysis, and HCO3 by titration with HC1. Analytical precision is estimated to be _+2% (_+5% for SO4-S).
Table 2 Properties of chalks at 'En Bardi - - concentration of water-soluble components, and 15N/14N ratio of water-soluble nitrate, in bituminous chalks from above 'En Bardi Sample
Br- 1 Br-2
meq per kg rock Na +
K÷
Ca 2+
Mg 2+
SO2-
CI
HCO 3
NO 3
Si
1.14 1.13
1.86 2.35
14.8 28.5
0.61 1.72
8.05 25.0
0.22 0.19
9.22 6.85
0.05 0.17
2.66 2.42
a 615N, in mil = [ ( R s a m p l e - Rstandard ) -- 1] x 103; where R b Proportion of rock insoluble in acetic acid.
=
15N/14N
and the standard is atmospheric N2.
~ 15N
Insoluble
(%0)"
(%) ~
- 1.9 -0.7
16.4 17.3
72
A. Burg, T.H.E. Heaton/Journal o f Hydrology 204 (1998) 6 8 - 8 2
Table 3 Data for 'En Bardi: discharge, 180/160 and D/H ratios, and the concentrations and 15N/14N ratios of nitrate Sample
Date
Discharge (1 min -1)
6180 (%0)"
6D (%o)a
NO3 (meq 1-l)
GAB/1 GAB/4 GAB/18 GAB/19 GAB/20 GAB/21 GAB/26 GAB/30 GAB/60 GAB/62 GAB/63 GAB/64 GAB/65 GAB/66
20/02/92 21/04/92 10/11/92 27/11/92 03/12/92 12/12/92 15/01/93 08/02/93 07/02/94 21/02/94 28/02/94 19/03/94 28/07/95 13/09/95
>220 5.7 2.9 4.6 10.5 5.4 >400 177 6.4 6.7 24 37
-6.0 -6.0 -6.0
-29 -25 -28
-6.2 -6.3 -6.1 -6.2 -6.2 -6.0 -6.1 -6.0 -6.1
-31 -30 -30 -29 -28 -31 -31 -30 -29
0.82 0.92 1.18 1.31 1.00 1.62 0.58 1.14 1.78 1.17 0.61 0.48 0.99 1.23
t5ISN (%o)b
+8.8
+7.2 +9.0 +8.4 +7.4
a 6D or 6180, in mil = [(Rsample-Rstandard)-I] × 103; where R = 2H/IH or lSO/160,respectively, and the standard is VSMOW. b See Table 1. D / H a n d 180/160 ratios o f w a t e r w e r e d e t e r m i n e d on hydrogen produced by reaction with zinc turnings at 500°C, a n d c a r b o n d i o x i d e e q u i l i b r a t e d at c o n t r o l l e d r o o m t e m p e r a t u r e , r e s p e c t i v e l y . S o l u t i o n s for 15N/14N a n a l y s i s w e r e c o n c e n t r a t e d b y rotary e v a p o r a tion, w i t h nitrate c o l l e c t e d as a m m o n i u m sulfate b y distillation w i t h M g O plus D e v a r d a ' s alloy, a n d N2
p r o d u c e d b y c o m b u s t i n g the d r i e d a m m o n i u m sulfate w i t h CuO. D/H, 180/160 a n d 15N/laN ratios w e r e d e t e r m i n e d in V G S I R A and O p t i m a m a s s s p e c t r o meters, and are r e p o r t e d in the usual (5 notation (Tables 1 - 4 ) . Analytical replication is typically _+ < 2%o, <0.1%o and <0.2%o, for 6D, 6180 and 615N, respectively.
Table 4 Data for 'En Kaukabe, 'En Neria and 'En Retet: discharge, and the concentrations and 15N/14N ratios of nitrate Spring
Sample
Date
Discharge (1 min 1)
NO3 (meq 1-l)
6 JSN (%o)~
•En Kaukabe
GAK/23-27 GAK/63
19/12/92-14/01/93 14/02/94
1.0-2.7 0.3
ca. O. 10
+4.3 b
0.14
+3.5
GAN/5 & 6 GAN/50 & 51 GAN/54 & 55 GAN/60 GAN/64 GAN/66 GAN/65 & 67
11/03 & 02/04/92 17 & 29/08/93 05 & 24/10/93 06/01/94 07/02/94 21/02/94 14 & 28/02/94
6.7-16 1.1-1.2 1.0-1.1 2.4 2.2 2.2 5.6-8.8
ca. 0.15 ca. 0.24 ca. 0.24
+4.8 b +3.3 b +3.6 b +3.9 +3.6 +3.9 +3.9 b
GAR/48-51 GAR/64 & 66 GAR/65
29/07 31/08/93 14 & 28/02/94 21/02/94
7.8-9.0 24-28 14
ca. 0.12 ca. 0.11
'En Neria
'En Retet
0.27 0.22 0.20 ca. 0.15
0.16
+4.0 b +3.5 b +3.3
a See Table 1. b615N/14N analysis of composite sample. Sample number and dates joined by " - " or " & " indicate that composite was of all samples in range, or the two samples, respectively. Discharge is the range during the indicated dates; nitrate concentrations are means of the individual samples.
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68 82
73
a) discharge (lit/min)
rain amount (mm) 140
discharge
120
I O0 1 O0
~°
~
40
10
~
20 '
i I,I,
, I,
o
, I
10
,ill
I
11 12 1992
I
3
date
,I I 4 5 ~month 1993 I
b) rain amount (ram)
discharge (lit/min)
140
120
/
1O0
discharge 10
6O
420 0 0
I, 10
i
'1 I
,,11
,
II, 11 1993
I
,
I 12
I date
,
2 1994
3 ~month !>
Fig. 3. Temporal relationship between discharge o f 'En Bardi (logarithmic scale), and rainfall measured at the Neria field school: (a) October 1992 to June 1993; (b) October 1993 to March 1994. Discharge was measured every one to two weeks; rainfall bars represent totals over three consecutive days.
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
74
a variety of flow regimes, the ranges in hydrogen and oxygen isotope compositions are very small, typically only 3%0 and 0.3%o, respectively (Table 3). In comparison, isotopic variations between rainfall events and between mean monthly rainfall in Israel commonly range over 40%0 in 6D and 6%0 in 6180 (Gat and Dansgaard, 1972; IAEA, 1990). The relationship between nitrate concentrations and discharge at 'En Bardi is shown in Fig. 4. There is a general pattern of low nitrate concentrations associated with seasons of high discharge during the wet winter months, and of high concentrations associated with low discharge during the dry summer months. However, while discharge varies by a factor of up to c a . 100, nitrate concentrations vary by a factor of only c a . 4. Comparing the intervals January-March 1993 and August-September 1992, for example (Fig. 4): average discharge was 50 times higher (150 cf. 3 1 rain-l), but nitrate concentrations were reduced by only 35% (0.7 cf. 1.1 meq l-l). Detailed examination also reveals additional features: (1) the lowest nitrate concentrations occur at the onset of a period of high discharge rates; 2) nitrate concentrations then rise progressively - - both during the period of high discharge, and during the ensuing dry season when a prolonged period of low, stable "base-flow" discharge is established; (3) at the end of the 1993
3. Results
Fig. 3 shows the temporal relationship between the discharge of 'En Bardi and the amount of rainfall measured at a station 6 km away (Neria Field School, Fig. 2(a)). When the summer dry period was broken by heavy rainfall in November 1992, the flow immediately started to increase, but only reached high levels in December after several large rainfall events. In the following year the first rains were relatively small and sparse, and the flow only rose after heavier rains in January. Since the discharge was only measured weekly, temporal relationships on a shorter time scale are difficult to assess. During the period of high discharge in the winter of 1992/93, however, the discharge rate did appear to respond quickly to large rain storms, and declined between rain storms (but not to base-flow levels). Hydrogen and oxygen isotopic compositions o f ' E n Bardi water are shown in Table 3. They fall within the range of values reported for other groundwaters in the same area of Israel and, in common with meteoricderived waters in the eastern Mediterranean, have a " d parameter" (d = 6D - 8 x 6180) of c a . 20 compared with the value of 10 typical of global meteoric waters (Gat and Dansgaard, 1972). Although the spring was sampled over a period of 2 years, under o
discharge
m
(lit/min)
(meq/lit)
NO
3.5
iiiiiiii
!ii!ii!iii!i
iiiiiii~
............
iiiiJii
IO0
:
iiiiiii
"
i!i!ill
z.s
10
-
~--- ;. ~ 0.5 ~
•
0.1
llI213141~IrtTIBIglI011~Ilzl
>
2 3
~ 1992
date
4
5 fl71a191~011~1121112 I
~ 1993
month
~
--
--
:~
1994
Fig. 4. Discharge (logarithmic scale) and nitrate concentrations of 'En Bardi from February 1992 to March 1994.
75
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
1.4 1.2 1.0 0.8 0.6 0.4 0.2
Na + . . . , . +
~
+
+
I
I
!
K÷
1.2
+
+
+ +
1.4 1.6
1.0 0.8
.if.~.:l~..ik ~+ +
"~'1" +
0.6 0.4 0.2 13"
I
E
I
~ + 4 + +
f
.i..~-Ik'~
I
I
Ca 2+ -
Cl"
1.2 1.0 0.8 0.6 " 0.4 " 0.2
+
I
I
SO~- .~..~+
1.0
+
+ 4"+
+
0.8 " ~
.6 0,4 0,2
I
Mg2+
1.2 1
I
-
I
.
!
++ +
I
I
HCO~
"~~+
-
0.8
+
0.6 0.4 0.2 0
I
I
I
0.5
1
1.5
2
NO 3 (meq.1-1)
0
I
I
I
0.5
1
1.5
NO 3 (meq.I q )
Fig. 5. Relationship between major ions and nitrate concentrations, for 64 samples of 'En Bardi water from February 1992 to March 1994.
dry season nitrate concentrations were fully restored to their levels at the end o f the 1992 dry season (see, also the 1995 analysis, Table 3); and (4) m a x i m u m nitrate concentrations occur at the beginning o f the rain season when discharge rates start to increase. Relationships between nitrate and other ions at 'En Bardi are shown in Fig. 5. Nitrate shows a marked positive correlation with Na, K, M g and
C1 (r 2 > 0.85), and to some extent with SO 4 (r 2 = 0.77). The nitrogen isotope compositions o f 'En Bardi (Table 3) and the other springs (Table 4) are shown as a function o f nitrate concentration in Fig. 6. The three dolomite springs have 6 ~SN values in the range from about +3 to +5%o. Samples from ' E n Bardi, the spring with the highest nitrate concentrations, have 615N values in the range +7 to +9%o.
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
76 +14
,
,
,
+12
,
._~
,
,
3
.........................
.-'"
+10 ,. ,._,
,
+8
,"
,,
_".~ _ ~ . . . . . . . . I-
~ .
-*
--"
2
,' ,,"
+6
J
+2 O
I
0
I
0.5
I
I
1.0
I
I
1.5
I
2.0
NO3(meq/1)
Fig. 6. Variations in the 15N/14Nratios (6 ~SNin %oversus atmospheric nitrogen) and concentrations of nitrate in water from four springs: solid diamonds = 'En Bardi; open diamonds = 'En Kaukabe, 'En Neria and 'En Retet. Broken lines show the calculated compositions of waters resulting from mixing a low-nitrate end-member, having a composition similar to the springs shown as open diamonds, with different highnitrate end-members (see text).
4. Discussion
4.1. Hydrology The data in Fig. 3 indicate that the earliest rainfall, at the very beginning of the winter wet season, does not result in an immediate substantial increase in discharge. During the wet season, however, peaks in discharge do apparently occur soon after heavy rainfall. This pattern suggests that for the earliest rainfall the infiltrating portion (i.e. that part not lost by evaporation/transpiration and runoff) is taken up in saturating the sub-surface field capacity. Thereafter, during the remainder o f the wet season, the response of discharge to rainfall is more immediate. Because the 6D and 6180 values lie on the local meteoric water line there is no evidence that the spring water represents recharge o f rainfall which has undergone significant partial evaporation at the surface (Gat, 1980). Any significant evaporative losses which do occur either involve complete drying out o f the surface, or loss o f sub-surface water by transpiration. With the cessation o f rainfall in the spring, the discharge declines. For an isotropic system a
"depletion" on the spring hydrograph, under conditions o f zero recharge, can be described by a relationship of the form (Mero, 1964):
Qt = Qo e x p [ - t/to] where Q0 and Ot are the discharge rates o f the spring at the start o f drainage and at time t, respectively; and to is a depletion time characterising the physical properties o f the aquifer - - the amount o f time it would take to drain the aquifer (with no replenishment) if the discharge rate was constant at Q0. A semi-log plot o f discharge (log) versus time (linear) would yield a straight line o f slope related to -0.43/t0. In carbonate aquifers, however, such semi-log plots tend to be curved (cf. Fig. 7), and may be regarded as a summation o f two or more single exponential curves, i.e.
Ot = O0t e x p [ - t/tot ] + 002 e x p [ - t/to2 ] +... where Q0], Q02, etc., are the initial discharge rates of the individual flow components; and t01, t02, etc., are the characteristic depletion times o f these components. Following Mero (1964), graphical analysis of the semi-log plot of discharge versus time for 'En
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
77
o discharge (lit/rain)
100
i
100 o" I
10]
I I0
i 02
1
oi
,,.,.~,..,,,,..,...,.,o,,,,,2,
IC>
o2
j
,,~,,,,,,,,,,,o,,,,o,,,,,,,,,,-°~,
1>
months. Analysis based on methods of Mero (1964) suggests depletion patterns can be approximated by three components, with depletion times of: t03 = 0.25 month; to2 = 1 month; t0t = 50 months.
--0"--%0.oi (1992)
0 %0.o' (1993)]
o %2(,993) I % of discharge
~)-o3
(1993]
(1992) "-~-%Qo3
100 1 ~
100
80
80
60
60
40
40
20
20 E
0
Iz
3
4
S
6
r
8
1992
~
io
,t
iz
t
~
date
~
4
s
6
r
e
~
io
H
month
1993
Fig. 8. The proportion (%) of each one of the different flow components of Fig. 7, during the dry summer months.
Bardi, for depletion during two dry seasons, is shown in Fig. 7. The depletion in discharge can be described by the sum of three components: t03 and to2 having short depletion times of c a . 0.25month and 1 month, respectively, and t01 with a long depletion time of c a . 50 months. This separation of the depletion curves allows the total flow of water at a particular time (Qt), during the dry season, to be apportioned among the three component flows (Fig. 8). Discharge of Q03 and Q02 represents flow through the chalk via fissures of different width, and is the dominant flow during the period of winter rainfall and at the beginning of the dry season. The rapid depletion of Q03 and Q02 during the dry season (Fig. 8), together with the rapid response of spring discharge to rainfall during the wet season (Fig. 3), testify to these fissures being a small but highly transmissive reservoir. By May, about two months after the end of the main season of rainfall, the greatly reduced discharge is mainly derived from Q01. This component is the source of the sustained, low, base-flow discharge from 'En Bardi during the last 6 to 7 months of the dry summer period. Q0L probably represents "piston flow" of water through, and draining from the chalk's matrix porosity - - the main reservoir of water, but having a very low transmissivity.
78
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
4.2. Sources of nitrate 4.2.1. Dolomite springs Nitrate concentrations in the springs of 'En Kaukabe, 'En Neria and 'En Retet (Table 4) are below the normal recommended limit for nitrate in drinking water in Israel (45 mg or 0.73 meq NO3 1-1). Their 615N values, between +3.3 and +4.8%0, are within the range typically found for nitrate in rainwater (ca. -8 to +4%0; Heaton, 1987), and for nitrate derived from nitrification of organic matter in undisturbed, non-forested soils (ca. +4 to +9%0; Heaton, 1986; Macko and Ostrom, 1994). They are therefore compatible with a "natural", non-pollutant origin. While limited data for fertilizers in Israel suggest that they have 615N values in the range -2.3 to +2.3%0, in other countries values range up to ca. +5%o (Kaplan and Magaritz, 1986; Macko and Ostrom, 1994; Sandler and Heaton, 1997). To the best of our knowledge, however, fertilizers are not used in the catchment areas of these springs, and are therefore not considered as a likely source of nitrate. 4.2.2. 'En Bardi Over a 2 year sampling period 'En Bardi had nitrate concentrations ranging from 0.44 to 1.78 meq l -~. For most of this period the concentrations exceeded the 0.73 meq 1-1 recommended limit for drinking water (Fig. 4) and, on a spring discharge-weighted basis, averaged 0.78 meq 1-1. These high concentrations are associated with 615N values which are higher than in the dolomite springs (Tables 3 and 4). Increases in nitrate at 'En Bardi, moreover, are correlated with increases in 15N (Fig. 6). This suggests that there may be mixing between sources of nitrate with different concentrations and isotopic compositions. For a process involving mixing of two endmembers, mass and isotopic balance requires that: Cm = Ch)g+ CI(1 - f ) and Cm~ m = Ch~hf + C1~1(1 - f )
where the subscripts m, h and 1 refer to the mixed water, high-nitrate water and low-nitrate water endmembers, respectively; C and ¢5 are the concentrations and 615N values, respectively; and f is the fraction of the high-nitrate end-member in the mixture.
615N values of a mixture can then be calculated as a function of the concentration (Cm) from: ~m = { [(fro - Cl)(Chrh - Cl~l)/(Ch - Cl)] + f i l l } / C m
(1) The absence of waters with very low nitrate concentrations at 'En Bardi makes it difficult to estimate the likely composition of the low-nitrate end-member from the 'En Bardi data alone. Noting that the concentrations and isotopic compositions of the nitrate in the other, dolomite springs ('En Kaukabe, 'En Neria and 'En Retet) are fairly "typical" of groundwater, however, the low-nitrate end-member for 'En Bardi is assumed to have a composition similar to these springs - - i.e. on average, Cl = 0.18 meq 1-1 and 61 = + 3.8%o. Three mixing lines are then calculated from Eq. (1) and displayed in Fig. 6 assuming three different highnitrate end-members: Line 1: a high-nitrate end-member similar to the highest nitrate sample analysed at 'En Bardi (GAB 60: C h = 1.8meq1-1 a n d r h = +9%o). Line 2: a hypothetical high-nitrate end-member with the same isotopic composition, but an arbitrary, 10 times higher concentration (Ch = 18 meq 1-1 and bh = + 9%0). Line 3: a hypothetical high-nitrate end-member with both a higher concentration and an arbitrary higher 615N value (Ch -- 18 meq 1-1 and 6h = + 12%o). Distribution of the 'En Bardi water nitrate concentrations and 615N values in Fig. 6 fits mixing lines 1 or 2, but not 3. The variations in nitrate concentrations in 'En Bardi can therefore be explained in terms of mixing between water of low nitrate concentration and ¢515N value - - of the type found in the other springs - - and water with higher nitrate concentration and 615N value. The data do not allow an estimation of the nitrate concentration of the highnitrate end-member at 'En Bardi, other than it must be at least 1.8 meq 1-I, but a value of about +9%0 can be placed on the 615N value of this high-nitrate end-member. A 6 ~SN value of +9%0 eliminates fertiliser as a possible source for the high-nitrate end-member at 'En Bardi, and is at the upper end of the range of 6~5N values normally found for soil-derived nitrate.
A. Burg. T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
Organic matter in the 'En Zetim bituminous chalk itself might also be considered as a potential source of nitrate (Rosenthal et al., 1987). In two samples of this chalk, however, nitrate concentrations were only 0.05 and 0.17 meq kg -1, and were moreover lower than the concentrations of other soluble ions (Table 2). In contrast, whilst Na, K, C1, Mg and SO4 are clearly associated with nitrate in 'En Bardi water, increases in nitrate concentrations in the spring are always larger than those for these other ions (Fig. 5). In addition, 615N values for the chalk nitrate (-1.9 and -0.7%o, Table 2) were much lower than those for 'En Bardi water. On the basis of both the chemical and 15N/laN data, therefore, the 'En Zetim bituminous chalk is not the source of the nitrate in the spring water. Na, K and C1, in addition to nitrogen compounds, are known to be strongly associated with pollution derived from sewage (Twort et al., 1974). Nitrate derived from sewage commonly has high 15N/14N ratios, with 615N values in the range +10 to +20%0 often being regarded as typical (Kreitler and Jones, 1975; Gormly and Spalding, 1979; Heaton, 1984; Exner and Spalding, 1994). It must be stressed, however, that the degree of 15N enrichment is very dependent on a variety of environmental factors operating during the decomposition of sewage urea to form nitrate, especially those controlling the volatilization of ammonia. Thus lower 615N values, e.g. +9%o, might well result if loss of ammonia was inhibited by subsurface containment of sewage (Aravena et al., 1993). The chemical and isotopic evidence are therefore compatible with the high-nitrate end-member of the 'En Bardi water being derived from sewage waste. Since the perched water table draining at 'En Bardi sits on a marly aquiclude which extends below the northern part of the village of Jish (Fig. 2(a)), we conclude that nitrate and other ions from some of the village's septic tanks flow towards 'En Bardi. Variations in the concentrations and 615N values of the nitrate at 'En Bardi can be explained in terms of dilution of this sewage-contaminated water by infiltrating rainwater and by Ca + HCO3 waters typical of groundwater in carbonate aquifers. 4.3. Relationship between nitrate concentrations and hydrology
If Jish has been populated from at least the 12 th
79
century sewage contamination of the perched aquifer may have been occurring for several hundred years. In this case the nitrate will probably be distributed throughout the depth of the stratigraphic section in a relatively localised area below Jish, and dispersed in the lower part of the section over a wider area in the direction of groundwater flow. The fact that nitrate concentrations at 'En Bardi at the end of the 1993 dry season were fully restored to their levels of the previous year, and the high concentration in 1995 (Table 3), testifies to the size of the nitrate reservoir. Most of the nitrate would be in the matrix porosity, which represents the main storage reservoir of the chalk and, below the surficial zone, may remain close to saturation even during the dry summer months (Price et al., 1993; Nativ et al., 1995). If the nitrate is present in the matrix one might expect nitrate concentrations to be high in water representing drainage of the chalk matrix during the summer (discharge component Q0t), and to be low - i.e. diluted - - in water representing fissure flow in the winter (discharge components Q03 and Q02). Although this is generally true (Figs. 4, and 7), the pattern is not simple. Thus, whilst data in Fig. 6 point to the existence of a "low-nitrate end-member", water of low nitrate concentrations similar to those of the dolomite springs was never found at 'En Bardi. Even during the periods of heaviest discharge, with large volumes of water passing rapidly through fissures, nitrate concentrations do not drop below 0.44 meq 1-1. Such concentrations are more than 10 times higher than those typical of rainwater in the Galilee (ca. 0.03 meq 1-1; Herut, 1992). Evaporation/transpiration may concentrate rain- or soil-derived nitrate salts in the surficial zone of the aquifer during the summer months; but these should be removed by the first recharging winter rains and not persist in the fissure water throughout the winter months. In addition, the relatively narrow overall range in nitrate concentrations, compared with the hundredfold range in discharge, must also be explained. Thus water derived from drainage of the matrix with a discharge depletion time of ca. 50 months, has nitrate concentrations which are only up to four times higher than those of the fissure water with a discharge depletion time of ca. 0.25 to 1 month. This relative similarity between the nitrate concentrations of fissure and matrix flow implies a ready
80
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
transfer of nitrate from the matrix to the fissures. Two mechanisms may contribute to this. If the hydraulic pressure of water in the matrix is higher than that of water in the fissures, by an amount sufficient to overcome the capillary suction of the matrix pores, pollutant water may flow from the matrix into the fissures. Alternatively, where there is no net flow of water between matrix and fissures, but the concentration of the pollutant is higher in the matrix water, then the fissure water may gain pollutant by diffusion (Foster, 1993). Starting at the beginning of the wet season, the following sequence is proposed. The narrowness of the range of 6D and c5180 values for 'En Bardi, compared with the wide range normally found in rainwater, suggests that very little rainfall passes immediately through the aquifer via fissures. Mixing of rainfall probably occurs in the surficial zone of the aquifer where the effective storage capacity and transmissivity are enhanced by weathering. As noted above the earliest rainfall, at the beginning of the wet season, is largely taken up in saturating the field capacity of this zone. Subsequent movement of water downward via piston flow in the chalk matrix, however, may be limited by its low hydraulic conductivity; particularly if dry fissures interrupt the capillary tension between matrix blocks (Nativ et al., 1995). With the onset of heavy rainfall the rate of infiltration may exceed this conductivity, and result in oversaturation of the matrix, and possible temporary ponding in the surficial layer. The high hydraulic pressure developed in the matrix will then force water into the fissures, with a consequent rapid increase in flow through the aquifer and discharge at the spring. Thus in the early part of the high-discharge period much of the fissure water may have passed through the matrix in the upper part of the aquifer. Some of this fissure water can re-saturate that part of the matrix which is close to fissures in lower parts of the aquifer, where dispersion from Jish may have produced higher nitrate concentrations. Initially, this movement of water from the fissure into the matrix will inhibit movement of nitrate in the opposite direction. With the continuation of infiltration and fissure flow, however, the matrix in the lower parts of the aquifer eventually becomes fully re-saturated, allowing greater exchange of nitrate from the matrix to the fissures. Nitrate concentrations in the fissure waters
discharging at 'En Bardi therefore progressively increase during the high discharge period. Depletion of the discharge at 'En Bardi with the onset of the dry season has been described above. Fissure-discharge components Q03 and Qo2 are replaced by drainage of water from progressively smaller microvoids, and the increasing relative importance of slow piston flow through the matrix from deeper parts of the chalky blocks. Such water, having a greater residence time and being less accessible to dilution by fissure water, may be expected to have the highest nitrate concentrations. This may explain the continued increase in nitrate concentrations during the long period of stable, Q0, matrix drainage. The cause of the final increase in nitrate concentrations, corresponding to the first increase in discharge of the following winter (Fig. 4), is not clear. It may represent piston displacement of nitrate-rich matrix water, due to an increase in the hydrostatic head prior to the activation of fissure flow. In addition to variations imposed by these temporal changes in hydrology, the nitrate concentrations at 'En Bardi could also be influenced by the spatial differences in the recharge area. The septic tanks in the northern part of Jish effectively represent a point source of nitrate dispersed in the direction of groundwater flow. Discharge at 'En Bardi represents water which has recharged near Jish, or passed through the dispersal plume, mixed with water which has recharged through a wider area of less polluted aquifer. The different path lengths and residence times of these waters will also control, therefore, the temporal variations in nitrate concentrations at 'En Bardi. Thus the first fissure water to discharge at 'En Bardi after the beginning of the wet season may contain a higher proportion of water recharged in the immediate, less polluted vicinity of the spring (Fig. 2(a)). Water recharged from Jish, and having a higher nitrate concentration, will tend to appear later.
5. Summary and conclusions
The perennial spring, 'En Bardi, drains a small, perched aquifer of Upper Cretaceous chalks and has a temporal pattern of discharge which closely reflects that of the rainfall. The spring's hydrograph allows two main flow types to be characterised: slow
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
piston-flow drainage of the main storage reservoir - the chalk's matrix porosity; and rapid drainage via fissures. The matrix drainage, with a depletion time of ca. 50 months, is responsible for sustaining the low levels of base-flow discharge at the spring during the dry summer months. The fissure flow, with a depletion time of --<1 month, is responsible for the up to 100fold increase in discharge during the wet winter months. Chemical and 15N/14N isotopic data suggest that elevated levels of nitrate in the 'En Bardi water are derived from septic tanks situated in part of a village which lies in the spring's recharge area. Some of the other ions which are strongly correlated with nitrate (Na, Mg, K and C1) may also have been derived from sewage pollution. The antiquity of the village's sewage system, and the annual recovery of nitrate concentrations at the end of the dry summer, suggest that the nitrate is now dispersed in the matrix porosity of the chalk over a large part of the aquifer. Annual variations in nitrate concentrations at the spring reflect mixing between the high-nitrate, high 6 ~SN, sewagederived end-member, with low-nitrate, low 615N, Ca + HCO3 water typical of non-polluted groundwater in other carbonate aquifers in the region. In a general fashion the highest nitrate concentrations occur during the period of matrix drainage in the summer, with lower concentrations associated with fissure flow during the winter. The differences between the concentrations in these two periods, however, does not relate to a simple dilution of polluted matrix water by large volumes of freshly recharged, non-polluted fissure water. Thus a 50-fotd increase in spring discharge, associated with a change from matrix drainage to fissure flow (August-October 1992 to JanuaryMarch 1993), resulted in only a 35% reduction in nitrate concentrations. Although the spring's discharge shows a close temporal response to rainfall, this does not require that there is an immediate rapid passage of rainfall from the surface to the spring, solely via fissures. The similarity between the chemistry of very high-discharge fissure-flow waters and low-discharge matrixdrainage waters implies that much of the fissure water has mixed with, or been expelled from the matrix. Thus the spring's response to rainfall may be a largely hydraulic one. A close association between fissure and matrix waters would also account
81
for the very restricted range in the 6D and 6180 values of the spring water. A gradual increase in nitrate concentrations during the winter rainfall period of fissure flow may reflect changes in the depth, or depth interval over which this matrix-fissure interchange takes place. Mixing of waters which have followed different flow paths through the aquifer - - through and not through the zone of heavy sewage pollution - - will also influence the temporal pattern of nitrate concentrations at the spring. The recent completion of the central sewage drainage system at Jish will remove the primary source of nitrate contamination. Future monitoring of the subsequent changes in the nitrate concentrations and 615N at 'En Bardi should be continued, as this would allow an assessment of the rate at which the chalk aquifer recovers from several hundred years of pollution. In many cases the physical properties of chalks - - a large matrix storage capacity which can be "bypassed" by fissure flow - - should tend to prolong the persistence of this pollution (Foster, 1993). The similarity of the fissure and matrix waters of 'En Bardi reported here, however, suggests that they are intimately related. This should tend to speed up the flushing out of the nitrate contamination.
Acknowledgements
Professors A. Katz and A. Starinsky of the Hebrew University, and Dr A. Bein of the Geological Survey are thanked for their invaluable advice to the first author during his postgraduate studies, of which this work forms a part. The technical work and assistance of Professor Katz, Mr M. Amon and Mr R. Knafo, and the collaboration of Dr B. Spiro, are also gratefully acknowledged. Partial funding of the study was provided by the joint German-Israeli Research Fund ( B M F T / M O S T , project DISUM-20).
References
Aravena, R., Evans, M.L., Cherry, J.A., 1993. Stable isotopes of oxygen and nitrogen in source identification of nitrate from septic systems, Ground Water, 31, 180-186. Burg, A., 1997. The sources of soluble salts in perched carbonate aquifers in northern and central Israel: a key to understanding the evolutionof water in carbonateaquifers. Unpublished Ph.D.
82
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
dissertation, Hebrew University of Jerusalem (in Hebrew, English absract), in preparation. Exner, M.E., Spalding, R.F., 1994. N- 15 identification of nonpoint sources of nitrate contamination beneath cropland in the Nebraska Panhandle: two case studies, Appl. Geochem., 9, 73-81. Foster, S.S.D., 1993. The chalk aquifer - - its vulnerability to pollution. In: Downing, R.A., Price, M. and Jones, G.P. (Eds), The Hydrogeology of the Chalk in North-West Europe. Clarendon Press, Oxford, pp. 93-112. Gat, J.R., 1980. The isotopes of hydrogen and oxygen in precipitation. In: Fontes, J.Ch. and Fritz, P. (Eds), Handbook of Environmental Isotope Geochemistry, vol. 1. Elsevier, Amsterdam, pp. 21-47. Gat, J.R., Dansgaard, W., 1972. Stable isotope survey of fresh water occurrences in Israel and the northern Jordan Rift Valley, J. Hydrol., 16, 177-212. Gormly, J.R., Spalding, R.F., 1979. Sources and concentrations of nitrate-nitrogen in ground water of the Central Platte Region, Nebraska, Ground Water, 17, 291-301. Heaton, T.H.E., 1984. Sources of the nitrate in phreatic groundwater in the western Kalahari, J. Hydrol., 67, 249-259. Heaton, T.H.E., 1986. Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere: a review, Chem. Geol. (Isotope Geoscience section), 59, 87-102. Heaton, T.H.E., 1987. 15N/JaN ratios of nitrate and ammonium in rain at Pretoria, South Africa, Atmos. Environ., 21,843-852. Herut, B., 1992. The chemical composition and sources of dissolved salts in rainwater in Israel. Unpublished Ph.D. dissertation, Hebrew University of Jerusalem, 95 pp. (in Hebrew, English abstract). IAEA, 1990. Environmental Isotope Data No. 9: World Survey of isotope Concentration in Precipitation (1984-1987). International Atomic Energy Agency (Vienna, 1990) Technical Reports Series no. 311, 188 pp.
Kaplan, N., Magaritz, M., 1986. A nitrogen-isotope study of the sources of nitrate contamination in ground water of the Pleistocene Coastal Plain aquifer, Israel. Water Res., 20, 131-135. Kreitler, C.W., Jones, D.C., 1975. Natural soil nitrate: the cause of the nitrate contamination of groundwater in Runnels County, Texas, Ground Water, 13, 53-61. Macko, S.A. and Ostrom, N.E., 1994. Pollution studies using stable isotopes. In: Lajtha, K. and Michener, R.H. (Eds), Stable Isotopes in Ecology and Environmental Science. Blackwell Scientific Publications, pp. 45 62. Mero, F., 1964. Application of the groundwater depletion curves in analysing and forecasting spring discharges influenced by well fields. World Meteorological Organization and International Association of Scientific Hydrology, Symposium Surface Waters, publication no. 63, pp. 107-117. Nativ, R., Adar, E., Dahan, O., Geyh, M., 1995. Water recharge and solute transport through the vadose zone of fractured chalk under desert conditions, Water Resour. Res., 31,253-261. Price, M., Downing, R.A. and Edmunds, W.M., 1993. The chalk as an aquifer. In: Downing, R.A., Price, M. and Jones, G.P. (Eds), The Hydrogeology of the Chalk in North-West Europe, Clarendon Press, Oxford, pp. 35--58. Rosenthal, E., Magaritz, M., Ronen, D., Roded, R., 1987. Origin of nitrates in the Negev Desert, Israel, Appl. Geochem., 2, 347354. Sandier, A. and Heaton, T.H.E., 1997. The origin of an unusual form of cave niter; Source of nitrate and hydrologic origin of an unusual cave niter dripstone; Israel. J. Geol. in press. Schwarz, J., 1992. Management of the water resources of Israel, Isr. J. Earth Sci., 39, 57-65. Twort, A.C., Hoather, R.C. and Law, F.M., 1974. Water Supply, 2nd edn. Edward Arnold, London, 478 pp. Vengosh, A., Rosenthal, E., 1994. Saline groundwater in Israel: its bearing on the water crisis in the country, J. Hydrol., 156, 389 430.
Hydrology ELSEVIER
Journal of Hydrology 204 (1998) 68 82
The relationship between the nitrate concentration and hydrology of a small chalk spring; Israel Avi Burg a'b, Tim H.E.
Heaton
c
alnstitute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel bGeological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel CNERC Isotope Geosciences Laboratory, Keyworth, Nottingham NGI2 5GG, U.K
Received 2 April 1997; revised 28 August 1997; accepted 28 August 1997
Abstract
Discharge from a spring draining a small, perched, Cretaceous chalk aquifer in the Upper Galilee, Israel, was monitored over a period of two years. The water has elevated nitrate concentrations, with ~5N/14Nand chemical data suggesting that it is a mixture of low-nitrate and high-nitrate end-members; the latter derived from the sewage of a centuries-old village served by septic tanks. Hydrograph data allowed distinction between fissure flow during the period of winter rainfall, and matrix drainage during the dry summer months. These different flow types, however, did not have markedly different nitrate concentrations: a 50-fold increase in spring discharge due to fissure flow, compared with matrix drainage, was reflected in only a 35% decrease in nitrate concentrations. The relatively high nitrate concentrations in the fissure waters suggests that they have had close contact with, and are possibly displaced from the matrix. This should help to accelerate the decline in the spring's nitrate concentrations following the recent completion of the village's central sewage drainage system. © 1998 Elsevier Science B.V. Keywords: Nitrate; Pollution; Chalk; Spring; Ground water; Isotopes
1. Introduction
More than 60% o f the total water supply in Israel is provided by groundwater, but the continued exploitation o f this resource is being compromised by a deterioration in its quality (Sehwarz, 1992). A large part o f the country is covered by Upper Cretaceous carbonates (Fig. 1). O f these, the limestones and dolomites o f the Judea Group form significant aquifers, and springs issuing from them have been important supplies o f water throughout history. The aquifers, however, are vulnerable to pollution by infiltration of surface contaminants, including nitrate derived from sewage - - many o f the villages, especially in the Galilee region o f northern Israel, having been served by old sewer systems. Immediately above the 0022-1694/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved Pll S0022-1694(97)00105-4
Judea Group, the chalks and marls of the Mount Scopus Group are generally considered to be aquitards or aquicludes (Vengosh and Rosenthal, 1994). Under ideal conditions such rocks might afford some protection from the infiltration o f surface contaminants into underlying aquifers. There is particular interest in the manner by which pollutants such as nitrate pass through chalk (Foster, 1993), and the effectiveness o f chalk as a hydrologic barrier to pollutants in Israel (Nativ et al., 1995). As part o f a range o f studies on the hydrology of carbonate aquifers (Burg, 1997), the chemistry of water from a spring issuing from chalks of the Mount Scopus Group in the Upper Galilee region of Israel (Figs. 1 and 2) was monitored over a period of 2 years. The spring, 'En Bardi, drains a small perched
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
69
STRATIGRAPHY
I .=-
FOR.ATIO.
r,, ~ ~ z ~ . < ~ ~ E i] -~ ~-
P_.
Timrat Taqiye f---~
z
~ ~
~]
|
'En Bardi ~
~
'En Zeti
Limonite ~
Z <
.
-'.,'-;,, y ,
B'ina
~
~
3g----------------~;-: s
Sakhnin
~
w _< < "" ~:< =
"
u~ o = z ~ ~- ~
1 . ~ 1 Marl ~
Chalk
~
Chert nodules
I'Ll
~
V
:~ ; - l c ~~ Deir Hanna
'En Kaukabe ',En Retet ,
-~" ~
i;~
Karst phenomena
~' .i,p ........
Yanuh
Z
Quartz geodes
~ ,,,,
., ,~, ,,,,,v,,,
O e.~
"<
Megafossils
~
7-
o
Bituminousrock Phosphatic rock
o ~ tad
Legend
Ghareb I
__~: ~° ~
LIT.OLOOY
~
En Neria ~
Yagur
I
~Vf'l Dolomite
~
Limestone
i
...... / . - / / 3. / /
/
='
." /
.;/ .:;F:
Rama
(Schematic and not to scale) Fig. 1. Compositecolumnarsectionof the Judea and MountScopusGroupsin the Galilee,withthe fourspringsmarkedin their stratigraphic positions. aquifer with a well-defined recharge area. Its hydrochemistry therefore largely reflects processes in the unsaturated zone, which is of particular importance in relation to controls on the migration of pollution through chalk. The nitrate concentrations displayed regular seasonal variations associated with changes in discharge, with a weighted mean nitrate concentration above the limit recommended for drinking water. With chemical and isotopic data we discuss the origin of the nitrate at 'En Bardi, and consider
how temporal changes in its chemistry can be related to the hydrology.
2. Study area and methodology 2.1. Spring sites
Four springs in the Galilee were studied: 'En Kaukabe, 'En Neria and 'En Retet, which drain
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
70
190
1711
D
m recharge
ti "talk te .d area age of Jish)
A b )
A'
- SW - Jish 19179/27000 / m/ /
'En Bardi
- NE 19251/27088 /m
I 750
750
/a.
o I
50o 18 81 Bituminous
z5o I
I
I
rock
I m
~"~
Marl
-~
Chalk
Fig. 2. (a) Locations of the four springs and the rainfall station at Neria field school, in the Upper Galilee, Israel. (b) Diagrammatic cros: section of the perched aquifer drained by the spring '"En Bardi'.
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
71
Table 1 Properties of chalks at 'En Bardi - - physical properties of hand specimens from outcrops at different heights above 'En Bardi. (Maximum stratigraphic thickness above 'En Bardi = ca. 101 m; analyses by Laboratory of the Geological Survey of Israel) Lithology
Height (m)
Porosity (%)
Permeability (millidarcy)
Density (g c m 1)
Bituminous chalk Chalk Chalk Chalk
5 52 71 86
17 18 31 36
0.03 1.13 0.03 0.03
2.10 1.82 1.70 1.58
southern part is occupied by the village of Jish (current population ca. 2200) which has been populated since at least the 12 th century and, up to the 1970s, discharged its sewage to septic tanks dug into the chalk. Since that time a central system draining sewage to the Wadi Gush Halav has been gradually introduced, with the last area of the village being completed in 1995 - - after the collection of samples reported here.
dolomites of the Judea Group; and 'En Bardi, situated in the Upper Galilee, draining chalks of the overlying Mount Scopus Group (Fig. 1). This report concentrates on the latter spring. The Upper Galilee experiences a Mediterranean climate characterised by precipitation in the winter (November-March), with a warm dry summer (April-September). Total precipitation amounts to ca. 850-900 mm year -~, and the mean annual air temperature is 15-17°C. 'En Bardi is a small perennial spring having the highest discharge of a group of five springs emerging on the western slope of the Wadi Gush Halav (Fig. 2(a)). The spring is located at a horizon where bituminous chalks lie on a marly aquiclude (Fig. 2(b)), and its perennial flow is from a perched water table. The topography and structural geology allow the recharge area to be fairly well defined (Fig. 2(a)). It is estimated to be no more than 1.5 km 2, and is composed of bituminous chalks, chalks and clayey chalks of the 'En Zetim and overlying Ghareb and Taqiye Formations (Fig. 1). Porosities of hand specimens representing the main rock types in the stratigraphic section above 'En Bardi range from 17 to 36%, and their permeability is low (Table 1). Numerous fractures, however, are visible where these rocks are exposed above the spring. The recharge area is partly cultivated (mainly olives), but not irrigated. Its
2.2. Sampling and analysis
The four springs were monitored and sampled once per week during the winter, and once per fortnight during the dry summer season, over a 2 year period from winter 1991/2 to spring 1994. Discharge was measured with pail and stopwatch. Water samples were collected directly after emergence from the rock, and stored refrigerated until filtering and analysis. Samples of 'En Zetim bituminous chalk were crushed and stirred for 60 h with deionised water to release water-soluble components. Cations and SO4-S were determined by atomic absorption and inductively coupled plasma spectrometry, NO3 and CI by colorimetry using Flow Injection Analysis, and HCO3 by titration with HC1. Analytical precision is estimated to be _+2% (_+5% for SO4-S).
Table 2 Properties of chalks at 'En Bardi - - concentration of water-soluble components, and 15N/14N ratio of water-soluble nitrate, in bituminous chalks from above 'En Bardi Sample
Br- 1 Br-2
meq per kg rock Na +
K÷
Ca 2+
Mg 2+
SO2-
CI
HCO 3
NO 3
Si
1.14 1.13
1.86 2.35
14.8 28.5
0.61 1.72
8.05 25.0
0.22 0.19
9.22 6.85
0.05 0.17
2.66 2.42
a 615N, in mil = [ ( R s a m p l e - Rstandard ) -- 1] x 103; where R b Proportion of rock insoluble in acetic acid.
=
15N/14N
and the standard is atmospheric N2.
~ 15N
Insoluble
(%0)"
(%) ~
- 1.9 -0.7
16.4 17.3
72
A. Burg, T.H.E. Heaton/Journal o f Hydrology 204 (1998) 6 8 - 8 2
Table 3 Data for 'En Bardi: discharge, 180/160 and D/H ratios, and the concentrations and 15N/14N ratios of nitrate Sample
Date
Discharge (1 min -1)
6180 (%0)"
6D (%o)a
NO3 (meq 1-l)
GAB/1 GAB/4 GAB/18 GAB/19 GAB/20 GAB/21 GAB/26 GAB/30 GAB/60 GAB/62 GAB/63 GAB/64 GAB/65 GAB/66
20/02/92 21/04/92 10/11/92 27/11/92 03/12/92 12/12/92 15/01/93 08/02/93 07/02/94 21/02/94 28/02/94 19/03/94 28/07/95 13/09/95
>220 5.7 2.9 4.6 10.5 5.4 >400 177 6.4 6.7 24 37
-6.0 -6.0 -6.0
-29 -25 -28
-6.2 -6.3 -6.1 -6.2 -6.2 -6.0 -6.1 -6.0 -6.1
-31 -30 -30 -29 -28 -31 -31 -30 -29
0.82 0.92 1.18 1.31 1.00 1.62 0.58 1.14 1.78 1.17 0.61 0.48 0.99 1.23
t5ISN (%o)b
+8.8
+7.2 +9.0 +8.4 +7.4
a 6D or 6180, in mil = [(Rsample-Rstandard)-I] × 103; where R = 2H/IH or lSO/160,respectively, and the standard is VSMOW. b See Table 1. D / H a n d 180/160 ratios o f w a t e r w e r e d e t e r m i n e d on hydrogen produced by reaction with zinc turnings at 500°C, a n d c a r b o n d i o x i d e e q u i l i b r a t e d at c o n t r o l l e d r o o m t e m p e r a t u r e , r e s p e c t i v e l y . S o l u t i o n s for 15N/14N a n a l y s i s w e r e c o n c e n t r a t e d b y rotary e v a p o r a tion, w i t h nitrate c o l l e c t e d as a m m o n i u m sulfate b y distillation w i t h M g O plus D e v a r d a ' s alloy, a n d N2
p r o d u c e d b y c o m b u s t i n g the d r i e d a m m o n i u m sulfate w i t h CuO. D/H, 180/160 a n d 15N/laN ratios w e r e d e t e r m i n e d in V G S I R A and O p t i m a m a s s s p e c t r o meters, and are r e p o r t e d in the usual (5 notation (Tables 1 - 4 ) . Analytical replication is typically _+ < 2%o, <0.1%o and <0.2%o, for 6D, 6180 and 615N, respectively.
Table 4 Data for 'En Kaukabe, 'En Neria and 'En Retet: discharge, and the concentrations and 15N/14N ratios of nitrate Spring
Sample
Date
Discharge (1 min 1)
NO3 (meq 1-l)
6 JSN (%o)~
•En Kaukabe
GAK/23-27 GAK/63
19/12/92-14/01/93 14/02/94
1.0-2.7 0.3
ca. O. 10
+4.3 b
0.14
+3.5
GAN/5 & 6 GAN/50 & 51 GAN/54 & 55 GAN/60 GAN/64 GAN/66 GAN/65 & 67
11/03 & 02/04/92 17 & 29/08/93 05 & 24/10/93 06/01/94 07/02/94 21/02/94 14 & 28/02/94
6.7-16 1.1-1.2 1.0-1.1 2.4 2.2 2.2 5.6-8.8
ca. 0.15 ca. 0.24 ca. 0.24
+4.8 b +3.3 b +3.6 b +3.9 +3.6 +3.9 +3.9 b
GAR/48-51 GAR/64 & 66 GAR/65
29/07 31/08/93 14 & 28/02/94 21/02/94
7.8-9.0 24-28 14
ca. 0.12 ca. 0.11
'En Neria
'En Retet
0.27 0.22 0.20 ca. 0.15
0.16
+4.0 b +3.5 b +3.3
a See Table 1. b615N/14N analysis of composite sample. Sample number and dates joined by " - " or " & " indicate that composite was of all samples in range, or the two samples, respectively. Discharge is the range during the indicated dates; nitrate concentrations are means of the individual samples.
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68 82
73
a) discharge (lit/min)
rain amount (mm) 140
discharge
120
I O0 1 O0
~°
~
40
10
~
20 '
i I,I,
, I,
o
, I
10
,ill
I
11 12 1992
I
3
date
,I I 4 5 ~month 1993 I
b) rain amount (ram)
discharge (lit/min)
140
120
/
1O0
discharge 10
6O
420 0 0
I, 10
i
'1 I
,,11
,
II, 11 1993
I
,
I 12
I date
,
2 1994
3 ~month !>
Fig. 3. Temporal relationship between discharge o f 'En Bardi (logarithmic scale), and rainfall measured at the Neria field school: (a) October 1992 to June 1993; (b) October 1993 to March 1994. Discharge was measured every one to two weeks; rainfall bars represent totals over three consecutive days.
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
74
a variety of flow regimes, the ranges in hydrogen and oxygen isotope compositions are very small, typically only 3%0 and 0.3%o, respectively (Table 3). In comparison, isotopic variations between rainfall events and between mean monthly rainfall in Israel commonly range over 40%0 in 6D and 6%0 in 6180 (Gat and Dansgaard, 1972; IAEA, 1990). The relationship between nitrate concentrations and discharge at 'En Bardi is shown in Fig. 4. There is a general pattern of low nitrate concentrations associated with seasons of high discharge during the wet winter months, and of high concentrations associated with low discharge during the dry summer months. However, while discharge varies by a factor of up to c a . 100, nitrate concentrations vary by a factor of only c a . 4. Comparing the intervals January-March 1993 and August-September 1992, for example (Fig. 4): average discharge was 50 times higher (150 cf. 3 1 rain-l), but nitrate concentrations were reduced by only 35% (0.7 cf. 1.1 meq l-l). Detailed examination also reveals additional features: (1) the lowest nitrate concentrations occur at the onset of a period of high discharge rates; 2) nitrate concentrations then rise progressively - - both during the period of high discharge, and during the ensuing dry season when a prolonged period of low, stable "base-flow" discharge is established; (3) at the end of the 1993
3. Results
Fig. 3 shows the temporal relationship between the discharge of 'En Bardi and the amount of rainfall measured at a station 6 km away (Neria Field School, Fig. 2(a)). When the summer dry period was broken by heavy rainfall in November 1992, the flow immediately started to increase, but only reached high levels in December after several large rainfall events. In the following year the first rains were relatively small and sparse, and the flow only rose after heavier rains in January. Since the discharge was only measured weekly, temporal relationships on a shorter time scale are difficult to assess. During the period of high discharge in the winter of 1992/93, however, the discharge rate did appear to respond quickly to large rain storms, and declined between rain storms (but not to base-flow levels). Hydrogen and oxygen isotopic compositions o f ' E n Bardi water are shown in Table 3. They fall within the range of values reported for other groundwaters in the same area of Israel and, in common with meteoricderived waters in the eastern Mediterranean, have a " d parameter" (d = 6D - 8 x 6180) of c a . 20 compared with the value of 10 typical of global meteoric waters (Gat and Dansgaard, 1972). Although the spring was sampled over a period of 2 years, under o
discharge
m
(lit/min)
(meq/lit)
NO
3.5
iiiiiiii
!ii!ii!iii!i
iiiiiii~
............
iiiiJii
IO0
:
iiiiiii
"
i!i!ill
z.s
10
-
~--- ;. ~ 0.5 ~
•
0.1
llI213141~IrtTIBIglI011~Ilzl
>
2 3
~ 1992
date
4
5 fl71a191~011~1121112 I
~ 1993
month
~
--
--
:~
1994
Fig. 4. Discharge (logarithmic scale) and nitrate concentrations of 'En Bardi from February 1992 to March 1994.
75
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
1.4 1.2 1.0 0.8 0.6 0.4 0.2
Na + . . . , . +
~
+
+
I
I
!
K÷
1.2
+
+
+ +
1.4 1.6
1.0 0.8
.if.~.:l~..ik ~+ +
"~'1" +
0.6 0.4 0.2 13"
I
E
I
~ + 4 + +
f
.i..~-Ik'~
I
I
Ca 2+ -
Cl"
1.2 1.0 0.8 0.6 " 0.4 " 0.2
+
I
I
SO~- .~..~+
1.0
+
+ 4"+
+
0.8 " ~
.6 0,4 0,2
I
Mg2+
1.2 1
I
-
I
.
!
++ +
I
I
HCO~
"~~+
-
0.8
+
0.6 0.4 0.2 0
I
I
I
0.5
1
1.5
2
NO 3 (meq.1-1)
0
I
I
I
0.5
1
1.5
NO 3 (meq.I q )
Fig. 5. Relationship between major ions and nitrate concentrations, for 64 samples of 'En Bardi water from February 1992 to March 1994.
dry season nitrate concentrations were fully restored to their levels at the end o f the 1992 dry season (see, also the 1995 analysis, Table 3); and (4) m a x i m u m nitrate concentrations occur at the beginning o f the rain season when discharge rates start to increase. Relationships between nitrate and other ions at 'En Bardi are shown in Fig. 5. Nitrate shows a marked positive correlation with Na, K, M g and
C1 (r 2 > 0.85), and to some extent with SO 4 (r 2 = 0.77). The nitrogen isotope compositions o f 'En Bardi (Table 3) and the other springs (Table 4) are shown as a function o f nitrate concentration in Fig. 6. The three dolomite springs have 6 ~SN values in the range from about +3 to +5%o. Samples from ' E n Bardi, the spring with the highest nitrate concentrations, have 615N values in the range +7 to +9%o.
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
76 +14
,
,
,
+12
,
._~
,
,
3
.........................
.-'"
+10 ,. ,._,
,
+8
,"
,,
_".~ _ ~ . . . . . . . . I-
~ .
-*
--"
2
,' ,,"
+6
J
+2 O
I
0
I
0.5
I
I
1.0
I
I
1.5
I
2.0
NO3(meq/1)
Fig. 6. Variations in the 15N/14Nratios (6 ~SNin %oversus atmospheric nitrogen) and concentrations of nitrate in water from four springs: solid diamonds = 'En Bardi; open diamonds = 'En Kaukabe, 'En Neria and 'En Retet. Broken lines show the calculated compositions of waters resulting from mixing a low-nitrate end-member, having a composition similar to the springs shown as open diamonds, with different highnitrate end-members (see text).
4. Discussion
4.1. Hydrology The data in Fig. 3 indicate that the earliest rainfall, at the very beginning of the winter wet season, does not result in an immediate substantial increase in discharge. During the wet season, however, peaks in discharge do apparently occur soon after heavy rainfall. This pattern suggests that for the earliest rainfall the infiltrating portion (i.e. that part not lost by evaporation/transpiration and runoff) is taken up in saturating the sub-surface field capacity. Thereafter, during the remainder o f the wet season, the response of discharge to rainfall is more immediate. Because the 6D and 6180 values lie on the local meteoric water line there is no evidence that the spring water represents recharge o f rainfall which has undergone significant partial evaporation at the surface (Gat, 1980). Any significant evaporative losses which do occur either involve complete drying out o f the surface, or loss o f sub-surface water by transpiration. With the cessation o f rainfall in the spring, the discharge declines. For an isotropic system a
"depletion" on the spring hydrograph, under conditions o f zero recharge, can be described by a relationship of the form (Mero, 1964):
Qt = Qo e x p [ - t/to] where Q0 and Ot are the discharge rates o f the spring at the start o f drainage and at time t, respectively; and to is a depletion time characterising the physical properties o f the aquifer - - the amount o f time it would take to drain the aquifer (with no replenishment) if the discharge rate was constant at Q0. A semi-log plot o f discharge (log) versus time (linear) would yield a straight line o f slope related to -0.43/t0. In carbonate aquifers, however, such semi-log plots tend to be curved (cf. Fig. 7), and may be regarded as a summation o f two or more single exponential curves, i.e.
Ot = O0t e x p [ - t/tot ] + 002 e x p [ - t/to2 ] +... where Q0], Q02, etc., are the initial discharge rates of the individual flow components; and t01, t02, etc., are the characteristic depletion times o f these components. Following Mero (1964), graphical analysis of the semi-log plot of discharge versus time for 'En
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
77
o discharge (lit/rain)
100
i
100 o" I
10]
I I0
i 02
1
oi
,,.,.~,..,,,,..,...,.,o,,,,,2,
IC>
o2
j
,,~,,,,,,,,,,,o,,,,o,,,,,,,,,,-°~,
1>
months. Analysis based on methods of Mero (1964) suggests depletion patterns can be approximated by three components, with depletion times of: t03 = 0.25 month; to2 = 1 month; t0t = 50 months.
--0"--%0.oi (1992)
0 %0.o' (1993)]
o %2(,993) I % of discharge
~)-o3
(1993]
(1992) "-~-%Qo3
100 1 ~
100
80
80
60
60
40
40
20
20 E
0
Iz
3
4
S
6
r
8
1992
~
io
,t
iz
t
~
date
~
4
s
6
r
e
~
io
H
month
1993
Fig. 8. The proportion (%) of each one of the different flow components of Fig. 7, during the dry summer months.
Bardi, for depletion during two dry seasons, is shown in Fig. 7. The depletion in discharge can be described by the sum of three components: t03 and to2 having short depletion times of c a . 0.25month and 1 month, respectively, and t01 with a long depletion time of c a . 50 months. This separation of the depletion curves allows the total flow of water at a particular time (Qt), during the dry season, to be apportioned among the three component flows (Fig. 8). Discharge of Q03 and Q02 represents flow through the chalk via fissures of different width, and is the dominant flow during the period of winter rainfall and at the beginning of the dry season. The rapid depletion of Q03 and Q02 during the dry season (Fig. 8), together with the rapid response of spring discharge to rainfall during the wet season (Fig. 3), testify to these fissures being a small but highly transmissive reservoir. By May, about two months after the end of the main season of rainfall, the greatly reduced discharge is mainly derived from Q01. This component is the source of the sustained, low, base-flow discharge from 'En Bardi during the last 6 to 7 months of the dry summer period. Q0L probably represents "piston flow" of water through, and draining from the chalk's matrix porosity - - the main reservoir of water, but having a very low transmissivity.
78
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
4.2. Sources of nitrate 4.2.1. Dolomite springs Nitrate concentrations in the springs of 'En Kaukabe, 'En Neria and 'En Retet (Table 4) are below the normal recommended limit for nitrate in drinking water in Israel (45 mg or 0.73 meq NO3 1-1). Their 615N values, between +3.3 and +4.8%0, are within the range typically found for nitrate in rainwater (ca. -8 to +4%0; Heaton, 1987), and for nitrate derived from nitrification of organic matter in undisturbed, non-forested soils (ca. +4 to +9%0; Heaton, 1986; Macko and Ostrom, 1994). They are therefore compatible with a "natural", non-pollutant origin. While limited data for fertilizers in Israel suggest that they have 615N values in the range -2.3 to +2.3%0, in other countries values range up to ca. +5%o (Kaplan and Magaritz, 1986; Macko and Ostrom, 1994; Sandler and Heaton, 1997). To the best of our knowledge, however, fertilizers are not used in the catchment areas of these springs, and are therefore not considered as a likely source of nitrate. 4.2.2. 'En Bardi Over a 2 year sampling period 'En Bardi had nitrate concentrations ranging from 0.44 to 1.78 meq l -~. For most of this period the concentrations exceeded the 0.73 meq 1-1 recommended limit for drinking water (Fig. 4) and, on a spring discharge-weighted basis, averaged 0.78 meq 1-1. These high concentrations are associated with 615N values which are higher than in the dolomite springs (Tables 3 and 4). Increases in nitrate at 'En Bardi, moreover, are correlated with increases in 15N (Fig. 6). This suggests that there may be mixing between sources of nitrate with different concentrations and isotopic compositions. For a process involving mixing of two endmembers, mass and isotopic balance requires that: Cm = Ch)g+ CI(1 - f ) and Cm~ m = Ch~hf + C1~1(1 - f )
where the subscripts m, h and 1 refer to the mixed water, high-nitrate water and low-nitrate water endmembers, respectively; C and ¢5 are the concentrations and 615N values, respectively; and f is the fraction of the high-nitrate end-member in the mixture.
615N values of a mixture can then be calculated as a function of the concentration (Cm) from: ~m = { [(fro - Cl)(Chrh - Cl~l)/(Ch - Cl)] + f i l l } / C m
(1) The absence of waters with very low nitrate concentrations at 'En Bardi makes it difficult to estimate the likely composition of the low-nitrate end-member from the 'En Bardi data alone. Noting that the concentrations and isotopic compositions of the nitrate in the other, dolomite springs ('En Kaukabe, 'En Neria and 'En Retet) are fairly "typical" of groundwater, however, the low-nitrate end-member for 'En Bardi is assumed to have a composition similar to these springs - - i.e. on average, Cl = 0.18 meq 1-1 and 61 = + 3.8%o. Three mixing lines are then calculated from Eq. (1) and displayed in Fig. 6 assuming three different highnitrate end-members: Line 1: a high-nitrate end-member similar to the highest nitrate sample analysed at 'En Bardi (GAB 60: C h = 1.8meq1-1 a n d r h = +9%o). Line 2: a hypothetical high-nitrate end-member with the same isotopic composition, but an arbitrary, 10 times higher concentration (Ch = 18 meq 1-1 and bh = + 9%0). Line 3: a hypothetical high-nitrate end-member with both a higher concentration and an arbitrary higher 615N value (Ch -- 18 meq 1-1 and 6h = + 12%o). Distribution of the 'En Bardi water nitrate concentrations and 615N values in Fig. 6 fits mixing lines 1 or 2, but not 3. The variations in nitrate concentrations in 'En Bardi can therefore be explained in terms of mixing between water of low nitrate concentration and ¢515N value - - of the type found in the other springs - - and water with higher nitrate concentration and 615N value. The data do not allow an estimation of the nitrate concentration of the highnitrate end-member at 'En Bardi, other than it must be at least 1.8 meq 1-I, but a value of about +9%0 can be placed on the 615N value of this high-nitrate end-member. A 6 ~SN value of +9%0 eliminates fertiliser as a possible source for the high-nitrate end-member at 'En Bardi, and is at the upper end of the range of 6~5N values normally found for soil-derived nitrate.
A. Burg. T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
Organic matter in the 'En Zetim bituminous chalk itself might also be considered as a potential source of nitrate (Rosenthal et al., 1987). In two samples of this chalk, however, nitrate concentrations were only 0.05 and 0.17 meq kg -1, and were moreover lower than the concentrations of other soluble ions (Table 2). In contrast, whilst Na, K, C1, Mg and SO4 are clearly associated with nitrate in 'En Bardi water, increases in nitrate concentrations in the spring are always larger than those for these other ions (Fig. 5). In addition, 615N values for the chalk nitrate (-1.9 and -0.7%o, Table 2) were much lower than those for 'En Bardi water. On the basis of both the chemical and 15N/laN data, therefore, the 'En Zetim bituminous chalk is not the source of the nitrate in the spring water. Na, K and C1, in addition to nitrogen compounds, are known to be strongly associated with pollution derived from sewage (Twort et al., 1974). Nitrate derived from sewage commonly has high 15N/14N ratios, with 615N values in the range +10 to +20%0 often being regarded as typical (Kreitler and Jones, 1975; Gormly and Spalding, 1979; Heaton, 1984; Exner and Spalding, 1994). It must be stressed, however, that the degree of 15N enrichment is very dependent on a variety of environmental factors operating during the decomposition of sewage urea to form nitrate, especially those controlling the volatilization of ammonia. Thus lower 615N values, e.g. +9%o, might well result if loss of ammonia was inhibited by subsurface containment of sewage (Aravena et al., 1993). The chemical and isotopic evidence are therefore compatible with the high-nitrate end-member of the 'En Bardi water being derived from sewage waste. Since the perched water table draining at 'En Bardi sits on a marly aquiclude which extends below the northern part of the village of Jish (Fig. 2(a)), we conclude that nitrate and other ions from some of the village's septic tanks flow towards 'En Bardi. Variations in the concentrations and 615N values of the nitrate at 'En Bardi can be explained in terms of dilution of this sewage-contaminated water by infiltrating rainwater and by Ca + HCO3 waters typical of groundwater in carbonate aquifers. 4.3. Relationship between nitrate concentrations and hydrology
If Jish has been populated from at least the 12 th
79
century sewage contamination of the perched aquifer may have been occurring for several hundred years. In this case the nitrate will probably be distributed throughout the depth of the stratigraphic section in a relatively localised area below Jish, and dispersed in the lower part of the section over a wider area in the direction of groundwater flow. The fact that nitrate concentrations at 'En Bardi at the end of the 1993 dry season were fully restored to their levels of the previous year, and the high concentration in 1995 (Table 3), testifies to the size of the nitrate reservoir. Most of the nitrate would be in the matrix porosity, which represents the main storage reservoir of the chalk and, below the surficial zone, may remain close to saturation even during the dry summer months (Price et al., 1993; Nativ et al., 1995). If the nitrate is present in the matrix one might expect nitrate concentrations to be high in water representing drainage of the chalk matrix during the summer (discharge component Q0t), and to be low - i.e. diluted - - in water representing fissure flow in the winter (discharge components Q03 and Q02). Although this is generally true (Figs. 4, and 7), the pattern is not simple. Thus, whilst data in Fig. 6 point to the existence of a "low-nitrate end-member", water of low nitrate concentrations similar to those of the dolomite springs was never found at 'En Bardi. Even during the periods of heaviest discharge, with large volumes of water passing rapidly through fissures, nitrate concentrations do not drop below 0.44 meq 1-1. Such concentrations are more than 10 times higher than those typical of rainwater in the Galilee (ca. 0.03 meq 1-1; Herut, 1992). Evaporation/transpiration may concentrate rain- or soil-derived nitrate salts in the surficial zone of the aquifer during the summer months; but these should be removed by the first recharging winter rains and not persist in the fissure water throughout the winter months. In addition, the relatively narrow overall range in nitrate concentrations, compared with the hundredfold range in discharge, must also be explained. Thus water derived from drainage of the matrix with a discharge depletion time of ca. 50 months, has nitrate concentrations which are only up to four times higher than those of the fissure water with a discharge depletion time of ca. 0.25 to 1 month. This relative similarity between the nitrate concentrations of fissure and matrix flow implies a ready
80
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
transfer of nitrate from the matrix to the fissures. Two mechanisms may contribute to this. If the hydraulic pressure of water in the matrix is higher than that of water in the fissures, by an amount sufficient to overcome the capillary suction of the matrix pores, pollutant water may flow from the matrix into the fissures. Alternatively, where there is no net flow of water between matrix and fissures, but the concentration of the pollutant is higher in the matrix water, then the fissure water may gain pollutant by diffusion (Foster, 1993). Starting at the beginning of the wet season, the following sequence is proposed. The narrowness of the range of 6D and c5180 values for 'En Bardi, compared with the wide range normally found in rainwater, suggests that very little rainfall passes immediately through the aquifer via fissures. Mixing of rainfall probably occurs in the surficial zone of the aquifer where the effective storage capacity and transmissivity are enhanced by weathering. As noted above the earliest rainfall, at the beginning of the wet season, is largely taken up in saturating the field capacity of this zone. Subsequent movement of water downward via piston flow in the chalk matrix, however, may be limited by its low hydraulic conductivity; particularly if dry fissures interrupt the capillary tension between matrix blocks (Nativ et al., 1995). With the onset of heavy rainfall the rate of infiltration may exceed this conductivity, and result in oversaturation of the matrix, and possible temporary ponding in the surficial layer. The high hydraulic pressure developed in the matrix will then force water into the fissures, with a consequent rapid increase in flow through the aquifer and discharge at the spring. Thus in the early part of the high-discharge period much of the fissure water may have passed through the matrix in the upper part of the aquifer. Some of this fissure water can re-saturate that part of the matrix which is close to fissures in lower parts of the aquifer, where dispersion from Jish may have produced higher nitrate concentrations. Initially, this movement of water from the fissure into the matrix will inhibit movement of nitrate in the opposite direction. With the continuation of infiltration and fissure flow, however, the matrix in the lower parts of the aquifer eventually becomes fully re-saturated, allowing greater exchange of nitrate from the matrix to the fissures. Nitrate concentrations in the fissure waters
discharging at 'En Bardi therefore progressively increase during the high discharge period. Depletion of the discharge at 'En Bardi with the onset of the dry season has been described above. Fissure-discharge components Q03 and Qo2 are replaced by drainage of water from progressively smaller microvoids, and the increasing relative importance of slow piston flow through the matrix from deeper parts of the chalky blocks. Such water, having a greater residence time and being less accessible to dilution by fissure water, may be expected to have the highest nitrate concentrations. This may explain the continued increase in nitrate concentrations during the long period of stable, Q0, matrix drainage. The cause of the final increase in nitrate concentrations, corresponding to the first increase in discharge of the following winter (Fig. 4), is not clear. It may represent piston displacement of nitrate-rich matrix water, due to an increase in the hydrostatic head prior to the activation of fissure flow. In addition to variations imposed by these temporal changes in hydrology, the nitrate concentrations at 'En Bardi could also be influenced by the spatial differences in the recharge area. The septic tanks in the northern part of Jish effectively represent a point source of nitrate dispersed in the direction of groundwater flow. Discharge at 'En Bardi represents water which has recharged near Jish, or passed through the dispersal plume, mixed with water which has recharged through a wider area of less polluted aquifer. The different path lengths and residence times of these waters will also control, therefore, the temporal variations in nitrate concentrations at 'En Bardi. Thus the first fissure water to discharge at 'En Bardi after the beginning of the wet season may contain a higher proportion of water recharged in the immediate, less polluted vicinity of the spring (Fig. 2(a)). Water recharged from Jish, and having a higher nitrate concentration, will tend to appear later.
5. Summary and conclusions
The perennial spring, 'En Bardi, drains a small, perched aquifer of Upper Cretaceous chalks and has a temporal pattern of discharge which closely reflects that of the rainfall. The spring's hydrograph allows two main flow types to be characterised: slow
A. Burg, T.H.E. Heaton/Journal of Hydrology 204 (1998) 68-82
piston-flow drainage of the main storage reservoir - the chalk's matrix porosity; and rapid drainage via fissures. The matrix drainage, with a depletion time of ca. 50 months, is responsible for sustaining the low levels of base-flow discharge at the spring during the dry summer months. The fissure flow, with a depletion time of --<1 month, is responsible for the up to 100fold increase in discharge during the wet winter months. Chemical and 15N/14N isotopic data suggest that elevated levels of nitrate in the 'En Bardi water are derived from septic tanks situated in part of a village which lies in the spring's recharge area. Some of the other ions which are strongly correlated with nitrate (Na, Mg, K and C1) may also have been derived from sewage pollution. The antiquity of the village's sewage system, and the annual recovery of nitrate concentrations at the end of the dry summer, suggest that the nitrate is now dispersed in the matrix porosity of the chalk over a large part of the aquifer. Annual variations in nitrate concentrations at the spring reflect mixing between the high-nitrate, high 6 ~SN, sewagederived end-member, with low-nitrate, low 615N, Ca + HCO3 water typical of non-polluted groundwater in other carbonate aquifers in the region. In a general fashion the highest nitrate concentrations occur during the period of matrix drainage in the summer, with lower concentrations associated with fissure flow during the winter. The differences between the concentrations in these two periods, however, does not relate to a simple dilution of polluted matrix water by large volumes of freshly recharged, non-polluted fissure water. Thus a 50-fotd increase in spring discharge, associated with a change from matrix drainage to fissure flow (August-October 1992 to JanuaryMarch 1993), resulted in only a 35% reduction in nitrate concentrations. Although the spring's discharge shows a close temporal response to rainfall, this does not require that there is an immediate rapid passage of rainfall from the surface to the spring, solely via fissures. The similarity between the chemistry of very high-discharge fissure-flow waters and low-discharge matrixdrainage waters implies that much of the fissure water has mixed with, or been expelled from the matrix. Thus the spring's response to rainfall may be a largely hydraulic one. A close association between fissure and matrix waters would also account
81
for the very restricted range in the 6D and 6180 values of the spring water. A gradual increase in nitrate concentrations during the winter rainfall period of fissure flow may reflect changes in the depth, or depth interval over which this matrix-fissure interchange takes place. Mixing of waters which have followed different flow paths through the aquifer - - through and not through the zone of heavy sewage pollution - - will also influence the temporal pattern of nitrate concentrations at the spring. The recent completion of the central sewage drainage system at Jish will remove the primary source of nitrate contamination. Future monitoring of the subsequent changes in the nitrate concentrations and 615N at 'En Bardi should be continued, as this would allow an assessment of the rate at which the chalk aquifer recovers from several hundred years of pollution. In many cases the physical properties of chalks - - a large matrix storage capacity which can be "bypassed" by fissure flow - - should tend to prolong the persistence of this pollution (Foster, 1993). The similarity of the fissure and matrix waters of 'En Bardi reported here, however, suggests that they are intimately related. This should tend to speed up the flushing out of the nitrate contamination.
Acknowledgements
Professors A. Katz and A. Starinsky of the Hebrew University, and Dr A. Bein of the Geological Survey are thanked for their invaluable advice to the first author during his postgraduate studies, of which this work forms a part. The technical work and assistance of Professor Katz, Mr M. Amon and Mr R. Knafo, and the collaboration of Dr B. Spiro, are also gratefully acknowledged. Partial funding of the study was provided by the joint German-Israeli Research Fund ( B M F T / M O S T , project DISUM-20).
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