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A stable isotope study of recharge processes in the English Chalk
Journal of Hydrology, 101 (1988) 31-46
31
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
[3] A S T A B L E ISOTOPE S T U D Y OF RECHARGE P R O C E S S E S IN THE E N G L I S H CHALK
W.G. DARLING and A.H. BATH
British Geological Survey, Maclean Building, Crowmarsh Gifford, Wallingford, Oxon OXIO 8BB (U.K.) British Geological Survey, Keyworth, Notts NG12 5GG (U.K.) (Received August 11, 1987; revised and accepted December 18, 1987)
ABSTRACT Darling, W.G. and Bath, A.H., 1988. A stable isotope study of recharge processes in the English Chalk. J. Hydrol., 101: 31-46. The 52H and 51sO values of water contained in unsaturated Chalk have been measured in drillcore profiles taken over a period of 28 months on arable land in Cambridgeshire, eastern England. The isotopic composition of rainfall and lysimeter drainage on the same site has also been monitored over a period of four years. The distribution of isotope values in the upper few metres of soil and unsaturated Chalk shows that the mechanism of infiltration is not simple piston-like downward displacement since the scale of vertical fluctuations is incompatible with estimated annual infiltration and the apparent rates of movement of matrix water derived from thermonuclear tritium studies. Lysimeter drainage is not isotopically identical to matrix water at the same depth suggesting that different routes to water table are associated with consistently different isotopic content. Although there is an apparent difference between matrix water beneath arable and permanent grassland, these differences appear to cancel out in the composition of bulk recharge beneath these land types. The regional aquifer has a uniform isotopic composition similar in value to weighted average rainfall and lysimeter drainage, suggesting that the rechargeabstraction system has been in isotopic equilibrium for many years despite changes in pumping rates and land use. Two other Chalk sites in southern England have been investigated in less detail but indicate clear difference in infiltration behaviour from that seen in Cambridgeshire; this seems to be related to a higher matrix conductivity and annual rainfall amount.
INTRODUCTION
Knowledge of infiltration pathways in the Chalk unsaturated zone is of importance in attempting to quantify both recharge and transfer of pollutants to the aquifer (Foster et al., 1982). Chalk in southern England has a fine-grained carbonate matrix with high porosity but low intergranular permeability, though abundant near-surface jointing can permit rapid fissure flow under intense rainfall conditions. Well hydrograph analysis (Headworth, 1972), lysimeter studies (Kitching and Shearer, 1982), thermonuclear tritium profiling (Smith et al., 1970) and soil physical techniques (Wellings and Bell, 1980) have all been used to investigate this recharge process. These different methods
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32 suggest that the dominant flow mode may be either via the macrofissures or via the matrix, depending on the site. In particular, soil physical techniques demonstrate the spatial variability in the hydraulic properties of unsaturated Chalk; this reflects differences between the storage and conductivity of the matrix and of the fissures. Environmental tritium studies in southern England have attempted to separate fissure and intergranular recharge components and have yielded estimates of up to 20% by fissure flow (Smith et al., 1970). However, this division of flow pathways is inconsistent with the relatively nondispersed movement of tritium in interstitial water (Barker and Foster, 1981) and the tritium balance based on seasonal discrimination of inputs (Foster and SmithCarington, 1980). Seasonal relationships between unsaturated water tensions and storage (Wellings and Cooper, 1983) and balances of fertiliser-derived nitrate between saturated and unsaturated zones (Foster et al., 1982) support this conclusion. A composite model, assuming a continuum of pore and fissure sizes, may be more appropriate than bimodal flow models (see discussion in Beven and Germann, 1982). This study has used stable isotope ratio measurements of water from precipitation, unsaturated chalk and the underlying aquifer in an attempt to deduce flow mechanisms. The following questions have been addressed: (1) To what extent are seasonal fluctuations in isotopic composition of rainfall propagated into the unsaturated zone? (2) What differences, if any, occur in infiltration behaviour between different Chalk sites? (3) Is the isotopic composition of recharge to the Chalk aquifer related to seasonal or whole-year rainfall? (4) Is there evidence for long-term drift in the stable isotopic composition of groundwater due to climatic and/or land use changes? Previous investigators employing stable isotope methods to elucidate unsaturated zone flow have used both natural rainfall and artificially-enriched tracers as inputs to the system. Dispersion of 2H1HO, and also 3H1HO, tracer pulses in loamy and sandy soils in Germany demonstrated a range of porewater velocities, although most flow was sufficiently uniform to result in a ~piston" displacement of the tracer peak (Zimmermann et al., 1967a). This study concluded that soil water is not significantly fractionated by plant transpiration, a result confirmed by later workers (F5rstel, 1982; Allison et al., 1984; Sharma and Hughes, 1985; Turner et al., 1987). Stable isotope trends could therefore be used to estimate the depths of penetration of evaporative processes, these being greater under bare soil (Zimmermann et al., 1967a). More detailed mathematical, physical and experimental analysis of these evaporative enrichment patterns has permitted quantitative estimates of evaporation rates for semi-arid soils (Allison, 1982; Allison and Barnes, 1983; Barnes and Allison, 1983, 1984; Allison et al., 1983, 1984). Field studies elsewhere in arid conditions have shown that isotopic fractionation caused by evaporation from soils, or selective infiltration related to land use, can modify infiltration water (Gat and Tzur, 1967; Dinqer et al., 1974; Vogel and Van Urk, 1975). Zimmermann et al. (1967a) also suggested that selective infiltration due
33
to plant cover may counteract the effect of bare soil evaporation on 2H/1H in soil water in the European temperate climate. That such a selection mechanism can cause isotopic modification is supported by lysimeter experiments (Sauzay, 1974) and has been elaborated in detail for semi-arid soils (Allison and Hughes, 1983). Movement of labelled infiltration waters (3H, 2H, 180) has been modelled by means of the diffusion equations and a multi-layer "box" model (Zimmermann et al., 1967a, b; Mfinnich, 1983). A satisfactory agreement between observed and modelled isotope profiles required the assumption that effective diffusion coefficients were one or two orders of magnitude greater than predicted owing to additional dispersion. Similar conclusions, and the influence of soil heterogeneities on isotopic mixing patterns, are described by Fontes (1983). SITE DESCRIPTION AND SAMPLING
Detailed environmental isotope studies have been carried out on Middle Chalk at the Fleam Dyke research site, near Cambridge in eastern England, in conjunction with investigations of fertiliser-derived nitrate leaching (Foster et al., 1982), soil physical measurements (Wellings and Cooper, 1983) and recharge measurements by lysimeter (Kitching and Shearer, 1982). The research site incorporates a meteorological station and is adjacent to a pumping station for public water supply. Location and geology of the site are shown in Fig. 1; a hard Oq ~
Sol I
'~ ~I:~,.~Cr,o° 24 ~;~ Chalk ®
turbated
Flearn Dyke
Middle
3-(
Chalk
-~ 4-1
E
5-1
F
~
...... Water
table
below
1Sin
~
ulc
°
u.
~'0
120
Arable
Oll
Lysimeter
90
Fig. 1. Plan of the Fleam Dyke research site.
station
==
gauge==
Grassland
land
100
r-]
Weather Rain
80
30
50
H
Gus s a ~ : ~
Lysimeter
Pumping
lO met
res
J
station 60rn
"==~
34 band of Chalk lies at 2 m, and water table is below 15 m depth. Recharge at this site has been estimated by several methods: lysimeter measurements between 1978-83 indicate averages of 170mm a ' out of 606mm a -~ precipitation (T.R. Shearer, pers. commun.), whereas soil physical studies indicate 154 mm a- ' over the same period (J.D. Cooper, pers. commun.). The UK Meteorological Office model for evaporation in the area indicates 89mma -~ recharge out of 599 mm a ~ average rainfall between 1979 and 1983. Duplicated drillcores for extraction of water were taken from a plot of less than 50 × 50m on arable land on four occasions over a period of 29 months between 1979 and 1981 (Fig. 1). The acid-insoluble residue (quartz and clay minerals) of Chalk samples is around 5% below 2 m depth, but reaches 10-15% nearer the surface; corresponding porosities are around 45-35% but the submicron pore sizes of Chalk restrict its saturated hydraulic conductivity typically to 10 3md-~ (Foster and Bath, 1983). A drillcore profile was also taken in May 1979 on permanent grassland at a site (Golf Course) about 6 km WSW of Fleam Dyke, where about 0.25 m of rendzina soil overlies Chalk and the hard band observed at Fleam Dyke is absent. The research lysimeter at Fleam Dyke is a 5 m cube of undisturbed Chalk overlain by grassed soil (Fig. 1). It is sealed by sheet piling at the sides and base, where drainage is channelled into a metering device accessible in a trench from which aggregated water samples have been collected from 1979. High-angle joints in the Chalk were observed during the construction of the lysimeter (Kitching and Shearer, 1982).
METHOD Four sources provided water for stable isotope analysis: (1) Rainfall was collected in a standard 5-in gauge daily from October 1979, then weekly from April 1981 and fortnightly from July 1983. (2) Unsaturated zone water was extracted by centrifuge from drillcores (Edmunds and Bath, 1976). Yield by this method is 40~0% total water, but tests have shown no significant isotopic fractionation between extracted and residual water in Chalk samples. (3) Drainage at the lysimeter base collects in a tipping-bucket measuring device. Aggregated samples were collected at intervals during the periods of significant lysimeter flow (approximately November/December-May/June). (4) Groundwater from the area was collected from local wells and pumping stations, including Fleam Dyke, in October 1983; samples were measured for tritium in addition to stable isotopes. One research site drillhole, FD6, reached water table. The 180/I60 ratios were measured on CO2 prepared by equilibration with a 5 ml water sample at constant temperature. The 2H/~H ratios were measured on H2 prepared initially by reduction of 10#l of water in a uranium furnace at 850°C, though latterly by zinc in sealed reaction tubes at 450°C with direct
35 TABLE
1
Monthly
w e i g h t e d m e a n s o f ~2H a n d 3 ' 8 0 f o r r a i n f a l l a t F l e a m D y k e
Month
E rain
52H
5'sO
(mm)
(%0)
(?/co)
Month
66 - 86
9.9 - 11.9
37 42 52 26 8 44 100 33 30 67 37 46
-
69 68 68 57 55 47 52 41 33 48 52 56
- 10.3 - 9.4 - 9.8 - 8.6 - 7.3 - 6.8 - 7.7 - 4.7 - 5.3 - 7.5 - 7.9 - 8.9
39 12 101 69 63 20 58 53 50 82 23 34
-
47 56 46 23 50 33 77 24 30 60 42 89
E rain
52H
51sO
(mm)
(%0)
(%0)
44 17 41 18 75 ~1 6~ 24 47 138 79 43
-
1979 Nov Dec
47 90
1980
1982
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
1981
attachment Isotopic analysers.
of these ratios
WTW,
enabled
Harwell of
automated
to the
1H).
mass
measured
to to be
with
Chloride
colorimetry
of the
be
tritium
8.3 7.3 6.7 5.7 6.1 6.1 6.0 5.7 5.5 9.3 9.3 9.2
-
of
to
a precision
in of
(Coleman
the
extracted
(1 TR water
=
1 atom
samples
+ 1 mg 1-'.
6.9 - 10.8 6.2 8.5 - 9.5 3.7 5.2 - 4.3 - 5.7 - 5.4 9.2 - 8.9
et al., 1982). with
working
from Isotope
scale.
_+ 29/00 5 2 H . drillcores
by
Laboratory
of 3H to every were
twin
standard
V-SMOW-SLAP
Environmental
+_ 2 T R
concentrations
45 76 41 55 67 26 31 27 36 40 63 55
spectrometer
+ 0.2%o 5180 and
in water
by the
-
Wallingford on
than
-
inlet 602E
calibrated
better
performed a precision
27 35 44 105 110 22 71 22 61 34 33 51
a VG-Isogas
measurements results
was
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
spectrometer
on
of thermonuclear
distillation
of AERE
7.3 8.3 6.7 3.7 7.4 4.6 - 10.5 - 4.6 - 4.9 - 8.3 - 6.8 - 12.7
is estimated
Measurement vacuum
-
tubes
were
Simultaneous
Reproducibility
atoms
-
1983
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
water,
55 49 45 37 30 43 40 39 38 66 65 64
measured
1 0 ~8 by
36 2O
14.4_-1.8 mg/I
mg 1-1 15 10
62H -40 %o
E
mm
-47.7-*1.6%o
-so
80 4¢ 13 -20
%o
S - Or ~ -90 L
~V'-54°~ z-.-.'522 m m
~-'-48%o L.,604m m
I I ~
r-51%o z--,632 m m
~-'-50%o l--, 6 1 5 m m
8O mm
40 o
I
1980
1
1981
1
1982
I
1983
I
Fig. 2. The stable isotopic composition of rainfall and lysimeter drainage at Fleam Dyke.
-20 + .¢.
-30
÷
÷
+
+ ÷÷
-40
÷
2 o
+
+
-50
÷
÷
+÷
"I-
¢u
-60 ~::~
6'H = 7.30 6'~0 + 4 . 0 4
-70 +
-80
-90
-t3
-12
+
- 1t
÷
I
I
I
1
I
I
1
- t0
-9
-8
-7
-6
-5
-4
6'80%o
Fig. 3. 62H versus 6180 for calculated monthly rainfall values at Fleam Dyke, 1979-83.
-3
37
8 2 H % 0 ESMOWl -70 -60 -SO -40 -80 -S0 -40 -30 -20 -S0 -SO -40 -30 -20 -60 -SO -40 -30 -20 I
I May
I
I
1979
I
r
I
Oct 1980
I
i
I
I Feb
I
198,1.
I
I
I
I
1
l
I
Sept 1981
2 3 ~4
¶"t
~5 E
Bib positionof
~ the 1963-64 /' 3H peak in ~,~' FD 3,7& GMI//
K6 ~7
l e
i
'
/
'~ pl
• FD9 =FD10
°FD11 "FD12
,
8
9 10 arable
land
grassland
i ° FD 3 "FD5 OGM1
"FD7 "FD8
DFDG
Fig. 4. Depth profiles of $2H for unsaturated zone water at Fleam Dyke (FD) at four dates between 1979 and 1981. Solid symbols are profiles below arable land (see Fig. 1); open symbols are profiles below permanent grassland at Fleam Dyke (FDG) and at a site 6km away (GM1).
Rainfall
The weighted monthly mean isotopic compositions of rainfall over fifty months from late 1979 to 1983 are listed in Table 1. From April 1981 these averages are calculated from weekly sampling, but before this collections were made daily and extreme day-to-day variations of up to 60 or 70%0 ~2H were observed. Such short-term variations, the causes of which are not apparent from simple comparisons with meteorological data, must reflect the complexity of water vapour sources and air mass movements in the maritime climate of England (Heathcote and Lloyd, 1986; Lawler, 1987). It is clear, too, from the longer term record (Fig. 2) t h a t there is no simple relationship between isotope ratio and amount of rainfall. However, the correlation between ~2H and 5180 for monthly rainfall (Fig. 3) is strong: 52H = 7.305180 + 4.04
(r 2 = 0.96, n = 50)
Although there is a fairly well developed tendency for summer rainfall to be isotopically enriched relative to winter rainfall, Fig. 2 shows that it is not always the case: 1981 for example was unsettled in this respect. Unsaturated zone water
Interstitial water samples from duplicated drillcores were taken from the research site (Fig. 1) in May 1979, October 1980, and February and September
38 1981, and were ana|ysed in each case for 1~0 and ~H content. For clarity only ~2H is shown in Fig. 4, in which isotope compositions are plotted against depth. Profiles from two permanent grassland sites are also shown: FDG from the lysimeter block at Fleam Dyke, and GM1 from the Golf Course site 6 km away. Thermonuclear tritium levels were also measured in porewaters from boreholes FD3, FD7 and GM1, and the position of the 1963-64 peak is shown in Fig. 4; the peak is well-defined at between 6 and 7m deep with values up to 190TR (Foster and Bath, 1983). The close similarity between data from duplicate drillcores, shown in Fig. 4, suggests that the data are representative of this site at their respective sampling dates. The chloride content of all samples except those from FDG was measured, but because of fertiliser application only the grassland borehole GM1 is representative of the natural state. This profile has a "steady-state" drainage C1- content of 23.4 + 1.4 mg l- 1.
Lysimeter drainage Samples of lysimeter drainage collected between 1979 and 1983 had an almost constant composition compared to fluctuations observed in rain (Fig. 2). Isotopic values varied from - 5 0 to -45%o ~2H and f r o m - 7 . 3 to -6.5%0 ~lso. Concentrations of chloride were in the range 11.3-18.5mg1-1. Unweighted means are - 47.7%0~2H, - 7.0%0~lso and 14.4 mg 1-~ C1-. Two samples collected in J a n u a r y and March, 1983, were analysed for tritium and found to contain 85 and 75 TR, respectively.
Groundwater Groundwater from the catchment of the Chalk aquifer present at Fleam Dyke was sampled in October 1983 at six pumping stations (including Fleam Dyke) and two farm boreholes, all within a radius of 7 km of the research site. Compositions were uniform: - 5 0 to -47%o62H, - 7 . 3 to -7.1%o~1so and 1530mg1-1C1-. Tritium in five of these samples varied between 3 and 7TR (Table 2). DISCUSSION
Rainfall Weighted average rainfall for the calendar years 1980-83 has isotopic compositions of - 54, - 48, - 51 and - 50% 52H, respectively (Fig. 2). The weighted average for the whole four years is -50.4%0 52H, close to the mean of pumping station output (-48.4%o52H). From the low tritium values measured at the pumping stations, quite apart from simple hydraulic considerations, it is clear that little or none of this rainwater could have reached the aquifer in time for sampling in late 1983, and therefore it is inferred that
39 TABLE 2 Stable isotope, tritium and chloride data for groundwater from Fleam Dyke and surrounding area Source
Date of sampling
5~H (%o)
5180 (%o)
3H (TR)
C1 (mg l- 1)
Fleam Dyke PS FDG, water table Babraham PS Fleam Dyke PS Fulbourne PS Linton PS Westley PS Wilbraham PS Dotterel Hall Farm Valley Farm, Fulbourne FD6, water table
June June Oct Oct Oct Oct Oct Oct Oct Oct Apt
- 51 - 50 - 49 -48 - 47 - 50 -49 - 47 - 48 - 49 - 48
- 7.2 - 7.2 - 7.2 - 7.3 - 7.3 - 7.3 - 7.2 - 7.2 - 7.1 - 7.1 -7.3
nd nd nd 7 7 5 3 nd 3 nd nd
nd nd 30 19 19 16 16 15 18 20 nd
1979 1979 1983 1983 1983 1983 1983 1983 1983 1983 1984
PS - pumping station, nd = not determined. recharge to the aquifer has remained isotopically constant for tens of years, if not much longer. The similarity of rainfall composition to that of groundwater also suggests that rainfall from all months of the year must contribute to groundwater replenishment. Unsaturated zone
Isotope ratios in temperate-climate unsaturated zone profiles might be expected to reach near-surface minima in winter-spring owing to isotopicallydepleted winter rainfall, and maxima in summer-auttunn owing to isotopicallye n r i c h e d s u m m e r r a i n f a l l a n d / o r e v a p o r a t i o n a l p r o c e s s e s . T h i s p a t t e r n is l a r g e l y a d h e r e d t o a t F l e a m D y k e ( F i g . 4), t h o u g h t h e F e b r u a r y 1981 p r o f i l e s (FD9/10) are anomalous, probably because the preceding relatively dry autumn and early winter caused little modification of the previous October's profiles. I n d e e d t h e l y s i m e t e r d r a i n a g e f o r t h e 1980-81 s e a s o n w a s b y F e b r u a r y 1981 t h e lowest for four years, at only half the average of the previous three winters ( K i t c h i n g a n d S h e a r e r , 1982). S i g n s o f e v a p o r a t i o n a l e n r i c h m e n t a r e s e e n i n t h e p r o f i l e s o f O c t o b e r 1980 ( a v e r a g e 5H2-~lsO g r a d i e n t o f 5.7 f o r s a m p l e s a b o v e t h e i s o t o p i c m i n i m u m ) a n d t o a l e s s e r e x t e n t i n t h o s e o f S e p t e m b e r 1981 ( a v e r a g e s l o p e 6.4). O t h e r w i s e , i s o t o p i c e n r i c h m e n t is p r e s u m e d t o b e d u e t o a n t e c e d e n t rainfall. Beneath a depth of about 1 m all the arable profiles have similar shapes and v a l u e s , r e a c h i n g a g r e a t e s t d e p l e t i o n ( a b o u t - 54%0 5~H) b e t w e e n 1 a n d 2 m a n d a g r e a t e s t e n r i c h m e n t ( a b o u t - 4 0 % o ) b e t w e e n 5 a n d 7 m. T h i s c h a r a c t e r i s t i c shape could be due simply to climatic variability or soil physical effects, or both combined. If variations are caused by climatic effects than these are on a t i m e s c a l e o f s e v e r a l y e a r s , b e c a u s e t h e p o s i t i o n o f t h e 1963 t h e r m o n u c l e a r
40
tritium peak in profiles FD3, 7 and GM1 shows that apparent drainage velocity is only about 0.4 m a 1, whereas the scale of variation is of the order of a metre or more. Rainfall isotopic records for Fleam Dyke do not go back far enough to confirm or disprove a climatic effect, but there is some apparent downward movement of the isotopic minimum between May 1979 and September 1981 (Fig. 4), and the distance (0.8 m) is consistent with rates of movement suggested by the tritium peak. The apparent isotopic consistency of recharge, however, implies that any climatic influences must average themselves out fairly rapidly, and therefore shorter-term effects are more likely to be responsible for profile minima. It is considered by Barnes and Allison (1984) that the non-isothermal conditions existing in the field soil column can cause development of an isotopic profile which reaches a minimum value several per rail (~2H) more negative than the steady-state drainage composition. While this may well be the case for warmer semi-arid climates, the theory appears to demand soil temperature gradients greater than the English climate is able to provide. A more likely cause at Fleam Dyke is that heavy, isotopically depleted rainfall may penetrate more deeply than lighter rainfall (an explanation also considered by Turner et al. (1987) for some rather pronounced isotopic minima in profiles from Western Australia). There is no good correlation between rainfall amount and isotopic depletion apparent from Fig. 2, and even examination of daily data from 1980 (the only full calendar year available) shows that the weighted mean of rainfall events exceeding twice the daily average of 4.9 mm is scarcely different from the mean of all 1980 rainfall. However, if these larger events are divided into ~'winter" (January-March, October-December) and ~'summer" (April-September) categories, weighted means are - 6 4 and 46%052H, respectively, from comparable amounts of rain. It therefore appears that the isotopic minima are the product of rainfall during the winter, when soil physical conditions are also likely to be more conducive to deeper penetration, as indicated by the periodic nature of lysimeter discharge (Fig. 2). -
62H, %o -50
- 4 0L
-30
o
~°
~ 1
ID\ ~ 2
"0.
3
b
4 1'5
2'0
2'5
moisture content, %
Fig. 5. Averaged moisture content (% dry weight) and J2H data for arable profiles FD7-FD12.
41 Whatever the explanation of the characteristic profile shape, examination of averaged moisture contents in the top 4 m of profiles FD7 to 12 (Fig. 5) reveals that the isotopic minimum is associated with a soil water content well below the maximum possible of around 25-30% by weight. This implies that the isotopic deviations of the top few metres are of proportionately small importance to the mass balance of the profile as a whole. The two grassland profiles, GM1 and FDG, suggest that recharge beneath permanent grass cover is somewhat isotopically-depleted relative to the arable plots; the reasons for this would presumably be related to land use. Because neither profile extends beyond 4m depth nor is duplicated the evidence is somewhat equivocal, but the GM1 profile, taken 6 km away, is of significance because it closely matches the FD3/5 profiles. This proves that factors peculiar to the Fleam Dyke site alone are not involved in profile formation.
Lysimeter drainage The average lysimeter drainage of approximately - 48%o/52H at 5 m depth is virtually identical to the composition of groundwater measured in 1979 and 1984, both in the general area and beneath (or close by) the research plots (Table 2). This does not agree with the drillcore profiles at the same depth (5 m), which range between - 4 0 and -45% o~2H. However, the profiles sample the micropore end of a probable continuum of drainage routes and can only represent the bulk isotopic composition of all water drainage through the unsaturated zone if an isotopic equilibrium has been reached with any fastermoving water, which on this evidence is doubtful. The lysimeter has a surface area of 25 m 2, over three thousand times greater than the cross-section of a 100 mm diameter drillcore and thus much more likely to contain a range of pore and fissure sizes typical of the unsaturated zone throughout the catchment. Therefore, though the lysimeter walls and base must inevitably distort natural soil physical conditions, lysimeter drainage should be reasonably representative of the recharge water at least in the vicinity of a depth of 5 m. As the isotopic composition is identical to t h a t at the water table some 10m or so below it implies that possible short-to-medium term climatic fluctuations have no discernible effect on the isotope ratios of recharge water. Lysimeter discharge rates are known to respond to particularly heavy rainfall events within a few days (T.R. Shearer, pers. commun.). If this is the result of direct penetration, rather than a piston effect, the isotopic composition of percolate does not respond significantly to it, suggesting that either direct infiltration is of little importance in volumetric terms, or that rapid mixing with water in the larger pores has occurred.
Groundwater and the flow mechanism of infiltration While the characteristic shape of the first few metres of each profile is of problematical origin, lysimeter drainage and steady-state composition of
42 porewater are of more importance in terms of the bulk recharge to the Chalk aquifer. Within measurement limits groundwater isotopic ratios are indistinguishable from those of lysimeter percolate, close to those of weighted annual average rainfall, and have remained essentially unchanged for at least four years both at Fleam Dyke pumping station and at the water table beneath the research site (a borehole, FD6, was drilled to water table in 1979). Low thermonuclear tritium values in pumped wells in the area indicate only a small component of post-1964 water in the aquifer (Table 2). If lysimeter percolate is the result of admixture between slowly-moving micropore water and faster drainage through fissures and macropores, then the bulk of faster moving water must necessarily be isotopically lighter to bring about the sampled compositions. It is not clear, owing to a lack of samples down to water table, whether the arable porewater profiles really do reach a steadystate drainage composition, or whether they are slowly becoming more isotopically negative towards the water table. The evidence from the deepest profiles, FD3, together with the water table samples from FD6 in 1979 and 1984 suggests the latter may be the case. If true, it is probable that a large proportion of the faster-moving water is achieving an isotopic equilibrium with the profile water before water table is reached. The chloride balance of the system offers some support to this theory of unsaturated zone flow. Lysimeter drainage is more dilute, at an average of 14.4 + 1.8mgl-lC1 -, than "steady-state" porewater C1 at about 23.4 + 1.4mg1-1 beneath grassland. Local pumping station and farm well water (with the exception of Babraham) contains 17.6 + 1.9 mg l- 1C1- (Table 2). It is clear that as with isotopic ratios, lysimeter drainage is not chemically identical to micropore water composition. The kind of conditions t h a t promote this faster flow are therefore those that favour lower levels of chloride and lighter isotopic composition. These might be presumed to occur in winter when evapotranspiration is low and there is a tendency (already demonstrated) for rainfall to be isotopically depleted. The profiles GM1, and FDG (Fig. 4) suggest that grassland somehow causes a slightly isotopically-lighter infiltration. Even if this is so it cannot have a great influence on the catchment as a whole, because permanent grassland has covered 10% or less of the catchment since the early 1940's (Carey and Lloyd, 1985). It is in any case contradicted by the lysimeter percolate which is isotopically identical to groundwater, most of which must be the result of recharge under arable land. The implication therefore is that while grassland may result in isotopically-lighter micropore water this is balanced by a modified fissure/ macropore flow, with net recharge closely similar to that under arable land in terms of isotope ratio. COMPARISON WITH OTHER CHALKSITES The stable isotopic composition of infiltration water has been investigated at two further sites in southern England, the locations of which are shown in the inset to Fig. 1. The site at Bridgets Experimental Husbandry Farm (EHF)
43
has also been studied by detailed soil physical measurements (Wellings and Bell, 1980), and in addition a small irrigation experiment using deuterated water followed by cored profile sampling was carried out (Wellings, 1982). The Chalk at Gussage was the location of early studies on thermonuclear tritium distribution in the unsaturated zone which established the concept of downward piston displacement of most infiltration with minor by-pass flow (Smith and Richards, 1972). Further tritium measurements at this site have been reported and the simple model of piston displacement and preservation of tritium mass balance critically examined (Foster and Smith-Carington, 1980). At Bridgets EHF, 0.25 m of brown silty loam overlies rubbly chalk to 1.5 m, below which is solid Upper Chalk. The water table is deep, at between 38 and 42 m depth. The 25-year mean annual rainfall is 794 ram. Samples of rain collected from November 1978 to April 1981 had isotopic compositions related by the equation: 62H = 7.38~1so+ 4.99
(r 2 = 0.97, n = 41)
Annual net infiltration is equivalent to about 0.9 m downward movement. A profile of 52H is shown in Fig. 6; the scale of fluctuations corresponds to that of annual infiltration, and seasonal variations are preserved with progressive attenuation. The tracer experiments (Wellings, 1982) showed that the injected pulse was almost completely dispersed by 2 m depth. Interstitial water samples from the Chalk at Gussage were collected simultaneously with a further suite of samples for tritium analysis. Two profiles were measured, OG9A and 10A (Fig. 6), which were below rough grassland, passing through about 0.3 m of brown calcareous soil. Borehole OG9A reached water 62H%o 30
-6o-so-,o
i
= I i July 1979
11
[sMow~ -6o-so-,o -3o F
~ i Dec 1978
,
Gussage
2I 3
Bridget's
4
EHF
P •$ 5 E
~6 i
7 S
~
position o1 the 1 9 6 3 - 6 4 3H peak in 1970
• OG 9 A • OG IOA
10 all" 3H peak in 1977
12 13
iJ~
3 H peak at - 1 5 m in 1 9 7 9 'C-
Fig. 6. Typical depth profiles of 52H for u n s a t u r a t e d zone water at two other sites on Chalk in southern England (see Fig. 1).
44 table at around 15m depth whereas OG10A terminated above this point. Positions of the thermonuclear tritium peak in 1970, 1977 and 1979 are shown in Fig. 6; these suggest somewhat irregular downward penetration of just under 1 m a - ' (Smith and Richards, 1972; Foster and Smith-Carington, 1980). Values of fi2H show that seasonal cyclicity is preserved with decreasing amplitude to about 6 m depth (Fig. 6). Similar results have been demonstrated for the Chalk of the Champagne region, northern France, which is south of southern England in terms of latitude but has a mean annual rainfall of 630 mm, similar to Fleam Dyke though with higher infiltration (Vachier et al., 1987). There is a clear contrast between the characteristic isotopic composition at the top of the Fleam Dyke profiles and the propagation of seasonal rainfall variations into the Bridgets E H F and Gussage Chalk profiles. This may be attributed principally to higher infiltration rates in southern (> 400mm a 1) than in eastern England (<200mma-1), but also to different infiltration mechanisms. These latter would be influenced by bulk differences in lithology due to position in the Chalk sequence and also conceivably by postdepositional history, for example the absence of glaciation in southern England. Wellings et al. (1982) report that matrix conductivity at Bridgets E H F is about five times higher than at Fleam Dyke; this and other physical variables may be responsible for allowing fissure flow more readily at Fleam Dyke than at Bridgets EHF. CONCLUSIONS Aquifer recharge appears from several years' records to be closely related to weighted annual average rainfall and to have remained constant for many years. This implies that rain the year round contributes to the isotopic composition of recharge. There are signs that different land use can modify the balance of isotopic distribution between infiltration flow paths, but no evidence that it can affect the bulk composition of recharge. The piston displacement mechanism of unsaturated zone flow adequately explains isotopic profiles developed in southern England but is inappropriate to the Fleam Dyke site, where the porewater isotope record shows no sign of seasonal cyclicity. This variation in behaviour appears to depend primarily on matrix conductivity, though rainfall amount must also contribute. A conclusion relevant to pollution studies is that differences in infiltration behaviour and, by implication, solute transfer have been shown to occur between the three sites considered, and therefore results of site-specific studies should be generalised with some caution. ACKNOWLEDGEMENTS We are grateful to Adrian Brunsdon for the majority of isotope measurements. These studies were carried out in collaboration with the Groundwater Diffuse Pollution Section and Infiltration Studies Section of the BGS Hydrogeology Group, and the Soil Physics Section of the Institute of Hydrology. Our
45
colleagues are thanked for collecting drillcore samples, extracting interstitial water and collecting rainfall. A large amount of subsidiary information has been made freely available to us. We particularly thank Sam Boyle, Lionel Bridge, Jenny Cook, David Cooper, Stephen Foster, Amanda Geake, Adrian Lawrence and Bob Shearer, and we pay tribute to the memory of the late Steve Wellings, a valued colleague who contributed much to the knowledge of water movement in the Chalk unsaturated zone. The comments of Brian Payne and Brian Smith were helpful in improving this paper, which is published with the permission of the Director, British Geological Survey (NERC). REFERENCES Allison, G.B., 1982. The relationship between 180 and deuterium in water in sand columns undergoing evaporation. J. Hydrol., 55: 163-169. Allison, G.B. and Barnes, C.J., 1983. Estimation of evaporation from non-vegetated surfaces using natural deuterium. Nature, 301: 143-145. Allison, G.B. and Hughes, M.W., 1983. The use of natural tracers as indicators of soil.water movement in a temperate semi-arid region. J. Hydrol., 60:157 173. Allison, G.B., Barnes, C.J. and Hughes, M.W., 1983. The distribution of deuterium and oxygen-18 in dry soils. II: Experimental. J. Hydrol., 64:377 397. Allison, G.B., Barnes, C.J., Hughes, M.W. and Leaney, F.W.J., 1984. The effect of climate and vegetation on oxygen-18 and deuterium profiles in soils. In: Isotope Hydrology 1983. Proc. Symp. IAEA, Vienna, pp. 105-123. Barker, J.A. and Foster, S.S.D., 1981. A diffusion exchange model for solute movement in fissured porous rock. Q. J. Eng. Geol., 14: 17-24. Barnes, C.J. and Allison, G.B., 1983. The distribution of deuterium and ~sO in dry soils. I: Theory. J. Hydrol., 60: 141-156. Barnes, C.J. and Allison, G.B., 1984. The distribution of deuterium and ~80 in dry soils. III: Theory for non-isothermal water movement. J. Hydrol., 74: 119-135. Beven, K. and Germann, P., 1982. Macropores and water flow in soils. Water Resour. Res., 18: 1311-1325. Carey, M.A. and Lloyd, J.W., 1985. Modelling non-point sources of nitrate pollution of groundwater in the Great Ouse Chalk, UK. J. Hydrol., 78: 83-106. Coleman, M., Shepherd, T.J., Durham, J.J., Rouse, J.E. and Moore, G.R., 1982. Reduction of water with zinc for hydrogen isotope analysis. Anal. Chem., 54: 993-995. Dinner, T., A1-Mugrin, A. and Zimmermann, U., 1974. Study of the infiltration and recharge through the sand dunes in arid zones with special reference to the stable isotopes and thermonuclear tritium. J. Hydrol., 23: 79-110. Edmunds, W.M. and Bath, A.H., 1976. Centrifuge extraction and chemical analysis of interstitial waters. Environ. Sci. Technol., 10: 467472. Fontes, J.-Ch., 1983. Some examples of isotope studies of the unsaturated zone. In: Review of French Hydrogeology. Rep. Inst. Geol. Sci., 82(6): 60-70. F6rstel, H., 1982. ~80/~O-ratio of water in plants and in their environment (results from Fed. Rep. Germany). In: H.L. Schmidt, H. FSrstel and K. Heinzinger (Editors), Stable Isotopes. Elsevier, Amsterdam, pp. 503-516. Foster, S.S.D. and Smith-Carington, A., 1980. The interpretation of tritium in the Chalk unsaturated zone. J. Hydrol., 46: 343-364. Foster, S.S.D. and Bath, A.H., 1983. The distribution of agricultural soil leachates in the unsaturated zone of the British Chalk. Environ. Geol., 5: 53-59. Foster, S.S.D., Cripps, A.C. and Smith-Carington, A., 1982. Nitrate leaching to groundwater. Phil. Trans. R. Soc. London, 296: 477~489.
46 Gat, J.R. and Tzur, Y., 1967. Modification of the isotope composition of rainwater by processes which occur before groundwater recharge. Proc. Symp. lsot. Hydrol., IAEA, Vienna, pp. 49-60. Headworth, H.G., 1972. The analysis of n a t u r a l groundwater fluctuations in the Chalk of Hampshire. J. Inst. Water Eng., 26: 107-124. Heathcote, J.A. and Lloyd, J.W., 1986. Factors affecting the isotopic composition of daily rainfall at Driby, Lincolnshire. J. Climatol., 6: 97-106. Kitching, R. and Shearer, T.R., 1982. Construction and operation of a large undisturbed lysimeter to measure recharge to the Chalk aquifer, England. J. Hydrol., 58:267 277. Lawler, H.A., 1987. Sampling for isotopic responses in surface waters. E a r t h Surf. Proces. Landforms, 12: 551-559. Mfinnich, K.O., 1983. Moisture movement in the u n s a t u r a t e d zone. In: Guidebook on Nuclear Techniques in Hydrology. Tech. Rep. Ser., IAEA, Vienna, 91: 203-222. Sauzay, G., 1974. Sampling of lysimeters for environmental isotopes of water. In: Isotope Techniques in Groundwater Hydrology. Proc. Symp. IAEA, Vienna, pp. 61~8. Sharma, M.L. and Hughes, M.W., 1985. Groundwater recharge estimation using chloride, deuterium and oxygen-18 profiles in the deep coastal sands of Western Australia. J. Hydrol., 81: 93-109. Smith, D.B. and Richards, H.J., 1972. Selected environmental studies using radioactive tracers. Proc. Symp. Peaceful Uses At. Energ., IAEA, Vienna, pp. 467480. Smith, D.B., Wearn, P.L., Richards, H.J. and Rowe, P.C., 1970. Water movement in the unsaturated zone of high and low permeability strata by measuring natural tritium. Proc. Symp. Isot. Hydrology, IAEA, Vienna, pp. 73 86. Turner, J.V., Arad, A. and Johnston, C.D., 1987. Environmental isotope hydrology of salinized experimental catchments. J. Hydrol., 94: 8~107. Vachier, P., Dever, L. and Fontes, J.C., 1987. Mouvements de l'eau dans la zone non satur~e et alimentation de la nappe de la craie de Champagne (France). Isot. Tech. Water Resour. Dev., Proc. Symp. IAEA, Vienna, pp. 367-379. Vogel, J.C. and Van Urk, H., 1975. Isotopic composition of groundwater in semi-arid regions of southern Africa. J. Hydrol., 25:23 36. Wellings, S.R., 1982. The use of deuterium oxide (2H20) as a water tracer in a study of hydrology of the u n s a t u r a t e d zone of the English Chalk. In: H.L. Schmidt, H. FSrstel and K. Heinzinger (Editors), Stable Isotopes. Elsevier, Amsterdam, pp. 167-171. Wellings, S.R. and Bell, J.P., 1980. Movement of water and nitrate in the unsaturated zone of Upper Chalk near Winchester, Hants, England. J. Hydrol., 48: 11~136. Wellings, S.R. and Cooper, J.D., 1983. The variability of recharge of the English Chalk aquifer. Agric. Water Manage., 6:243 253. Wellings, S.R., Cooper, J.D. and Bell, J.P., 1982. The physics of solute movement in the unsaturated zone of the British Chalk. Proc. Symp. Impact Agric. Act. Groundwater, IAH, Prague, pp. 3 7 ~ 388. Zimmermann, U., Ehhalt, D. and Miinnich, K.O., 1967a. Soil water movement and evapotranspiration: changes in the isotopic composition of the water. Proc. Symp. Isot. Hydrol., IAEA, Vienna, pp. 567 585. Zimmermann, U., Mfinnich, K.O. and Roether, W., 1967b. Downward movement of soil moisture traced by means of hydrogen isotopes. In: Isotope Techniques in the Hydrologic Cycle. Am. Geophys. Union, Geophys. Monogr. Ser., 11: 2~36.
31
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
[3] A S T A B L E ISOTOPE S T U D Y OF RECHARGE P R O C E S S E S IN THE E N G L I S H CHALK
W.G. DARLING and A.H. BATH
British Geological Survey, Maclean Building, Crowmarsh Gifford, Wallingford, Oxon OXIO 8BB (U.K.) British Geological Survey, Keyworth, Notts NG12 5GG (U.K.) (Received August 11, 1987; revised and accepted December 18, 1987)
ABSTRACT Darling, W.G. and Bath, A.H., 1988. A stable isotope study of recharge processes in the English Chalk. J. Hydrol., 101: 31-46. The 52H and 51sO values of water contained in unsaturated Chalk have been measured in drillcore profiles taken over a period of 28 months on arable land in Cambridgeshire, eastern England. The isotopic composition of rainfall and lysimeter drainage on the same site has also been monitored over a period of four years. The distribution of isotope values in the upper few metres of soil and unsaturated Chalk shows that the mechanism of infiltration is not simple piston-like downward displacement since the scale of vertical fluctuations is incompatible with estimated annual infiltration and the apparent rates of movement of matrix water derived from thermonuclear tritium studies. Lysimeter drainage is not isotopically identical to matrix water at the same depth suggesting that different routes to water table are associated with consistently different isotopic content. Although there is an apparent difference between matrix water beneath arable and permanent grassland, these differences appear to cancel out in the composition of bulk recharge beneath these land types. The regional aquifer has a uniform isotopic composition similar in value to weighted average rainfall and lysimeter drainage, suggesting that the rechargeabstraction system has been in isotopic equilibrium for many years despite changes in pumping rates and land use. Two other Chalk sites in southern England have been investigated in less detail but indicate clear difference in infiltration behaviour from that seen in Cambridgeshire; this seems to be related to a higher matrix conductivity and annual rainfall amount.
INTRODUCTION
Knowledge of infiltration pathways in the Chalk unsaturated zone is of importance in attempting to quantify both recharge and transfer of pollutants to the aquifer (Foster et al., 1982). Chalk in southern England has a fine-grained carbonate matrix with high porosity but low intergranular permeability, though abundant near-surface jointing can permit rapid fissure flow under intense rainfall conditions. Well hydrograph analysis (Headworth, 1972), lysimeter studies (Kitching and Shearer, 1982), thermonuclear tritium profiling (Smith et al., 1970) and soil physical techniques (Wellings and Bell, 1980) have all been used to investigate this recharge process. These different methods
0022-1694/88/$03.50
© 1988 Elsevier Science Publishers B.V.
32 suggest that the dominant flow mode may be either via the macrofissures or via the matrix, depending on the site. In particular, soil physical techniques demonstrate the spatial variability in the hydraulic properties of unsaturated Chalk; this reflects differences between the storage and conductivity of the matrix and of the fissures. Environmental tritium studies in southern England have attempted to separate fissure and intergranular recharge components and have yielded estimates of up to 20% by fissure flow (Smith et al., 1970). However, this division of flow pathways is inconsistent with the relatively nondispersed movement of tritium in interstitial water (Barker and Foster, 1981) and the tritium balance based on seasonal discrimination of inputs (Foster and SmithCarington, 1980). Seasonal relationships between unsaturated water tensions and storage (Wellings and Cooper, 1983) and balances of fertiliser-derived nitrate between saturated and unsaturated zones (Foster et al., 1982) support this conclusion. A composite model, assuming a continuum of pore and fissure sizes, may be more appropriate than bimodal flow models (see discussion in Beven and Germann, 1982). This study has used stable isotope ratio measurements of water from precipitation, unsaturated chalk and the underlying aquifer in an attempt to deduce flow mechanisms. The following questions have been addressed: (1) To what extent are seasonal fluctuations in isotopic composition of rainfall propagated into the unsaturated zone? (2) What differences, if any, occur in infiltration behaviour between different Chalk sites? (3) Is the isotopic composition of recharge to the Chalk aquifer related to seasonal or whole-year rainfall? (4) Is there evidence for long-term drift in the stable isotopic composition of groundwater due to climatic and/or land use changes? Previous investigators employing stable isotope methods to elucidate unsaturated zone flow have used both natural rainfall and artificially-enriched tracers as inputs to the system. Dispersion of 2H1HO, and also 3H1HO, tracer pulses in loamy and sandy soils in Germany demonstrated a range of porewater velocities, although most flow was sufficiently uniform to result in a ~piston" displacement of the tracer peak (Zimmermann et al., 1967a). This study concluded that soil water is not significantly fractionated by plant transpiration, a result confirmed by later workers (F5rstel, 1982; Allison et al., 1984; Sharma and Hughes, 1985; Turner et al., 1987). Stable isotope trends could therefore be used to estimate the depths of penetration of evaporative processes, these being greater under bare soil (Zimmermann et al., 1967a). More detailed mathematical, physical and experimental analysis of these evaporative enrichment patterns has permitted quantitative estimates of evaporation rates for semi-arid soils (Allison, 1982; Allison and Barnes, 1983; Barnes and Allison, 1983, 1984; Allison et al., 1983, 1984). Field studies elsewhere in arid conditions have shown that isotopic fractionation caused by evaporation from soils, or selective infiltration related to land use, can modify infiltration water (Gat and Tzur, 1967; Dinqer et al., 1974; Vogel and Van Urk, 1975). Zimmermann et al. (1967a) also suggested that selective infiltration due
33
to plant cover may counteract the effect of bare soil evaporation on 2H/1H in soil water in the European temperate climate. That such a selection mechanism can cause isotopic modification is supported by lysimeter experiments (Sauzay, 1974) and has been elaborated in detail for semi-arid soils (Allison and Hughes, 1983). Movement of labelled infiltration waters (3H, 2H, 180) has been modelled by means of the diffusion equations and a multi-layer "box" model (Zimmermann et al., 1967a, b; Mfinnich, 1983). A satisfactory agreement between observed and modelled isotope profiles required the assumption that effective diffusion coefficients were one or two orders of magnitude greater than predicted owing to additional dispersion. Similar conclusions, and the influence of soil heterogeneities on isotopic mixing patterns, are described by Fontes (1983). SITE DESCRIPTION AND SAMPLING
Detailed environmental isotope studies have been carried out on Middle Chalk at the Fleam Dyke research site, near Cambridge in eastern England, in conjunction with investigations of fertiliser-derived nitrate leaching (Foster et al., 1982), soil physical measurements (Wellings and Cooper, 1983) and recharge measurements by lysimeter (Kitching and Shearer, 1982). The research site incorporates a meteorological station and is adjacent to a pumping station for public water supply. Location and geology of the site are shown in Fig. 1; a hard Oq ~
Sol I
'~ ~I:~,.~Cr,o° 24 ~;~ Chalk ®
turbated
Flearn Dyke
Middle
3-(
Chalk
-~ 4-1
E
5-1
F
~
...... Water
table
below
1Sin
~
ulc
°
u.
~'0
120
Arable
Oll
Lysimeter
90
Fig. 1. Plan of the Fleam Dyke research site.
station
==
gauge==
Grassland
land
100
r-]
Weather Rain
80
30
50
H
Gus s a ~ : ~
Lysimeter
Pumping
lO met
res
J
station 60rn
"==~
34 band of Chalk lies at 2 m, and water table is below 15 m depth. Recharge at this site has been estimated by several methods: lysimeter measurements between 1978-83 indicate averages of 170mm a ' out of 606mm a -~ precipitation (T.R. Shearer, pers. commun.), whereas soil physical studies indicate 154 mm a- ' over the same period (J.D. Cooper, pers. commun.). The UK Meteorological Office model for evaporation in the area indicates 89mma -~ recharge out of 599 mm a ~ average rainfall between 1979 and 1983. Duplicated drillcores for extraction of water were taken from a plot of less than 50 × 50m on arable land on four occasions over a period of 29 months between 1979 and 1981 (Fig. 1). The acid-insoluble residue (quartz and clay minerals) of Chalk samples is around 5% below 2 m depth, but reaches 10-15% nearer the surface; corresponding porosities are around 45-35% but the submicron pore sizes of Chalk restrict its saturated hydraulic conductivity typically to 10 3md-~ (Foster and Bath, 1983). A drillcore profile was also taken in May 1979 on permanent grassland at a site (Golf Course) about 6 km WSW of Fleam Dyke, where about 0.25 m of rendzina soil overlies Chalk and the hard band observed at Fleam Dyke is absent. The research lysimeter at Fleam Dyke is a 5 m cube of undisturbed Chalk overlain by grassed soil (Fig. 1). It is sealed by sheet piling at the sides and base, where drainage is channelled into a metering device accessible in a trench from which aggregated water samples have been collected from 1979. High-angle joints in the Chalk were observed during the construction of the lysimeter (Kitching and Shearer, 1982).
METHOD Four sources provided water for stable isotope analysis: (1) Rainfall was collected in a standard 5-in gauge daily from October 1979, then weekly from April 1981 and fortnightly from July 1983. (2) Unsaturated zone water was extracted by centrifuge from drillcores (Edmunds and Bath, 1976). Yield by this method is 40~0% total water, but tests have shown no significant isotopic fractionation between extracted and residual water in Chalk samples. (3) Drainage at the lysimeter base collects in a tipping-bucket measuring device. Aggregated samples were collected at intervals during the periods of significant lysimeter flow (approximately November/December-May/June). (4) Groundwater from the area was collected from local wells and pumping stations, including Fleam Dyke, in October 1983; samples were measured for tritium in addition to stable isotopes. One research site drillhole, FD6, reached water table. The 180/I60 ratios were measured on CO2 prepared by equilibration with a 5 ml water sample at constant temperature. The 2H/~H ratios were measured on H2 prepared initially by reduction of 10#l of water in a uranium furnace at 850°C, though latterly by zinc in sealed reaction tubes at 450°C with direct
35 TABLE
1
Monthly
w e i g h t e d m e a n s o f ~2H a n d 3 ' 8 0 f o r r a i n f a l l a t F l e a m D y k e
Month
E rain
52H
5'sO
(mm)
(%0)
(?/co)
Month
66 - 86
9.9 - 11.9
37 42 52 26 8 44 100 33 30 67 37 46
-
69 68 68 57 55 47 52 41 33 48 52 56
- 10.3 - 9.4 - 9.8 - 8.6 - 7.3 - 6.8 - 7.7 - 4.7 - 5.3 - 7.5 - 7.9 - 8.9
39 12 101 69 63 20 58 53 50 82 23 34
-
47 56 46 23 50 33 77 24 30 60 42 89
E rain
52H
51sO
(mm)
(%0)
(%0)
44 17 41 18 75 ~1 6~ 24 47 138 79 43
-
1979 Nov Dec
47 90
1980
1982
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
1981
attachment Isotopic analysers.
of these ratios
WTW,
enabled
Harwell of
automated
to the
1H).
mass
measured
to to be
with
Chloride
colorimetry
of the
be
tritium
8.3 7.3 6.7 5.7 6.1 6.1 6.0 5.7 5.5 9.3 9.3 9.2
-
of
to
a precision
in of
(Coleman
the
extracted
(1 TR water
=
1 atom
samples
+ 1 mg 1-'.
6.9 - 10.8 6.2 8.5 - 9.5 3.7 5.2 - 4.3 - 5.7 - 5.4 9.2 - 8.9
et al., 1982). with
working
from Isotope
scale.
_+ 29/00 5 2 H . drillcores
by
Laboratory
of 3H to every were
twin
standard
V-SMOW-SLAP
Environmental
+_ 2 T R
concentrations
45 76 41 55 67 26 31 27 36 40 63 55
spectrometer
+ 0.2%o 5180 and
in water
by the
-
Wallingford on
than
-
inlet 602E
calibrated
better
performed a precision
27 35 44 105 110 22 71 22 61 34 33 51
a VG-Isogas
measurements results
was
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
spectrometer
on
of thermonuclear
distillation
of AERE
7.3 8.3 6.7 3.7 7.4 4.6 - 10.5 - 4.6 - 4.9 - 8.3 - 6.8 - 12.7
is estimated
Measurement vacuum
-
tubes
were
Simultaneous
Reproducibility
atoms
-
1983
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
water,
55 49 45 37 30 43 40 39 38 66 65 64
measured
1 0 ~8 by
36 2O
14.4_-1.8 mg/I
mg 1-1 15 10
62H -40 %o
E
mm
-47.7-*1.6%o
-so
80 4¢ 13 -20
%o
S - Or ~ -90 L
~V'-54°~ z-.-.'522 m m
~-'-48%o L.,604m m
I I ~
r-51%o z--,632 m m
~-'-50%o l--, 6 1 5 m m
8O mm
40 o
I
1980
1
1981
1
1982
I
1983
I
Fig. 2. The stable isotopic composition of rainfall and lysimeter drainage at Fleam Dyke.
-20 + .¢.
-30
÷
÷
+
+ ÷÷
-40
÷
2 o
+
+
-50
÷
÷
+÷
"I-
¢u
-60 ~::~
6'H = 7.30 6'~0 + 4 . 0 4
-70 +
-80
-90
-t3
-12
+
- 1t
÷
I
I
I
1
I
I
1
- t0
-9
-8
-7
-6
-5
-4
6'80%o
Fig. 3. 62H versus 6180 for calculated monthly rainfall values at Fleam Dyke, 1979-83.
-3
37
8 2 H % 0 ESMOWl -70 -60 -SO -40 -80 -S0 -40 -30 -20 -S0 -SO -40 -30 -20 -60 -SO -40 -30 -20 I
I May
I
I
1979
I
r
I
Oct 1980
I
i
I
I Feb
I
198,1.
I
I
I
I
1
l
I
Sept 1981
2 3 ~4
¶"t
~5 E
Bib positionof
~ the 1963-64 /' 3H peak in ~,~' FD 3,7& GMI//
K6 ~7
l e
i
'
/
'~ pl
• FD9 =FD10
°FD11 "FD12
,
8
9 10 arable
land
grassland
i ° FD 3 "FD5 OGM1
"FD7 "FD8
DFDG
Fig. 4. Depth profiles of $2H for unsaturated zone water at Fleam Dyke (FD) at four dates between 1979 and 1981. Solid symbols are profiles below arable land (see Fig. 1); open symbols are profiles below permanent grassland at Fleam Dyke (FDG) and at a site 6km away (GM1).
Rainfall
The weighted monthly mean isotopic compositions of rainfall over fifty months from late 1979 to 1983 are listed in Table 1. From April 1981 these averages are calculated from weekly sampling, but before this collections were made daily and extreme day-to-day variations of up to 60 or 70%0 ~2H were observed. Such short-term variations, the causes of which are not apparent from simple comparisons with meteorological data, must reflect the complexity of water vapour sources and air mass movements in the maritime climate of England (Heathcote and Lloyd, 1986; Lawler, 1987). It is clear, too, from the longer term record (Fig. 2) t h a t there is no simple relationship between isotope ratio and amount of rainfall. However, the correlation between ~2H and 5180 for monthly rainfall (Fig. 3) is strong: 52H = 7.305180 + 4.04
(r 2 = 0.96, n = 50)
Although there is a fairly well developed tendency for summer rainfall to be isotopically enriched relative to winter rainfall, Fig. 2 shows that it is not always the case: 1981 for example was unsettled in this respect. Unsaturated zone water
Interstitial water samples from duplicated drillcores were taken from the research site (Fig. 1) in May 1979, October 1980, and February and September
38 1981, and were ana|ysed in each case for 1~0 and ~H content. For clarity only ~2H is shown in Fig. 4, in which isotope compositions are plotted against depth. Profiles from two permanent grassland sites are also shown: FDG from the lysimeter block at Fleam Dyke, and GM1 from the Golf Course site 6 km away. Thermonuclear tritium levels were also measured in porewaters from boreholes FD3, FD7 and GM1, and the position of the 1963-64 peak is shown in Fig. 4; the peak is well-defined at between 6 and 7m deep with values up to 190TR (Foster and Bath, 1983). The close similarity between data from duplicate drillcores, shown in Fig. 4, suggests that the data are representative of this site at their respective sampling dates. The chloride content of all samples except those from FDG was measured, but because of fertiliser application only the grassland borehole GM1 is representative of the natural state. This profile has a "steady-state" drainage C1- content of 23.4 + 1.4 mg l- 1.
Lysimeter drainage Samples of lysimeter drainage collected between 1979 and 1983 had an almost constant composition compared to fluctuations observed in rain (Fig. 2). Isotopic values varied from - 5 0 to -45%o ~2H and f r o m - 7 . 3 to -6.5%0 ~lso. Concentrations of chloride were in the range 11.3-18.5mg1-1. Unweighted means are - 47.7%0~2H, - 7.0%0~lso and 14.4 mg 1-~ C1-. Two samples collected in J a n u a r y and March, 1983, were analysed for tritium and found to contain 85 and 75 TR, respectively.
Groundwater Groundwater from the catchment of the Chalk aquifer present at Fleam Dyke was sampled in October 1983 at six pumping stations (including Fleam Dyke) and two farm boreholes, all within a radius of 7 km of the research site. Compositions were uniform: - 5 0 to -47%o62H, - 7 . 3 to -7.1%o~1so and 1530mg1-1C1-. Tritium in five of these samples varied between 3 and 7TR (Table 2). DISCUSSION
Rainfall Weighted average rainfall for the calendar years 1980-83 has isotopic compositions of - 54, - 48, - 51 and - 50% 52H, respectively (Fig. 2). The weighted average for the whole four years is -50.4%0 52H, close to the mean of pumping station output (-48.4%o52H). From the low tritium values measured at the pumping stations, quite apart from simple hydraulic considerations, it is clear that little or none of this rainwater could have reached the aquifer in time for sampling in late 1983, and therefore it is inferred that
39 TABLE 2 Stable isotope, tritium and chloride data for groundwater from Fleam Dyke and surrounding area Source
Date of sampling
5~H (%o)
5180 (%o)
3H (TR)
C1 (mg l- 1)
Fleam Dyke PS FDG, water table Babraham PS Fleam Dyke PS Fulbourne PS Linton PS Westley PS Wilbraham PS Dotterel Hall Farm Valley Farm, Fulbourne FD6, water table
June June Oct Oct Oct Oct Oct Oct Oct Oct Apt
- 51 - 50 - 49 -48 - 47 - 50 -49 - 47 - 48 - 49 - 48
- 7.2 - 7.2 - 7.2 - 7.3 - 7.3 - 7.3 - 7.2 - 7.2 - 7.1 - 7.1 -7.3
nd nd nd 7 7 5 3 nd 3 nd nd
nd nd 30 19 19 16 16 15 18 20 nd
1979 1979 1983 1983 1983 1983 1983 1983 1983 1983 1984
PS - pumping station, nd = not determined. recharge to the aquifer has remained isotopically constant for tens of years, if not much longer. The similarity of rainfall composition to that of groundwater also suggests that rainfall from all months of the year must contribute to groundwater replenishment. Unsaturated zone
Isotope ratios in temperate-climate unsaturated zone profiles might be expected to reach near-surface minima in winter-spring owing to isotopicallydepleted winter rainfall, and maxima in summer-auttunn owing to isotopicallye n r i c h e d s u m m e r r a i n f a l l a n d / o r e v a p o r a t i o n a l p r o c e s s e s . T h i s p a t t e r n is l a r g e l y a d h e r e d t o a t F l e a m D y k e ( F i g . 4), t h o u g h t h e F e b r u a r y 1981 p r o f i l e s (FD9/10) are anomalous, probably because the preceding relatively dry autumn and early winter caused little modification of the previous October's profiles. I n d e e d t h e l y s i m e t e r d r a i n a g e f o r t h e 1980-81 s e a s o n w a s b y F e b r u a r y 1981 t h e lowest for four years, at only half the average of the previous three winters ( K i t c h i n g a n d S h e a r e r , 1982). S i g n s o f e v a p o r a t i o n a l e n r i c h m e n t a r e s e e n i n t h e p r o f i l e s o f O c t o b e r 1980 ( a v e r a g e 5H2-~lsO g r a d i e n t o f 5.7 f o r s a m p l e s a b o v e t h e i s o t o p i c m i n i m u m ) a n d t o a l e s s e r e x t e n t i n t h o s e o f S e p t e m b e r 1981 ( a v e r a g e s l o p e 6.4). O t h e r w i s e , i s o t o p i c e n r i c h m e n t is p r e s u m e d t o b e d u e t o a n t e c e d e n t rainfall. Beneath a depth of about 1 m all the arable profiles have similar shapes and v a l u e s , r e a c h i n g a g r e a t e s t d e p l e t i o n ( a b o u t - 54%0 5~H) b e t w e e n 1 a n d 2 m a n d a g r e a t e s t e n r i c h m e n t ( a b o u t - 4 0 % o ) b e t w e e n 5 a n d 7 m. T h i s c h a r a c t e r i s t i c shape could be due simply to climatic variability or soil physical effects, or both combined. If variations are caused by climatic effects than these are on a t i m e s c a l e o f s e v e r a l y e a r s , b e c a u s e t h e p o s i t i o n o f t h e 1963 t h e r m o n u c l e a r
40
tritium peak in profiles FD3, 7 and GM1 shows that apparent drainage velocity is only about 0.4 m a 1, whereas the scale of variation is of the order of a metre or more. Rainfall isotopic records for Fleam Dyke do not go back far enough to confirm or disprove a climatic effect, but there is some apparent downward movement of the isotopic minimum between May 1979 and September 1981 (Fig. 4), and the distance (0.8 m) is consistent with rates of movement suggested by the tritium peak. The apparent isotopic consistency of recharge, however, implies that any climatic influences must average themselves out fairly rapidly, and therefore shorter-term effects are more likely to be responsible for profile minima. It is considered by Barnes and Allison (1984) that the non-isothermal conditions existing in the field soil column can cause development of an isotopic profile which reaches a minimum value several per rail (~2H) more negative than the steady-state drainage composition. While this may well be the case for warmer semi-arid climates, the theory appears to demand soil temperature gradients greater than the English climate is able to provide. A more likely cause at Fleam Dyke is that heavy, isotopically depleted rainfall may penetrate more deeply than lighter rainfall (an explanation also considered by Turner et al. (1987) for some rather pronounced isotopic minima in profiles from Western Australia). There is no good correlation between rainfall amount and isotopic depletion apparent from Fig. 2, and even examination of daily data from 1980 (the only full calendar year available) shows that the weighted mean of rainfall events exceeding twice the daily average of 4.9 mm is scarcely different from the mean of all 1980 rainfall. However, if these larger events are divided into ~'winter" (January-March, October-December) and ~'summer" (April-September) categories, weighted means are - 6 4 and 46%052H, respectively, from comparable amounts of rain. It therefore appears that the isotopic minima are the product of rainfall during the winter, when soil physical conditions are also likely to be more conducive to deeper penetration, as indicated by the periodic nature of lysimeter discharge (Fig. 2). -
62H, %o -50
- 4 0L
-30
o
~°
~ 1
ID\ ~ 2
"0.
3
b
4 1'5
2'0
2'5
moisture content, %
Fig. 5. Averaged moisture content (% dry weight) and J2H data for arable profiles FD7-FD12.
41 Whatever the explanation of the characteristic profile shape, examination of averaged moisture contents in the top 4 m of profiles FD7 to 12 (Fig. 5) reveals that the isotopic minimum is associated with a soil water content well below the maximum possible of around 25-30% by weight. This implies that the isotopic deviations of the top few metres are of proportionately small importance to the mass balance of the profile as a whole. The two grassland profiles, GM1 and FDG, suggest that recharge beneath permanent grass cover is somewhat isotopically-depleted relative to the arable plots; the reasons for this would presumably be related to land use. Because neither profile extends beyond 4m depth nor is duplicated the evidence is somewhat equivocal, but the GM1 profile, taken 6 km away, is of significance because it closely matches the FD3/5 profiles. This proves that factors peculiar to the Fleam Dyke site alone are not involved in profile formation.
Lysimeter drainage The average lysimeter drainage of approximately - 48%o/52H at 5 m depth is virtually identical to the composition of groundwater measured in 1979 and 1984, both in the general area and beneath (or close by) the research plots (Table 2). This does not agree with the drillcore profiles at the same depth (5 m), which range between - 4 0 and -45% o~2H. However, the profiles sample the micropore end of a probable continuum of drainage routes and can only represent the bulk isotopic composition of all water drainage through the unsaturated zone if an isotopic equilibrium has been reached with any fastermoving water, which on this evidence is doubtful. The lysimeter has a surface area of 25 m 2, over three thousand times greater than the cross-section of a 100 mm diameter drillcore and thus much more likely to contain a range of pore and fissure sizes typical of the unsaturated zone throughout the catchment. Therefore, though the lysimeter walls and base must inevitably distort natural soil physical conditions, lysimeter drainage should be reasonably representative of the recharge water at least in the vicinity of a depth of 5 m. As the isotopic composition is identical to t h a t at the water table some 10m or so below it implies that possible short-to-medium term climatic fluctuations have no discernible effect on the isotope ratios of recharge water. Lysimeter discharge rates are known to respond to particularly heavy rainfall events within a few days (T.R. Shearer, pers. commun.). If this is the result of direct penetration, rather than a piston effect, the isotopic composition of percolate does not respond significantly to it, suggesting that either direct infiltration is of little importance in volumetric terms, or that rapid mixing with water in the larger pores has occurred.
Groundwater and the flow mechanism of infiltration While the characteristic shape of the first few metres of each profile is of problematical origin, lysimeter drainage and steady-state composition of
42 porewater are of more importance in terms of the bulk recharge to the Chalk aquifer. Within measurement limits groundwater isotopic ratios are indistinguishable from those of lysimeter percolate, close to those of weighted annual average rainfall, and have remained essentially unchanged for at least four years both at Fleam Dyke pumping station and at the water table beneath the research site (a borehole, FD6, was drilled to water table in 1979). Low thermonuclear tritium values in pumped wells in the area indicate only a small component of post-1964 water in the aquifer (Table 2). If lysimeter percolate is the result of admixture between slowly-moving micropore water and faster drainage through fissures and macropores, then the bulk of faster moving water must necessarily be isotopically lighter to bring about the sampled compositions. It is not clear, owing to a lack of samples down to water table, whether the arable porewater profiles really do reach a steadystate drainage composition, or whether they are slowly becoming more isotopically negative towards the water table. The evidence from the deepest profiles, FD3, together with the water table samples from FD6 in 1979 and 1984 suggests the latter may be the case. If true, it is probable that a large proportion of the faster-moving water is achieving an isotopic equilibrium with the profile water before water table is reached. The chloride balance of the system offers some support to this theory of unsaturated zone flow. Lysimeter drainage is more dilute, at an average of 14.4 + 1.8mgl-lC1 -, than "steady-state" porewater C1 at about 23.4 + 1.4mg1-1 beneath grassland. Local pumping station and farm well water (with the exception of Babraham) contains 17.6 + 1.9 mg l- 1C1- (Table 2). It is clear that as with isotopic ratios, lysimeter drainage is not chemically identical to micropore water composition. The kind of conditions t h a t promote this faster flow are therefore those that favour lower levels of chloride and lighter isotopic composition. These might be presumed to occur in winter when evapotranspiration is low and there is a tendency (already demonstrated) for rainfall to be isotopically depleted. The profiles GM1, and FDG (Fig. 4) suggest that grassland somehow causes a slightly isotopically-lighter infiltration. Even if this is so it cannot have a great influence on the catchment as a whole, because permanent grassland has covered 10% or less of the catchment since the early 1940's (Carey and Lloyd, 1985). It is in any case contradicted by the lysimeter percolate which is isotopically identical to groundwater, most of which must be the result of recharge under arable land. The implication therefore is that while grassland may result in isotopically-lighter micropore water this is balanced by a modified fissure/ macropore flow, with net recharge closely similar to that under arable land in terms of isotope ratio. COMPARISON WITH OTHER CHALKSITES The stable isotopic composition of infiltration water has been investigated at two further sites in southern England, the locations of which are shown in the inset to Fig. 1. The site at Bridgets Experimental Husbandry Farm (EHF)
43
has also been studied by detailed soil physical measurements (Wellings and Bell, 1980), and in addition a small irrigation experiment using deuterated water followed by cored profile sampling was carried out (Wellings, 1982). The Chalk at Gussage was the location of early studies on thermonuclear tritium distribution in the unsaturated zone which established the concept of downward piston displacement of most infiltration with minor by-pass flow (Smith and Richards, 1972). Further tritium measurements at this site have been reported and the simple model of piston displacement and preservation of tritium mass balance critically examined (Foster and Smith-Carington, 1980). At Bridgets EHF, 0.25 m of brown silty loam overlies rubbly chalk to 1.5 m, below which is solid Upper Chalk. The water table is deep, at between 38 and 42 m depth. The 25-year mean annual rainfall is 794 ram. Samples of rain collected from November 1978 to April 1981 had isotopic compositions related by the equation: 62H = 7.38~1so+ 4.99
(r 2 = 0.97, n = 41)
Annual net infiltration is equivalent to about 0.9 m downward movement. A profile of 52H is shown in Fig. 6; the scale of fluctuations corresponds to that of annual infiltration, and seasonal variations are preserved with progressive attenuation. The tracer experiments (Wellings, 1982) showed that the injected pulse was almost completely dispersed by 2 m depth. Interstitial water samples from the Chalk at Gussage were collected simultaneously with a further suite of samples for tritium analysis. Two profiles were measured, OG9A and 10A (Fig. 6), which were below rough grassland, passing through about 0.3 m of brown calcareous soil. Borehole OG9A reached water 62H%o 30
-6o-so-,o
i
= I i July 1979
11
[sMow~ -6o-so-,o -3o F
~ i Dec 1978
,
Gussage
2I 3
Bridget's
4
EHF
P •$ 5 E
~6 i
7 S
~
position o1 the 1 9 6 3 - 6 4 3H peak in 1970
• OG 9 A • OG IOA
10 all" 3H peak in 1977
12 13
iJ~
3 H peak at - 1 5 m in 1 9 7 9 'C-
Fig. 6. Typical depth profiles of 52H for u n s a t u r a t e d zone water at two other sites on Chalk in southern England (see Fig. 1).
44 table at around 15m depth whereas OG10A terminated above this point. Positions of the thermonuclear tritium peak in 1970, 1977 and 1979 are shown in Fig. 6; these suggest somewhat irregular downward penetration of just under 1 m a - ' (Smith and Richards, 1972; Foster and Smith-Carington, 1980). Values of fi2H show that seasonal cyclicity is preserved with decreasing amplitude to about 6 m depth (Fig. 6). Similar results have been demonstrated for the Chalk of the Champagne region, northern France, which is south of southern England in terms of latitude but has a mean annual rainfall of 630 mm, similar to Fleam Dyke though with higher infiltration (Vachier et al., 1987). There is a clear contrast between the characteristic isotopic composition at the top of the Fleam Dyke profiles and the propagation of seasonal rainfall variations into the Bridgets E H F and Gussage Chalk profiles. This may be attributed principally to higher infiltration rates in southern (> 400mm a 1) than in eastern England (<200mma-1), but also to different infiltration mechanisms. These latter would be influenced by bulk differences in lithology due to position in the Chalk sequence and also conceivably by postdepositional history, for example the absence of glaciation in southern England. Wellings et al. (1982) report that matrix conductivity at Bridgets E H F is about five times higher than at Fleam Dyke; this and other physical variables may be responsible for allowing fissure flow more readily at Fleam Dyke than at Bridgets EHF. CONCLUSIONS Aquifer recharge appears from several years' records to be closely related to weighted annual average rainfall and to have remained constant for many years. This implies that rain the year round contributes to the isotopic composition of recharge. There are signs that different land use can modify the balance of isotopic distribution between infiltration flow paths, but no evidence that it can affect the bulk composition of recharge. The piston displacement mechanism of unsaturated zone flow adequately explains isotopic profiles developed in southern England but is inappropriate to the Fleam Dyke site, where the porewater isotope record shows no sign of seasonal cyclicity. This variation in behaviour appears to depend primarily on matrix conductivity, though rainfall amount must also contribute. A conclusion relevant to pollution studies is that differences in infiltration behaviour and, by implication, solute transfer have been shown to occur between the three sites considered, and therefore results of site-specific studies should be generalised with some caution. ACKNOWLEDGEMENTS We are grateful to Adrian Brunsdon for the majority of isotope measurements. These studies were carried out in collaboration with the Groundwater Diffuse Pollution Section and Infiltration Studies Section of the BGS Hydrogeology Group, and the Soil Physics Section of the Institute of Hydrology. Our
45
colleagues are thanked for collecting drillcore samples, extracting interstitial water and collecting rainfall. A large amount of subsidiary information has been made freely available to us. We particularly thank Sam Boyle, Lionel Bridge, Jenny Cook, David Cooper, Stephen Foster, Amanda Geake, Adrian Lawrence and Bob Shearer, and we pay tribute to the memory of the late Steve Wellings, a valued colleague who contributed much to the knowledge of water movement in the Chalk unsaturated zone. The comments of Brian Payne and Brian Smith were helpful in improving this paper, which is published with the permission of the Director, British Geological Survey (NERC). REFERENCES Allison, G.B., 1982. The relationship between 180 and deuterium in water in sand columns undergoing evaporation. J. Hydrol., 55: 163-169. Allison, G.B. and Barnes, C.J., 1983. Estimation of evaporation from non-vegetated surfaces using natural deuterium. Nature, 301: 143-145. Allison, G.B. and Hughes, M.W., 1983. The use of natural tracers as indicators of soil.water movement in a temperate semi-arid region. J. Hydrol., 60:157 173. Allison, G.B., Barnes, C.J. and Hughes, M.W., 1983. The distribution of deuterium and oxygen-18 in dry soils. II: Experimental. J. Hydrol., 64:377 397. Allison, G.B., Barnes, C.J., Hughes, M.W. and Leaney, F.W.J., 1984. The effect of climate and vegetation on oxygen-18 and deuterium profiles in soils. In: Isotope Hydrology 1983. Proc. Symp. IAEA, Vienna, pp. 105-123. Barker, J.A. and Foster, S.S.D., 1981. A diffusion exchange model for solute movement in fissured porous rock. Q. J. Eng. Geol., 14: 17-24. Barnes, C.J. and Allison, G.B., 1983. The distribution of deuterium and ~sO in dry soils. I: Theory. J. Hydrol., 60: 141-156. Barnes, C.J. and Allison, G.B., 1984. The distribution of deuterium and ~80 in dry soils. III: Theory for non-isothermal water movement. J. Hydrol., 74: 119-135. Beven, K. and Germann, P., 1982. Macropores and water flow in soils. Water Resour. Res., 18: 1311-1325. Carey, M.A. and Lloyd, J.W., 1985. Modelling non-point sources of nitrate pollution of groundwater in the Great Ouse Chalk, UK. J. Hydrol., 78: 83-106. Coleman, M., Shepherd, T.J., Durham, J.J., Rouse, J.E. and Moore, G.R., 1982. Reduction of water with zinc for hydrogen isotope analysis. Anal. Chem., 54: 993-995. Dinner, T., A1-Mugrin, A. and Zimmermann, U., 1974. Study of the infiltration and recharge through the sand dunes in arid zones with special reference to the stable isotopes and thermonuclear tritium. J. Hydrol., 23: 79-110. Edmunds, W.M. and Bath, A.H., 1976. Centrifuge extraction and chemical analysis of interstitial waters. Environ. Sci. Technol., 10: 467472. Fontes, J.-Ch., 1983. Some examples of isotope studies of the unsaturated zone. In: Review of French Hydrogeology. Rep. Inst. Geol. Sci., 82(6): 60-70. F6rstel, H., 1982. ~80/~O-ratio of water in plants and in their environment (results from Fed. Rep. Germany). In: H.L. Schmidt, H. FSrstel and K. Heinzinger (Editors), Stable Isotopes. Elsevier, Amsterdam, pp. 503-516. Foster, S.S.D. and Smith-Carington, A., 1980. The interpretation of tritium in the Chalk unsaturated zone. J. Hydrol., 46: 343-364. Foster, S.S.D. and Bath, A.H., 1983. The distribution of agricultural soil leachates in the unsaturated zone of the British Chalk. Environ. Geol., 5: 53-59. Foster, S.S.D., Cripps, A.C. and Smith-Carington, A., 1982. Nitrate leaching to groundwater. Phil. Trans. R. Soc. London, 296: 477~489.
46 Gat, J.R. and Tzur, Y., 1967. Modification of the isotope composition of rainwater by processes which occur before groundwater recharge. Proc. Symp. lsot. Hydrol., IAEA, Vienna, pp. 49-60. Headworth, H.G., 1972. The analysis of n a t u r a l groundwater fluctuations in the Chalk of Hampshire. J. Inst. Water Eng., 26: 107-124. Heathcote, J.A. and Lloyd, J.W., 1986. Factors affecting the isotopic composition of daily rainfall at Driby, Lincolnshire. J. Climatol., 6: 97-106. Kitching, R. and Shearer, T.R., 1982. Construction and operation of a large undisturbed lysimeter to measure recharge to the Chalk aquifer, England. J. Hydrol., 58:267 277. Lawler, H.A., 1987. Sampling for isotopic responses in surface waters. E a r t h Surf. Proces. Landforms, 12: 551-559. Mfinnich, K.O., 1983. Moisture movement in the u n s a t u r a t e d zone. In: Guidebook on Nuclear Techniques in Hydrology. Tech. Rep. Ser., IAEA, Vienna, 91: 203-222. Sauzay, G., 1974. Sampling of lysimeters for environmental isotopes of water. In: Isotope Techniques in Groundwater Hydrology. Proc. Symp. IAEA, Vienna, pp. 61~8. Sharma, M.L. and Hughes, M.W., 1985. Groundwater recharge estimation using chloride, deuterium and oxygen-18 profiles in the deep coastal sands of Western Australia. J. Hydrol., 81: 93-109. Smith, D.B. and Richards, H.J., 1972. Selected environmental studies using radioactive tracers. Proc. Symp. Peaceful Uses At. Energ., IAEA, Vienna, pp. 467480. Smith, D.B., Wearn, P.L., Richards, H.J. and Rowe, P.C., 1970. Water movement in the unsaturated zone of high and low permeability strata by measuring natural tritium. Proc. Symp. Isot. Hydrology, IAEA, Vienna, pp. 73 86. Turner, J.V., Arad, A. and Johnston, C.D., 1987. Environmental isotope hydrology of salinized experimental catchments. J. Hydrol., 94: 8~107. Vachier, P., Dever, L. and Fontes, J.C., 1987. Mouvements de l'eau dans la zone non satur~e et alimentation de la nappe de la craie de Champagne (France). Isot. Tech. Water Resour. Dev., Proc. Symp. IAEA, Vienna, pp. 367-379. Vogel, J.C. and Van Urk, H., 1975. Isotopic composition of groundwater in semi-arid regions of southern Africa. J. Hydrol., 25:23 36. Wellings, S.R., 1982. The use of deuterium oxide (2H20) as a water tracer in a study of hydrology of the u n s a t u r a t e d zone of the English Chalk. In: H.L. Schmidt, H. FSrstel and K. Heinzinger (Editors), Stable Isotopes. Elsevier, Amsterdam, pp. 167-171. Wellings, S.R. and Bell, J.P., 1980. Movement of water and nitrate in the unsaturated zone of Upper Chalk near Winchester, Hants, England. J. Hydrol., 48: 11~136. Wellings, S.R. and Cooper, J.D., 1983. The variability of recharge of the English Chalk aquifer. Agric. Water Manage., 6:243 253. Wellings, S.R., Cooper, J.D. and Bell, J.P., 1982. The physics of solute movement in the unsaturated zone of the British Chalk. Proc. Symp. Impact Agric. Act. Groundwater, IAH, Prague, pp. 3 7 ~ 388. Zimmermann, U., Ehhalt, D. and Miinnich, K.O., 1967a. Soil water movement and evapotranspiration: changes in the isotopic composition of the water. Proc. Symp. Isot. Hydrol., IAEA, Vienna, pp. 567 585. Zimmermann, U., Mfinnich, K.O. and Roether, W., 1967b. Downward movement of soil moisture traced by means of hydrogen isotopes. In: Isotope Techniques in the Hydrologic Cycle. Am. Geophys. Union, Geophys. Monogr. Ser., 11: 2~36.