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Relation of Groundwater Quantity and Quality (Proceedings of the Hamburg Symposium, August 1983). IAHS Publ. no, 146.
Detection of subsurface seepage between aquifers by hydrochemical and environmental isotopic techniques - a case study from South Australia
L. M. RAMAMURTHY & J. W, HOLMES The Flinders University of South Australia, Bedford Park, South Australia 5042
ABSTRACT The rising salinities and sources of recharge to a confined aquifer by vertical seepage from an overlying unconfined aquifer and by lateral seepage from a fresh-water lake were investigated using major ions and environmental isotopes 23l*U, 2 3 8U, 2H and 3H. Estimates obtained by flownet analyses were not substantiated by physical groundwater modelling. A comprehensive interpretation based on the above environmental tracers proved useful in understanding the hydrological relationships between the aquifers and associated water bodies. A linear correlation between U-concentration and HCOg content observed for the groundwater proved useful in delineating aquifers and in inferring subsurface seepage between them.
Detection de l'infiltration souterraine entre les couches aguifères par des techniques hydrochimiques et isotopiques - un cas étudié en Australie du Sud RESUME On a étudié la salinité croissante d'une nappe captive et ses sources d'alimentation par l'infiltration verticale venant d'une nappe libre au-dessus et par l'infiltration latérale venant d'un lac d'eau douce, en se servant des ions principaux et des isotopes U, 2 3 8U, 2H et 3H. Des estimations obtenues par des analyses des courbes isopièzes n'ont pas pu être justifiées par des observations sur place. Une interprétation détaillée basée sur les traceurs du milieu (cités plus haut) s'est révélée utile pour la compréhension des rapports hydrologiques entre les couches aquifères et les masses d'eau associées. Une corrélation linéaire entre la concentration U et le contenu HCOg mesurés pour l'eau souterraine s'est révélée utile pour la délimitation des couches aquifères et l'interprétation de l'écoulement entre elles.
INTRODUCTION
A major limitation to the understanding and modelling of groundwater flow is the estimation of seepage between aquifers. Information is often lacking about the extent, thickness and hydraulic conductivity of the confining beds which may be more complex and variable in their physical and hydraulic characteristics than the aquifers themselves. The problem is widespread and assumes great
267
268 L.M.Ramamurthy & J.W.Holmes
importance in the context of pollution, notably salinity, transmitted in groundwater aquifers by vertical and lateral seepage. Improved insights into flow behaviour commonly rely on physical modelling but use is often made of environmental isotopes such as 2H, 1 8 0 , 3H, lhC, 2 3 8U, 23l*U, and chemical ionic species as groundwater tracers - sometimes with limited success.
We present the results of an investigation in the Angas-Bremer irrigation areas of South Australia to illustrate, what we consider to be a novel technique, not only for identifying and delineating groundwater bodies but also for qualitative and quantitative estimation of mixing (seepage) between them.
HYDROGEOLOGICAL SETTING
The Angas-Bremer irrigation district, located in South Australia, is approximately 200 km2. It is bound to the north by the Mount Lofty Ranges and to the south by Lake Alexandrina, as shown in the location map (Fig.l). Two ephemeral streams, the Angas and the Bremer traverse the area and discharge into Lake Alexandrina, a natural, shallow lake of presumed tectonic origin, which also possesses features of a coastal lagoon. The two streams have a very variable flow, as shown in Table 1, and neither of them have salinities below 1000 mg I - 1, except during short periods of high flow.
TABLE 1
Flow variability
Bremer River Angas River
Area of catchment (km)2
732 126
M in. flow (Ml) year 1
4 500 1 300
Max. flow (Ml) year 1
52 000 4 000
The irrigation area is characterized by two aquifers; the upper water table aquifer with high salinities - in some places more than 20 000 mg 1 , and a lower confined aquifer of comparatively low salinities which is tapped for irrigation. The lower salinity zones in both aquifers are associated with the two rivers and are considered to result from flushing effects of river recharge. Parts of the lower aquifer, especially in the southern parts of the study area, are showing trends of increasing salinity in recent years, presumably as a consequence of induced vertical seepage from the unconfined aquifer. However, the trend is enigmatic, since it is not uniform in all the boreholes sampled - some showing a decrease, others an increase, within short distances of each other. The hydrology of the area is described in Table 2 and the hydrological setting described in a self-explanatory diagram shows a cross section of the study area in a north-south direction (Fig.l). Potentiometric contour maps for the two aquifers are shown in Fig.2, and indicate large head differences between the aquifers in the
LANGHORNECREEK
U l l M I S T O N E
B A S E M E N T
^ A i T E R WATEf lHOUSE 1978)
J J K-T
t 1 S A N D
FIG.l Map showing the location of Angas-Bremer irrigation area; and the sampling locations of groundwater for chemical and isotopic analysis and schematic north-south cross section of the irrigation area indicating the hudrological setting.
270 L.M.Ramamurthy S J.W.Holmes
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Detection of subsurface seepage 271
FIG.2 (a) Potentiometric contour map of the unconfined aquifer. (b) Potentiometric contour map of the confined aquifer. (c) Map showing the potentiometric head differences between the two aquifers in the Angas-Bremer irrigation area (unconfined > confined) (m a.m.s.l.).
northern parts of the irrigation area, and small head differences in the southern part, indicating possibly the absence or a feeble presence of a confining layer. Recharge to the aquifers from the two rivers has been observed to occur across an inferred fault zone to the northwest of the study area (Smith, 1978; Sinclair, 1976). The potentiometric levels in the confined aquifer are for the most part of the year below that of Lake Alexandrina whose level is maintained at 0.75 m a.m.s.l. by the construction of barrages.
272 L.M.Ramamurthy S J.N.Holmes
The water balance study in response to this problem yielded the following result for the confined aquifer (Waterhouse et al., 1978).
INFLOW OUTFLOW
5600 + 3000 + 3400 + 13 000 = 25 000 ml year-1
rivers lake lateral vertical seepage seepage from from confined unconfined aquifer aquifer
In this study, both vertical and horizontal seepage and recharge from Lake Alexandrina were quantitatively inferred by flow net analysis based on the configuration of potentionietric heads in the two aquifers and aquifer characteristics determined from pumping tests and other such techniques. Sheard (1979) attempted a steady stage finite element numerical model for the aquifer system based on the water balance proposed by Waterhouse, but the results did not match the observed configuration of potentiometric contours.
There is, thus, a need to gather more information on the following specific aspects of the hydrology of the study area:
(a) Whether vertical seepage from the unconfined aquifer in the southern regions of the irrigation area occurs uniformly at a regional scale or from point sources. Localized seepage from point sources may be possibly caused by artificial factors like imperfect and leaky boreholes, as believed by the farmers and relevant Government authorities who have since passed legislation regulating the construction of boreholes.
(b) Whether there is a means of distinguishing parts of the confined aquifer which have been polluted by vertical seepage, from those that have not. If so, would it be possible to quantify in absolute or relative terms the extent of such seepage? The salinity of the groundwaters would be of little help in this regard, since although, in general, the unconfined aquifer is relatively more saline than the confined aquifer, there are many parts of the confined aquifer where the reverse holds.
(c) Whether increased salinity at any point in the confined aquifer caused by vertical seepage from the unconfined aquifer could be distinguished from a similar trend caused by lateral seepage from the eastern and western margins of the confined aquifer which are also characterized by very saline waters (>2000 ppm).
(d) As a natural corollary to the above, is there any seepage to the confined aquifer, at its southern margin, from Lake Alexandrina, which is a fresh water lake?
In view of the enigmatic salinity trends observed in the area, and the failure of physical modelling to explain the water balance, it was considered expedient to study the area in terms of its hydrochemistry and environmental isotopes such as uranium-series isotopes, deuterium and tritium in ground and surface waters. Besides their use as hydrological tracers, uranium isotopes in water are good indicators of leaching and rock water interaction; deuterium, a good indicator of micrometeorological conditions at the time of recharge; and tritium a semi-quantitative indicator of residence time. The hydrochemistry of the waters provides an
Detection of subsurface seepage 273
infrastructure for interpreting all the results. Analytical techniques for determining uranium concentration
involved ion exchange separation and a-spectrometry using 10-15 litre samples filtered through 0.45 u millipore filters (Osmond & Cowart, 1976). Major ions were measured by spectrophotometry, electrometric titration and colorimetric methods according to standard analytical procedures described in ASTM Book of Standards Vol. 30. 2H concentration was measured by mass-spectrometer and results expressed relative to the Vienna SMOW standard.
RESULTS AND DISCUSSION
The results of analyses for all major ions, together with the results of concentrations of deuterium, uranium and 23"*u/238U activity ratio are given in Table 3. The locations of sampling points in the confined and unconfined aquifers are shown in Fig.l.
The results indicate that although the absolute concentrations of major ions show a large variation in different parts of the aquifers, the chemical constitution of the water in terms of the relative proportions of ions is not markedly different in the aquifers, both being sodium-chloride-type waters. It is therefore apparent that the two aquifers cannot be distinguished on the basis of their hydrochemical nature.
Deuterium analyses indicate that the mean 6D values for the confined and unconfined aquifers are -25.03 and -26.13o/0o
respectively. The results also indicate that there is a general enrichment of deuterium isotopes in the part of the confined aquifer underlying and associated with the River Angas, compared to that part associated with the River Bremer. This can be due to the fact that the catchment area of the River Angas is both smaller and at a lower elevation compared to the River Bremer, as a consequence of which greater fractionation of hydrogen isotopes is to be expected (Table 1). The 2H (<5D) content of the River Angas and the River Bremer measured during flood conditions is -31.6 and -37.4% respectively.
It is observed that the concentration of deuterium in the confined and unconfined aquifers along the lake front, shows no enrichment of deuterium relative to the rest of the aquifers, with the exception of BRM 233 in the upper aquifer, thereby indicating that lake water, which is highly enriched (<5D = -3.9°/oo), is not seeping into the aquifers. This could be due to any one of the following factors:
(a) Hydraulic connection between the lake and the aquifers is either absent or significantly impaired.
(b) Hydraulic connection is present but is located considerably offshore in the lake so that insufficient time must have elapsed for recharge to have reached the lake shore.
(c) There could possibly be a preferred path for recharge which might be located outside the study area, either towards the west or towards the east. Such a preferred path could be the buried palaeo-channel of the River Murray, which is as yet undiscovered.
The significant enrichment of deuterium in BRM 233, being the only exception to the above trend, indicates that, even with recharge
274 L.M.Ramamurthy S J.W.Holmes
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Detection of subsurface seepage 275
from the lake, it would be to the unconfined aquifer and not to the confined aquifer. In addition, the significant different between <5D concentration of BRM 233 (unconfined) and BRM 35 (confined), adjoining sampling sites in the study area, implies that there is no hydraulic connection between the two aquifers and as a consequence there is no vertical seepage.
p Q h O Q Q
Variation in uranium concentration and U/ U activity ratio in the aquifers appears to be random and lacks coherence for physical interpretation, except for noting consistencies with 2H for an absence of lake recharge. Similarly, a plot of uranium
9 0 h 0 3 Q
concentration vs_. U/ U activity ratio for the two aquifers, given in Fig.3, is unrewarding for deductions relevant to flow patterns in the aquifers or for its possible use in delineating the two aquifers. An area, high in uranium concentration at BRM 35 and BRM 36, is located in the southwestern corner of the study area. An inspection of the bore logs reveals no peculiarities in the stratigraphy or lithology of the aquifer at that location, nor is there any evidence of any external source of uranium.
It is obvious from the above discussion that the deuterium concentration and U-concentration in the aquifer along the lake
a
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276 L.M.Ramamurthy S J.W.Holmes
front indicate that there is no recharge from Lake Alexandrina. In regard to the vertical seepage from the unconfined aquifer, no inferences could be made, since the deuterium, uranium and hydrochemical composition of the aquifers are not characterized by any significant contrasts. In view of this, a comprehensive interpretation of the environmental isotopes and hydrochemistry has been done to yield additional insight into the hydrology of the Angas-Bremer irrigation area.
The relationship between environmental isotopes and hydrochemistry can be explored by plotting uranium concentration (U) vs. (HCO3) content of groundwater. This is consistent with, and is based on the present understanding that, as a result of leaching under normal oxidizing conditions and in the pH range 5-8, uranium is mobilized into groundwater in its dissolved form as uranyl carbonate complexes U02(C03)2
_2 and W2(.C03)3~b' (Hostetler & Garrels, 1962; Borovec et
al., 1979; Garrels & Christ, 1965; Osmond & Cowart, 1976). It_is therefore possible to expect a correlation between U and (HCCO3 in a given aquifer, as an indication of the extent of leaching and rock water interaction. This could also be supplemented by examining the relationship of U with electrical conductivity where the hydrochemistry of an aquifer is uniform and homogeneous, as in the present case. It is found that there is correlation between U and both HCO3 and electrical conductivity for the confined aquifer and based on the few analyses available, for the unconfined aquifer as well (Figs 4 and 5). It is noteworthy that the relationship of U with HCO~ differs between aquifers mainly in terms of the intercept. The generality, significance and quantitative implications of these relationships are being explored using additional data, both local and international. The respective relationships indicate a linear increase in uranium concentration with both leaching and rock water interactions. BRM 35 and BRM 36, which appear to have anomalously high uranium concentrations are, in fact, found to be normal on the basis of such a correlation, indicating that the high uranium content of these samples have been derived directly as a result of leaching and rock water interactions. Instead, it is found that samples 7(FRL3610), 4(FRL3556), 17(FRL65) and 5(BRM2771) are anomalous in this regard. Their anomalous nature cannot be explained in terms of chemical constitution, since, as already discussed, the chemical constitution of groundwater in both aquifers throughout the study area is reasonably uniform. The llthology of the aquifer at these sampling points, as inferred from bore logs, does not reveal any peculiarities. It is possible that the anomaly is due to contamination from external sources, which in the present context could be vertical seepage from the unconfined aquifer, As indicated from the HCO3 vs. U plots, the anomalous samples would lie between the two regression lines for the two aquifers.
Figure 6 is a plot of U-concentration vs. ZH concentration of the confined aquifer at the various sampling locations. One of the obvious features of the diagram is the fact that samples 7, 4, 17 and 5 which were found anomalous on the U vs. HCO3 plot, also exhibit a comparatively depleted 2H content. In fact, all the samples from the aquifer could be divided into three groups as shown in the diagram, with groups I and II associated with, and underlying the River Angas and the River Bremer respectively and group III
Detection of subsurface seepage 277
URANIUM CONCENTRATION (pg 1 "^
FIG.4 Uranium concentration vs. HC03 content; electrical conductivity for the confined aquifer in the Angas-Bremer irrigation area.
representing samples that could have resulted from seepage from the overlying unconfined aquifer. The higher ZH content (<5D = -21 to -23.5) of group I, compared to group II (-23.5 to -25.5) is consistent with the higher 2H content of the River Angas compared to the River Bremer, as discussed earlier.
The concentration of 2H, U and HCO3 from all parts of the confined aquifer can thus be explained. It is therefore possible that any part of the aquifer with a HCOo, U content anomalous in comparison to the aquifer as a whole as depicted in Fig.5, could indicate mixing from external sources, possibly the unconfined aquifer. In the present case, this would be also accompanied by a
278 L.M.Ramamurthg <S J.W.Holmes
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depletion of 2H content (ÔD < -25.3) at least in those parts of the aquifer close to the two streams from where most of the samples for the present study have been obtained.
The above interpretation gains support from the 3H content of the confined aquifer sampled at various locations (Fig.l) and shown in Table 4. The 3H content of the aquifer appears to be rather uniform showing no increase along the lake front (BRM 35, BRM 36, FRL 65) as might be expected, if recharge were to occur from the lake. An increase in 3H content for FRL 63, as compared to adjacent sampling sites (FRL 62 and FRL 64) in the middle of the area, could, on the other hand, signify vertical seepage and hydraulic connection with the River Bremer from point sources. This deduction is consistent with the previous inference drawn from measurements of uranium and 2H isotopes.
While extending the general validity of the technique based on (HCOÔ, U) correlation to groundwater aquifers, it is useful to note that the concentration of HCO3 and uranium are characteristic properties of the groundwater obtained as a result of interaction
p 18 3
with the aquifer matrix. The concentration of H (and 0, H ) , on the other hand, are external properties obtained almost exclusively due to geographic and meteorological conditions, before recharge. As a consequence the use of these environmental isotopes as
Detection of subsurface seepage 279
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FIG.6 Plot of U-concentration vs. SD content of the confined aquifer in the Angas-Bremer irrigation area.
groundwater tracers is based on totally independent factors and therefore useful for mutual correlation of results derived from them.
In the case of the Angas-Bremer irrigation area, an increase in salinity observed in the confined aquifer, could be due to vertical seepage from the overlying unconfined aquifer, or lateral seepage from the more saline eastern and western flanks of the confined aquifer itself, or a combined effect of both. Both these factors can be resolved, based on the derived uranium relationships, since
TABLE 4 Tritium analytical results
Sample number
Borehole notation
Tritium concentration (TU)
Tl T2 T3 T4 T5 T6 T7 T8
BUM 35 BUM 36 FRL 62 FRL 63 FRL 64 FRL 65 DM 26 Bremer River
0.7 0.7 1.0 1.8
.0
.9
.9
.4
Samples collected by Hall (1977).
280 L.M.Ramamurthy S J.W.Holmes
the deviation of the sample from the line of best fit would continue to increase with time due to vertical seepage, while the deviation would remain constant for lateral seepage from within the aquifer itself, as depicted schematically in Fig.7. In the latter case, however, the absolute concentration of uranium and HCO3 for the sample would increase as salinity increases, but the coordinates would trace a path parallel to the line of best fit for the aquifer as a whole. The above effect could be possibly proved by monitoring the aquifer over an extended period of time.
U- CONCENTRATION
FIG.7 _ Schematic diagram illustrating the possible use of HCO3 vs. U~concentration plot in Identifying vertical seepage to the confined aquifer from the overlying unconfined aquifer; and lateral seepage from the outer saline flanks from within the aquifer itself.
The discussion above is heavily dependent on the premise that, while the dissolved U content in a given aquifer is etched proportionally with the HCOo content of the water, any deviations from the proportionality is an indication of mixing with waters of a different origin. While this could be true, it should be noted that the geochemistry of uranium is not fully understood, and several factors have been known to affect the mobility of uranium in groundwater. Uranium concentration in groundwater could be influenced by the presence of organic acids (Halback et al., 1980); organic matter (Swanson, 1961); trace elements such as vanadium (Hostetler & Garrels, 1962; Langmuir, 1978); the oxidation state of uranium (U* or U ); presence of phosphatic rocks and other U-ore bodies, etc.
It can be seen that all the above factors imply either an abrupt change in the lithology of an aquifer, an anomalous presence of "foreign matter", and/or a sudden change in Redox potential induced by these two factors; all of which have a very low probability of erratic occurrence rather than in a phased manner; and which in any case can to a large extent be physically verified. Where no anomalies exist in the aquifer matrix itself, it is reasonable to ascribe the deviations from proportionality, to mixing of
Detection of subsurface seepage 281
groundwaters of different origins and to confirm this on the basis of other conservative environmental isotopes such as H, 180 and 3H which are immune to chemical changes in the aquifer. In the Angas-Bremer irrigation area, samples found anomalous on the HCO3, U plot also show a H content different from the rest of the aquifer, which would not be the case if there were to be no mixing of waters of different origins. There is obviously more work to be done in the Angas-Bremer irrigation area and other such areas, to document the various aspects of the application of the technique to groundwater problems.
CONCLUSIONS
(a) Studies described above, based on environmental isotopes, H, U, U, H, and hydrochemistry indicate that there is no detectable recharge to the confined aquifer from Lake Alexandrina.
(b) The studies also reveal that diffused vertical seepage on a regional scale in the southern parts of the study area, as suggested by small potentiometric head differences between the aquifers, is unlikely. Rather, the studies support the prevailing opinion of the authorities and farmers, that vertical seepage is from point sources possibly due to faulty, corroded and leaky borehole casings which form an effective hydraulic connection between the two aquifers. It is feasible to identify these sources of leakage on the basis of HCO3, U, and H content of groundwater, as discussed in the text.
(c) The increase in salinity at any particular point in the aquifer, over a period of time could be caused by vertical seepage of saline water from the unconfined aquifer or by lateral flow of high salinity waters from the outlying flanks of the aquifer itself. Both of these effects could be resolved, and even quantified on the basis of a (HCO3, U) plot. This, of course, involves extensive sampling and analysis of the aquifer for environmental isotopes and hydrochemistry, both in space and time, the viability of which could only be determined by its practical use.
(d) The study reveals that a good correlation between HCO3 and uranium concentration, if found in groundwater aquifers, could be effective in fingerprinting groundwater bodies and sometimes in qualitative and quantitative determination of mixing that might occur between them. Such a concept, which should invariably be used in the context of the overall hydrogeological setting and with complementary data on environmental isotopic tracers, and major ions, could be a useful tool in hydrological investigations.
REFERENCES
Borovec, Z., Kribec, B. & Tolar, V. (1979) Sorption of uranyl by humic acids. Chem. Geol. 27, 39-46.
Garrels, R.M. & Christ, C.L. (1965) Solutions, Minerals and Equilibria. Freedman-Cooper.
Halback, P., von Borstel, D. & Gundermann, K.D. (1980) The uptake of uranium by organic substances in a peat bog environment on a
282 L.M.Ramamurthy s J.W.Holmes
granitic bedrock. Chem. Geol. 29, 117-138. Hall, J.W. (1977) The hydrology of the Bremer River, South
Australia. Unpublished BSc (Hons) thesis, the Flinders University of South Australia.
Hostetler, P.B. & Garrels, R.M. (1962) Transportation and precipitation of uranium and vanadium at low temperature, with special reference to sandstone type uranium deposits. Economic Geol. 57, 137-167.
Langmuir, D. (1978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochim. Cosmochim. Acta 42, 547-569.
Osmond, J.K. & Cowart, J.B. (1976) The theory and uses of natural uranium isotopic variations in hydrology. Atom. Energy Rev. 144, 621-670.
Sheard, M.J. (1979) Angas-Bremer irrigation area, Milang groundwater basin, groundwater modelling. Geological Survey of South Australia Report Bk no. 79/133.
Sinclair, J. (1976) The water balance of the Milang basin, South Australia. Unpublished BSc (Hons) thesis, the Flinders University of South Australia.
Smith, R. (1978) Recharge to the Angas-Bremer irrigation area, South Australia. Unpublished BSc (Hons) thesis, the Flinders University of South Australia.
Swanson, V.E. (1961) Geology and geochemistry of uranium in marine black shales - a review. USGS Prof. Pap. 356-C, 67.
Waterhouse, J.D., Sinclair, J.A. & Gergis, N.J. (1978) The hydrogeology of the Angas-Bremer irrigation area. Department of Mines and Energy of South Australia Report Bk no. 78/8, vols 1, 2, 3.
