Cuba Molerio A Iaea Cn 151 125

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International Symposium on Advances in Isotope Hydrology and its role in sustainable Water Resources Management, Vienna, Austria, 21-25 May, 2007

IAEA-CN-151/125

Tritium as an indicator of groundwater overexploitation in a tropical karst aquifer L. F. Molerio León CESIGMA, S.A., PO Box 6219, CP 10600, Habana 6, La Habana, Cuba Abstract. Overexploitation of two huge Cuban karst aquifers is derived from Tritium sampling and isotope balance modeling. During the dry season no 3H activity was measured in the ground waters discharged at the springs or either at some of the observation wells. Occasionally very high values for 3H were recorded. These values are strongly correlated with rainfall that took place at the 1980 decade or a little older. On the other hand, 3H activity linked with present precipitation has been also recorded. Therefore a good mixture of water of different origin and residence time moves through the aquifer. The change in the isotopic composition therefore, has been interpreted as an indicator of overexploitation as far as waters that are not replenished during the present hydrological cycle are pumped out of the aquifer. Residence time modeling applying Maloszewski and Zuber, [1,2] and Zuber and Maloszewski, [3] lumped model approach fits these no Tritium spring waters with waters with around 100 years of residence time. A more general conclusion is derived from the results obtained in the last years in both poljes relative to the mixing hydrodynamics in karst aquifers were flow stratification is associated with cave levels thus allowing that under certain conditions, old ground waters prevail in the mixing allowing important losses of ground water reserves that are not replenished during the present hydrological cycle.

1. Introduction During the last decade Cuba has suffered a large hydrological drought. In the particular case of the Vento Basin, a karstic polje of about 325 km2, ground water levels has shown a sustained decline, several wells became completely dry, and the yield of the Vento springs averaging 3 m3/s in the dry season has reduced to about 1 m3/s. The isotopic composition of groundwater has dramatically changed and very old waters (with no Tritium) began to appear systematically during the last months of each dry season (FebruaryApril) at the springs that constitute the natural discharge of the aquifer. The Vento Basin (Fig. 1) is one of the four aquifers supplying fresh water to La Habana, the capital of Cuba (2,1 million inhabitants) since the last 100 years. The recorded change in the isotopic composition therefore, has been interpreted as an indicator of overexploitation as far as waters that are not replenished during the present hydrological cycle are pumped out of the aquifer. Residence time modeling [1-3] lumped model approach fits these no Tritium spring waters with waters with around 100 years of residence time. The continuous use of waters with so slow natural replenishment has largely contributed to the depletion of the aquifer. During the IAEA´s project ARCAL XIII a similar result was previously obtained by the author and his colleagues [4,5] in the contiguous polje of Jaruco in 1993, were the hydrogeological structure is very similar to that of the Vento Basin. In that case environmental isotope techniques provided a useful tool to the decision-makers to sustain the exploitation of the El Gato well field, an important abstraction system with a summary yield of about 3 m3/s.

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After these results the application of isotope techniques to determine the mean residence time of ground waters involved in the supply of the Capital city of Cuba was extended to the involved aquifers. The main objective of those studies [6-12] was to determine if the same problem detected in Jaruco-Aguacate basin was eventually a more generalized hydrogeological problem and, in that case, to identify the corrective measures to sustain the exploitation of those aquifers. The contiguous Vento basin was selected because of its similar geologic, hydrogeologic and geomorphologic structure. FIG. 1. Vento Basin showing sampling points and reference locations (after [10]).

. However, a more general conclusion is derived from the results obtained in the last years in both poljes relative to the mixing hydrodynamics in karst aquifers were flow stratification is associated with cave levels thus allowing that under certain conditions, old ground waters prevail in the mixing allowing important losses of ground water reserves that are not replenished during the present hydrological cycle. Overexploitation of groundwater is a primary focus of attention for decision makers. The effect of groundwater resources depletion on food security and health is of particular importance in islands and small islands dependent primarily of rain to recharge the aquifers. In karst regions were surface waters does not exist the problem becomes strong dependent both from the adequate managing of the groundwater resource and of the availability of recharge water. The effect of the sustained droughts experienced by the Humid Tropics in the last years has largely affected the availability of drinking water of adequate quality. Some discussions around the problem focused their attention whether or not those droughts are random phenomena or are linked with climate change. Despite theoretical considerations, the onset of old waters not linked with the present hydrological cycle clearly shows that those aquifers are becoming more and more stressed by overexploitation. Records show that in several aquifers of Western Cuba, old waters become systematically prevalent in the water mixture. When they dominate the mixture, groundwater mining is an irreversible result. A recent review of the capability of isotope techniques for groundwater exploitation studies is due to Seiler [13]. 2. The differentiated response of karst aquifers to recharge and overexploitation It is supposed that when more water than that entering the aquifer is extracted, the available resources become systematically exhausted or its quality deteriorated, but the concept is incomplete because it is known that overexploitation can occur even when recharge is greater than abstraction because of bad management practices. A more general agreement states that overexploitation of groundwater takes place when a limit in volume, yield or water quality is surpassed. Therefore beyond those limits, some side effects -usually ambiguously defined as “undesirable affects”- due to overexploitation appear, e.g: systematic groundwater level decline that in turn causes the exhaustion of springs and the abandon of water wells; loss of water quality and, in turn, contamination of the productive aquifer and subsidence.

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For water supply purposes, this limit is often called “safe yield” or “groundwater resource”. Conventionally overexploitation should take place when this boundary is surpassed, but experience show that sometimes water is abstracted beyond this limit and no side “undesirable” effects become apparent. Therefore the concept of overexploitation which is important to know with the kind of problem managers are dealing with, falls into a category of ambiguous definitions, v.gr.: the limit established by the “safe yield” and the safe yield itself; the limit established by the managing practices and the limit established by the water quality that is particularly needed. The safe yield of the aquifers is defined as the quantity of water that can be extracted annually of an aquifer without producing undesirable effects [14]. As Todd points out, this intuitive definition is more complex of that than at first sight it seems, since “there can be more than an undesired result effect … the safe may be limited to an amount less than the net amount of water supplied to the basin and that the safe yield can vary as the conditions governing it vary” [14:201]. The concept is completed when he points out that "the safe yield cannot exceed the long time mean annual water supply to the basin…Extractions exceeding this supply must come from storage within the aquifer”. And continues “… in any one year the draft can exceed the recharge without causing permanent depletion. But on a long-term basis, when series of wet and dry years would tend to average out, the draft becomes an overdraft if the mean supply is exceeded” [14:201]. But as Adams and MacDonald [15] have pointed out, “certain aquifers are more susceptible to overexploitation than others –equally certain managing practices are conducive to overexploitation”. Karst aquifers, particularly those of the Humid Tropics fall in this category because the particular way in which groundwater flow is organized in them. In a regional karst aquifer system groundwater converging from different local and subregional flow systems do not necessarily contributes continuously to ground water resources (Fig. 2). This singularity means that different parts of the aquifer behave differently in time and space. In fact, according to the degree of karst development, its distribution within the aquifer, its relation with local erosion base levels and the way they module the recharge inputs, karst aquifers show very complex hydrodynamic responses. Under certain boundary conditions some of the local flow systems and even its associated epikarst could seasonally or inter annually become saturated by water or be completely dry. This changing behavior is not necessarily reflected in the yield of springs or in the water level decline. The generally big fluctuations recorded in groundwater levels -sometimes associated with the effect of hurricanes or heavy rains- of large regional karst flow systems masks the actual behavior of the aquifer. Some cave levels or local flow systems become hydrological active by several months or years leading to an erroneous assessment of groundwater reserves and therefore to the establishment of an abstraction plan based on an erroneous safe yield estimation that eventually could lead to overexploitation. FIG. 2. Recharge patterns of a karst aquifer (slightly modified after [16]).

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On the other hand, groundwater abstraction could isolate local flow systems interrupting its contribution to major systems that could become dried. Sometimes an active epikarst could be completely drained producing a local and sometimes abrupt lack of water supply. River runoff could be shortened and even interrupted when feed by karst springs (concentrate or diffuse) that become dry when their local flow systems are drained. The inverse is also true and some episodic rivers use to flow only when their local flow systems become hydrological active. The typical heterogeneity of the system of collectors and conducts of ground waters and the differences in the natural recharge patterns in karst systems is the main cause of the differentiated space and time distribution of the replenishment of ground water resources. In fact, two extreme recharge patterns are recognized in karst systems. A rapid, fast concentrated recharge along vertical shafts and open cracks in bare karst areas and a slow, diffuse, recharge trough joints and soil cover in buried karst systems. This means that different recharge rates exist for a same recharge event. Even more, different arrival times to the outlet are then recognized depending on the degree of water mixing. In turn this allows to different isotopic behavior. This difference is the cause that in particular flow domain water with different age coexists. The sustained extraction of ground waters belonging to the current hydrological cycle causes, inexorably, the exhaustion of the available resources and this relation could be rigorously determined with the use of the proper isotopic tracers. In general, the variable "residence time" is not taken into account in the water balance and in the assessment of groundwater resources. But the abstraction of “old” ground waters or the exploitation of water of very slow replenishment leads to the sometimes very fast exhaustion of the resources, -particularly in karst aquifers-. Remediation works like artificial recharge, the protection against the contamination of the water supply wells and springs or the deep injection of waste liquids are other aspects in which the residence time is fundamental and, neither, regrettably, it is usually considered. As a basic principle it is highly encouraged that waters not participating of the current hydrological cycle should not be exploited in a way to guarantee its replenishment. However, the term “current hydrological cycle” is ambiguous by nature and has to be more precisely defined. Anyway, the isotopic techniques provide appropriate resources to know the residence time of ground waters. Quantifying this variable provides criteria to adopt appropriate management practices without damaging the aquifer by the exhaustion of its resources or the artificial recharge of contaminated waters [17-19].

Final remarks The Tritium based isotopic balance for the different karst flow systems evidenced the overexploitation of the ground waters. As an indicator of groundwater overexploitation in two Cuban karst aquifers the following behavior could be observed: 1. The isotopic balance of the ground water showed that in the discharge area and in some points of the aquifer converge waters with different residence time, indicating a stratification of the aquifer system associated to the development of different cave levels (Figs. 3-4). 2. During the dry season in certain observation boreholes and at the system’s outlet conveys waters with no Tritium activity. Is highly probable that these waters are not linked with the natural replenishment associated to the current hydrological cycle or at least that takes place in the last fifty years. Model shows the best fit for waters of 100 years of residence time. 3. During the dry season the exploitation of the eventual volumes that might be recharged associated to the “cold fronts” rains does not reach the aquifer, limiting or impeding its the natural regulation and, in turn, contributing to the exhaustion of the aquifer because of the systematical lack of replenishment.

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4. In some cases, the difference among transit times varies from three months in the upper part of the flow system to 100 years in the lowest levels. The exploitation of these waters during the dry season represents a constant reduction of water resources. 5. Because of the nature of the integration of the cavernous system a good mixing of waters exists. The fact that successive events of recharge are not consecutively identified is a good evidence of it. This that recharge waters arrive to the outlet and to other sampling points in a differentiated way, some first one than others, but without following a strict chronological order, in what seems to be a typical feature of the Cuban karst systems. 6. Most recharge events that possibly can be identified with a well-known Tritium input in rain waters basically takes place during the rainy period (May to October). This fact confirms that these karst system receives some fresh recharge annually but not in the whole extension of the aquifer. 7. The isotopic results show that, at least during a part of the year losses by evaporation take place in several parts of the aquifer, mainly close to the outlet or in karst depressions where groundwater outcrops. Therefore some of these karst features behaves as points of loss of groundwater and not of recharge as intuitively can be supposed. FIG. 3. Cave levels identified at the Vento Basin

FIG. 4. Wells drilled at different depths intersect different cave levels and provide differentiated information on groundwater residence time.

Acknowledgements. The author is particularly indebted to his former Professors at the 1991 IAEA´s Training Group in Isotope Hydrology: R. Gonfiantini. D. Louvat, K. Rozanski, L. Araguas-Araguas, A. Plata-Bedmar, P. Maloszewski, W. Stchler, D.T. Dubinchuk, Y. Yurtsever, J.Ch. Fontes and K. Frohlich. P. Maloszewski also provided updated versions of the modeling software. Special thanks to those colleagues who shared the field work and data interpretation in the studied aquifers: C. Dapeña and H. Panarello (INGEIS-Argentina), J.L. Peralta, J. Carrazana, R. Gil, M. Peña, D. Leyva and I. Fernández (CPHR-Cuba), M. Guerra and M.Pin (formerly at INRHCuba). IAEA´s support for this presentation is also acknowledged as well as the friendly cooperation of its staff members C. Devia-Torres and L. Araguas-Araguas. Cooperation of my wife Ana, was permanent in field sessions and data processing.

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