Introduction

The study of the interaction between surface waters and aquifers through oxygen-18 measurements was applied in the Araripe Plateau in the semiarid zone of the Brazilian Northeast. In spite of rainfall amounting to 1,000 mm, due to the high hydraulic conductivity of sandy soils there are practically no surface water resources on top of the Araripe Plateau, except for the water stored in excavations covered with clayey soil (“barreiros”) trampled for impermeabilization by livestock during drought periods. Part of the rainfall water that infiltrates on the plateau is responsible for recharging the superior aquifer system. Main groundwater discharge from this system is through some 300 springs along the northern scarp of the plateau that produce about 40.5×106 m3 per year (DNPM 1996).

Groundwater exploitation through drilled wells in the superior aquifer system is difficult due to the average water-table depth of 120 m. However, the “Jardim” Fault, in the very east of the plateau, with an extension of about 34 km and 400 m in width in a topographic depression, defines a strip-like area where the water-table depth is only 3 m (Marques and others 1984), thus allowing the construction of dug wells.

At present, in the Araripe Plateau, there are five pioneer wells, recently drilled for exploiting the medium aquifer system with a static water level of 360 m.

The aquifers studied are subject to a regional climate that is characterized by two distinct seasons: rainfall is limited to the 5 months from February to June, and the rest of the year is practically dry.

Oxygen-18 has been used in recent years, e.g., by Matheney and Gerla (1996) and Yehdegho and others (1997), as a natural tracer to investigate the hydraulic relationship between surface waters and groundwater. McCarthy and others (1997) used deuterium and 18O to study the dynamic relationship between groundwater and the Columbia River. The authors made simple mixing calculations for the determination of the contribution of the river to the water produced by five wells.

In the present study, investigating the hydraulic relationship between surface waters and groundwater, measurements of 18O and electrical conductivity were used for the characterization of infiltrating, modern meteoric water, evaporated modern water from “barreiros”, and aquifer water. These parameters and their seasonal variations permit the detection of mixtures and mixing ratios for these components.

Area of study

The area of study consists of a plateau, the “Chapada do Araripe”, and a valley, the “Vale do Cariri”, north of it (Fig. 1). The Araripe Plateau, in the interior of the Brazilian Northeast, covers an area of about 8,000 km2, has an average altitude of 800 m, and is partitioned between the three states of Ceará, Pernambuco and Piaui. This area is delimited by the following geographical coordinates: 39°05′E and 40°55′E, and 7°10′S and 7°50′S. The Araripe Plateau coincides with the outcropping area of the Exu Formation. A schematic profile is depicted in Fig. 2.

Fig. 1
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Location map of the Araripe Plateau, along with pluviometer locations and sampling points

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Fig. 2
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Schematic profile of the Araripe sedimentary basin. Shaded zones are aquifers

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According to Ponte and Appi (1990), the sedimentary sequence is composed, from the top to the crystalline base, by the formations Exu, Arajara, Santana, Rio da Batateira, Abaiara, Missão Velha, Brejo Santo and Mauriti. The Exu and Arajara formations compose the superior aquifer system, the Rio da Batateira, Abaiara and Missão Velha formations the medium aquifer system, and the Mauriti F. the inferior aquifer system. The first system is separated from the second by the Santana aquitard, and the inferior aquifer system is separated from the medium aquifer system by the Brejo Santo aquitard.

In the northern margin of the superior aquifer system, there is an average hydraulic gradient of −7‰ that induces a groundwater flow to the scarp in the Cariri Valley, where a large number of springs emerge from the geological contacts Exu/Arajara and Arajara/Santana. In the central part, groundwater flow is from east to west (SUDENE 1967; Mendonça and others 2001a), driven by a gradient of about −2‰. On this pathway, flow is being intercepted by manifold tectonic features.

Methodology

The 18O/16O ratio in rainfall water of a specific region depends on several factors, distance to the coast, altitude, atmospheric temperature and the amount of rainfall being the most important. Furthermore, seasonal variations and latitude dependence also are observed to affect the 18O/16O ratio. Surface water is enriched in 18O due to evaporation and, therefore, in the semiarid Brazilian Northeast exhibits a strong seasonal variation in its isotopic composition. In deep groundwaters, 18O/16O is conserved and characterizes the recharge water.

Isotopic 18O measurements are expressed in units of δ(‰), defined as the relative permil deviation of the isotope ratio 18O/16O in a sample from that of a standard:

$$ \delta ^{{18}} {\text{O}}{\left( {\raise0.5ex\hbox{$\scriptstyle {\text{o}}$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle {{\text{oo}}}$}} \right)} = \frac{{{\left( {{{}^{{18}}{\text{O}}} \mathord{\left/ {\vphantom {{{}^{{18}}{\text{O}}} {{}^{{16}}{\text{O}}}}} \right. \kern-\nulldelimiterspace} {{}^{{16}}{\text{O}}}} \right)}_{{{\text{sample}}}} - {\left( {{{}^{{18}}{\text{O}}} \mathord{\left/ {\vphantom {{{}^{{18}}{\text{O}}} {{}^{{16}}{\text{O}}}}} \right. \kern-\nulldelimiterspace} {{}^{{16}}{\text{O}}}} \right)}_{{{\text{standard}}}} }} {{{\left( {{{}^{{18}}{\text{O}}} \mathord{\left/ {\vphantom {{{}^{{18}}{\text{O}}} {{}^{{16}}{\text{O}}}}} \right. \kern-\nulldelimiterspace} {{}^{{16}}{\text{O}}}} \right)}_{{{\text{standard}}}} }}*10^{3} $$

Samples were collected monthly, from September 1999 to August 2000, in four barreiros, three dug wells, and five tubular wells indicated in Fig. 1.

δ18O (‰) measurements were performed at the Centro de Energia Nuclear na Agricultura (CENA) in Piracicaba/São Paulo, with a precision better than ±0.15‰, in relation to the standard VSMOW (Vienna Standard Mean Ocean Water).

Results

Barreiros and dug wells

Results for 18O and electrical conductivity (EC) on samples from four barreiros and three dug wells in the east of the Araripe Plateau (see Fig. 1) are listed in Table 1, and plotted in Fig. 3b, c together with precipitation (Fig. 3a).

Table 1 Measurements of δ18O (‰) and electrical conductivity (EC) for samples taken from barreiros and dug wells on the top of the Araripe Plateau (barreiros: 1B Filemón Teles, 2B Dr. Raimundo, 3B Vila Cacimbas, 4B Antônio Roriz; dug wells: 1PA Dr. Raimundo, 2PA Cacimbas, 3PA Vila Cacimbas/B)
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Fig. 3a–c
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Monthly precipitation in the eastern part of the Araripe Plateau (a), and δ18O for water samples taken from barreiros (b) and dug wells (c) in the period February 1999 to September 2000

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Tubular wells

The δ18O values for water samples taken from tubular wells in the eastern and western parts of the Araripe Plateau are listed in Table 2 and plotted in Fig. 4.

Table 2 Measurements of δ18O (‰) for tubular wells on the top of the Araripe Plateau
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Fig. 4
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δ18O as a function of time for tubular wells located in the eastern and western sections of the Araripe Plateau

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Figure 5 shows 18O and electrical conductivity (EC) from February 1999 to September 2000 for the barreiro in Vila Cacimbas and for the dug well named Vila Cacimbas/B.

Fig. 5a, b
figure 5

Electrical conductivity (EC, left axis) and δ18O (right axis) as a function of time for the barreiro in Vila Cacimbas (a) and for the dug well Vila Cacimbas/B (b)

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Discussions

Barreiros and dug wells

By analyzing Fig. 3 it is observed that all barreiros have a similar temporal behavior. In drought periods, from September 1999 to December 1999 and from June 2000 to July 2000, the concentration of 18O increases due to evaporation. In rainy periods, from February 1999 to April 1999 and from January 2000 to May 2000, δ18O decreases, thereby indicating the renewal of water through rainfall waters, which have lower δ18O than the water in the reservoir. Small differences between these barreiros are due to their different usage and construction procedure.

The rainfall contribution to the barreiros is made through direct precipitation in the storage area, and mainly by temporary surface runoff guided into small drains known as “water pathways”. The existence of such small creaks efficiently promotes the conduction of rainfall water that otherwise would infiltrate into the sandy soils of the plateau.

The barreiro Antonio Roriz does not dispose of an efficient clay revestment for keeping water stored during the dry season, thus drying up rapidly after its onset.

Contrasting with the barreiros Dr. Raimundo and Filemon Teles, the barreiro Vila Cacimbas presents smaller seasonal variations in δ18O. The fact that in the dry period δ18O is less positive reveals that this barreiro is connected to the water of the aquifer. In the rainy season, δ18O is less negative, indicating that the renewal through rainfall water is of less importance than in the barreiros Dr. Raimundo and Filemon Teles.

At the onset of the rainy period, δ18O for the barreiros Dr. Raimundo and Filemon Teles reacts without delay, whereas barreiro Vila Cacimbas exhibits a 2-month delay (Fig. 3b). These differences in the behavior are certainly due the coupling of the latter water body to the much greater reservoir that the superior aquifer system represents. However, there are other reasons as well: the barreiros Dr. Raimundo and Filemon Teles constitute the exclusive source of water for the respective locations and are being used for small-scale irrigation and cattle breading; their runoff collecting “water pathways” are therefore maintained in good condition by permanent care. In contrast, the barreiro Vila Cacimbas is in a topographic depression where the water table is near surface and the population disposes of dug and drilled wells. Under these conditions, maintenance of the surface water reservoir is neglected and “water pathways” are taken over by vegetation.

δ18O values for water collected from the dug wells Dr. Raimundo and Vila Cacimbas did not present significant variations from September 1999 to July 2000, scattering closely around an average of −3.58‰ (Fig. 3c). This is consistent with the average δ18O value of −3.76‰ for wells exploiting waters from the Arajara aquifer in the eastern sector of the plateau (Mendonça and others 2001b).

The dug well Vila Cacimbas was not being pumped from February 1999 to April 1999. Thus, the water presented an average δ18O value of −1.72‰ in this period, typical of evaporated water.

The dug well Vila Cacimbas/B, at a distance of 30 m from the barreiro Vila Cacimbas, produces water of δ18O=−0.67‰. This value is much higher than the average value for waters exploited by other wells in the neighborhood. However, it is similar to δ18O measured in barreiros during the rainy period (Fig. 3), i.e., when the water level is elevated. This result indicates an injection of surface water into the groundwater. This assertion is further confirmed by comparing the temporal behavior of the electrical conductivity and δ18O values for the barreiro Vila Cacimbas with those values for the dug well Vila Cacimbas/B. The barreiro exhibits strong changes in salt concentration and isotopic enrichment through evaporation. The highest δ18O concentration occurred in February 2000, which also corresponds to the highest salt concentration (see Fig. 5). The minimum values for EC and δ18O occur at the end of the rainy season in May. Similar variations in EC and δ18O occur in the dug well, but with a delay of about 7 months corresponding to a groundwater flow velocity of about 4.5 m per month.

Tubular wells

Figure 4 shows that the waters exploited in the eastern sector of the plateau are isotopically distinct from those exploited in the western sector.

δ18O values for the tubular well Betânia, closest to the scarp, vary only slightly around an average of −3.15‰ (see Table 2). This is consistent with the average value of −3.24‰ for springs at the interface Exu/Arajara (Mendonça and others 2001b). Also, the EC of 30 μS/cm for the well is very close to that for waters from these springs.

The tubular wells Vila Cacimbas and Serrolândia I exploit water from the Arajara aquifer, presenting an EC of 85 μS/cm, exactly the same as observed for waters from springs at the Arajara/Santana contact of the scarp of the plateau (Santiago and others 1997). In spite of this chemical similarity, the waters from the two wells present δ18O values completely different from each other, namely, −3.56 and −4.70‰. The former value agrees with that for the springs, −3.59‰ (Mendonça and others 2001b), the latter not.

The tubular well 4-BO-01-PE exploits the Rio da Batateira aquifer confined on its top by the Santana aquiclude. The average electrical conductivity is 1,098 μS/cm (Mendonça and others 2001b), and δ18O remains practically constant, with an average of −4.70‰ (Fig. 4). The tubular well Serrolândia I also presents δ18O values in the same range. It is noteworthy that these values are much lower than the average for modern rainfall in the region, which is between −3.20 and −3.70‰. Frischkorn and others (1984) have shown that significantly lower δ18O in deep sedimentary aquifers of the northeast of Brazil is due to the presence of paleowaters older than about 10,000 years. These waters accumulated before a +5 °C climate change at the Pleistocene/Holocene transition.

The tubular well Serrolândia II (242 m in depth) produces water from a sandstone lens (9 m in thickness) inside the Santana Formation. In the dry period this well exhibits δ18O similar to that of the wells Serrolândia I and 4-BO-01-PE. However, systematic changes are observed in the rainy season. Considering an error of 0.15‰ for these isotope measurements, the observed changes could seem to be negligible. However, systematic, simultaneous changes are monitored also in the water quality, as illustrated in Fig. 6. EC varies from over 1,000 μS/cm in the dry season to about 250 μS/cm at the end of the rainy season.

Fig. 6
figure 6

Electrical conductivity (EC, left axis) and δ18O values (right axis) for the water samples taken from the tubular well Serrolândia II

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Lowest δ18O values of −4.69 and −4.84‰, observed in November 1999 and August 2000, respectively, are marked by the presence of paleowaters in the superior aquifer system in the western sector of the plateau. The maximum δ18O value of −4.24‰ was observed in March 2000, evidencing a contribution from waters isotopically very different, originating from modern waters in the superior aquifer system, indicating downward leakage. By plotting δ18O vs. EC (Fig. 7), one observes a linear relation (R=0.941) that can be satisfactorily represented as a mixture of one component with high EC and low δ18O with another one with low EC and high δ18O.

Fig. 7
figure 7

Linear plot of EC vs. δ18O for samples from the well Serrolândia II

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For a quantitative analysis of the mixture, the authors adopted end members of EC and δ18O of 1,040 μS/cm and −5.00‰, respectively (for the paleocomponent), and of 85 μS/cm and −3.70‰. These latter values characterize the modern recharge waters in the superior aquifer system, as measured for the wells 1PA and 2PA (Table 1). The intermediate values are averages weighted with the concentration of each component.

The temporal variation of EC in the well Serrolândia II was used for the determination of the percent contribution of the modern component, by using the values predicted by the mixing line shown in Fig. 7. The calculated results are listed in Table 3.

Table 3 Percent contribution, X, from the modern component to the water produced by well Serrolândia II, together with measured and calculated EC and δ18O
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A comparison of the percent contribution from the modern component with rainfall (Fig. 8) illustrates the fast reaction of the well to recharge to the upper aquifer system with a delay of about one month only. This may occur either due to leaks in the well sealing or to deep, open faults in the sedimentary package.

Fig. 8a, b
figure 8

Comparison between precipitation (a) and the modern contribution, X, to samples from well Serrolândia II (b)

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Hydraulic connections through geologic faults in the Santana aquiclude were recently discussed by Santiago and others (1997), using a phenomenological model for groundwater circulation in the Araripe sedimentary basin, and by Mendonça and others (2001c), reporting on MODFLOW simulation of the depression cone for well 4-BO-01-PE on the top of the Araripe Plateau.

Conclusions

Oxygen-18 and EC measurements permit the characterization of surface waters and groundwater in the Araripe Plateau, and an understanding of the relationship between them.

Measurements of 18O in samples from four “barreiros”, three dug wells and five drilled wells reveal that groundwaters from the eastern part of the Araripe Plateau are derived from present-day rainfall, characterized by δ18O between −3.2 and −3.7‰, while groundwaters from the western portion are isotopically different, with δ18O approximately −5.0‰, revealing the presence of paleowaters. Due to elevated evaporation, the surface reservoirs “barreiros” are strongly marked seasonally by elevated 18O and EC during the dry period. The dug well Vila Cacimbas/B, at a distance of approximately 30 m from a “barreiro”, exhibits δ18O and EC similar to that of the reservoir in the rainy season, indicating strong interaction between groundwater and surface water. Thus, special care should be taken with respect to water quality of these surface reservoirs, as they constitute a risk to groundwater contamination. Despite a depth of 242 m, the tubular well Serrolândia II, located in a fault, exhibits strong seasonal changes in δ18O and EC, revealing downward leakage between aquifers. As the waters from the two aquifers involved are different in terms of isotope composition and salinity, the water produced by this well may be identified as a mixture of two components: an older one, of high EC (1,040 μS/cm) and low δ18O (−5.00‰), and a younger component, of low EC (85 μS/cm) and high δ18O (−3.70‰). Mixing calculations (Table 3) reveal that the Serrolândia II well receives a contribution of up to 68% from the young component in the rainy period (March 2000), whereas this contribution amounts to just 1% in the dry period (August 2000).