Isotopic and geochemical evidence of regional-scale anisotropy and interconnectivity of an alluvial fan system, Cochabamba Valley, Bolivia

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Abstract

A study of groundwater chemistry and isotopic composition was conducted in Cochabamba Valley, Bolivia of an alluvial fan aquifer system located along the northern edge of the valley. In total, 75 wells were sampled for major and minor (F, Br, Si4+, NO3, Fetotal, H2S, PO34−) ion chemistry, 18O and 2H, and 3H. In the alluvial fan aquifer, groundwater flow patterns appear to be controlled by the unique hydrostratigraphy of the fan sediments. The radial morphology of alluvial fans and the entrenchment of channels in the apex of the fan are two probable causes of deposition of coarser, more permeable material along the axis of the fan. Geochemical and isotopic parameters give indirect evidence of these more permeable zones. Ratios of Na+Ca++ are higher off the axis of the fan, which is most likely due to longer residence times so that diffusion of saline residual pore water or ion exchange reactions create more Na+-rich groundwater. Also, 3H concentrations down the flow system are higher along the axis of the alluvial fan, a direct indication of younger groundwater age along the axis. The distribution of other minor ions, such as F and Si4+, and the redox parameters NO3, Fetotal, SO42− and H2S suggest a shorter residence times along the axes. The even pattern of Na+Ca++ and Na+K+ ratios, and Si4+, NO3, Fetotal, SO42−, H2S, and 3H concentrations along the flow system from wells varying 20 to 80 m in depth suggest that the aquifer–aquitard alluvial fan system is interconnected between different depths. Enriched groundwater 18O and 2H signatures in the distal end of the flow system in Area A are most likely the result of the introduction of evaporated waters from the surface, further evidence that the system is interconnected.

Introduction

Alluvial fans, given their permeable nature, form important aquifer systems worldwide. In many alluvial fan settings, the steep topographic gradient, the presence of many aquitards and the inclined angle of strata create confined aquifers that can produce flowing artesian conditions (Bull, 1977). Alluvial fan aquifers are important water resources in semi-arid intermontane basins, such as the Bolivian valleys of Cochabamba, Cliza-Punata, Sacaba and Tarija, the Trans-Mexican Volcanic Belt of central Mexico (San Vicente, 1990), and the Basin and Range Province of southwestern USA (Robertson, 1991). These aquifer systems have a very complex stratigraphy. Fluvial deposition creates lenses of sand, gravel, silt and clay. Given the intricate geometry of aquifer boundaries and the complicated flow systems that develop in the alluvial sediments, isotopic and geochemical tools often prove useful in evaluating regional groundwater flow in this type of aquifer system, where the physical characteristics of the flow system are mostly unknown and very complex.

The isotopic and geochemical composition of groundwaters in arid to semi-arid alluvial fan settings have not been studied in great detail. In Cochabamba Valley and 3 associated graben valleys nearby, a study of basin hydrology and hydrogeology was conducted (UN-GEOBOL, 1978). The major ion geochemistry was examined in groundwater sampled from 73 wells in Cochabamba Valley as part of the UN-GEOBOL (1978) study. Similarily, the major and minor ion geochemistry of the alluvial basins of the Basin and Range Province of Arizona, USA have been studied and modelled extensively (AWRA (America Water Resources Association, 1985, Robertson, 1991) and stable isotope studies in alluvial fan settings in Northern Chile (Fritz et al., 1981), Central Mexico (Issar et al., 1984) and Central Spain (Vera et al., 1981) have been completed, but often the emphasis has been at the reconnaissance level concerned more with basinal units than with alluvial fans in particular.

Cochabamba Valley is a semiarid valley in the Eastern Andes of Bolivia (Fig. 1). The city of Cochabamba (2540 m above sea level) has a population of approximately 800,000. This population had doubled in the past 10 a, due to the resettlement of miners to Cochabamba from the highland areas of Bolivia, where, due to decreasing metal prices, the mining industry has collapsed in past 3 decades. At present, only some 40% of the population receives water and sewage service. Most of the water for agricultural and domestic use is supplied by reservoirs, located outside of the valley. These surface water sources are limited and, recently, groundwater has been exploited as an alternative supply. Over the last 20 a, the city has developed well fields in some of the principal alluvial fan systems along the northern edge of the valley. A drought in the early 1990s forced the city to install additional water supply wells. The increased groundwater extraction has caused significant changes in the potentiometric surfaces of the associated aquifers and impacted flowing artesian conditions and spring discharges. The long-term production capacity of the alluvial fan aquifers should be evaluated to determine if they will meet the needs of the city in the future.

A collaborative project between Corporación Regional de Desarrollo de Cochabamba (CORDECO) and the International Development Research Centre (IDRC) began in 1989 to investigate the groundwater resources in Cochabamba Valley. The principal objectives of the geochemical portion of the project were: (1) to develop a conceptual model of the behaviour of an alluvial fan groundwater flow system, in order to better predict the location of more permeable zones for groundwater exploitation; and (2) to determine the potential for groundwater contamination of the principal aquifer system by urban development over recharge areas.

As part of the first objective, the geochemistry and isotopic composition of groundwater was measured to develop a basic understanding of the physical pattern of groundwater flow within alluvial fan aquifers (Area A; Fig. 2). The reconstruction of the 3H precipitation record and its use in analyzing the groundwater flow system of Cochabamba Valley as examined in a previous paper (Stimson et al., 1996). The present paper explores the thesis that alluvial fan stratigraphy controls in great part the geochemical and isotopic evolution of groundwater in Cochabamba Valley.

In order to address the second objective, groundwater inorganic geochemical parameters were compared to determine if geochemical parameters were largely uniform across the alluvial fan complexes and therefore the major aquifers were interconnected, or if discrete zones of differing geochemistries were observed within alluvial fans. In addition, the groundwater under the city of Cochabamba (Area B; Fig. 2) was studied, in order to examine the degree to which urban development on the alluvial fans has impacted the groundwater quality.

In the low-lying southeastern corner of the valley (Area C, Fig. 2), increased groundwater salinization, which had begun to affect agricultural production, was examined in a conference proceeding (Stimson et al., 1993), where groundwater was found to be much older and salinization appears to be due to natural processes rather than agricultural practices.

Section snippets

Hydrostratigraphy of alluvial fan complexes

In order to understand geochemical and isotopic trends found in groundwater, a new hydrogeological analysis of the stratigraphy of alluvial fans is presented, so that one can understand how geological controls of deposition create zones of higher hydraulic conductivity along the apexes of alluvial fans. Alluvial fan deposits form in areas of decreased confinement of streamflow, referred to as the apex of the fan, at a point where a stream emerges from a mountain area (Bull, 1977) (Fig. 3). In

Geology and hydrogeology of Cochabamba Valley

Cochabamba Valley is a semi-arid graben valley in the Eastern Andes of Bolivia (Fig. 1). The valley was created in the Pliocene by extensional tectonics occurring within metamorphosed sandstone and siltstone formations of Ordovician age (Isaacson, 1975). In the western corner of the valley, Tertiary formations composed of marls, limestones, and shales overlie the Ordovician strata.

As the graben block is hinged along the southern normal fault, the depth to basement varies across the valley, from

Methods

Geochemical and isotopic sampling campaigns were conducted during two field seasons: June to August 1989 and June to November 1990. Given the real extent of Cochabamba Valley, sampling was concentrated into 3 specific areas of interest (Fig. 2). Seventy-five existing wells were sampled in Area A (38 wells), Area B (25) and Area C (12).

All wells that were not flowing were pumped for 5–10 min before sampling. Well depths were ascertained from a well inventory conducted by CORDECO and well owners.

Results and discussion

The evolution of groundwaters in the alluvial fan groundwater flow systems will be discussed in the first section of the results, with particular attention paid to the differences in geochemistry between groundwater located along and between axes of alluvial fans. A second section will cover the geochemical evidence of aquifer interconnectivity. Representative isotopic and inorganic chemistry data for Areas A and B are presented in Table 1, and for Area C in Table 2. In the following

Degree of interconnection between aquifer units in alluvial fans in areas A and B

Given that rapid urban development on recharge areas that is occurring along the northern edge of the city of Cochabamba (Area B), an understanding of the potential effect of future contamination to the alluvial groundwater systems must be examined. Septic ponds are common in those areas not served by the sewage system, and the city's sewage system is known to have significant losses due to leakage. Thus, contamination of the recharge areas of some alluvial aquifers is inevitable.

Given the

Conclusions

Geochemical and isotopic data indicate that groundwater velocities are faster and residence times shorter along the axis of the Chocaya alluvial fan. Lower Na+Ca2+ and Na+K+ ratios, higher 3H concentrations, lower Si4+ and Fl concentrations, and lower Fetotal concentrations in groundwater found along the axis of alluvial fans also suggest faster groundwater velocities and shorter residence times in these zones. The depositional history of alluvial fans indicates that zones of high permeability

Acknowledgements

We would like to thank Dr. Robert Farvolden for his hard work and foresight in conceiving and organizing the research project in the Cochabamba Valley, and giving his advice on our research plans. The support of field activities offered by Ing. Victor Ricaldi, Ing. Leonardo Anaya, and the staff at CORDECO (Corporación Regional de Desarrollo de Cochabamba) was invaluable. We also wish to acknowledge the co-operation of members of the Alcaldı́a de Cochabamba, the Water Laboratory at the

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