Isotopic and geochemical evolution of ground and surface waters in a karst dominated geological setting: a case study from Belize, Central America
Introduction
Sixty-nine percent of Belize's rural population has access to safe drinking water (UNICEF, 1997). Contamination of aquifers is known to exist in several locations in Belize due to increasing agrichemical use and the dissolution of SO4-rich minerals (Day, 1996, Alacron et al., 1998). The expansion of countrywide groundwater supply and improvement of drinking water quality is national priority for Belize (UNICEF, 1997). In order to determine the extent and potential for groundwater contamination in Belize, a region dominated by karst geology, it is important to determine the origin of groundwater in aquifers and the dominant geochemical reactions responsible for surface and groundwater quality in this type of environment.
The isotopic compositions of O and H in water, modified by meteoric processes, are useful tracers for determining groundwater origin and movement. Stable isotopes of O and H are particularly useful as tracers because the isotopic composition of O and H in groundwater does not change as a result of rock–water interactions at low temperatures (e.g., Sidle, 1998). Carbon isotope ratios of dissolved inorganic C can be used to trace multiple C sources, and enables the identification of biogeochemical processes in the hydrosphere (Clark and Fritz, 1997). When stable isotopes and geochemical data are examined for both surface water and groundwater systems, important information about processes related to water cycling and rock–water interaction that may affect water quality are readily discernable.
This study was conducted to understand hydrogeologic processes governing water quality in Belize, Central America. The authors report the stable isotope ratios of H, O and C, and concentrations of select ions (Ca2+, Mg2+ and SO42−) of surface waters and groundwaters in Belize that are essential in understanding important atmospheric and geochemical processes affecting water quality. The results of this study add to the understanding of water resources management.
Belize lies in the subtropical geographical belt between 15° 53′ to 18° 30′ N and 77° 30′ to 89° 20′ W (Fig. 1). Rainfall in Belize increases with increase in elevation and is largely affected by the topography of the Maya mountains. Orographic lifting of the Easterly Trade Winds, thermal convection and cyclones result in rainfall totals ranging from 130 cm/a in the north to 450 cm/a in the south of the county (Miller, 1996, Heyman & Kjerfve, 1999). The wettest months of the year are June – September while the driest months are February – April.
The southern half of Belize is dominated by the Maya mountains. The Maya mountains are uplifted over 900 m above sea level and composed of Pennsylvanian to early Cretaceous granites, quartz and shales. Flanking the Maya mountains to the north and south are sedimentary units of mostly Cretaceous limestones and dolomites with rugged topography and well-developed karst features (Miller, 1996). In the north of the country at elevations of 200–300 m, the Cretaceous limestone hills terminate into Paleocene–Eocene carbonate formations extending northward as rolling plains. Insoluble clays, carbonates, marls and poorly consolidated sands are the most recent sediments of the typically flat north (Miller, 1996). In the low lands of the south, Tertiary gravels, sands, and muds dominate in the landscape beyond the limestone hills and ridges of the southern slopes of the Maya mountains. Uplifting and NE–SW faulting of the Yucatan platform defines the N–NE course for surface water drainage north of the Maya mountains. Streams draining the north, east and SE slopes of the Maya mountains form well-developed branching patterns with relatively straight, steep courses in the mountainous areas.
Carbonate aquifers in the north of the country have shown high permeabilities where tested (Hartshorn, 1984). High hydraulic conductivities typical of well-developed karst terrains are measured in many areas. Agricultural contamination of the karst aquifers in the north of the country is of increasing concern in the Orange Walk District (Day, 1996). Groundwater from the karst aquifers in some rural villages are reported to have high level of pathogens responsible for severe illness (Panton, 1998). Water quality of groundwater is also impaired by high SO4 content with concentrations as high as 450 mg/l from domestic wells in the Orange Walk District (Hartshorn, 1984).
Section snippets
Field sampling
Surface and groundwater samples were collected from several locations throughout Belize (Fig. 1). Surface water samples were collected using the grab technique. Groundwater samples were collected from public supply wells after purging a minimum of 3 estimated casing volumes. Water for δ18O analyses was collected and stored in leak proof 25 ml scintillation vials with inverted cone closures. Waters for DIC and stable C isotope analysis were collected in pre-evacuated septum tubes pre-loaded with
Stable oxygen and hydrogen isotopes
Stable O and H isotope values are given in Table 1. The mean δD and δ18O values are −12.4 and −3.9‰ for surface water and −13.8 and −4.4‰ for groundwater samples. A plot of δD vs.δ18O for surface and groundwater samples is shown in Fig. 2. The distribution of δD and δ18O in surface and groundwater samples are similar. The similarity of δD and δ18O for both surface and groundwater samples is not surprising due to high hydraulic conductivities in karst regions and suggest rapid recharge of the
Conclusions
An isotopic and chemical analysis of the surface waters and groundwaters from Belize, Central America was carried out to study and improve upon the current knowledge of atmospheric and geochemical processes governing water quality in a karst dominated geological setting. Stable isotope ratios of H and O have similar ranges for both surface and groundwater suggesting rapid recharge of groundwater aquifers. The O and H isotope measurements show d-excess values between 10 and 40.8‰, suggesting a
Acknowledgements
Partial funding for this study was provided by the Graduate College at Western Michigan University, Kalamazoo, Michigan. We thank Mark Menzies and Rudolph Williams for assistance in the design and execution of this study. We also thank Tanya Rauss and Edith McClintoch for field assistance and Madhav Machavaram for ion analysis. Reviews by Dr. Corinne le Gal la Salle and an anonymous reviewer helped improve the original version significantly.
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