H51F-01
Water Resource Assessment in Spring Catchment Areas
Water resource management of karst spring systems is difficult due to the complex, dynamic hydraulic character of karst aquifers. Numerical models are typically used to evaluate the impact of pumping within watersheds, however conventional numerical groundwater models are not capable of simultaneously replicating the rapid response of the conduits and the slow response of the rock matrix in karst aquifers. Because of this limitation, evaluating the impact of pumping on spring discharge is difficult when significant water is withdrawn at wells located in the catchment area of the spring. A recently-developed dual conductivity karst aquifer flow modeling tool, MODFLOW-DCM, is used to simulate discharge from the Blue Springs system located in the Floridan Aquifer to allow assessment of the impact of pumping in the spring catchment area. The ability of MODEFLOW-DCM to simultaneously accommodate both rapid turbulent flow in conduits and slow diffuse flow through the matrix allows for more representative assessment of the water resources in the spring catchment area.
H51F-02
Integrated Hydrogeochemical and Geophysical Interpretation of Groundwater Salinization in an Uplifted Pleistocene Carbonate Aquifer of Barbados
Understanding the processes that influence spatial and temporal distributions of aquifer salinity are essential to the development of a groundwater salinity management plan. In this paper, we integrate geophysical, hydrogeochemical and submarine seepage measurements to develop a conceptual hydrogeological model of groundwater salinization of a Pleistocene carbonate aquifer that has experienced Quaternary glacio-eustatic sea- level changes and tectonic uplift. The Pleistocene carbonate rock mantles moderately folded and faulted Tertiary marine sedimentary rocks of early Eocene to middle Miocene age. The main issues to be addressed are (1) an understanding of the hydrogeological regime of the karst aquifer, (2) the origin and extent of aquifer salinization, and (3) groundwater provenance. Non-invasive Time Domain and Resistivity soundings were used to map the subsurface electrical resistivity structure to infer the distribution of aquifer salinity and geologic structure. An analysis of the major and minor ions was used to evaluate groundwater chemistry patterns and the main mineralization processes. Submarine seepage measurements were taken from random locations in the near- shore region including a region of spring discharge. The results suggest (1) a heterogeneous distribution of fresh and saline groundwater that deviates from the idealized freshwater/saltwater transition zone on the decimeter scale, (2) a transition from Ca- HCO3 to Na-Cl type waters towards the coast indicating mixing with saline groundwater, (3) an Mg/Ca ratio that suggest aquitard-influenced saline groundwater (4) seepage of recirculated saline groundwater at locations where seepage springs are absent, and (5) an aquifer that has not been adequately flushed. In order to support these concepts, further work will utilize stable and environmental isotopes to age-date both fresh and saline groundwater and to evaluate the effects of water-rock and aquifer- aquitard interactions on the spatial and temporal distribution of aquifer salinity.
H51F-03
Water Resources Management in Coastal Karst Aquifers in the Caribbean
Coastal karst aquifers represent significant sources of water on several Caribbean islands including Barbados, Jamaica, and Tobago and an important auxiliary source of water on several others. In Barbados, for example, approximately 20 percent of the annual water demands comes from coastal karst aquifers. Sustainable management of freshwater resources contained in coastal karst aquifers is severely lacking on many Caribbean islands due to limited data collection, insufficient knowledge of hydrologic conditions, and limited institutional capacity. These limitations in turn result in poorly defined and, in some cases, non-existent policies and legal frameworks to support sustainable aquifer management. Challenges associated with the management of coastal karst aquifers in the Caribbean are expected to increase in response to (i) future climate change and climate variability, (ii) changes in population demographics, and (iii) competition for available resources. This presentation discusses these emerging challenges and current work aimed at improving water resources management in the Caribbean.
H51F-04
Relevance of the Halocline in the Diet of the Troglobic Shrimp T.mitchelli in the Yucatan Peninsula
The anchihaline cave systems are characterized by subsurface connections with the sea and the sinkholes, have polyhaline underground water masses and are locally influenced by the rainforest organic input during the rainy season. The specific physical characteristics of the two water masses (fresh and marine) define the presence of karst fauna and their diet, the halocline represents a physicochemical barrier for many organisms and retains suspended particles creating food source storage along a cline of geochemical reactions. We studied the occurrence of the shrimp Typhatya mitchelli, an endemic Decapod Crustacean from the Yucatan Peninsula anchihaline ecosystems. We analyzed its presence in the two water masses and related its muscle d13C and d15N stable isotopic composition to its diet and specific habitat preference. The presence of specialized setae in P1 and P2 and depleted C and N values (d13C -43.11 min. -21.05 max.; d15N -0.54 min. 9.34 max.) confirm a diet sustained on bacteria with similar signatures as those from chemoautotrophic environments where the dominant morphological structures show that scraping on the cave walls, cave floor and the halocline are the prevailing feeding strategy. The density differences of water masses (rainwater rho 0.996 to 0.997 kg m-3, halocline rho 0.997 to 1.009 kg m-3; marine water rho 1.09 to 1.024 kg m-3) at the halocline allow the species (rho 1.12 to 1.13) to find the interface as a suitable substrate where organic matter can concentrate and sustain a dense population of shrimps.
H51F-05
A Regional-Scale Groundwater Model Supporting Management of the Sian Ka'an Biosphere Reserve and its Catchment, Quintana Roo, Mexico
The Caribbean Coast of the Yucatan Peninsula is a rapidly developing area featuring a booming tourism industry. The number of hotel rooms in the Riviera Maya has increased from 2600 in 1996 to 26,000 in 2005, while the total population in the Mexican federal state of Quintana Roo has grown from 500,000 in 1990 to 1,115,000 in 2005. This explosive growth threatens the region's water resources, which primarily consist of a less than 50m thick freshwater lens residing in the regional karst aquifer underlying the entire Yucatan Peninsula. The Sian Ka'an Biosphere Reserve, a 6400 km2 combined marine/terrestrial nature protection area is situated south of Tulum (approx. 87.3° - 88° W, 19° - 20° N). The site is listed as a UNESCO world heritage site and is protected under the Ramsar Convention. It includes extensive freshwater wetlands, saline/brackish mangrove swamps, tropical rainforests and parts of the world's second largest coral reef. The freshwater supply to the system occurs primarily via subsurface inflow. Large freshwater springs emerge through vertical sinkholes (cenotes) in the lagoons of Sian Ka'an. Management of this unique ecosystem in view of the rapid development and urbanization of the surrounding areas requires detailed knowledge on the groundwater flow paths in and around the reserve. Moreover, mapping and delineation of its groundwater catchment zone and groundwater traveling time zones is essential. To this end, a regional-scale steady-state groundwater flow model of the Sian Ka'an Biosphere reserve and its catchment was developed. The model is implemented in MIKE SHE with a finite-difference cell size of 1 km2 and is driven with temporally averaged climate forcings. The karst aquifer is treated as an equivalent porous medium. Darcy's law is assumed to be valid over regional scales and the main structural elements of the karst aquifer are included in the model as zones of varying hydraulic conductivity. High conductivity zones in the Sian Ka'an catchment area were identified from existing geological maps and remote sensing data. Selected high conductivity zones were surveyed using geophysical techniques (EM-34, multi-electrode profiling) to confirm their existence on the ground. Modeled groundwater catchment zones and groundwater traveling times to Sian Ka'an were shown to sensitively depend on the location and parameterization of the high conductivity zones. In order to target groundwater and wetland protection efforts, regional-scale mapping of the aquifer structure using airborne geophysical techniques is recommended.
H51F-06
Hydrobiogeochemical Description of Remipede Habitat in a Coastal Karst Cave
In anchialine caves, the marine water layer beneath the density interface is usually hypoxic limiting the presence of fauna. Remipede Crustaceans survive under the limiting conditions of the hypoxic marine water mass in addition of being restricted in food supply. The primary objective of this study is to describe the hydrobiogeochemical conditions of the Crustacea anchialine cave system where large numbers of individuals of cave adapted crustaceans occur, increasing our understanding this coastal groundwater habitat and helping conservation strategies by providing basic information to foretell changes by coastal development in the remipede habitat. The Crustacea anchialine cave system is located inland at 1 km from the coast and links 3626 m of surveyed passages with 3 sinkholes. The system is characterized by a two level configuration with a shallow 2 m deep fresh to brackish water (n=40, 16.1±0.0 g/L; n=40, 24.7±0.0°C) passage and an 18 m deep full of salt water (n=11, 35.7±0.1 g/L, n=11, 25.3±0.0°C, pH 7.7±0.0) passage. A halocline (n=7, 25.1±8.6 g/L; n=7, 24.9±0.2°C) occurs at 12 to 14 m depth. These passages are connected by vertical pipes. A preliminary census of the crustacean fauna along the cave main line, identified that shrimps congregate close to the ceiling and the halocline of the deep passages, remipedes are restricted to bottom water mass. Three different types of bacterial mats (brown, orange and white) were recorded to cover the sediment of the passages. Other information is being processed and will be available during the presentation.