Effects of interbasin groundwater transfer on water and chemical budgets in lowland tropical watersheds- La Selva, Costa Rica

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Title: Effects of interbasin groundwater transfer on water and chemical budgets in lowland tropical watersheds- La Selva, Costa Rica
Author: Jordan, Michael Terence
Advisors: Dr. David Genereux, Committee Chair
Dr. Edward Stoddard, Committee Member
Dr. John Fountain, Committee Member
Abstract: Small watershed budget studies are a major research tool in hydrology, ecology, and geochemistry. Most watershed hydrology and geochemistry studies attempt to avoid sites with groundwater seepage beneath topographic divides, known as interbasin groundwater transfer (IGT), due to the difficulty in quantifying it. However, IGT is an important and relatively common hydrological process that merits study. This project identified and quantified IGT by making detailed physical and chemical measurements in two adjacent lowland rainforest watersheds in Costa Rica: the Arboleda watershed, which receives IGT, and the Taconazo watershed, which is not affected by IGT. Physical hydrologic data allowed total IGT (mm/year) to be quantified in the context of water budgets for the watersheds, and the combination of physical and chemical data together allowed the IGT to be quantitatively separated into two components (high-solute bedrock groundwater and low-solute local water). Physical measurements of change in groundwater storage using piezometers, rainfall using a tipping bucket rain gauge, and stream discharge using V-notch weirs were made for four consecutive budget years. Major ion concentrations were measured in bulk rainfall samples, streamwater samples, and groundwater samples. The physical and chemical data were used to calculate annual water and chemical (Cl, SO4, Na, K, Mg, Ca) budgets for the two study watersheds. The watersheds had equal annual rainfall and ET (averaging 4,973 mm/yr and 2,107 mm/yr, respectively), but the Arboleda also had additional large water inputs by IGT (averaging about 4,367 mm/yr from bedrock groundwater and 5,590 mm/yr from local water). IGT to the Taconazo (all local water) was negligible (328 mm/yr). IGT of bedrock groundwater was mainly responsible for the Arboleda watershed receiving 18 times more Cl, 11 times more SO4, 36 times more Na, 54 times more K, 220 times more Mg, and 71 times more Ca input than the Taconazo watershed. Total solute input to the Arboleda watershed is dominated by bedrock groundwater, which accounts for an average of 84 percent (SO4) to 99 percent (Mg) of total solute input as compared with the Taconazo watershed where total solute input is controlled by rainfall accounting for on average of 77 percent (Mg) to 91 percent (SO4) of total solute input. The Arboleda watershed was in a steady state condition (i.e., the difference between inputs and outputs was within the range of uncertainty) for each solute during the 12/00-11/01 and 12/02-11/02 budget years, as was the Taconazo for Na and Ca in both budget years and Cl in the second budget year. The Taconazo chemical budgets showed an excess of SO4 inputs over outputs (+88 mol/ha and +115 mol/ha), a net loss of K (-165 mol/ha and -162 mol/ha), and a net loss of Mg (-256 mol/ha and -330 mol/ha). Traditionally, most watershed budget studies are conducted on the assumption that the watershed is 'tight'; however, the results of this study clearly indicate that caution should be used in making such an assumption. Results also suggest a linkage between deep groundwater systems and lowland rainforest may be important to watershed science, water quantity and quality, water management, and conservation of lowland rainforest ecosystems.
Date: 2003-11-13
Degree: MS
Discipline: Marine, Earth and Atmospheric Sciences
URI: http://www.lib.ncsu.edu/resolver/1840.16/2304

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