groundwater

Geochemical Model of Redox Reactions in a Tropical Rain Forest Stream Riparian Zone: DOC Oxidation, Respiration and Denitrification

Jiménez R.A., Geochemical Model of Redox Reactions in a Tropical Rain Forest Stream Riparian Zone: DOC Oxidation, Respiration and Denitrification. Master's Capstone and Thesis. University of Pennsylvania, 2011.

Abstract: 
A geochemical equilibrium model was used to quantify Dissolved Organic Carbon (DOC) electron donors during aerobic respiration and denitrification in a tropical stream riparian zone of the Luquillo Experimental Forest, Puerto Rico. DOC electron donors were measured across three general redox zones (Oxic: slope, Transitional: slope-riparian interface and Anoxic: riparian-floodplain) of the Icacos watershed. Model results suggest that nitrate and oxygen are completely reduced after approximately 10.1 mg/L of DOC have reacted with an initial ground water solution. In order to reach the observed mean oxygen concentration of 3.79 mg/L in the Oxic zone from the modeled equilibrium oxygen concentration of 9.46 mg/L, approximately 5.33 mg/L of DOC need to be oxidized. Additionally, 2.06 mg/L of DOC are oxidized in order to reach the observed mean oxygen concentration of 1.6 mg/L in the Transitional zone. In order to reach the observed mean Anoxic zone oxygen concentration of 1.27 mg/L from the observed mean Transitional zone oxygen concentration, an additional 0.309 mg/L of DOC are oxidized. From modeled equilibrium concentrations of oxygen (9.46 mg/L), approximately 8.8 mg/L of DOC are oxidized by oxygen before nitrate becomes more thermodynamically favorable as the electron acceptor and begins decreasing in concentration. Model simulations suggest that 1.19 mg/L of DOC reduce the observed mean nitrate concentration of 0.47 mg/L found in the Oxic zone to the lowest observed mean nitrate concentration of 0.01mg/L found in the Transitional zone. Differences between the observed DOC concentrations in the field and the modeled DOC concentrations needed to reach zone levels of oxygen and nitrate suggest that field reported values for DOC electron donors could represent residual or unused electron donors. Results also indicate that between 8.68 mg/L and 10.7 mg/L of DOC oxidation, 0.42 mg/L of dissolved N2 are produced, HCO3 increases from 0.33 mg/L to 2.64 mg/L and CO2 concentrations decrease from 13.8 mg/L to 13.7 mg/L before continuing to increase. This pronounced interval of DOC oxidation at which denitrification occurs and beyond which CO2 continues increasing suggests a specific range at which denitrifiers metabolize versus a larger range at which a general heterotrophic population metabolizes.

GROUND WATER OPERATIONAL MODEL YAUCO, PUERTO RICO

Abstract: 
A numerical groundwater model was constructed to simulate groundwater flow in the Yauco Alluvial Valley aquifer. The groundwater flow model was constructed to evaluate future management options, including the potential to increase aquifer firm yield through a conjunctive management of ground and surface water. The aquifer within the Yauco Valley consists of river alluvium deposited over the incised Juana Díaz formation and Ponce limestone. A finite-difference, numerical model was developed to simulate ground water flow in the Yauco Valley. The single-layer model encompasses the entire alluvial deposits of the valley which extends from the Yauco town to the Caribbean Sea. The model was calibrated to October 1960 and 1970 to 1974 water levels. Different management scenarios were modeled to analyze and determine how much water can be extracted from the aquifer and evaluate the conjunctive use potential. Results demonstrate that the aquifer could be subjected to a total extraction in the order of 4.6 to 4.8 mgd (1-1.25 mgd above current extractions) without reducing the water levels to a point that could produce saltwater intrusion. Simulations showed that groundwater extractions could be increased by 5 mgd to 6 mgd during the dry season (March-August) if artificial recharge is provided in the range of 1.3 mgd to 1.95 mgd on a year-around basis. This demonstrates that the potential exists to conjunctively use ground and surface water to increase aquifer yield

Effects of Hurricane Disturbance on Groundwater Chemistry and Riparian Function in a Tropical Rain Forest

Effects of Hurricane Disturbance on Groundwater Chemistry and Riparian Function in a Tropical Rain Forest
William H. McDowell, Claire P. McSwiney and William B. Bowden
Biotropica
Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances (Dec., 1996), pp. 577-584

Abstract: 
The long-term response of shallow groundwater chemistry to the canopy disturbance and defoliation associated with Hurricane Hugo was studied at two sites in the Luquillo Experimental Forest, Puerto Rico. The sites differed in bedrock type, dominant vegetation, and availability of pre-hurricane data. At the primary study site, the Bisley catchment, hurricane disturbance resulted in increased concentrations of NO3 -, NH4 +, dissolved organic N, base cations, Cl-, and SiO2 in groundwater within 5 mo of the hurricane. The largest relative change in concentration occurred for K+, which increased from 0.7 to as high as 13 mg/L, concentrations were still 1.3 mg/L 5.5 yr after the hurricane. Most other solutes had returned to background levels within 1-2 yr of the hurricane. At the secondary study site, the Icacos catchment, NO3 - concentrations peaked at 1.1 mg/L one yr after the hurricane and decreased to nearly zero 5.5 yr after the hurricane. At both sites, NO3 - concentrations were higher in upslope wells than in those closer to the stream. Overall, riparian processes appear to reduce but not eliminate hydrologic losses of N following catastrophic disturbance. The nature of the long-term biogeochemical response to disturbance in this tropical rain forest ecosystem is similar to that observed in some montane temperate forests, and the time course of recovery appears to be associated with the speed with which vegetation regrows following disturbance.
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