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Morphology and Hydrologic Behavior of Exfiltration Sites in a Small Watershed in Costa Rica R. Hamid 1 , K. Brumbelow 2 , M. Richardson, P. S. K. Knappett 3 , M. Zapata 2,4 ,D. Riddle 5 , G. W. Moore 6 1 Queens College (CUNY); 2 Texas A&M University (TAMU), Civil & Environmental Engineering; 3 TAMU, Geology & Geophysics; 4 Lone Star College; 5 Utah Valley University; 6 TAMU, Ecosystem Science and Management Acknowledgements/ References Funding for this Research Experiences for Undergraduates program is provided by the National Science Foundation’s Division of Earth Sciences (EAR-1659848). 1) Guillermo, Alvarado & , Alvarado & , Cárdenes. (2017). Geology, Tectonics, and Geomorphology of Costa Rica: A Natural History Approach. 2) Jiménez, Blanca & Lopardo, Raul & Daniel Bacchiega, Jorge & E. Higa, Luis & Urquidi'barrau, Fernando & Tundisi, Jose & Tucci, Carlos & Rosadospilki, Fernando & Hespanhol, Ivanildo & Cirilo, José & Cortesao Barnsley Scheuenstuhl, Marcos & Periotto, Natalia & Ormeci, Banu & D´andrea, Michael & Mcphee, James & Gironás, Jorge & Pastén, Pablo & Vargas, José & Vega, Alejandra & Lara-Borrero, Jaime.(2015). Urban water challenges in The Americas. A perspective from the Academies of Sciences. 3) Rojas, Lyner Chavarria. (2013). Geological Surveys for Exploration in Costa Rica. Introduction About 68% of Costa Rica’s drinking water is groundwater exfiltrated from local springs and is responsible for approximately 4.3 million m 3 of water per year (Jiménez et al., 2015 ). Used in Costa Rica’s homes, agriculture, livestock and industry water, is essential for their lifestyle. Groundwater is stored in unique geological features created by the Cocos plate subducting under the Caribbean plate at a rate of 78 mm/yr. (Guillermo et al., 2017) and channeled by Costa Rica’s topographic extremes, slope variations and rocks. In this study we investigated the geological history of Costa Rica and its effect on the hydrology and flowrate of exfiltration sites in a small watershed due to the current drought in the San Isidro de Peñas Blancas region. Geologic History & Study Site The study site is a 2.2 ha watershed southwest of the Texas A&M Soltis Center in San Isidro de Peñas Blancas The andesite/dacite bedrock of the San Isidro de Peñas Blancas region was created by the Arenal Volcano's Northern migration from Cerro Los Perdidos (Rojas, 2013) The watershed is in the Monteverde formation and is made up of feldspathic andesite/dacite lava flows with andesite fragments The Monteverde Formation is made up of andesitic-basaltic to andesite lavas and pyroclastic products from the Pliocene and Pleistocene in the South and more recent eruptions of similar composition towards the North (Rojas, 2013) The Peñas Blancas normal fault crosses south of the area Abundant cliffs or escarpments within the watershed could be caused by: I. Land slips of weathered volcanic rocks (clay) II. Major events related to plate tectonics III. Margins of the volcanic Poco Sol caldera Methodology Exfiltration sites were found, mapped, and characterized throughout the watershed An instrument was created to capture water and measure flowrate once a day for 15 days To capture as much water as possible from the springs an epoxy sealant was used on the device as well as between the rocks and the funnel of the device To measure flow rate, a stopwatch was used to determine how much time it took for a spring to fill a 1000 mL graduated cylinder Flowrates were classified into three magnitudes: (1)>1000 mL/min, (2) 500-1000 mL/min, and (3) <500 mL/min Results/Discussion Conclusion Andesite lava flow with encased andesite fragments Springs were classified into four types according to their overall composition and location of their orifice: (a) Cooling fractures in lava bedding planes, (b) All soil, (c) Fractures in bedrock, and (d) Half soil/saprolite and bedrock Rock sample of feldspathic andesite/dacite Geological map of La Fortuna NW of San José, Costa Rica. An ”X” is marked at the location of the watershed. Adapted from Rojas (2013) Contour map of the watershed with the forty-six springs found (CSB = “Community Spring Box”; SCSB = “Soltis Center Spring Box) b c d a Objective Relate the hydrologic behavior of the exfiltration sites to geological characteristics within the Howler Monkey Watershed in Costa Rica The flow rate of spring location are not directly related to precipitation, but may be affected by: Soil thickness and permeability Cooling fractures in lavas can set up large networks for water storage and flow Tops of massive lava flows form aquitards for water flow (low porosity and permeability) Pyroclastic layers allow water to flow (high porosity and permeable) We were not able to determine a direct response of spring flow to precipitation. This may indicate a disconnect between surface and groundwater during this period. Spring flowrate may be indirect due to several reasons: Rain events were insufficient to recharge groundwater in the aquifer(s) (spring 25 and 41) Water experiences long residence times due to soil thickness, landslides, and random assortment of permeable and impermeable rock underground (spring 43) Large ancient aquifers are present that supply water to springs causing them to flow continuously (spring 19) Graph of selected springs and their response to rainfall classified into three magnitudes. All soil (spring 41), crack in bedrock (Spring 25), and half soil/saprolite and bedrock (Springs 43 and 19) Elevation and soil depth map near the weir

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Page 1: Morphology and Hydrologic Behavior of Exfiltration Sites ... · •Flowrates were classified into three magnitudes: (1)>1000 mL/min, (2) 500-1000 mL/min, and (3)

Morphology and Hydrologic Behavior of Exfiltration Sites in a Small Watershed in Costa Rica R. Hamid1, K. Brumbelow2, M. Richardson, P. S. K. Knappett3, M. Zapata2,4 ,D. Riddle5, G. W. Moore6

1Queens College (CUNY); 2Texas A&M University (TAMU), Civil & Environmental Engineering; 3TAMU, Geology & Geophysics; 4Lone Star College; 5Utah Valley University; 6TAMU, Ecosystem Science and Management

Acknowledgements/ ReferencesFunding for this Research Experiences for Undergraduates program is provided by the National Science Foundation’s Division of Earth Sciences (EAR-1659848).

1) Guillermo, Alvarado & , Alvarado & , Cárdenes. (2017). Geology, Tectonics, and Geomorphology of Costa Rica: A Natural History Approach.2) Jiménez, Blanca & Lopardo, Raul & Daniel Bacchiega, Jorge & E. Higa, Luis & Urquidi'barrau, Fernando & Tundisi, Jose & Tucci, Carlos & Rosadospilki, Fernando & Hespanhol, Ivanildo & Cirilo, José & Cortesao Barnsley Scheuenstuhl, Marcos & Periotto, Natalia & Ormeci, Banu & D´andrea, Michael & Mcphee, James & Gironás, Jorge & Pastén, Pablo & Vargas, José & Vega, Alejandra & Lara-Borrero, Jaime.(2015). Urban water challenges in The Americas. A perspective from the Academies of Sciences.3) Rojas, Lyner Chavarria. (2013). Geological Surveys for Exploration in Costa Rica.

IntroductionAbout 68% of Costa Rica’s drinking water is groundwater exfiltrated from local springs and is responsible for approximately 4.3 million m3 of water per year (Jiménezet al., 2015 ). Used in Costa Rica’s homes, agriculture, livestock and industry water, is essential for their lifestyle. Groundwater is stored in unique geological features created by the Cocos plate subducting under the Caribbean plate at a rate of 78 mm/yr. (Guillermo et al., 2017) and channeled by Costa Rica’s topographic extremes, slope variations and rocks. In this study we investigated the geological history of Costa Rica and its effect on the hydrology and flowrate of exfiltration sites in a small watershed due to the current drought in the San Isidro de Peñas Blancas region.

Geologic History & Study Site• The study site is a 2.2 ha watershed southwest of the Texas A&M Soltis Center in San Isidro de Peñas Blancas• The andesite/dacite bedrock of the San Isidro de Peñas Blancas region was created by the Arenal Volcano's

Northern migration from Cerro Los Perdidos (Rojas, 2013) • The watershed is in the Monteverde formation and is made up of feldspathic andesite/dacite lava flows with andesite

fragments• The Monteverde Formation is made up of andesitic-basaltic to andesite lavas and pyroclastic products from the

Pliocene and Pleistocene in the South and more recent eruptions of similar composition towards the North (Rojas, 2013)

• The Peñas Blancas normal fault crosses south of the area• Abundant cliffs or escarpments within the watershed could be caused by:

I. Land slips of weathered volcanic rocks (clay)II. Major events related to plate tectonics III. Margins of the volcanic Poco Sol caldera

Methodology• Exfiltration sites were found, mapped, and characterized throughout the watershed• An instrument was created to capture water and measure flowrate once a day for

15 days• To capture as much water as possible from the springs an epoxy sealant was

used on the device as well as between the rocks and the funnel of the device • To measure flow rate, a stopwatch was used to determine how much time it took

for a spring to fill a 1000 mL graduated cylinder• Flowrates were classified into three magnitudes: (1)>1000 mL/min, (2) 500-1000

mL/min, and (3) <500 mL/min

Results/Discussion

Conclusion

Andesite lava flow with encased andesite fragments

Springs were classified into four types according to their overall composition and location of their orifice:

(a) Cooling fractures in lava bedding planes, (b) All soil, (c) Fractures in bedrock, and (d) Half soil/saprolite and bedrock

Rock sample of feldspathic andesite/dacite

Geological map of La Fortuna NW of San José, Costa Rica. An ”X” is marked at the location of the watershed. Adapted from Rojas (2013)

Contour map of the watershed with the forty-six springs found (CSB = “Community Spring Box”; SCSB = “Soltis Center Spring Box)

b

c d

a

ObjectiveRelate the hydrologic behavior of the exfiltration sites to geological characteristics within the Howler Monkey Watershed in Costa Rica

The flow rate of spring location are not directly related to precipitation, but may be affected by:

• Soil thickness and permeability • Cooling fractures in lavas can set up large networks for water storage and flow• Tops of massive lava flows form aquitards for water flow (low porosity and permeability) • Pyroclastic layers allow water to flow (high porosity and permeable)

We were not able to determine a direct response of spring flow to precipitation. This may indicate a disconnect between surface and groundwater during this period. Spring flowrate may be indirect due to several reasons:• Rain events were insufficient to recharge groundwater in the aquifer(s) (spring 25 and 41)• Water experiences long residence times due to soil thickness, landslides, and random

assortment of permeable and impermeable rock underground (spring 43) • Large ancient aquifers are present that supply water to springs causing them to flow

continuously (spring 19)

Graph of selected springs and their response to rainfall classified into three magnitudes. All soil (spring 41), crack in bedrock (Spring 25), and half soil/saprolite and bedrock (Springs 43 and 19)

Elevation and soil depth map near the weir