| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | February 10, 2012 |
| Latest Amendment Date: | February 8, 2017 |
| Award Number: | 1141768 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
Holly Barnard
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | March 1, 2012 |
| End Date: | February 28, 2018 (Estimated) |
| Total Intended Award Amount: | $261,540.00 |
| Total Awarded Amount to Date: | $261,540.00 |
| Funds Obligated to Date: |
FY 2013 = $184,684.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
801 LEROY PL SOCORRO NM US 87801-4681 (575)835-5496 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
801 Leroy Place Socorro NM US 87801-4681 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Hydrologic Sciences |
| Primary Program Source: |
01001314DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Karst Conduit Hyporheic Zone Exchange
John L. Wilson, New Mexico Institute of Mining and Technology
This project will attempt to measure, for the first time, hyporheic exchange at the margin of a karst conduit (cave below the water table). Hyporheic exchange in surface streams is a relatively new field, but related publications are growing exponentially as the implications for flow contaminant transport, residence time distributions, biogeochemistry and biology are realized. One definition of the hyporheic zone is the subsurface volume where water enters from the stream, travels through the subsurface, and returns to the stream. Hyporheic zones in streams exist at a variety of nested spatial and temporal scales whether there is net gain, net loss, or even no net exchange along the course. The fundamental fluid dynamics of karst conduits are not different from streams, and karst conduits have the same features that cause hyporheic flow in fluvial systems. This project will investigate whether, as with streams, hyporheic exchange occurs in karst conduits, regardless of larger-scale net exchange with the aquifer. Karst-conduit hyporheic exchange, and related biogeochemical processing, then becomes a fundamental component of the karst water cycle. The project will be conducted in a phreatic conduit field site in the unconfined Floridan Aquifer; additionally there will be a nearby air-filled analog site that was exposed to similar processes at some time in the geologic past. Cores will be taken from the phreatic and analog sites. Porosity, permeability, and petrophysical measurements will be used a proxy indicators of past conduit hyporheic flow. The core holes at the phreatic site will be equipped with multilevel samplers and injectors, and a series of dye traces will be conducted to observe karst-conduit hyporheic flow. Modeling will be used throughout the project, first to help plan the field work and then to synthesize the data. Results from the data and models will give insights into the temporal and spatial scales of hyporheic flow. The project will end with modeling as a predictive tool to generalize our results.
Karst aquifers supply water to 25% of the United States, and almost all water to some regions, e.g. 90% of Florida's population. New mathematical models and field studies will be used to illuminate a previously unrecognized process, karst hyporheic exchange at the margin of a flowing karst conduit, in which the conduit and surrounding karst rock matrix exchange water with consequences for karst chemistry and biology. As does hyporheic exchange in surface streams, karst hyporheic exchange may impact water supply, water quality, and ecology, and will be of interest to environmental agencies and interest groups at the local, state and federal level.
PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Caves Have Hyporheic Exchange, Too.
Hyporheic exchange from streams occurs where stream water infiltrates the banks, flows through surrounding sediments, mixes with aquifer water, and then returns to the stream. This has important implications for the biogeochemistry and ecology of streams and surrounding aquifers, as demonstrated by the exponentially-growing scientific literature on this subject. We now know that a similar hyporheic exchange occurs in caves carrying flowing water, with the cave water infiltrating porous-cave walls, flowing through the matrix of the surrounding porous rock, mixing with resident water, and eventually returning to the cave, with implications not only for the biogeochemistry and ecology of caves, but also their future growth into the surrounding rock. Hyporheic exchange of water occurs over a variety of nested spatial and temporal scales whether there is net gain, net loss, or even no net exchange along the length of the cave, which is more technically referred to as a karst conduit. (Karst is rock, e.g., limestone that has been eroded by dissolution.) This research used mathematical modeling and field observations to hypothesize, characterize, and attempt to quantify the karst-conduit hyporheic exchange process for phreatic conduits, which are caves that are fully filled with flowing water.
Modeling used computational fluid dynamics (CFD) simulations of flow through the complex shape of cave conduits, creating pressure gradients along the conduit wall which drives Darcian flow through the rock matrix and carries conduit water into the matrix and away from, and eventually back to, the conduit. The depth of hyporheic exchange into the rock matrix is controlled by the length scale of cave-wall morphological features, like scallops and cupolas, and the length scale of variations in cave cross-sectional area. While exchange depth is proportional to the length of these features along the cave wall, feature height and shape also play a role. The rate of exchange and the residence time within the rock matrix needed for biogeochemical reactions to occur are controlled by these features, the conduit diameter, the conduit flow rate, and the matrix permeability and porosity. Hyporheic exchange enters the matrix near locations of conduit flow reversal and eddy reattachment, and returns to the conduit in the trough (low spot) of wall features. The exchange is faster and the residence time lower near the peak of wall features that stick out into the flow; the exchange is slower near the trough and deeper into the matrix. These findings also apply to surface bedrock streams. However, unlike surface streams, phreatic karst conduits are surrounded on all sides by walls (e.g., ceiling above and floor below). Morphological features on the floor affect the pressure distribution on the ceiling above, and vice versa. Consequently, morphological features on the floor drive hyporheic exchange in the ceiling above, even if the ceiling is flat, and vice versa. When the morphology of the opposing walls is different, for example, scallops below and larger cupolas above, nested hyporheic zones are created with a dramatic impact on the depths and rates of hyporheic exchange and the spatial distribution of exchange residence times.
Modeling was supplemented through field observations in Florida karst. Limestone rock was cored one meter into the matrix with laboratory characterization of cored material to test for any alterations of rock properties, like porosity, that might carry a signature of hyporheic exchange. Multilevel samplers were deployed in the sealed core holes for a series of quantitative dye traces aimed at demonstrating hyporheic exchange. A new coring technology used a submersible (to 50 meters) electric drill and a drill guide that greatly enhanced core recovery, allowing intact 2? cores to be collected through either vertical or horizontal drilling while wading bedrock streams or by submerged cave divers. A new mechanical packer design for multilevel sampling of bedrock allowed passive or active sampling (or tracer injection) of discrete intervals within the rock matrix, isolating hyporheic exchange pathways from each other. Dye tracers were used in conjunction with the multilevel samplers to demonstrate hyporheic flow through the matrix. Cores collected from limestone bedrock streams and phreatic caves were tested for properties that indicate past hyporheic flown through the matrix, e.g. enhanced permeability development or systematic mineralogical variations along the length of cores. These various field tests confirmed the presence of hyporheic flow but were not sufficiently exhaustive to quantify it, principally due to the significant heterogeneity of the Floridan aquifer rock matrix and the limited number of samples and tests performed. Future work should address this limitation.
Last Modified: 07/11/2019
Modified by: John L Wilson
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