Relationship between saprolite-hosted groundwater chemistry, baseflow, and weathering depth at Redlair Observatory, a critical zone research site on the North Carolina Piedmont

Search for this publication on Google Scholar
Horgan, J. (2021). Relationship between saprolite-hosted groundwater chemistry, baseflow, and weathering depth at Redlair Observatory, a critical zone research site on the North Carolina Piedmont. Unc Charlotte Electronic Theses And Dissertations.
Download PDF

Analytics

36 views ◎
9 downloads ⇓


Abstract

Crystalline bedrock weathering profiles in the southern Piedmont terrane have thicker soils and gentler slopes than mountainous terranes with their thin soils and steep slopes. The objective of this study was to understand watershed-scale baseflow and groundwater chemistry in relationship to weathering depth, and to assess landscape position (ridgetop, midslope, and base of slope) as a potential critical zone factor within the Piedmont. This study took place at the recently established (2019-present) Redlair Observatory (RO) in Gaston County, North Carolina in the southern Piedmont. RO is part of the 1,200-acre Redlair Preserve, which protects Magnolia macrophylla and Helianthus schweinitzii. The climate at RO is humid and subtropical, with a mean annual temperature of 14.5°C. The geology consists of felsic and mafic metavolcanic rocks of the Charlotte Terrane overlain by Ultisols (Cecil soil series) and Entisols (Pacolet soil series). Seventeen wells were installed among four zero-order to first-order watersheds, ~4-25 m deep. Monthly surface water baseflow sampling, seasonal groundwater sampling, high-resolution continuous loggers, and geospatial analyses were used to examine critical zone processes. Samples were analyzed for major ion concentrations and water isotopes (δD, δ18O). Groundwater ranged from -25.4 to -35.8‰ (δD) and from -4.6 to -7.9‰ (δ18O). Manual groundwater measurements and 10-minute increment loggers revealed that groundwater levels were shallowest in the late spring-early summer months and deepest in late fall-early winter months. However, not all wells responded to these warmer groundwater peaks and colder groundwater minimums on the same timeline. Water level and temperature patterns suggest that mean groundwater residence times are on the order of a season to a few months. Well depth seemed to be the most important factor influencing groundwater chemistry in this study. Most notably, the calcium/sodium (Ca/Na) molar ratios of the shallow wells versus the deep wells reveal a varying degree of weathered material at depth. The deep wells had higher Ca/Na ratios of 0.34-1.72 (5-24 m depth below land surface) while the shallow wells had Ca/Na ratios ranging from 0.7-1.79 (4-21 m depth below land surface). This suggests that the deep wells were screened in less weathered material than the shallow wells, consistent with weathering of Na-plagioclase shallower in the weathering profile and Ca-plagioclase deeper in the weathering profile. This study found that hydrologic processes occur quickly in these small (4.4-15.5 ha) headwater watersheds of the Piedmont. Understanding of critical zone processes is important for predicting how small watersheds will respond to climate change and urbanization influences. Locally, the results of this study may contribute to management of Redlair Preserve.

Details
Author

Horgan, Julianna

Title

Relationship between saprolite-hosted groundwater chemistry, baseflow, and weathering depth at Redlair Observatory, a critical zone research site on the North Carolina Piedmont

Physical Description

1 online resource (137 pages) : PDF

Date

2021

Degree Granting Institution

University of North Carolina at Charlotte

Abstract

Crystalline bedrock weathering profiles in the southern Piedmont terrane have thicker soils and gentler slopes than mountainous terranes with their thin soils and steep slopes. The objective of this study was to understand watershed-scale baseflow and groundwater chemistry in relationship to weathering depth, and to assess landscape position (ridgetop, midslope, and base of slope) as a potential critical zone factor within the Piedmont. This study took place at the recently established (2019-present) Redlair Observatory (RO) in Gaston County, North Carolina in the southern Piedmont. RO is part of the 1,200-acre Redlair Preserve, which protects Magnolia macrophylla and Helianthus schweinitzii. The climate at RO is humid and subtropical, with a mean annual temperature of 14.5°C. The geology consists of felsic and mafic metavolcanic rocks of the Charlotte Terrane overlain by Ultisols (Cecil soil series) and Entisols (Pacolet soil series). Seventeen wells were installed among four zero-order to first-order watersheds, ~4-25 m deep. Monthly surface water baseflow sampling, seasonal groundwater sampling, high-resolution continuous loggers, and geospatial analyses were used to examine critical zone processes. Samples were analyzed for major ion concentrations and water isotopes (δD, δ18O). Groundwater ranged from -25.4 to -35.8‰ (δD) and from -4.6 to -7.9‰ (δ18O). Manual groundwater measurements and 10-minute increment loggers revealed that groundwater levels were shallowest in the late spring-early summer months and deepest in late fall-early winter months. However, not all wells responded to these warmer groundwater peaks and colder groundwater minimums on the same timeline. Water level and temperature patterns suggest that mean groundwater residence times are on the order of a season to a few months. Well depth seemed to be the most important factor influencing groundwater chemistry in this study. Most notably, the calcium/sodium (Ca/Na) molar ratios of the shallow wells versus the deep wells reveal a varying degree of weathered material at depth. The deep wells had higher Ca/Na ratios of 0.34-1.72 (5-24 m depth below land surface) while the shallow wells had Ca/Na ratios ranging from 0.7-1.79 (4-21 m depth below land surface). This suggests that the deep wells were screened in less weathered material than the shallow wells, consistent with weathering of Na-plagioclase shallower in the weathering profile and Ca-plagioclase deeper in the weathering profile. This study found that hydrologic processes occur quickly in these small (4.4-15.5 ha) headwater watersheds of the Piedmont. Understanding of critical zone processes is important for predicting how small watersheds will respond to climate change and urbanization influences. Locally, the results of this study may contribute to management of Redlair Preserve.

Genre

masters theses

Subjects--Topics

Environmental sciences
Geology
Hydrology

Degree

M.S.

Keywords

Critical Zone
Groundwater Chemistry
Landscape Position
Piedmont
Watershed Hydrology
Weathering Depth

Subject Area

Earth Sciences

Advisor(s)

Vinson, David

Committee Members

Eppes, Martha Cary
Allan, Craig

Degree Note

Thesis (M.S.)--University of North Carolina at Charlotte, 2021.

Rights Statement

This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). For additional information, see http://rightsstatements.org/page/InC/1.0/.

Rights Holder Information

Copyright is held by the author unless otherwise indicated.

Identifier

Horgan_uncc_0694N_12811

Permalink

http://hdl.handle.net/20.500.13093/etd:2440