Moore et al 2009 - conduit, matrix exchange

Article Overview

• Title: Geochemical and Statistical Evidence of Recharge, Mixing, and Controls on Spring Discharge in an Eogenetic Karst Aquifer

• Authors: Paul J. Moore, Jonathan B. Martin, Elizabeth J. Screaton

• Publication Details: Department of Geological Sciences, University of Florida, received Nov 20, 2008, revised Jun 7, 2009, accepted Jul 20, 2009.

• Keywords: Floridan aquifer, Karst springs, Chemistry, Mixing, Recharge.

Introduction to Study

• Focuses on understanding recharge sources, flow path distributions, and water-rock reactions in karst aquifers.

  • from physical and chemical monitoring

• Method: Monitoring spring chemistry and discharge to unravel complexities in karst systems.

  • major-element chemistry: Na+, Mg2+, K+, Cl, and SO24

  • physical conditions including river stage

  • precipitation, evapotranspiration (ET)

  • temperature gradients of groundwater

  • multivariate statistical method (principal component analysis; PCA)

• Quotes:

  “Consequently, a question we explore in this paper is what additional insight can be gained from physical and chemical monitoring of spring flow and chemical composition.”

• Key Parameters Analyzed:

  • Surface and groundwater chemistry.

  • Physical conditions: River stage, precipitation, and evapotranspiration (ET).

  • Area of study: 6 km section of the Upper Floridan aquifer (UFA), North-Central Florida.

Sources and Methods

Principal Component Analysis (PCA)

• Highlights three main sources affecting spring discharge:

  • Allogenic recharge through a swallet.

  • Diffuse recharge via a thin vadose zone.

  • Deep water upwelling from within the aquifer influencing major ions (Na+, Mg2+, K+, Cl-, SO4).

• Each source contributes to the water chemistry:

  • Deep water provides the majority of ionic content.

  • High temperature readings indicate significant vertical flow (1 m/year).

• Mass-balance calculations indicate diffuse recharge and deep-water contributions can exceed 50% of spring discharge.

Study Area Characteristics

• Geography: Santa Fe River watershed, 3600 km², primarily agricultural land use.

• Aquifer Composition: Oligocene and Eocene carbonate rocks comprising the UFA.

• Thin unconsolidated sediment cover (about 4 m) overlying the aquifer in the area leading to a significant interaction between hydraulic head and spring discharge.

Flow Path Dynamics

• **Types of Aquifer Flow: **

  • Dominant flow is conduit-based, yet storage primarily in matrix porosity impacting recharge dynamics.

  • Recharge can happen seasonally or during storms via point-source or diffuse methods.

• Upward water flows from depths contribute to shallow water budgets and influence chemical compositions.

Temporal Variability in Spring Characteristics

Spring Behavior Insights

• Springs with larger discharge variations are typically allogenic, while smaller variations point towards diffuse recharge.

• Understanding ramifications of changing flow paths and sources is vital for a comprehensive analysis of karst systems.

Monitoring Methods

• River stage monitored using pressure transducers.

• Data collected to analyze relationships between river discharge and environmental conditions, especially precipitation effects.

• Field sampling conducted through various instruments, ensuring accurate collection and analysis of major ion concentrations.

  Quotes: “eight groundwater monitoring wells, one sinking stream (River Sink), one first-magnitude spring (River Rise), and four intermediate karst windows.”

Findings on Water Chemistry

Chemical Analysis

• Groundwater temperature recorded at approximately 21°C, indicating stability over periods but variable depending on well site.

• Notably, differences in chemical composition noted across sampling sites, indicating varying contributions from distinct water sources (Piper diagram analysis).

• Each sampling site represented a continuum of water chemistry from three identified end-member sources:

  • Ca-HCO3 Water

  • Ca-Mg-SO4 Water

  • Na-Cl Water (influenced primarily by allogenic inputs).

Principal Component Analysis Application

• PCA shows significant variance explained by major ions, highlighting major influences on spring chemistry from different sources through clustering.

• Factors Evaluated:

  • Strong positive loadings indicate influences of Na+, Cl-, and SO4 ions on water samples.

  • Negative loading on stage suggests inverse relationships between discharge rates and measured water levels at spring sites.

Conclusion Insights

• Monitoring both spring and groundwater sources effectively enhances understanding of karst aquifer behavior.

• Identifies the importance of multiple water sources and how their interactions shape the discharge and overall chemistry at spring sites.

• Key Point: Acknowledges that neglecting deep-water contributions may lead to misinterpretations concerning aquifer characteristics.

Allogenic recharge

• “Positive loadings of stage, K+, Na+, and Cl and negative loading of pH, Ca2+, and alkalinity on PC 2 suggest allogenic recharge at the River Sink delivers increasing concentrations of K+, Na+, and Cl, but dilutes pH, Ca2+, and alkalinity as stage increases”

• River Sink during high flow is evolved rain water flowing overland or in the shallow subsurface during storm events with minimal groundwater contribution (

• sea water evapotranspiration: Na+, Cl, and K+

  • seaspray