ERCZO -- Rainfall Chemistry, Stream Water Chemistry, Throughfall Chemistry, Groundwater Chemistry -- Solute chemistry -- Rivendell -- (2007-2015)

OVERVIEW

Description/Abstract

Significant solute flux from the weathered bedrock zone - which underlies soils and saprolite - has been suggested by many studies. However, controlling processes for the hydrochemistry dynamics in this zone are poorly understood. This work reports the first results from a four-year (2009-2012) high-frequency (1-3 day) monitoring of major solutes (Ca, Mg, Na, K and Si) in the perched, dynamic groundwater in a 4000 m2 zero-order basin located at the Angelo Coast Range Reserve, Northern California. Groundwater samples were autonomously collected at three wells (downslope, mid-slope, and upslope) aligned with the axis of the drainage. Rain and throughfall samples, profiles of well headspace pCO2, vertical profiles and time series of groundwater temperature, and contemporaneous data from an extensive hydrologic and climate sensor network provided the framework for data analysis.

All runoff at this soil-mantled site occurs by vertical unsaturated flow through a 5-25 m thick weathered argillite and then by lateral flows to the adjacent channel as groundwater perched over fresher bedrock. Driven by strongly seasonal rainfall, over each of the four years of observations, the hydrochemistry of the groundwater at each well repeats an annual cycle, which can be explained by two end-member processes. The first end-member process, which dominates during the winter high-flow season in mid- and upslope areas, is CO2 enhanced cation exchange reaction in the vadose zone in the more shallow conductive weathered bedrock. This process rapidly increases the cation concentrations of the infiltrated rainwater, which is responsible for the lowest cation concentration of groundwater. The second-end member process occurs in the deeper perched groundwater and either dominates year-round (at the downslope well) or becomes progressively dominant during low flow season at the two upper slope wells. This process is the equilibrium reaction with minerals such as calcite and clay minerals, but not with primary minerals, suggesting the critical role of the residence time of the water. Collectively, our measurements reveal that the hydrochemistry dynamics of the groundwater in the weathered bedrock zone is governed by two end-member processes whose dominance varies with critical zone structure, the relative importance of vadose versus groundwater zone processes, and thus with the seasonal variation of the chemistry of recharge and runoff.

Creator/Author

Kim, Hyojin|Bishop, Jim|Dietrich, William|Fung, Inez

CZOs

Eel

Contact

hyojin820.kim@gmail.com

Subtitle

Water samples from rainfall, stream, and wells. Major solutes (Ca, Mg, Na, K, and Si).

SUBJECTS

Disciplines

Water Chemistry

Topics

Rainfall Chemistry|Stream Water Chemistry|Throughfall Chemistry|Groundwater Chemistry

Subtopic

Solute chemistry

Keywords

solute chemistry|cation|Ca|Mg|Na|K|Si|rivendell|angelo reserve|weathered bedrock|argillite|isco sampler|CO2

Variables

magnesiumDissolved|calciumDissolved|sodiumDissolved|potassiumDissolved|silicaDissolved

Variables ODM2

Calcium, dissolved|Magnesium, dissolved|Potassium, dissolved|Silica|Sodium, dissolved

TEMPORAL

Date Start

2007-09-23

Date End

2015-02-24

SPATIAL

Field Areas

Rivendell

Location

Rivendell

North latitude

39.72957

South latitude

39.727871

West longitude

-123.644457

East longitude

-123.64366000000001

REFERENCE

Citation

Kim, Hyojin Bishop, James K B Dietrich, William E. Fung, Inez Y. (2014): Process dominance shift in solute chemistry as revealed by long-term high-frequency water chemistry observations of groundwater flowing through weathered argillite underlying a steep forested hillslope. Geochimica et Cosmochimica Acta 140: 1-19.

Publications of this data

Kim, Hyojin Bishop, James K B Dietrich, William E. Fung, Inez Y. (2014). Process dominance shift in solute chemistry as revealed by long-term high-frequency water chemistry observations of groundwater flowing through weathered argillite underlying a steep forested hillslope. Geochimica et Cosmochimica Acta 140: 1-19.

Publications using this data

Kim, H, JKB Bishop, TJ Wood, IY Fung (2012). Autonomous water sampling for long-term monitoring of trace metals in remote environments. Environmental Science & Technology, V.46 (20): 11220-11226 http://dx.doi.org/10.1021/es3006404

CZO ID

4742