Investigating the effects of water chemistry on carbonates and biota in a northern California spring region

Abstract

Sedimentological observations and hydrogeochemical measurements of Ringtail Spring Creek, Felton, California were used to understand seasonal effects on groundwater chemistry and carbonate formation along a ~250 spring flow transect. Determining the effects of hydrochemistry and groundwater inputs on carbonate formation is significant for assessing the use of spring-associated carbonates (tufa) as an archive of regional climatic events. Ringtail Creek is a perennial spring that flows for an approximate 250 m stretch and exhibits a ~ 96 m elevation change from the outflow channel to the distal-most pool facies. This work presents hydrochemistry results from two sites (proximal and distal) and hydrochemical parameters measured in situ from three additional sites with distinct sedimentary facies along the flow path. Facies observations and hydrochemistry results suggest tufa deposition is strongly controlled by slope variations along the flow path, calcite saturation state, and water turbulence. Measurements of seasonal water parameters recorded from October 2019 to March 2021 indicate the following: (1) water temperature remains constant at the spring source (12.5-13.0°C), but exhibits strong seasonal trends at the distal-most sites with warming to 13.5°C in the summer and cooling to 10.5°C in the winter, (2) pH ranges from 7.3 to 8.0 at the source and all sites exhibit a rise in pH irrespective of the season, and (3) there is a notable rise in calcite saturation state from the source to the distal-most pool which reflect observations of modern carbonate deposition in the distal-most sites. Hydrochemistry data collected in the winter of 2020-2021 nearly doubles in concentration for organic carbon (TOC and DOC), and the redox potential of the waters measured in situ reflect the lowest values recorded. We attribute these changes to an influx of organic carbon resulting from the CZU Lightning Complex Fire which burned the Ringtail Spring Creek catchment from August 16, 2020 to September 22, 2020 signifying a relatively rapid groundwater infiltration response following the first post-fire rainfall of the winter season. This work represents a first look at the effects of spring chemistry on tufa deposition to serve as a baseline and reference for future analyses on currently-forming and ancient tufa deposits in the region.