Identifying short-term and seasonal trends in cave drip water trace element concentrations based on a daily-scale automatically collected drip water dataset

Abstract

A 13-month long, daily-scale drip water dataset collected by an automatic water sampling device placed in a small cave in Ireland provides evidence for seasonal shifts in hydrochemistry, including a pulse of colloidally-associated elements in the late summer. Ca, Sr, Mg, Na, Ba, P, Cu, Zn, Rb, Y, Cs, U, Th, and Pb concentrations of the collected water samples were determined, and flow rates were also calculated. Alkali and alkali earth metals decreased in concentration during a summer water deficit, whereas colloidally-associated element concentrations increased during the same interval and spiked dramatically in the late summer/early autumn. The observed increase in colloidally-associated element concentrations may have coincided with increased breakdown of soil organic material by microorganisms in the late summer/early autumn, which led to an increased flux of organic colloids in the drip water, or it may relate to increased rates of dry deposition from cave air. The decrease in alkali and alkali earth metal concentrations in the summer most likely resulted from the addition of dilute water linked to condensation of water vapor above the drip site. Drip water Sr and P concentrations are anticorrelated, and their variation over the study period resembles seasonal trends observed in stalagmite calcite at other sites. Because the Sr minima and P maxima are interpreted as reflecting different phenomena (increased proportion of condensation water relative to karst water, and an increased flux of organic colloids, respectively), the relative timing of these two mechanisms in the past may explain the shifting polarity of their correlation observed in some stalagmite trace element records.