CoWS: Continuous Water Sampler for CRDS-based, real-time measurements of water isotopes

Abstract:

Stable isotopes of water (δ¹⁸O and δD) are unique tracers for studying hydrological and associated processes. High spatial and temporal resolution measurements of water isotopes are necessary to follow the dynamics in rapidly changing systems and to map out the spatial heterogeneity of water circulation and mixing. Here we present results of the first commercially available Continuous Water Sampler Module (CoWS) that can be coupled to a Picarro L2130-i Cavity Ring-Down Spectrometer (CRDS) for real-time measurements of water isotopes. The CoWS is a compact and fully automated system with its core method modified from that of Munksgaard et al. (2011). Liquid water is continuously pumped into an extraction chamber, where water vapor diffuses through a micro-poruous polytetrafluoroethylene (ePTFE) membrane. The vapor is then carried by a dry carrier gas to the L2130-i for high precision measurements of δ¹⁸O and δD. The inlet water, carrier gas, and surface of the ePTFE membrane are actively temperature controlled to maintain a stable amount of fractionation of water isotopes across the membrane, which minimizes measurement drift. We have tested the CoWS-CRDS system with various inlet water types (tap water, deionized water, and seawater), and under operational conditions with variable ambient water and air temperatures. CoWS-CRDS has high precision (< 0.05 and < 0.15 ‰ 1σ, 5 minute average for δ¹⁸O and δD, respectively) and low drift water isotope measurements, with short response time (<5 minutes to eliminate 98% of the memory). The CoWS software is user configurable; allowing automated sampling among up to four water sources with user defined sampling durations. Additionally, we will present isotopic measurements with high-temporal resolution of an estuarine system where tidal changes affected the isotopic composition of the estuary.