Developing a multiport storage device for soil water vapor

Recently developed membrane inlet systems have created the possibility of measuring bulk soil water isotopes (i.e. δ¹⁸O_vapor and δ²H_vapor) with minimal soil profile disruption, which provides the potential for higher temporal resolution records (Oerter et al., 2016). Daily records of soil water isotopes that complement daily soil moisture and temperature observations will greatly elucidate the complexity of water movement in the vadose zone. However, current systems rely on the field deployment of cavity ring down spectrometers (CRDS) to measure those samples in real time, which requires daily calibration, temperature control and reliable power in the field (e.g. Oerter et al, 2016; Rothfuss et al., 2015). These constraints limit the practicality of higher temporal resolution sampling over long time periods. Here, we describe the development of a water vapor storage system that will allow for the collection of serial soil water vapor measurements without daily intervention. The storage system will eliminate the need for field deployment of a CRDS and enable collection of water samples that can be used for a broad array of measurements beyond stable isotope ratios. Our current system utilizes a membrane inlet probe system described by Oerter and others (2016) to push water vapor from a soil into a Valco valve coupled with a 1000 ml glass flask. After storage, soil water vapor is measured using a Picarro l2130-i. Initial laboratory results show after storage for up to one-week, δ¹⁸O_vapor values are within 0.5‰, and δ²H_vapor values are within 6.5‰ of the vapor the bottle was filled with. This demonstrates recovery of the original oxygen isotopic signal within error of the overall system. We will also present further laboratory testing using longer storage times and initial daily field sampling that will be done in the Fall of 2018.