Department Water Resources and Drinking Water

Non-targeted liquid chromatography tandem high-resolution mass spectrometry (LC-MS/MS) is increasingly applied for the structure-resolved chemical analysis of dissolved organic matter (DOM). With new developments in MS instrumentation and analysis software, the approach has gained substantial momentum over the past decade. However, achieving high-quality analytical data that is reproducible and comparable across laboratories can be a bottleneck in non-targeted metabolomics and organic matter chemical analysis, especially for data reuse in repository-scale analyses. Understanding the capabilities as well as challenges of comparing LC-MS/MS data from different laboratories is necessary for inferring global trends from public data sets. To illuminate instrumentation factors that drive differences and variability, we used a standardized data analysis pipeline, including classical (CMN) and feature-based molecular networking (FBMN), to analyze data from a ring trial by 24 laboratories on identical sample sets of algal and DOM extracts that were mixed in predefined concentrations and spiked with standards. Our results showed that data sets from similar mass spectrometer types with unified instrument parameters were qualitatively comparable, resolving the same general trends and shared mass spectral features. Interlaboratory comparability was best for high-intensity features, while low-intensity features showed greater detection variability. Our analysis also highlights challenges when comparing data from instruments with different acquisition rates or operating with less standardized methods. Lastly, we provide recommendations for data integration, public data sharing, standardization, and best practices for standardized LC-MS/MS data acquisition, which will be critical for long-term time series and intercomparability of DOM chemical analyses.

In natural environments, bacteria often encounter low concentrations of nutrient mixtures that are continuously replenished by physical processes such as fluid flow. Studying bacterial physiology under such conditions is experimentally challenging because it is difficult to maintain steady, low nutrient concentrations with rapid renewal. Most studies on nutrient limitation have used approaches such as the chemostat, which rely on long renewal times to sustain low concentrations. We developed a Millifluidic Continuous Culture Device (MCCD), inspired by microfluidics, that enables bacterial cultivation in nutrient mixtures at low micromolar concentrations with rapid renewal driven by fluid flow. Unlike microfluidic systems, the MCCD retains sufficient culture volume to support batch-scale 'omic analyses. Using the MCCD, we cultured Escherichia coli in a mixture of amino acids and nucleobases at three concentration ranges spanning a fivefold difference in growth rates. Surprisingly, at the lowest concentration range, cells exhibited proteomic signatures of iron limitation despite equal total ferrous iron across conditions. Uptake experiments with labeled iron–histidine and iron–cysteine complexes confirmed that amino acids facilitated ferrous iron acquisition. Under continuous flow, siderophores were washed out, rendering this pathway ineffective and revealing a previously unrecognized mechanism of iron acquisition via soluble ferrous iron–amino acid complexes. These findings highlight the importance of studying bacterial physiology at low nutrient concentrations and also suggest a broader role for other organic substrates capable of complexing iron as potential iron sources in environments with rapid renewal.

Hydrological models are calibrated on specific periods based on how well simulations match streamflow observations by selecting the parameter vector(s) that provide the highest accuracy. This accuracy can decrease significantly during extrapolation to periods not seen during calibration, especially when they are characterized by different climate conditions. Here, we develop a novel calibration objective that relies on the joint calibration of model accuracy and time-invariance of residuals, hypothesizing that such invariance can improve the temporal generalizability of hydrological models. We test this approach using a hydrological model and 208 catchments in Western Germany. We evaluate it based on (a) the accuracy loss between calibration and evaluation, (b) the compromise between accuracy and residual invariance, (c) the dependence of residuals on variations in forcings, and (d) the instability of parameters between two calibration periods. We find that our approach reduces the loss in accuracy between calibration and extrapolation as compared to calibration approaches solely based on accuracy. Furthermore, our method combined with the Boxcox streamflow transformation weakens the dependence of residuals on variations in the forcing. However, we find that invariant residuals do not necessarily imply improved parameter stability between different calibration periods. Furthermore, the gain in temporal generalizability comes at the cost of a decrease in accuracy, which should be considered based on the application. Our results highlight that the new RESIdual STability (RESIST) calibration objective has the potential to improve the temporal generalizability of hydrological models for climate-impact studies.

Dissolved noble gases have been recognized for decades as ideal artificial hydro(geo)logical tracers, as they are chemically inert, invisible, and non-toxic. However, their widespread adoption has historically been limited by the difficulty of tracer injection, sampling, and analysis procedures. Developments in portable, high-resolution dissolved gas measurement technology over the last two decades have rekindled interest in the use of gas tracer methods for routine hydrogeological investigations, such as well-to-well tracer tests, intra-well tests, or studies of river infiltration towards alluvial aquifers. The application of gases in aqueous environments still poses unique challenges compared to other tracer methods, as potential exsolution and degassing need to be accounted for, and, if possible, avoided during tracer injection. Here, we present a simple and efficient methodology that addresses these challenges and allows the efficient, on-site preparation and injection of highly concentrated tracer solutions with controlled dissolved gas concentrations. We applied the method in a large drinking water wellfield and performed well-to-well tracer tests in an unconfined aquifer using helium-4 (⁴He), neon-20 (²⁰Ne) and krypton-84 (⁸⁴Kr). Known tracer quantities were injected together with fluorescent dyes into an observation well upgradient of a pumping well. Gas tracer breakthrough was monitored in the pumping well with a portable mass spectrometer. Breakthrough curves of ⁴He and ⁸⁴Kr compared favorably with fluorescent dye tracers, and enabled reliable estimates of groundwater flow velocities, travel times, and tracer recovery. These findings illustrate how noble gases can substitute or complement other artificial tracer methods, even in large-scale settings. The methodology can be extended to other gases (e.g., neon-22, xenon isotopes, light hydrocarbons), significantly expanding the range of artificial tracers available for routine hydrogeological investigations.