Abstract
Biogeochemical gradients in streambeds are steep and can vary over short distances often making adequate characterisation of sediment biogeochemical processes challenging. This paper provides an overview and comparison of streambed pore-water sampling methods, highlighting their capacity to address gaps in our understanding of streambed biogeochemical processes. This work reviews and critiques available pore-water sampling techniques to characterise streambed biogeochemical conditions, including their characteristic spatial and temporal resolutions, and associated advantages and limitations. A field study comparing three commonly-used pore-water sampling techniques (multilevel mini-piezometers, miniature drivepoint samplers and diffusive equilibrium in thin-film gels) was conducted to assess differences in observed nitrate and ammonium concentration profiles. Pore-water nitrate concentrations did not differ significantly between sampling methods (p-value = 0.54) with mean concentrations of 2.53, 4.08 and 4.02 mg l−1 observed with the multilevel mini-piezometers, miniature drivepoint samplers and diffusive equilibrium in thin-film gel samplers, respectively. Pore-water ammonium concentrations, however, were significantly higher in pore-water extracted by multilevel mini-piezometers (3.83 mg l−1) and significantly lower where sampled with miniature drivepoint samplers (1.05 mg l−1, p-values <0.01). Differences in observed pore-water ammonium concentration profiles between active (suction: multilevel mini-piezometers) and passive (equilibrium; diffusive equilibrium in thin-film gels) samplers were further explored under laboratory conditions. Measured pore-water ammonium concentrations were significantly greater when sampled by diffusive equilibrium in thin-film gels than with multilevel mini-piezometers (all p-values ≤0.02).The findings of this study have critical implications for the interpretation of field-based research on hyporheic zone biogeochemical cycling and highlight the need for more systematic testing of sampling protocols. For the first time, the impact of different active and passive pore-water sampling methods is addressed systematically here, highlighting to what degree the choice of pore-water sampling methods affects research outcomes, with relevance for the interpretation of previously published work as well as future studies.
Highlights
The nitrate depth profiles observed varied depending upon which sampling technique was used (Fig. 3); the greatest individual porewater nitrate concentrations were observed in the Diffusive Equilibrium in Thinfilm (DET) gel samples, more samples taken with the Minipoints had relatively high concentrations
The three samplers discussed in this study mainly differ with respect to the absolute sampling depth they can reach, the achievable vertical spatial resolution and the pore sizes samples are predominantly extracted from
Disturbances in subsurface flow may vary between sampling techniques depending on sample volumes and sampling rates used for active sampling and the relatively large horizontal sampler footprint of the DET gel, which may affect nutrient profiles near the sediment-water interface where strong gradients are observed
Summary
A field study comparing three commonly-used pore-water sampling techniques (multilevel mini-piezometers, miniature drivepoint samplers and diffusive equilibrium in thinfilm gels) was conducted to assess differences in observed nitrate and ammonium concentration profiles. Porewater nitrate concentrations did not differ significantly between sampling methods (p-value = 0.54) with mean concentrations of 2.53, 4.08 and 4.02 mg l−1 observed with the multilevel mini-piezometers, miniature drivepoint samplers and diffusive equilibrium in thin-film gel samplers, respectively. There are various technical differences between the most commonly used porewater sampling methods, with respect to their spatial and temporal resolution, sampling volume and rates (few millilitres to several litres) (Bou and Rouch, 1967; Conant Jr. et al, 2004; Duff et al, 1998; Hunt and Stanley, 2000; Kalbus et al, 2006; Krause et al, 2013; Palmer et al, 2007; Rivett et al, 2008), maximum sampling depths (mm's to 2 m) and sampling intervals (Bou and Rouch, 1967; Duff et al, 1998; Hunt and Stanley, 2000; Krause et al, 2011a; Krom et al, 1994; Metzger et al, 2016; Palmer et al, 2007; Rivett et al, 2008; Sanders and Trimmer, 2006)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
