Development of a flexible dialysis pore water sampler placement system: easy handling and related error sources

<p>Investigations in the context of greenhouse gas production measurements in sub-tropical reservoirs brought up the necessity to survey the <em>in situ</em> pore water gas and ion concentrations at many positions within a relatively short time. As several sediment cores were taken, the interest in analyzing the pore water at the same time and at the same positions forced us to develop a cost- and time saving method for the placement of dialysis pore water samplers (DPS). General prerequisites were the ability to place several DPS <em>per</em> day, within a flexible depth range of up to 40 m and with a low cost budget. To meet these requirements, a DPS placing system (DPSPS) was developed, which would allow the precise placement of DPS in water with a depth of up to 40 m and assessing the biases of on-board measurements and possible methodological improvements. The DPSPS was transported to Brazil and tested in a measurement campaign for 10 days. The measurements were carried out during two campaigns in December 2012 and March 2013 in the Capivari Reservoir north-east of Curitiba in the State of Paraná. The system worked properly and several DPS could be placed from a 5 m class aluminum boat. The placement was performed with high accuracy regarding the positioning as well as the penetration depth of the DPS. After the recovery of the DPS, the possible biases during sampling were analyzed. Possible back-diffusion was investigated, taking oxygen concentration as one representative parameter for estimation of the sample behavior. Laboratory as well as field results showed that special care has to be taken to minimize the influence of diffusion processes during post-recovery sampling. The results also suggested that the used membranes are affected by clogging which is likely to influence the diffusion times of various ions and gases. It can be stated that the DPSPS was developed successfully as the demands in terms of handling as well as monitoring efficiency and sample quality were met. With this deployment and measurement technique, a valuable part in the assessment of greenhouse gas emissions from surface water bodies could be achieved.