Filtration
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wash in 10% HNO3 for a week, prepared from analytical grade 65% HNO3; (ii) then
bottles were emptied and rinsed (x5) with Milli-Q water (Figure I.7.); (iii) bottles are then filled with pH 2 (from 65% HNO3 Merck Suprapur
®
) Milli-Q water; (iv) bottles are then double-bagged (zip-lock) until sampling. Other plastic material (tweezers, vials, spatulas, filter systems,...) is washed inside plastic containers in a similar way.
Figure I.7. Milli-Q water system.
All water samples were collected either using 30-L Niskin bottles (X-Niskin, General Oceanics) or with the aid of a telescopic arm for surface waters. Both methods are proposed in the protocols of clean techniques developed by the EPA. Niskin bottles were previously washed with Milli-Q water and left filled with Milli-Q at pH 2 (from HNO3 analytical grade) overnight. Before sampling collection, bottles
were emptied and rinsed several times with ambient water at the sampling station before taking the final sample. A similar procedure was used when using telescopic rod. The "dirty-hands" person opened the external zip bag while the "clean-hands" wearing polyethylene gloves opened the inner zip bag to extract the polyethylene bottle (500 mL or 1 L) and re-close the bag until ready to return the bottle with the final sample. First of all, the content of the bottles (Milli-Q at pH 2 from HNO3) was
emptied and bottles homogenized (x3) with the sample before collecting the final sample which was stored in a fridge (~4°C) pending filtration and analysis. In the case of sampling with telescopic rod, samples were collected facing upstream in the case of rivers and sewage treatment plants and upwind avoiding the shadow of the boat in case of surface water samples collected from the boat (EPA guidelines).
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Surface sediment samples were collected using a Van Veen grab, sub- sampling the first centimetre with a pre-washed plastic spatula. Subsequently the samples were introduced in vials previously washed in acid or zip bags and kept in refrigerator until drying and storage. Sediment cores were collected using a Rouvilloise grab with PVC cores for surface ones and a gravity corer for deeper ones. The cores were stored at 4°C until being sectioned or sliced and sub- sampled in the laboratory.
c) Filtration
Filtration of samples was carried out inside a laminar flow hood in the lab clean using a vacuum pump located outside the hood to avoid contamination. Filtration systems (Nalgene) were previously acid washed (see above), whereas polycarbonate filters (Pall) were washed by immersion in 1% HCl overnight and extensively rinsed Milli-Q water before drying, weighing and storage in acid- washed petri dishes. The first 50 mL of sample were discarded after homogenization of the receiving cup; the subsequent filtrate was stored in acid pre- washed 500-1000 mL polyethylene bottles.
d) Storage
Filtered samples for total dissolved metal concentrations were acidified to pH 2 (HCl Trace Select, Fluka) pending analysis, whereas those for chemical speciation were stored frozen at their ambient pH in order to avoid altering the distribution of metals among different species (Capodaglio et al., 1995).
The loaded filters were stored frozen in Petri dishes until analysis. Sediments were dried in an oven (50ºC) and sieved through 2000 and 63 µm nylon sieves. Sediments were then stored in acid-washed vials or zip-lock bags until their acid digestion prior to analysis.
e) Reagents and Analysis
All reagents used for the analysis of the samples were Trace Select (Fluka) grade or Suprapur (Merck). To ensure non-contamination of samples during analysis, a series of blanks (i.e. analytical blanks) were measured. These blanks were treated in the same way as the samples (filtration, acidification, addition of reagents, etc.). Blank values were substracted to the result of each sample. Besides the analytical blanks, a series of field blanks, consisting of Milli-Q water transported to the sampling point and back to the lab with the rest of the sampling material, were also obtained. This determines the potential contamination of the sample during the sampling procedure.
Analytical methods for water, sediment and suspended particulate matter, are described in detail in each chapter.
f) Certified Reference Materials
In order to check the precision and accuracy of the measurements, certified reference materials were analysed. Results appear in each chapter.
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CASS-4 (Reference Material for trace metals in coastal waters), SLRS-4 (reference material of river water for trace metals), and PACS-2 (reference material of marine sediments for trace elements) were used in this work.
1.3. State of the Art. Previous Studies on Trace Elements in Galician Rias and Coastal Waters
As it has been reported in a recent review about the state of the knowledge of trace elements in the Galician Rias (Prego and Cobelo-García, 2003), there is still an important lack of studies on the distribution, behaviour, speciation and biogeochemical cycles of these elements in this particular environment of the coast of the Northwest Iberian Peninsula. Despite of the number of papers published in the last decade that tried to overcome this lack of information on trace metals in sediments (Cobelo-García and Prego., 2003a; Alvarez-Iglesias et al., 2003; Evans et al., 2003; Prego et al., 2006a; Marmolejo-Rodriguez et al., 2007 ), estuarine waters (Cobelo-García and Prego, 2004a; Cobelo-García et al., 2005 y Prego et al., 2006b), continental and pluvial waters (Cobelo-García and Prego, 2003b; Cobelo-García et al., 2004; Filgueiras and Prego, 2007) and organic speciation (Cobelo-García and Prego, 2004b), some aspects have not been addressed yet or suffer from scarce data.
Most part of the previous work (Prego and Cobelo-Garcia, 2003) was focused on the study of the levels of trace metal in the sediments, and although an effort has been made on the chemical speciation of metals in the different sediment fractions (Alvarez-Iglesias et al., 2003; using the sequential extraction methods: BCR or Tessier), a more complete study (wider sampling area, more metals and more fractions) is still required. In addition, there are no studies about the geochemistry of trace elements in porewaters of the sediments and benthic fluxes to the overlying waters.
Regarding trace metal studies in different estuarine or continental waters, an important advance has been made but there is still a lack of studies on trace metal distribution and baseline levels in Galician costal and open-ocean waters that will serve to evaluate the impact of any contaminant events (i.e. oil spills). Moreover, these studies were focused on the ‘classic’ elements (Cu, Pb, Zn and Cd; Cobelo-Garcia and Prego, 2004a) so an extension of the studies to a greater number of elements will highly enrich the knowledge about trace metal behaviour of trace elements in this region.
Some attempts have been made to study the chemical speciation of trace elements in Galician Rias (Cobelo-Garcia and Prego, 2004b) and a more complete work about organic speciation of trace metals in continental or estuarine waters would be beneficial.
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