I was unable to perform the sampling and analysis for 40 surface water and 20 groundwater samples, so I hired an in-country consultant firm to do it. The results that they returned were difficult to interpret and had extremely high levels of iron and manganese. I spoke with a former professor and he expressed concern from his long career of working in SE Asia that the subcontractor/lab might not have filtered the samples. How can I know that the results are for the water and not the particulates?
Water samples are filtered only when dissolved nutrients are measured (e.g DRP, dissolved inorganic-N) since total nutrients such as total-N and total-P and microbial analyses require unfiltered samples. The presence of Mn and Fe could be an issue with groundwater than surface water, since groundwater under reduced conditions contain excess amount of Mn or Fe. Such waters contain little or no nitrate-N because of reducing conditions.
The contractor may not be the owner of the lab hence you could contact the lab directly and find out how they performed the analyses. Most such labs (even if they are located in SE Asia) must be accredited. If they are accredited labs, they will follow normal protocols for samples preparation and analyses. Your subcontractor can provide all the above info for you to follow up.
I agree with Selva. Most Labs are accredited, and they would be open to share the process that they followed.
Dear Liz
I agree with Selva. if you sent your sample in good lab, you can trust them.
To add, you can send your own QA samples including duplicates, spiked samples, standards, and blanks and request the specifics of the analyses (SOPs). Most labs should have their own QA/QC reports that are included with the results.
Do a charge balance (anions = cations in meqv) and plot them in a piper diagram
Compare results for manganese and others to solubility. This may not be definitive because of potential co-solubility or chealation etc, but would be suggestive of a problem.
Well
In the case of analysis of the samples without filtration in the laboratory as you mentioned, and if you keep any number of available samples you can nominate samples and analyze the concentration of iron and manganese in the suspended solids and obtained from the filter paper devices, then you can subtract these values from the values of analysis of elements ( iron and manganese) in water samples to reach their values as dissolved cations
Sincerely.
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