Filtering in the Field
by Jeffrey Gerard, AmeriCorps OSM/VISTA
Filtering water samples in the field can be a pain in the butt, but it’s also a crucial step in getting the most accurate analysis data.
Filtering involves forcing water through a membrane filled with tiny holes, about 25 microns across (ΒΌ of a human hair). The filter removes suspended solids from the water, and in fact, the weight of the solids caught in the filter is one common analysis parameter: total suspended solids (TSS). Such solids can be virtually invisible to the naked eye, but not dissolved, so they’ll eventually settle out if the sample is left undisturbed.
AMD-impacted water can have tiny bits of iron hydroxide (a.k.a. yellowboy) suspended in it. In a water sample fixed with acid but not filtered in the field, this iron hydroxide will dissolve into the water before it arrives at the laboratory, which will give incorrect test results for metals. Alternatively, in unacidified water samples, dissolved oxygen can cause dissolved ferrous iron to oxygenate into ferric iron en route to the lab. Unless the insoluble ferric iron is filtered out at sampling time, the lab can’t determine how much ferrous vs. ferric iron the water actually contained when it was sampled. Ferrous iron oxygenation also consumes acid, so certain lab acidity or pH measurements will be wrong.
Although filtering water samples eliminates these errors, it requires special equipment and training. Many groups — especially those that rely heavily on volunteers — leave filtering to the analysis laboratory, accepting any inaccuracies that creep in beforehand. Ultimately, the decision to filter relies on a group’s capacity and requirements, and it should be discussed with the chemical analysis lab that receives the water samples.
October 3rd, 2006 at 10:22 am
Read a follow-up discussion about sample filtering in the October 2, 2006 Abandoned Mine Posts entry. It clarifies some of the science in this article and also relates filtering to mine drainage treatment.