Original question:
I need to collect water samples (about 250 ml) from a headwater stream. The water depth is on the order of a few mm and I need to be sure I do not get sediment in the sample, or as little sediment as possible. How have others done this? Clarification: I probably should have emphasized in my original comment that disturbing the sediments as little as possible is an absolute priority. We will ultimately filter the samples but that cannot be the only technique for getting sediment-free samples. Thanks to everyone for taking the time to respond. Responses: You can 'engineer' the stream a little by slightly constricting the steam and putting a rock or two strategically so that water will flow over them - so you can then get the mouth of a bottle under the flow as if goes over the rock. Let the stream clear after your engineering. We also use flat sided wedge bottles that fit into autosamplers (like ISCO) that you can lay flat in the stream. Use a 60 mL syringe. It works very well. Make sure to rinse it (and your sample bottle) a few times with stream water, tho. I recommend a 60 ml plastic syringe (needle removed) either by itself or connected to a short length of tubing. This is particularly convenient if you need to filter samples. I routinely use filter canisters that hold 25 mm filters that engage with the syringe and filter samples in the field. Since you need a larger volume you might also consider using a small battery-powered peristaltic pump. we typically vacuum filter our samples before analyzing if they have a high amount of organic matter/sand in them. I'm not sure if this is applicable to your situation but I hope it might help. This may be a brute-force method, but depending on what you'll be measuring, you could field collect water as best as possible and filter it thru a course filter, such as a GF-C. This would of course get rid of any sediments and suspended solids and attached constituents, but should retain the dissolved components as well as the general chemistry. We've done this before for some basic chemical analyses. Just an idea... If your stream channel is narrow enough to make it feasible, why not install a small V-notch weir, made of plywood or sheet metal, or even pile some stones to dam the flow and create a small pool to sample from? Every stream has pools and riffles, no matter how small, that should give you enough depth to draw from with a large syringe. The amount of water you are collecting is relatively small. After collecting your sample, let it sit in the refrigerator overnight, then decant to avoid any sediment you picked up. Hopefully that sits well with the methodology for whatever parameter you are measuring (you don't say in your note). can you not collect the sample without worrying too much about sediment (I might try a 60 mL syringe) and then filtering through a GFF filter with a manual vacuum pump? I collect samples from glacial rivers (loaded with sediment) and this works for me. Having done a LOT of water quality sampling over the years (including sites like you mention) I have a couple of suggestions. If you don't mind doing a little simple modification of a small portion of the stream bed and you can revisit: I have had success digging out a small hole with your boot heel or trowel or whatever implement that will accommodate a small sample bottle that will allow you to dip the neck of the bottle without getting bottom sediment. Of course, when you do this you stir up sediment and you cannot sample there. But wait an hour, of a few hours, or overnight and then go back to your temporary hole and you will have the perfect spot to sample. You make it a steep sided hole on the upstream end where it pours in and basically let it sit till it clears out (which will happen quite rapidly). This causes no significant damage to the stream and obviously no damage of any long term. Alternatively, you can scoop multiple VOA bottle out of a very shallow stream bed without disturbing the bottom. Maybe anchor down a clean, cut-off bottle with the broad cut off base facing upstream and a clean poly hose coming out the neck end. run it down 10 ft or so to someplace down gradient where you can just get the hose end higher than your bottle opening of the one you need to fill. The neck of the one you are filling is now just above the stream surface and so cannot be affected by the TSS that occurs when you push it down. Once again, let it flow through the bottle/tubing combo and clear prior to use. Also, any kind of a temporary weir structure, too, might work...with hard plastic V shape facing upstream that comes out at a spout with just enough elevation to fill your bottle on the downstream end. Once again...let it flush for a while after installation You either do it with a syringe provided by filter membrane device (like those for microbiological samples) or more practical, collecting the water samples in a bigger, clean recipient, you wait until sediments deposit (few moments); then you take your sample from bigger recipient (sampling plastic tubes can do as well). How about using a flexible rubber hose laid gently on the streambed (parallel to flow) to divert water into some sort of a pan a ways downstream? You probably thought of that..... Regards, Daniel L. Tufford, Ph.D. University of South Carolina Department of Biological Sciences 209A Sumwalt (office) 715 Sumter St, Room 401 (mail) Columbia, SC 29208 Ph. 803-777-3292, Fx: 803-777-3292 e-mail: tuff...@sc.edu web: http://www.biol.sc.edu/~tufford P Please think before you print.
