How is this typically done in terms of location on the lake/river. There will obviously be different concentrations at the edges/middle or depths? How is all this information conformed into a clear picture of the concentration within a lake/river? Links to journal studies would be appreciated or a clear explanation. Thanks
To make things more clear, I would like to compare excess nutrient concentrations in various lakes or rivers in relation to the population density of a certain aquatic plant. Would it be fair to take water samples near the plant and compare it to various other locations?
It should not really matter that much where you take the sample. Nitrates (potassium nitrate, sodium nitrate, etc.) will disassocate in water so that you will have ionic nitrates and they will tend to disperse relatively evenly through the water.
A question right up my alley. Origin's answer is a good quick answer, but it's not always true. The simplest technique is a mid-stream grab sample, but it rests on the assumption that the composition is spatially uniform, which isn't always true (where plumes or diffuse discharges are present, for example). I'll do a more detailed post after work tonight.
As I alluded yesterday, there are a couple of different approaches. The first, and the most straight forward is to assume that the waterbody is well-mixed and take a single mid stream grab sample. Ideally this is done by taing the sample from as close to the middle of the stream as possible, holding the sampling jar so that it is upside down, placing it under the surface of the water {I have 20 cm in my head}and then filling it. If you suspect the existence of a plume then you can tae multiple samples across the width of the stream. You can combine them or not, but then you loose the spatial resolution. Combining gives you an average value for your nanalytes which in and of itself can be useful - for example, some wastewater plant operators will take a combined sample over a 24 hour period. These combined samples can be constant volume or constant time depending on whether or not you want the sample weighted according to flow. The same weighting techniques can be used in a stream. Equally important is the number of samples you tae and the period of time they're taken over. If you're wanting to get a handle on the overall quality of a water body then you might take one sample a week for a season, or one sample a month for a couple of years. The advantage of doing a few samples seperated by weeks or months over a period of a year or two is that water quality in most water bodies varies on a seasonal basis - it's often worse during winter because of increased surface run-off, for example.
Trippy This post started with: "You are abusing your powers." But I will apologize for that and now accept that trippy just wants the topic so narrowly defined that comments on the seriousness and main source of the nitrate water pollution are "too far off thread." As he is with little doubt the most expert active here in measurement of nitrate pollution in water, basically only his comments will be "on thread." I think such a narrow interpretation of the comment in allow posts is mistaken and counter productive in post open to the general reader. I think that reader needs to be made aware of the issues I tried to raise. If only measurement techniques are allowed, why not have closed the thread (as it effectively is) and replied by PM? Without comments very closely related to nitrate in water and the environmental damage I tried to make and carefully document but now only briefly mentioned below, there is very little of general interest to the sciforums reader. My msg you removed started with link showing that the main source of nitrate pollution of the Mississippi river was the production of heavily fertilized corn; and the nitrate load delivered into the Gulf of Mexico, creates {via algae bloom) a O2 depleted, fish killing zone that is growing larger every year - now more than 7,000 acres. The next quote reference, as I recall, documented {with reference to Noble Prize winning expert's data and text) that corn based alcohol is more polluting, makes more global warming {as NOx from the corn fertilizer has 300 times the Global Warming, GW, potential of CO2), increases taxes and food cost with more than 1/3 of corn, the largest farm subsidy, being used to produce alcohol, etc. others consideration, than just using gasoline fuel in cars. I admit that I did then mention fact I started thread: "How DUMB can US voters be?" before GWB was even elected for his second term in effort to get his foolish corn to alcohol program to die - that may be "off thread" but was just a continuation showing how hard its is to get government to become concern about public needs than their re-election - Global Warming has that problem now. Please at least restore the first half of my post telling that most of US's problem with nitrates is due to fertilizing corn to speed its growth in Iowa's short growing season. BTW, one of my links* tells that fertilizer's nitrates is also the main nitrate pollution of the Chesapeake Bay drainage basin too. - Problem is not just in the mid west. * I think my first link, has a whole sub chapter on just the nitrate problem in the Chesapeake Bay drainage basin. Chapter 6.4 or 6.5 as I recall, but I did not quote that part as unlike Iowa's corn alcohol there is no much better direct replacement for most of the crops over fertilized there.
Here's what I would be tempted to do, obviously it's dependent on your budget. Grid the water body {neccessary for a lake, not so much for a stream}. Within each grid use a floating quadrat to take a random sample. Take the sample from the quadrat, have it tested for Total oxidised nitrogen, and dissolved reactive phosphorus {not all catchments are Nitrogen limited, as I recall, most aren't}. Measure the cover of the plant within the quadrat. The size of the grid squares is up to you - more squares means better resolution, but will also cost more because you will have more samples to test. You can take more than one quadrat per grid square if you want. This will enable you to derive an average calue for each grid-square, however, again it will cost more. You will want to take a hand held GPS with you to record the location of the quadrat, but remember that it can take a couple of minutes to get an accurate reading. You'll also want to take a camera with you to try and get a photo of the quadrat in place on the water surface. With regards to sampling protocols. First and foremost, if you have a laboratory in mind for doing the tests, contact them. They may be able to provide you with bottles for storing your sample in and will be able to give you the details of any pre-treatment of the bottles that may be neccessary (some labs require you to filter the sample before you submit it for testing, for example). Secondly. Label everything rigourosly. Thirdly, if possible take your samples from approximately 20cm under the surface of the water (about 8 inches if you're american). If the water body is not deep enough to achieve this, then try to take your sample from as close to the middle as possible. Fourthly, if you're working in a stream. BE SAFE. I can not emphasize this enough. Sampling from streams can be dangerous, I know of someone who was trying to get a sample to submit to the organization I work for that slipped, fell, and drowned to to get a sample. Consider a life-jacket and an EPLB as saftey equipment - depending, of course, on the circumstance. Take someone with you if you can, if you can't, select someone to act as a buddy - someone you can say in advance to "I should be back by this time" who can contact you and potentially raise the alarm should you fail to check in. Fifthly when you're sampling from a stream, move from the most down stream location to the most up stream location. When you sample you risk disturbing the stream bed, in some cases disturbing the stream bed is inevetable. Sampling the water when it's carrying the stream bed sediment can give you a misleading result. The easiest way of avoiding this is to start downstream and work upstream, that way any sediment plumes you create will be behind you and affect only areas you have already sampled. Finally, and this is perhaps the most important point write EVERYTHING down. You don't know in advance which results might be anomalous, and what you write down might be key to explaining any anomaly. The sample forms that we use when we submit a sample for testing at work include things like the weather (wind direction and speed, is it cloudy sunny or raining), water characteristics (turbidity, colour, flow rate) and other things like odour. When things are a little less crazy here at home I'll see if I can track down some standardized macrophyte survey protocols for you.
