Pond Boss Forums GENERAL QUESTIONS Muddy water Understanding the Causes of Secchi Disk Readings

I'm interested in better understanding the causes of Secchi Disk readings. Essentially I'm confused about differentiating between silt, plankton, and humic stain turbidity. Is this just done via observation/estimation of water color?

Some of all 3 plus experience. One of the benefits of secchi readings is to watch the changes over time. A lot of it is common sense. If there has been no big rain lately then its probably not silt. If a new pond with not much veg ( in the pond or watershed) to decay then not humic. If water is warming and greening is occurring then probably plankton bloom. The scientific process is always about observation , hypothesis and testing. Over time and observation you get a sense of if it is colloidal suspension , veg decay or plankton. What type of info would help you ?

That right there was VERY helpful - Thanks!

When I first read about Secchi Disk Readings, it made no sense to me because I wasn't aware of the multiple causes of turbidity or even that some were beneficial and healthy. I assumed they were all bad. "You actually want the disk to disappear? That's a good thing???"

Once I figured that part out, the next logical question was, "If the disk disappears at a good distance, how do I know it disappeared for a good reason?" That's why I posed the question I did.

Now I'm ready to make a disk and wait for the thaw...

Two other things that contribute to secchi disk visibility are suspended bacteria and suspended organic particulates.

You can also get a water test done to determine what is causing the turbidity. Basically you take a sample of water to a water testing lab and ask them to quantify Total Suspended Solids (TSS) and Total Volatile Solids (TVS). They will take a measured quantity or your sample and "cook" it so that all the water evaporates. Then they will weigh the remaining solids. This will give you TSS per quantity of water. Then they "cook" the remaining solids at a higher temperature so that all the organics (algae, bacteria, etc..) are vaporized. The remaining solids are inorganic (silt). Now you have a ratio of organic to inorganic suspended solids. This will give you an idea of types of treatment methods to apply. For example if the ratio shows high organics, an aeration system, algaecide, or bacteria might work for you. If the water is high in inorganics you may consider alum or gypsum.

Reading this thread has helped me more to understand turbidity more than any of the scientific mumbo-jumbo EVER could!

Thanks for the question NChad and the straight forward simple answers from Bill, ewest, and WaterWizard. You 3 made it simple enough for even this hillbilly to understand!

Waterwizard, when you get the numbers for TVS/TSS where can you get info on how to interpret them? Is there a table or some discussion source that tells you what they mean?

Here is a pretty good explanation: http://www.bfhd.wa.gov/info/tss.php

The interpretation is dependent on your reason for taking the test. I (being an aeration/ozone guy) am most interested in TVS and TDS (Total Dissolved Solids) because aeration and or ozone can treat these things. We cannot treat inorganics with our technology. So, if a client has a very high ratio of TSS and low TVS and TDS, I know that there is not much that we can do for them.

Thanks for the link which further explains TSS/TVS. Unless it links to another page it does not exactly address my question.
What I am wondering is what kinds of numbers do you get when you run these tests? For example, 30 mg TSS/ 20mg TVS per whatever volume would be mostly(66%) TSS, but is 30mg per whatever volume a high reading? So what kind of TSS readings do you get from a really muddy pond vs a really clear one? And what is a lot of organic matter by ratio?

Now I understand your question. Again, the answer depends on why your testing. Lets say you have an irrigation reservoir that your want to treat. An irrigation manager wants the water as close to crystal clear distilled water as possible so that the suspended and dissolved solids do not clog filters, emmiters, etc... However, if you are trying to create fish habitat you want a healthy balance of planktonic algae and other things for fish to graze on. In this case, your water will not be crystal clear. It will probably have a nice green cast to it with 2+ feet of visibility. To my knowledge, inorganic suspended solids do not really provide any service to your ecosystem aside from maybe blocking solar radiation therefore reducing weed growth (this is a stretch). This is a generalization of course. Site conditions, and other factors dictate how well you will be able to achieve your desired water clarity or quality. Maybe this would be a good topic to bring up in the fish stocking forum since there are probably more fisheries biologists that can tell you the values that would indicate a healthy population of algae.

LRunkle,
You ask some very good questions.

The sechi disk only tells you the amount of light absorbed or scattered across a water column. That is all. It is essentially a rough semi-quantitative method to determine the level of TSS. A turbidimeter would give a more accurate direct measurement, but still only tells you quantitative, not qualitative data. Neither differentiate between mineral solids, organic solids (TVSS), and do not discount color bodies such as organic stains from planktonic algae and zooplankton. In the end, you are trying to establish a base of the food chain consisting of a mix of plankton. The sediment doesn't do you any good, and may be doing harm if too high, but usually makes up some of the sechi disk reading. Color also adds to the reading, but does not directly help your needs, other than to provide some additional organic matter that acts to feed the plankton.

If you want a more scientific and direct measurement of the plankton levels, you will need to use a test that is specific for that. That would possibly include a TSS/TVSS anaylsis to back out the amount of mineral solids. You might then need to use microscopic analysis to get an idea of the levels and types of zooplankton and phytoplankton. All of this seems too academic and cumbersome for what you really need. One way to get some idea of what makes up the TSS is to take a sample, add bleach or peroxide to reduce the impact of organics on the clarity of the water, and the residual will be the mineral clays suspended in the water. In my experience, when adding peroxide, if the color changes from tan/green to just turbid opaque, you had the plankton you want.

Probably the best way to meet your needs is to do what Erik says and just use visual observation of color and the sechi disk along with experience to tell you where you are. Use the peroxide test to start to get an idea of what the sechi is telling you, then use that device as an easy way to get data on the fly. As always, the more data you collect, the better the determination you get from the analytics

Mike, I like that idea about adding bleach. Very simple and inexpensive. I'll have to try it soon.