Sampling Pore Water

[Dan_P]

Back in January I posted a question about what was considered normal for pore water chemistry, i.e., the water in between the substrate particles. The particles in my case are very fine beach sand collected from Long Island Sound.

http://www.reefcentral.com/forums/showthread.php?t=2476332

Forum responses were useful and led me to believe that this was not a hot topic in the hobby. The scientific literature about the chemistry that occurs in marine sediments was an eye opener. But when the frenzy of information gathering came to an end, I was still left with my original question unanswered: is having levels of phosphate and iron much higher then the overlaying water an issue? Does it predict a future problem or tank crash? Does it explain the persistent cyanobacteria growth even though phosphate and nitrate were undetectable? I decided to embark on a "research project".

My first report to the forum is how I collected pore water. It might be of interest to other aquarists wondering what's happening in their system's substrate. The picture below is the sample probe I use to siphon pore water. It is a length of 3/16 rigid tubing with a little filter floss at the end which is pushed into the sand. The other end is attached to some air line. Typically, I slowly push it into the sand until it touches the bottom glass. Then I use a 5 mL syringe to start the siphon. I then use a syringe to reverse the flow and very slowly push 1 mL of water backwards to unplugged the tip of the sample probe. To give you a sense of how slowly the water flows, it takes approximately three hours to collect 1 gallon of water.

Pore water collection is tricky and there are a couple approaches used by scientists to ensure that they are collecting water at exactly the spot in the sediment they intend. In my case, I have a less precise objective in mind, to collect water from the sand and not the overlaying water. The graph below shows the phosphate level in the pore water collected from the same spot in the aquarium on consecutive days. Two things stand out. One is that on both days the phosphate level starts out high and levels off but still remains much higher than the concentration level in the overlaying water which is <0.05 ppm. The other point is that on day two, the highest level is approximately equal to the low value recorded on day one. In other experiments, the level does rebound given enough time. This may be a combination of biological activity and diffusion from higher concentration areas. From all this, I conclude that the method I am employing to sample pore water is primarily extracting pore water, especially the first 10 mL, and not overlaying water.

My next step is to look at the chemistry of pore water in more detail and to monitor how it changes over time.

 

[bertoni]

Interesting! One gallon of water is a lot of interstitial water. I'd be worried about changing the water parameters significant when drawing that much.

 

[Dan_P]

Interesting! One gallon of water is a lot of interstitial water. I'd be worried about changing the water parameters significant when drawing that much.

That is exactly right! It will be a subject for a later post or two. Just imagine what siphoning a substrate does!

Here is a quick teaser. I was surprised that the phosphate levels rebounded after siphoning the pore water off. Is the rebound biological, diffusion, or driven by both mechanisms? O, to be able to measure dissolved organic carbon.

 

[bertoni]

Good question. I remember that my interstitial water measurements always showed some amount of phosphate, although the water column measured zero. I always thought that biological action (infauna) must have been moving organics into the sand, but whether the rebound is rapid due to decay or diffusion is beyond my knowledge. I suppose you start to test the diffusion hypothesis by sterilizing a sand sample and then checking for diffusion, although you might need a large amount of sand. I'm not sure how to sterilize the sand without affecting diffusion, though. I'm not good with chemistry.

 

[Dan_P]

Good question. I remember that my interstitial water measurements always showed some amount of phosphate, although the water column measured zero. I always thought that biological action (infauna) must have been moving organics into the sand, but whether the rebound is rapid due to decay or diffusion is beyond my knowledge. I suppose you start to test the diffusion hypothesis by sterilizing a sand sample and then checking for diffusion, although you might need a large amount of sand. I'm not sure how to sterilize the sand without affecting diffusion, though. I'm not good with chemistry.

Diffusion is very slow, but knowing this does not help much. I looked at how quickly methylene blue diffused in sand. My photographic evidence is a couple of millimeters in three hours. That suggests that after twenty four hours, not much ground is covered. Maybe biological processes are generating it and/or moving it along. I might consider siphoning some sand out and to see if it generates phosphate, though that is not a very strong experimental design(if it does not generate phosphate I learn nothing).

 

[CStrickland]

How deep is the sand?
I would be curious to see diff readings at various depths, though I guess that would be even harder to control for contaminated sampling due to the water flowing in to fill the void.

Are you pulling a gallon for each sample, like on day one 7 gallons was removed, or was the gallon measure just a test of your drip speed?

 

[dkeller_nc]

Diffusion is very slow, but knowing this does not help much. I looked at how quickly methylene blue diffused in sand. My photographic evidence is a couple of millimeters in three hours. That suggests that after twenty four hours, not much ground is covered. Maybe biological processes are generating it and/or moving it along. I might consider siphoning some sand out and to see if it generates phosphate, though that is not a very strong experimental design(if it does not generate phosphate I learn nothing).

If you wish, you could treat a small sample of sand with an acid solution, then measure the phosphate in the supernatant. The only question I'd have about such an experiment would be whether the colorimetric phosphate assay would have issues with an acidic sample. You could potentially test this by making up a couple of phosphate standards with monobasic sodium phosphate in newly made saltwater, add the same amount of acid by volume you'd use with the sand sample, and run a molybdate phosphate test.

One other thing to keep in mind is that even if one could consistently measure 10X phosphate concentration in the interstitial water in the sand bed, that would have relatively little effect on the tank water, as the total interstitial volume in an average sand bed will be quite small.

 

[Dan_P]

How deep is the sand?
I would be curious to see diff readings at various depths, though I guess that would be even harder to control for contaminated sampling due to the water flowing in to fill the void.

Are you pulling a gallon for each sample, like on day one 7 gallons was removed, or was the gallon measure just a test of your drip speed?

Nominally, two inches, but current and animals made it hilly. I sample where the sand is deep.

I agree sampling at various depths could be iffy. I am putting that off for a rainy day and maybe a different sampling probe.

The amount I siphon depends on the purpose. 30 mL for testing and 1-4 liters for the experiment to reduce phosphate levels. I will post these results as soon as I given them a little more thougt.

 

[Dan_P]

If you wish, you could treat a small sample of sand with an acid solution, then measure the phosphate in the supernatant. The only question I'd have about such an experiment would be whether the colorimetric phosphate assay would have issues with an acidic sample. You could potentially test this by making up a couple of phosphate standards with monobasic sodium phosphate in newly made saltwater, add the same amount of acid by volume you'd use with the sand sample, and run a molybdate phosphate test.

What is the purpose of the acid? I have beach sand.

One other thing to keep in mind is that even if one could consistently measure 10X phosphate concentration in the interstitial water in the sand bed, that would have relatively little effect on the tank water, as the total interstitial volume in an average sand bed will be quite small.

Agree, if it all mixed with the overlaying water, I estimate that it would raise the phosphate level about 0.05 ppm. I am very tempted to upgrade to a test that would detect poly and organo phosphates. I am interested in this because phosphate can be undetectable and yet hair algae and cyanobacteria can grow very well. I don't fully buy into explanations like "the organism is soaking up the PO4" or "the organism does not need much PO4". But I need to convince myself this is worth the cash to pursue.

 

[dkeller_nc]

What is the purpose of the acid? I have beach sand.

You were asking whether the sand contained/leached phosphate. Incubation in a strong acid will solubilize the phosphate (as well as convert most organic phosphates to inorganic phosphates).

 

[CStrickland]

I though beach sand didn't bind like aragonite?
Mines from near Long Island sound, and it is more like play sand. It does not fizz in vinegar

 

[dkeller_nc]

Depends on what beach it comes from. Many beaches in the Bahamas are nearly pure aragonite; beaches on the Eastern coast of the US tend to be mostly silicates, and many beaches on the shores of Hawaii are eroded basaltic lavas high in iron and magnesium.