"Form Follows Function"

The title of this post, a phrase often associated with architecture, is a concise way of saying that the appearance or aesthetics of a functional object often is constrained by its ultimate use. Synthetic sediment can be thought of in this way.  Its composition and to a lesser extent its appearance may be constrained by its specific application.  This might be a good working assumption for those developing synthetic sediment: decide a batch’s functionality and work backwards to develop a formulation to achieve that functionality. 

We think up many possible uses or functions for sediment in a laboratory setting: aquarium decoration, substrate for testing interactions between chemicals and sediment components, substrate for growing aquatic plants or culturing aquatic macro-invertebrates, a clean reference sediment to control test variables associated with physical properties of test sediment, a chemical-spiked positive control to confirm the observed response to a test sediment, and others.  Each of these uses, and others we might imagine, requires a different “minimum” level of detail or complexity of the sediment substrate; a substrate cannot be too complex for a given use, but complexity less than the minimum will not work.  We could think of natural sediment as having the highest level of complexity, so any sample of natural sediment will work for any use.  The accuracy of that statement might be debatable, but for our purposes the complexity of natural sediment is the benchmark to shoot for and to which synthetic substrate complexity is compared.

The first response might be to develop one, universally-complex synthetic substrate and use it for all possible functions and applications.  In practice, that might be an ambitious approach because of the difficulty in assuring that the complexity of the synthetic material matches the natural.  Secondly, replicating that level of complexity by multiple labs in multiple locations throughout the world would require a lot of time and expense by the participating labs.  For many applications, such effort would be over-kill.  Lastly, regional differences in natural sediment characteristics, or organism differences in micro-habitat requirements, would make a single universal synthetic substrate inappropriate.  This is one of the drawbacks with current synthetic sediment formulations.

So thinking of synthetic substrates as use-specific can simplify how specific labs might approach formulating it.  Recipes and components (type, number) would be specific to end-use, regional environment, data quality objectives and organism habitat needs.  To apply this approach requires a clear understanding of what components can or should be used for which end-use substrate.  This means knowing how each individual component affects and contributes to the characteristics of the whole, and how each interacts with the other components.

Most of my work with synthetic sediment so far is focused on researching, documenting and testing how individual components affect characteristics of the whole material.  This knowledge will help end-users of synthetic sediment select appropriate components and recipes for their specific needs.  Some components have little influence on substrate characteristics; large particle-size sand (silica; quartz) is an example.  Most have a strong affect on the characteristics of the whole material.  Examples include small particle-size geochemical clay (layered alumina-silicate minerals), geochemical silt (simple oxides, simple silicates, carbonates, feldspar and granite minerals), all types of organic matter, and mineral components like metal-sulfides.  One set of experiments, presented at a technical conference in Long Beach, California, demonstrated how the type and amount of mineral oxides included in a mixture can have a profound effect on pore water pH and on the cohesion and compaction properties of whole substrates.  You can find the summary of that presentation on page 313 of the 2012 conference abstract book, found online HERE. 

More tests will be done with other minerals and organic matter.  Stay tuned for more posts to this blog. 

In-house testing with organisms.

Well, I'm back.
After over a year of attempts (and $$), I've finally succeeded in starting and maintaining a healthy culture of Hyalella organisms.  This is an enormously important milestone!

The dark spots on the white gauze are Hyalella..

 Here's what they look like really up close:

[I did not take this one... photo was borrowed from an online source]

In fact, it's been so successful... they're proliferating rather quickly.  I went from one small bag of 100 organisms, to this:

and now I'm up to this:

We start excavation in the backyard for the pool later this summer... just kidding.

I now will be able to more frequently test synthediment recipes on real live biological organisms, the type that live in natural sediment, and get feedback information much quicker.  This is the break I've been needing to make quicker progress.  My wonderful Partner-Labs have gone above and beyond their everyday work to provide me with some very good data-feedback.  I will continue working with them because they have access to biological species that I do not.  But this Hyalella culture breakthrough will help things move along at a faster pace.

I have worked out a certain combination of fine particles (in terms of types and amounts) that make the material "sticky" or cohesive.  This is very important to some organisms who need that kind of sediment for successful construction of their "burrow" habitats.  They do not like sediment that is loose and flowing (like sand) because their excavation only collapses as quickly as they dig.   I'm presenting this work at a scientific conference in November.

I had a bit of bad timing with that cohesion work: the batches I sent to labs with access to "burrowing" organisms were sent just a few weeks BEFORE I finally worked out the composition and combination.  Here are two photos that illustrate how the early synthetic material failed to function like natural sediment. 

One of my early synthediment recipes...

... looks nothing like the real stuff!!

Those little burrows and tracks in the lower photo are the tell-tell signs of healthy living benthos. They are conspicuously absent from my synthediment batch!  Although these results were "negative" in the sense that these batches were not suitable to the organisms, they were positive in that they provide valuable information to guide future adjustments.  As has been said, "science IS a continuing process..."

So the next stages of development will involve lots of experiments exposing Hyalella to batches of synthediment in my own lab, and will focus on adding organic material to the most successful mineral recipes.  Onward!

Uses for synthetic sediment: Part 2

Happy New Year, dear Readers (... in April?).  I must post more frequently than once per quarter!  Thanks to all who have told me they enjoy reading this blog.  Pass the word on to your friends and family.

Today's post explores another possible use of synthetic sediment: to strengthen the reliability of data generated from samples of natural sediment.

Environmental Data
Environmental samples are collected and analyzed, and environmental professionals interpret that data to guide their decisions about how to manage and steward the environment.  The analyses are done by chemists at environmental laboratories.  Part of the chemists' job is to check that the instruments used to analyze samples are giving them reliable readings, and to report their "instrument reliability" findings to the client who submitted the samples.  There are many types of instrument-checks, but one is to analyze a material (a reference standard) that contains a precisely known amount of what they're measuring.  Doing this isn't to find out what's in the reference standard; it's to "test" the instrument.  By comparing the known amount of material in the reference standard to the reading given by the instrument, the chemists get a sense for how well the instrument is working (... how efficiently and accurately it is detecting the material they're measuring).  This information is the only way the chemists know how much confidence they can put in the instrument's readings, and whether those readings truly and accurately reflect the actual composition of a field sample containing an unknown amount of material.

Natural Sediment is Alive
Natural sediment is an enormously complex mixture of components (living organisms and non-living material) and those components are actively changing all the time.  Very few of those components are stable (almost all components in natural sediment change to some degree, over time).  Because of this, natural sediment makes an extremely lousy laboratory reference standard.  One interesting component of natural sediment is a material that contains sulfur.  It's called acid-volatile sulfide (AVS, for short) because it becomes a gas (volatilizes) when it reacts with acid.  AVS is one of those sediment components that constantly changes: it occurs in different amounts from location to location within a sediment bed, and the amount found at any one location changes at a over time (perhaps seasonally to some degree).  It's created by the activity of micro-organisms in the sediment munching on organic material and "inhaling" dissolved sulfate ions (think, Epsom salts).  The organisms "exhale" sulfide (think, rotten-egg odor), and this sulfide is very reactive and very different than the original sulfate.  Sulfide quickly combines with many types of metals in the water to form a new chemical, one that doesn't dissolve in water very well (a precipitate).  Sulfide is different from the original sulfate also in that sulfide does not tolerate being around dissolved oxygen.  When it reacts with dissolved oxygen in the water, it reverts back to sulfate.. which is again available for those sulfate-"breathing" bacteria to use in their part of the cycle.

One of the most abundant metals in the earth's crust is iron (Fe).  Its presence in natural sediment is not usually considered contamination.  The natural metal iron (and also another natural metal, manganese; Mn) combines with sulfide to form a dark-brown or black precipitate which stays associated with the sediment (iron sulfide - FeS; manganese sulfide - MnS).  In simplistic terms, chemicals like these constitute AVS in natural sediment.

Because of AVS's instability around dissolved oxygen, what happens in natural sediment is that the very thin surface layer of sediment along the sediment bed is in contact with so much dissolved oxygen that almost no FeS or MnS exists.  There is a point within sediment where the migration of oxygen from the water above is just balanced by the biological activity of the sulfate-"breathing" bacteria living and growing in the deeper parts of the sediment.  At this depth, and below, the sulfate is converted to sulfide faster than the oxygen from above can migrate down, and the AVS can persist without being oxidized.

This "boundary" depth (where the rate of at which sulfide is created (by bacteria) just balances the rate at which oxygen migrates down from the overlying water above) fluctuates wildly based on all sorts of factors and conditions.  An important one is the temperature of the water, which dictates not only how much oxygen can dissolve in the water, but also the activity of the bacteria in the water.  In cold temperatures (fall/winter), water can hold more dissolved oxygen and bacteria are lethargic: that combination favors a net loss of sulfide (more is oxidized than is produced  = low AVS concentrations).  During periods of warm temperature (spring/summer), the combination of lower dissolved oxygen in the water and high bacteria activity favors more production than oxidation (high AVS concentrations).  So not only do AVS concentrations vary from season to season, but it also varies from place to place, depending on available organic material and dissolved sulfate and how easily dissolved oxygen can penetrate into the sediment.

So... how do chemists and environmental professionals ensure that their AVS instrumentation equipment is accurate if it's always changing??  They don't use a natural sediment sample containing AVS.. that's for sure.  They rely on measurements of pure chemical FeS or MnS or other stable metal-sulfide compounds.  That's not very realistic when you consider that the "real" samples being analyzed are soil/mud type materials, whereas the "reference" is a pure chemical scooped from a lab bottle.  Wouldn't it be more relevant to measure AVS that's associated with a sediment sample?

Sediment Alternative for Reference Standard
In theory, synthetic sediment may be constructed to simulate the complexity of natural sediment, without all the complex changes.  In other words, synthetic sediment could be prepared to represent a "snap-shot" in time of a sample of natural sediment.  The materials in the synthetic sediment would be consistent in composition and stable through time.  Such a material would make an excellent reference standard for sediment analysis.  Also, such a synthetic sediment would simulate natural sediment more closely than just measuring a pure sample of the material being measured (which currently is the only way to "instrument-check" for some sediment components).  The stability of synthetic sediment against uncontrolled changes like those experience by natural sediment is only part of the equation for a synthetic sediment reference standard.  The other part is finding a chemical that acts like AVS in all aspects EXCEPT in its instability when exposed to oxygen.  That's the focus of one part of my work: finding a metal sulfide that reacts with acid like natural sediment, but that does NOT oxidize in air or when in contact with dissolved oxygen unlike natural sediment.


Other Stuff.. if you're interested...

Why is reliable data important?
Making an accurate "diagnosis" of the sediment that was sampled depends on how reliable are the lab data.  Here's why...

Reliability in the world of scientific data doesn't exactly mean "dependability" like one might think ("It's critical that you be there on time... I'm relying [depending] on you.").  Reliability of data is closer in meaning to "confidence" in data.  It's the way scientists address the question: "Can I trust this measurement?"  Another way of asking this question is: "Does this measurement REALLY reflect what is actually going on in this sample?"  I think these and other variations of this question are at the heart of science.  We explore the universe by poking and prodding and measuring tiny parts of it, to see what it tells us about itself.  If we want an accurate understanding of that part of the universe, the response we get from our poking and prodding needs to accurately reflect what is there.  On a very basic level, imagine the consequences of reading the temperature of an oven with a broken (inaccurate; uncalibrated) thermometer.  A reading of 70*F on that thermometer may mean the difference between a second degree burn or just a call to the appliance repair shop.

It's the same for scientific endeavors.  A measurement of contamination level in soil from a lot that is slated to be converted into a playground needs to accurately reflect what really is there: the "reliability" of that measurement could be the difference between years of fun and laughter, or years of unexplainable skin rashes.  In a less dramatic example, data from sediment in a river bottom could suggest possible hazard to the ecology of that river (based on the level of contamination present in the sediment), or the data could indicate a self-adjusting/self-attenuating river with no hazard to the ecology.  The interpretation of the data depends heavily on the answer to the question: "How confident are we that those data are accurately reflecting actual conditions in the part of the environment we sampled?"  A lot rides on the level of confidence in scientific data.

How do materials get buried under sediment?
Sediment under water bodies (streams, creeks, lakes, estuaries, bays, oceans) are collections of particles of various sizes and compositions.  The same way late-season snow storms cover the remnants of earlier snow storms, sediment particles continually settle on the bottom, covering "older" sediment particles.  The oxygen and other dissolved material in the water above the sediment surface can migrate into the pore-water between sediment particles.  The sediment bed contains a whole host of components, both living and non-living.  The living part includes bacteria and the non-living part includes organic material and dissolved salts that are food and "oxidants" for the bacteria.  So the more particle material "rains" down onto the sediment surface, the deeper the sediment bed gets, the further away the deeper layers of sediment get from oxygen-rich water.  These deep areas are devoid of oxygen and are called anoxic.

Uses for synthetic sediment: Part 1

Seasons greetings after a long absence from the blogging world.  Since my last post, circumstances have limited the time available for making progress.  The little bit of time spent on synthediment has brought some challenges, and on-going trials with my two partner-laboratories have been very insightful to me.  Although the trials "failed" to achieve the original goals, they uncovered factors that I had not recognized to be so important to synthetic sediment uses.  Edison made lemonade from his lemons in similar fashion.

To get back into a more regular posting schedule, I am starting a mini-series of blog posts dealing with theoretical uses and benefits of using synthetic sediment.  Not only will this provide me with blog material during lulls in lab-development, but ideas presented in these blog entries might seed your (the reader's) mind with other ideas for possible uses of synthetic sediment in the real world.  Theoretical applications shared in this and future "uses and benefits" posts may be so specific that they would require extremely detailed and nuanced synthetic sediment formulations (recipes).   It is my belief that these theoretical applications might only be realized through my "synthediment" procedural system of synthetic sediment preparation [... sorry for the crass commercial statement].

The first application envisioned for synthetic sediment is as a microcosm habitat for organisms that are kept and cultured in laboratories.  Here's why:

[1] Commercial laboratories exist that provide testing services for assessing sediment quality. A common activity those labs do is to test a field-sediment sample for toxicity to live organisms (a bioassay).  Scientists try to set up their tests so that all conditions are controlled and none of their test conditions will cause negative effects on the live organisms in the test.  The only negative effects (if any) need to be attributed to the sediment sample with a high level of confidence.  Since live organisms are complex entities, they can be a challenge to "control."  If they are not, negative effects in the tests can be caused by poor health of the organisms.  For this reason, organisms captured in the wild are seldom used in sediment toxicity tests. 

[2] To minimize the chance of poor organism health being the cause for negative test results, labs cultivate and maintain their own population of organisms.  Some of the more common test organisms cultured in biological laboratories are called aquatic macro- (visible to the unaided eye) or micro- (undetectable to the unaided eye) invertebrates ("lacking a vertebral column").  They are the river/lake/stream equivalent of spiders, worms and beetles we find on land.  Proper care for these lab cultures requires expertise in the biology and physiology of these organisms.  Such cultures are meticulously cared-for by maintaining consistent feeding, water, lighting and temperature conditions, among others. 

[3] Aquatic systems (lakes, rivers, streams, ponds, etc.) are complex systems containing a range of habitats suitable for an even wider array of organisms.  Some aquatic invertebrates are free-floating, swimming organisms that live and feed in the water column (e.g., plankton, water fleas, bacteria).  Others exist almost entirely on the surface bottom sediment, at the junction of the water column above and the sediment material below.  Those organisms (called epibenthic invertebrates) may burrow into the sediment a bit, but only into the very surface layer and never more than a few body lengths deep before coming back up to the surface.  They may swim in the water column a times, but always very close to the sediment surface (e.g., snails, shrimp-like amphipods, copopods, etc.).  A third group of organisms are true benthic organisms, spending the majority of their development cycle "underground" within the sediment material (e.g., worms, midge larvae, clams, etc.).  Each type of organism, and at times even individual species, require their own specific "living conditions" or habitat for maximum growth, metabolism and reproduction.

[4] Organisms that naturally associate with sediment material as part of their normal development cycle in nature (the epibenthic and benthic invertebrates) will require some form of sediment substrate for maximum growth, metabolism and reproduction.  If these types of organisms are cultured in the laboratory, their growing tanks or containers must contain some form of sediment substrate.  The question is: What substrate can be used?  If field sediment is used, there is a chance the sediment is contaminated and detrimental to the organisms in the lab.  Material not collected in the field may be used to act as a surrogate substrate or artificial sediment.  In this case, although the material can be guaranteed to be "clean" (not chemically harmful to the organisms), there is a chance that the artificial material is not suitable for the organisms' growth and health.  It could be too fine, not fine enough, too fluid, or not containing organic material in sufficient quantities or of appropriate composition, for examples.  The more realistic that substrate is, the better the benthic or epibenthic organisms may respond during culturing.

[5] Synthetic sediment, composed of a set of components that represent natural components in field sediment, can be created and conditioned in such a way that organisms will thrive in the laboratory.  Substrates used currently are good enough; they include sand, nylon mesh, shredded paper towel, etc.  However, could those same cultures be improved with more realistic synthetic sediment used as their culture substrate?  Would organisms cultured in the realistic synthetic sediment provide better response when used in sediment bioassays?  If so, those organisms can provide good quality data that helps environmental managers make good decisions about the true conditions of sediment in the field.  That's the theory, at least...

What is Synthediment(TM)???

I've had so many people ask me what exactly am I doing in the garage, and why exactly are you doing it, I started thinking how I might get the concept across to non-technicals.  There are so many fields of study involved in sediment toxicology and sediment geochemistry, that it can be an overwhelming chore for a non-technical person to grasp.  So after some thought and re-thought, here's my summary explanation for synthediment(TM):

Short Explanation:

I am developing a process/method to create a synthetic sediment (artificial lake/river/stream mud) that will act almost exactly like real lake/river/stream mud.  This material can be used by scientists who want to test real-world mud but don’t have a good reference to which to compare their results.  My artificial sediment can provide them the ability to get better test data and to make better decisions based on those data.

Long Explanation:

Cleaning the environment depends on having good data for three items: what part of the environment is contaminated (e.g., air?, soil?, water?, mud?), what is the identity of the contaminants present, and how much of each contaminant is out there.  Sometimes, environmental scientists suspect that the bottom mud of some lakes and rivers/streams may be contaminated.  In those cases, before clean-up can start, testing is done to find out what contaminants are in the mud, and in what amounts. 

Many small organisms (worms, clams, etc.) live in lake/river/stream mud; it is their preferred habitat.  When contaminant amounts in the mud get too high, the small critters either move or suffer biological harm.  Environmental scientists use this fact in their testing of lake/river/stream mud.  They collect samples of the mud and send them to a lab.  The lab scientists culture “clean and healthy” lab-organisms in their laboratory, and they use these lab-critters to test the mud samples.  A known number of lab-critters are added to smaller portions of the mud sample, and those lab-critters are fed for a known number of days.  At the end of the test period, lab scientists look through the mud to see if they find any of the lab-critters they added, and (if some are found) how they are doing biologically.  Contaminated mud can sometimes cause harm to the lab-critters, anything from DNA damage to slow growth or reproduction, all the way up to mortality. 

However, all good scientists know that there are many factors, or “experimental variables,” that can affect the outcome of a lab test, not just contaminants.  In the mud testing example, not only can contaminants cause harm to the lab-critters, but other things like improper habitat can also seem to harm lab-critters.  For instance, if a particular organism likes sandy textured river mud, it will not be healthy if forced to live in clay textured river mud.  In order to tell what factor is causing the outcome, all factors expect one must be matched.  Unfortunately, nature is so diverse that no two lakes, no two streams, or even no two mud locations in the same creek, are exactly the same.  So for mud testing, there is a lot of question as to what REALLY caused the outcome of a lab test.  Currently, lab scientists make educated guesses, apply reasonable assumptions, or they use mud from a lake/river/stream that they THINK is clean as the test “reference.”

The only way for a mud sample to be the same as a real/natural mud sample (except for the contaminants) is to create that mud sample.  A very few number of scientists have thought about this since 1980, and have proposed a very simple “mud” material made up of only 4 or 5 ingredients.  This simple artificial mud material is no where near equivalent to a real mud sample found in nature, which is a highly complex mixture of several dozen components or ingredients.  So it is my opinion that using this one recipe as a “reference” for testing all the different kinds of mud found in nature is not much better than using a mud sample from a lake/river/stream that they THINK is clean.

This is the problem that my project aims to address.  My method/procedure will create synthetic mud using as many as 30 ingredients, each ingredient designed to be the equivalent of a component found in real mud in nature.  The exact procedures I am developing for selecting ingredients, pre-cleaning or pre-treating raw materials, creating other complex ingredients found in real mud, and combining all these ingredients are based on over 10 years of researching scientific reports and papers in scientific fields of study such as toxicology, geology, marine biology, environmental engineering, and others.  My methods can produce not just one artificial mud material, but can be adapted to match almost any real mud sample found in nature.  This will allow me to offer the equivalent of hundreds of artificial mud materials, tailor-made to meet the lab scientists’ needs.

I am at the stage of development where I have developed all my procedures on paper, and now I am testing the implementation and scale-up of these procedures.  I am marketing my ideas to laboratories in hopes that they will agree to test some samples of my material in their laboratories, side-by-side with real mud samples.  The information I hope to gather from these collaboration tests with labs is whether my material is suitable as habitat for lab-critters they may have growing in their laboratories, and whether I can reproduce the results consistently.  Eventually, I hope to be a supplier of synthetic mud to all commercial laboratories, university departments of environmental studies, and government agencies (both State, Federal and international) involved with dealing with contaminated lake/river/stream mud.

Automation and scale-up

So this month, I have had a fair bit of activity on the synthediment development front.  First of all, I had the privaledge of giving a platform presentation on Synthediment(TM) at the 21st Tennessee Water Resources Symposium.  Also, in addition to a continuing collaboration trial with a toxicology lab using Hexagenia species, I've made contact with two other potential trial-partners.  With this increase in synthetic sediment development activity, I'm starting to think about how I might process larger volumes of material.  This would include not only receiving and handling raw materials ("ingredients") such as sand, silt particles and clays, but also (and more importantly) washing/purifying prior to use in a composition, and homogenizing large volumes of ingredients during the synthediment composition process.

At this point, I'm graduating from jars, tubes and other bench-scale equipment to plastic pails (e.g., 5-gallon buckets).  The relative volumes of materials I'm dealing with at this point are in the 1-20 kilogram range, with corresponding volumes in the 1-20 liter range.  For now, these amounts are easily processed in small pails.

One of the most important activities I have to do on raw materials is washing/rinsing/purification, and I greatly wish to automate this process.  For now, I have configured a closed-loop rinse process consisting of a small submersible aquarium pump, two plastic pails and a 100-micron filter sock.  Here are some photographs of the current setup that shows the major components. 

The submersible pump (~2 gallons per minute) is placed into the bucket containing some type of rinse fluid (distilled water, dilute acid, hydrogen peroxide, dilute caustic solution, etc., according to the type of rinse process required).  I'm not at all thrilled with having to use a submersible pump due to the unavoidable contact of the pump with my rinse fluid.  I'm forced to do this for now based on available funds, but at some time in the future I plan to replace this with an external chemincal feed-type pump that will not be in contact with the rinse fluid except for the inner surface of the pump tubing. 

The outlet of the fluid pump is fitted with a length of chemically-resistent tubing, and the end of the tubing is directed into the second pail which contians the raw material to be rinsed/treated.  This is the only part of this particular setup that requires some hands-on manipulation to ensure that the fluid flow coming out of the tubing reaches the entire volume of solids to be rinsed. 

The second container holding the raw material has been fitted with an outlet spigot, created by drilling a hole into the side roughly 4 inches from the top and inserting a threaded PVC tube (obtained from a local aquarium store).  A PVC nut is screwed down the threads up against the outside of the pail to secure a liquid-tight seal.  This configuration allows fluid (and ligher than water fines) to exit that pail and fall back into the fluid-source bucket by gravity. 

In order to trap unwanted particles and fines leaving the raw material pail, I placed a 100-micron filter sock (also obtained from a local aquarium store) directly below the outlet spigot.  Thus, before the fluid completes the circuit, it is strained of any suspended material (such as residual organic matter) larger than 100 micrometers. 

Of course dissolved materials captured by the rinse water will be recycled back through the raw material over time.  In the case of acids, bases, or oxidizing agents, this is actually preferred since total reagent usage is minimized.  Released metals, dissolved organic matter, or oxidation byproducts are NOT wanted to be returned to the raw material, but this can easily be addressed with a post-treatment, water-only rinse to remove those treatment byproducts.

The raw sand rinse that I have performed has removed a substantial amount of extraneous dirt and organic solids, as shown in these photographs.

So, the development process for synthediment is moving into the scale-up phase.  More information on the hexagenia-exposure trials and other field trials will be posted when available.  Visit again soon for more updates!

Trial No. 1 - Hexagenia culture

This post describes the first application trial using a batch of synthediment(TM) I developed for a government toxicity laboratory.  The director of the tox lab provided specifications of the natural sediment currently used to culture hexagenia in their laboratory.  Their field-collected sediment (from a northern-climate pond/lake) is heavy in the silt fraction and averages about 3.6% total organic carbon.  The challenge with this batch, and all batches of synthediment(TM), is finding a suitable surrogate material for the silt fraction particle size.  Sand and clay components are easy to replicate geochemically; natural silts on the other hand are a mixture of large clay clusters and weathered sand particles.  I'm not aware of one single material composed of such a mixture of geochemical components.

For this batch of synthediment(TM), some of the silt phase included a low-density silica-based material which gave the final dry-solids product an almost fluffy texture.  I was concerned that the solids would float upon hydration, but apparently that did not occur.  I received a photo (below) from the toxicity lab showing the hydrated synthediment(TM) solids (grey material on the left) side-by-side with their natural sediment (reddish-brown material on the right), being prepared for an hexagenia culture trial. 

The texture appears to be replicated rather well.  The coloring is definitely off, very likely due to a greater proportion of iron oxides in the natural sediment than in the synthediment(TM) batch.  This can be addressed very easily in future batches.  The grey color of the synthediment(TM) batch is due primarily to all 16 components in this batch being colored either white or black (with the exception of the minor component of dark brown peat powder).

At last report, overlying water and hexagenia larvae were added to the tanks sometime during the last week of February.  We'll have an update on the progress soon..