Good questions. I'll make a feeble stab at answers, without writing a book - the reading of such would actually be my suggestion for in-depth explanations.
A half-life represents the amount of time required for a substance to degrade into half of its original amount (or concentration); which continues - in theory - until minute detection-limits are exceeded.
A pesticide's half-life is affected by many site-variables; namely sunlight intensity (photo-degradation), thermal degradation, hydrolysis and microbial-degradation. Therefore, a pesticide's half-life is rarely a linear time-line, yet it follows a reasonably predictable path under specific site-conditions.
Many folks may remember chlordane, and how well it worked for controlling termites beneath structural foundations. Its half-life generally ranged from 1 to 10 and sometimes 20 years, but site-variables - and application methods, such as sub-slab applications and soil-incorporation, both of which eliminate sunlight exposure - greatly increased that compound's half-life. Based on industry lore, it wasn't until chlordane sustained significant misuse by the general public - i.e. applied to exposed areas where its use was never intended, namely for ants - that it began to show up in surface and groundwater, which ultimately brought about its demise. But I digress...
Suffice to say that every prospective pesticide faces a barrage of site-scenario testing during its lab and R&D phases to determine its half-life potential and many other potential environmental and toxicological issues, ultimately to determine if it should be pursued for marketable uses.
I forget the actual statistics from manufacturers, but if memory serves me, roughly 10,000 compounds are screened for efficacy, toxicology and possible environmental issues for every single compound that actually makes it to the marketplace. I remember a figure of $32 million, from at least a decade or two ago, as the amount that a pesticide manufacturer must invest in a promising compound before they actually sell the first pound or gallon. No doubt this figure has grown exponentially since that time. I know of at least three instances in the past ten years where products had cleared the hurdles (and consumed huge investments), only to be "canned" due to factors that weren't discovered until the final "field development" (pre-sale) phase. Is it any wonder why we see so few "new chemistries" entering the marketplace?
It is for the very reason that my industry strives so hard to insure that currently available products are properly utilized, since there's virtually no new chemistries in the "pipeline" to replace them should they be lost due to user-negligence - or ignorance.
As for comparable active-ingredients possessing different site-labels, several reasons exist for this occurrence. Glyphosate is a perfect example, since it is one of the most common active-ingredients that possesses specific use-site labels. As has been mentioned and previously covered in other posts, some glyphosate formulations contain "other ingredients" that have no place in aquatic sites - particularly those that contain tallow-amine surfactants. This particular type of surfactant (wetting agent) greatly enhances the uptake and activity of glyphosate and is deemed suitable and desirable in many terrestrial sites - but NOT in aquatic sites. Glyphosate products intended for aquatic-use (with one exception) do not contain an integrated surfactant, and require the user to add a suitable (labeled) aquatic surfactant to the mix-tank.
It is generally true that aquatic formulations may cost slightly more to use than their terrestrial counterparts, but not nearly as much as one might believe when making price-per-gallon comparison.
For example: Aquatic-labeled glyphosate formulations generally contain 25% more active-ingredient that their terrestrial-labeled cousins. Assuming one chooses to use an aquatic-labeled glyphosate to treat cattails, the recommended mix-rate is a 3/4% solution, which has (for simplicity sake) the equivalent amount of active-ingredient as a 1% mix-solution of the terrestrial glyphosate formulations.
By the way, mixing a higher rate than what is prescribed on the label is akin to shooting a cottontail with a 30-06, when a .22 will do the job. In fact, mixing some herbicides "too hot" can actually defeat the product's normal mode-of-action, resulting in a plant that may appear "dead" overnight, even though the application simply "burned" the plant's leaf-tissue and put it into "shock". Always remember, YOU CAN ONLY KILL SOMETHING SO DEAD!
Now for a little math-exercise.
I located a 2.5 gal jug of an aquatic-labeled glyphosate herbicide (RODEO) on Amazon for $77, delivered! (I have no affiliation with the seller) That price equates to $30.80/gal, or 24-cents per ounce. Assuming that one follows the labeled mix-rate of 1-oz per gallon of water, the cost per gallon of sprayable-mixture is 24-cents; or $2.40 for 10 gallons of mixture, and so on. At that mixing-ratio, a 2.5-gal container will produce 320-gallons of spray-mix; which may be used in either ponds OR terrestrial sites - legally, effectively and with relative safety. I'm not rich by any means, but that cost seems very affordable - especially when considering that the legitimate aquatic formulation doesn't represent a risk to non-targeted aquatic life within the treated pond or lake.
The other referenced product was 2,4-D (aka 2,4 dichlorophenoxyacetic acid / sometimes confused with 2,4,5-T / aka 2 4 5-trichlorophenoxyacetic acid / aka "agent orange". They're NOT the same!). Numerous brands and formulations of 2,4-D are available. One of major distinction between the various 2,4-D brands is determined by whether it is an "amine", "ester" or "acid" formulation, as well as its "inert ingredients". Suffice to say that a liquid 2,4-D ESTER should never be used in aquatic sites. Other than that, check the product's label to insure that it is intended for aquatic sites.
I could drone on until I TOO fall asleep, so I'll stop here. Hope this sheds some light on the subject.
