Pesticides have been detected in high-altitude regions, demonstrating sufficient persistence to survive transport across hundreds of kilometers in the atmosphere.
Similarly, photochemical transformations require sunlight, available only in the topmost meter(s) of lakes or rivers, plant surfaces or submillimeter soil layers.
Such studies further fail to cover unusual environmental conditions such as strongly sulfidic environments such as estuaries or prairie potholes, nor do they reveal transformations at low residual concentrations at which biodegradation may stop.
For example, organophosphate esters that interfere with nerve signal transmission in insects do not affect microbial processes and offer nourishment for microorganisms whose enzymes can hydrolyze phosphotriesters.
[2] In addition, genes move horizontally within microbial populations, spreading newly evolved degradation pathways.
For example, the hydrolytic dechlorination of atrazine to hydroxyatrazine in soil by atrazine-dechlorinating bacterial enzymes reached a second-order rate constant of 105/mole/second, likely dominating in the environment.
In other situations (e.g., in agricultural wastewater treatment), microorganisms mostly grow on other, more readily assimilable carbon substrates, whereas pesticides present at trace concentrations are transformed through fortuitous metabolism, producing potentially recalcitrant intermediates.
[1] Pesticides persist over decades in groundwater, although bacteria are in principle abundant and potentially able to degrade them for unknown reasons.
This may be related to the observation that microbial degradation appears to stall at low pesticide concentrations in low-nutrient environments such as groundwater.
Pesticide electronic absorption spectra typically show little overlap with sunlight, such that only a few (e.g., trifluralin) are affected by direct phototransformation.
[5] In the absence of such rate constants, quantitative structure–activity relationships(QSARs) may allow their estimation for a specific pesticide from its chemical structure.
For example, aqueous abiotic hydrolysis degrades organophosphates, carboxylic acid esters, carbamates, carbonates, some halides (methyl bromide, propargyl) and many more.
Conditions such as high pH or low-redox environments combined with in situ catalyst formation including (poly)sulfides, surface-bound Fe(II) or MnO2.
Chemical reactions may also prevail in compartments such as groundwater or lake hypolimnion, which have hydraulic retention times on the order of years and where biomass densities are lower due to the almost complete absence of assimilable organic carbon.
Carbon 14-labeled pesticides do enable mass balances, but investigations with radioactively tagged substrates cannot be conducted in the field.
In such a case, transformation mechanisms are identifiable from plots of 13C/12C versus 15N/14N parent compound data, reflecting different underlying carbon- and nitrogen-isotope effects.
For the special case of chiral pesticides, enantiomer analysis may substitute for isotopes in such analyses as a result of stereoselective reactions.
E.g., N, N-dimethylaniline, used as a probe for the carbonate radical reacts very quickly with DOM-excited triplet states and its oxidation is hampered by DOM.
[1] 13C-labeled parent pesticides were used in the nontarget analysis of degraders by stable isotope probing (SIP) to demonstrate biotransformation potential in soil and sediment samples.
AtzD was unambiguously identifiable and hence quantifiable, as unusually, it belongs to a protein family that largely consists of biodegradative enzymes.
Another factor confounding gene-based approaches is that biodegradative function can arise independently in evolution, such that multiple unrelated genes catalyze the same reaction.
Some consider the higher limit acceptable as no imminent health risk can be proven, whereas others regard it as a fundamental deviation from the precautionary principle.