Pesticides and Groundwater

Traces of Oxadixyl have been found in local water, despite it not being used in pesticides in the Island since around 2003, although it is of course likely that any stockpiles may have been in use for a year or two after that.  It is a fungicide, designed to prevent potato blight.

The manufacturers decided not to renew its licensing in 2003, meaning that its license lapsed. Part of the reason for this was that the EU produced a list of active ingredients to be removed in July 2003 under Directive 91/414/EEC. Oxadixyl was one of 320 pesticide active ingredients removed as a result, which is why its licence was not renewed.

Interestingly, back in 2003, taking into account the 19 pesticides already banned by the Commission, the 320 about to go and the 150 followed, represented a loss of more than 60% of all the pesticides that were on the market in 1993.

Pesticides themselves have a limited effectiveness because of evolution. As the Berkeley Understanding Evolution site notes:

“Pest insects have short generation times and large population sizes — which means that they evolve quickly. If pesticides are widely applied, or if fields are widely planted with pesticide-producing plants, insects resistant to the pesticide will evolve. Some degree of resistance has been documented for every major class of insecticide used in agriculture”

But this process can be slowed by what are termed “refugia” — they are fields without pesticides (sprayed or plant-produced) located near fields planted with pesticide-producing crops. They provide havens in which providing havens in which the non-resistant insects survive.

If a resistant insect (rr) surviving field mates with a non-resistant insect (RR) surviving in the refuge, their offspring may either be mixed, some non-resistant (Rr), or if the non-resistant gene is recessive, as has been found with some insects, all of their offspring will be non-resistant (Rr).

Recognizing how our short-term solutions are likely to affect evolving insect populations, and the evolutionary impact can lead us to manage pesticides better.

So why have the traces of Oxadixyl been seen now?

As USGS reported in 2006:

“Ground water responds more slowly than stream water to changes in pesticide use. A persistent pesticide or degradate can remain in ground water long after its use is discontinued because of the slow rates of ground-water flow and the resulting long residence time of water and pesticides in ground-water flow systems.”

It suggested the following priorities:

- Improve tracking of pesticide use in agricultural and non-agricultural areas, including amounts, locations, and timing
- Add assessments of pesticides not yet studied, including some already in use as well as new pesticides
- Improve assessment and understanding of degradates, including their distribution and potential effects
- Evaluate toxicities of mixtures and their potential to affect humans, aquatic life, and wildlife
- Evaluate the performance of management practices and their effects on concentrations and transport of pesticides
- Improve methods for prediction of pesticide levels in unmonitored areas
- Sustain and expand long-term monitoring for trends

Cornell University has an article from 2012 - “Pesticides and Groundwater: A Guide for the Pesticide User”: which notes that:

“Cleanup of groundwater contaminated by pesticides often is impossible, and the contamination may last for many years. The cold temperatures and low microbial activity in groundwater cause pesticide degradation to occur more slowly than at the soil surface. The slow movement of groundwater means that it may take decades for the contaminated water to flow beyond the affected wells.”

“Even determining which wells will be affected and for how long is a difficult problem, necessitating expensive long-range monitoring to ensure the safety of drinking water supplies. Clearly, the best solution is to keep pesticides out of groundwater through careful storage, use, and disposal practices.”

“Many pesticides bind strongly to soil and are therefore immobile. For those that are mobile in soil, their leaching to groundwater can be thought of as a race in time between their degradation into nontoxic by-products and their transport to groundwater.”

“If the pesticide is not readily degraded and moves freely with water percolating downward through the soil, the likelihood of it reaching groundwater is relatively high. If, however, the pesticide degrades quickly or is tightly bound to soil particles, then it is more likely to be retained in the upper soil layers until it is degraded to nontoxic by- products.”

This particular contaminant in Jersey water is low levels, and does not pose a threat to water users of mains water, although bore-hole users need to get their water checked. As a precautionary measure, however, Val de la Mare reservoir has been shut off from the mains water supply.

But it does serve perhaps as a wake-up call to what has already been known elsewhere about pesticides and groundwater, and how the pesticide can remain there for decades. Next time, we may not be so lucky.