Return of the rain

2 May 2012

Right up to 28 April (last Saturday) the rainfall in Cambridge was below the monthly average. Then over Saturday and Sunday we had nearly 25 mm of rain. This means that at last we’ve had a month where rain has exceeded the monthly average.Raining

Much to the surprise of my wife, I decided to play golf on Sunday afternoon. The course was quite busy because the wind and the temperature were very acceptable. Underfoot the soil wasn’t soggy, because the course is built on shallow chalk.

I was playing a doctor who expressed surprise on how well the course walked. This led to a discussion on rain, soil moisture deficit and transpiration. At first, he couldn’t understand why it is most unlikely that summer rainfall will not recharge the watertables. I explained that in the summer the transpiration losses can be around 25 mm a week, and so it needs rainfall consistently above that figure to get water moving down the profile.

There has been sufficient rainfall locally for some drains to start running and, of course, tramlines are very wet. This can mean only one thing - pesticides in watercourses. In the West Midlands, where there was more rain earlier in the month, pesticides are appearing in feeder streams to reservoirs at levels above those specified in the good old Drinking Water Directive.

The usual suspects for this time of year are being recorded; clopyralid and the hormone herbicides mecoprop and MCPA. Fortunately, water companies are prepared to treat raw water in order to reduce pesticides in tap water to below the levels specified in the legislation.

These herbicides currently appearing in water are not effectively taken out of water by activated charcoal filters. This is because they are relatively water soluble and activated carbon is more effective on the more water insoluble herbicides, the classic one being isoproturon. But, the hormone herbicides are more effectively removed by the other process adopted to reduce pesticides in water - passing ozone through the water which oxidises chemicals in the water and breaks them down.

Unfortunately, clopyralid is one of two major pesticides detected in water that can get through both processes relatively unscathed. Metaldehyde is the other one. As they cannot be removed from water, their levels have to be reduced in raw water to below the threshold specified in the Drinking Water Directive. At the doses commonly used, this presents a particular challenge to metaldehyde, hence the importance of the Pelletwise campaign.

The problem is that all herbicides can occur in water at very high levels for a short time after a significant rain, particularly if the soil is already wet and the water treatment works cannot cope with these spikes of concentrations. This is where grass buffer zones to reduce immediate run off of surface water, particularly from tramlines, are particularly useful.

There is a great fear that the Water Framework Directive, which includes the Drinking Water Directive, will eventually result in the withdrawal of some key herbicides. Many now consider that these fears are over-egged. The Water Framework Directive says that the position of pesticides in water should not get worse, with the implication that an indication of the situation getting worse would be water companies installing additional facilities to remove pesticides.Winter cereal spraying

So the future may not be so bleak, provided that each and every farmer adopts sensible practice, both for filling and cleaning out sprayers as well as for the spraying operation itself. Identifying fields with a high risk of run-off and only spraying them when the risk is low is also an essential step.

Buffering watercourses with sufficiently wide grass strips is required in most cases. These strips are only as good as the weakest point and so they shouldn’t have breaks in them. Most breaks are where vehicles enter the field, but check to see if they are sufficiently wide and robust by walking them during periods of heavy rain to see if there is a breach from run-off.

The weather over the last few years has shown that we can get periods of very wet and very dry weather. Should this type of variable weather become even more exaggerated then there is the prospect that water companies may not be able to cope with pesticides in water with their current facilities. This really could spell trouble unless the farming industry reacts promptly to the situation.