Recently I came across a research paper prepared by the Interfaculty Environmental Science Department (IVAM) of the University of Amsterdam. They completed two studies, in 2002 and 2005, comparing various methods of weed control and their impacts in the environment. They considered a large number of factors including human health, eutrophication of water and soil, global warming, ozone effects, acidification of soil or water, effects on biodiversity, among a number of other factors.

The systems they tested for controlling weeds were conventional chemical control, mechanical control by using steel brushes, burning weeds using a flamethrower, a type of hot water applicator, and Ultra-Low Volume spraying. The results were consistent between the two studies: ULV spraying has the least overall negative effect on the environment.

It is largely due to this study that Mankar systems are so widely used in the Netherlands - indeed there is only one other system (which is between 5-10 times as costly) that meets the strict criteria the government has set out for weed control.

You can read the full studies on the University of Amsterdam site here, however the studies were only published in the Dutch language. If you have an interest please let me know and I will translate the relevant sections for you.