In iran, uranium is being enriched using centrifuges. in the 1940’s and 1950’s usa uranium was enriched using mass spectrometry. why did the iranians choose not to use mass spectrometry?

This is due to the low cost, in energy, of enrichment per unit  of uranium. It's cheaper to use centrifugal isotope separation than the use of mass spectrometry. However, mass spectrometry produces highly enriched uranium compared to centrifugal. Additionally, mass spectrometry requires a less mass of natural uranium (NU) to produce the desired enriched uranium.

Nps pollution is caused by rainfall or snowmelt moving over and through the ground. nonpoint source pollution can include: excess fertilizers, herbicides and insecticides from agricultural lands and residential areas. oil, grease and toxic chemicals from urban runoff and energy production