The concerns about leaching of fertilizer nitrogen (N} into groundwater which were raised in the late 1960's were largely silenced during the 1970's as our agricultural economy was massively expanded. But since the early 80's they have been raised again to the point that national legislation (the Clean Water Act) was put into place to bring purity of the nation's water supplies into compliance with EPA-established standards. Beyond the apprehension largely about nitrate (NO3) levels in groundwater, the groundwater issue today also includes concerns about pesticides. At the current time, the public is apprehensive about drinking water and this is being fanned by the media with information from various sources. Some of this information is reliable and some isn't, but all of it is pressing our governmental agencies toward more restrictive use of agricultural chemicals. This is coupled with advances in analytical techniques by which some substances can now be measured accurately in parts per trillion. It hasn't been many years since parts per million was the acceptable norm.
Wells, Kenneth L., "Managing Soil Content of Nitrate Nitrogen" (1991). Soil Science News and Views. 150.