Between 1974 and 1984, concentrations increased at all depths greater than about 30 feet. The most dramatic increases were in shallow ground water, between 30 and 100 feet below the land surface. Nitrate in ground water under the Delmarva Peninsula of eastern Maryland has responded to changes in fertilizer use. The steep increase in concentration beginning about 1968 follows a sixfold rise in fertilizer application rates between 1951 and 1971. Since then, fertilizer use has remained fairly constant.
Much more data exist for determining nutrient trends in surface waters.
A consistent change between 1970 and 1992 is seen in many urban streams across the country. In streams affected by sewage-treatment plants, the proportion of nitrogen present as ammonia has decreased, and the proportion present as nitrate has increased. This pattern appears to be widespread in urban areas throughout the Nation and is a direct result of improvements in sewage-treatment practices mandated by the Clean Water Act. Sewage-treatment processes were improved during the late 1970's and 1980's to convert ammonia into nitrate. As a result, nutrient releases from sewage treatment rarely pose a concern for fishkills, but still may be a concern for eutrophication.