Are nitrate levels in groundwater stable over time

摘要: Epidemiologists often use a retrospective study design to examine for associations between an exposure and the occurrence of adverse health effects. Several these studies used this approach association elevated levels nitrate in drinking water related effects such as methemoglobinemia, cancer, neural tube effects, or spontaneous abortions. Often, data on exposures that occurred before outcomes were not available. Consequently, researchers measurements at time represent people developed conditions. An opportunity stability during survey private wells nine Midwestern states. In survey, samples from 853 homes with drilled collected May 1994 September 1995 nitrate-nitrogen (nitrate-N) was measured by colorimetric cadmium reduction method. Nitrate-N same well over assessed mixed-effects analysis variance. Analysis showed no significant difference initial level those 16 months later. Furthermore, most variance concentrations due well-to-well variation (89%) rather than residual error (12%). This observation single measurement could longer periods exposure.