Are nitrate levels in groundwater stable over time?

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