Temporal analysis of groundwater nitrate concentrations from wells in Prince Edward Island, Canada: application of a linear mixed effects model

Changes in nitrate concentration in groundwater from wells in Prince Edward Island, Canada were investigated over time using two datasets. Temporal trends in groundwater nitrate concentrations were assessed annually during 1981-1996 (1,299 observations), and both seasonally and monthly during 1988-1991 (1,868 observations). Data were analysed using linear mixed models with random effects and correlation structures. The average nitrate concentration in the monthly dataset was 3.99 mg/L as NO3-N, with January, May, and November concentrations being higher (p = 0.018). A seasonal effect was present when season was combined with land use type in an interaction term (p = 0.004). Wells located in agricultural areas had greater nitrate concentrations than urban areas, which in turn, had greater values than low human-impact areas. Row-cropped areas had higher groundwater nitrate concentrations in the summer, whereas manure storage areas were higher in the spring and autumn. Nitrate in groundwater in areas with low human impact and with centralized sewage disposal infrastructure remained relatively low and stable throughout the seasons. There was no significant annual trend (p = 0.954), but for individual sites, 9.6% significantly increased in nitrate concentration over time, and 6.6% significantly decreased over time. © Springer-Verlag 2007