DIURNAL AND SEASONAL VARIATION IN GROUNDWATER NITRATE-N CONCENTRATION IN A SATURATED BUFFER ZONE

Surface water pollution by nitrate (NO₃^-) is identified as a critical problem in agricultural land-use areas. Excess NO₃^- loading causes eutrophication and hypoxia in near-shore marine waters such as the Gulf of Mexico. Diversion of agricultural runoff into saturated buffer zones reduces NO₃^- loading. Although the mechanisms responsible for NO₃^- reduction in saturated buffer zones are well characterized, little is known about how NO₃^- concentrations vary temporally and what factors are involved. The objective of this study is to understand NO₃^- concentration variability in a saturated buffer zone on a diurnal scale within and among seasons. Sampling events occurred weekly between September 2016 and August 2017 with groundwater collected from a saturated buffer zone located in central Illinois. Water samples were withdrawn from a well screened in an unconfined aquifer at 1.5m below the surface. Each collection event drew samples every hour for 24-hours; each sample was analyzed for NO₃^-. Seasons are defined as follows Spring: Mar., Apr., May; Summer: June, July, Aug.; Fall: Sept., Oct., Nov.; Winter: Dec., Jan., Feb. While daily mean NO₃^- concentrations ranged from 2.18 mg/L in the fall to 3.81 mg/L in the summer, the daily mean NO₃^- concentration did not vary statistically (F(3, 26)=1.89, p=0.16). The differences between maximum and minimum NO₃^- concentration measured over 24 hours were statistically significant in all seasons. The daily timing of maximum (early- to mid-morning) and minimum NO₃^- (mid-afternoon) concentrations did not vary seasonally. While the magnitude of mean difference between daily maximum and minimum NO₃^- concentration was highest in the summer (0.74 mg/L), no statistical differences were noted across the seasons (F(3, 26)=3.35, p=0.34). The magnitude of difference between daily maximum and minimum NO₃^- concentration had: zero correlation with daily average air temperature, negative correlation with solar intensity, and weak correlation with daily average water temperature. The results of this study show observable diurnal variation in NO₃^- concentration controlled by factors operating independently of the growing season.