Graduation Term

2017

Degree Name

Master of Science (MS)

Department

Department of Geography-Geology: Hydrogeology

Committee Chair

Eric Peterson

Abstract

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 control those changes. The objective of this study is to understand NO3- concentration and environmental factor variability in a saturated buffer zone on a diurnal scale within and among the different seasons. Weekly 24-hour sampling events occurred between September 2016 and August 2017 with groundwater collected hourly from a saturated buffer zone located in central Illinois. Water samples were withdrawn from a well screened in an unconfined aquifer at 1.5 m below the surface. Seasons were defined by the 2017 solstices and equinoxes: Spring: March, 20 – June, 20; Summer: June, 21 – September, 21; Fall: September, 22 – December, 20; Winter: December, 21 – March, 19. Each collection event drew samples every hour for 24-hours; each sample was analyzed for nitrate as nitrogen (NO3--N). Mean daily NO3--N concentration ranged from 2.18 mg/L in the fall to 4.63 mg/L in the summer and varied by a statistically significant difference for spring-fall (t(15.90)=2.70, p=0.02) and summer-fall (t(10.91)=4.83, p=0.00) combinations. The differences between maximum and minimum NO3--N concentration measured over the 24 hour periods were statistically significant within spring (t(12)=2.76, p=0.01), summer (t(8)=6.83, p=0.00), fall (t(4)=4.34, p=0.01), and winter (t(5)=3.33, p=0.01). Vegetation uptake was identified as a process controlling 24-hour NO3--N concentration variability by the presence of a sinusoidal trend and daily timing of maximum (dark period) and minimum NO3--N (photoperiod) concentrations that coincide with photoperiod. NO3- leaching, ET, and nitrification were identified as processes controlling NO3--N concentration increases by the presence of an increase trend and NO3--N:Cl- rates of increase over 24-hours. There were no statistically significant differences between the magnitude of mean difference between daily maximum and minimum NO3--N concentration among seasons, indicating the amount of NO3- available does not influence the magnitude of change over 24-hours. The magnitude of difference between daily maximum and minimum NO3--N concentration had: no correlation with daily average air temperature, no correlation with solar intensity, and no correlation with mean daily water temperature. This study has demonstrated that variation in NO3- concentration exists on both the seasonal and diurnal scale, multiple processes operate to produce variation, and that variation over 24-hours is consistent across processes.

Access Type

Thesis-Open Access

DOI

http://doi.org/10.30707/ETD2017.Miller.J

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