ASSESSMENT OF SPATIAL AND TEMPORAL VARIATIONS IN CHLORIDE CONCENTRATION IN AN AGRICULTURAL TILE-DRAINED AREA IN CENTRAL ILLINOIS
COMMANDER, Okiemute1, PETERSON, Eric1, O'REILLY, Catherine M.1, THAYN, Jonathan1, PERRY, William L.2 and TWAIT, Rick3, (1)Department of Geography, Geology, and the Environment, Illinois State University, Felmley Hall 206, Campus Box 4400, Normal, IL 61790, (2)School of Biological sciences, Illinois State University, School of Biological Sciences Julian Hall 210, Normal, IL 61761, (3)Public Works Water Division, City of Bloomington, Bloomington, IL 61701
Increase in chloride (Cl-) concentration in surface water and groundwater from anthropogenic sources, including deicing salts, agriculture, septic effluents, and wastewater treatment plants is a growing cause for concern all over the world. In rural areas with less impervious surface cover, agriculture may serve as a source for Cl- in water systems. A saturated buffer zone (SBZ) installed adjacent to a Central Illinois stream (tributary of Lake Evergreen) to reduce nutrient losses was used to identify groups of Cl- in the SBZ and temporal Cl- variation within the SBZ. Water samples collected from 37 wells, a diversion box and stream over a 7-year period within the SBZ were analyzed for major anions. Based upon well depth and locations within the SBZ, subgroups were delineated into deep groundwater, downgradient shallow groundwater, upgradient shallow groundwater, diversion box, and stream. Seasons were divided to correspond with agricultural practices: spring/planting, summer/growing, fall/harvest, and winter/fallow.
A cumulative probability plot indicates three populations of Cl-, and two-way ANOVA results identify three distinct groups: 1) stream, 2) upgradient shallow groundwater, and 3) diversion box, downgradient shallow groundwater, and deep groundwater. The ANOVA identified seasonal differences in only the stream, while both the stream and upgradient shallow subgroups were spatially different from the others. Principal component analysis (PCA) of the water chemistry data indicated that water-rock interaction explained 28% of the variance while surface processes explained 23% of the variation. Classifications from the PCA correspond to the three groups of Cl- population within the SBZ, hence supporting results from the ANOVA and the cumulative probability plot. Understanding the different populations and seasonal variations of Cl- within the SBZ will help in reducing chloride and other nutrients loading into the stream and further downstream into Lake Evergreen (City of Bloomington water reservoir).
Keywords: ANOVA, Chloride, Cumulative probability plot, Principal component analysis (PCA), Saturated buffer zone.
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