GROUNDWATER FLOW ALONG A GRAVEL-SAND LENSE IN AGLACIATED TERRAIN

Quaternary glacial terrains are characterized by complex stratigraphy and heterogeneous lithology, which influence groundwater flow. In the agricultural Midwest where most of the surficial geology consists of glacial deposits, understanding groundwater flow pattern is crucial in designing efficient drainage systems. Drainage systems are needed to increase agriculture profitability, but release nutrients to surface waters. The purpose of this study was to investigate the geologic control of groundwater flow in a glaciated terrain in central-Illinois to better understand the role of tile drainage systems.

Thirty –two (32) borings were drilled at various depth to characterize the subsurface geology. These borings served also as monitoring wells, allowing for water level measurement and water sampling. The groundwater samples were analyzed for major ions and nitrates. The local geology , based on the borings, consists of: a 60 cm thick dark clay with high organic matter content,  a brown clay with lenses of sand and gravel with an approximate thickness of 160 cm, and a gray tight clay of undetermined thickness. The water table resides in the medium sandy and gravel unit. An analysis of hydraulic head data suggests that the medium sandy unit receives water input from the upper and lower layers.  Upon convergence of the water vertically, the waters move horizontally and discharge to the nearby stream. A piper plot of the major anions revealed two hydrofacies: sulfates - chloride rich water in the deeper wells, and carbonates –bicarbonates rich water in the shallower wells. Higher nitrates concentration were found in wells that are screened in the medium unit as compared to those screened in the upper and lower units. The results of this study provide the evidence that the medium sandy unit serves as a preferential groundwater flow medium and a potential route for nitrates transport.

 

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