CHARACTERIZING GROUNDWATER-SURFACE WATER CONNECTIONS AND GEOCHEMICAL FLUXES WITHIN AN URBAN WATERSHED USED FOR PUBLIC WATER SUPPLY

The Spring Valley subwatershed to the Kalamazoo river is located within Kalamazoo Township, Michigan and consists of a series of small natural lakes that are hydraulically connected through a combination of surface and subsurface flow pathways. The subwatershed trends approximately northeast to southwest and is surrounded by residential, commercial, and industrial development. A wellfield comprised of five supply wells rated at 350 gpm is located near the shoreline of Spring Valley lake, a man-made impoundment. The close proximity of the supply wells to the lake and the shallow well depths (< 30 m) may induce groundwater recharge by surface water. The wellfield contributes to the City of Kalamazoo’s water supply and this study is motivated by the potential for eutrophication and to better understand if applications of road salt may impact surface water and groundwater quality within the subwatershed. To address these concerns, this project seeks to identify sources and interpret spatial and seasonal trends of chloride and primary nutrients within the watershed. The watershed boundary was delineated using QGIS, USGS NED elevation data, and a municipal storm sewer system. Groundwater-surface water interaction is characterized by measuring vertical hydraulic gradients in Spring Valley lake, and stable isotope analysis of ¹⁸O collected from both the Lake and water supply wells. A sampling transect for major ions, nutrients, and other geochemical parameters within the watershed was established using locations of suitable mixing in the surface water flow system. A local-scale numerical groundwater flow model is being developed to test various hypotheses that may explain hydraulic connections between the impoundment, deeper water supply aquifer, and the water supply wells. Water chemistry results indicate sodium and chloride concentrations in surface water generally increase downstream with the highest concentrations occurring in Spring Valley Lake, and are likely associated with winter applications of road salt. Nitrogen and phosphorus concentrations are highest in the northeastern region of the watershed and may be related to agricultural and golf course land usage. Downward vertical gradients measured in Spring Valley lake near the pumping station provide preliminary support for the induced recharge.