ORIGIN OF HIGH LEVELS OF AMMONIUM IN GROUNDWATER, OTTAWA COUNTY, MI

Wells located in and around Hemlock Crossing Park in Ottawa County, MI have elevated levels of ammonium (NH₄⁺) in the groundwater. Ammonium can interfere with drinking water disinfection processes and lead to the eutrophication of surface water bodies. Elevated NH₄⁺ concentrations generally originate from anthropogenic sources. Agricultural activity is common around Hemlock Crossing Park. Manure, spread on fields for fertilizer purposes, could be leaching into the subsurface and providing a source for the NH₄⁺. Another possible source of the NH₄⁺ is from the decay of buried organic matter that was deposited during the ice-free mid-Wisconsin glacial episode.

To determine if organic matter is present in the subsurface below Hemlock Crossing Park, a complete core of the glacial drift was recovered during the installation of a monitoring well using Rotasonic drilling. The boring was advanced to bedrock (Coldwater Shale), with the intent of acquiring a complete record of glacial sediment in the area. Grain size analysis reveals that the local glacial stratigraphy includes sediment from Glacial Lake Chicago, the Saugatuck till, possible Ganges till, and Glenn Shores till. Of interest in the core was a layer of compacted peat. Organic matter in the peat layer was dated at ~41,000 yr before present (B.P.) using radiocarbon analysis. The peat layer is bounded by two confined aquifers; both of which park and nearby residential wells draw water from for potable purposes. Results from water samples collected from park wells confirm that both aquifers have elevated concentrations of iron (Fe²⁺) and NH₄⁺. This is indicative of a favorable environment for anaerobic microbes, which can break down buried organic material into NH₄⁺.

Groundwater samples from park and nearby residential wells were analyzed for major ions and redox parameters to evaluate differences in water quality for the various aquifer settings. An isotopic investigation, involving ³H and ¹⁴C dating as well as δ¹⁸O-δ²H and δ¹⁵N-NH₄⁺ signatures, provided lines of evidence suggesting that the elevated NH₄⁺ originates from the decay of the buried peat layer and not from agricultural practices. Results are applicable to formerly glaciated regions with similar hydrological conditions where significant amounts of buried organic matter and NH₄⁺ may be present.