DELINEATION OF FLOW PATHS AND PROCESSES AFFECTING CHEMICAL VARIABILITY, ALLEQUASH BASIN, WISCONSIN

Most lakes, streams, and wetlands in Wisconsin are well connected to the groundwater system. Many lakes are flow-through lakes, in which groundwater discharges to the lake and is also recharged by water from the lake. In the Allequash Basin in northern Wisconsin, the exchange between surface water and groundwater affects the entire catchment ecosystem. Geochemical sampling and particle tracking simulations were used to identify the source areas of recharge, and trace groundwater flow paths from point of origin to discharge. Monthly (June-November, 2004) groundwater samples collected from 75 nested piezometers along three transects were analyzed for the isotopes of δ¹⁸O and δ⁸⁷Sr, as well as major ion concentrations, iron, and inorganic and organic dissolved carbon species. An existing three-dimensional groundwater flow model was used for particle tracking.

Flow paths delineated on the basis of the chemistry data showed groundwater derived from terrestrial, wetland, and lake source areas, which exhibited differing degrees of transient effects. Monthly/seasonal chemical variability observed along two transects with aquatic sources was caused by fluctuations in lake levels, which significantly affect the short-term orientation of the flow paths. These effects were also demonstrated in the particle tracking simulation where path lines pass both through and under a lake before finally discharging into strong sinks at the downgradient end of the basin. Thus, groundwater at the upgradient end of a flow path may discharge in different areas of the basin under different lake stage conditions. Transient effects in a discharge area near a 2-m wide stream was larger than transience near a relatively wide (> 400 m) lake. The study demonstrates that both aquatic and terrestrial components of flow are important in this watershed.