THE RELATION OF LAKE CHEMISTRY TO GROUND-WATER FLOW SYSTEMS

Four sites in the north-central United States have been the focus of research on lake and wetland hydrology since the early 1980s. Three of the sites, the Trout Lake area in Wisconsin, the Williams Lake area in Minnesota, and the Cottonwood Lake area in North Dakota, are in glacial terrain. The fourth, the Island Lake area in Nebraska, is in dune terrain. All of the sites contain closed lakes that lie within ground-water flow systems. Two questions that can be addressed at the four sites are (1) do the lakes become more mineralized in a down-gradient direction within their respective ground-water flow fields? And (2) Does the major-ion chemistry of the lakes reflect the mineralogy of the underlying geology? The Trout Lake area is underlain by about 30 m of relatively uniform sand composed largely of silica and no carbonate minerals. The Williams Lake area is underlain by glacial deposits that are greater than 120-m thick. The surficial unit consists of sand and gravel that is as much as 22-m thick and contains abundant carbonate minerals. The Cottonwood Lake area is underlain by 140 m of clayey till containing small, isolated sand units. The Island Lake area is underlain by about 40 m of dune and fluvial sand containing abundant quartz and feldspar. Commonalities and differences in the chemistry of the lakes between the four sites are related to differences in their position within ground-water flow systems, the mineralogy and permeability of their geological substrate, and climate. In the Trout Lake and Williams Lake areas, the lakes maintain a common major-ion water type (CaHCO3 in the Trout Lake area and CaMgHCO3 in the Williams Lake area) but they become more mineralized downgradient in their ground-water flow systems. In the Island Lake area, the lakes maintain a common major-ion water type (NaHCO3) but they become less mineralized downgradient in the ground-water flow system. The Cottonwood Lake area is more complex. Similar to the Trout Lake and Williams Lake areas, most of the wetlands contain more mineralized water downgradient in the ground-water flow field. However, the chemistry of several wetlands is unique because of local geological conditions or biological processes.