Cases and solutions Seepage measurements from Long Lake, Indiana Dunes National Lakeshore S. A. Isiorho . F. M. Beeching . P. M. Stewart . R. L. Whitman Abstract Long Lake, located near Lake Michigan within the dune-complexes of Indiana Dunes Na- tional Lakeshore, USA, was formed some time dur- ing the Pleistocene and Holocene epochs. A surfi- cial aquifer underlies Long Lake, which is either a source or sink for the later. The hydrologic proc- esses in the lakeshore and surrounding environs have been significantly altered during the agricul- tural, municipal, and industrial development of the region. Limited data suggest that the organisms of Long Lake have elevated levels of several contami- nants. This study attempts to quantify seepage within the lake to assess the potential threat to groundwater quality. Seepage measurements and minipiezometric tests were used to determine see- page within the lake. Seepage measurements and minipiezometric tests suggest that water seeps out of Long Lake, thus recharging the groundwater that flows southwest away from the lake. There is a great deal of variability in the seepage rate, with a mean of 11.5 X 10 -4± 11.2 X 10 -4 m d -1. The mean seepage rate of 0.3 m yr -1 for Long Lake is greater than the 0.2 m yr -1 recharge rate estimated for the drainage basin area. The Long Lake re- charge volume of 2.5 X 105 m 3 yr -1 is approximate- ly 22% of the volume of the lake and is significant when compared to the total surface recharge vol- ume of 4.8 X 105 m 3 yr -1 to the upper aquifer of the drainage area. There is a potential for contami- nation of the groull(fwater system through seepage from the lake from contaminants derived from aer- ial depositions. Key words Long Lake . Seepage . Minipiezometers . Indiana Dunes National Lakeshore Received: 16 August 1995 . Accepted: 18 September 1995 S. A. Isiorho (181) . F. M. Beeching Geosciences Dept., Indiana University - Purdue University Fort Wayne (IPFW), Fort Wayne, IN 46805, USA P. M. Stewart . R. L. Whitman National Biological Service, 1100 N. Mineral Springs Raod, Porter, IN 46304, USA Introduction Wetlands once covered 5.6 million ha of Indiana, USA, but only 13% of these wetlands remain today (National Research Council 1992). The wetlands of Indiana Dunes National Lakeshore (INDU) are exceptional relics of the extensive wetlands that once existed in the Calumet Re- gion of northern Indiana. The original wetlands were drained by ditching; then filled, dredged, and dissected by railways and highways, and by industrial and munici- pal development. Even in their presently reduced state, these wetlands remain an outstanding natural feature of the INDU. Long Lake, the largest inland lake at INDU, is located ap- proximately 3 km south of Lake Michigan (Fig. l). The lake is approximately 0.5 km wide and 2 km long, oriented east to west. Long Lake is relatively shallow with a mean depth of 1.4 m and is approximately 3 m at its deepest point. During the late summer of 1992, the lake was approximately 75% covered with aquatic vegetation. According to Shedlock and others (1994), the geologic cross section near Long Lake shows a relatively simple hydrogeologic framework (Fig. 2). The cross section indi- cates that a broad water table mound passes through the dune complexes between the Little Calumet River and Lake Michigan. Limited data suggest that the organisms of Long Lake have elevated levels of several contaminants. Steffeck (1989) found several contaminants with levels as high as those found in the biota from three nearby Superfund sites located in Gary, Indiana. Levels of mercury and sele- nium from Long Lake biota were higher than that found in the biota from the Superfund sites. In addition, moni- toring efforts at national park units indicates that INDU has the highest nitrate and sulfate levels in precipitation of any monitored park unit in the country (NADP/NTN Coordination Office 1992). It is difficult to assess the full impact of such contamination on the region without in- formation obtainable from extensive hydrological re- search. The quantification of seepage in lakes to assess the po- tential threat to groundwater quality could be important in areas with regard to industries that discharge their li- quid wastes to surface water. Seepage can be an impor- tant component in the water budget, and chemical bal- ance of a lake and seepage measurements are often used to study lake/groundwater interactions (Winter 1976, Environmental Geology 28 (2) September' 996 . © Springer-Verlag 99