THE DISTRIBUTION OF SEEPAGE WITH DISTANCE FROM SHORE REVISITED

Much of our current understanding regarding the connectivity of ground water and surface water stems from the mid 1970s when research first indicated that water flows readily between surface water and ground water in most lake and wetland settings. Hans Olaf Pfannkuch and several of his graduate students determined that discharge of ground water to surface water should be concentrated near the shoreline of most lakes and decrease exponentially with distance from shore. This revelation was significant because it allowed scientists to ignore the areally extensive deep-water regions and focus measurements in the shallow near-shore margins, simplifying logistical challenges. Following publication of several papers on this topic, field measurements of distribution of seepage relative to distance from shore provided mixed results. Heterogeneity of the sediment was commonly invoked to explain unexpected deviations from an exponential distribution. It is now generally recognized that aquifers adjacent to and beneath surface-water bodies are rarely homogeneous, and usually are not isotropic. Sediments at the interface between ground water and surface water typically are deposited, eroded, reworked, and interlayered on event-based, seasonal, and inter-annual time scales, resulting in increased aquifer heterogeneity with proximity to the surface-water body. In addition, organic deposits are often buried by subsequent deposition of inorganic material, resulting in interlayering of greatly different sediment types. In areas that have seasonal ice cover, ice rafting can substantially rework sediments at the downwind shoreline. Benthic vertebrates and invertebrates constantly rework sediments, particularly organic sediments, as they carry out their life cycles. Fish also rework sediments in the creation of spawning redds. Although seepage has proven difficult to quantify because of the plethora of processes that create heterogeneity at the sediment-water interface, the thesis that seepage is focused near the margins of surface-water bodies has generally held up during the decades since Olaf Pfannkuch and others first advanced this tenet.