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Using groundwater modeling to understand the factors controlling lake seepage patterns
Mikochik, James S.
Mikochik, James S.
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2008
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Earth and Environmental Science
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Mikochik-Thesis-2008.pdf
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https://doi.org/10.34944/4ecr-0316
Abstract
Understanding groundwater-lake interactions requires detailed knowledge of seepage patterns. Simple numerical models predict an exponential decrease of seepage with increasing distance offshore, but field measurements have shown that many lakes deviate from this pattern. To examine how hydrogeologic factors influence lake seepage patterns, two-dimensional steady state numerical models were constructed using the USGS modeling code MODFLOW. Sensitivity was tested to recharge, groundwater basin size and thickness, and lake morphology, as well as heterogeneities in lake sediment thickness and hydraulic conductivity, bedrock fracture zoning, and the effect of combining factors. The models were hypothetical. but many of the hydrogeologic parameters were based on those seen at Mirror Lake, a small glacial lake in New Hampshire. The models showed that increasing the thickness of lake bed sediments or decreasing their conductivity, increasing aquifer anisotropy, and decreasing the penetration of a lake into the groundwater system resulted in seepage rates that were more evenly distributed across a lakebed rather than focused near shore. Changing overall lake slope without increasing the penetration of the lake into the aquifer system produced little noticeable change in seepage patterns. Localized geologic heterogeneities, including a pinch-out in lake sediment and the presence of a high-K fracture zone, as well as a change in lake slope produced localized peaks in seepage. The peak produced by a pinch-out in lake sediment increased in magnitude as the degree of thinning increased, with seepage rates that locally were magnified by a factor of 150 where lake sediment was absent. A high-K fracture zone produced a less significant peak whose size was controlled by several factors, including the overall length of the fracture, its connectivity to the lake, its orientation, and the presence or lack of low-K lake sediment. A change in lake slope produced a smaller peak compared with the pinch-out and fracture zone cases except when modeled using highly anisotropic conditions in which seepage rates were magnified by a factor of 20. Heterogeneities modeled closer to shore are more likely to contribute a significant amount of seepage to a water budget than those further from shore. One important factor controlling the degree of change in seepage was the contrast between lake bed sediments and the bedrock aquifer. Thus, the conductance of lake bed sediment and anisotropy should be measured to predict whether heterogeneities or breaks in lake slope will produce detectable localized changes in seepage rates. This modeling can help guide field studies by providing an understanding of the geologic conditions that control the rate of exponential decline of seepage away from shore.
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