3.5.4 Infiltration Capacity
Recharge to the water table occurs when surface water from rain or snow fall infiltrates into the groundwater system. The groundwater recharge rate depends on many different factors. Less water seeps through areas of relatively impervious surface material such as tight clayey glacial till than areas of highly permeable soils such as loose sandy or gravely glacial outwash. In some areas, such as buried bedrock valleys, bedrock aquifers are directly exposed to surficial drift materials. The HHPLS used soils, geology, and topographic information to assess infiltration and recharge potential within the watershed. Figure 6 shows that this potential ranges widely across the watershed, with some highly permeable areas capable of high infiltration rates to areas of variability where the soils are organic in nature and less permeable.
The Lake Minnetonka vicinity provides an important recharge area for the Jordan aquifer. Water tables in the glacial drift near Lake Minnetonka for all practical purposes equal the lake level, which normally ranges from elevation 928.5 to 930.0 feet above mean sea level. A study performed in 1971 prepared a water budget for Lake Minnetonka that included seepage losses through lake bottom sediment. This water budget succeeded a similar analysis completed by the Watershed District in 1969. Study results reported an annual seepage loss (recharge) of about 3 to 4 inches in Lake Minnetonka. Based on these estimates, Lake Minnetonka contributes about 4,700 acre-feet of water annually as recharge to underlying aquifers.
Areas where the drift material is relatively thin, transmissivity is high, and water table depth is minimal are critical recharge areas. Areas with these characteristics have a greater potential to transport contaminants to the drift aquifers than other areas. Figure 31 classifies aquifer sensitivity from the Hennepin County Geologic Atlas and the Carver County Environmental Services data.
Recharge can also occur from surface water bodies. Wetlands and floodplains can function as recharge, discharge, or flow-through areas depending on the level of the water within the wetland or floodplain relative to the water table. Discharge can occur naturally through springs, seeps, and directly into streams and lakes. Percolation into underlying aquifers is also a form of discharge from drift aquifers. The configuration of these subsurface aquifers and the position of the water table are highly variable and complex. More information will become available as cities complete their wellhead protection plans.