The upper 350 to 400 feet of rocks underlying Lake County, Indiana, form a single but complex hydrologic system. The rock units composing this system consist (in ascending order) of dolomite, clay till (unit 4), glaciofluvial sand (unit 3), clay till (unit 2), and lacustrine sand, silt, and clay (unit 1). The dolomite and unit 3 form the principal aquifers and the clay tills, units 4 and 2, the confining layers. The geohydrology of the confining layers controls to a large extent the rate at which the aquifers are recharged from local precipitation and, thereby, their potential yield. Unit 4, the dolomite's confining layer, has an estimated average vertical permeability of about 0.003 gpd (gallon per day) per square foot. Under present conditions of head difference, the rate of recharge to the dolomite through unit 4 is estimated to average 20,000 gpd per square mile. Unit 2, the confining layer for unit 3, has an average estimated vertical permeability of about 0.007 gpd per square foot. Under present conditions of head difference the recharge through unit 3's confining layer is estimated to average about 100,000 gpd per square mile. However, these rates of recharge can be expected to increase as the head difference across each confining layer increases with extensive development of the aquifer.
