Federal Way wells tap into an aquifer which extends under the Sea-Tac airport. The interactions of the high iron content, hydrogen sulfide and natural gas identified in the area during drilling need to be evaluated. The assumptions that some have made in technical discussions, in technical reports related to airport construction and the project's EIS regarding effectiveness of till to protect the aquifers are unconservative. That degree is dependent upon the degree of transmissivity of the semi-confining units. One common misunderstanding is that semi-confining units, such as till or clays, are impermeable.
Water can indeed flow through tills and clays, just much more slowly than through sands and gravels.
NBC News Extra. It contains the Univ of Florida fill calculations.