Water moves in response to this hydraulic gradient, from high areas to low areas, where it is discharged from the aquifer back to the land surface or directly to the ocean. Under natural conditions, the cells are hydraulically independent ground-water flow systems. The configuration of the flow cells remains approximately the same from year to year. A computer analysis of the aquifer estimated that the total flow through the six systems is about 270 million gallons per day. Ground-water flow in the vertical dimension is illustrated by the hydrogeologic section through the cape shown in figure 57 .
Flow is nearly horizontal in much of the aquifer.
Vertical flow occurs mainly at recharge areas near water-table highs and at major discharge areas, such as the coast. The lower boundary of the ground-water flow system is bedrock or slightly permeable glacial deposits, such as silt and very fine sand, or the transition, or mixing, zone between freshwater and saltwater in the aquifer. In the northern part of the cape, freshwater is underlain everywhere by saltwater. At Truro, the freshwater zone is about 200 feet thick and it might be thicker elsewhere on the outer cape. Freshwater and saltwater in the aquifer are separated by a narrow transition zone where mixing occurs.