“1. Infiltration—by intercepting underground currents through natural formations of beds or banks of water-courses.
“2. Filtration—mechanically by artificial beds of sand, gravel, etc., chemically by charcoal, iron, etc.
“3. Subsidence—clarification by deposition; storage reservoirs.
“4. Aeration—spontaneous purification by oxidation.
“5. Covered reservoirs—to prevent atmospheric influences.
“6. Precipitation of carbonates—Clark’s System.”
The infiltration system is resorted to where natural means for permeation are found; the galleries for intercepting the water being constructed in the sand or gravel banks or bed.
The clarification, however, is necessarily restricted, owing to the general high rate of filtration.
Lowell, Mass., has a gallery in the gravel banks of the Merrimack River, 1,300 feet in length, 8 feet by 8 feet, the bottom 8 feet below the river bed. The capacity is six million gallons, and rate of flow 150 gallons per square foot in twenty-four hours.
Lawrence, Mass., has a similar gallery.