Drilled vs. dug vs. driven wells

Not every water well is drilled. Older properties in particular may have a dug or driven well, and the type matters for water safety, reliability in drought, and what a buyer or owner should check. This comparison draws on the U.S. Environmental Protection Agency (EPA) and the National Ground Water Association (NGWA).

What are the types of water wells?

Sources for the table: EPA: Learn About Private Water Wells and NGWA (Wellowner.org): Types of Wells. [EPA-LEARN source note] [NGWA-TYPES source note]

What is a dug well?

Historically, dug wells were excavated by hand below the water table, lined and capped. Their large diameter exposes a big area of the aquifer, and bored wells can draw water from fine materials such as silt and clay. But they are shallow and lack continuous casing and grout, and they can go dry during drought if the water table drops below the bottom of the well. NGWA (Wellowner.org): Types of Wells [NGWA-TYPES source note]

What is a driven well?

Driven wells are made by driving a small-diameter pipe into shallow water-bearing sand or gravel. They are relatively simple and economical, but they can only reach shallow water and are easily contaminated from nearby surface sources because they are not sealed with grout. NGWA (Wellowner.org): Types of Wells EPA adds that they draw water from aquifers near the surface. EPA: Learn About Private Water Wells [NGWA-TYPES source note] [EPA-LEARN source note]

Why are drilled wells the usual choice?

Drilled wells are built with cable tool or rotary rigs and can exceed 1,000 feet. In loose material they need casing and a screen, and the space around the casing is sealed with grout to keep contaminants out. NGWA (Wellowner.org): Types of Wells EPA notes their lower contamination risk comes from their depth and continuous casing. EPA: Learn About Private Water Wells For how the drilling itself is done, see well drilling methods. [NGWA-TYPES source note] [EPA-LEARN source note]

Drilled vs. dug well: which is safer?

Generally the drilled well, for two reasons EPA gives: depth and continuous casing. EPA: Learn About Private Water Wells A dug well's wide, shallow opening and unsealed lining let surface water reach the supply more easily, which is why dug wells are more prone to bacteria after heavy rain or snowmelt. That doesn't make every dug well unsafe, but it does make regular testing more important. The CDC recommends testing well water at least once a year for total coliform bacteria, nitrates, total dissolved solids and pH. CDC: Guidelines for Testing Well Water For a shallow well, also test after floods and any time the water changes; see how often to test well water. [EPA-LEARN source note] [CDC-TEST source note]

Replacing a dug or driven well

If a shallow well keeps failing tests or running short, replacing it with a drilled well is a common fix. Plan the new well's location to meet current setbacks, budget the full system, and arrange for the old well to be properly decommissioned by a qualified contractor, as NGWA's owner checklist advises. NGWA (Wellowner.org): Homeowner's Maintenance Checklist Deepen an existing well or drill a replacement covers the decision. [NGWA-CHECKLIST source note]

What should owners and buyers of each type check?

• Testing matters more for shallow wells. Test at least yearly, and after floods or changes in taste, color or smell. See private well water testing.
• Watch shallow wells in drought. A dug well's supply depends on the water table staying above its bottom.
• Check local rules before building. Some jurisdictions restrict shallow well types for drinking water or require specific casing depths; EPA notes that some states and localities set minimum casing lengths.
• Replacing an old well means the old one must be properly decommissioned, not just left in place.

For the components that make a drilled well sanitary, read well casing and completion.