Surface water is vulnerable to a wide range of natural and human contamination, and will almost certainly need extensive treatment in order to be potable.
Assessing water quality can be difficult, as the water will vary greatly in quality depending on season, weather events, human influence, and natural cycles and issues. Before choosing to use a surface water source, determine the origin of the water body and find out what kinds of activities happen upstream from your intake, in particular industries, sewage treatment facilities, and major highways. Each of these can introduce contaminants. If the water body is subject to seasonal flooding and/or drought, this will also affect quality by introducing new contaminants or concentrating those already existing.
There are two types of wells, but both operate on the same principle. An underground aquifer is accessed by digging or drilling an intake point into an area with sufficient flow to provide the required quantity of water.
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Dug Wells These wells tend to be shallow, between 10-40 feet. A hole is excavated (by hand or mechanically), and the sides of the hole shored up and retained by sidewalls. Water in the ground is able to collect inside the well, and it is pumped or lifted from this reservoir.
Drilled Wells Used to access deep aquifers (between 40-400 feet), these wells are mechanically drilled (through any type of soil or rock) until sufficient water has been reached. A metal well pipe is fitted into the hole, and water from the aquifer fills some portion of this pipe. A submersible pump is lowered into the pipe and sits in the water.
Drilled wells may supply potable water with no need for treatment. Deeper wells tend to have fewer issues with bacterial contamination, but are often rich in mineral content. Depending on the composition of the rock around the aquifer, odor and taste of the water can be affected, and the interior of piping can suffer build-up of mineral scale. Treatment may be required to remove mineral content.
Drilled wells may be contaminated by sources far from the intake. The movement of underground aquifers is not well mapped, and water can travel long and circuitous paths. Deep wells tend not to be affected by seasonal changes, and a well that provides clean water is likely to continue to do so unless new human activity somehow affects it.