Have you ever noticed that the temperature of water from wells is remarkably constant? In wells that are from 30 to 60 feet deep, the water temperature is 2° to 3°F above the annual mean temperature of the locality. Water decreases in temperature about 1°F for every 64 feet of depth to the well.
In general, deep wells extend down through an impervious layer to reach an underlying supply. Shallow wells, in contrast, are sunk in easily penetrated strata to a point where they are below the water table. In terms of depth, deep wells are classified as those extending below 25 feet; those going less than this are considered shallow wells. Actually, deep wells vary from 100 to 3,000 feet. The vast majority are in the 100 to 1,000-foot range. Deep well water usually shows but a slight change in composition over a long period of time. In one study of some wells in Florida over a 24 year period, hardness ranged from a high of 342 to a low of 304 parts per million. Alkalinity went from a high of 168 to a low of 148.
Springs provide another source of groundwater. It is a popular belief that spring waters are clear, colorless, sparkling, and absolutely pure. While these facts hold true for many springs, others show a marked degree of turbidity, especially after a heavy rainfall.
Spring waters further contain rather large amounts of dissolved mineral matter and are hard. On the score of potability, no spring water should be considered safe to drink unless it is given periodic bacterial examination.
Other groundwaters could also include mine waters and connate waters. Large quantities of water are found in many mines and must be removed by pumping. In some cases, mine waters are no different than other ground supplies. Generally, however, they have a high sulfuric acid and iron content. As a result, they may be extremely corrosive. Connate water or oil field brines are the remains of ancient seas in which sedimentary rock was at one time deposited. These "fossil waters," as they are sometimes called, are generally highly saline. In the operation of oil fields, they have only nuisance value and present serious disposal problems when brought to the surface.
While groundwater supplies have definite advantages, they also present problems. The important disadvantages are:
- Larger amounts of hardness mineral compounds are found in the groundwater supplies than surface water.
- Iron and manganese are present in many good supplies.
- Hydrogen sulfide is sometimes present.
- The cost of pumping well water is usually greater than that for pumping surface water.
- The mineral content of several wells may differ widely even though located close to each other.
- The supply may be uncertain.
- They may contain nitrate or detergent contamination. The presence of nitrates or detergents in a groundwater supply can indicate pollution from sewage.