Wells, Geology and Contamination

from USGS

A well is simply a hole dug or drilled into the ground from which water can be removed. The hole is called the borehole and is lined with a well casing, which is typically a plastic or metal pipe. The well casing prevents the side walls of the borehole from collapsing into the well and closing the hole. The casing is sealed into place with grout which is usually neat cement or bentonite that was pumped into the annular space between a well casing and the borehole. This grouting seals the well and prevents water from flowing into the well directly from land surface down the side of the well casing pipe or from the shallowest part of the aquifer where the water quality may be less desirable.

Depending on geology, the casing will be open at the bottom or perforated at a specific depth with a screen, allowing water to flow into the well where it can be pumped to the surface. In sandy soils well screens are common. In fractured rock systems and bedrock screens are not necessary on low volume domestic wells. In clay or loam coarse sand or gravel can be placed around the well screen to help improve the flow of water into the well. These sand or gavel packs create larger pore spaces for water to accumulate. Gravel or sand packs are rarely used for domestic wells.

In the United States almost half of all drinking water is supplied by wells. About a third of the population obtains its drinking water from public supply wells which they never think about, and about 15% of the population obtains their household water from private domestic wells. Domestic well owners need to think about their wells and the groundwater that supplies them. Domestic wells have pumps that can pump 10-15 gallons a minute into the pressure tank when needed for household use. These pumps draw groundwater from the area immediately surrounding the well. Depending on the depth of the well and the local geology groundwater drawn into a private domestic drinking water well is typically young water-it could be weeks, months or several years old.

Typically rain water and snow melt percolate into the ground and the deeper the well the further away is the water origination and the older the water. The groundwater age is a function of the depth of the well, the geology of the area, the precipitation, recharge of the aquifer and pumping rates of the aquifer that control the rate of flow of water to a well. The age of the water in an aquifer provides insight into the likelihood of contamination from both anthropogenic and natural sources. Very young groundwater that has recently infiltrated into the aquifer is more vulnerable to contamination from human activities near the land surface than older, deeper groundwater that has had more time to be filtered by soils. Old groundwater, however, is not necessarily free of contaminants. The older groundwater can contain naturally occurring chemical elements and contamination from years past. The land surface through which groundwater is recharged must remain open and uncontaminated to maintain the quality and quantity of groundwater.

Though the most common sources of pollution to groundwater supplies come from two categories; naturally occurring and human activities, groundwater and domestic well water vulnerability to contamination depends on three factors:

1. The presence of man made or natural contaminant sources; for example, a failing septic system or chemicals poured down the drain, or underlying sediments can be sources of contaminants entering groundwater. From the underlying rocks radionuclides and heavy metals can enter the groundwater. There are areas with natural occurring arsenic, cadmium, chromium, lead, selenium and fluoride. 
2. The natural processes in the subsurface that can filter or cleanse the groundwater; for example, microorganisms can break down some chemical contaminants in groundwater like nitrate, contaminants can attach to soil particles and unsorted sediments can cause dispersion of contaminants as they move through an aquifer. 
3. The ease with which water and contaminants can travel to and through an aquifer; for example microorganisms in the soil and from wildlife can travel into groundwater supplies through cracks, fissures, other pathways of opportunity or even through sedimentary and basaltic rocks that are highly fractured and overlain by a thin cover of overburden, while a dense clay layer can reduce groundwater vulnerability by acting as a barrier to the movement of water and contaminants.

The vulnerability and water quality of a well can be vastly different from the quality and productivity of nearby wells. The most common sources of pollution to groundwater supplies come from two categories; naturally occurring ones and those cause by human activities. Naturally occurring contamination are those that are produced from the underlying soil and rock geology and wildlife. Once within an aquifer, contaminants that dissolve in water will travel with the flowing groundwater. What happens next is dependent on the chemical properties of a contaminant, the geology of the area and the flow rate of groundwater. Natural processes such as sorption/desorption, dissolution/precipitation, ion exchange, or biodegradation can reduce contaminant concentrations to effectively clean the groundwater. Many contaminants in the shallow groundwater remain in solution because of presence of oxygen, and the short travel times between the water table and the domestic wells allows contaminants to easily reach the well. In addition, a well might have one or more pathways of opportunity. One of the most common contaminant pathways is the failure of the grouting on the well casing allowing rainwater and snowmelt (carrying dirt and other contaminants) directly enter the well.

Nitrate concentration are often elevated in shallow groundwater because of agricultural and suburban development. Bacteria and nitrates contamination to groundwater can be caused by human and animal waste. In our own neighborhoods septic systems, horses, backyard poultry can cause these problems perculating into the ground or finding an opportunistic pathway through a fissure or other geological entry. On a regional level small lots and dense population of septic systems or large animal or fertilized farm operations can cause problems. Heavy local use of pesticides for ornamental gardens, leaks from underground fuel tanks can be sources of contamination. Households can introduce solvents, motor oil, and paint, paint thinner, water treatment chemicals and others substances by not maintaining our septic systems, or pouring chemicals into the ground or down the drain. Groundwater-quality protection depends on the entire community, what my up gradient neighbor does could impact my water quality. If residents and businesses take steps to reduce input of anthropogenic contaminants to the groundwater, water quality can be improved because of the short travel times between the water table and the well. The opposite is also true. 

from USGS