Evaluating factors for testing groundwater quality
The source for drinking water is an important consideration when determining which water quality tests to run. Groundwater sources, especially private wells that are not tested on a regular basis, are susceptible to various naturally occurring and manmade contaminants.
The depth of a well is an important factor when determining which contaminants are present in groundwater sources. Shallow wells are more vulnerable to surface contaminants, such as gasoline from a spill, because the contaminant has less distance to travel.
The deeper the well, the more the soil and surrounding geological formations act as a filter for surface contaminants. Additionally, certain geologic formations can slow contaminants from entering aquifers. For example, contaminants can easily pass through sand, but clay offers a layer of protection. Deeper wells may be subject to more naturally occurring contaminants, such as minerals, arsenic, iron, manganese and various radiologicals.
The location of a well also can help determine possible contaminants. Consider which potential sources of contaminants are present in the vicinity of the well. Some common sources include septic tanks, underground storage tanks, animal feedlots, gas stations, laundromats, landfills, golf courses and farmland.
Septic tanks and animal feedlots pose bacteria and nitrate concerns, while gas stations and underground storage tanks typically contribute volatile organic contaminants like benzene, toluene or xylene.
When dealing with farmland and golf courses, pesticides and herbicides are a concern. Keep in mind that there are thousands of pesticides and herbicides in use, so try to determine what is or was being used in the area, as testing can become expensive if you cannot narrow down the possibilities.
As discussed, geologic formations can contribute contaminants to groundwater, so it is important to determine what is present in your area. When it comes to determining local geological formations, the U.S. Geological Survey is a primary resource for studies and maps of naturally occurring and manmade contaminants. These maps can help determine which contaminants may be present in your geographic area. Visit www.usgs.gov  for more information.
When considering testing for manmade contaminants such as volatile organics, pesticides or herbicides, there are several properties to take into account. These properties include persistence, adsorption, solubility, volatility and molecular size.
Persistence. Persistence refers to how long a contaminant stays in the environment. Certain contaminants do not break down easily and will stay in the environment for a long time. Manufacturing and distribution of polychlorinated biphenyls, for example, have been prohibited since 1976, yet they are still found in the environment today.
Adsorption. Adsorption refers to how tightly a compound attaches to soil particles. Compounds that are strongly adsorbed are less likely to leach into groundwater.
Solubility. Solubility is the ability of a substance to dissolve in a solvent. Compounds that are highly water soluble easily dissolve in water that is percolating through soil down to the water table. Methyl tertiary butyl ether (MTBE) is an example of a contaminant with high water solubility, which is one of the reasons MTBE contamination is so widespread.
Volatility. Volatility of a substance refers to its tendency to change from a liquid or solid into a gas. One group of contaminants called volatile organic chemicals readily changes from liquid or solid to gas when exposed to the atmosphere. The more volatile a substance is, the more likely it is to be lost to the atmosphere.
Molecular size. Finally, the size of the molecule can play a role in contamination. The smaller the molecule, the more likely it is to travel between soil particles.
Another factor to take into account is the weather. Heavy rainfall or drought play a role in groundwater contamination. Significant rainfall means more water will percolate down into the aquifer; more rain can carry more contaminants. Heavy rainfalls or flood conditions that cause water to pool around a wellhead may contribute bacteria to the well itself, so bacteria testing is recommended.
Areas experiencing drought conditions also can experience a dramatic change in water quality. During a drought, the water table can drop, exposing the water to different geologic formations that may add more minerals or radiologicals to the aquifer.
Physical, environmental and chemical factors all affect groundwater quality and need to be considered when testing source water. Considering all of the factors mentioned above will help in choosing a test package that best fits local groundwater sources and provides a thorough analysis of all possible contaminants.