Schematic cross section of the Edwards Aquifer during storm events
Schematic cross section of the Edwards Aquifer during storm events

Water quality from wells drilled into  the Edwards Aquifer can vary substantially depending on location, well depth, well construction, and hydrologic conditions (high flows, low flows, storm events).  Recharge, which refers to water replenishing an aquifer, can also dramatically influence water quality—this is especially true in the Edwards Aquifer.

If we assume wells are constructed properly and have not deteriorated, there is a lot of natural variability in water quality from wells drilled into the Edwards Aquifer. The Edwards Aquifer has a very high recharge rate compared to other aquifers in Texas due to the presence of caves and sinkholes (known as karst).  This has both good aspects, and potentially negative ones.

The positive aspect of rapid recharge into the karstic Edwards Aquifer is that the aquifer can be replenished rapidly, and also can store a lot of water. However, the negative aspect of rapid recharge is the lack of natural filtration, and removal of contaminants, through porous materials such as thick soil and sands, before entering the aquifer.  So the positive is that water can quickly replenish the water supply, but the negative is that the quality of that water can be directly influenced by contaminants in storm water (Figure 6).

Storm water carries a variety of contaminants from the surface.  The most common contaminants include sediment, bacteria, nutrients, and other trace metals.  Sampling well water for bacteria and nitrates, especially after rain events, can indicate if there is a rapid groundwater and surface water connection.  Bacteria and nitrates are good indicators for a direct connection with surface water flows and can alert a well owner that additional testing and treatment (such as reverse osmosis or chlorination) is warranted.

The District facilitates an annual Water Well Check-up in the Spring to screen samples from private water wells for common contaminants, including fecal coliform bacteria and nitrates.  Additionally, there are a number of local laboratories that offer residential well water analyses.  If the well is determined to have a surface water connection, additional water treatment may be appropriate to make the water safe to drink.  A comprehensive water sample analysis could help identify the level of treatment.

The U.S. Environmental Protection Agency’s rules that protect public drinking water systems do not apply to individual water systems, such as privately owned wells.  As an individual water system owner, it is up to the well owner to make sure that the water is safe to drink.  Wells should be checked and tested annually for mechanical problems, cleanliness, and the presence of certain contaminants, such as coliform bacteria and nitrates.  Contaminants such as these can jeopardize the health of its users, especially vulnerable populations like children, the elderly, or those with compromised immune systems.  Well water should be tested more than once a year if there are recurrent incidents of gastrointestinal illness among household members or visitors and/or a change in taste, odor, or appearance of the well water.  With increasing development, the quality of groundwater in our area is at greater and greater risk of contamination from a variety of sources.

For more resources for well owners including the District Well Owner Guide, information on the 2014 Water Well Check-up, and details about local laboratories accepting residential water well samples visit: