As populations increase around areas with public water-supply wells in the northern Tampa Bay region, there are corresponding increases in contamination. According to a study by the U.S. Geological Survey (USGS), human activities are impacting ground water resources.
In the first phase of the study, 30 randomly selected public-supply wells were sampled prior to treatment and analyzed for the presence of 258 compounds generated by humans such as pesticides and volatile organic compounds (VOCs). The northern Tampa Bay area was selected for study because a large percentage of the population relies on ground-water resources from the Upper Floridan aquifer for drinking water supply.
Of the 258 sampled compounds, 31 were detected in wells prior to treatment. Samples from the wells generally contained a mixture of compounds (average of 4 compounds) and 70 percent of the samples had at least one compound detected. Concentrations were low (less than 1 microgram per liter), well below the potential for human health concern, and were several orders of magnitude below the level of toxicity for drinking-water standards set by the U.S.
Environmental Protection Agency. The relative levels of contamination were closely linked to land-use type and to the amounts and types of chemicals used in each setting. For example, urban areas showed widespread occurrence of pesticides commonly used around the home and gardens, golf courses and public road right-of-ways. These common pesticides included atrazine (including the breakdown products 2-hydroxyatrazine and deethylatrazine), simazine and prometon.
In the second phase of the study, wells that had the highest levels of contamination were resampled before and after treatment. The pesticides most frequently detected both prior to and after treatment were atrazine and its breakdown products. All detections were at very low concentrations (less than 0.03 micrograms per liter).
"We are seeing the effect of human activity and land-use practices on our ground water supplies," says Patricia Metz, USGS hydrologist and lead author on the report. "Although concentrations are very low, their presence indicates the relatively rapid mobility of these contaminants to the ground water system and the vulnerability of ground water supplies to contamination from human activities."
The study examined the relation between the occurrence of the contaminants and land-use, population and local hydrogeologic conditions. In the northern Tampa Bay area the Upper Floridan aquifer ranges from being unconfined to semi-confined. In areas where the aquifer is unconfined, it is more open to recharge from land surface and therefore more vulnerable to impact from human land-use activities. In this study, half of the 30 water supply wells were located in areas where the aquifer was unconfined, the other half in areas where the aquifer was semi-confined. Compounds associated with human activity were found at almost double the rate in water from wells where the aquifer was unconfined as compared to semi-confined conditions.
The study also found that a significant relation exists between population and the number of contaminants detected. Where population and human development was limited, such as large well fields, little to no anthropogenic compounds were detected.
"Concentrations of specific compounds in ground water depend on a number of factors," says Metz. "The hydrogeology plays an important role in allowing these compounds to migrate from land surface into the ground water system. Both the unconfined nature of the aquifer and the higher population are determining factors in the number of anthropogenic compounds detected." Metz adds, "For the past several decades we've seen land-use changes that may affect the future of our potable ground-water supplies." Studies like these can provide information to resource managers and decision makers.
Other key findings include:
- Chloroform (disinfection by-product)
was the most commonly detected VOC and compound detected in the study
(detection frequency 43 percent); it was most frequently detected in
residential areas. Chloroform detected in residential areas may be
associated with lawn irrigation, leaking of supply lines, pools, and spas.
- Atrazine (herbicide used in lawn care) and its degradates (deethyatrazine; 2-hydroatrazine, and deisopropylatrazine) were the most commonly detected pesticides (54 percent combined detection frequency); most frequently detected in residential areas. Atrazine's detection in residential areas is commonly associated with application of lawn maintenance chemicals.
- DEET was detected in five source-water samples
(17 percent detection frequency); commonly found when sampled wells were near
septic systems.
- Wells sampled in well fields such as Cross Bar,
Cypress Creek, Eldridge Wilde and Cosme where the population and the land-use
development is limited, little or no human-generated compounds were detected in
the ground water.
- One-on-one comparison between source water and
the associated finished water (non-blended) found certain compounds still were
detected after the treatment process (for example, atrazine and its breakdown
products, bentazon, imidacloprid, tebuthiuron and caffeine were found in the
treated ground water); again, these were detected at very low levels.
- Treated water generally had higher concentrations of human-generated compounds due to the disinfection treatment process.
