Land-Use, Human Activity Impact Aquifers and Drinking Water Supplies

A study by the U.S. Geological Survey (USGS) has determined that 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, Fla., 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% 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.

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," said 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 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 semiconfined. Compounds associated with human activity were found at almost double the rate in water from wells where the aquifer was unconfined as compared to semiconfined 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.