The decrease can be seen in the more recent sediment deposits in the upper part of the core compared to the baseline concentrations in the oldest sediments in the bottom of the core. Radiochemical dating is used to determine the time represented by specific points along the length of the core. Analysis of this signature can determine the time frame when leaded gasoline was being used in the basin. By tracing the history of the lead concentrations in the core, USGS hydrologists can tell if there has been a change in the percentage of lead present in sediments deposited in more recent times, as compared to the earlier sediments. Looking at trends can improve understanding of the influence of human activities on water-quality conditions.
Trends can also indicate the effectiveness of environmental regulations, like the Clean Air Act.
Another important function of information on water-quality trends is to signal a warning of additional degradation of water quality in the future, for which additional study or action might be taken. Note recent declines due to switch to unleaded gas. Zinc is a heavy metal that can be toxic. An intriguing finding that came out of the USGS reservoir coring study was the perplexing pattern of the zinc curve. While lead showed a reassuring and rather dramatic drop in its presence in the environment, strongly correlated to the removal of lead from gasoline, the picture for zinc is quite different.