Twenty of the 270 wells sampled in 1993-99 as part of the U.S. Geological Survey (USGS) National Water-Quality Assessment Program in Pennsylvania were resampled by USGS as part of a monitoring program conducted in cooperation with the Pennsylvania Department of Agriculture. These 20 wells all had waters with detectable concentrations of pesticides. Twelve of these wells representing limestone and dolomite (carbonate-rock) aquifers in the Piedmont and Ridge and Valley Physiographic Provinces and eight wells representing siliciclastic aquifers in the Piedmont Physiographic Province were sampled twice in 2003-05. Water samples were analyzed by gas chromatography/mass spectrometry to determine concentrations of up to 83 parent compounds and degradation products. Alachlor and metolachlor along with atrazine, prometon, and simazine were the parent compounds detected. Well waters with the largest atrazine, simazine, and metolachlor concentrations (greater than 1 microgram per liter) in 1993-99 had significantly smaller concentrations by an order of magnitude when resampled in 2003-05. Overall, changes were generally of less magnitude with a consistent tendency to smaller concentrations with time. Median decrease in concentration for samples with atrazine and simazine detected was 25 and 4 percent, respectively. Median decrease in concentration for samples with metolachlor detected was 2 percent. Results from the resampling also indicated waters with small concentrations (less than 0.02 micrograms per liter) of a parent compound or degradation product retained the small concentrations over time.

Three additional wells completed in carbonate-rock aquifers with waters having unusually large concentrations (greater than 5 micrograms per liter) of pesticides are being sampled for five years (2003-07) at four water-level conditions (stable low, winter rise, rise due to spring flush, and declining) to evaluate long-term and seasonal changes in pesticide concentration. Results for the first three years show overall declines in concentration for alachlor in water from one well and metolachlor in waters from three wells. Acetochlor, atrazine, and simazine show little change in concentration.