Concentrations of total (unfiltered) mercury (Hg) exceed the Maximum Contaminant Level (2 µg/L) in the acidic water withdrawn by more than 700 domestic wells from the areally extensive unconfined Kirkwood-Cohansey aquifer system. Background concentrations of Hg generally are <0.01 µg/L. The source of the Hg contamination has been hypothesized to arise from Hg of pesticide-application, atmospheric, and geologic origin being mobilized by some component(s) of septic-system effluent or urban leachates in unsewered residential areas. Initial results at many affected wells were not reproducible upon later resampling despite rigorous quality assurance, prompting concerns that duration of well flushing could affect the Hg concentrations. A cooperative study by the U.S. Geological Survey and the New Jersey Department of Environmental Protection examined variability in Hg results during the flushing of domestic wells. Samples were collected at regular intervals (about 10 minutes) during flushing for eight domestic wells, until stabilization criteria was met for field-measured parameters; the Hg concentrations in the final samples ranged from about 0.0005 to 11 µg/L. Unfiltered Hg concentrations in samples collected during purging varied slightly, but particulate Hg concentration (unfiltered – filtered (0.45 micron capsule) concentration) typically was highly variable for each well, with no consistent pattern of increase or decrease in concentration. Surges of particulates probably were associated with pump cycling. Pre-pumping samples from the holding tanks generally had the lowest Hg concentrations among the samples collected at the well that day. Comparing the newly obtained results at each well to results from previous sampling indicated that Hg concentrations in water from the Hg-contaminated areas were generally greater among samples collected on different dates (long-term variations, months to years) than among samples collected on the same day (short-term variations, minutes to hours). The long-term variations likely are caused by changes in local pumping regimes and time-varying capture of slugs of Hg-contaminated water moving on flowpaths.