As a well is pumped through time, concentrations of specific constituents in the water discharging from the well may change as a result of their transport within the well and the aquifer. A series of experiments conducted at a research site on Cape Cod, Massachusetts, examined the effects of transport on the chemistry of water samples obtained from a long-screened well. Analyses of time series of constituent concentrations in water pumped from the long-screened well showed persistent temporal trends during the first experiment. Iron concentrations decreased over a five-hour test (15 casing volumes), whereas the calcium and magnesium concentrations increased. In contrast, the time series of constituent concentrations of water discharging from the same well showed less change with time during a later experiment. Numerical simulations were undertaken to test the relative importance of several possible factors affecting the temporal variations of these constituents. During the process of quantitatively explaining the changes in concentrations over time observed in the two experiments, different system conceptualizations were used, including (1) flow and transport in the aquifer without wellbore transport, (2) flow and transport in the aquifer with advective flow and transport in the wellbore, and (3) flow and transport in the aquifer with advective flow and transport in the wellbore and a thin layer (skin) of water surrounding the well with constituent concentrations that had been altered by the presence of the well. The conjectured skin of wellbore water, which could have invaded the aquifer because of nearby sampling or dispersion and diffusion near the wellbore, in conjunction with flow and transport in the aquifer and advective transport within the wellbore, produced a reasonable match between the simulated and observed concentrations. The data analysis confirms the known fact that long-screened wells provide mechanisms for the redistribution of chemical constituents in the aquifer. The analysis provides new insight into the mechanisms responsible for the observed changes in concentrations during the sampling of long-screened wells. The analysis quantifies the effects of the various conceptual models of flow and transport in and around wells on the interpretation and meaning of chemical analyses of water withdrawn from long-screened wells.A series of experiments was conducted at a research site on Cape Cod, Massachusetts, to examine the effects of transport on the chemistry of water samples obtained from a long-screened well. A time-series analysis of the constituents concentration in pumped water revealed a persistent temporal trend during the first experiment. Iron concentrations decreased over a five-hour test, while calcium and magnesium concentrations increased. Numerical simulations were undertaken to test the relative importance of several possible factors affecting the temporal variations of the constituents to quantify the effects of the various conceptual models of flow and transport in and around the wells.