New Mexico Geological Society Annual Spring Meeting — Abstracts

Chemical analyses of 46 time-dependent samples of ground water from 11 wells in 4 geologic settings illustrate the problem of obtaining a "representative water sample." Samples were obtained the first time 7 new wells were pumped and during pumping tests of 4 old wells. Pumping rates ranged from <1 gpm to >200 gpm. Pumping periods ranged from <60 minutes to 2880 minutes. From 3 to 6 samples were collected from each well. The constituents analyzed for included: SiO₂, Ca, Mg, Na, K, Fe, Mn, HCO₃ + CO₃, SO₄, Cl, F, NO₃, P₂O₅, Se, As, Pb, Zn, Co, and Cu plus pH and specific conductance. Not all samples were analyzed for all constituents. The concentration of a specific constituent in serial samples from a well might: (1) remain the same in all samples, (2) vary systematically, or (3) differ inexplicably between samples. The systematic variations seem to represent the production of water from the casing, water in close proximity to the well, or water with components that have longer flow paths. The systematic variations reflect the use of galvanized pipe for the pump column, the effect of the atmosphere on the dissolved gasses in the water stored in the casing and terrain factors. The random variations may reflect sampling procedures, analytical techniques, or actual variations in the water sampled.