COMPARING IN SITU SAMPLING METHODS IN A DEEP GROUNDWATER WELL

Collecting groundwater samples that are representative of deep formation waters with reproducible results over time can be a challenge. Wells with depths greater than 500 feet create a larger challenge due to limitations related to pump capabilities and the large volumes of purged water that must be managed. Two different in situ sampling technologies were evaluated to determine their usefulness for groundwater sampling from great depths. Two groundwater samples collected with equilibrium-membrane type passive samplers were compared to six groundwater samples consecutively collected using a custom-built depth-discrete bailer. Samples were collected at 590 feet below water in a 5 ½ inch uncased well. Evaluating major ion concentrations across all samples and methods showed a coefficient of variation <4% for each major ion. Concentrations of trace element ions showed more variability. Comparison of all samples and methods for 25 different trace elements revealed 10 trace element ions had a coefficient of variation >10%. Looking at the individual in situ sampling methods, the two equilibrium-membrane type passive samples had 4 ions and the six depth-discrete bailed samples had 9 ions with coefficient of variation >10%. Evaluating the last three depth-discrete bailed samples improved the reproducibility by having only 2 ions with coefficient of variation >10%. These preliminary findings indicated that both sampling methods provided stable major ion results that were reproduced with each consecutive sample. Trace ion concentrations were relatively stable for the equilibrium-membrane type passive sampler, but the depth-discrete bailer results indicated that some removal of well water was necessary before stability was reached. Additional work is being completed to assess isotope ratios and concentrations, as well as to compare these results to a pumped and purged sample.