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Sample preservation and analysis of ground waters follow rigid protocols governed by well-established quality assurance/quality control procedures. Well purging requirements are less rigorously defined, making chemical sampling critical in obtaining representative data.
Purging a well for chemical sampling causes local changes in water chemistry in terms of temperature, pH, solubility of gases, and redox potential. Subtle changes over a more widespread area may also occur. Before sampling a new or existing well, hydraulic and chemical stability should be established between the aquifer water and the water within the well. This becomes more important as the concentration of contaminants being evaluated becomes lower or nears the detection limit of the test. The actual physical process of retrieving representative water samples, therefore, is complicated by the chemical implications of short-term aquifer dewatering.
In developing new wells, stability must be verified so that all traces of drilling fluid (where it has been used) and drilling debris are removed from the well and only aquifer water is sampled. This is typically performed by making volume-by-volume analysis of pH, specific conductance, and temperature until successive samples show consistent values. Properly developed existing wells usually reach chemical stability relatively rapidly.
Each monitoring well or set of wells has its own unique hydraulic and chemical characteristics. All wells, after first being drilled, must be purged to determine when equilibrium is reached. It cannot be assumed that removal of a fixed number of casing volumes of water will guarantee stability. Once this stability has been achieved, further tests are still required. These tests will determine the volume of water which must be removed to obtain a representative formation water sample, for each individual well or group of wells, in the same aquifer or overburden situation. It is essential that the evaluation of the hydraulic character of a well be made in conjunction with water chemistry. This evaluation should be made in light of the level of analytical accuracy required, and prior to establishing a standard number of volumes to be removed for purging prior to sampling for any well.
ground water, sampling, water quality, purging, chemical analysis, observation wells, well completion, water removal
Project scientist, Acres International Ltd., Niagara Falls, ON
senior projects manager, ICF-SRW Associates, Inc., Pittsburgh, PA