Ground-water tracers, development

Water-Tracing Compounds. Another application of these fluorescent pyridones has been the development of a series of coaipounds useful as ground-water tracers. Smart and Laidlaw have outlined several qualities desirable in fluorescent water-tracing compounds (.18). 

The more recent development of sophisticated ground-water flow and transport modeling software and the availability of powerful computers allow a much more detailed modeling of the tracer evolution in ground-water systems. Consequently, applications of tracer data in conjunction with multi-dimensional flow and transport models began to show up in the 1990s (Reilly et al. 1994 Szabo et al. 1996 Sheets et al. 

Of course, inverse-modeling approaches can also be used to determine the parameters of numerical ground-water transport models from fits to observed tracer data. Estimates of residence times for a number of locations in an aquifer provide a powerful calibration target for numerical ground-water transport models. If enough data are available to constrain the numerous unknowns in such models, this is presumably the most effective way to extract useful information from tracer data, in particular because the numerical models can be used to make predictions for the future development of the investigated system. Such predictions may become much more constrained and trustworthy if the model has been calibrated against tracer data. 

As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

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