Sampling in groundwater hydrochemical and isotopic studies: the importance of well logs knowledge for data interpretation

Abstract

This work discusses about the importance of knowledge of the well logs for groundwater sampling to hydrochemical and isotopic studies. Among the most important aspects of soil and geological logs, the following stand out: detailed description of the soil, saprolite and rock profile, including fractures, contacts, lithology changes, etc. Within the scope of the construction logs, the following information are essential: static level, casing depth, screen sections positioning, not-cased portion in rock, sanitary protection insolation, grain size and composition of gravel pack among others. The lack of adequate or minimal knowledge of well profiles can generate ambiguity in the results, erroneous interpretations or even make the evaluation of analytical data unfeasible. Although, apparently, it is an obvious aspect in groundwater studies, numerous technical and scientific papers and reports do not even refer to the descriptions of the wells used to obtain samples. In addition to the knowledge of the logs, care should be given to good practices during sampling, with special attention to the purging of the wells prior to obtaining the samples. This work also brings situations in which sampling compromises interpretations and others in which, even with reduced data, it was possible to achieve satisfactory results (due to the care in the sampling points and the use of appropriate techniques).