Leveraging Drilled Well Data into a Modified DRASTIC Framework for Groundwater Vulnerability Mapping in Estonia

Groundwater is the primary source of Estonia’s drinking water, but its vulnerability remains under-characterized across regions that lack detailed mapping. Current assessments rely on a modified DRASTIC method based on field-based geological mapping, which so far covers only about a third of Estonia. The EU Water Framework Directive and the ongoing development of a new nationwide, data-driven risk assessment methodology have highlighted the need for alternative approaches to assess groundwater vulnerability – particularly in areas where existing maps are outdated or unavailable.

This study introduces a further adaptation of the modified DRASTIC method, leveraging Estonia’s extensive database of drilled wells to evaluate groundwater vulnerability on a national scale. Drilled well logs contain detailed information on local geological and hydrogeological conditions, which, once interpreted, inform DRASTIC parameter values.

A Python-based data processing workflow, incorporating a natural language processing routine, will be used to automatically extract and classify thousands of unique Quaternary sediment descriptions. Subsequently, a combination of Python and open-source GIS tools will be used to develop a semi-automated geospatial model to compute vulnerability indices for each individual drilled well site. The model’s performance will be evaluated in regions with established vulnerability maps to ensure calibration against existing field-based results. Finally, a customized kriging-based interpolation method will be used to generate region-wide vulnerability surfaces from the data points, which will undergo further validation and refinement by comparison with known maps.

Preliminary results indicate that well-based vulnerability scores align closely with those produced by the current, more detailed DRASTIC methodology, suggesting this approach could be a viable alternative for assessing groundwater vulnerability in unmapped areas. Using data from drilled wells allows for the flexible inclusion of multiple layers of Quaternary deposits, rather than limiting assessments to the uppermost layer. This enables the consideration of deep layers of clays and silts, potentially offering more accurate assessments compared to the current method in some areas. However, findings also suggest that certain DRASTIC parameters may require adjusted weightings or redefinition to better capture local variability.

By integrating digital drilled well logs with open-source GIS and coding tools, this methodology provides a practical alternative for large-scale groundwater vulnerability mapping in areas where other relevant spatial datasets are not available. The approach offers a broad coverage and leverages an already available, constantly growing dataset, possibly enabling continuous, near-automatic vulnerability reassessments using the most up-to-date data. This study reaffirms the value of modifying established methodologies like DRASTIC to account for new data formats, providing a flexible framework for improving groundwater management practices.