A concept for calculating accumulated clay thickness from borehole lithological logs and resistivity models for nitrate vulnerability assessment

We present a concept that combines lithological information from boreholes with resistivity information from geophysical data to produce an accumulated clay thickness (ACT) estimate as a proxy for assessing the vulnerability of the groundwater to contamination from nitrate. The groundwater's vulnerability to nitrate is strongly dependent on the hydraulic conductivity and the thickness of the protective layers. Low permeable clays in the overburden offer good protection to underlying aquifers by increasing the transit time. This means that the accumulated clay thickness is a good indicator for aquifer vulnerability to nitrate. In geophysically derived resistivity models clays are characterized by low electrical resistivity, but non-unique clay–sand resistivity transition prevents direct mapping of resistivity models to clay thickness. Within the ACT concept, a translator model linking the resistivity to the accumulated clay thickness is calibrated by borehole information, ensuring consistency between the resistivity and the borehole data. An accumulated clay thickness map of the aquifer overburden (e.g., top 30 m) is then calculated, based on the calibrated translator model and geophysically derived resistivity models. We demonstrate the concept on a large-scale nitrate vulnerability assessment survey in Denmark. The concept successfully delineates the clay-dominated areas that play a key role in the assessment of the aquifer's vulnerability to nitrate pollution.