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. Effective porosity plays an important role in contaminant management.However, the effective porosity is often assumed to be constant in space and henceheterogeneity is either neglected or simplified in transport modelcalibration. Based on a calibrated highly parametrized flow model, athree-dimensional advective transport model (MODPATH) of a 1300 km 2 coastal area of southern Denmark and northern Germany is presented. Adetailed voxel model represents the highly heterogeneous geologicalcomposition of the area. Inverse modelling of advective transport is used toestimate the effective porosity of 7 spatially distributed units basedon apparent groundwater ages inferred from 11 14C measurements inPleistocene and Miocene aquifers, corrected for the effects of diffusion andgeochemical reactions. By calibration of the seven effective porosity units,the match between the observed and simulated ages is improved significantly,resulting in a reduction of ME of 99 % and RMS of 82 % compared to auniform porosity approach. Groundwater ages range from a few hundred years inthe Pleistocene to several thousand years in Miocene aquifers. The advectiveage distributions derived from particle tracking at each sampling well showunimodal (for younger ages) to multimodal (for older ages) shapes and thusreflect the heterogeneity that particles encounter along their travel path.The estimated effective porosity field, with values ranging between 4.3 %in clay and 45 % in sand formations, is used in a direct simulation ofdistributed mean groundwater ages. Although the absolute ages are affected byvarious uncertainties, a unique insight into the complex three-dimensionalage distribution pattern and potential advance of young contaminatedgroundwater in the investigated regional aquifer system is provided,highlighting the importance of estimating effective porosity in groundwatertransport modelling and the implications for groundwater quantity and qualityassessment and management.

hydrology and earth system sciences

Estimation of effective porosity in large-scale groundwater models by combining particle tracking, auto-calibration and &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;C dating