Coupled MODFLOW–MT3DMS simulation of nitrate transport in the Plana de Valencia coastal aquifer (Spain): calibration to monitoring wells and implications for groundwater–wetland interactions

Nitrate contamination is one of the dominant groundwater-quality pressures in Mediterranean irrigated plains. In the Plana de Valencia (eastern Spain), intensive agriculture and irrigation return flows coexist with strong groundwater–surface water connectivity and ecologically sensitive receptors such as the Albufera wetland system. We present a regional-scale numerical framework that couples groundwater flow and conservative solute transport to reproduce observed nitrate dynamics and to provide a basis for scenario testing of mitigation measures.

A previously developed transient groundwater-flow model of the Plana de Valencia was implemented in the ModelMuse graphical environment and used as the hydraulic driver for MT3DMS multi-species transport simulations. The hydrogeological conceptualization comprises four model layers representing (i) highly permeable Quaternary detrital deposits, (ii) Tertiary formations of intermediate permeability (limestones/sandstones/conglomerates), (iii) low-permeability Miocene marls, and (iv) permeable Mesozoic carbonates over an impermeable Keuper basement. External stresses include spatially distributed recharge (rainfall infiltration and irrigation returns), exchanges with rivers, channels and wetlands (“ullals”), pumping abstractions, and lateral boundary transfers, consistent with a predominantly inland-to-coast hydraulic gradient.

Agricultural nitrate inputs were spatially allocated using land-use information (CORINE) and fertilization constraints from the regional regulatory framework, translated into gridded source terms (1 km × 1 km) and applied through the sink/source mixing package. Simulations covered 1980–2017 and were evaluated against nitrate time series from the Júcar River Basin Authority monitoring network at multiple observation wells across both the northern and southern sectors of the aquifer. Manual multi-well calibration produced acceptable agreement in most wells; a classification of fit quality indicates good performance for 8 wells and poor performance for 3 wells, with no clear spatial clustering of misfits, suggesting the need for local refinement or parameter regionalization. Model performance improved after reducing the fraction of applied nitrate reaching groundwater from 10% to 7%, and the best correspondence commonly occurred in the second layer (typical monitoring depths ~40–50 m).

Results confirm widespread exceedance of the 50 mg/L threshold in the majority of wells and a generally increasing temporal trend even under regulated application rates, highlighting the risk of persistent degradation and potential downstream impacts on the Albufera system. The proposed model constitutes a transferable decision-support baseline for testing management scenarios (fertilization control, irrigation efficiency, drought sequences, and saltwater intrusion) and for advancing toward automated calibration and uncertainty quantification.