Laudi et al. (2026) Tracing nitrate fate in Malta’s hydrogeological system using an intensive vadose-groundwater monitoring network

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2026
• Date: 2026-02-02
• Authors: Luca Laudi, Ofer Dahan, Manuel Sapiano, Michael Schembri, Luke Daniel Galea, Ella Busuttil, John Mangion, Tuvia Turkeltaub
• DOI: 10.1016/j.ejrh.2026.103162

Research Groups

• The Zuckerberg Institute for Water Research, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Israel
• Energy and Water Agency, Malta
• Private Consultant, Malta

Short Summary

This study integrated five years of vadose zone and groundwater monitoring in Malta to identify dominant non-point nitrate pollution sources from various agricultural systems. It found that potato cultivation is the primary source of nitrate loading to the Mean Sea Level Aquifer, while intensive vegetable and greenhouse farming create contamination hotspots in perched and coastal aquifers, with vadose zone thickness and nitrate storage being key controls on aquifer vulnerability.

Objective

• To assess the impact of agricultural land use on nitrate concentrations in Malta’s groundwater.
• To identify the key factors governing the vulnerability of groundwater to nitrate contamination in the Maltese Islands.

Study Configuration

• Spatial Scale: Maltese Islands, focusing on three main groundwater systems (Mean Sea Level Aquifer, Blue Clay perched aquifers, Coastal aquifers). Sixteen vadose zone monitoring stations were installed beneath six representative crops (potatoes, fodder, vegetables, vineyards, greenhouses, orchards). Groundwater samples were collected from 30 boreholes in the MSLA, 10 in Coastal aquifers, and 12 in Blue Clay perched aquifers/springs.
• Temporal Scale: Five years of integrated vadose zone and groundwater monitoring. Groundwater samples were collected from 2021 to 2024 (bi-annually for MSLA/Perched, monthly for Coastal from November 2022 to November 2024). Vadose zone pore water samples were extracted monthly. Meteorological data covered October 2021 to September 2024.

Methodology and Data

• Models used: Modified DRASTIC (specifically DRSTVZNCL for the Mean Sea Level Aquifer and DRSTVZN for Coastal and Blue Clay perched aquifers).
• Data sources:
  • Vadose Zone Monitoring System (VMS) network: 16 stations providing continuous measurements of soil and rock water content (Flexible Time-Domain Reflectometry sensors) and monthly pore-water sampling for chemical analysis (Vadose Zone Sampling Ports) from various depths (up to 20 meters or to the water table).
  • Groundwater monitoring: Over 300 groundwater samples collected from 52 boreholes/springs across three aquifer systems. Nitrate concentrations were analyzed using Ion Chromatography (EPA 300.1 1999, CSN EN ISO 10304-1).
  • Agricultural land use: CORINE Land Cover (CLC) 2018 dataset and Malta’s agricultural census reports (2000 onwards, primarily 2010 Census of Agriculture).
  • Meteorological data: Daily data (solar radiation, wind speed, minimum and maximum air temperature, relative humidity, precipitation) from the national weather-station network (Malta International Airport) used to calculate Penman–Monteith reference evapotranspiration (ET0) and crop evapotranspiration (ETc).
  • Agricultural practices: Information on crop-specific irrigation practices and nitrate recommendations from existing literature and National Statistics Office (NSO) reports.

Main Results

• Nitrate accumulation in the vadose zone varied significantly with crop type and geology. Potato cultivation showed the highest average vadose zone nitrate concentration (967 milligrams per liter), followed by fodder (675 milligrams per liter) and mixed outdoor vegetables/greenhouses (443 milligrams per liter). Orchards and vineyards exhibited the lowest accumulation.
• Potato cultivation was identified as the dominant source of nitrate loading to the Mean Sea Level Aquifer (MSLA), particularly in eastern areas where nitrate concentrations reached 100 to 200 milligrams per liter, exceeding the EU Nitrates Directive limit of 50 milligrams per liter.
• Intensive vegetable and greenhouse farming created local nitrate contamination hotspots in the Blue Clay perched aquifers and Coastal aquifers, which exhibited the highest overall groundwater nitrate concentrations.
• The Modified DRASTIC modeling (DRSTVZNCL/DRSTVZN) indicated that depth to groundwater and vadose zone nitrate storage are the most critical factors influencing aquifer vulnerability to nitrate contamination.
• While the Blue Clay layer was initially considered a barrier, the model suggested that the significant depth to groundwater in the north-western MSLA is a more critical factor for low nitrate levels, implying delayed rather than prevented contamination.

Contributions

• Established the first national-scale, integrated vadose zone and groundwater monitoring network in Malta to trace nitrate fate from agricultural land use.
• Quantified nitrate accumulation and storage in the vadose zone for key agricultural crops in a semi-arid island environment.
• Developed and applied a refined groundwater vulnerability assessment model (Modified DRASTIC, DRSTVZNCL) tailored to Malta's specific hydrogeological conditions by incorporating vadose zone nitrate storage and the impact of the Blue Clay layer.
• Provided a practical framework for identifying non-point nitrate pollution sources and enhancing groundwater management strategies in water-stressed island regions by clarifying the coupled role of land use and subsurface structure in nitrate transport.

Funding

• EU Cohesion Fund (for the national Vadose Zone Monitoring Systems network)

Citation

@article{Laudi2026Tracing,
  author = {Laudi, Luca and Dahan, Ofer and Sapiano, Manuel and Schembri, Michael and Galea, Luke Daniel and Busuttil, Ella and Mangion, John and Turkeltaub, Tuvia},
  title = {Tracing nitrate fate in Malta’s hydrogeological system using an intensive vadose-groundwater monitoring network},
  journal = {Journal of Hydrology Regional Studies},
  year = {2026},
  doi = {10.1016/j.ejrh.2026.103162},
  url = {https://doi.org/10.1016/j.ejrh.2026.103162}
}