Arborea (Italy): Nature-based Solutions for Groundwater Nitrate Reduction

This Case Study is located in Arborea, a town and municipality in the Oristano province, in Sardinia (Italy) and it is characterised for being a farming district. Arborea plain was initially a swamp which was reclaimed between 1920 and 1930: sand dunes were flattened and brackish and salted wetlands were drained, and it was all transformed into agricultural land. The resulting plain was divided in rectangular fields with the long side-oriented North-South, delimited and protected by eucalyptus edges and surrounded by a drainage network consisting of main channels and a dense network of smaller ones. 

Nowadays the Arborea plain represents one of the most productive agricultural sites in Italy. Part of the forage cropping is based on local production of fibre from the double cropping of silage maize and Italian ryegrass for hay or a winter cereal for silage, representing over 80% of the irrigated plain. Almost all the remaining area in the district is grown with meadows of alfalfa and horticultural crops.

The Arborea plain is surrounded by several marsh wetlands hosting significant biodiversity, including migratory birds, amphibian, and reptile species classified as community interest breeding vertebrates. The climate is classified as semi-arid Mediterranean, characterized by warm summers, mild winters, and a high-water deficit from May to September. Environmental conditions are further influenced by the designation of part of the Arborea Municipality as a Nitrate Vulnerable Zone (NVZ) of agricultural origin. The Arborea plain includes two Hydrogeological units - the Sandy Hydrogeological Unit and the Alluvial Hydrogeological Unit - both containing interconnected aquifers. Nearly 50% of the groundwater samples from these aquifers exceed the 50mg/l nitrate concentration threshold, a value recommended by the WHO. The main source of nitrate pollution in the groundwater of the area are the effluents from intensive dairy cattle farming system. Nitrate groundwater pollution therefore represents the main environmental challenge faced by the region.

The main objective of this case study was to improve groundwater storage and quality by upgrading existing nature-based practices to mitigate groundwater nitrate contamination.

NATMed project, EU-funded project through the PRIMA Programme (n. 2221)

• The FIA demonstrated a very high and persistent denitrification performance, achieving nitrate removal efficiencies of 80–90%. Nitrate concentrations in groundwater were effectively reduced, confirming strong and sustained denitrification processes within the infiltration area, consistently bringing concentrations below the 50 mg/L benchmark.
• Improved groundwater quality in the study area by reducing nitrate loads. 
• Negligible production of nitrite, ammonium, and nitrous oxide during the denitrification process, indicating an environmentally safe treatment process with limited secondary impacts.
• Implementation and evaluation of new materials for phosphate sequestration. Although laboratory investigations indicated good phosphate adsorption potential without interfering with denitrification, no clear evidence of phosphate removal by kaolin was observed at field scale.
• The economic valuation of the FIA shows that, when assessed solely against its primary objective using standard evaluation parameters, the project presents a negative Net Present Value (NPV) and is not socio-economically viable. However, when the full range of ecosystem services provided by the FIA is considered, and when longer time horizons and discount rates that reflect long-term benefits and intergenerational impacts are applied, the overall socio-economic performance becomes positive. This outcome indicates that future FIA implementations should be designed to maximise co-benefits by enhancing ecological complexity and social value

• Forested Infiltration Area (FIA) upgrade: Enhancement of an existing FIA system through recharge trenches filled with passive treatment systems to promote microbial denitrification and reduce nitrate concentrations before groundwater infiltration.
• Tree plantations: Planting of eucalyptus and poplar trees along infiltration trenches to progressively sustain long-term natural denitrification through root–microbe interactions.
• Monitoring and impact assessment: Implementation of a comprehensive monitoring framework to assess nutrient removal, groundwater quality and levels, infiltration rates, soil moisture, evapotranspiration and socio-economic performance.
• Stakeholder engagement and governance alignment: Active involvement of stakeholders through co-design processes and coordination with regional initiatives to harmonise interventions in Nitrate Vulnerable Zones.

• The socio-economic value of NbS cannot be assessed solely on immediate financial returns. While short-term economic indicators may appear unfavourable, a comprehensive valuation that includes co-benefits such as biodiversity enhancement, carbon sequestration, and recreational value is essential to demonstrate overall positive socio-economic performance. 
• The FIA system relies on locally sourced labour, materials, and expertise and act as a local economic catalyst, fostering community acceptance and generating additional value through potential biomass or timber use.
• Addressing complex environmental challenges such as nitrate pollution requires multidisciplinary collaboration and alignment with other regional and EU initiatives. Integrating expertise from different fields and building synergies across projects allows more comprehensive and robust solutions than single-approach interventions.
• Embedding NbS within regional governance structures is key for long-term sustainability.
• Early and continuous consultation with key local stakeholders, including farmers’ cooperatives and local authorities, ensured that the FIA intervention was well aligned with local economic, social, and ecological conditions and increased local acceptance and ownership of the solution.

University of Sassari (Case Study Leader)

University of Cagliari (Case Study Leader)

NBSCLIMATE (IUCN Assessmente Leader)

CARTIF Technology Center (Project Coordinator)

Alessandra Paulotto (UNISS), Mario Pirastru (UNISS), Alberto Carletti (UNICA), Elisabetta Strazzera (UNICA), Giampiero Branca (UNISS), Claudia Sánchez (NBSCLIMATE), Raquel Marijuan (CARTIF)