Groundwater protection analysis for Malta using C-O-P method

Abstract

Groundwater underpins Malta’s water security, contributing to about 54% of production in 2022. Long-term declines in the mean sea level aquifers combined with increased contamination levels underscore the need for further research in this area; defensible, map based guidance is a potential solution. This study develops an island-wide groundwater vulnerability map for Malta using the COP framework: integrating Concentration of flow (C), Overlying layers (O) and Precipitation (P) on a harmonised 10 metre grid. The O factor combines a soil layer and unsaturated-zone lithology in a five-class protection score. The C factor employs a two-domain strategy: within delineated swallow hole catchments, basin constrained Euclidean distances to sinks and sinking streams are modulated by a slope – vegetation factor; elsewhere, surface features and slope – vegetation are similarly applied. The P factor is parameterised using rainfall data from 2005–2024, yielding a uniform climatic scalar of 0.87. The resulting COP surface reproduces the expected vulnerability spectrum: very high vulnerability along sinking streams and within swallow hole basins; lowest vulnerability over Blue Clay and marly units; predominantly moderate values on fissured carbonate; and generally lower vulnerability across sealed urban tracts. Internal diagnostics—such as buffer-ring gradients and basin-wise rank correlations around sinks— confirm the intended index behaviour. An overlay with Malta’s Groundwater Protection Zones adds finer-scale detail, revealing both hotspot areas and zones that may warrant deprioritisation. Principal uncertainties stem from soil generalisation, DEM resampling, approximations in perched-aquifer floors, and gaps in near-shore soil data; these are retained and flagged as limited-confidence areas. The product offers a planning-ready baseline for prioritising protection and guiding field validation. Recommended further research includes testing alternative flow-routing algorithms and associated along-flow metrics; incorporating impedance-based (cost-distance) routing with explicit anthropogenic barriers and connectors; simulating event-based storm scenarios; and cross-comparing with other protection methods such as DRASTIC. A separate section outlines targeted field validation methods, including hydrochemistry, stable isotopes (δ¹⁸O, δ²H), dye-tracing tests, and nitrate measurements in monitoring wells.