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From Dust to Groundwater: How Atmospheric Particles Shape Water Quality in Drylands
Article contributors: Dr. Francisco Javier Alcalá and MSc Raúl Segura, Spanish National Research Council (CSIC), Spain
Introduction
Water quality is becoming one of the defining sustainability challenges of our time, especially as climate change intensifies pressure on already water-scarce regions. In drylands, groundwater is often the main source of water and its quality is a function of salinity from atmospheric and lithological sources. In polluted areas, human activities contribute additional salinity, whereas in pristine areas with negligible contribution of lithological salinity, groundwater salinity depends exclusively on the fraction of atmospheric salinity that reaches the aquifer after deducting the fraction of water lost via evapotranspiration. So, groundwater salinity is a function of these three salinity additive forces, where each of them can individually exceed legal quality standards, as shown in Figure 1.
Figure 1. Conceptualization of the groundwater salinity as a function of three salinity additive forces, where each of them can individually exceed legal quality standards.
Groundwater salinity due to atmospheric forces depends on the salinity input from precipitation (wet deposition) and dust and aerosols (dry deposition). The relevance of atmospheric salinity derived from dry deposition has increased in recent decades, as shown in Figure 2 for the chloride ion, necessitating further research to identify its role in global groundwater salinity.
Figure 2. Growing relevance of the atmospheric chloride dry deposition (dust and aerosols) in two Spanish stations of the EMEP Network (EBAS database: https://ebas.nilu.no/).
Measurements and methodology
`Within the framework of the CiROCCO project, groundwater salinity is investigated through an innovative model that integrates atmospheric salinity input (a combined function of wet and dry deposition), water lost via evapotranspiration, and aquifer geometry and hydraulics to predict groundwater salinity output. The model has been implemented in the groundwater body occupying the southeast versant of the Sierra de Gádor Mountains. Sierra de Gádor Mountains act as the recharge area of a regional groundwater body bounded by drylands/deserts that support one of the most important groundwater-dependent agroeconomic system of Europe (Figure 3).
Figure 3. Aerial view of Sierra de Gádor Mountains (right area) and Campo de Dalías coastal plain (left area), which host one of the most important groundwater-dependent agroeconomic system of Europe, the so called “Sea of Plastic” of the Almeria’s greenhouse horticulture system.
Over the last two decades, CSIC operates two networks to investigate how atmospheric salinity controls groundwater salinity. The first is an Atmospheric Bulk Deposition Network operated since 2008 (Figure 4a), whereas the second is a Groundwater Quality Network operated since 2006 (Figure 4b). Both networks are currently an intrinsic part of the CiROCCO project in Sierra de Gádor Mountains. Data pairs of atmospheric salinity (input data used for model implementation; Figure 4a) and groundwater salinity (output data used for model calibration; Figure 4b) are combined with evapotranspiration deduced from spatial climatic covariates and other geographical and spatial environmental covariates (different territorial thematic maps) generated by State Agencies to achieve the spatial regionalization of groundwater salinity associated with atmospheric forces in the study area.
Figure 4. Integration of the CSIC infrastructure and CiROCCO sensors (red triangles and black crosses) in Sierra de Gádor Mountains. (a) The CSIC Atmospheric Bulk Deposition Network provides monthly precipitation and dissolved inorganic chemical substances from wet (precipitation) and dry (dust and aerosols) deposition. (b) The CSIC Groundwater Quality Network provides physicochemical parameters of groundwater. The selected groundwater body (blue contour) is also indicated.
What CiROCCO adds to the picture
The CiROCCO Network is providing spatial information on key variables to calculate the rate of dry deposition (dust and aerosols), such as wind speed and particulate matter of different size, as a step to segregate its salinity fraction from the bulk deposition salinity that measures the CSIC Network. Data from the Llano de los Juanes FLUXNET station, which is operated by EEZA-CSIC since 2006, are used for other detailed evaluations. Figure 5 shows the integration of CiROCCO sensors within the equipment of the FLUXNET station.
Figure 5. View of the Llano de los Juanes FLUXNET station in the upper reaches of Sierra de Gádor Mountains, which includes an Eddy Covariance tower for fine water balance evaluations together with the node HE2 from CiROCCO network installed in January 2026 (see location in Figure 4).
Results
To date, a thirteen-month record of bulk deposition collected by the CSIC Atmospheric Bulk Deposition Network (Figure 4a) and a three-month combined record of bulk and dry deposition respectively collected by the CSIC and CiROCCO Networks (Figure 4b) at different elevation and distance from the coast are available. Chemical analyses performed to both types of deposition cover various common saline substances. Focusing on chloride, as a good proxy for atmospheric salinity, the atmospheric bulk chloride deposition rate ranges from 1.1 g m² yr⁻¹ at 800 m a.s.l. to 0.8 g m² yr⁻¹ at 1600 m a.s.l. For the three months with paired bulk-dry deposition data, the atmospheric dry chloride deposition rate is around 20% (at 800 m a.s.l.) and 18% (at 1600 m a.s.l.) of the bulk deposition. This comparison period has been too short and predominantly wet, so a greater impact in atmospheric salinity from dry deposition is expected when the dust events during dry season without influence of precipitation can be computed, possibly on the order of 40%.
The observation, conceptualization of the influence of atmospheric salinity on groundwater salinity depends on the groundwater transit time through the unsaturated and saturated zones, from rainfall infiltration to groundwater discharge. In large groundwater bodies this influence may remain unnoticed for decades, but not so in small aquifers with average transit time on the order of a few months. This is the case of El Berchul spring (Figure 6), which belongs to the CSIC Groundwater Quality Network. This spring discharges a small perched aquifer hydraulically disconnected from the main Sierra de Gádor Mountain groundwater body. In El Berchul spring, with an average transit time of 2-3 months, the influence of atmospheric dust events is identified as a transient groundwater salinity increase in the range of 5–10% when compared to the chloride background value produced by typical succession of precipitation events during the wet season.
These initial tentative results appear to corroborate the role of salinity from dry deposition (dust and aerosols) in the groundwater salinity of atmospheric origin. Investigation is ongoing to extend the monitoring period in order to corroborate these statements and refine the magnitudes.
Figure 6. View of El Berchul spring during periodical monitoring (P03, green triangle in Figure 4b). This spring belongs to the CSIC Groundwater Quality Network and is located near to the node LC6 from CiROCCO network (see location in Figure 4).