Hydrogeological, hydrodynamic and anthropogenic factors affecting the spread of pharmaceuticals and pesticides in water resources of the Granada plain (Southern Spain)

The anthropogenic organic contaminants contemplated in the environmental legislation, as well as those of emerging concern, threaten the quality of water resources to a degree that remains largely unknown. Contaminant exposure in the aquatic environment is a crucial element if a full understanding of the risk is pursued. There are still many uncertainties about the occurrence of organic contaminants and their behavior in the hydro(geo)logical media in large scale areas. The case study of the unconfined aquifer of the Granada Plain (approximately 200 km²) is presented here. Two groundwater and surface water monitoring campaigns were conducted (March 2017 and June 2018). Water samples were analysed for (i) 171 organic contaminants (e.g., pesticides, pharmaceuticals, drugs of abuse, PAHs); (ii) major and minor ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^-, SO₄^2-, HCO₃^-, NO₃^-) and (iii) isotopes of the water molecule (δ¹⁸O, δ²H) and δ¹³C from the dissolved inorganic carbon. Additionally, in situ measurements of physico-chemical parameters (pH, temperature, electrical conductivity, redox potential and dissolved oxygen) were carried out. In total, 41 organic pollutants were detected, at least once: 17 pharmaceuticals or drugs of abuse, 21 pesticides or their metabolites and three PAHs. Statistical tests confirmed the significance of seasonal changes for some of these parameters (e.g., EC, Cl^-, F^-, δ¹⁸O, δ¹³C), revealing the influence from snowmelt water input on streams and the intensification of irrigation. In March 2017, the group of pesticides (largely represented by triazines) predominated, whereas the frequency of detection of pharmaceuticals increased substantially in June 2018. Based on the obtained results, a qualitative evaluation has been made to suggest four main factors affecting the spatial and seasonal variation of organic pollutants in the aquifer: (i) the variation of the unsaturated zone thickness; (ii) the river-groundwater hydraulic connection; (iii) the hydraulic gradient; and (iv) the anthropogenic factor determining the period of contaminant release throughout the year and wastewater management practices. The river-groundwater hydraulic connection can be especially important in the case of those contaminants whose main path of entry into the aquatic environment occurs through wastewater discharge into streams (i.e., pharmaceuticals).