IWRA Proceedings

(a) Purpose or objectives and status of study or research hypothesis
Climate change is expected to affect groundwater recharge rates, altering hydrogeological dynamics and groundwater quality. Antibiotics and antibiotic resistance (AR) in the subsurface, derived from manure fertilization, are nowadays considered as contaminants of emerging concern. Their fate depends on complex geochemical & microbiological processes and conditions, whose detailed dynamics is still far from being properly understood.

(b) Key issue(s) or problem(s) addressed
This research addresses the hydrological behavior of these pollutants in groundwater under changing hydrological conditions so monitoring and prevention strategies can be efficiently delineated. It links two relevant issues that determine our capability to deal with such type of pollution; first, the effect of hydrological changes on their fate, and second, how to adequately include them in the groundwater quality management plans.

(c) Methodology or approach used
Based on a detailed monitoring survey in the Baix Fluvià alluvial aquifer (NE Catalonia), the spatial occurrence and temporal distribution of both, concentration of antibiotics and antibiotic resistance genes (ARG) to selected compounds, are analyzed and interpreted within the regional hydrogeological context. This work includes groundwater hydrogeochemical, isotopic and antibiotic data as well as detailed composition of groundwater bacterial communities and associated ARGs conferring resistance to sulfonamides, fluoroquinolones, macrolides, and tetracyclines.

(d) Results and conclusions derived from the project
Antibiotic data indicate that seasonal recharge has a strong influence on their occurrence. Given the high adsorption and degradation rates, some antibiotics can only be found when intense infiltration occurs, and their usual low concentration is the result of mixing (dilution) with the regional groundwater flow, explaining the sparsity of their occurrence. Notice that groundwater withdrawal rates also stay as an important factor of their identification in groundwater samples. Only a few compounds (i.e., those with large input loads and lower reaction conditions) are considered to be uniformly distributed along the aquifer, and define a background level.
Seasonal behavior of ARGs was not satisfactorily proved in the Baix Fluvià alluvial aquifer, which addresses the issue of whether these AR determinants were actually disseminated to the subsurface or largely depends on local environmental conditions (around the well capture zone). Indeed, no significant differences were observed in alfa diversity estimators at a seasonal basis, suggesting that studied communities showed minor variations in richness and diversity during the studied annual cycle.
Results indicate that finding antibiotics and ARGs in aquifers is largely influenced by the hydrodynamics of the system. Future lower average infiltration rates due to a rainfall decrease and temperature increase (i.e., larger evapotranspiration rates) would enhance the
accumulation of antibiotic residues in the uppermost layers of the vadose zone thus obstructing their arrival to the aquifer in  mediterranean areas. If rainfall intensity increases as predicted, the outcome can be reversed, and their occurrence be otherwise larger. Effects on AR are still unpredictable as it is unsure whether ARGs and bacteria can actually be treated as solutes.

(e) Implications of the project relevant to selected conference theme, theory and/or practice

The outcome of this research provides and explanation for the sparse spatial and temporal distribution of antibiotics and AR in aquifers, as observed in the Baix Fluvià area. Results indicate that identifying both pollutants will largely be influenced by the hydrological regime, which will vary with climate change This is paramount to establish trustable monitoring strategies upon which develop management plans.