Monitoring short-term transient groundwater hydrochemical and hydrodynamics changes following the onset of an early dry season and hydrological drought period

While there have been significant advances in the understanding of drought in the surface water domain, little knowledge is available for groundwater and the interactions with surface water. In particular, few studies have been run to understand the short-term transient changes in groundwater quality since the early onset of a hydrological drought period. This contribution presents data and information on the groundwater hydrochemical and hydrodynamics changes occurring in an aquifer following the onset of an early dry season in Spring 2021 and developed in a hydrological drought period lasted until December 2021 in the alluvial plain of the Cornia River in coastal Tuscany (Italy).

The Cornia plain hosts a Holocene coastal aquifer constituted, in the investigated area, mainly by gravel in silty matrix. We monitored groundwater chemical quality and hydrodynamics in a series of multi-depth piezometers in a recharge area covering three different depths from the soil surface (i.e., 8m, 12m, and 18m) in the near of a Managed Aquifer Recharge (MAR) scheme (Caligaris et al. 2022). We monitored these piezometers alongside with the existing network of piezometers and the relations with Cornia River surface water for nine months from April 2021 (when the max groundwater head was recorded) until December 2021 (when the minimum was recorded).

Ten sampling campaigns were performed in this period, covering the early end of the annual MAR operation period in May 2021, and monitoring every fifteen days in the initial phase of the dry season. The last effective rainfall occurred on 11 May 2021. A total of about 130 water samples were collected. The concentrations of the main ions in the water samples were determined using an Ion Chromatography (IC) instrument. The concentrations of trace elements were determined using an Inductively Coupled Plasma Mass Spectrometer (ICP). The concentration of Boron in water was determined using a Microwave Plasma Atomic Emission Spectrometer (MP-AES). Physico-chemical parameters were measured in the field with a multiparametric probe. This resulted on the measurement of the spatiotemporal variation of 49 different parameters at each of the study point.

An important groundwater table decline, ranging from 6 to 10 m, was observed in this period, which brought to relevant water stress even in trees at the end of October 2021. The statistical behavior of the different parameters as well as their relationships are studied and presented to define a robust conceptual model unifying hydrochemistry and hydrodynamics in order to describe the evolution of the aquifer.

Acknowledgement

This paper is presented within the framework of the project MARSoluT (www.marsolut-itn.eu), a four-year Marie Skłodowska-Curie Actions (MSCA) Innovative Training Network (ITN) funded by the European Commission (Grant Agreement 814066).

References

Caligaris, E.; Agostini, M.; Rossetto, R. Using Heat as a Tracer to Detect the Development of the Recharge Bulb in Managed Aquifer Recharge Schemes. Hydrology 2022, 9, 14. https://doi.org/10.3390/hydrology9010014