Chemical and isotopic composition of precipitation in the Piedmont Po Plain (NW Italy): preliminary evaluation of impacts on the groundwater quality

The precipitation constitutes one of the main sources of the groundwater resources. The chemical composition of precipitation is influenced both by natural and anthropic sources. For this reason, it is essential to monitor rainfall potentially able to influence groundwater quality. The Po Plain sector (NW Italy) is one of the most urbanized, industrialized and air polluted area in Europe but few studies have been conducted in this area, particularly in the Piedmont Region.

The main purpose of this study was: I) to provide a preliminary assessment of quality and isotopic composition of rainwater in the western Po Plain, II) to show the spatial and temporal differences of rain chemical composition between the monitoring points, and III) to define the influence of rain to groundwater chemistry.

A long-term trends on the groundwater concentration of NO₃^– and SO₄^2– in the shallow aquifer on 227 monitoring points of the Regional Monitoring network database were conducted (2000-2020 period).

In the last decades, in Europe a large effort was carried out to reduce sulphur and nitrogen emission in the atmosphere. This resulted in a sharp decrease in the deposition of SO₄^2– and nitrogen compounds.

The rain analysis of long-term trends in near regions, revealed a large proportion of significant decreasing trends in the concentration of both sulphate and nitrogen compounds.

Actually also the analysis of groundwater long-term trends revealed a significant decreasing trends in the concentration of NO₃^– and SO₄^2– in the shallow aquifer.

A sampling campaigns was carried out during one year (September 2021 – September 2022) in 4 monitoring points located in the western Po Plain. Rainfall collection occurred every 2 months, for a total of 20 samples. Physical-chemical analyzes of the main ions and isotopic analyzes (δ¹⁸O, δ²H) were conducted for all samples.

The period  September 2021 – September 2022 was characterized by a rainfall deficit in the winter period in the NW Italy, recording a 62% reduction in rainfall (compared to the climatic average of the thirty year period 1981-2010).

The processing of rainfall chemical data has shown different concentrations between the monitoring points and a temporal variability. High NO₃^– and SO₄^2– concentrations were observed.

Rainfalls sampled after the winter dry period (March-April samples) show higher ions concentrations (NO₃^– 13 mg/L, SO₄^2– 4 mg/L) respect to other periods. Differences in rainfall samples depend on the location of the monitoring point (urban or rural areas).

Isotopic data has shown different spatial and temporal isotopic signals, linked to the location and elevation of the monitoring points. In the δ¹⁸O/δ²H diagram all isotopic signals are not placed on the Local Meteoric World Line, potentially linked to climate change.

The isotopic signals are within the ranges of previous studies (δ¹⁸O: -12,6/-6,2; δ²H: -82,15/-35,1 ‰ (min/max)).

In conclusion, the rain ions concentrations are influenced by anthropic pollution, they are affected by dry periods and they appears to influence the concentration in groundwater.