Contribution of water geochemistry and isotopes (δ18O, δ2H, 3H, 87Sr/86Sr and δ11B) to the study of groundwater flow properties and underlying bedrock structures of a deep landslide

Inferring Hydrological Information at the Regional Scale by Means of δ18O–δ2H Relationships: Insights from the Northern Italian Apennines

“…This study aims to verify whether such discrepancies in slopes and intercepts from different regression methods are present (thus significant) or not in four water types (precipitation, surface water, groundwater collected in wells from the lowlands, groundwater from low-yield springs) from the northern Italian Apennines. For this reason, we exploited datasets already published in the literature, e.g., [7,11,[16][17][18], and we carried out visual inspection (heat maps) and statistical comparison of results from the three aforementioned approaches (OLS, RMA, MA) already tested in [15]. With reference to OLS approach, we further verified whether preliminary weighting of the isotopic data to the corresponding values of discharge or precipitation may have induced changes to our results or not.…”

“…Oxygen ( 18 O and 16 O) and hydrogen isotopes ( 2 H and 1 H) of water are commonly used in surface and subsurface hydrology [1][2][3]. They are considered environmental tracers in the form of δ 18 O and δ 2 H, where δ(‰) = R sample R standard − 1 × 1000 and R is the corresponding isotopic ratio ( 18 O/ 16 O or 2 H/ 1 H) in the water sample or in the standard (usually V-SMOW, i.e., the Vienna Standard Mean Oceanic Water). If a multitude of water samples is collected from the same source (a rain gauge, a river, a spring, or a well), the corresponding δ 18 O-δ 2 H pairs in a Cartesian graph will be aligned along a regression line in the form of y = mx + q, where y is δ 2 H, x is δ 18 O, m is the slope, and q the intercept.…”

“…The authors of [5] have substantially confirmed this slope and intercept by processing more recent data. Although this relationship is valid everywhere, more accurate regression lines (with different slope and intercept) can be obtained by selecting δ 18 O and δ 2 H values from restricted areas. This is due to the specific isotopic fractionation processes (i.e., vapour pressure and temperature conditions) controlling precipitation over each area.…”

“…Moreover, since further post-condensation and temperaturedriven processes such as evaporation and evapotranspiration could act prior to infiltration and/or during runoff, δ 18 O-δ 2 H regressions from rivers (river water lines, RWLs) and groundwater (groundwater lines, GWLs) may also differ from that of the precipitation occurring in their recharge areas (meteoric water lines, MWLs). In fact, evaporation and evapotranspiration lead to a fractionation between the different isotopologues of water, with lighter water molecules ( 1 H 2 16 O) vaporising faster than heavier ones ( 2 H 2 18 O) and inducing an enrichment of the latter into the liquid residual. In this case, as a water parcel evaporates, its isotopic composition usually evolves with a δ 18 O-δ 2 H regression line whose slope is lower than those of MWLs.…”

“…Starting with the pioneering works by [4,12] regarding meteoric water and up to this day, δ 18 O-δ 2 H regression lines are usually carried out by means of the ordinary least squares (OLS) method, i.e., an approach that minimises the sum of the squared vertical distances between the y data values and the corresponding y values on the fitted line (the predictions). Thus, the OLS design assumes that there is no variation in the independent variable (x) and is considered as the simplest method among the several available linear regression models.…”