Soil Water Stable Isotopes Reveal Surface Soil Evaporation Loss Dynamics in a Subtropical Forest Plantation

Line-conditioned excess (lc-excess), the deviation of the relationship between δD and δ18O in soil water from that of precipitation, is often used to indicate soil evaporation loss, but the conditions of using lc-excess under the influences of precipitation infiltration or percolation had not been identified. The interaction effects of climate, soil and vegetation on soil evaporation in forests are not well known. We collected soil water at 0–5, 15–20 and 40–45 cm depths and event-based precipitation from 2011 to 2015 in a subtropical forest plantation and calculated the lc-excess. Precipitation on the sampling day and percolation of upper soil water with low lc-excess affected the capacity of the lc-excess to indicate the soil evaporation fractionation signals. Lc-excess of soil water at 0–5 cm depth indicated a reliable soil evaporation loss estimate over 30 days prior to the sampling day. Soil evaporation loss was dominated by the cumulative soil temperature (Tss) during drought periods and was dominated by the relative soil water content (RSWC) during non-drought periods. High Tss decreased soil evaporation loss by increasing transpiration and relative humidity. Our results emphasize the importance of sampling the upper-most soil layer when there is no rain and vegetation during drought periods in forests when studying soil evaporation loss dynamics.

Compositions of hydrogen and oxygen isotopes of precipitation in Xiamen, Southeast China Coast: Seasonal variations, synoptic processes, and typhoons impact

In this study, the δD and δ18O values of 162 precipitation samples (including 33 typhoon-related precipitation samples), collected in Xiamen, Southeast China coast, during June 2018 to August 2019, were investigated and analyzed. The results show that there are obvious seasonal variations in the δD and δ18O, which are mainly controlled by the East Asia Monsoon with significant influence of typhoon events in summer. The influence of moisture sources on δ18O values overrides the influence of precipitation fractionation process on δ18O values which leads to an inverse temperature effect in the study area. In comparison to the seasonal scale, the synoptic time-series variation of δD and δ18O is much more complicated. In general, there are three types of isotopic variations in the normal precipitation processes, which are obviously affected by re-evaporation processes and continuing equilibrium fractionation during condensation. The local meteorological parameters during normal precipitation, which mainly control the re-evaporation process, are the dominant factors for the variation patterns of δD and δ18O, whereas moisture sources control the overall isotope values of precipitation. The differences between the time-series of normal and typhoon-related precipitation are mainly controlled by the changes of physical processes and meteorologic parameters during the precipitation process. However, due to the unique atmospheric structure and dynamic processes of typhoons, the δD and δ18O of typhoon-related precipitation changes in stages gradually as the distance between the typhoon’s center and the study area changes. The uniformity of typhoon structure leads to a similar staged changes in different typhoon-related precipitation. The moisture source trajectory of typhoon-related precipitation shows a clear spiral structure (except for typhoon Yutu), and the moisture sources at different heights control the δD and δ18O values of typhoon-related precipitation. This study is important for quantifying the global changes of typhoon processes and paleotempestology studies.

Recharge and Infiltration Mechanisms of Soil Water in the Floodplain Revealed by Water-Stable Isotopes in the Upper Yellow River

The stable isotopes (δD and δ18O) in soil water allow tracing of the flow and transportation of water in the soil. However, there are few studies on the use of soil water stable isotopes to explore the soil water in the floodplain, especially in determining the soil water source and infiltration mechanism. The Bayesian mixing model (MixSIAR) was integrated with the line conditioned excess (lc-excess) of stable isotopes (δD and δ18O) in precipitation, soil water (0–150 cm), river water, and groundwater to determinate the source and recharge mechanisms of two different soil profile types in the floodplain of the upper Yellow River in Lanzhou, China. The results showed that soil water below 80 cm was affected by river water recharge, affecting soil water content and stable isotopic composition at S1 sampling points (profile parallel to river water); this effect was not observed at S2 (profile is higher than the river water) sampling points. The isotopic compositions of the soil water sources at the two sampling points (S1: δD = −77.41‰, δ18O = −11.01‰; S2: δD = −74.02‰, δ18O = −10.56‰) were depleted more than those in the long-term amount-weighted precipitation isotopes (δD = −56.30‰, δ18O = −8.17‰). The isotope signatures of soil water sources are similar to the isotope characteristics of some high-intensity precipitation events (≥30 mm/day), indicating that soil water originates from a fraction of the total precipitation. The piston flow (60%) and the preferential flow (40%) coexist, but soil moisture and rainfall intensity will affect the sequence of the two infiltration methods. This study provides insights for understanding the hydrological process of the upper Yellow River and evaluating groundwater quality and protecting the floodplain environment.

Hydrogeochemistry and Groundwater Quality Assessment in the High Agri Valley (Southern Italy)

The High Agri Valley (southern Italy) is one of the largest intermontane basin of the southern Apennines affected by intensive agricultural and industrial activities. The study of groundwater chemical features provides much important information useful in water resource management. In this study, hydrogeochemical investigations coupled with multivariate statistics, saturation indices, and stable isotope composition (δD and δ18O) were conducted in the High Agri Valley to determine the chemical composition of groundwater and to define the geogenic and anthropogenic influences on groundwater quality. Twenty-four sampling point ( including well and spring waters) have been examined. The isotopic data revealed that groundwater has a meteoric origin. Well waters, located on recent alluvial-lacustrine deposits in shallow porous aquifers at the valley floor, are influenced by seasonal rainfall events and show shallow circuits; conversely, spring waters from fissured and/or karstified aquifers are probably associated to deeper and longer hydrogeological circuits. The R -mode factor analysis shows that three factors explain 94% of the total variance, and F1 represents the combined effect of dolomite and silicate dissolution to explain most water chemistry. In addition, very low contents of trace elements were detected, and their distribution was principally related to natural input. Only two well waters, used for irrigation use, show critical issue for NO3- concentrations, whose values are linked to agricultural activities. Groundwater quality strongly affects the management of water resources, as well as their suitability for domestic, agricultural, and industrial uses. Overall, our results were considered fulfilling the requirements for the inorganic component of the Water Framework Directive and Italian legislation for drinking purposes. The water quality for irrigation is from “good to permissible” to “excellent to good” although salinity and relatively high content of Mg2+ can occasionally be critical.

ISOTOPE COMPOSITION OF NATURAL WATERS IN THE VERKH-TULA SETTLEMENT (THE NOVOSIBIRSK REGION)

The first data obtained in the isotope hydrogeochemical studies of natural waters in the Verkh-Tula settlement are presented in the work. The composition of these waters is mainly HCO Na-Mg-Ca with TDS varying from 542 to 731 mg/dm, and silicon content 0.46 to 7.04 mg/dm. The geochemical parameters of the medium vary from reductive to oxidative with Eh -157.4 - +231, weakly alkaline pH (7,4 - 8,1) and О 0.29 - 5.52 mg/dm3. It was established that δD and δ18O of surface and ground waters differ from each other substantially and vary from -105 to -126 ‰ and from -13.2 to -16.3 ‰ - for the former, and from -136 to -138 ‰, from -18.3 to -18.8 ‰ - for the latter. According to the data obtained, for the majority of groundwaters, the time of water residence in the aquifer is not less than 5 years, and their feeding is independent of local surface waters. The isotope composition of water-dissolved carbon (δС from -14.3 to -12.5 ‰) points to the biogenic origin of carbon dioxide participating in carbonate-silicate weathering.

Characteristics and moisture sources of the stable isotopes in precipitation in the monsoon marginal region of north-central China

The stable hydrogen and oxygen isotopes in precipitation (δD and δ18O, respectively) are important source signatures for understanding the hydrological cycle and paleoclimatic reconstruction. In this study, 32 precipitation samples were collected from April to October 2014 at the Luya Mountain summit, a representative site in the monsoon marginal area of north-central China. The isotopic signatures of precipitation exhibited strong seasonal variations ranging from -185.61‰ to -18.50‰ and -25.51‰ to -4.59‰ for δD and δ18O, respectively, which were relatively higher in August and lower in September. The local meteoric water line was δD=(7.95±0.16) δ18O+(15.79±1.77) (R2 = 0.988, N = 32, p < 0.001), which had a similar slope but higher intercept than that of the global meteoric water line, indicating that the precipitation in this area is mainly sourced from the ocean surface transported via monsoons. Additionally, the secondary evaporated water by continental recycled moisture was identified by the significantly higher deuterium excess value (16.09‰). Backward trajectories generated via the Hybrid Single-Particle Lagrangian Integrated Trajectory model indicated seasonal moisture transport changes in the studied monsoonal marginal region, in which the main moisture sources were the Westerly winds in April, the Pacific Ocean in July, and the Indian Ocean in September. Secondary evaporated water from the alpine ecosystem may also influence the local atmospheric water cycle throughout the year. Positive temperature-isotopic signature effect (δD and δ18O) was observed in the cold season (before mid-May); however, the precipitation amount effect was observed in the monsoon season from June to August, and both effects became vague across the entire period. These findings suggest that the stable isotope compositions of precipitation can be utilized to determine the moisture sources in the monsoon marginal region of north-central China and potentially be utilized to reconstruct the precipitation signals in this region.

Calculating groundwater mixing ratios in multi-aquifers based on statistical methods: a case study

Samples of river water and groundwater from Quaternary (QA), sandstone (SA), Taiyuan formation (TA), and Ordovician limestone (OA) aquifers in the Suxian coal-mining district, Anhui Province, China were collected. Their physicochemical properties, and major ion and isotopic compositions were determined. The samples were alkaline, with pH values exceeding 8, and the total dissolved solids concentrations depended on the water source. The δD and δ18O contents were highest in the river water samples and lowest in the SA groundwater. The isotopic characteristics of the QA and OA groundwaters suggest recharge by rainfall and surface water. The isotopic characteristics of river water were controlled mainly by evaporation. Water–rock interactions, the flow rate, and the main water sources were the most important influences on groundwaters in QA, OA, and TA, but the properties of SA groundwater were controlled by static reserves. Two discriminant functions, explaining more than 98.2% of the total variances, indicated that QA, TA, and OA were hydraulically connected. Three groundwater sources were identified as end-members, and a conceptual model was established to calculate water mixing ratios.

Soil Water Dynamics Under Different Land Uses in Loess Hilly Region in China by Stable Isotopic Tracing

Exploring soil water dynamics under different land use types is important for water resource management and vegetation restoration in the Loess Plateau. In this study, we investigated the hydrogen and oxygen isotopic compositions of soil water from four different land use types to explore the mechanism of soil water movement and transformation and analyse the influence of land use. The results show that the range of stable isotopes (δD and δ18O) in soil water was smaller than that in precipitation. Values for δD and δ18O in soil water showed relatively similar temporal variation, heavy isotopes were enriched in the soil water in July and depleted in October. Stable isotope values in shallow (<100 cm depth) soil water and deep (>200 cm depth) soil water were low. The δD and δ18O values in woodlands decreased gradually with increasing depth. Across the four land use types, the maximum variation in δD and δ18O was in the shallow depth of the soil profile. Groundwater was recharged mainly from precipitation and then from soil water. The ratio of groundwater recharge by soil water under different land use types followed this rank order: woodland (35.70%) > grassland (31.14%) > shrubland (29.47%) > cropland (29.18%). Matrix flow and preferential flow coexisted during infiltration, and the occurrence of preferential flow was related to the land use type. The main reason for the variation in isotopic composition in soil water is the difference in soil evaporation, which is influenced by different vegetation cover. Owing to the difference in soil evaporation and fractionation, precipitation on cropland, shrubland, and grassland can recharge more soil water than on woodland.