Deep Unsaturated Zone Research Articles

Deep desert aquifers as an archive for Mid- to Late Pleistocene hydroclimate: An example from the southeastern Mediterranean

Many efforts have been made to illuminate the nature of past hydroclimates in semi-arid and arid regions, where current and future shifts in water availability have enormous consequences on human subsistence. Deep desert aquifers, where groundwater is stored for prolonged periods, might serve as a direct record of major paleo-recharge events. To date, groundwater-based paleoclimate reconstructions have mainly focused on a relatively narrow timescale (up to ∼40 kyr), limited by the relatively short half-life of the widely used radiocarbon (5.73 kyr). Here we demonstrate the usage of deep regional aquifers in the arid southeastern Mediterranean as a hydroclimate archive for earlier Mid-to-Late Pleistocene epochs. State-of-the-art dating tools, primarily the 81Kr radioisotope (t1/2 = 229 kyr), were combined with other atmosphere-derived tracers to illuminate the impact of four distinguishable wetter episodes over the past 400 kyr, with differences in climatic conditions and paleo-recharge locations. Variations in stable water isotope composition suggest moisture transport from more proximal (Mediterranean) and distal (Atlantic) sources to different parts of the region at distinct times. Large variability in the computed noble gas-based recharge temperature (NGT), ranging ~15–30 °C, cannot be explained by climate variations solely, and points to different recharge pathways, including geothermal heating in the deep unsaturated zone and recharge from high-elevation (colder) regions. The obtained groundwater record complements and enhances the interpretation of other terrestrial archives in the arid region, including a contribution of valuable information regarding the moisture source origin as reflected in the deuterium-excess values, which is unattainable from the common practice analysis of calcitic cave deposits. We conclude that similar applications in other deep (hundred-m-order) regional groundwater systems (e.g., the Sahara desert aquifers) can significantly advance our understanding of long-term (up to 1 Myr) paleo-hydroclimate in arid regions, including places where no terrestrial remnants, such as cave, lake, and spring sediments, are available.

High spatiotemporal asynchrony between water deficit and nitrate accumulation under apple orchards in deep loess deposits

Understanding the relationship between water and nitrate in soils is critical to agricultural production and aquifer vulnerability assessments, especially in regions with intensive agricultural activities and thick unsaturated zones. Deep soils usually record long-term variations in coupling water and nitrate processes, influenced by multiple environmental factors. As such, this study aims to investigate water and nitrate-nitrogen (NO3–−N) contents and their relationships in soil profiles of 13–23 m deep under farmland and neighboring apple orchards with different ages. Soil properties were considered to explore the individual effect of a single factor or combined effects of multiple factors by wavelet analysis and partial least squares path models. Compared with farmland, apple orchards had large water deficit and NO3–−N accumulation due to root water uptake and synthetic fertilizer inputs, with high spatiotemporal asynchrony in deep unsaturated zones. Specifically, water deficit and NO3–−N accumulation were respectively observed in relatively stable (5–15 m) and active layers (0–5 m, spatial asynchrony), with higher water deficit rates under young apple trees (e.g., 12 and 22 years old) but larger NO3–−N accumulation rates under old apple trees (e.g., 23–32 years old, temporal asynchrony), respectively. Under farmland and orchard, soil texture-organic carbon regulated soil water movement, while soil water-organic carbon affected nitrate transport and transformation processes, which further altered soil pH and electric conductivity conditions in deep vadose zones. The growth of apple trees leads to asynchronous water depletion and fertilizer accumulation in soils, and the > 20 years apple trees are limited by insufficient water despite the excessive fertilizer. The dried soil layers not only reduce potential groundwater recharge, but also prolong the residence time of nitrate bombs in thick unsaturated zones. This study provides essential information for sustainable management of vegetation and water resources, and benefits the complicated hydrological and biogeochemical cycles in thick vadose zones.

Low variation in microbial carbon sequestration between farmland and apple orchards in typical loess-covered regions

The cultivation of deep-rooted vegetation greatly alters the tradeoffs between water and carbon dynamics, yet their impact on microbial carbon sequestration (MCS) remains inadequately documented, particularly in thick vadose zones. This information is crucial for global climate mitigation and ecosystem service enhancement. As such, we collected triplicate samples for each land use type, exploring the abundance and structure of cbbL-harboring microbial communities within 10-m deep profiles under cultivated farmland and two apple orchards of varying ages (n = 72). Our findings indicated a shift from water-dominated (improved water retention at the expense of reduced carbon sequestration) to carbon-dominated benefits (enhanced carbon sequestration at the expense of decreased water retention) in deep soil layers (2–10 m) after the transformation from farmland to apple orchards. Despite the lack of significant differences among the three land use types, the gene abundances and microbial diversity indices reduced with soil depths because of variations in substrates, resources, and microenvironments. The cbbL-harboring microorganisms consisted of 5 phyla, 7 classes, 19 orders, 30 families, and 55 genera, with Proteobacteria as the predominant bacterial phylum. The relative contributions of different genera varied with soil depths, reflecting their distinct survival strategies. The microbial network was positively connected in various soil layers under farmland and apple orchards. Furthermore, the results of correlation, redundancy analysis, and structural equation modeling jointly indicated that plant roots and soil properties, including total nitrogen and organic carbon, governed MCS processes in deep unsaturated zones. Specifically, the low microbial variation between farmland and apple orchards can be attributed to the predominant effects of climatic and pedogenic factors on soil properties, the restricted contribution of plant roots to soil nutrient levels per unit depth, and the adverse effects of plant-induced ecohydrological consequences on root systems and microbial populations. This study offers new insights within thick vadose zones containing deep-rooted plants, thereby contributing to a better understanding of global terrestrial carbon cycle processes.

Microbial nitrogen mineralization is slightly affected by conversion from farmland to apple orchards in thick loess deposits

Organic nitrogen mineralization, indispensable to soil carbon and nitrogen cycles, is the largest contributor to nitrate reservoirs in deep vadose zones. The microbial nitrogen mineralization (MNM) within deep soils, particularly in regions with intensive agricultural activities and thick soil horizons, has been largely disregarded. As such, this study aims to address this knowledge gap by investigating the chiA-harboring microbial structure and network within nine 10-m profiles beneath cultivated farmland and two apple orchards. The results showed that apple orchards, compared to farmland, had considerable water deficit and nitrogen accumulation within deeper soil layers due to well-developed root systems and the overuse of chemical fertilizers. However, the chiA-harboring microbial diversity, composition, and abundance all exhibited significant variations with soil depths rather than being influenced by different land use types. Moreover, the diversity indices and gene abundances decreased with soil depths, and the related soil microbes included 19 phyla, 29 classes, 72 orders, 114 families, and 197 genera, with Actinobacteria and Proteobacteria being the two major bacterial phyla. The microbial co-occurrence network was simper beneath apple orchards. The chiA-harboring microorganisms within deep unsaturated zones were greatly influenced by the depth-dependent soil nutrients, such as total nitrogen, organic carbon, and available potassium. The limited plant root biomass and the inhibitory effects of dried soil layers both restricted the availability of carbon sources, which further interfered with the MNM processes within deep soils insignificantly. Therefore, despite the considerable plant-induced ecohydrological consequences, the depth-dependent MNM processes were slightly affected after the transformation from farmland to apple orchards within thick loess deposits. This study offers crucial insights into microbial dynamics of the deep biosphere, thereby contributing to our understanding of depth-dependent biogeochemical cycles within global deep unsaturated zones.

Large-scale paleo water-table rise in a deep desert aquifer recorded by dissolved noble gases

• Very high amounts of dissolved noble gases were identified in a deep desert aquifer. • Large water table fluctuations during past pluvial periods are reconstructed. • Hydrogeological settings highly affect the air entrapment and dissolution process. • Similar excess air dissolution patterns are identified in distinct desert aquifers. Illuminating past hydrological and climatological conditions in arid regions may provide insights into future trends in groundwater availability. The main objective of this study is to explore paleorecharge processes in the deep regional Nubian Sandstone Aquifer (NSA), which stretches below the hyperarid deserts of the Sinai Peninsula (Egypt) and the Negev (Israel), using dissolved noble gas data. Extremely high amounts of dissolved excess air (Ne concentrations up to 4 × solubility equilibrium) were observed in the ancient ( 81 Kr-depleted) groundwater that was recharged during past pluvial epochs in the basin. The observed unique excess air signal and the clear spatiotemporal structure of the noble gas compositions in the aquifer are hypothesized to reflect two major characteristics of the groundwater system: (a) large-scale, long-term rises in the water table that facilitate the entrapment and dissolution of air bubbles under increased hydrostatic pressure, and (b) the geological settings in the southern recharge area in Sinai, including an almost horizontal position of the layers and intercalation of low-permeability rock formations within the aquifer, which inefficiently reject air bubbles from groundwater, thereby allowing for substantial entrapment of bubbles that dissolve as the water table rises. Enhanced dissolution of entrapped air was also observed in other paleo-groundwater-containing regional sandstone aquifers across North Africa, which, together with the recent findings from the NSA, suggest that large-scale water table fluctuations have likely extensively occurred during past pluvial periods over these (and possibly other) arid regions. A preliminary assessment of noble gas recharge temperatures (NGTs) indicates an apparent decoupling of surface and water table temperatures in the case of deep aquifers with a thick (hundreds of m) unsaturated zone. This observed decoupling calls for a re-evaluation of previously obtained NGT records and a need for future work to consider the modification of NGTs due to soil air fractionation and geothermal heating of the underlying deep unsaturated zones.

Identifying multivariate controls of water and nitrate in deep loess deposits under different land use types

Identifying the environmental factors controlling deep soil water and nitrate is important for the sustainability of vegetation and ecosystems. However, the combined effects of multiple factors at varying scales have been poorly understood, especially in those regions with thick vadose zones and deep-rooted plants. The aim of this study is to identify the multivariate controls of water and nitrate in seven > 13 m deep boreholes under different land use types on China’s Loess Plateau. After measuring soil water and nitrate contents, the combined effects of climatic factors (precipitation, temperature, and potential evapotranspiration) and soil properties (soil texture, magnetic susceptibility, pH, EC, and soil organic carbon) were explored by wavelet analysis. Land use is the key factor regulating deep soil water and nitrate reservoirs, with high water deficit and low nitrate accumulation within 0–10 m under non-fertilized forestlands and shrublands. The positive and negative standardized ratios respectively suggested the synergistic and antagonistic relationships of water and nitrate under shallow- and deep-rooted vegetation. As the important agent for nitrate leaching, soil water is influenced by magnetic susceptibility and sand. Furthermore, soil water, pH, EC, and soil organic carbon are individually or simultaneously responsible for nitrate transport and transformation, especially at large scales (> 7.5 m). This study provides novel insights for vegetation and environmental management, and benefits the parameterizing of process-based hydrological and biogeochemical models at regional or global scales, especially in the deep unsaturated zones.

Fate and transport modelling framework for assessing risks to soil and groundwater from chemicals accidentally released during surface operations: An Australian example application from shale gas developments

Shale and tight gas developments in the Beetaloo (28,000 km2) and Cooper (139,000 km2) basins of Australia are subject to stringent State and Federal Government controls and assessments. Several scientific investigations are ongoing to improve the scientific basis of the risks from unconventional gas developments to water and the environment. In this study a framework was developed to derive estimates of chemical dilution associated with leakage to groundwater from accidental release of chemicals used for shale and tight gas extraction in Australia. The quantitative assessment accounted for key landscape parameters that determine natural attenuation: soil type, depth to groundwater and groundwater velocity. Both basins were discretised into 1000 × 1000 m2 grids for which the unsaturated zone and groundwater dilution factors were derived. Migration of chemicals through deep unsaturated zones was calculated with the HYDRUS-1D simulator, taking account of best-available hydraulic properties from a digital soil database. A three-dimensional analytical solution of the advection–dispersion equation provided estimates of dilution in groundwater after solutes travelled 500 m from the centre (source location) to the edge of every grid cell. The combined vadose zone-groundwater dilution factors were used to determine under which conditions concentrations of hydraulic fracturing chemicals or flowback water accidentally released into the environment would decrease to levels that are no longer considered harmful to the environment. When the method was applied to 39 hydraulic fracturing chemicals scheduled for stimulation of a shale gas well, ecotoxicological risk quotients (RQ) were calculated to indicate which chemicals were of no environmental concern. This work contributes to increasing the efficiency of quantitative impact assessments and provides a framework to develop dedicated monitoring and management practices to support regulation and management of the gas industry in Australia.

Modelling the attenuation of flowback chemicals for a soil-groundwater pathway from a hypothetical spill accident

Flowback water from shale gas operations contains formation-derived compounds, including trace metals, radionuclides, and organics. While accidental releases from storage tanks with flowback water are low-probability events if multiple containment barriers are put in place, they cannot be entirely excluded. Here the natural attenuation potential of deep unsaturated zones and groundwater was explored using predictive modelling involving a hypothetical leak from a storage tank. Actual chemical concentrations from flowback water at two shale gas wells with contrasting salinity (12,300 and 105,000 ppm TDS) in the Beetaloo Sub-basin (Northern Territory, Australia) served as input to the one-dimensional HYDRUS model for simulating chemical transport through the unsaturated zone, with groundwater at 50 and 100 m depth, respectively. Subsequent chemical transport in groundwater involved the use of a three-dimensional analytical transport model. For a total of 63 chemicals the long-term attenuation from dilution and dispersion in unsaturated sediments and groundwater was calculated. Predicted environmental concentrations for aquatic receptors were compared with no-effect levels of individual chemicals to derive risk quotients (RQ) and identify chemicals of no concern to ecosystem health (i.e. RQ <1). Except for salinity and radium-228 in one of the two wells, RQ < 1 for all other chemicals. The initial approach considered testing of toxicity to individual chemicals only. When direct toxicity assessments (DTAs) were used to account for effects of chemical mixtures, the required DTA-derived safe dilution factor for 95% species protection was 1.8 to 2.5 times higher than the dilution factor accounting for dispersion and dilution only. Accounting for biodegradation, sorption and radioactive decay decreased chemical concentrations in unsaturated sediments to safe levels using the DTA for all chemicals. The study highlighted the importance of incorporating DTA in chemical risk assessments involving complex chemical mixtures. Improved understanding of fate and transport of flowback chemicals will help effectively manage water-quality risks associated with shale gas extraction.

Stand age and precipitation affect deep soil water depletion of economical forest in the loess area

Success of afforestation efforts hinges on better understanding of roles of water in the deep unsaturated zone in plant water uptake. But previous studies have deep soil water measurement either not deep, or not long enough to understand annual deep soil depletion rate for reforested forests, especially the effect of stand age or precipitation year on annual deep soil water depletion rate. To address this question, we selected, in 2016, four apple orchards planted on cropland in 2008, 2005, 2001, and 1994 (i.e., young orchard AP2008, mature orchards AP2005 and AP2001, and old orchard AP1994, respectively) in a thick loess area. Soil water contents were measured in 2016 - 2020 to the depth of 19 m with an in-situ neutron probe technique. Meanwhile, leaf area indexes were measured in 2017 and 2019 to represent each orchard water demand. Results showed annual deep soil water in 5-19 m showed a peak depletion rate of 160 mm yr−1 at around 14 years of orchard age. After the age effects are removed, mature orchards AP2005 and AP2001 showed significant depletions in 5-19 m deep soil water in the dry year 2016; in normal years 2017/2018, the depletion rates were reduced; and no significant deep soil water depletions were found in all orchards in the wet year 2019. The greater deep soil water uses in mature orchards AP2005 and AP2008 agreed with greater leaf area indexes. The depleted deep soil water contributions to actual evapotranspiration (AET) were 27.0% and 13.6% in the dry year 2016 for orchards AP2005 and AP2001, respectively; but their respective contributions were 15.1% and 7.9% in normal years 2017/2018. This indicates both the rainwater supply and the stand age affect deep soil water use, which suggests that more rainwater saving strategies and branch pruning are needed to avoid excess deep soil water depletion in dry and normal precipitation years for the mature forest on the Chinese Loess Plateau.

Groundwater recharge mechanisms on the Loess Plateau of China: New evidence for the significance of village ponds

Numerous studies have been conducted to understand the mechanism of groundwater recharge for water resources management. But the roles of wide-spread village ponds on the Chinese Loess Plateau on groundwater recharge remains uncertain. Here, to evaluate the groundwater recharge under village ponds, we measured environmental tracers (2H, 18O, Cl-) in rainwater, groundwater, and pore waters of the deep unsaturated zone under three land-cover types (cropland, a village pond, and two apple orchards with stand ages of 10 and 27 years). The results show that shallow groundwater is recharged by both diffusive recharge and focused recharge. Specifically, diffusive recharge occurs beneath farmlands, grasslands, orchards and similar land-cover types, but diminishes in old orchards due to deep root water uptake. The diffusive recharge beneath crop field had an infiltration velocity of 10.3–12.5 cm yr−1; the resulting recharge rate was 30–33 mm yr−1, accounting for 5–6% of the annual precipitation. However, focused recharge occurs mainly beneath village ponds with low-lying terrain, and had an infiltration velocity of 26 cm d−1 during the rainy season; the resulting mean recharge rate was 134 mm yr−1 in the catchment of the pond, accounting for 23.1% of the annual precipitation. Given that the water table is generally more than 30 m below the soil surface, it takes hundreds of years for rainwater to reach the water table via the diffusive recharge but only days to months via the focused recharge. Diffusive recharge contributed 55–68% to the total recharge while focused recharge contributed 32–45%. However, the conversion of croplands to apple orchards and the decrease in the number of village ponds are occurring, which diminishes diffusive recharge, and reduces focused recharge, respectively. These findings are critical to understanding hydrological processes and protecting groundwater resources. We should preserve, reclaim, and construct village ponds to ensure groundwater recharge, in addition to optimizing areal ratio of large farmland to apple orchards in the region for the sustainable utilization of groundwater resources.

Groundwater recharge in hillslopes on the Chinese Loess Plateau

Study regionYuanzegou Watershed in Qingjian, Wangdonggou watershed in Changwu, the Chinese Loess Plateau. Study focusHillslopes are the predominant land form unit on Earth, and despite its importance in maintaining groundwater supplies in many parts of the world, groundwater recharge on hillslopes is poorly understood. We hypothesize that groundwater recharge in the deep unsaturated zones of hillslopes is facilitated mainly by soil matrix flow, and the recharge rates are much smaller in hillslopes than on flat landscapes. To test the hypothesis, seven 15–20 m-long cores were collected from areas of different land uses to determine the groundwater recharge rates and its controls in deep unsaturated zones of hillslopes at a sub-humid and a semi-arid watershed located on the Chinese Loess Plateau. New hydrologic insights for the regionThe tritium distribution in each soil profile exhibits a well-defined bell-shape, signifying that soil matrix displacement is the main recharge mechanism. The tritium peak was located between depths of 9.33 m–11.01 m at the semiarid watershed and 6.29 m–7.22 m at the sub-humid watershed, and the recharge rates varied from 24.5 to 33.8 mm yr−1. These recharge rates account for 4 %–7 % of the long-term precipitation, which is approximately 56 % of the recharge typically seen on flat landscapes. Groundwater recharges were mainly controlled by soil texture, and land use changes, but exhibited little impact from the climatic difference between the two watersheds.

Origin of sulphate in the unsaturated zone and groundwater of a loess aquifer

AbstractThe use of the sulphate mass balance (SMB) between precipitation and soil water as a supplementary method to estimate the diffuse recharge rate assumes that the sulphate in soil water originated entirely from atmospheric deposition; however, the origin of sulphate in soil and groundwater is often unclear, especially in loess aquifers. This study analysed the sulphur (δ34S‐SO4) and oxygen (δ18O‐SO4) isotopes of sulphate in precipitation, water‐extractable soil water, and shallow groundwater samples and used these data along with hydrochemical data to determine the sources of sulphate in the thick unsaturated zone and groundwater of a loess aquifer. The results suggest that sulphate in groundwater mainly originated from old precipitation. When precipitation percolates through the unsaturated zone to recharge groundwater, sulphates were rarely dissolved due to the formation of CaCO3 film on the surface of sulphate minerals. The water‐extractable sulphate in the deep unsaturated zone (&gt;10 m) was mainly derived from the dissolution of evaporite minerals and there was no oxidation of sulphide minerals during the extraction of soil water by elutriating soil samples with deionized water. The water‐extractable concentration of SO4 was not representative of the actual SO4 concentration in mobile soil water. Therefore, the recharge rate cannot be estimated by the SMB method using the water‐extractable concentration of SO4 in the loess areas. This study is important for identifying sulphate sources and clarifying the proper method for estimating the recharge rate in loess aquifers.

Oak Transpiration Drawn From the Weathered Bedrock Vadose Zone in the Summer Dry Season

AbstractThe spatiotemporal dynamics of plant water sources are hidden and poorly understood. We document water source use of Quercus garryana growing in Northern California on a profile of approximately 50 cm of soil underlain by 2–4 m of weathered bedrock (sheared shale mélange) that completely saturates in winter, when the oaks lack leaves, and progressively dries over the summer. We determined oak water sources by combining observations of water stable isotope composition, vadose zone moisture and groundwater dynamics, and metrics of tree water status (potential) and use (sapflow). During the spring, oak xylem water is isotopically similar to the seasonal groundwater and shallow, evaporatively enriched soil moisture pools. However, as soils dry and the water table recedes to the permanently saturated, anoxic, low‐conductivity fresh bedrock boundary, Q. garryana shifts to using a water source with a depleted isotopic composition that matches residual moisture in the deep soil and underlying weathered bedrock vadose zone. Sapflow rates remain high as late‐summer predawn water potentials drop below −2.5 MPa. Neutron probe surveys reveal late‐summer rock moisture declines under the oaks in contrast to constant rock moisture levels under grass‐dominated areas. We therefore conclude that the oaks temporarily use seasonal groundwater when it occupies the weathered profile but otherwise use deep unsaturated zone moisture after seasonal groundwater recedes. The ample moisture, connected porosity, and oxygenated conditions of the weathered bedrock vadose zone make it a key tree water resource during the long summer dry season of the local Mediterranean climate.

Modelling Recharge from Irrigation Developments with a Perched Water Table and Deep Unsaturated Zone

Modelling of recharge under irrigation zones for input to groundwater modelling is important for assessment and management of environmental risks. Deep vadose zones, when coupled with perched water tables, affect the timing and magnitude of recharge. Despite the temporal and spatial complexities of irrigation areas; recharge in response to new developments can be modelled semi-analytically, with most outputs comparing well with numerical models. For parameter ranges relevant to the western Murray Basin in southern Australia, perching can reduce the magnitude of recharge relative to irrigation accessions and will cause significant time lags for changes to move through vadose zone. Recharge in the vicinity of existing developments was found to be similar to that far from existing developments. This allows superposition to be implemented spatially for new developments, thus simplifying estimation of recharge. Simplification is further aided by the use of exponential approximants for recharge responses from individual developments.

Hydrochemical evolution of groundwater in gabbro of the Troodos Fractured Aquifer. A comprehensive approach

Ophiolites often form complex, fractured, multi-lithological and highly compartmentalized aquifers that are difficult to characterize. Groundwater chemistry reflects water-rock interactions and can provide insights to groundwater recharge mechanisms, flow paths and relative residence time. A comprehensive hydrogeochemical modelling approach with PHREEQC has been developed to identify water-rock interactions and simulate groundwater evolution and mixing with connate water in fractured aquifers. The methodology was applied in the gabbro aquifers of Troodos Massif (Cyprus). The geochemical processes generating the most frequently observed water types were reproduced through modelling. Based on the overall results, a generic evolution path has been identified: CaMgHCO3 → MgCaHCO3 → CaMgNaHCO3 → Ca/Mg/Na/HCO3/SO4 → NaMgHCO3SO4 → NaSO4/Cl group. CaMgHCO3 and MgCaHCO3 types were modelled to occur predominantly in early stages of recharge. Deep unsaturated zones and longer flow paths result in gradual removal of Ca+2 and Mg+2 from groundwater, through secondary mineral precipitation, and in an increase of Na+2 and SO4−2. The most frequently occurring water types, CaMgNaHCO3 and MgCaNaHCO3, were modelled to occur at later stages of hydro-chemical evolution, after secondary mineral precipitation, and in closed conditions. In regional flow, where closed conditions with secondary mineral precipitation prevail, water types progressively shift from the HCO3 to the NaSO4 end-member group. In 38% of the 536 samples from gabbro aquifers in Troodos, groundwater mixing with connate water at a ratio of ≥0.999:0.001 occurs.

How does precipitation recharge groundwater in loess aquifers? Evidence from multiple environmental tracers

Characterization of the rainfall infiltration mechanism, whether piston or preferential flow, through an unsaturated zone is necessary for groundwater resource assessment and for analysis of environmental geological hazards. However, several limitations and difficulties remain in determining the infiltration mechanism in arid and semiarid areas. This study combines the evaluations of soil water infiltration characteristics, recharge rate, and determination of groundwater age at a water table in a semiarid loess tableland to examine the recharge processes. The case study is the Xifeng loess tableland in the Loess Plateau of China, which has an unsaturated zone depth of 40–75 m and an obvious 1963-3H peak at a depth of 7.5 m. According to the piston flow, the soil water infiltration velocity is 0.12–0.14 m/yr, corresponding to a recharge rate of 37–41 mm/yr, based on the 1963-3H peak and chloride mass balance method. All of the groundwater from the water table and wells is old (tritium-free), with ages ranging from hundreds of years to 20,000 years, which suggests that no rapid modern recharge has occurred since the 1950s. The chloride content of soil water in the deep unsaturated zone, with an average value of 10.8 mg/L in the unsaturated zone below 30 m, overlaps with that in groundwater, at 4.0–10.9 mg/L. The comprehensive isotopic and chemical compositions for soil water and groundwater showed that piston flow was dominant during soil water infiltration at the field scale, resulting in the delay of modern water and related solutes entering the groundwater. Combined evaluations of the unsaturated zone with those of the saturated zone and the use of multiple environmental tracers, such as Cl, Br, 2H, 18O, 13C, 3H and 14C, can be regarded as a generic framework for studying the recharge processes in arid and semiarid areas with thick unsaturated zones at macroscopic and field scales.

Estimation of evapotranspiration rate by using the Penman–Monteith and Hargreaves formulas for the loess in Northeast Bulgaria complex with HYDRUS-1D

Loess and loess-like sediments cover approximately 13% of the Bulgarian territory, mainly within the Danubian plain. From the Danube River to the Fore-Balkan, the loess soils form a loess complex where its depth varies from 50–60 meters in the north to few meters in the south, respectively. Widespread loess sediments possess a specific feature: they typically form deep unsaturated zones. Quantification of the near surface water balance is extremely important for evaluating land-atmosphere interactions, and the impact of land-use change on the subsurface flow and the evapotranspiration rate is an essential term in this quantification. In the frames of a scientific project, an automatic weather station was installed in a typical plain terrain of the loess complex in Northeast Bulgaria, recording meteorological data from September 2015 to February 2017. This study provides a mathematical description of processes (i.e., Penman-Monteith and Hargreaves Methods) used to estimate daily evapotranspiration rates implemented into the numerical model HYDRUS-1D, as well as a respective rate investigation of months with and without intensive rainfalls. Overall results indicate that using the Hargreaves formula for evaluation of the potential evapotranspiration leads to overestimation between 10% and 20%, respectively for a “wet” and “dry” month.

Chloride tracer of the loess unsaturated zone under sub-humid region: A potential proxy recording high-resolution hydroclimate

A high-resolution hydroclimate archive is critical to understanding the past changes, but such hydroclimatic reconstructions are extremely limited. Our study aims to examine the potential of the chloride tracer (Cl) within unsaturated zone (USZ) to reconstruct high-resolution hydroclimate records. We investigated a representative sub-humid monsoon area of the Chinese Loess highland, where piston flow recharge dominates and a constant rate of Cl input occurs. We successfully reconstructed a 1007-yr hydroclimate record with a 2-yr sampling resolution using a 95-m USZ Cl profile. Excluding the recycling and mixing zone, and the groundwater affecting zone, six relatively humid phases and five relatively dry phases were discernible. The uncertainty in the time of wet/dry phase decreases as the accumulated time increases, and the minimal recharge uncertainty is 16% over a 10-yr time scale. The reconstructed hydroclimate record from the semi-humid region has a much higher resolution than that of the arid zones, likely because the semi-humid, fine-grained thick USZ, possesses higher velocity piston flow, relatively to diffusion and dispersion of Cl signals. The record compared well with other related records, suggesting that the Cl proxy can be used in sub-humid areas, and is sensitive to wet/dry alternations that are largely driven by the Asian monsoon intensity. To the best of our knowledge, this is the first investigation of the USZ Cl to hydroclimatic reconstruction in a non-arid region. The high-resolution hydroclimate record may deepen our understanding of the hydrological process in the deep USZ, past climate and water resources, and promote developments of the hydropedology and global change science.

Nitrogen stock and leaching rates in a thick vadose zone below areas of long-term nitrogen fertilizer application in the North China Plain: A future groundwater quality threat

Nitrate in groundwater is increasing in many aquifers worldwide due to excessive use of fertilizers. However, the behavior of N-based fertilizer as it leaches through thick vadose zones is still poorly understood, and the long-term effects on groundwater quality in such settings may yet to be fully realized. In this study, deep soil cores were taken below experimental plots where N-fertilizer was applied for nearly 20 years under wheat-maize double cropping in the North China Plain (NCP). N-fertilizer application rates of zero, 400, and 600 kg N ha−1 yr−1 were applied to different plots. Water and N balances were quantified to estimate the leaching rate of NO3−-N below the root zone, and NO3−-N loss via denitrification. Transport pathways of nitrate in the vadose zone were identified using stable isotopes of water (δ18O-H2O & δ2H-H2O) and nitrate (δ18O-NO3− & δ15N-NO3−) from the sampled pore-water and groundwater. The average N transport velocity through the vadose zone ranged from 38 to 41 cm yr−1 and was relatively constant regardless of fertilizer application rate. The transport rates are significantly lower than the long-term rate of groundwater level decline (100 cm yr−1), meaning it is unlikely much of the leachate from chemical N-fertilizers (abundantly applied throughout the region since the 1970s) has entered groundwater via matrix flow. Nitrate isotopes appear to indicate that manure and septic waste (not chemical N-fertilizer) are the major sources of nitrate in groundwater to date. However, stable isotopes show that increased nitrate concentrations in groundwater may also result due to preferential flow in sand layers and/or lateral flow from headwater areas. Denitrification (particularly in the upper 4 m depth of soil) appears to have limited the flux of fertilizer-derived nitrate to some degree. However, the large N stock and generally weak denitrification potential in the deep unsaturated zone, pose a major threat to future groundwater quality. The results have major implications for groundwater quality management in the future.

Elucidating controls of the variability of deep soil bulk density

Information on the variability of bulk density (BD) in deep soil is critical to understand the hydrological dynamics of the deep unsaturated zone (USZ), but studies have mainly focused on quantifying BD in shallow soils (<1 m). The objective of this study was to quantify the spatial variability of soil BD in deep USZ and their controls at multiple scales. Three undisturbed soil cores (98.0, 18.4, 22.8 m) were collected from three land-use areas (grassland, orchard, and farmland) from the Changwu tableland of the Chinese Loess Plateau; then, BD, magnetic susceptibility, particle size and organic carbon were determined in the laboratory. The spatial patterns and scale- and location-specific controls of BD variability were quantified using traditional statistics and wavelet analyses. BD distribution in the deep USZ was consistent across the cores indicating the minimal influence of land-use. For the vertical distribution of the USZ BD, the coefficient of variation (CV) in the upper 0–40 m was higher (8%) compared to the broadly steady BD (CV = 3%) in the lower 40–90 m depth. BD with depth mainly exhibited two-type variations: an exponentially increasing trend at the large scale (13–30 m) and a cyclical trend at the medium scale (3–13 m). While soil depth was the major factor contributing to BD variability at the large scale, magnetic susceptibility contributed at the medium scale and exhibited a stronger effect than organic carbon and particle size composition. This information on the deep USZ BD is distinctly different than that of the shallow soil from previous studies and enhanced our understanding of BD spatial patterns with depth and identified the dominant factor(s) to improve predictability. This can provide an objective evaluation of soil characteristics, accurate estimation of soil water, nutrients and carbon stocks and fluxes, and the simulation of related hydraulic processes in the deep USZ.