Exploring the coupling coordination relationship of water resources, socio-economy and eco-environment in China.

Water resources (W), socio-economy (S), and eco-environment (E) have incredibly intricate linkages of interaction, and the coordination of them is crucial to the long-term sustainability of a nation. Thus, we considered "water resources, socio-economy, and eco-environment" (W-S-E) as a composite system and constructed an evaluation model to quantitatively analyze the coupling coordination degree (CCD) of W-S-E system in China from 2011 to 2020. Then, the spatial correlation characteristics were analyzed by using spatial autocorrelation method. To analyze the time evolution patterns of the W-S-E system, this paper divided the stages from the perspective of clustering, which is more scientific and interpretable than the CCD fixed-value division. We found that: (1) W subsystem, S subsystem and E subsystem were closely connected and its CCD was enhanced with relatively higher growth rates in the development of S subsystem but slower growth rates in the W subsystem. (2) The CCD of W-S-E system had spatial correlation. The areas with low CCD were concentrated in the west of China, forming poor coordinated development phenomena. Conversely, most of provinces had relatively high CCD in the east of China with the coastal region playing radiative driving function. (3) The temporal change of W-S-E system followed four transforming patterns including "policy-oriented type", "resource problems constraint type", "socio-economy leading type", and "special location controlling type". Furthermore, we also put forward some advice and policy suggestions. The findings provide research basis and guidance for the sustainable and coordinated development of water, society and ecology.

Dynamics and control mechanisms of inorganic nitrogen removal during wetting-drying cycles: A simulated managed aquifer recharge experiment.

Managed aquifer recharge (MAR) systems can be operated intermittently through wetting-drying cycles to simultaneously improve the water supply and quality. Although MAR can naturally attenuate considerable amounts of nitrogen, the dynamic processes and control mechanisms of nitrogen removal by intermittent MAR remain unclear. This study was conducted in laboratory sandy columns and lasted for 23 d, including four wetting periods and three drying periods. The hydraulic conductivity, oxidation reduction potential (ORP), and leaching concentrations of ammonia nitrogen and nitrate nitrogen of MAR systems were intensively measured to test the hypothesis that hydrological and biogeochemical controls play an essential role in regulating nitrogen dynamics at different stages of wetting-drying cycles. Intermittent MAR functioned as a sink for nitrogen while providing a carbon source to support nitrogen transformations; however, it occasionally became a source of nitrogen under intense flushes of preferential flow. Nitrogen dynamics were primarily controlled by hydrological processes in the initial wetting phase and were further regulated by biogeochemical processes during the subsequent wetting period, supporting our hypothesis. We also observed that a saturated zone could mediate nitrogen dynamics by creating anaerobic conditions for denitrification and buffering the flush effect of preferential flow. The drying duration can also affect the occurrence of preferential flow and nitrogen transformations, which should be balanced when determining the optimal drying duration for intermittent MAR systems.

Analysis of Groundwater Overexploitation Based on Groundwater Regime Information.

Although groundwater overexploitation is a global problem, there are no unified standards for its identification and determination. To date, groundwater overexploitation has mainly been evaluated using the groundwater quantity balance and effect of groundwater exploitation on the environment. However, it is difficult to determine groundwater exploitation for an agricultural irrigation area owing to the lack of detailed environmental monitoring data. We used the experience of previous studies to introduce a novel identification model for groundwater overexploitation by relying on groundwater regime information and multi-factor analysis. The model was applied to the Songhua River-Naoli River area, Sanjiang Plain (China). In the demonstrated model, the study area was divided in the context of the risk of overexploitation into natural and non-natural regime areas according to groundwater regime characteristics. The areas were identified by analyzing groundwater flow fields, cones of groundwater depression, and storage variation. The analysis demonstrated that the study area could be divided into high-risk, medium-risk, low-risk, and non-overexploitation areas with corresponding area ratios of 10.12%, 1.38%, 54.8%, and 33.7%. Moreover, the total amount of overexploited groundwater was estimated to be 30.41 × 108 m3 (average annual decrease = 1.69 × 108 m3 ). Overall, the proposed identification model was shown to be applicable to agricultural irrigation areas, thereby offering promising implications.

Air-water interfacial adsorption of C4-C10 perfluorocarboxylic acids during transport in unsaturated porous media.

The impact of chain length on air-water interfacial adsorption of perfluorocarboxylic acids (PFCAs) during transport in unsaturated quartz sand was investigated. Short-chain (C4-C7: PFBA, PFPeA, PFHxA, PFHpA) and long chain (C8-C10: PFOA, PFNA, PFDA) PFCAs were selected as a representative homologous series. Surface tensions were measured to characterize surface activities of the selected PFCAs. Miscible-displacement column experiments were conducted for each of the PFCAs to characterize the magnitudes of air-water interfacial adsorption under transport conditions. The transport of the long-chain PFCAs exhibited greater retardation than the short-chain PFCAs. Air-water interfacial adsorption (AWIA) was the predominant source of retention (≥63%) for the long-chain PFCAs. Conversely, AWIA contributed less to retention than did solid-phase sorption for the short-chain PFCAs, with the former contributions ranging from 4% to 40%. Direct examination of the breakthrough-curve profiles as well as mathematical-modeling results demonstrated that transport of the two longest-chain PFCAs was influenced by nonlinear AWIA, whereas that of the shorter-chain PFCAs was not. This disparate behavior is consistent with the input concentration used for the transport experiments in comparison to the respective surface activities and critical reference concentrations of the different PFCAs. Quantitative-structure/property-relationship (QSPR) analysis was applied to characterize the influence of molecular size on air-water interfacial adsorption. The logs of the air-water interfacial adsorption coefficients (Kia) determined from the transport experiments are a monotonic function of molar volume, consistent with prior QSPR analyses of surface-tension measured values. The Kia values determined from the transport experiments are very similar to those measured from surface-tension data, indicating that the transport experiments produced robust measurements of AWIA.

Clogging and Water Quality Change Effects of Typical Metal Pollutants under Intermittent Managed Aquifer Recharge Using Urban Stormwater.

Managed aquifer recharge (MAR) using urban stormwater facilitates relieving water supply pressure, restoring the ecological environment, and developing sustainable water resources. However, compared to conventional water sources, such as river water and lake water, MAR using urban stormwater is a typically intermittent recharge mode. In order to study the clogging and water quality change effects of Fe, Zn, and Pb, the typical mental pollutants in urban stormwater, a series of intermittent MAR column experiments were performed. The results show that the type of pollutant, the particle size of the medium and the intermittent recharge mode have significant impacts on the pollutant retention and release, which has led to different clogging and water quality change effects. The metals that are easily retained in porous media have greater potential for clogging and less potential for groundwater pollution. The fine medium easily becomes clogged, but it is beneficial in preventing groundwater contamination. There is a higher risk of groundwater contamination for a shallow buried aquifer under intermittent MAR than continuous MAR, mainly because of the de-clogging effect of porous media during the intermittent period.

Analysis of Potential Risks Associated with Urban Stormwater Quality for Managed Aquifer Recharge.

Managed aquifer recharge (MAR) can be used to increase storage and availability of groundwater resources, but water resources available for recharge are constrained due to a surface water shortage. Alternative resources, like stormwater, are receiving increasing attention as sustainable resources for reuse in MAR. However, pollutants in stormwater can impact groundwater quality, and cause clogging of the infiltration system. Based on the stormwater data in the literature, the physicochemical stormwater properties of data were analyzed. The results showed that concentrations of pollutants from different underlying surfaces varied widely. The main pollutants of stormwater were as follows: Total suspended particles (TSSs), organic matter represented by the chemical oxygen demand (COD), nutrients (total nitrogen, TN; total phosphorus, TP; and NH3-N), and metals (Zn, Pb, Cu, Cd, Fe, and Mn). Based on the simulation of TOUGHREACT, the contamination risk of pollutants for each type of stormwater was assessed. The risk of contamination was divided into four categories due to the different migration times of ions through the sand column. The iron ion has the highest risk of contamination, followed by Zn and Mn, and the contamination risk of nutrients and other metals (Pb, Cu, and Cd) are relatively low. Besides, the physical, biological, and chemical clogging risk were evaluated. The physical clogging potential of all types of stormwater is very high because of the high concentration of TSS. According to the concentration of TN that can spur the growth of bacteria and algae, the relative risk of biological clogging for stormwater is greenbelt stormwater < road stormwater < roof stormwater. However, only road stormwater has high chemical clogging due to the existence of iron, which can generate precipitation that blocks the pore volume.

Mechanism of Suspended Kaolinite Particle Clogging in Porous Media During Managed Aquifer Recharge.

Managed aquifer recharge is an effective strategy for urban stormwater management. Chemical ions are normally retained in stormwater and groundwater and may accelerate clogging during the recharge process. However, the effect of water chemistry on physical clogging has not previously been investigated. In this study, we investigated the hydrogeochemical mechanism of saturated porous media clogging in a series of column experiments. The column was packed with river sand and added suspensions of kaolinite particles. Calcium chloride and sodium chloride are used as representative ions to study chemical effects. We found that an increase in ionic strength resulted in retention of kaolinite solids in the column, with a breakthrough peak of C/C0 value of 1 to 0.2. The corresponding hydraulic conductivity decreased with increased solids clogging. Divalent cations were also found to have a greater influence on kaolinite particle clogging than monovalent cations. The enhanced hydrochemical-related clogging was caused by kaolinite solids flocculating and increasing the deposition rate coefficient by 1 to 2 times in high ionic strength conditions. Three clogging mechanisms of kaolinite solids are proposed: surface filtration, inner blocking, and attachment. This study further deepens the understanding of the mechanisms of solids clogging during aquifer recharge and demonstrates the significance of ionic strength on recharge clogging risk assessments.

Effect of snowmelt infiltration on groundwater recharge in a seasonal soil frost area: a case study in Northeast China.

The effect of spring snowmelt infiltration in a seasonal soil frost area on groundwater recharge was evaluated by systematically monitoring meteorological factors, soil temperature and humidity, groundwater table and temperature, electrical conductivity, and the value of δ18O in a small field site over a 2-year period. The variation of soil temperature and humidity, groundwater table during the freezing period, and the snowmelt period respectively, as well as their correspondence to the relevant environmental factors, and the influencing factors of the permeability of frozen layer were analyzed. The results showed that the evaluation of precipitation infiltration in seasonal soil frost areas should be divided into three stages: a non-freezing period, a freezing period, and a snowmelt period. Snow is the main form of precipitation during the freezing period, and groundwater cannot be recharged. During the snowmelt period of spring, the snow cover that accumulated during the freezing period infiltrates together with rainfall and has a significant effect on groundwater recharge. The general precipitation infiltration process occurs after the frozen soil thaws completely. These research results can improve the accuracy of groundwater recharge calculations for snowmelt infiltration in the seasonal soil frost area of Northeast China and provide a scientific basis for the evaluation and management of regional water resources.

Identification of key influence factors and an empirical formula for spring snowmelt-runoff: A case study in mid-temperate zone of northeast China.

Because of the unique climate characteristics, the runoff law in mid-temperate zone is very different from other regions in spring. Accurate runoff simulation and forecasting is of great importance to spring flood control and efficient use of water resources. Baishan reservoir is located in the upper Second Songhua River Basin in Northeast China, where snowmelt is an important source of runoff that contributes to the water supply. This study utilized long-term hydrometeorological data, in the contributing area of Bashan reservoir, to investigate factors and time-lag effects on spring snowmelt and to establish a snowmelt-runoff model. Daily precipitation, temperature, and wind data were collected from three meteorological stations in this region from 1987-2016. Daily runoff into the Baishan reservoir was selected for the same period. The snowmelt period was identified from March 23 to May 4 through baseflow segmentation with the Eckhardt recursive digital filtering method combined with statistical analyses. A global sensitivity analysis, based on the back propagation neural network method, was used to identify daily radiation, wind speed, mean temperature, and precipitation as the main factors affecting snowmelt runoff. Daily radiation, precipitation, and mean temperature factors had a two-day lag effect. Based on these factors, an empirical snowmelt runoff model was established by genetic algorithm (GAS) to estimate the snowmelt runoff in this area. The model showed an acceptable performance with coefficient of determination (R2) of 73.6%, relative error (Re) of 25.10%, and Nash-Sutcliffe efficiency coefficient (NSE) of 66.2% in the calibration period of 1987-2010, while reasonable performance with R2 of 62.3%, Re of 27.2%, and NSE of 46.0% was also achieved during the 2011-2016 validation period.

[Research Progress on the Sources of Inorganic Nitrogen Pollution in Groundwater and Identification Methods].

The identification of the main inorganic nitrogen (MIN, referring to NH4+-N, NO3--N, and NO2--N) pollution sources in groundwater is of great significance to its control and repair. A review of the MIN sources in groundwater and the main identification methods was conducted. The main sources of MIN pollution in groundwater (atmospheric nitrogen deposition, soil natural organic nitrogen mineralization, nitrogen from streams, and nitrogen emission from human activity), and its distribution in China were expounded. The common methods for tracing MIN sources include hydrochemical analysis, statistical estimation, regional nitrogen balance evaluation, stable isotopes tracer, and new types of tracers. Because of the variety of nitrogen sources and the complexity of the MIN pollution formation mechanism, the single identification methods shared limitations in application, whereas more comprehensive ones, especially the stable isotope tracer integrated with other methods, were mainstream. Furthermore, future research prospects, including the development of new types of tracing methods, the optimization of quantification methods, the integration of research on pollution source identification, transformation mechanism, groundwater recharge and discharge condition, and groundwater-surface water conversion, have been put forward.

Unraveling the complexities of the velocity dependency of E. coli retention and release parameters in saturated porous media.

Escherichia coli transport and release experiments were conducted to investigate the pore-water velocity (v) dependency of the sticking efficiency (α), the fraction of the solid surface area that contributed to retention (Sf), the percentage of injected cells that were irreversibly retained (Mirr), and cell release under different (10-300mM) ionic strength (IS) conditions. Values of α, Sf, and Mirr increased with increasing IS and decreasing v, but the dependency on v was greatest at intermediate IS (30 and 50mM). Following the retention phase, successive increases in v up to 100 or 150mday-1 and flow interruption of 24h produced negligible amounts of cell release. However, excavation of the sand from the columns in excess electrolyte solution resulted in the release of >80% of the retained bacteria. These observations were explained by: (i) extended interaction energy calculations on a heterogeneous sand collector; (ii) an increase in adhesive strength with the residence time; and (iii) torque balance consideration on rough surfaces. In particular, α, Sf, and Mirr increased with IS due to lower energy barriers and stronger primary minima. The values of α, Sf, and Mirr also increased with decreasing v because the adhesive strength increased with the residence time (e.g., an increased probability to diffuse over the energy barrier) and lower hydrodynamic forces diminished cell removal. The controlling influence of lever arms at microscopic roughness locations and grain-grain contacts were used to explain negligible cell removal with large increases in v and large amounts of cell recovery following sand excavation. Results reveal the underlying causes (interaction energy, torque balance, and residence time) of the velocity dependency of E. coli retention and release parameters (ksw, α, and Sf) that are not accounted for in colloid filtration theory.

Surface clogging process modeling of suspended solids during urban stormwater aquifer recharge.

Aquifer recharge, which uses urban stormwater, is an effective technique to control the negative effects of groundwater over-exploitation, while clogging problems in infiltration systems remain the key restricting factor in broadening its practice. Quantitative understanding of the clogging process is still very poor. A laboratory study was conducted to understand surface physical clogging processes, with the primary aim of developing a model for predicting suspended solid clogging processes before aquifer recharge projects start. The experiments investigated the clogging characteristics of different suspended solid sizes in recharge water by using a series of one-dimensional fine quartz sand columns. The results showed that the smaller the suspended particles in recharge water, the farther the distance of movement and the larger the scope of clogging in porous media. Clogging extents in fine sand were 1 cm, for suspended particle size ranging from 0.075 to 0.0385 mm, and 2 cm, for particles less than 0.0385 mm. In addition, clogging development occurred more rapidly for smaller suspended solid particles. It took 48, 42, and 36 hr respectively, for large-, medium-, and small-sized particles to reach pre-determined clogging standards. An empirical formula and iteration model for the surface clogging evolution process were derived. The verification results obtained from stormwater recharge into fine sand demonstrated that the model could reflect the real laws of the surface clogging process.