Influence of velocity distribution conditions of coastal aquifer on nitrate contamination and seawater intrusion in the presence of subsurface physical barriers.

The velocity distribution is an important factor that affects seawater intrusion (SI) and nitrate (NO3-) pollution. However, there are few studies on the impact of subsurface physical barriers (SPBs) on the velocity distribution of the whole aquifer and the impact of velocity distribution on SI and NO3- pollution. Especially, the quantitative method of velocity distribution has not been studied. By the methods of laboratory experiments and numerical simulations, effects of the NO3- concentrations of the pollution source, hydraulic gradients (HGs), the location of the SPB and relative heights of SPBs (HP') on the SI, NO3- pollution levels and velocity in the presence of SI and SPBs were investigated. The velocity distribution was first quantified to better describe the relationships between the velocity and degrees of SI and NO3- pollution. The results showed that the HG and HP' were the main factors that affected the velocity, NO3- pollution and SI. The higher the HG, the smaller the HP', and the decreased SI inferred a more serious NO3- pollution. The influence of SPBs on NO3- pollution and SI was mainly affected by the changes in the velocity distribution in the aquifer. With increasing HGs, for the region with flow rate less than 0.5 m/d (A0.5), the smaller its distribution area is, the smaller the relative area of SI (TLs') is. With an increase in the HG or decrease in the HP', the relative area of NO3- pollution (Ns') is proportional to the distribution area where the flow velocity is greater than 1 m/d (A1). When the flow velocity distribution condition was A'1 (the relative area of A1) > A'0.5-1 (A'0.5-1 is the ratio of the area where the flow velocities are greater than 0.5 m/d and less than 1 m/d to the total area of the aquifer) > A'0.5 (the relative area of A0.5), NO3- pollution was serious; when the flow velocity distribution condition was A'0.5 > A'0.5-1 > A'1, the levels of NO3- pollution were the lowest.

A hybrid spatio-temporal data indexing method for trajectory databases.

In recent years, there has been tremendous growth in the field of indoor and outdoor positioning sensors continuously producing huge volumes of trajectory data that has been used in many fields such as location-based services or location intelligence. Trajectory data is massively increased and semantically complicated, which poses a great challenge on spatio-temporal data indexing. This paper proposes a spatio-temporal data indexing method, named HBSTR-tree, which is a hybrid index structure comprising spatio-temporal R-tree, B*-tree and Hash table. To improve the index generation efficiency, rather than directly inserting trajectory points, we group consecutive trajectory points as nodes according to their spatio-temporal semantics and then insert them into spatio-temporal R-tree as leaf nodes. Hash table is used to manage the latest leaf nodes to reduce the frequency of insertion. A new spatio-temporal interval criterion and a new node-choosing sub-algorithm are also proposed to optimize spatio-temporal R-tree structures. In addition, a B*-tree sub-index of leaf nodes is built to query the trajectories of targeted objects efficiently. Furthermore, a database storage scheme based on a NoSQL-type DBMS is also proposed for the purpose of cloud storage. Experimental results prove that HBSTR-tree outperforms TB*-tree in some aspects such as generation efficiency, query performance and query type.

Distinguishing and quantifying the fate of nitrate in irrigation water and nitrate produced by ammonium nitrification.

When the sources of nitrogen include not only ammonium (NH4+) fertilizer (ANF) but also nitrate (NO3-) from groundwater and rainfall (NRI), if the proportions of various types of NO3- are still based on the amount of ANF, the corresponding calculation method may be complicated. This paper established a water flow-nitrogen migration transformation model for the unsaturated zone in grain-planting and vegetable-planting areas, and studied the migration and transformation of NH4+ and NO3- in the unsaturated zone when ANF and NRI coexist. This paper proposed for the first time the proportional coefficient method (PCM) and hypothetical assignment method (HAM) to distinguish and quantify the fate proportions of NO3- from NO3- produced by NH4+ nitrification (NNR) and NRI. The results showed that the PCM was more practical than the HAM in quantifying the fate of NO3- from different sources. If only the root absorption ratio was used to evaluate the degree of nutrient supply to crops, the ratios of root absorption were as high as 40% (44.75-50.85%). NRI provided more nutrients in grain-growing areas than those in vegetable-growing areas. If the sum of the proportion of other fates was regarded as the degree of groundwater NO3- mitigation through irrigation in the unsaturated zone, except for the ratio of NO3- leaching to groundwater, the proportion of NO3- pollution mitigation was as high as 57.89% (57.89-92.99%), and the mitigation ability of groundwater NO3- pollution in grain-growing areas was higher than that in vegetable-growing areas.

Quantitative analysis of dominant mechanisms in improving fluid sweeping uniformity in a layered heterogeneous system via xanthan gum addition.

Although it has been proven that the addition of shear-thinning polymers can lead to an improvement in the sweeping efficiency of the remedial agent in heterogeneous aquifers, the related mechanisms require further investigation. This study revealed the mechanisms associated with the improvement of fluid sweeping uniformity. Under the assumption of no transverse flow existed between layers, the variation patterns of the fluid sweeping uniformity in strip layered heterogeneous media (RP/RK) with the change of the fluid hydraulic conductivity were investigated. The outcomes showed that, for the hydraulic conductivity dominated fluid sweeping uniformity control, the performance was satisfactory only when the ratio of the hydraulic conductivities of viscous fluid in porous media (PM) layers (K1P/K2P) is less than 2.5 times that of pure oxidant solution (K1K/K2K), and PM combinations with higher permeability contrast usually present a wider applicability and better performance. In addition, transverse fluid movement was more likely to occur in layers with low permeability contrasts (2.3-5) than in those with relatively higher permeability contrasts (> 11). The observed transverse pressure difference directly confirmed the hypothesis of the dynamics of the cross-flow mechanism that existed during viscous fluid flow, and the relationships between the fluid fronts and the pressure head difference at each point (Px) were proposed. Our study exemplifies an effective strategy for determining the most economically effective co-injection concentration of xanthan to achieve highly efficient delivery of remedial agents.

Reference ion method: A simple and fast method for quantitatively identifying the source of nitrate and denitrification rate in groundwater.

The effective control and management of nitrate (NO3-) pollution requires the identification of the sources of NO3- pollution in groundwater and quantification of their contribution rates. In this study, the molar concentration ratio of NO3-/Cl- (n(NO3-)/n(Cl-)) and the molar concentration of Cl- (n(Cl-)) (reference ion method; RIM) was first used to identify the NO3- sources and estimate their contribution rates in groundwater. The relationship between the Cl- concentration and NO3- concentration (reference ion method; RIM) was used to judge whether denitrification had occurred and to estimate the denitrification rate in groundwater. It was proved that homology analysis was the prerequisite for applying the RIM. The main NO3- sources included chemical fertilizers (CF), sewage/manure (M&S) and soil nitrogen (SN). The contribution rate of CF in the vegetable planting area (upstream regions) (69.12%) was significantly higher than that in the grain planting area (midstream regions and downstream regions) (14.29% and 14.29%). The difference in the contribution rates of NO3- in the grain planting area was greater than that in the vegetable planting area. The results indicated that denitrification rate in the grain planting area was higher than that in the vegetable planting area, while the temporal variations in the denitrification rate in the vegetable planting area were consistent with in the grain planting area. The RIM offers a useful and simple way to quantify the contribution rates of NO3- sources and denitrification rates in groundwater.

Influence of the subsurface physical barrier on nitrate contamination and seawater intrusion in an unconfined aquifer.

Coastal areas are facing not only environmental problems associated with seawater intrusion (SWI) but also health and ecological problems caused by excessive nitrate (NO3-) contamination. The installation of a subsurface physical barrier (SPB) is one of the common methods employed to reduce or prevent SWI, but there are few studies on the impact of SPBs on NP in groundwater. Through laboratory experiments and numerical simulations, the effects of the hydraulic gradient (HG), the nitrate concentration of the set groundwater nitrate pollution source, the relative height of the SPB (HP') and the relative distance between the SPB and the saltwater boundary on the NP of groundwater in the presence of SWI, subsurface dams and cut-off walls were studied. Evaluation indicators were established to evaluate the degree and shape of the SWI and NP. To better describe the relationship between the velocity distribution and changes in the velocity distribution area and the degree of NP and SWI, the velocity distribution in the presence of SWI and a SPB was summarized separately. The results showed that when there was SPB, low-velocity zones were formed on both sides of the SPB, which not only slowed the migration of NO3- but also changed the shape of the NO3--contaminated area. The closer to the SPB area the pollutants were, the more obvious the obstruction effect. The obstruction effect of adding the cut-off wall on NP was more obvious than that of adding the subsurface dam wall. The selected HG and Hp' were important factors affecting NP and SWI. The higher HG was, the more serious the NP, the lower the HG, and the stronger the degree of SWI. Adding SPBs reduces the impact of HGs on NP and SWI. Therefore, the design of SPBs in coastal areas should focus on aspects related to these two factors.