Evaluation on quality and health risk of groundwater in a highly urbanized watershed, China.

Urbanizations and industrializations may accelerate the contamination and deterioration of groundwater quality. This study aimed to evaluate the quality and potential human health risks associated with shallow groundwater in Shenzhen, China, a city characterized by high levels of urbanization and industrialization. The hydrochemistry characteristics, water quality levels, and human health risks of main ions, nutrient elements, and metals in 220 samples collected from Maozhou River Basin (MRB) located in the northwest of Shenzhen were investigated. It showed that chemical constituents of the groundwater were further complicated by seawater intrusion and urbanization expansion. Water quality evaluated by fuzzy comprehensive method showed that 21.05% of samples distributed around reservoirs were classified into grade II or better. Nearly 79% of samples distributed in the densely populated urban land were classified into grade III or worse, indicating pollution from anthropogenic factors cannot be ignored. For the river tidal reach where river stage fluctuated about 0.5 to 1.5 m within a tidal cycle, the chemical composition of groundwater was influenced by frequent water exchange with the river. The carcinogenic and non-carcinogenic health risks for different age groups, from the high to the low, were children, adult women, adult men, adolescent women, and adolescent men, respectively. Approximately 39% of groundwater samples distributed around the densely populations area with health risk larger than 5 × 10-5 were unacceptable for children. This investigation would be helpful for improving groundwater management and as a practical reference for sustainable groundwater exploitation in the MRB.

Dynamics in Diffusive Emissions of Dissolved Gases from Groundwater Induced by Fluctuated Ground Surface Temperature.

During the lateral transport with subsurface flow, amounts of manufactured volatile organic chemicals and gases dissolved in groundwater are emitted into the atmosphere via upward diffusion through soils. Quantifying gas emissions is important for assessing environmental risk associated with these constituents (e.g., air pollution and global warming). It is widely recognized that the temperature would affect gas spreading in soils, which in turn regulates the gas emission from groundwater. However, the upward diffusive gas emission induced by the fluctuated ground surface temperature (GST) remains unexplored. A coupled heat transfer and gas transport model is developed to investigate emissions of tetrachloroethylene (PCE) and N2O, a typical manufactured volatile organic chemical and a natural gas, from groundwater with seasonally fluctuating GSTs. The results indicate that both PCE and N2O emissions vary significantly from month to month. Moreover, fluctuations of emissions lag obviously behind the fluctuation of GST due to the damping effects of both capillary fringe and soil sorption. The proposed model agrees with the observed data from a monolith lysimeter experiment well. The model is also applied to the estimations of N2O emissions from 12 aquifers in Walloon Region, Belgium. The estimated N2O emission is 12.6 µg N/m2/d that falls in the estimated range (9.0-21.5 µg N/m2/d) using the IPCC emission factor approach that commonly accounts for the N2O emission of groundwater discharge to surface water only. It suggests that the upward diffusion is non-negligible for estimations of N2O emission from groundwater.

Fractions Transformation and Dissipation Mechanism of Dechlorane Plus in the Rhizosphere of the Soil-Plant System.

The fractions transformation and dissipation mechanism of Dechlorane Plus (DP) in the rhizosphere of soil-plant system were investigated and characterized by a 150-day experiment using a rhizobox system. The depuration, accumulation, and translocation of DP in rice plants were observed. The contributions of plant uptake, microbial degradation, and bound-residue formation to DP dissipation under the rhizosphere effect were modeled and quantified. The gradients of DP concentrations correlated well with microbial biomass in the rhizosphere (R2 = 0.898). The rhizosphere facilitated the bioavailability of DP (excitation) and modified the bound-residue formation of DP (aging). DP concentrations in roots were positively correlated with the labile fraction of DP in soil (R2 = 0.852-0.961). There were spatiotemporal variations in the DP fractions. Dissolved and soil organic carbon were important influences on fraction transformation. Contributions to total DP dissipation were in the following ranges: microbial degradation (8.33-54.14%), bound-residue formation (3.64-16.43%), and plant uptake (0.54-3.85%). With all of these processes operating, the half-life of DP in the rhizosphere was 105 days. The stereoselectivity of DP isomers in both rice and DP fractions in soil were observed, suggesting a link between stereoselective bioaccumulation of DP in terrestrial organisms and dissipation pathways in soil.

An analytical model of bubble-facilitated vapor intrusion.

Mass transfer from nonaqueous phase liquid (NAPL) to entrapped air induced by a fluctuating water table commonly occurs in residual NAPL zones in aquifers. Gas bubble expansion and vertical migration due to interphase mass transfer could facilitate the upward transport of volatile organic compounds (VOCs) in the aquifer and result in higher mass fluxes into a building relative to those of diffusion-limited (D-L) VOC transport. However, the current vapor intrusion models have not considered bubble migration. In this study, an analytical solution of bubble-facilitated (B-F) VOC transport in the unsaturated-saturated zone was developed. The analytical solution was tested by a numerical solution using the finite-difference method. Sensitivity analyses of model parameters were implemented to understand the VOC transport behaviors. The effects of bubble migration on vapor intrusion pathway completion time (tc) and the attenuation factor (AF) were investigated by comparison with the D-L VOC transport model. The results indicate that the D-L model significantly overestimates the tc and underestimates the AF because the model neglects the impacts of bubble migration. Therefore, one may make an inappropriate decision and set up an inappropriate response action schedule if using the D-L model to assess the risk of bubble-facilitated vapor intrusion. The analytical solution was applied to a laboratory experiment. The analytical model managed to interpret the laboratory experiment data, showing that the mass flux of B-F VOC transport is two orders of magnitude higher than that of D-L VOC transport.

[Hydrochemical Characteristics and Possible Controls of Groundwater in the Xialatuo Basin Section of the Xianshui River].

In order to study the characteristics of groundwater chemistry and groundwater flow system in the Xianshui River fault zone, samples of precipitation, surface water, groundwater, and hot spring samples in the Xialatuo Basin were collected and tested. Through the test data, the main ions and the sources of recharge were analyzed by means of ionic relations, correlation analysis, Gibbs plot, Piper triangular diagrams, and saturation index. The groundwater recharge sources in the basin were studied using combined hydrogen and oxygen isotope information. Results show that all the water samples in the study area were weakly alkaline. The predominant cations were Ca2+, Mg2+, and Na+. Among these, Ca2+ accounted for 2.6%-53.6%, with an average value of 28.84%, Mg2+ accounted for 2.7%-57%, with an average value of 40.6%, and Na+ accounted for 6.2%-93.1%, with an average value of 28.6%. The anions were mainly HCO3-, accounting for 82.4%-98% of the total anions and with an average value of 89.6%. HCO3- and Na+ accounted for most of the ions with 93.1% and 98%, respectively, in the Zhanggu hot spring. The total dissolved solids (TDS) of the groundwater ranged from 116.11 to 372.75 mg·L-1, and with an average value of 281.91 mg·L-1. The hydrogeochemical type of groundwater was HCO3-Mg·Ca and HCO3-Ca·Mg. It is controlled by carbonatite dissolution with a circulatory depth range in dozens of meters. The hot springs are controlled by the fault zone and are mainly distributed along the main stem of the Xianshui River fault. Their water is of the HCO3-Na type. The hydrogeochemical process is controlled by silicate dissolution with a circulatory depth range in thousands of meters.

Validation and Application of a 3-Step Sequential Extraction Method to Investigate the Fraction Transformation of Organic Pollutants in Aging Soils: A Case Study of Dechlorane Plus.

A 3-step sequential extraction method was developed to characterize the "labile," "stable-adsorbed," and "bound-residue" fractions of Dechlorane Plus (DP) in aging soils. Afterward, the proposed method was used to observe the transformation of DP fractions during aging. Slight decrease of total DP concentrations suggested there was a rather limited degradation, with only 4.2-8.2% of initial DP having degraded after 260 days. The labile fraction, which indicated the bioavailability of DP, decreased from 25.5% to 8.2%. The bound-residue fraction, usually regarded as a route for detoxification, increased from 0.1% to 18.5%. Model simulations were then developed to investigate the transformation, indicating that transformation rates were inconstant and distinguishable over time. Half-lives of DP were estimated to range from 1325 to 2948 days, indicating its environmental persistence in aging soils. Through Sobol Global Sensitivity Analysis (SGSA), degradation was evaluated to be the most sensitive factor of effecting the DP transformation in aging soils. Furthermore, the fsyn values increased from 0.26 to 0.37 in the labile fraction and decreased from 0.25 to 0.18 in the bound-residue fraction. The observed stereoselectivity difference might be the cause of the stereoselective accumulation of DP in terrestrial organisms.

Temporal Scaling Analytical Method to Identify Multi-Fractionality in Groundwater Head Fluctuations.

Natural dynamics such as groundwater head fluctuations may exhibit multi-fractionality, likely caused by multi-scale aquifer heterogeneity and other controlling factors, whose statistics requires efficient quantification methods. As a scaling exponent, the Hurst exponent can describe the temporal correlation or multifractal behavior in groundwater level fluctuation processes. However, the scaling behavior may change with time under natural conditions, likely due to the non-stationary evolution of internal and external conditions, which cannot be characterized by traditional methods using a single or several scaling exponents for the complex features of the overall process. This methods note quantifies the multi-fractionality using the timescale local Hurst exponent (TS-LHE) and then proposes a systematic statistical method to analyze groundwater head fluctuations. Time series of daily groundwater level fluctuations from three wells located in the lower Mississippi valley are analyzed, after removing the seasonal cycle, which leads to transient TS-LHE, implying multi-fractionality and multifractal-scaling behavior that changes with time and location. Therefore, the temporal scaling analysis proposed here may provide useful and quantitative information to understand the nature of dynamic hydrologic systems.

A simple method of transport parameter estimation for slug injecting tracer tests in porous media.

Slug (instantaneous injection) tracer tests can be used effectively to determinate solute transport parameters in porous media such as pore velocities and dispersivities, which are usually estimated with curve-fitting methods. This study proposes a simple method to estimate conservative and reactive solute transport parameters in one-, two- and three- dimensional domains with uniform flow fields based on peak times of slug tracer tests. This method requires fewer measured data than traditional curve-fitting methods. The accuracy of the method depends on the time-interval of measurement that is the time interval used in collecting observed concentrations of solutes. The error of the pore velocity estimate is very small (less than 3%) even for a relatively large time-interval of measurement. The error of the dispersivity estimate increases with the time-interval (Δt) of measurement significantly. For 1-D case, the relative error increases from 0.29% at ∆t of 0.1 min to 17.12% at ∆t of 6 min. Such an error can be reduced by refining the time-interval of measurement near the actual peak time of breakthrough curves. The error of the dispersivity estimate decreases when the retardation factor increases. The first-order decay rate constant in the liquid hardly influences the accuracies of both pore velocity and dispersivity estimates. The proposed method is applied on laboratory sand column tests. The results indicate that the estimated pore velocities and dispersivities are almost the same to that of the curves-fitting method. This method can be employed easily by scientists and practitioners for parameter estimations in laboratory column experiments if advection-dispersion equation is applicable. This method can also be used for parameter estimation of heat transport in a laboratory column experiment if a slug heat source is injected into a porous media with the presence of a uniform flow field. Limitations of the study have also been addressed.

Analytical solution for drainage and recession from an unconfined aquifer.

One-dimensional transient groundwater flow from a divide to a river in an unconfined aquifer described by the Boussinesq equation was studied. We derived the analytical solution for the water table recession and drainage change process described with a linearized Boussinesq equation with a physically based initial condition. A method for determining the average water table in the solutions was proposed. It is shown that the solution derived in the form of infinite series can be well approximated with the simplified solution which contains only the leading term of the original solution. The solution and their simplification can be easily evaluated and used by others to study the groundwater flow problems, such as drainage and base flow estimation, in an unconfined aquifer.

Temporal scaling of groundwater level fluctuations near a stream.

Temporal scaling in stream discharge and hydraulic heads in riparian wells was evaluated to determine the feasibility of using spectral analysis to identify potential surface and groundwater interaction. In floodplains where groundwater levels respond rapidly to precipitation recharge, potential interaction is established if the hydraulic head (h) spectrum of riparian groundwater has a power spectral density similar to stream discharge (Q), exhibiting a characteristic breakpoint between high and low frequencies. At a field site in Walnut Creek watershed in central Iowa, spectral analysis of h in wells located 1 m from the channel edge showed a breakpoint in scaling very similar to the spectrum of Q (∼20 h), whereas h in wells located 20 and 40 m from the channel showed temporal scaling from 1 to 10,000 h without a well-defined breakpoint. The spectral exponent (ß) in the riparian zone decreased systematically from the channel into the floodplain as groundwater levels were increasingly dominated by white noise groundwater recharge. The scaling pattern of hydraulic head was not affected by land cover type, although the number of analyses was limited and site conditions were variable among sites. Spectral analysis would not replace quantitative tracer or modeling studies, but the method may provide a simple means of confirming potential interaction at some sites.

Hydroxyl radical generation and oxidative stress in earthworms (Eisenia fetida) exposed to decabromodiphenyl ether (BDE-209).

Antioxidant responses induced by decabromodiphenyl ether (BDE-209) in the earthworms (Eisenia fetida) were studied after 7 days of exposure. Electron paramagnetic resonance (EPR) spectra indicated that hydroxyl radicals (•OH) in earthworms were significantly induced by 0.01-10 mg/kg of BDE-209. Malondialdehyde (MDA) and protein carbonyl (PCO) were stimulated at 0.5-10 mg/kg and 1-10 mg/kg, respectively. The reduced glutathione (GSH) was inhibited at 1-10 mg/kg while oxidized glutathione (GSSG) accumulated at 0.5-10 mg/kg. The GSH/GSSG ratio decreased at 0.5-10 mg/kg, and superoxide dismutase (SOD) and glutathione-S-transferase (GST) activities were induced at 0.1-1 and 0.5 mg/kg, respectively and both decreased at 10 mg/kg. Catalase (CAT) activities increased at 1-10 mg/kg. The results showed that severe oxidative stress occurred in E. fetida, and may play an important role in inducing the toxicity of BDE-209 on earthworms.

Particle tracking experiments in match-index-refraction porous media.

A low-cost, noninvasive, three-dimensional (3D), particle tracking velocimetry system was designed and built to investigate particle movement in match-index-refraction porous media. Both a uniform load of the glass beads of the same diameter and a binary load of the glass beads of two diameters were used. The purpose of the experiments is to study the effect of the two loads on the trajectories, velocity distribution, and spreading of small physical particles. A total of 35 particles were released and tracked in the uniform load and 46 in the binary load. The 3D trajectory of each particle was recorded with two video camcorders and analyzed. It is found that the particle's velocity, trajectory, and spreading are very sensitive to its initial location and that the smaller pore size or heterogeneity in the binary load increases the particles' velocity and enhances their spreading as compared with the uniform load. The experiments also verified the previous finding that the distribution of the particle velocities are lognormal in the longitudinal direction and Gaussian in two transverse directions and that the particle spreading is much larger along the longitudinal direction than along the traverse directions.