Vertical Hydrologic Exchange Flows Control Methane Emissions from Riverbed Sediments.

CH4 emissions from inland waters are highly uncertain in the current global CH4 budget, especially for streams, rivers, and other lotic systems. Previous studies have attributed the strong spatiotemporal heterogeneity of riverine CH4 to environmental factors such as sediment type, water level, temperature, or particulate organic carbon abundance through correlation analysis. However, a mechanistic understanding of the basis for such heterogeneity is lacking. Here, we combine sediment CH4 data from the Hanford reach of the Columbia River with a biogeochemical-transport model to show that vertical hydrologic exchange flows (VHEFs), driven by the difference between river stage and groundwater level, determine CH4 flux at the sediment-water interface. CH4 fluxes show a nonlinear relationship with the magnitude of VHEFs, where high VHEFs introduce O2 into riverbed sediments, which inhibit CH4 production and induce CH4 oxidation, and low VHEFs cause transient reduction in CH4 flux (relative to production) due to reduced advective CH4 transport. In addition, VHEFs lead to the hysteresis of temperature rise and CH4 emissions because high river discharge caused by snowmelt in spring leads to strong downwelling flow that offsets increasing CH4 production with temperature rise. Our findings reveal how the interplay between in-stream hydrologic flux besides fluvial-wetland connectivity and microbial metabolic pathways that compete with methanogenic pathways can produce complex patterns in CH4 production and emission in riverbed alluvial sediments.

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.

A national-scale assessment of land subsidence in China's major cities.

China's massive wave of urbanization may be threatened by land subsidence. Using a spaceborne synthetic aperture radar interferometry technique, we provided a systematic assessment of land subsidence in all of China's major cities from 2015 to 2022. Of the examined urban lands, 45% are subsiding faster than 3 millimeters per year, and 16% are subsiding faster than 10 millimeters per year, affecting 29 and 7% of the urban population, respectively. The subsidence appears to be associated with a range of factors such as groundwater withdrawal and the weight of buildings. By 2120, 22 to 26% of China's coastal lands will have a relative elevation lower than sea level, hosting 9 to 11% of the coastal population, because of the combined effect of city subsidence and sea-level rise. Our results underscore the necessity of enhancing protective measures to mitigate potential damages from subsidence.

Mapping global lake dynamics reveals the emerging roles of small lakes.

Lakes are important natural resources and carbon gas emitters and are undergoing rapid changes worldwide in response to climate change and human activities. A detailed global characterization of lakes and their long-term dynamics does not exist, which is however crucial for evaluating the associated impacts on water availability and carbon emissions. Here, we map 3.4 million lakes on a global scale, including their explicit maximum extents and probability-weighted area changes over the past four decades. From the beginning period (1984-1999) to the end (2010-2019), the lake area increased across all six continents analyzed, with a net change of +46,278 km2, and 56% of the expansion was attributed to reservoirs. Interestingly, although small lakes (<1 km2) accounted for just 15% of the global lake area, they dominated the variability in total lake size in half of the global inland lake regions. The identified lake area increase over time led to higher lacustrine carbon emissions, mostly attributed to small lakes. Our findings illustrate the emerging roles of small lakes in regulating not only local inland water variability, but also the global trends of surface water extent and carbon emissions.

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.

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.

[Spatial Variation of Ammonia-N, Nitrate-N and Nitrite-N in Groundwater of Dongshan Island].

In Dongshan Island, groundwater is the main resource of the local residents' drinking water, domestic water, agriculture irrigation and freshwater aquaculture. This study aimed to investigate the spatial distribution characteristic and its variation pattern of ammonia-N, nitrate-N and nitrite-N in groundwater, as well as its pollution source and influence factors. It is very important to understand the pollution level of ammonia-N, nitrate-N and nitrite-N in groundwater of Dongshan Island, the control and prevention of ammonia-N, nitrate-N and nitrite-N pollution, which is of great significance to the residents' health. In this study, the spatial variability characteristics of ammonia-N, nitrate-N and nitrite-N concentration in groundwater of Dongshan Island was analysed by geo- statistic method, the values of the non-observation points were determined by Kriging method, and the pollution characteristics of ammonia-N, nitrate-N and nitrite-N in groundwater of Dongshan Island was also analyzed. Our results showed that the ammonia-N and nitrite-N concentration in groundwater of Dongshan Island were at low levels, but their spatial variability were high, and their autocorrelation were poor; however, the nitrate-N concentration was general high, its spatial variability was moderate, and the autocorrelation was much good. The distribution characteristics of ammonia-N, nitrate-N and nitrite-N in groundwater of Dongshan Island were similar that the high concentration areas were all located in the coastal land. The domestic pollutants and human and animal wastes from towns and villages were the main sources of nitrogen pollution, which would be the first step to control the nitrogen pollution of Dongshan Island. Land use pattern, soil type, groundwater depth, pH, dissolved oxygen, season, and the existence of Fe2+, were the impact factors that influence the distribution and transformation of ammonia-N, nitrate-N and nitrite-N in groundwater, which could be the considerable factors in the control of nitrogen pollution in groundwater of Dongshan Island.

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.