Benthic Oxygen Consumption and Organic Matter Turnover in Organic-poor, Permeable Shelf Sands

The high permeability of sediments and strong near-bottom currents cause seawater to infiltrate the surface layers of Middle Atlantic Bight shelf deposits. In this study, sandy sediment cores from 11 to 12 m water depth were percolated with filtered seawater on shipboard. Sedimentary oxygen consumption (SOC) increased non-linearly with pore water flow, approaching maximum rates of 120 mmol m⁻² d⁻¹ (May 2001) or 75 mmol m⁻² d⁻¹(July 2001). The addition of acetate to the inflowing water promptly enhanced the release of dissolved inorganic carbon (DIC) from the cores. DIC production rates were a linear function of acetate concentration, ranging from 100 to 300 mmol m⁻² d⁻¹ without substrate addition to 572 mmol m⁻² d⁻¹ with 100 mM acetate. The sediments also hydrolyzed a glucose pseudopolymer, and the liberated glucose prompted an increase of SOC. Our results suggest that decomposition rates of organic matter in permeable sands can exceed those of fine-grained, organic-rich deposits, when water currents cause advective interstitial flow, supplying the subsurface microbial community with degradable material and electron acceptors. We conclude that the highly permeable sand beds of the Middle Atlantic Bight are responsive within minutes to hours and efficiently operate as biocatalytical filters.     

牡蛎壳改性及联合释氧复合材料修复地下水氨氮污染实验研究

为研究以牡蛎壳-释氧复合材料为填料的渗透反应格栅去除地下水氨氮污染的效果及其经济实用性,开展了相关室内实验:采用高温加热方式对牡蛎壳进行改性,通过测定牡蛎壳改性后的比表面积判定其吸附效果;以过氧化钙为释氧化合物,将其与水泥、石英砂/牡蛎壳粒、钙基膨润土以一定比例混合制成粒径约1.8 cm球型释氧材料,采用静态实验研究不同原料配比的释氧材料的释氧性能;最后研究了两种不同粒径牡蛎壳粒-释氧复合材料修复地下水氨氮的效果及不同供氧方式下不同吸附材料修复地下水氨氮的效果及其经济实用性。结果表明:对牡蛎壳粒进行高温改性,牡蛎壳粒高温条件下会产生团聚现象,比表面积随煅烧温度升高呈下降趋势,高温改性方式并不能有效改善牡蛎壳粒吸附性能;实验中制作的释氧材料在95天的实验期间,各实验柱的溶解氧量可以保持在18 mg/L左右,且在嗜碱菌作用下pH值得以有效降低,所制作的释氧材料可为硝化细菌长期在溶解氧低的地下水环境中生长提供氧气;以牡蛎壳-释氧复合材料为填料的渗透反应格栅通过耐碱硝化细菌的硝化作用可以将氨氮浓度从50 mg/L降至约35 mg/L,不同粒径牡蛎壳粒修复效果差异不明显。使用牡蛎壳-释氧复合材料渗透反应格栅长期修复地下水氨氮,不仅可以达到活性炭及沸石作为骨架的修复效果,而且更加具有经济实用性。     

Investigation of frost heave prevention using permeable subgrade structure

This paper set up a series of comprehensive targets based on the concept of 'anti-freeze filler', which include reasonable water retention rate, frost heave characteristics, and compaction characteristics of filling material. Then, a type of permeable graded gravel is proposed, suitable for high-speed railway subgrade. A series of in-door water retention, permeability, and frost heave tests were performed under different graded conditions. Water retention, permeability, and frost heave characteristic of different graded filling materials can be determined, in order to define the gradation range of permeable graded gravel. Relying on the frost-heave monitoring record of high speed railway in Northeast China, a series of experimental studies were performed, which included on-site filler production, compaction test, and the anti-frost effect test, in order to improve the production and compaction techniques of permeable graded gravel. From the research of this paper, the use of permeable graded gravel subgrade as the anti-frost structure for the high-speed railway subgrade in cold areas is feasible.     

Scattering of water wave by a submerged horizontal plate and a submerged permeable breakwater

Based on a two-dimensional linear water wave theory, this study develops the boundary element method (BEM) to examine normally incident wave scattering by a fixed, submerged, horizontal, impermeable plate and a submerged permeable breakwater in water of finite depth. Numerical results for the transmission coefficients are also presented. In addition, the numerical technique's accuracy is demonstrated by comparing the numerical results with previously published numerical and experimental ones. According to that comparison, the transmission coefficient relies not only on the submergence of the horizontal impermeable plate and the height of the permeable breakwater, but also on the distance between horizontal plate and permeable breakwater. Results presented herein confirm that the transmission coefficient is minimum for the distance approximately equal to four times the water depth.     

Denitrification-nitrification process in permeable coastal sediments: An investigation on the effect of salinity and nitrate availability using flow-through reactors

Permeable coastal sediments act as a reactive node in the littoral zone,transforming nutrients via a wide range of biogeochemical reactions.Reaction rates are controlled by abiotic factors,e.g.,salinity,temperature or solute concentration.Here,a series of incubation experiments,using flow-through reactors,were conducted to simulate the biogeochemical cycling of nitrate (NO_3~?) and phosphorus (P) in permeable sediments under differentNO_3~?availability conditions (factor I) along a salinity gradient (admixture of river and seawater,factor II).In an oligotrophic scenario,i.e.,unamendedNO_3~?concentrations in both river and seawater,sediments acted as a permanent net source ofNO_3~?to the water column.The peak production rate occurred at an intermediate salinity(20).IncreasingNO_3~?availability in river water significantly enhanced netNO_3~?removal rates within the salinity range of 0 to 30,likely via the denitrification pathway based on the sediment microbiota composition.In this scenario,the most active removal was obtained at salinity of 10.When both river and seawater were spiked with NO_3~?,the highest removal rate switched to the highest salinity (36).It suggests the salinity preference of theNO_3~?removal pathway by local denitrifiers (e.g.,Bacillus and Paracoccus) and thatNO_3~?removal in coastal sediments can be significantly constrained by the dilution relatedNO_3~?availability.Compared with the obtained variation forNO_3~?reactions,permeable sediments acted as a sink of soluble reactive P in all treatments,regardless of salinity andNO_3~?input concentrations,indicating a possibility of P-deficiency for coastal water from the intensive cycling in permeable sediments.Furthermore,the net production of dissolved organic carbon (DOC) in all treatments was positively correlated with the measuredNO_3~?reaction rates,indicating that the DOC supply may not be the key factor forNO_3~?removal rates due to the consumption by intensive aerobic respiration.Considering the intensive production of recalcitrant carbon solutes,the active denitrification was assumed to be supported by sedimentary organic matter.     

立管-水平板结构消浪特性试验研究

在现有的矩形方箱和透空结构的基础上,提出一种由立管和水平板组合而成的消浪结构。利用波浪水槽物理模型试验,测试该结构在单向规则波作用下的消浪性能。分别探讨了其单排布置和双排布置时的透射系数随结构宽度、相对吃水深 度、相对间距等因素的变化规律。结果表明：单排布置时,透射系数随结构宽度的增大而减小,总体上随吃水深度的增大而减小;双排布置时,透射系数相对总宽度相同的单体结构减少 10 %,透射系数随吃水深度的增大而减小,前后间距对透射系数的影响不明显。对试验结果数据进行曲线拟合,得到了透射系数关于各影响因素的函数表达式。     

Evaluating groundwater discharge to tidal rivers based on a Rn-222 time-series approach

The natural flux of groundwater into coastal water bodies has recently been shown to contribute significant quantities of nutrients and trace metals to the coastal ocean. Groundwater discharge and hyporheic exchange to estuaries and rivers, however, is frequently overlooked though it often carries a distinctly different chemical signature than surface waters. Most studies that attempt to quantify this input to rivers use multiple geochemical tracers. However, these studies are often limited in their spatial and temporal extents because of the labor-intensive nature of integrating multiple measurement techniques. We describe here a method of using a single tracer, ²²²Rn, to rapidly characterize groundwater discharge into tidally-influenced rivers and streams. In less than one week of fieldwork, we determined that of six streams that empty into the Indian River Lagoon (IRL), Florida, three (Eau Gallie River, Turkey Creek, and Main Canal) did not receive substantial groundwater inputs, one canal (C-25 Canal) was dominated by groundwater exchange, and the remaining two (Sebastian River system and Crane Creek) fell somewhere in between. For more detailed discharge assessments, we focused on the Sebastian River system, a stratified tidal river estuary, during a relatively dry period (June) and a wet period (July) in 2008. Using time-series ²²²Rn and current velocity measurements we found that groundwater discharge into all three branches of the Sebastian River increased by 1–2 orders of magnitude during the wetter period. The estimated groundwater flow rates were higher than those reported into the adjacent IRL, suggesting that discharge into these rivers can be more important than direct discharge into the IRL. The techniques employed here should work equally well in other river/stream systems that experience significant groundwater discharge. Such assessments would allow area managers to quickly assess the distribution and magnitude of groundwater discharge nature into rivers over large spatial ranges.     

Tidal sands as biogeochemical reactors

Sandy sediments of continental shelves and most beaches are often thought of as geochemical deserts because they are usually poor in organic matter and other reactive substances. The present study focuses on analyses of dissolved biogenic compounds of surface seawater and pore waters of Aquitanian coastal beach sediments. To quantitatively assess the biogeochemical reactions, we collected pore waters at low tide on tidal cross-shore transects unaffected by freshwater inputs. We recorded temperature, salinity, oxygen saturation state, and nutrient concentrations. These parameters were compared to the values recorded in the seawater entering the interstitial environment during floods. Cross-shore topography and position of piezometric level at low tide were obtained from kinematics GPS records. Residence time of pore waters was estimated by a tracer approach, using dissolved silica concentration and kinetics estimate of quartz dissolution with seawater. Kinetics parameters were based on dissolved silica concentration monitoring during 20-day incubations of sediment with seawater. We found that seawater that entered the sediment during flood tides remained up to seven tidal cycles within the interstitial environment. Oxygen saturation of seawater was close to 100%, whereas it was as low as 80% in pore waters. Concentrations of dissolved nutrients were higher in pore waters than in seawater. These results suggest that aerobic respiration occurred in the sands. We propose that mineralised organic matter originated from planktonic material that infiltrated the sediment with water during flood tides. Therefore, the sandy tidal sediment of the Aquitanian coast is a biogeochemical reactor that promotes or accelerates remineralisation of coastal pelagic primary production. Mass balance calculations suggest that this single process supplies about 37 kmol of nitrate and 1.9 kmol of dissolved inorganic phosphorus (DIP) to the 250-km long Aquitanian coast during each semi-diurnal tidal cycle. It represents about 1.5% of nitrate and 5% of DIP supplied by the nearest estuary.