Mobility of major ions and nutrients in the unsaturated zone during paddy cultivation: a field study and solute transport modelling approach

Study of the movement of water and solute within soil profiles is important for a number of reasons. Accumulation of prominent contaminants from agricultural chemicals in the unsaturated zone over the years is a major concern in many parts of the world. As a result, the unsaturated zone has been a subject of great research interest during the past decade. Hence, an intensive field study was conducted in a part of Palar and Cheyyar river basins to understand the variation of major ions and nutrients in the soil zone during paddy cultivation. The chloride and nitrate data were used to model the movement of these chemicals in the unsaturated zone using the HYDRUS‐2D model. The field study shows that fertilizer application and irrigation return flow increases the major ions and nutrients concentration in the unsaturated zone. Further, the nutrient concentrations are regulated by plant uptake, fertilizer application and infiltration rate. Additionally, denitrification and soil mineralization processes also regulate the nitrogen concentration in the unsaturated zone. The solute transport modelling study concluded that the simulated results match reasonably with the observed trends. Simulated concentrations of chloride and nitrate for a 5‐year period indicate that the concentrations of these ions fluctuate in a cyclic manner (from 60 to 68 mg l^?1 and from 3·4 to 3·5 mg l^?1 respectively in groundwater) with no upward and downward trend. The influence of excessive fertilizer application on groundwater was also modelled. The model predicts an increase of about 17 mg l^?1 of chloride and 2·3 mg l^?1 of nitrogen in the groundwater of this area when the application of fertilizers is doubled. The model indicates that the present level of use of agrochemicals is no threat to the groundwater quality. Copyright © 2007 John Wiley & Sons, Ltd.     

Impact of redox and transport processes in a riparian wetland on stream water quality in the Fichtelgebirge region,southern Germany

Biologically mediated redox processes in the riparian zone, like denitrification, can have substantially beneficial impacts on stream water quality. The extent of these effects, however, depends greatly on the hydrological boundary conditions. The impact of hydrological processes on a wetland's nitrogen sink capacity was investigated in a forested riparian fen which is drained by a first‐order perennial stream. Here, we analysed the frequency distributions and time‐series of pH and nitrogen, silica, organic carbon and oxygen concentrations in throughfall, soil solution, groundwater and stream water, and the groundwater levels and stream discharges from a 3‐year period. During baseflow conditions, the stream was fed by discharging shallow, anoxic groundwater and by deep, oxic groundwater. Whereas the latter delivered considerable amounts of nitrogen (～0·37 mg l^?1) to the stream, the former was almost entirely depleted of nitrogen. During stormflow, near‐surface runoff in the upper 30 cm soil layer bypassed the denitrifying zone and added significant amounts to the nitrogen load of the stream. Nitrate‐nitrogen was close to 100% of deep groundwater and stream‐water nitrogen concentration. Stream‐water baseflow concentrations of nitrate, dissolved carbon and silica were about 1·6 mg l^?1, 4 mg l^?1 and 7·5 mg l^?1 respectively, and >3 mg l^?1, >10 mg l^?1 and <4 mg l^?1 respectively during discharge peaks. In addition to that macroscale bypassing effect, there was evidence for a corresponding microscale effect: Shallow groundwater sampled by soil suction cups indicated complete denitrification and lacked any seasonal signal of solute concentration, which was in contrast to piezometer samples from the same depth. Moreover, mean solute concentration in the piezometer samples resembled more that of suction‐cup samples from shallower depth than that of the same depth. We conclude that the soil solution cups sampled to a large extent the immobile soil‐water fraction. In contrast, the mobile fraction that was sampled by the piezometers exhibited substantially shorter residence time, thus being less exposed to denitrification, but predominating discharge of that layer to the stream. Consequently, assessing the nitrogen budget based on suction‐cup data tended to overestimate the nitrogen consumption in the riparian wetland. These effects are likely to become more important with the increased frequency and intensity of rainstorms that are expected due to climate change. Copyright © 2006 John Wiley & Sons, Ltd.     

Denitrifying bacteria and hydrogeochemistry in a natural wetland adjacent to farmlands in Chiba,Japan

This study combined bacterial incubation and hydrogeochemical methods to investigate denitrification in the surface water, top soil (0–20 cm), and shallow groundwater of the Ochi wetland in Japan. Surface water was rich in nitrate (40 mg/l) and denitrifying bacteria (700 per ml). Three functional zones in the wetland were identified in the top soil and shallow groundwater. In the upstream portion of the wetland (Zone I), the counts of denitrifying bacteria in top soil increased from 5200 to 14 970 per ml and nitrate decreased from 25·4 to 1·8 mg/l. Organic carbon concentrations decreased as sulfate increased from 4·0 to 9·6 mg/l in this zone. In the middle‐stream of the wetland (Zone II), all concentrations of major anions, iron, organic carbon, and total nitrogen content in top soil were relatively constant, but the counts of denitrifying bacteria increased up to 70 200 per ml. In the downstream portion of the wetland (Zone III), complete removal of nitrate resulted in sharp reduction of counts of denitrifying bacteria. Correspondingly, dissolved oxygen (DO), organic carbon, and total nitrogen increased in this zone. Counts of denitrifying bacteria were lower in shallow groundwater than in top soil; nitrate concentrations in shallow groundwater were also very low in this zone. DO and the oxidation/reduction potential data suggest that groundwater flows to the surface along an extended flow path, thus providing nitrate for the denitrifying community. Copyright © 2011 John Wiley & Sons, Ltd.     

三峡支流澎溪河消落带落干初期N2O释放与反硝化特征初探

水库近岸湿地(消落带)土壤N2O释放和反硝化作用是消落带氮的生物地球化学过程的重要组成部分.以三峡水库支流澎溪河高阳平湖库湾消落带为研究对象,于2013年落干初期,采用C2H2抑制-原状土柱培养法研究该处自然植被恢复区、农耕区和对照组等不同土地类型土壤的N2O释放速率和反硝化速率,并测定了土壤p H值、氧化还原电位、温度、有机质、总氮、铵态氮、硝态氮和土壤孔隙含水量等环境指标.结果表明,自然植被恢复区土壤N2O释放速率为9.88±6.49 g N/(hm2·d),反硝化速率为58.94±52.84 g N/(hm2·d);农耕区土壤N2O释放速率和反硝化速率分别为7.71±4.44和30.70±25.68 g N/(hm2·d).不同土地类型间N2O释放速率差异显著,落干初期土壤氧含量、含水量及氮含量对不同土地类型N2O释放和反硝化作用影响明显.土壤氧含量的升高促进了自然植被恢复区的N2O释放,并在一定程度上抑制了该区域反硝化作用.农耕区土壤含水量高于自然植被恢复区,可能致使N2O释放速率低于自然植被恢复区,而反硝化速率高于自然植被恢复区.消落带土壤氮含量降低同反硝化速率降低有一定联系.     

湖泊底泥疏浚环境效应:Ⅲ.对沉积物反硝化作用的影响

通过为期一年的疏浚模拟试验,在试验室培养疏浚与对照柱样,研究了底泥疏浚对沉积物反硝化过程的影响.沉积物反硝化速率的测定采用经典的乙炔抑制法.研究结果表明,在一年的试验周期内,疏浚和对照柱沉积物的反硝化速率分别为6.9-26.9nmol/(g·h)和21.6-102.7nmol/(g·h),除2006年的2月外的其他月份,疏浚沉积物的反硝化速率显著(P<0.05)低于未疏浚对照沉积物,同时还研究了环境因子对沉积物反硝化速率的影响,结果表明,疏浚和对照沉积物的反硝化速率都受温度的控制.硝态氟浓度是疏浚和对照沉积物反硝化速率的主要限制因子,有机碳对疏浚沉积物的反硝化速率有影响,但对未疏浚对照沉积物的反硝化速率没有影响.疏浚后短期内沉积物反硝化速率低于未疏浚对照沉积物,可看作是底泥疏浚在富营养华水体脱氮方面的一个负效应.     

A large weighing lysimeter for evapotranspiration and soil‐water–groundwater exchange studies

A large weighing lysimeter was installed at Yucheng Comprehensive Experimental Station, north China, for evapotranspiration and soil‐water–groundwater exchange studies. Features of the lysimeter include the following: (i) mass resolution equivalent to 0·016 mm of water to accurately and simultaneously determine hourly evapotranspiration, surface evaporation and groundwater recharge; (ii) a surface area of 3·14 m² and a soil profile depth of 5·0 m to permit normal plant development, soil‐water extraction, soil‐water–groundwater exchanges, and fluctuations of groundwater level; (iii) a special supply–drainage system to simulate field conditions of groundwater within the lysimeter; (iv) a soil mass of about 30 Mg, including both unsaturated and saturated loam. The soil consists mainly of mealy sand and light loam. Monitoring the vegetated lysimeter during the growing period of winter wheat, from October 1998 through to June 1999, indicated that during the period groundwater evaporation contributed 16·6% of total evapotranspiration for a water‐table depth from 1·6 m to 2·4 m below ground surface. Too much irrigation reduced the amount of upward water flow from the groundwater table, and caused deep percolation to the groundwater. Data from neutron probe and tensiometers suggest that soil‐water‐content profiles and soil‐water‐potential profiles were strongly affected by shallow groundwater. Copyright © 2000 John Wiley & Sons, Ltd.     

乌梁素海冰封期溶解氧的平衡性分析

为探明中国北方浅水湖泊乌梁素海冰封期水体溶解氧平衡的内在机理,于2021年1-2月在湖心处布设了一台水质在线监测浮标,收集到包括溶解氧等在内的水质数据.通过对溶解氧数据的小波降噪处理,结合气象资料,模拟分析了冰生长及稳定期内水体溶解氧的变化趋势,定性分析了水体溶解氧的平衡机理.结果表明:湖泊的日均最高产氧速率为7.19 mg/(L·d),最低产氧速率为2.01 mg/(L·d);日均最高耗氧速率为7.13 mg/(L·d),最低耗氧速率为2.37 mg/(L·d).24 h的单位时间平均最高产氧速率为0.55 mg/(L·h),最低产氧速率为0 mg/(L·h);单位时间平均最高耗氧速率为0.36 mg/(L·h),最低耗氧速率为0.08 mg/(L·h).由此说明小时间尺度下溶解氧的补充消耗不均衡导致了大时间尺度下的溶解氧不平衡,进而产生了冬季湖泊的亏氧现象.通过进一步溶解氧驱动因素与水环境因子响应关系的分析发现,浊度、水温与产氧速率呈显著负相关,叶绿素a与产氧速率和耗氧速率均呈显著正相关,表明了这些限制性水环境因子在一定程度上影响了冰下水体的溶解氧平衡.     

Heterotrophic denitrification using fluidized bed reactor

Groundwater denitrification was carried out in a fluidized bed reactor. This type of reactor is suited well to conditions of low substrate concentrations and high flow rates which are typical in groundwater denitrification processes. The reactor is characterized by the efficient contact between biomass and substrate, a high biomass concentration, and the absence of clogging or channeling. These characteristics provide for high treatment capacities (10.8 g/(L·d) NO₃^?N) and short retention times (less than 3 minutes) resulting in small reactor volume. Start-up procedure, biomass and nitrate profiles, and biofilm characteristics are presented in this article.     

Spatial variability of N2O concentrations and of denitrification-related factors in the surficial groundwater of a catchment in Northern Germany

N₂O concentrations and denitrification-related factors (NO₃, SO₄, dissolved organic carbon (DOC) and CO₂) were investigated in the surface groundwater of a catchment in northern Germany, the Fuhrberger Feld Aquifer (FFA). We sampled 79 plots that were selected according to the three criteria of land use, historical land use conversion (1954–1995) and groundwater level. We sampled three sites within each plot. The sampling depth was 0.5 m below the groundwater surface.We found no indication for the occurrence of autotrophic denitrification in the surface groundwater. Heterotrophic denitrification was identified as the main process for N₂O accumulation. The variability of N₂O concentrations on the plot-scale was extremely high and was poorly explained by the three sampling criteria. Other denitrification-related variables such as NO₃, SO₄ and DOC were less variable. The selection criteria land use and groundwater level clearly influenced the order of magnitude of N₂O concentrations in the surface groundwater. Under arable land, high NO₃ concentrations resulted in high N₂O concentrations. The surface groundwater under forest and pasture was almost NO₃-free and had also very small N₂O concentrations. Plots where the distance from the soil surface to the groundwater surface was large (>1 m up to 3.4 m) showed higher N₂O concentrations in the surface groundwater than plots where the distance was small (<1 m). A larger distance from the soil surface to the groundwater leads to a longer residence time and more decomposition of DOC in the soil. Consequently the less bioavailable DOC could inhibit the efficiency of the heterotrophic denitrification in the groundwater, yielding more N₂O. Elevated organic carbon levels in plots with historic land use conversion (pasture to arable) were very stable and did not influence N₂O concentrations. The high within plot variability showed that an upscaling of N₂O from the plot-scale to the catchment-scale is possible as long as the groundwater level regime and the land use do not change.     

Post‐mining landform evolution modelling: 2. Effects of vegetation and surface ripping

Computer simulations of the topographic evolution of the proposed post‐mining rehabilitated landform for the ERA Ranger Mine, showed that for the unvegetated and unripped case, the landform at 1000 years would be dissected by localized erosion valleys (maximum depth = 7·6 m) with fans (maximum depth = 14·8 m) at the outlet of the valleys. Valley form simulated by SIBERIA has been recognized in nature. This indicates that SIBERIA models natural processes efficiently. For the vegetated and ripped case, reduced valley development (maximum 1000 year depth = 2·4m) and deposition (maximum 1000 year depth = 4·8m) occurred in similar locations as for the unvegetated and unripped case (i.e. on steep batter slopes and in the central depression areas of the landform). For the vegetated and ripped condition, simulated maximum valley depth in the capping over the tailings containment structure was c. 2·2 m. By modelling valley incision, decisions can be made on the depth of tailings cover required to prevent tailings from being exposed to the environment within a certain time frame. A reduction in thickness of 1 m of capping material over tailings equates to c. 1 000 000 Mm³ over a 1 km² tailings dam area. This represents a saving of c. $1 500 000 in earthworks alone. Incorporation of SIBERIA simulations in the design process may result in cost reduction while improving confidence in environmental protection mechanisms. Copyright 2000 © Environmental Research Institute of the Supervising Scientist, Commonwealth of Australia.     

A meta‐analysis of reef island response to environmental change on the Great Barrier Reef

Reef islands on the Great Barrier Reef are influenced by a range of environmental factors. A meta‐analysis of 103 islands is presented to express variation in island size (area and volume) as a function of latitudinal and cross shelf gradients in regional oceanographic factors (exposure to incident waves, tidal range and tropical cyclone frequency) and local physical factors (position on the shelf, area, length and depth of supporting reef platform, vegetative cover). Models performed well for unvegetated sandcays (R² = 0.89), vegetated sandcays (R² = 0·72) and low wooded islands (R² = 0.78), with a moderate level of variation explained when all islands were simultaneously regressed (R² = 0.58). Future island dynamics were simulated for anticipated changes in cyclone regime, wave activity and sea level. For 38 islands mapped on the 1973 Royal Society and Universities of Queensland Expedition to the Northern Great Barrier Reef, change over the same 22 year period (1973–1995) was determined and the relative magnitude of observed and modelled changes was compared and found to be consistent through rank correlation analysis (Γ = 0.84 for unvegetated sandcays, Γ = 0.81 for vegetated sandcays). Simulations of island area or volume change from 2000 to 2100 indicated that under a 30% decrease in tropical cyclone activity, unvegetated sandcays continue to accrete at a lower rate, whereas all island types erode under a 38% increase in tropical cyclone activity. Vegetated sandcays initially accrete at higher levels of cyclone activity, entering an erosive state with a 60% increase in activity. Low wooded islands are unresponsive to environmental changes modelled. A sensitivity analysis of vegetated and unvegetated sandcays indicated that the presence of vegetation increases the tropical cyclone activity threshold at which islands begin to erode. Greatest sedimentary losses occur within the central band of high cyclone activity between Cooktown and Mackay. Copyright © 2015 John Wiley & Sons, Ltd.     

太湖人工生态系统中氮循环细菌分布

对太湖五里湖的以水生高等植物为主的湖泊人工生态系统,用最大可能数法,测定了该系统内各种生态类型水生高等植物群落内的４类氮循环细菌的分布。结果表明反硝化细菌的最大可能数,在水生高等植物群落内水体中较敞水区湖水高２－５个数量级差异极显著;漂服植物各落内水体的反硝化细菌ＭＰＮ值较沉水和浮叶植物群落内水体高２－３个数量级,差异极显著;     

The impact of pipe flow in riparian peat deposits on nitrate transport and removal

The effect of preferential flow in soil pipes on nitrate retention in riparian zones is poorly understood. The characteristics of soil pipes and their influence on patterns of groundwater transport and nitrate dynamics were studied along four transects in a 1‐ to >3‐m deep layer of peat and marl overlying an oxic sand aquifer in a riparian zone in southern Ontario, Canada. The peat‐marl deposit, which consisted of several horizontal layers with large differences in bulk density, contained soil pipes that were generally 0.1 to 0.2 m in diameter and often extended vertically for 1 to >2 m. Springs that produced overland flow across the riparian area occurred at some sites where pipes extended to the peat surface. Concentrations of NO₃^?–N (20–30 mg L^?1) and dissolved oxygen (DO) (4–6 mg L^?1) observed in peat pipe systems and surface springs were similar to values in the underlying sand aquifer, indicating that preferential flow transported groundwater with limited nitrate depletion. Low NO₃^?–N concentrations of <5 mg L^?1 and enriched δ¹⁵N values indicated that denitrification was restricted to small areas of the peat where pipes were absent. Groundwater DO concentrations declined rapidly to <2 mg L^?1 in the peat matrix adjacent to pipes, whereas high NO₃^?–N concentrations of >15 mg L^?1 extended over a larger zone. Low dissolved organic carbon values at these locations suggest that supplies of organic carbon were not sufficient to support high rates of denitrification, despite low DO conditions. These data indicate that it is important to develop a greater understanding of pipes in peat deposits, which function as sites where the transport of large fluxes of water with low biogeochemical reaction rates can limit the nitrate removal capacity of riparian zones. Copyright © 2011 John Wiley & Sons, Ltd.     

Denitrification in riparian buffer zones: the role of floodplain hydrology

The broad purpose of the study described here was to assess the role of denitrification in riparian zones in ameliorating groundwater pollution through nitrate loss, and as a potential source of nitrous oxide to the atmosphere. A suitable riparian zone was identified at Cuddesdon Mill on the River Thame floodplain near Oxford, England. Measurements were made of water and nitrate moving from arable land through the riparian zone and into the river. Techniques to measure denitrification were tested and applied, and the factors controlling denitrification measured. While there was considerable potential for denitrification at the site, this was not realized because much of the water moving off the farmland bypassed the riparian zone, entering the river directly via springs or through gravel lenses beneath the floodplain soil. Management of this site would not reduce nitrate leaching unless the floodplain hydrology could be substantially modified, and the main conclusion is that nitrate buffer zones will only operate efficiently where the hydrology of the site is appropriate. Copyright © 1999 John Wiley & Sons, Ltd.     

Groundwater recharge in an arid grassland as indicated by soil chloride profile and multiple tracers

Previous studies have shown that shallow groundwater in arid regions is often not in equilibrium with near‐surface boundary conditions due to human activities and climate change. This is especially the case where the unsaturated zone is thick and recharge rate is limited. Under this nonequilibrium condition, the unsaturated zone solute profile plays an important role in estimating recent diffuse recharge in arid environments. This paper combines evaluation of the thick unsaturated zone with the saturated zone to investigate the groundwater recharge of a grassland in the arid western Ordos Basin, NW China, using the soil chloride profiles and multiple tracers (²H, ¹⁸O, ¹³C, ¹⁴C, and water chemistry) of groundwater. Whereas conventional water balance and Darcy flux measurements usually involve large errors in recharge estimations for arid areas, chloride mass balance has been widely and generally successfully used. The results show that the present diffuse recharge beneath the grassland is 0.11–0.32 mm/year, based on the chloride mass balance of seven soil profiles. The chloride accumulation age is approximately 2,500 years at a depth of 13 m in the unsaturated zone. The average Cl content in soil moisture in the upper 13 m of the unsaturated zone ranges from 2,842 to 7,856 mg/L, whereas the shallow groundwater Cl content ranges from 95 to 351 mg/L. The corrected ¹⁴C age of shallow groundwater ranges from 4,327 to 29,708 years. Stable isotopes show that the shallow groundwater is unrelated to modern precipitation. The shallow groundwater was recharged during the cold and wet phases of the Late Pleistocene and Holocene humid phase based on palaeoclimate, and consequently, the groundwater resources are nonrenewable. Due to the limited recharge rate and thick unsaturated zone, the present shallow groundwater has not been in hydraulic equilibrium with near‐surface boundary conditions in the past 2,500 years.     

High chloride concentrations in the soil and groundwater under an oak hedge in the West of France: an indicator of evapotranspiration and water movement

Chloride is a major anion in soil water and its concentration rises essentially as a function of evapotranspiration. Compared to herbaceous vegetation, high transpiration rates are measured for isolated trees, shelterbelts or hedgerows. This article deals with the influence of a tree hedge on the soil and groundwater Cl^? concentrations and the possibility of using Cl^? as an indicator of transpiration and water movements near the tree rows. Cl^? concentrations were measured over 1 year at different depths in the unsaturated zone and in the groundwater along a transect intersecting a bottomland oak hedge. We observed a strong spatial heterogeneity of Cl^? concentrations, with very high values up to 2 g l^?1 in the unsaturated zone and 1·2 g l^?1 in the upper part of the groundwater. This contrasts with the low and homogeneous concentrations (60–70 mg l^?1) in the deeper part of the groundwater. Cl^? accumulation in the unsaturated zone at the end of the vegetation season allows us to identify the active root zone extension of trees. In winter, upslope of the tree row, downwards leaching partly renews the soil solution in the root zone, while the slow water movement under the trees or farther downslope results in Cl^? accumulation and leads to a salinization of the soil and groundwater. This salinization is of the same order as experimental conditions produce negative effects on oak seedlings. The measurement of Cl^? concentrations in the unsaturated zone under tree rows at the end of the vegetation season would indicate whether certain topographic, pedological or climatic conditions are likely to favour a strong salinization of the soil, as observed in the present study. Copyright © 2009 John Wiley & Sons, Ltd.     

太湖人工生态系统中氮循环细菌分布

对太湖五里湖的以水生高等植物为主的湖泊人工生态系统,用最大可能数（ＭＰＮ）法,测定了该系统内各种生态类型水生高等植物群落内的４类氮循环细菌的分布．结果表明,反硝化细菌的最大可能数（ＭＰＮ）,在水生高等植物群落内水体中较敞水区湖水高２－５个数量级,差异极显著（Ｐ＜０．０１）,漂浮植物群落内水体的反硝化细菌ＭＰＮ值较沉水和浮叶植物群落内水体高２－３个数量级,差异极显著（Ｐ＜０．０１）;硝化细菌ＭＰＮ值,敞水区湖水高于凤眼莲、水花生群落内水体,差异显著（ Ｐ＜ ０． ０５）,菱群落与其他群落比较,亦有极显著性差异（ Ｐ＜ ０． ０１）;亚硝化细菌ＭＰＮ值,在水生高等植物群落内的水体中较敞水区高 ３－ ４个数量级,差异极显著（ Ｐ＜ ０． ０１） ;氨化细菌ＭＰＮ值,在水生高等植物群落内的水体中高于敞水区水体．除硝化细菌外,反硝化、亚硝化及氨化细菌均在根际处最为密集,且由根际向外呈现递减趋势．     

Factors controlling stable oxygen,hydrogen and carbon isotope ratios in regional groundwater of the eastern United Arab Emirates (UAE)

Increasing groundwater salinity and depletion of the aquifers are major concerns in the UAE. Isotopes of oxygen, hydrogen, and carbon concentrations in groundwater were used to estimate evaporation loss using the isotopes of oxygen and hydrogen, and using a carbon isotope to trace inorganic carbon cycling in two main aquifers in the eastern part of the United Arab Emirates. The δD‐δ¹⁸O of groundwater samples plotted on a line given by: δD = 4 δ¹⁸O + 4 ·4 (r² = 0·4). In comparison, the local meteoric water line (LMWL) has been defined by the line: δD = 8 δ¹⁸O + 15. In order to better understand the system investigated, samples were separated into two groups based on the δD‐δ¹⁸O relationship. These are (1) samples that plot above the LMWL (δD = 6·1 δ¹⁸O + 12·4, r² = 0·8) and which are located predominantly in the north of the study area, and (2) samples that plot below the LMWL (δD = 5·6 δ¹⁸O + 6·2, r² = 0·8) and which are mostly distributed in the south. Slopes for both the groups are similar and lower than that for LMWL indicating potential evaporation of recharging water. However, the y‐intercept, which differs between the two groups, suggests evaporation of return flow and evapotranspiration in the unsaturated zone to be more significant in the south. This is attributed to intense agricultural activities in the region. Samples from the eastern Gravel Plain aquifer have δ¹³C and dissolved inorganic carbon (DIC) values in the range from ? 10 to 17‰, and 12–100 mg C/l, respectively, while the range for those from the Ophiolite aquifer is from ? 11 to ? 16.4‰, and 16–114 mg C/l respectively. This suggests the control of C‐3 and C‐4 plants on DIC formation, an observation supported by the range δ¹³C of soil organic matter (from ? 18·5 to ? 22·1‰.) Copyright © 2007 John Wiley & Sons, Ltd.     

Ground retreat and slope evolution on regraded surface-mine dumps,waunafon, gwent

Ground retreat was monitored on two vegetated and two unvegetated profiles over a five-year period. The average annual retreat of the two unvegetated profiles was 5.84 mm and 3.62 mm; that of the two vegetated profiles 2.34 mm and 2.07 mm. Slope evolution was controlled by the mid-slope-ward migration of two zones of accelerated erosion and the resulting replacement of a central rectilinear slope segment by the upper and lower slope elements.     

Soil saturation index of salt marshes subjected to spring‐neap tides: a new variable for describing marsh soil aeration condition

Organized spatial distribution of plants (plant zonation) in salt marshes has been linked to the soil aeration condition in the rhizosphere through simplistic tidal inundation parameters. Here, a soil saturation index (ratio of saturation period to tidal period at a soil depth) is introduced to describe the soil aeration condition. This new index captures the effects of not only the tidal inundation period and frequency but also the flow dynamics of groundwater in the marsh soil. One‐dimensional numerical models based on saturated flow with the Boussinesq approximations and a two‐dimensional variably saturated flow model were developed to explore the behaviour of this new soil aeration variable under the influence of spring‐neap tides. Simulations revealed two characteristic zones of soil aeration across the salt marsh: a relatively well aerated near‐creek zone and a poorly aerated interior zone. In the near‐creek zone, soils undergo periodic wetting and drying as the groundwater table fluctuates throughout the spring‐neap cycle. In the interior, the soil remains largely water saturated except for neap tide periods when limited drainage occurs. Although such a change of soil aeration condition has been observed in previous numerical simulations, the soil saturation index provides a clear delineation of the zones that are separated by an ‘inflexion point’ on the averaged index curve. The results show how the saturation index represents the effects of soil properties, tidal parameters and marsh platform elevation on marsh soil aeration. Simulations of these combined effects have not been possible with traditional tidal inundation parameters. The saturation index can be easily derived using relatively simple models based on five non‐dimensional variables. Copyright © 2010 John Wiley & Sons, Ltd.