The dark side of the hyporheic zone: depth profiles of nitrogen and its processing in stream sediments

1. Although it is well known that sediments can be hot spots for nitrogen transformation in streams, many previous studies have confined measurements of denitrification and nitrate retention to shallow sediments (<5 cm deep). We determined the extent of nitrate processing in deeper sediments of a sand plains stream (Emmons Creek) by measuring denitrification in core sections to a depth of 25 cm and by assessing vertical nitrate profiles, with peepers and piezometers, to a depth of 70 cm. 2. Denitrification rates of sediment slurries based on acetylene block were higher in shallower core sections. However, core sections deeper than 5 cm accounted for 68% of the mean depth‐integrated denitrification rate. 3. Vertical hydraulic gradient and vertical profiles of pore water chloride concentration suggested that deep ground water upwelled through shallow sediments before discharging to the stream channel. The results of a two‐source mixing model based on chloride concentrations suggested that the hyporheic zone was very shallow (<5 cm) in Emmons Creek. 4. Vertical profiles showed that nitrate concentration in shallow ground water was about 10–60% of the nitrate concentration of deep ground water. The mean nitrate concentrations of deep and shallow ground water were 2.17 and 0.73 mg NO₃‐N L^?1, respectively. 5. Deep ground water tended to be oxic (6.9 mg O₂ L^?1) but approached anoxia (0.8 mg O₂ L^?1) after passing through shallow, organic carbon‐rich sediments, which suggests that the decline in the nitrate concentrations of upwelling ground water was because of denitrification. 6. Collectively, our results suggest that there is substantial nitrate removal occurring in deep sediments, below the hyporheic zone, in Emmons Creek. Our findings suggest that not accounting for nitrate removal in deep sediments could lead to underestimates of nitrogen processing in streams and catchments.     

Ground water flow paths in relation to nitrogen chemistry in the near-stream zone

Interactions between ground water flow paths and water chemistry were studied in the riparian zone of a small headwater catchment near Toronto, Ontario. Significant variations in oxygen — 18 and chloride indicated the presence of distinct sources of water in the ground water flow system entering the near-stream zone. Shallow ground water at the upland perimeter of the riparian zone had nitrate-N, chloride and dissolved oxygen concentrations which ranged between 100–180 µg L^–1, 1.2–1.8 mg L^–1 and 4.6–9.1 mg L^–1 respectively. Concentrations of nitrate — N in deep ground water flowing upward beneath the riparian wetland were < 10 µg L^–1, whereas chloride and dissolved oxygen ranged between 0.6–0.9 mg L^–1 and 0.4–2.2 mg L^–1 respectively. Ammonium — N concentrations (20–60 µg L^–1) were similar in shallow and deep ground water. Ground water was transported through the wetland to the stream by three hydrologic pathways. 1) Shallow ground water emerged as springs near the base of the hillslope producing surface rivulets which crossed the riparian zone to the stream. 2) Deep ground water flowed upward through organic soils and entered the rivulets within the wetland. 3) Deep ground water reached the stream as bed and bank seepage. Springs were higher in nitrate and chloride than rivulets entering the stream, whereas bank seeps had lower concentrations of nitrate and chloride and considerably higher ammonium concentrations than the rivulets. These contrasts in nitrate and chloride concentrations were related to initial differences in the ion chemistry of shallow and deep ground water rather than to element transformations within the riparian wetland. Differences in ammonium concentration between seeps and rivulets were caused by immobilization of ammonium in the substrates of aerobic rivulets, whereas little ammonium depletion probably occurred in deep ground water flowing upward through reduced subsurface organic soils around the stream perimeter.     

Nitrate depletion in the riparian zone of a small woodland stream

Field enrichments with nitrate in two spring-fed drainage lines within the riparian zone of a small woodland stream near Toronto, Ontario showed an absence of nitrate depletion. Laboratory experiments with riparian substrates overlain with nitrate enriched solutions revealed a loss of only 5–8% of the nitrate during 48 h incubation at 12°C. However, 22–24% of the initial nitrate was depleted between 24 and 48 h when a second set of substrate cores was incubated at 20°C. Short-term (3 h) incubations of fresh substrates amended with acetylene were used to estimate in situ denitrification potentials which varied from 0.05–3.19 g N g^–1 d^–1 for organic and sandy sediments. Denitrification potentials were highly correlated with initial nitrate content of substrate samples implying that low nitrate levels in ground water and riparian substrates may be an important factor in controlling denitrification rates. The efficiency of nitrate removal in spring-fed drainage lines is also limited by short water residence times of < 1 h within the riparian zone. These data suggest that routes of ground water movement and substrate characteristics are important in determining nitrate depletion within stream riparian areas.     

Effects of a nematode Meloidogyne incognita and its interaction with above‐ground herbivory on an invasive wetland plant,alligator weed (Alternanthera philoxeroides)

Invasive plants may be attacked both above ground and below ground. Few studies have, however, investigated the simultaneous effects of above‐ground and below‐ground herbivory. In the present study, we report the effects of beetle herbivory and nematode infection on alligator weed, Alternanthera philoxeroides, an invasive plant in China. We found that the root‐knot nematode Meloidogyne incognita widely occurred on the plant in south China. To examine its effect on the plant in conjunction with above‐ground herbivory, we conducted a field common garden experiment with a local insect defoliator, Cassida piperata. We also included the native congener Alternanthera sessilis in our experiments for a comparison of the response of invasive and native species. We found no significant effects on plant biomass of the nematode infection in conjunction with the above‐ground herbivory. Further chemical analysis, however, showed that the water‐soluble carbohydrate content in roots of A. philoxeroides was significantly increased in plants attacked by both the nematode and the herbivore compared with the water‐soluble carbohydrate content in plants attacked by only the nematode or herbivore alone. We found no such change in the native congener A. sessilis. Together these results may suggest that A. philoxeroides tolerates joint above‐ground and below‐ground damage by allocating more resources to below‐ground material.     

The chalk-stream environment

Chalk is a very pure and porous form of limestone. Water percolates down through the rock to form aquifers which overflow as springs forming chalk streams which receive little surface runoff. Passage through the rock is relatively slow and smooths out irregularities in rainfall and stabilises the water temperature. As a result the streams tend to have a regular annual hydrograph with a relatively small difference between winter and summer flows and with no spate conditions; and the temperature range in the streams remains closer to the annual mean than in rivers receiving more surface runoff. The water is calcareous with a pH of 7.4–8.0 and an ample supply of plant nutrients. The streams support diverse and productive communities of plants and animals and the restricted temperature range is particularly suitable for the growth of brown trout.     

A ground water nitrogen budget for a headwater swamp in an area of permanent ground water discharge

Ground water inputs and outputs of N were studied for a small ground water discharge swamp situated in a headwater drainage basin in southern Ontario, Canada. Darcy's equation with data for piezometers was used to measure inputs of shallow local ground water at the swamp margin and deep regional ground water beneath the swamp. Ground water flux was also quantified by measuring ground water discharge to the outlet stream draining the swamp in combination with a chemical mixing model to separate shallow and deep ground water components based on chloride differences. Estimates of shallow ground water flux determined by these two approaches agreed closely however, the piezometer data seriously underestimated the deep ground water input to the swamp. An average ground water input-output budget of total N (TN) total organic nitrogen (TON) ammonium (NH₄ ⁺-N) and nitrate (NO₃ ^--N) was estimated for stream base flow periods which occurred on an average of 328 days each year during 1986–1990. Approximately 90% of the annual NO₃ ^--N input was contributed by shallow ground water at the swamp margin. Deep ground water represented about 65% of the total ground water input and a similar proportion of TON and NH₄ ⁺-N inputs. Annual ground water NO₃ ^--N inputs and outputs were similar whereas NH₄ ⁺-N retention was 4 kg ha^-1 representing about 68% of annual ground water input. Annual TON inputs in ground water exceeded outputs by 7.7 kg ha (27%). The capacity of the swamp to regulate ground water N fluxes was influenced by the N chemistry of ground water inputs and the hydrologic pathways of transport within the swamp.     

Removal of Cr(VI) from ground water by Saccharomyces cerevisiae

Chromium can be removed from ground water by the unicellular yeast, Saccharomyces cerevisiae. Local ground water maintains chromium as CrO₄ ^2- because of bicarbonate buffering and pH and E _h conditions (8.2 and +343 mV, respectively). In laboratory studies, we used commercially available, nonpathogenic S. cerevisiae to remove hexavalent chromium [Cr(VI)] from ground water. The influence of parameters such as temperature, pH, and glucose concentration on Cr(VI) removal by yeast were also examined. S. cerevisiae removed Cr(VI) under aerobic and anaerobic conditions, with a slightly greater rate occurring under anaerobic conditions. Our kinetic studies reveal a reaction rate (V_max) of 0.227 mg h^-1 (g dry wt biomass)^-1 and a Michaelis constant (Km) of 145 mg/l in natural ground water using mature S. cerevisiae cultures. We found a rapid (within 2 minutes) initial removal of Cr(VI) with freshly hydrated cells [55–67 mg h^-1 (g dry wt biomass)^-1] followed by a much slower uptake [0.6–1.1 mg h^-1 (g dry wt biomass)^-1] that diminished with time. A materials-balance for a batch reactor over 24 hours resulted in an overall shift in redox potential from +321 to +90 mV, an increase in the bicarbonate concentration (150–3400 mg/l) and a decrease in the Cr(VI) concentration in the effluent (1.9-0 mg/l).     

Occurrence of moulds in drinking water

AIMS: In order to determine the occurrence of filamentous fungi in public drinking water systems in Norway, water from 14 water supply networks from all over the country was sampled and analysed. Networks with both ground and surface water sources were included in this study. METHODS AND RESULTS: During a one-year period, 273 water samples were collected. Frequencies of fungi in samples from raw water, treated water and from home and hospital installations were determined on the basis of incubation of 100 ml membrane-filtered samples on dichloran 18% glycerol agar media. Filamentous fungi were recovered from 62% of all samples. In ground water 42.3% of the samples were positive for mould growth, while surface waters yielded 69.7% positive samples. CONCLUSIONS: The risk to recover moulds from surface water is three times higher compared with ground water. It is more likely to detect moulds in cold waters and showers than in hot waters. SIGNIFICANCE AND IMPACT OF THE STUDY: By analysing the water reaching the consumers, the results reported in present study indicate that filamentous fungi in drinking water is not negligible, and that moulds should be considered as part of the microbiological analysis parameters in drinking water.     

Modeling the Impact of Oxygen Reaeration on Natural Attenuation

Based on studies of leaking petroleum storage tank (LPST) sites in Texas and California, the average plume of benzene, toluene, ethylene, and xylenes (BTEX) is between 61 and 132 m (200 and 400 ft) long. Standard modeling of BTEX plumes produces plumes well in excess of observed plume lengths. The amount of oxygen carried into the plume zone with clean upgradient water often is insufficient to account for the levels of biodegradation observed in these studies. Traditional recharge of oxygen-containing water into an aquifer adds insufficient oxygen to the system and cannot account for the observed plume lengths. Research has shown that anaerobic processes can contribute to biodegradation in certain cases; however, anaerobic pathways are not included in this work. Reaeration of oxygen-depleted aquifers by diffusive transport of oxygen through the vadose zone has generally been neglected as a way to introduce oxygen into surficial aquifers. The observed plume lengths and preliminary laboratory results indicate that this source of oxygen should be accounted for in any natural attenuation model of BTEX contamination. This approach to modeling reaeration has been incorporated into the finite-element groundwater flow and contaminant transport code, FLOTRAN. Adding diffusion-driven reaeration to the modeling process produces BTEX plumes consistent with observed plume lengths.     

Groundwater seepage stimulates the growth of aquatic macrophytes

1. The impact of groundwater seepage on the growth of submerged macrophytes was investigated in experiments on the isoetid Littorella uniflora and the elodeid Myriophyllum alterniflorum both in the laboratory and in the field. Isoetids rely mostly on sediment‐derived CO₂ and nutrients via root uptake, whereas elodeids acquire their inorganic carbon and nutrients from the water column. We thus hypothesised that L. uniflora would respond positively to seeping ground water as it should improve both CO₂ and nutrient supply. 2. Laboratory experiments were conducted by percolating vegetated cores containing natural sediment or technical sand with artificial ground water of high CO₂ concentrations and with either high or low levels of nutrients. Field experiments were conducted in the oligotrophic Lake Hampen, Denmark, with custom‐built seepage‐growth chambers that permitted a near‐natural flow‐through of seeping ground water. Chambers with a solid bottom, and thus no flow‐through of seeping ground water, served as controls in both laboratory and field experiments. In the field, seepage chambers were installed at a site with relatively high seepage fluxes (ground water from forest catchment), at a site with much lower seepage fluxes but with higher nutrient concentrations (ground water from agricultural catchment) and at a reference site with no net discharge or recharge of ground water. 3. Positive growth responses were observed in the field at transects with high groundwater discharge compared to the control chambers with no seepage. No growth response was observed at the reference transect with low or alternating direction of groundwater seepage. The growth rates of L. uniflora in the field were significantly higher in seepage treatments compared to control treatments, and final plant mass was up to 70% higher than that for plants where seepage was excluded. In areas with high groundwater discharge, a strong positive correlation was found between groundwater seepage fluxes, growth rates, and final plant mass for L. uniflora, while there was no such relationship at the reference transect. The growth of M. alterniflorum was also significantly affected by groundwater seepage, but to a lesser degree than L. uniflora. Laboratory experiments generally showed the same trend for both L. uniflora and M. alterniflorum, and the positive influence of seeping ground water was apparently related to increased inorganic carbon supply and, to a lesser degree, improved nutrient availability. 4. Groundwater discharge results in enhanced growth of isoetids and to some extent elodeids inhabiting a groundwater‐fed softwater lake. We propose that the shallow dense vegetation present where most of the discharge takes place acts as a biological filter that retains nutrients that otherwise would end up in the water column and could result in increased algal growth.     

Nitrate transformation and water movement in a wetland area

The NO₃ ^– transformation capacity of a riparian zone at Rabis stream, Denmark, was investigated for a period of 2 years. The riparian zone of 15–25 m received NO₃ ^–-containing groundwater from the adjoining agricultural areas. The water flows as surface runoff along the surface of the wetland and in the root zone towards the stream. Changes in water chemistry, water balance and mass transport were investigated. The riparian zone acted as a buffer zone for NO₃ ^–, PO₄ ^3– and dissolved Fe²⁺. The NO₃ ^–-transformation capacity of the wetland was about 400 kg N ha^–1 y^–1, but varied seasonally. A simple rearrangement of drain systems in wetland areas can probably reduce the NO₃ ^– content of Danish surface waters by 20 000–50 000 t N y^–1.     

Drinking water quality and arsenic health risk assessment in Sistan and Baluchestan,Southeastern Province,Iran

Access to drinking water is one of the most important indicators determined by the World Health Organization (WHO). This investigation surveyed the concentration of various pollutants in drinking water and its health risk attribute to Arsenic in Sistan and Baluchistan province, Iran. Water samples were collected from ground water and analyzed for physical parameters, anions, and heavy metals using the standard procedures. The concentrations of sulfate (269 ± 127 mg/l) in five sites exceeded the permissible limit (250 mg/l), while chlorine concentrations (223 ± 100 mg/l) in four sites exceeded the permissible limit (250 mg/l) set by WHO. Similarly, the concentrations of Mg (30 ± 11 mg/l) in four sites exceeded the permissible limit (30 mg/l), while Na concentrations (222 ± 99 mg/l) in five sites exceeded the permissible limit (200 mg/l) set by Institute of Standards and Industrial Research of Iran (ISIRI). In addition, arsenic was in acceptable levels recommended by WHO and local regulations. Based on the calculated indices of hazard qutient (HR) and excess lifetime cancer risk (ELCR), the in-use drinking water has no adverse effects on the consumer's health. Excessive use of fertilizers and pesticides, unsuitable sewerage systems, and inappropriate sludge and solid waste disposal in this province can lead to drinking water pollution. Also, excessive pumping of ground water should be managed as an effective method for supply of safe drinking water.     

Environmental effects of nationwide selenium fertilization in Finland

To study the possible pollution of natural waters by selenium added to agricultural fertilizers, water selenium concentrations were determined in lakes and ground water pools during the year 1992, when the enrichment of fertilizers by selenium had continued for 8 yr. Water samples were preconcentrated by evaporating, and selenium concentrations were analyzed fluorometrically using the dye 2,3-diaminonaphthalene after wet digestion. In lake water samples from 13 lakes, no significant correlations were found between selenium and pH, chlorophyll A, total nitrogen, or phosphorus. Agriculturally affected and nonaffected lakes had no differences in their selenium concentrations. In the lakes that were surrounded by cultivated fields mean water selenium was 83.4 ng/L (range: 75.2–93.4 ng/L,n=9). Correspondingly, in lakes situated in forests without agricultural surroundings, mean water selenium concentration was 76.5 ng/L (range: 51.2–110 ng/L,n=3). Only one lake in the close vicinity of a coal power plant had a high water selenium concentration (mean 272 ng/L). In ground water samples, there was a big variation in selenium levels between different pools, the levels varying between 33 and 260 ng/L. This variation may partly be explained by different selenium concentrations of the bedrock and sediments. However, simultaneous increase of total nitrogen, phosphorus, and selenium levels in consecutive samples from some ground water pools indicates leaching of selenium from the fertilizers into the ground water in certain areas.     

基于稳定同位素技术的塔里木河下游不同林龄胡杨的水分利用来源

水分是制约很多陆地生态系统植物生长和繁殖的重要因素, 在干旱地区尤为明显。利用稳定同位素技术探究塔里木河下游不同林龄胡杨(Populus euphratica)的水分来源情况, 了解生态输水背景下荒漠河岸林的水分利用循环与利用策略, 可为生态输水提供科学依据, 同时也可对同类地区的生态恢复提供借鉴。本研究通过测定塔里木河下游胡杨茎干水和各潜在水源(土壤水、地下水)的稳定氢氧同位素值(δD、δ¹⁸O), 应用多源线性混合模型(IsoSource)分析了各潜在水源对不同林龄胡杨的贡献比例, 并结合3种林龄胡杨不同土壤深度含水量的变化, 分析了胡杨的主要吸水层位。结果表明: (1)不同林龄胡杨样地的不同深度区间上的土壤水δ¹⁸O值存在显著差异(P < 0.05): 胡杨幼龄木、成熟木、过熟木木质部δ¹⁸O分别为-7.83 ± 0.07‰、-8.53 ± 0.11‰、-9.36 ± 0.21‰; 而δD值不存在显著差异(P > 0.05)。可据此来推断胡杨的主要吸水层位。(2)总体上, 三种林龄胡杨土壤水δ¹⁸O值随土壤深度增加而减小, 并趋于接近地下水的δ¹⁸O值。其中, 0-60 cm土壤水受蒸发影响比较大, 其同位素组成经历了强烈的蒸发分馏过程, 土壤含水量极少, 土壤水δ¹⁸O值偏正。(3)不同林龄胡杨所利用的水分来源不同: 胡杨幼龄木对于地表80 cm以下的土壤水以及地下水均有一定程度的利用, 对80-140 cm、140-220 cm和220-340 cm的土壤水平均利用比率依次为16.2%、21.4%和24.6%, 对地下水平均利用比率为24.5%; 成熟木主要利用220-340 cm的土壤水及地下水, 平均利用比率分别为36.9%和42.3%; 过熟木主要利用140-340 cm的土壤水及地下水, 平均利用比率分别为32.8%和49.3%。     

Energy control of matter fluxes through land-water ecotones in an agricultural landscape

Ecotones play and important role in control of matter input into water bodies. The impacts of shelterbelt and meadow ecotones on ground water passage from cultivated fields to pond were studied. The reduction of water flux due to evapotranspiration by shelterbelts and meadows on slopes of different steepness were estimated. The horizontal passage of heat energy between cultivated fields and ecotones, which enhances evaporation in shelterbelts and meadows was demonstrated. The reduction of ground water flux by a ten meter wide shelterbelt or meadow surrounding a pond can reach as much as 100 per cent when the slope is about 1 degree, during a sunny day. Shelterbelts are a more effective measure for control of cycling matter than meadows. The greater the slope of the water table and the more intensive the radiation and advective processes, the more distinct the differences between shelterbelt and meadow impacts on groundwater flow are.     

Biodiversity indicators in European ground waters: towards a predictive model of stygobiotic species richness

1. Estimates of species richness obtained from exhaustive field inventories over large spatial scales are expensive and time-consuming. For this reason, efficiency demands the use of indicators as 'surrogates' of species richness. Biodiversity indicators are defined herein as a limited suite of taxonomic groups the species richness of which is correlated with the species richness of all other taxonomic groups present in the survey area.
2. Species richness in ground water was assessed at different spatial scales using data collected from six regions in Europe. In total, 375 stygobiotic species were recorded across 1157 sites and 96 aquifers. The taxonomic groups collected from more than one site and with more than two species (Oligochaeta, Gastropoda, Cyclopoida, Harpacticoida, Ostracoda, Isopoda, Amphipoda, Bathynellacea and Acari) were used to develop nonparametric models to predict stygobiotic biodiversity at the aquifer scale.
3. Pair-wise correlations between taxonomic groups were low, i.e. variation in species richness of a single taxonomic group did not usually reflect variation of the other groups. In contrast, multiple regressions calculated between species richness of any combination of taxa and extra-group species richness along the six regions resulted in a number of significant relationships.
4. These results suggest that some taxonomic groups (mainly Copepoda and Amphipoda and, to a lesser extent, Oligochaeta and Gastropoda) combined in different ways across the regions, were good biodiversity indicators in European groundwater ecosystems. However, the uneven distribution of taxonomic groups prevented selection of a common set of indicators for all six regions. Faunal differences among regions are presumably related to both historical and ecological factors, including palaeogeography, palaeoecology, geology, aquifer fragmentation and isolation, and, less clearly, anthropogenic disturbance.     

Riparian control on NO3 −, DOC, and dissolved Fe concentrations in mountainous streams, northern Japan

We evaluated (1) the longitudinal pattern of stream chemistry and (2) the effects of the riparian zone on this longitudinal pattern for nitrate (NO₃ ⁻), dissolved organic carbon (DOC), and total dissolved iron (Fe). We selected two small watersheds; the “southern watershed” had an extending riparian wetland and the “northern watershed” had a narrow riparian area. Stream NO₃ ⁻ concentrations decreased from the spring to outlet of both watersheds. In the southern watershed, stream DOC concentration decreased from the spring to midstream and then increased to the outlet. Stream Fe concentration in the southern watershed longitudinally increased. On the other hand, the northern watershed exhibited no longitudinal pattern for DOC and Fe concentrations. In both watersheds, while NO₃ ⁻ concentrations in the soil and ground water were lower than those in the stream waters, DOC and Fe concentrations exhibited the opposite patterns. The longitudinal decreases of NO₃ ⁻ concentrations in both streams and increase of stream Fe in the southern watershed mainly resulted from the inflow of the soil and ground water to the stream. The decrease in stream DOC from the spring to midstream in the southern watershed was due to the deep groundwater having low DOC, while the subsequent increase to the surrounding soil and ground water. Moreover, considerations of stream solute flow with soil and ground water chemistry suggested other mechanisms adding NO₃ ⁻ and removing/diluting DOC and Fe, especially for the northern watershed; coexistence of oxidizing and reducing conditions in the riparian zone might control the longitudinal concentration change in the stream water chemistry.     

Mosquito fauna inhabiting water bodies in the urban environment of Córdoba city,Argentina, following a St. Louis encephalitis outbreak

An understanding of urban aquatic environments as mosquito larval habitats is necessary to prioritize sites for surveillance and control of arbovirus vectors in urban areas. Natural and artificial water bodies at ground level that may be larval mosquito habitats in Córdoba city, Argentina were surveyed. Data on the characteristics of aquatic sites and the presence and abundance of mosquito larvae and pupae were collected in the summer of 2006, coinciding with the first report of human WNV and following an outbreak of St. Louis encephalitis in 2005. Eight species in the genera Aedes, Culex, and Mansonia were identified. At 64.2% (34 of 53) of the sites, only one species was collected, while 3.8% (2 of 53) had three associated species, the highest richness found per site. Culex quinquefasciatus represented over 99% (out of 32,729) of the specimens. It was also the most widely distributed and detected under diverse habitat conditions. Although puddles and semi‐permanent pools harbored a greater number of species, drainages and channels may be more relevant as risk factors from an epidemiological point of view because they showed the highest larval densities, mainly of Cx. quinquefasciatus (vector of SLE and WNV). Also, higher densities of this species were associated with stormwater runoff and sewage water, thus water management systems should be targeted and closely monitored for mosquito control purposes.     

Geomorphic influence on intraspecific genetic differentiation and diversity along hyporheic corridors

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Formation of nesquehonite and other minerals as a consequence of biofilm dehydration

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