The current knowledge about nitrogen removal processes in suboxic water columns will be reviewed. The most recent development in the understanding of these processes is the documentation of anaerobic ammonium oxidation with nitrite (anammox) in these areas, and the balance between the traditional denitrification and anammox will be in focus.<br>Se revisa el conocimiento actual sobre los procesos de remoción del nitrógeno en columnas de agua subóxicas. En estas áreas el más reciente desarrollo en la comprensión de estos procesos es la documentación sobre la oxidación anaeróbica del amonio con nitrato (anammox). Además enfocaremos el balance entre la tradicional denitrificación y el anammox.

Nitrogen (N) pollution in shallow groundwater within the Jianghan Plain—a depositional basin along the Yangtze River in China—is a serious issue affecting the local water supply. There is little concern regarding temporal variation patterns of groundwater N in regions with intensive groundwater/surface-water interactions thus far. In this study, a 2-year field monitoring program and a set of incubation experiments were jointly applied to characterize the seasonal variation mechanisms of groundwater N. Groundwater monitoring suggested that temporal N variations in monitoring wells of 25 and 10-m depth were correlated with fluctuations in water levels, with lower NH₄-N and higher NO₃⁻-N concentrations corresponding to groundwater discharge during the dry season, and with higher NH₄-N and lower NO₃⁻-N concentrations corresponding to groundwater recharge during the wet season. Batch sediment incubation experiments showed that buried organic carbon in sediments from the field site could not promote NH₄-N release, but exogenous organic carbon and oxidant input could catalyze organic N mineralization, resulting in NH₄-N release. Nevertheless, NO₃⁻-N release was limited in all incubations. Thus, the temporal variation patterns could be divided into (1) groundwater N in type-1 wells (25 and 10 m), controlled by the annual reversal of groundwater flow gradient, which determined whether there was input of exogenous organic carbon or oxidants, ultimately affecting organic N mineralization, denitrification and other transformation processes, and (2) groundwater N in type-2 wells (<4 m depth), which showed no significant seasonal variation, implying the process is mainly controlled by surface inputs in the shallower subsurface.

The use of fluidized-bed bioreactors in waste and drinking water treatment has several advantages, the most significant of which is the specific removal rate, which is an order of magnitude higher than that of equivalent activated sludge processes. In this paper, the usual concept of nitrification-denitrification in separated units is replaced by a new concept in which the two processes are performed together in a single high-performance fluidized-bed. Based on the former nitrifying reactor, new equipment was designed. This reactor contained a fluidized bed with separated aerobic and anoxic sections for nitrification and denitri fication respectively. This was accomplished by the use of different-diameter sand as carrier material and appropriate reactor shape, recirculation, feed and aeration conditions. The reactor (20 L fluidized-bed volume) was operated for 3 months. It was fed with synthetic waste water (50 L/h) containing 25-40 mg NH₄ ⁺-N/L. Propionic acid and ethanol in a 1:4 ratio were used as the carbon source (2.3 g C/L) for deni trification, fed in at different points of the reactor. Ammonium removal reached 50%, while denitrification was 75%. The total nitrogen removal rate was 0.8-1.2 kg N/m ³.d. A new simple hydrostatic pressure method was used to monitor biofilm thickness in the fluidized bed. During the experiments the oxidation-reduc tion potential (ORP) was tested as a tool to monitor reactor performance; its use for the control of the process was found to be limited.

Las transformaciones en regadio suponen el cambio a una agricultura mas intensiva con lo que aumentan considerablemente los aportes al suelo de sales en el agua de riego, fertilizantes y pesticidas. Estos aportes tienen un impacto en la fertilidad del suelo y en la calidad de las aguas subterraneas que conviene tener en cuenta. Los principales aspectos que se consideran en este trabajo son la salinizacion del suelo, la contaminacion de las aguas subterraneas por nitratos y la contaminacion del suelo por metales pesados y otros elementos traza.

Investigation of surface water and groundwater interaction (SW–GW interaction) provides basic information for regional water-resource protection, management, and development. In this survey of a 10-km-wide area along both sides of the Songhua River, northeast China, the hydrogeochemical responses to different SW–GW interactions were studied. Three types of SW–GW interactions were identified—“recharge”, “discharge”, and “flow-through”—according to the hydraulic connection between the surface water and groundwater. The single factor index, principal component analysis, and hierarchical cluster analysis of the hydrogeochemistry and pollutant data illuminated the hydrogeochemical response to the various SW–GW interactions. Clear SW–GW interactions along the Songhua River were revealed: (1) upstream in the study area, groundwater usually discharges into the surface water, (2) groundwater is recharged by surface water downstream, and (3) discharge and flow-through coexist in between. Statistical analysis indicated that the degree of hydrogeochemical response in different types of hydraulic connection varied, being clear in recharge and flow-through modes, and less obvious in discharge mode. During the interaction process, dilution, adsorption, redox reactions, nitrification, denitrification, and biodegradation contributed to the pollutant concentration and affected hydrogeochemical response in the hyporheic zone.

Groundwater flow and the geochemical evolution of a freshwater lens in an aquifer on a Pacific atoll were investigated by hydrogeochemical surveys. Sulfur hexafluoride measurements showed that deeper groundwater and groundwater at the periphery of the lens are older, consistent with a downward and outward groundwater flow scheme. This is the typical flow scheme on Pacific atolls where a Holocene–Pleistocene unconformity restricts the shape of the freshwater lens. Enrichment of Mg/Ca in the groundwater is another indicator of a longer residence time, because contact between the groundwater and the carbonate sediments composing the aquifer leads to the release of Mg from high-Mg calcite and the precipitation of Ca as low-Mg calcite. Groundwater quality was also affected by anthropogenic nitrogen loading and aboveground organic matter, which were altered by denitrification and sulfate reduction in the aquifer, especially in the older groundwater. The chemical composition of the groundwater in the center of the island, where saline water is up-coning, implies that freshwater recharge dilutes the older saline water, which as time passes will eventually be replaced by newly recharged freshwater.

Well vulnerability assessment is essential for groundwater source protection. A quantitative approach to assess well vulnerability in a well capture zone is presented, based on forward solute transport modeling. This method was applied to three groundwater source areas (Jiuzhan, Hadawan and Songyuanhada) in Jilin City, northeast China. The ratio of the maximum contaminant concentration at the well to the released concentration at the contamination source (cₘₐₓ/c₀) was determined as the well vulnerability indicator. The results indicated that well vulnerability was higher close to the pumping well. The well vulnerability in each groundwater source area was low. Compared with the other two source areas, the cone of depression at Jiuzhan resulted in higher spatial variability of cₘₐₓ/c₀and lower minimum cₘₐₓ/c₀by three orders of magnitude. Furthermore, a sensitivity analysis indicated that the denitrification rate in the aquifer was the most sensitive with respect to well vulnerability. A process to derive a NO₃−N concentration at the pumping well is presented, based on determining the maximum nitrate loading limit to satisfy China’s drinking-water quality standards. Finally, the advantages, disadvantages and prospects for improving the precision of this well vulnerability assessment approach are discussed.

Agricultural contamination of groundwater in northwestern Mississippi, USA, has not been studied extensively, and subsurface fluxes of agricultural chemicals have been presumed minimal. To determine the factors controlling transport of nitrate-N into the Mississippi River Valley alluvial aquifer, a study was conducted from 2006 to 2008 to estimate fluxes of water and solutes for a site in the Bogue Phalia basin (1,250 km2). Water-quality data were collected from a shallow water-table well, a vertical profile of temporary sampling points, and a nearby irrigation well. Nitrate was detected within 4.4 m of the water table but was absent in deeper waters with evidence of reducing conditions and denitrification. Recharge estimates from 6.2 to 10.9 cm/year were quantified using water-table fluctuations, a Cl– tracer method, and atmospheric age-tracers. A mathematical advection-reaction model predicted similar recharge to the aquifer, and also predicted that 15% of applied nitrogen is leached into the saturated zone. With current denitrification and application rates, the nitrate-N front is expected to remain in shallow groundwater, less than 6–9 m deep. Increasing application rates resulting from intensifying agricultural demands may advance the nitrate-N front to 16–23 m, within the zone of groundwater pumping.