Nitrogen Cycling in Freshwaters VI Posters
Presiding: M N Gooseff, Colorado School of Mines; D Scott, U.S. Geological Survey
NB33F-01 1330h
Influence of Hydrogeology on Nitrogen Transformations and Ground Water-Surface Water Interactions in a Coastal Plain Watershed
Nitrogen transport and ground water-surface water interactions were examined in both first- and third-order streams in the Contentnea Creek basin, a coastal plain watershed in the southeastern United States. Nitrogen transport and transformations in the ground-water system were determined by analyzing ground water along 1- to 2-km flow paths for nitrogen and other redox-active species and by using age-dating indicators. Nitrate was typically found only in recently recharged water in the upper few meters of saturated thickness in the upland portion of an unconfined surficial aquifer. Ground water with a residence time between 10 and 30 years typically had low nitrate concentrations and N2 concentrations in excess of that expected from atmospheric sources, indications that denitrification has reduced nitrate concentrations. Ground water older than 30 years also had low nitrate concentrations but contained little or no excess N2, suggesting that this water never contained elevated concentrations of nitrate along its flow path. Nitrate transport to the first-order stream differed from transport to third-order streams. Flood-plain geomorphology and the presence or absence of a confining unit at shallow depth affected the distribution of oxic conditions and residence times of ground water in the near-stream environment. Beneath the third-order stream and its broad alluvial valley, an areally extensive shallow confining unit has been removed by stream erosion. This process has created a thick, unconfined alluvial aquifer with oxic conditions and nitrate stability extending to greater depths. Age-dating and chemical information suggest that water in the alluvial aquifer is derived from short flow paths through the riparian zone and (or) from adjacent streams during high surface-water discharge periods when a hydrologic gradient reversal occurs. Pore waters from the bed and banks of these streams are strongly reducing (e.g., iron-reducing, methanogenic). As a result, nitrate in water that moves from streams to ground water during high-flow periods may be denitrified prior to discharging back to streams when flows recede. In contrast to the third-order stream, the first-order stream is steeply incised into the landscape, lacks a broad alluvial valley and is underlain by the areally extensive confining unit. These factors result in very little bank storage as indicated by older ground water adjacent to this stream than in the third-order streams. Redox conditions suggest that direct ground-water discharge of nitrate to this stream is unlikely; rather, artificial drainages (e.g., tile drains) and storm driven flow are indicated as major sources of nitrogen.
NB33F-02 1330h
Whole-stream and Sediment Nitrification of Four Streams in Grand Teton National Park and Jackson, Wyoming
Increasing levels of nitrate in stream and river ecosystems due to anthropogenic inputs make the need for nitrate control more apparent. Elevated streamwater nitrate concentrations can result from nitrate inputs from agricultural fertilizer application; however, nitrate can also be produced within the stream via nitrification. Estimating the relative contributions from these sources will allow managers to make more informed decisions to promote ecosystem health. Sediment and whole-stream nitrification rates were measured in four streams during the summer of 2004. The streams were located in Grand Teton National Park and Jackson, Wyoming. Nitrification rates in the sediment were estimated using the nitrapyrin assay method, and whole-stream nitrification rates were estimated using short-term NH4Cl enrichments. The background water column nitrate concentration ranged from 4.86Μg NO3 L-1 to 134.59Μg NO3 L-1. Average sediment nitrification rates varied, ranging from 1.95 to 9.89 Μg N m-2 min-1. In contrast, whole-stream nitrification (16 Μg N m-2 min-1) may be higher than sediment nitrification. This study suggests that sediment nitrification rates are similar but less than the whole-stream nitrification rates. Variation in nitrification estimates raises questions for further investigation.
NB33F-03 1330h
Investigation of Hyporheic Nitrate Retention Using 15NO3- Additions: Preliminary Results from Mack Creek, Oregon
We have initiated an investigation to understand the factors controlling nitrate (NO3-) retention and denitrification in hyporheic zones of small streams and to quantify the fraction of nitrate retention in these streams due to hyporheic exchange. We are testing three hypotheses: (1) hyporheic denitrification in headwater, forested streams will be low because of substrate and rate limitations, yet biotic assimilation will be high, relative to agricultural and urban streams, because of inorganic N-limitation; (2) hyporheic denitrification will be greatest in mid-network locations where surrounding land use is predominantly agricultural, however, total loss of NO3- will be transport-limited and biotic assimilation will be reduced because nitrogen is less limiting; and (3) potential rate of denitrification in the hyporheic zone will be high in the urban stream reaches, but total nitrate retention in the hyporheic zone will be low because both denitrification and biotic assimilation will be severely transport-limited. We will present preliminary data from the LINX II 15NO3- injection in an old-growth reach of 3rd-order Mack Creek, Oregon, where we installed a network of 17 hyporheic wells. Prior to and following the LINX II injection of 15NO3-, the wells were sampled for 14N- and 15N-species including 15N2 and 15N2O. Additionally, in-well salt tracer tests were conducted, and a preliminary groundwater flow model of the site has been developed.
NB33F-04 1330h
Surface-Subsurface Hydrologic Exchange and the Supply of Nitrogen to the Hyporheic Zone of the Tanana River in Interior Alaska
Vegetation in the taiga forest is commonly limited by nitrogen, with limitation especially acute in early successional floodplain stands. Hyporheic flow has been hypothesized as an important source of nitrogen to developing floodplain vegetation. A series of hyporheic well transects were sampled over the growing season to examine how hyporheic flow of the Tanana River in interior Alaska influences the flux and transformation of nitrogen in ground water of the floodplain. A two-end member mixing model was then used to estimate mixing of river and ground water in the hyporheic zone. During peak river flow (mid-July to early-August), the ground water rose on average 1 m from base flow conditions (May to June). With this increase in water table height, nitrate concentration in hyporheic water increased from 1.40 ΜM during low flow to 4.38 ΜM during high flow. In general, subsurface nitrate concentration changed with river concentration. Hyporheic nitrate concentration tended to decline with distance from the river, dissolved organic nitrogen and dissolved organic carbon concentrations tended to increase with distance from the river, and ammonium concentration exhibited little change. This study suggests river-floodplain connectivity as an important regulator of hyporheic nitrogen concentration.
NB33F-05 1330h
Quantifying Nitrogen Sources and Cycling Along the Upper Rio Grande
Synoptic sampling of a 1200km reach of the Upper Rio Grande has been performed in January and August from 2000 to present. The objective of this sampling has been to develop seasonal relationships between discharge, land use, and major water quality parameters including salinity and nutrients. In general, water quality, both salinity and nutrient concentrations, degrades with distance downstream. Increased salinity is explained largely by gradual downstream increase due to evapoconcentration punctuated by localized inputs of saline groundwater. Both total dissolved nitrogen (TDN) and Dissolved Organic Carbon (DOC) concentrations gradually increase with distance downstream, however for TDN this trend is punctuated by large, localized inputs primarily from urban areas. Somewhat surprisingly, surface water draining from areas of intensive, irrigated agriculture during the growing season often had lower nutrient and DOC concentrations than the river. Increased spatial and temporal sampling of the 250km reach between Cochiti Dam and Elephant Butte Reservoir was conducted in June, July and August of 2004 to quantify the relationships between agricultural and urban land use and nutrient loading as well as nutrient sinks within the surface water, hyporheic and riparian systems. Summer 2004 data indicate that wastewater treatment plants are the largest and most consistent sources of inorganic nitrogen to the river. In both June and July there was a net removal of nitrogen from the reach as discharge decreased 26%, concentrations decreased 39%, and TDN loads decreased 56%. Interestingly, the diversion of river water for irrigated agriculture reduced discharge 25%, TDN loads 60% and concentrations 47% along the same reach before draining back to the main stem of the river suggesting that agricultural diversions were serving as a sink for nitrogen. However in August, TDN loads were higher in returning drains suggesting that agricultural systems had switched to a net source of nitrogen to the river. Possible reasons for this increase include higher rainfall in august flushing accumulated solutes coupled with increased soil available nutrients associated with the end of the growing system. Both conservative hydrologic tracers and labeled nitrogen tracer work in 2005 will be designed to identify primary controls on nitrogen removal along this reach.
NB33F-06 1330h
Simulating Nitrate Retention Patterns in Stream Networks: An Interbiome Comparison
Recent studies indicate that a substantial fraction of nitrogen input to the landscape may be stored or denitrified within stream networks, and that the rate of in-stream nitrogen removal decreases as stream size increases from 4th order to large rivers. We hypothesize that nitrate retention in small streams comprises a large component of nitrate retention in stream networks. To test this hypothesis, we are developing a model of interactions among stream size, in-stream nitrate processing, and nitrate transport in stream networks. In conjunction with the second Lotic Intersite Nitrogen Experiment (LINX2), this model will serve as a null model for an inter-site comparison of catchment-scale in-stream nitrate retention patterns among eight different biomes across the United States and Puerto Rico. Since nutrient retention patterns and mechanisms may vary among different landscapes, we expect that the model's predictive power will be substantially higher for some biomes than others. By comparing modeled and measured patterns of nitrate variability in stream networks across biomes, the inter-site comparisons will provide a basis for identifying variation in mechanisms that control nitrate processing across our study streams, a crucial step in understanding the fate of nitrogen delivered to stream networks.
NB33F-07 1330h
Hydrologic Controls on Slough-scale Nitrogen Processing Within Southeastern U.S. Floodplains
Floodplains are biogeochemically active ecosystems where riverine nitrogen undergoes a series of transformations during flooding, including uptake, mineralization, nitrification, and denitrification. The extent of coupled N-transformations and ultimate denitrification of riverine nitrogen within floodplains imparts a positive impact to downstream water bodies by reducing riverine nitrogen loads. We measured N-fluxes of nitrate, ammonia, and dissolved & particulate organic nitrogen into and out of 2 adjacent floodplain sloughs during 2 flood events along the Tangipahoa River in Northeastern Louisiana. Isotope and slough mass-balance measurements demonstrate that the incoming nitrate was removed from each slough at the sediment-water interface, mostly through denitrification. Net organic nitrogen fluxes were zero between the incoming/outgoing flood water. Although the monitored sloughs are less than 100m apart, NH3 export from each slough was distinct. The directional component of vertical exchange within each slough controlled NH3 by (1) replenishing O2 required for nitrification to the sediment-water interface and/or (2) transporting NH3 rich subsurface water into the slough. Isotopic measurements from the 15NO3 tracer experiment within floodplain mesocosms suggest limited nitrification under upwelling conditions, consistent with the slough mass-fluxes. These results suggest that when surface-subsurface exchange promotes nitrification, overall floodplain N-removal is enhanced by coupled nitrification-denitrification. In conclusion, N-dynamics within this Southeastern floodplain are largely influenced by both hydrology and geomorphology, which control both the incoming N mass-flux and coupling of N-reactions.