H44B-01 INVITED 16:00h
Hydrologic and Biogeochemical Connections between Uplands and Streams in Contrasting Landscapes
We used combinations of hydrometric, chemical, and isotopic evidence to evaluate linkages between upland and riparian zones at the 5 small watersheds of the U.S. Geological Survey Water Energy and Biogeochemical Budget (WEBB) program. These sites span a broad range of climate and topography. At Sleepers River, Vermont, snowmelt induced the water table on hillslopes to rise into the highly transmissive upper soil. The close timing of the groundwater and stream hydrographs suggests a large contribution of hillslope water to the stream. However, the chemistry of these upland groundwaters indicates that only limited areas of convergent groundwater flow directly contribute to streamflow. At Panola Mountain, Georgia, a thin saturated zone develops on the hillslope during large rainstorms. This hillslope groundwater is chemically distinct from riparian groundwater, and transits the riparian zone near land surface with little mixing. Based on chemical mixing analysis, the hillslope contributes up to 30% of the streamwater during moderate to large-sized rainstorms. The Trout Lake site in Wisconsin is a low-lying landscape in highly conductive sandy glacial outwash.Hillslope water chemistry is considerably more dilute (i.e. less evolved) than the regional groundwater that supplies baseflow. The lack of chemical response in streamwater during storms suggests that hillslope water makes a minimal contribution relative to regional groundwater flow. In the alpine/subalpine watershed of Loch Vale, Colorado, much of the subsurface flow occurs on steep slopes of talus. Water in the talus flow has a wide range of residence times. The talus deposits are biogeochemically active and play an important role in maintaining summer baseflow, regulating seasonal changes in streamwater chemistry, and exporting nitrogen from atmospheric deposition. The tropical Icacos watershed in the Luquillo mountains of Puerto Rico receives 4 meters of rainfall annually and has high physical and chemical weathering rates. Streamwater chemistry during baseflow is strongly controlled by groundwater interaction with weathered bedrock. Most hillslope runoff occurs through near-surface macropores with limited soil interaction. This source dominates during storms resulting in stream chemistry that resembles that of the extremely dilute precipitation.We will compare these field observations at each site with the aid of TOPMODEL-based simulation of residence times and observed water quality on the hillslope and riparian saturated zones.
H44B-02 16:12h
Investigating Hydrological Processes Controlling Streamflow Generation at the Panola Mountain Research Watershed, Georgia
Runoff simulations of the Panola Mountain Research Watershed (PMRW) using Dynamic TOPMODEL improved compared to those of the original version of TOPMODEL. These improvements, in part, were due to the ability to define structurally different landscape units that better represent the spatial hydrological characteristics of the PMRW catchment. The model also allows for a dynamically variable saturated zone. However, model simulations were still deficient, especially with respect to capturing seasonality. An analysis of seasonal responses (dry, wetting, wet and drying periods) during 3 years resulted in the rejection of all model simulations within the Generalized Likelihood Uncertainty Estimation (GLUE procedure), a Monte Carlo based simulation methodology. The rejection of the simulations resulted from non-overlapping behavioral parameter distributions for the different seasonal periods, i.e. the parameter sets for the best models differed seasonally. The spatial characterization of the three landscape units (bedrock outcrop, hillslope and riparian zone/valley bottom) were re-evaluated in more detail with respect to hydrological and hydrochemical responses (e.g., subcatchment discharge and water quality, ground-water level and quality, hillslope discharge and water quality, TDR soil moisture) at different spatial scales over a 19-year period from 1985 to 2004 to gain a better representation of the connectivity and linkages among landscape units. The analyses of hydrometric data are used to restructure the hydrologic model of PMRW, with the aim of improving the model's spatial representation. The paper will discuss the data analyses, the reformulation of the model, and present the results from new simulations within the GLUE procedure. The important questions are: (1) does the new model predict runoff better than the previous models; (2) how do we know that the new model has a more realistic physically based structure; and (3) are temporal and spatial water-quality variations consistent with the hydrological model results?
H44B-03 16:24h
The Water, Energy, and Biogeochemical Model (WEBMOD): A TOPMODEL application developed within the Modular Modeling System
Process-based flow and transport simulation models can help increase understanding of how hydrologic flow paths affect biogeochemical mixing and reactions in watersheds. This presentation describes the Water, Energy, and Biogeochemical Model (WEBMOD), a new model designed to simulate water and chemical transport in both pristine and agricultural watersheds. WEBMOD simulates streamflow using TOPMODEL algorithms and also simulates irrigation, canopy interception, snowpack, and tile-drain flow; these are important processes for successful multi-year simulations of agricultural watersheds. In addition, the hydrologic components of the model are linked to the U.S. Geological Survey's (USGS) geochemical model PHREEQC such that solute chemistry for the hillslopes and streams also are computed. Model development, execution, and calibration take place within the USGS Modular Modeling System. WEBMOD is being validated at ten research watersheds. Five of these watersheds are nearly pristine and comprise the USGS Water, Energy, and Biogeochemical Budget (WEBB) Program field sites: Loch Vale, Colorado; Trout Lake, Wisconsin; Sleepers River, Vermont; Panola Mountain, Georgia; and the Luquillo Experimental Forest, Puerto Rico. The remaining five watersheds contain intensely cultivated fields being studied by USGS National Water Quality Assessment Program: Merced River, California; Granger Drain, Washington; Maple Creek, Nebraska; Sugar Creek, Indiana; and Morgan Creek, Delaware. Model calibration improved understanding of observed variations in soil moisture, solute concentrations, and stream discharge at the five WEBB watersheds and is now being set up to simulate the processes at the five agricultural watersheds that are now ending their first year of data collection.
H44B-04 INVITED 16:36h
Riparian Zone Controls on the Chemical Dynamics of DOC-Rich Runoff from a Boreal Hillslope
The chemistry of streamwater is often quite different from that of soil water and shallow groundwater in the extensive upslope areas of many catchments, particularly with regards to the amount and character of dissolved organic carbon. The riparian zone (RZ), which runoff traverses immediately before entering the stream channel, has often been invoked to explain the differences between upslope water and the stream, as well as the changes in runoff chemistry during episodes. A "riparian convolution model" (RCM) is proposed to conceptualize the interaction of hydrology and soil in the RZ to give observed stream dynamics. A powerful feature of the RCM is that simple hydrological observations to define GW level- flow relations yield predictions of vertical profiles of RZ soil solution (which are rarely observed) from the much more common observations of stream chemistry. This model was tested on a catchment in boreal Sweden. The variation of DOC and many other constituents during runoff events could be explained by the activation of more superficial lateral flow paths in the riparian soil profiles, in accordance with the RCM. The riparian soil solution concentrations of Al and Fe could not be explained by such simple mass balances between the stream and the riparian soil profile. (Indeed a simple mass balance would not have worked for DOC if one looks at the soil profiles 12 m and 25 m from the stream). While this study demonstrates the importance of the RZ, the study also illustrates the great difficulty of quantitatively observing the interaction of hydrology and hydrochemistry.
H44B-05 16:48h
The Influence of Subsurface Processes on the Concentration and Composition of Dissolved Organic Matter
Microbial and geochemical interactions in the subsurface can result in chemical alteration and fractionation of organic matter thereby altering the nature and composition of dissolved organic matter (DOM) in groundwater and in surface waters dominated by ground water inflow. In this paper, the results of a study designed to determine the effectiveness of subsurface processes for removing DOM from two surface waters in Southern California will be presented. The recharge zones immediately underneath two infiltration basins, Anaheim Lake and Kraemer Basin, were found to be very active with respect to changes in the amounts and reactivities of the DOM. In all cases, dissolved organic carbon (DOC) concentrations decrease as water moves from the basins into the regional aquifer system. Data obtained from relatively shallow wells located near the infiltration basins (travel times less than 30 days) indicate that a large amount of DOM is removed during the first stages of transport in the subsurface. Parcels of water from both basins were followed for up to 360 days as the water moved away from the infiltration basins. DOC concentrations and specific ultraviolet absorbance (SUVA), an excellent indicator of aromatic carbon content of DOM, continued to decrease substantially with a general decrease in DOC concentration of about 70%. Regardless of initial DOC concentrations present in the infiltration basins, values decreased to approximately 1.3 mg C/L at the furthest points sampled. Analyses of organic matter isolates obtained by chromatographic methods indicated greater removal of aromatic molecules and preservation of branched chain aliphatic and alicyclic structures more resistant to biodegradation. Compared to samples from a wide range of environments, the DOM in the down gradient wells most closely resembled similar materials obtained from other groundwater systems and those of microbial origin. These results suggest that subsurface processes are significant in the metabolism of terrestrially derived material and contribute to the evolution of DOM pools from ones dominated by terrestrially derived material to those of microbial origin. Ultimately, these processes exert strong controls on the composition and reactivity of DOM in ground water and contribute to the pools of compounds comprising DOM in surface water systems receiving ground water discharge.
H44B-06 17:00h
Hydrogeologic and Geochemical Controls on The Transport and Fate of Agricultural Chemicals in Ground Water Beneath Riparian Zones
Numerous studies have documented the potential water-quality benefits provided by riparian buffer zones. However, many of the hydrogeologic and geochemical processes controlling the transport and fate of solutes in riparian zones are poorly documented. Over the past decade, the National Water-Quality Assessment (NAWQA) Program has investigated the transport and fate of agricultural chemicals along ground-water flow paths in study areas across the United States. In these studies, riparian zone efficiency in removing nitrate from ground water flowing from uplands to streams, varied from negligible to 100 percent as a result of variations in hydrogeologic and geochemical factors. These factors include (1) total denitrification in the upgradient aquifer; (2) long residence times (>50 years) along ground-water flow paths allowing even slow reactions to completely remove nitrate; (3) dilution of nitrate-enriched waters with older ground water containing low concentrations of nitrate; (4) bypassing of riparian zones due to extensive use of drains and ditches; (5) movement of ground water along deep flow paths below shallower, organic-rich reducing zones; and (6) movement of ground water through riparian-zone sediments along preferential flow paths. An important outcome of these studies has been an increased understanding of the effect of long residence times of ground water along flow paths on the transport and fate of nitrate, with some exceeding 50 years in shallow aquifers less than 20 m thick. Such long residence times have important implications for nutrient management programs since it may take several decades before the full benefits of reduced inputs are realized in receiving surface waters. Another important finding is the impact of widespread use of drains and ditches as a management tool for lowering seasonally high water tables. Such artificial drainage circumvents natural hydrologic flow paths, and routes shallow ground water directly to surface waters, bypassing riparian buffer zones in the process. Increased understanding of the hydrogeologic settings in which riparian buffer zones are likely to be inefficient at nitrate removal can aid managers in developing improved nutrient management plans.
H44B-07 17:12h
Denitrification and patterns of electron donors and acceptors in 8 riparian zones with contrasting hydrogeology
A better understanding of nitrate removal mechanisms is important for managing the water quality function of stream riparian zones. We examined the linkages between hydrologic flow paths, patterns of electron donors and acceptors and the importance of denitrification as a nitrate removal mechanism in 8 riparian zones on glacial till and outwash landscapes in southern Ontario, Canada. Nitrate-N concentrations in shallow groundwater from adjacent cropland declined from levels that were often 10-30 mg L-1 near the field-riparian edge to <1 mg L-1 in the riparian zones throughout the year. Chloride data suggest that dilution cannot account for most of this nitrate decline. Despite contrasting hydrogeologic settings, these riparian zones displayed a well-organized pattern of electron donors and acceptors that resulted from the transport of oxic nitrate-rich groundwater to portions of the riparian zones where low DO concentrations and an increase in DOC concentrations were encountered. The natural abundances of d15N and in situ acetylene injection to piezometers indicate that denitrification is the primary mechanism of nitrate removal in all of the riparian zones. Our data indicate that effective nitrate removal by denitrification occurs in riparian zones with hydric soils as well as in non-hydric riparian zones and that a shallow water table is not always necessary for efficient nitrate removal by denitrification. The location of "hot spots" of denitrification within riparian areas can be explained by the influence of key landscape variables such as slope, sediment texture and depth of confining layers on hydrologic pathways that link supplies of electron donors and acceptors.
H44B-08 17:24h
Trace Metal Cycling Within the Riparian Wetland and Hyporheic Zones of a Northern Temperate Stream Catchment
Our study addresses watershed linkages to streams and lakes, focusing on trace element cycles and metal-binding ligand production and transport. The research is being carried out at an extensively instrumented stream catchment in the Northern Temperate Lakes Region of north-central Wisconsin - a system that is also under investigation by the NSF-funded Long Term Ecosystem Research (LTER) program and the USGS Water Energy and Biogeochemical Budgets (WEBB) program. Within the study system we have identified flow paths of contrasting redox, in addition to zones of positive and negative ground water recharge. Detailed temporal data sets of water and ligand chemistry data are being obtained from dense networks of nested piezometers and hyporheic samplers placed in these contrasting regions/flow paths. In addition to major element chemistry, data on over 25 trace metals are acquired using high-resolution ICP-MS. A suite of oxyanionic metals is included to assist in our interpretation of DOC and iron oxide/sulfide partitioning mechanisms. Along a predominantly oxic flow path from regional ground water to stream riparian porewaters, concentrations of both Mn and Zn drop markedly (10-100 fold). Arsenic is also lost to riparian and wetland sediments. In contrast, levels of Cu increase 2-3 fold from upland hillslope groundwater to stream riparian porewaters. The importance of sulfide and iron in regulating ground water fluxes of certain trace metals to the stream is evident when oxic and anoxic flow paths are compared. Levels of Cu in many anoxic regions (0.06 nM Cu, 50-700 nM sulfide, 30-40,000 nM Fe) are strikingly reduced from that observed in oxic zones (4-6 nM, Cu, $<$20 nM sulfide, 100 nM Fe). Studies are in progress to characterize the oxidation state of Cu in the contrasting micro-environments. Concentrations of both Zn and Tl are significantly lower (10 fold) along anoxic flow-paths than in oxic regions. Large seasonal variations in Pb levels are observed, with particularly elevated concentrations seen in spring, coinciding with greater DOC fluxes. Temporal variations in metal levels and fluxes, as driven by seasonality in hydrologic-forcing, will be emphasized in the paper.
H44B-09 17:36h
Rainfall-Runoff Modeling to Compare Hydrological Processes Governing Solute Transport in Five Different Agricultural Watersheds
Non-point sources of fertilizers and pesticides from agricultural areas can impair the water quality of streams. A better understanding of how hydrological processes affect the transport of agricultural chemicals to streams is required to improve management of fresh water resources. Examining how hydrological processes vary in agricultural landscapes and subsequently affect solute transport is a focus of the U.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Program. As part of the NAWQA Program, the quantities of precipitation, irrigation, recharge, ground water, seepage and streamflow are being monitored along with reported application rates of agricultural chemicals within five agricultural watersheds in California, Washington, Nebraska, Indiana, and Maryland. Two quasi-distributed hydrological models capable of simulating solute transport (the Soil and Water Assessment Tool (SWAT) and the Water, Energy, Biogeochemical Model (WEBMOD)) were applied to each watershed. Although the underlying environmental processes were represented differently in each model, both models were capable of simulating the expected hydrological characteristics and dominant flow paths in each watershed. In each model, the relative contributions of infiltration-excess overland flow, saturation-excess overland flow, shallow subsurface flow, preferential flow, and deep ground water flow were estimated. These fluxes then were compared to measured in-stream solute concentrations to assess how each hydrological flow path affected water quality. Generally, soil profile characteristics, land management practices, and irrigation methods regulated hydrological connections between watershed hillslopes and receiving streams.
http://water.usgs.gov/nawqa-only/act/index.html
H44B-10 17:48h
Hydro-Ecological Linkages in Urbanizing Watersheds: The Role of Small Streams in Controlling Nitrogen Export
The terrestrial-aquatic interface plays an important role in watershed nitrogen cycling. We assess the relative role of terrestrial and in-stream processes in the retention, transformation and mobilization of nitrogen, by combining hydro-ecological modeling with field measurements, geographic information systems and remote sensing, to address relevant processes and related patterns across a range of spatial and temporal scales. The Regional Hydro-Ecological Simulation System (RHESSys), a terrestrial hydro-ecological watershed model, is coupled within a geographic information system to a flow and water quality model for streams. Model development and application focuses on Baisman Run, a small, urbanizing watershed, located within the Baltimore Ecosystem Study Long Term Ecological Research program. Here we mainly report on results from our field data collection component. Field measurements include nutrient monitoring, solute additions and nitrate stable isotopes. To estimate nutrient uptake rates from solute additions, we adapted a transient storage model (OTIS) to account for nutrient saturation during the addition. Over time (i.e., several years), we observe a strong relationship between nitrate loss and flow conditions; nitrate loss was detected only at low to medium flows, while dilution dominates higher flows. Over space, stream characteristics exercise a strong control on nitrogen uptake. Ammonium uptake is related to stream size and reflects spatial variation in water/sediment contact. Transient storage, although it effects hydraulic residence time, does not necessarily translate into higher rates of biogeochemical processing in these headwater streams. Overall, our measurements and preliminary modeling results suggest that in urbanizing watersheds, small streams play a spatially and temporally complex role in controlling watershed nitrogen export. Subsequent work will further high-level integration between process-based models, field data collection and other data sources, to highlight the importance of ecotone processing and assess the impact of land-use change on watershed nitrogen export.