H21H-0918
How is dissolved iron produced and transported in the Amur River basin?
Recent research showed that iron limits phytoplankton growth in the western subarctic Pacific and that
significant part of it comes from the Amur River, which forms the boundary of China and Russia and finally
flows into the Sea of Okhotsk. Our research group confirmed that the crucial processes of producing
dissolved iron are reductive release of ferrous iron and formation of iron complex with organic compound in
wetland. However, drastic conversion of wetland into cultivated land in China part during the last half of 20th
century may have great impact on iron production in the future. To assess the land conversion impact on
dissolved iron production, we constructed a semi-distributed hydrological model incorporating dissolved iron
production mechanism.
The model consists of two modules; one for dealing with the physical process that calculates runoff (TOP-
RUNOFF), and the other for dissolved iron production process (TOP-FE). Performance level of TOP-
RUNOFF without any calibration assessed by Nash and Sutcliff criteria against observed discharge at several
points are fairly well except for catchments affected by anthropogenic impact such as dam. On the basis of
TOP-RUNOFF, TOP-FE was formulated as a function of water content, organic compound, air temperature,
and one parameter representing the degree of redox condition. Calculated values were compared with
observed value at the several tens of points. The result shows that the model reached the level which can
predict seasonal trend of dissolved iron concentration.
However, the present model can not simulate abrupt increase of dissolved iron concentration which was
observed at many points during the late 1990s. Possible reasons of this anomaly are now under
consideration. At present, promising reasons are climate change and anthropogenic impact such as
agricultural activity, but not yet resolved. Thus, to clarify unknown mechanism which governs the long-term
trend of dissolved iron concentration is needed to improve predictability of our model.
http://www.chikyu.ac.jp/AMORE/index_e.html
H21H-0919
Phosphorus Accumulating Organisms and Biogeochemical Hotspots
Despite extensive research, many of the processes that control phosphorus (P) movement from agricultural fields to streams and lakes are not well understood. This limits our ability to develop management strategies that will mediate P contamination of freshwater ecosystems and subsequent eutrophication. Recent advances in molecular microbiology have prompted a paradigm shift in wastewater treatment that recognizes and exploits the ways specific microbial processes influence P solubility. Central to this enhanced biological phosphorus removal in wastewater treatment plants is a relatively recently discovered microorganism, Candidatus accumulibacter, which takes-up P and stores it internally as polyphosphate under alternating aerobic and anaerobic conditions. Within the past few months we have discovered this organism in the natural environment and its role in P biogeochemistry is unclear. We speculate that it may function similarly in variable source areas, which experience cycles of saturation and desaturation, as it does in the anaerobic- aerobic cycles in a wastewater treatment plant. If so, there may be potential opportunities to realize similarly new perspectives and advancements in the watershed context as have been seen in wastewater technologies. Here we present some of our preliminary findings.
H21H-0920
Spatial and Temporal Variability in Groundwater Phosphorus Concentrations in an Agricultural Riparian Zone in Response to Changes in Flow Regime
Phosphorus (P) concentrations were measured in a stream riparian area during a range of flow regimes including a flood event resulting from the drawdown of an upstream reservoir. Stream flow, groundwater levels and water chemistry (total P, soluble reactive P) were measured along two transects (T1 and T4) located in a riparian zone located between an agricultural field and the stream. The direction of groundwater flow throughout the riparian zone was lateral between T1 and T4, rather than along each transect from the field to stream. The riparian zone was a source of TP and SRP to groundwater under all three flow regimes. Concentrations of TP varied considerably under different hydrologic regimes and increased by nearly an order of magnitude at the downstream transect (T4) during flooding during reservoir release, indicating that this riparian zone was a source of phosphorus to surface water and groundwater during this period. In contrast, groundwater SRP concentrations decreased during flooding, likely resulting from the influx of oxygen-rich water from the stream. Our results indicate that autumn flooding of riparian zones downstream from impoundments may mobilize phosphorus in groundwater systems, thereby reducing the nutrient retention potential of riparian zones.
H21H-0921
The fate of Nitrate under the natural groundwater flow system in a volcanic aquifer composed by pyroclastic flow
Nitrate in drinking water causes health problems, and causes eutrophication. Miyakonojo basin is a well- known agricultural area in Southern Kyushu, Japan and highly depends on groundwater resources for everyday use. The detailed three dimensional groundwater flow system study has been done in the basin to understand the three dimensional distribution of Nitrate-Nitrogen in the aquifer. Field water sampling was conducted between Sep. 2006 and July 2007. The result suggests that the tributary river water mainly recharges to the shallow groundwater in the eastern part of the basin. On the other hand, the summer precipitation recharges the shallow groundwater which flows from upland down toward the river in the western part of the basin. This means that the shallow groundwater flow system is primarily affected by the surface geomorphology, which is completely different between eastern and western parts of the studied basin. For nitrate concentration, the shallow groundwater shows gradual increasing tendency along the flow line in the eastern part of the basin, while the nitrate concentration decreases and the reduction product increases along the flow in the western part. Especially in the western part of the basin, the major land use are the intensive cattle farming and dry farm land using artificial fertilizer, both are the potential sources of the nitrate for the local shallow groundwater. Also geomorphologic upland of the western part of the basin works as the recharge area of the shallow groundwater, and high concentration of nitrate is supplied at these areas. To understand this site-specific nitrate reduction process in the western basin, it is important to understand the nitrate origin and nitrogen process including denitrifcation process. For this, a multi-Isotope approach of dN and dO to tracing the sources of nitrate is applied in the study area with the relation of groundwater flow system. The field sampling was done in July 2007 and Sep. 2008 to analyze inorganic water chemistry, dD and d18O stable isotopes in water andd15N and d18O in Nitrate. We intend to report this result and related discussion at the meeting.
H21H-0922
The Influence of Tidal Forcing and Groundwater Flow on Nutrient Exchange Between Salt Marsh Sediments and Adjacent Estuaries
Unlike freshwater wetlands, tidal salt marshes export significant dissolved reactive phosphorus (DRP) and ammonium to adjacent surface waters. Here we used long-term nutrient and salinity records from porewaters and tidal creeks collected at North Inlet, SC and numerical models of tidally-driven groundwater flow to investigate the role of tidal forcing in nutrient export. Long-term nutrient and salinity records were found to be highly correlated with variations in the tidal signal over a wide range of time scales. Numerical models of tidally-driven groundwater flow were then developed to independently estimate the impact of variations in mean water level (MWL) and tidal amplitude on porewater discharge to tidal creeks. Simulation results are consistent with observed trends and suggest that an increase in MWL of as little as 5 cm can increase groundwater flushing, and hence nutrient export, by more than 40 percent when marsh platforms are equilibrated near mean high water. Should MWL continue to increase faster than the marsh surface can accrete, however, the rate of flushing decreases steadily with increasing MWL. Increases in tidal amplitude also increase groundwater flushing, particularly when increasing the tidal amplitude causes the marsh platform to be inundated at high tide. Mass balance calculations based on these simulations suggest that the total DRP and ammonium discharged from salt marsh sediments at North Inlet are in excess of that needed to support observed exports to the coastal ocean. Results have important implications for marsh restoration, estuarine nutrient budgets, and sea level rise.
H21H-0923
Evaluating the human impact on groundwater quality discharging into a coastal reef lagoon
The Eastern coast of the Yucatan Peninsula has the fastest growth rate in Mexico and groundwater is the only source of drinking water in the region. The consequences of the lack of proper infrastructure to collect and treat wastewater and the impact of human activities on the quality of groundwater are addressed. The groundwater in the coastal aquifer of Quintana Roo (SE Mexico) discharges directly into the ocean. In addition, the coral reef of the Eastern Yucatan Peninsula is part of the Mesoamerican Coral Reef System, one of the largest in the world. The interaction of the reef-lagoon hydraulics with the coastal aquifer of Puerto Morelos (NE Yucatan Peninsula), and a major input of NH4, SO4, SiO2, as a consequence of the use of septic tanks and the lack of modern wastewater treatment plants are presented. No seasonal parameters differences were observed, suggesting that groundwater composition reaching the reef lagoon is not changing seasonally. A conceptual model of the coastal aquifer was developed, in order to explain how the human activities are impacting directly on the groundwater quality that, potentially, will have a direct impact on the coral reef. The protection and conservation of coral reefs must be directly related with a policy of sound management of coastal aquifers and wastewater treatment.
H21H-0924
Processes, Controls, and Potential for In-situ Nutrient Removal During Managed Aquifer Recharge in an Agricultural Basin
We are conducting research on rates and dynamics of water quality improvement that occur during managed aquifer recharge (MAR), with a focus on reducing the load of nitrate exported during recharge. Nitrate is the most common nonpoint source pollutant in surface and ground water in the United States, and is a problem particularly in basins developed for agriculture. Our study site is located in central coastal California, where diversion from a slough (wetland) is permitted during periods of high flow for use in MAR. Diverted water is recharged into an eolian and fluvial, unconfined aquifer using a 3-km2 percolation pond, then subsequently recovered and distributed to local farmers. As a result of agricultural and other activities in the basin, diverted slough water is often rich in nitrate (historical values as high as 4 mM); similarly high nitrate values have been measured in water from the underlying aquifer. Prior to the start of the 2007-08 water year, we surveyed, sampled, and instrumented the recharge pond in order to quantify local seepage rates and sample recharging water to assess changes in water quality during infiltration through the base of the pond. Nests of piezometers and lysimeters were screened at depths of 50 to 150 cm beneath the base of the pond and sampled weekly throughout the recharge season. Total MAR was 7.4 × 105 m3 (600 ac-ft) during the 2007-08 water year, with initial nitrate concentrations of 10 μM to 100 μM in the diverted water. Point-specific infiltration rates were greater than 10 m/day in some locations below the pond, and much lower in other locations. Nitrate concentrations were reduced by 50 to 90% beneath the pond, with the greatest reductions occurring at lower concentrations and slower infiltration rates. Suboxic conditions developed beneath the pond during recharge, which is consistent with removal of nitrate by denitrification. Dissolved organic carbon concentrations were elevated during the recharge season throughout much of the shallow subsurface, suggesting that there is no carbon limitation on denitrification. Isotopic analyses will be used to determine the extent and dynamics of denitrification in this system, and to explore controls and pathways responsible for improvements to water quality.
H21H-0925
Effective Removal of Nitrogen and Phosphorus from Surface Water Using Constructed Comprehensive Floating Remediation Islands
Nitrogen and phosphorus are the chief pollutants of our aquatic systems which may be resulted from different contamination sources and could cause serious environmental and ecological problems. For example, nitrate contamination of the water systems from agricultural practices may be contributing to the eutrophication of the Chesapeake Bay, Maryland, USA, degrading water quality and aquatic habitats. Effective approaches for removal of nitrogen and phosphorus from our aquatic systems, particularly from surface water, is called for imminently. An in-situ remediation measure by constructed floating remediation islands has been developed and tested through the field experiments recently. Four pilot-scale settings with the different components and structures were constructed and operated in parallel in which a new type of the constructed floating remediation islands with multi-layers of substrate fillers, called the constructed multi-layer comprehensive floating remediation island, was included. The contaminated water taken directly from a river containing richly nitrogen and phosphorus was used for those experiments. The experiment results obtained from the four different experiment settings were examined. It was noticed that the degradation rates of both nitrogen and phosphorus in water in the setting with the constructed multi-layer comprehensive floating remediation island was greater than those in others. The mean removal rate of phosphorous in the experiment setting with the constructed multi-layer comprehensive floating remediation island was considerably higher than the removal rates of phosphorous in the other three experiment settings.
H21H-0926
Mitigation efficacy of vegetated buffers in reducing non-point source pollution: A critical review and meta-analysis
Vegetated buffers are the most studied and widely used agricultural management practice for reducing non- point source pollution. A wealth of existing literature provides experimental data on their mitigation efficacy. This paper aggregated many of these results and performed a meta-analysis on them to quantify the relationships between pollutant removal efficacy and buffer width and buffer slope. Theoretical models for removal efficacy (Y) vs. buffer width (w) were derived and tested against data from the surveyed literature using statistical analyses. A model of the form \Y = K (1 - exp (-bw)) , (0< K ≤100) is shown to successfully capture the relationship between buffer width and pollutant removal, where K reflects the removal capacity of the buffer and b reflects its probability to remove any single particle of pollutant in a unit distance. The estimates of K are 90.9, 93.2, 92.0, and 89.5 for sediment, pesticides, nitrogen (N) and phosphorus (P), respectively. Buffer width alone explains 37, 60, 44 and 35% of the total variance in removal efficacy for sediment, pesticides, N and P. Buffer slope is linearly associated with sediment removal efficacy either positively (when slope ≤9%) or negatively (when slope >9%). A sediment removal model based on buffer width and buffer slope explains 55.8% of the total variation in removal efficacy. Models for all the studied pollutants are statistically significant with P-values <0.001. Based on our analysis, a 30 m buffer removes over 80% of all the studied pollutants. These models predicting optimal buffer width/slope could be instrumental in the implementation and design of vegetated buffers for treating agricultural runoff to meet specific water quality objectives. The quantitative relationship established also provides valuable information for modeling buffer efficacy at the watershed scale.
H21H-0927
The effectiveness of nitrate vulnerable zones for limiting surface water nitrate concentrations – the failure of nutrient input management.
This study considers the surface water concentrations of nitrate in areas designated as nitrate vulnerable zones for between 12 and 5 years. The study proposes a range of test of the effectiveness of nitrate management within the NVZ. First, that surface water trends should be downward, second that the trend should be less than observed over the same time period in a neighbouring control catchment. Third, that the effectiveness of NVZ designation can be measured relative to the control catchment and relative to the nitrate concentrations prior to designation. The tests shows that 69% of NVZs showed no significant improvement in surface water concentrations even after 15 years. In comparison to a control catchment, 29% of NVZs showed a significant improvement but 31% showed a significant worsening of nitrate concentrations. The average improvement due to NVZ designation was 0.02 ±0.08 mg N/l/yr but this was not significantly different from zero. The differences betweens NVZs could not be significantly related to the size of the NVZ; uptake of the scheme; extent of uptake; land use change or geology of the aquifer. The lack of objective success from NVZ designation suggests that strategies based on input management need to be rethought.
H21H-0928
The Fluvial Flux of Nitrogen from Great Britain 1974 – 2005 in the context of the terrestrial nitrogen budget of Great Britain.
This study compiles long term records of the concentration of nitrogen species from British rivers to assess temporal change in the total fluvial nitrogen flux compared to the other inputs to, and outputs from, the UK terrestrial biosphere. The following nitrogen species are considered: ammonia, nitrate, nitrite, dissolved organic nitrogen and particulate organic nitrogen. Concentration and flow records were reconstructed from 1974 to 2005 for ammonia, nitrate, nitrate and dissolved organic nitrogen (DON) and from 1992 for particulate organic nitrogen (PON). The reconstructed fluvial nitrogen time series was compared to records for inorganic fertiliser, atmospheric emissions, industrial and sewage effluent, and imports. .The results of the study show that the total dissolved nitrogen flux over the study period, after flow correction, varied from 470 to 980 ktonnes N, which, on average, comprised: 69% nitrate-N; 26% dissolved organic-N; 4% ammonia-N; and 1% nitrite-N. Including PON varied from 504--1004 ktonnes N/yr which made the UK the largest exporter of nitrogen to the oceans yet reported. The flux of ammonia-N shows a significant decline over the study period, but significant increases in both nitrate-N and dissolved organic-N mean that the total dissolved nitrogen flux still shows a significant increase at a rate of 6.3 ktonnes N/yr2. The dissolved nitrogen flux record shows both a steady increase over the period 1974 to 2005 and sharp discrete rises in response to severe droughts. The rise in response to severe droughts is not consistent with a storage effect of reduced flows but appears to represent new production in the year of the drought. The long-term rise of fluvial nitrogen flux from British rivers is in contrast to declines in inputs and other N outputs, so that, although Great Britain is a net sink of total nitrogen, the size of this sink is diminishing.
H21H-0929
Understanding nitrogen fixation in two shallow eutrophic lakes in Central Florida
Our objective is to improve our understanding of the physicochemical and biological influences on cyanobacterial N2 -fixation in Lakes Jesup and Monroe, Florida. The overall goal of this project was to determine rates of nitrogen added to the water column due to nitrogen fixation and thus contributing to a more accurate assessment of nitrogen loading from the watershed. The isotopically enriched 15 N2 method was used to mearsure N2 fixation in a sub-set of samples to calibrate the acetylene reduction approach used at all stations. One station from each lake was pre-selected for calibration purposes. To calibrate, we compared linear regression of N2 fixation nmol L-1 h-1 as calculated by the 15 N-method compared with ethylene produced (nmol L--1 h-1) via the acetylene reduction method in the same water collected at both stations for each sampling. A statistically significant relationship was found between the two methods in both lakes however, the regression was a better fit for Lake Jesup than for Lake Monroe. We therefore built our calibration data set by using all the data from Lake Jesup and several points that were < 5 nmol L-1 h-1 as determined by the 15N-method. In general, Lake Monroe had much lower rates of fixation than Lake Jesup. It was therefore difficult to calibrate the Lake Monroe methods independently. Thus we found the best calibration resulted from a combination of the data from the two lakes. Lake Monroe provided the N-fixation values between 0 to <5 nmol L-1 h-1, and Lake Jesup data included values >5 nmol L-1 h-1. The calibration slope of 6.37, is within the range of previously published measurements/calibrations. The approach used to measure the enriched samples also provides natural abundance δ15 N values from the "untreated" POM. Isotopic values less than 1 permil and near 0 permil , typically reflect an atmospheric N source via fixation of N2, versus nitrate utilization. All data in the time series from both Lakes were <5 permil , except for one value from Lake Monroe in September of 2007, which had an extremely enriched value of 9.3 permil, indicating a different N source for this sampling. δ15N values near this level of relative enrichment are typically associated with denitrification, and or DIN coming into the watershed from pasture lands (manure). However, our data do not fall within the typical range of δ15N values for denitrification when Lake Monroe's DIN concentrations are considered.
H21H-0930
Isotope Hydrology and Stream Nutrient Chemistry of the Cuyahoga River in Northeast Ohio, USA
Recent increasing occurrence of disruptive eutrophic conditions in Lake Erie highlights a shortcoming of our understanding on the dynamics of nutrients in aquatic ecosystems. In spite of significant reduction in point- source nutrient discharge, the loadings of bioavailable nutrients such as ammonia (NH4+), nitrate (NO3-), and soluble reactive phosphorus (SRP) from tributaries continue to sustain such harmful events. But little is known about how much and how the structure of nutrients changes along the Cuyahoga River, a major nutrient contributor to Lake Erie. To better understand the stream hydrology and nutrient dynamics in this impaired river, we proposed to 1) examine the stream hydrology through stable isotopic analysis, 2) determine the concentrations of various nutrient variables, and 3) evaluate the bioavailability of P in river bed sediments by chemical extraction tests. River waters and sediments were collected in a dozen locations from the Cuyahoga River and its major tributary five times from July 2007 to May 2008. As a result, we found that 1) river water becomes isotopically heavier from upstream to downstream, 2) nutrient concentrations increase stepwise downstream, and 3) changes in water soluble phosphorus (WSP) of river bed sediments are consistent with those of nutrient concentrations in river waters. The results of this study further attest that the majority of nutrient inputs are from point sources like effluents of municipal wastewater treatment plants.
H21H-0931
A Multi-isotope Tracer Approach Linking Land Use With Carbon and Nitrogen Cycling in the San Joaquin River System
The San Joaquin River (SJR) is a large hypereutrophic river located in the Central Valley, California, a major agricultural region. Nutrient subsidies, algae, and other organic material from the San Joaquin River contribute to periods of low dissolved oxygen in the Stockton Deep Water Ship Channel, inhibiting salmon migration. We used a multi-isotope approach to link nitrate and particulate organic matter (POM) to different sources and related land uses. The isotope data was also used to better understand the physical and biological processes controlling the distribution of nitrate and POM throughout the river system. Samples collected from the mainstem SJR and tributaries twice-monthly to monthly between March 2005 and December 2007 were analyzed for nitrate, POM, and water isotopes. There are many land uses surrounding the SJR and its tributaries, including multiple types of agriculture, dairies, wetlands, and urban areas. Samples from SJR tributaries containing both major and minor contributions of wetland discharge generally had distinct nitrate and POM isotope signatures compared to other tributaries. Unique nitrate and POM isotope signatures associated with wetland discharges may reflect anaerobic biological processes occurring in flooded soils. For the mainstem SJR, we applied an isotope mass balance approach using nitrate and water isotopes to calculate the expected downstream isotope values based upon measured inputs from known water sources such as drains and tributaries. Differences between the calculated downstream isotope values and the measured values indicate locations and time periods when either biological processes such as algal uptake, or physical process such as the input of unidentified water sources, significantly altered the isotope signatures of water, POM, or nitrate within the SJR. This research will provide a better understanding of how different land uses affect the delivery of carbon and nitrogen to the SJR, and will provide a better understanding of the physical and biological processes occurring within the mainstem SJR.
H21H-0932
Isotopic Responses to Processes Related to Oxygen Cycling During Diel Studies in the San Joaquin River, California
Episodic conditions of low dissolved oxygen (DO) occurring in the San Joaquin River deep water shipping channel (DWSC) at Stockton, California inhibit salmon migration during late summer and early fall. As part of a CALFED study to determine the sources of organic matter and nutrients related to the low DO problem, four diel studies were undertaken: two at the DWSC in 2006 and 2007 and two about 40 miles upstream at Crows Landing in 2005 and 2007. The purpose of the diel studies was to gain a better understanding of the transient processes controlling oxygen concentrations and to compare the range of diel variations of various isotopic measurements with those collected on a less frequent basis. The DWSC is dredged to about 40 feet deep and is tidally influenced. The river at Crows Landing is about 5 feet deep and well above tidal influence. Crows Landing was therefore used for comparison with the DWSC as a hydrologically less complicated portion of the river that has not been dredged. Samples were collected at two hour intervals from a one meter depth at Crows Landing. Values of d18O-DO, DO concentrations and pH showed a strong diel response linked to photosynthesis and the effects of bacterial respiration. The d13C of POM (composed mostly of algae) increased at night as expected while the C:N ratio of POM unexpectedly decreases, possibly due to an increased presence of zooplankton at night. Opposing trends of d15N and d18O of nitrate unrelated to a diurnal cycle suggest that source mixing was largely responsible for nitrate composition and concentration through much of the study. Samples were collected at two hour intervals from 1, 5, and 8 meter depths at Rough and Ready Island in the DWSC. Isotope and concentration data showed a distinct diurnal photosynthetic response at the 1 meter depth only. The 2007 DIC and POM isotopic data suggested that nitrification was significantly responsible for oxygen consumption through the duration of the study. The particularly low flow conditions in 2007 increased residence time and allowed water temperatures to rise accentuating the conditions for low DO concentrations. Compared to Crows Landing, where DO concentrations were near and above air saturation, DO concentrations at the DWSC were almost all below saturation reflecting the greater role of respiration and nitrification in the deeper, slower moving water where photosynthesis is restricted by light penetration and organic debris can accumulate. The diel studies illustrate that isotopes of DO, DIC, nitrate and POM provide unique information about the complex behavior of the DWSC. Furthermore, these data illustrate that sampling at a single depth or at a single time of day may not capture an accurate picture of conditions related to oxygen cycling.
H21H-0933
Potential effect of algal productivity in the San Joaquin River on nitrate concentrations and isotope ratios
The d15N of algae in nitrate-rich rivers is often about 4 to 5 permil lower than the d15N of the nitrate used by the algae. In cases where the algal productivity significantly depletes the available nitrate pool, the uptake of nitrate can cause significant increases in the d15N and d18O of the residual nitrate, resulting in isotope values similar to what would be expected for a major contribution of human or animal waste to the river. Furthermore, progressive algal uptake also causes nitrate d18O and d15N values that plot along slopes of about 1:2, consistent with assimilation and/or denitrification. One way to resolve the question of whether the high nitrate d15N and d18O values reflect a waste source, assimilation, or denitrification is to compare the simultaneous changes in nitrate concentrations, algal quality and loads, nitrate d15N and d18O, and the d15N, d13C, and C:N of the particulate organic matter, which is often dominated by algae in large rivers. As part of a recent investigation of nitrate and organic matter sources to the San Joaquin River (SJR), samples were collected twice-weekly to monthly for over 2 years from 7 mainstem sites (as well as many major and minor tributary sites) and analyzed for a wide range of chemical constituents and isotope ratios. The average nitrate d15N of mainstem sites was +11 permil, with a range of +2 to +17; the average d18O was +5 permil, with a range of -1 to +18. The potential impact of algal uptake on isotope ratios in the SJR was modeled using isotope and chemical data from 2 Lagrangian experiments in the San Luis Drain, a simple concrete-lined canal which drains into the SJR, has only a single input of water, and has algae similar to that in the SJR and a high productivity rate (Volkmar et al., in prep.).
H21H-0934
Nutrient sources to urban streams in three metropolitan areas of the United States using dual nitrate isotopes
Urbanization can have considerable impacts on streamwater quality, discharge, and biota. In particular, high population densities and impervious surface cover make urban streams susceptible to excess nutrients, particularly nitrate, derived from human waste inputs and also from surface runoff of atmospherically- deposited nitrogen. Here we use dual nitrate isotopes to examine nutrient sources to a 88 streams in 3 metropolitan areas of the U.S.: Atlanta (Georgia), Denver (Colorado), and Raleigh-Durham (North Carolina). For each metropolitan area, streams were selected in basins spanning a gradient of urban intensity and to minimize natural variability. This study was part of the National Water-Quality Assessment (NAWQA) Program's assessment of the effects of urbanization on stream ecosystems across urban areas of the U.S. Periodic samples were collected from 30 basins in the Raleigh-Durham and Atlanta areas and 28 basins in the Denver area between October 2002 and September 2003. In 260 analyses, δ15N and δ18O values varied widely. In this presentation, we examine potential causes for the large range in observed isotope values and discuss how these values may vary with indicators of urbanization including road and population density, urban intensity, and land cover. For example, preliminary analyses indicate that δ18O values are generally higher in basins with high road density, suggesting that urban drainage, transportation infrastructure, and impervious surface cover may expedite the delivery of atmospherically- deposited nitrogen to surface waters. These results are expected to further our understanding of nitrate dynamics in urban systems and provide essential information for the successful management of urban water quality.
H21H-0935
Nutrient Input and Dynamics in a Restored Urban Stream Impacted by Mixed Sewer Systems
Export and retention of nutrients in urban watersheds remains poorly constrained. Available data is often based on studies conducted on large-scale, forested and mixed use watersheds rather than small urbanized systems. Additionally, there is a lack of data on the amount and impact of nutrients introduced into urban waterways as the result of stream-sewer interactions and a varied flow regime. In order to address this knowledge gap, water was sampled during baseflow (bi-weekly from April 2007 to present) and stormflow from a restored urban stream in Pittsburgh, Pennsylvania (USA) impacted by both Sanitary Sewer and Combined Sewer Overflow (SSO and CSO, respectively) networks. Nine Mile Run (NMR), a restored urban stream, drains a 1600 hectare urban watershed characterized as 38% impervious. Analysis of post-restoration water quality data suggests that atmospheric deposition and sewage both contribute nutrient pollution to the stream. We estimate input of atmospheric nitrate deposition to the watershed is 18.96 kg NO3- ha-1yr-1, yet a preliminary nitrogen budget suggests that nitrate export from the basin is consistently higher (~30 kg NO3- ha-1yr-1). Mean baseflow nitrate concentrations are substantially higher during the wetter portions of 2008 (12.07 mg NO3 -/L) as compared to the drier 2007 year (7.3 mg NO3 -/L). This suggests increased stream/sewer interactions during wetter periods. These results document the effect of Sanitary Sewer systems on an urban stream and highlight the challenges inherent in improving urban water quality through physical stream restorations.
H21H-0936
Impact of urbanization on regional-scale Nitrogen transport
This research focuses on the relationship between land use change, in particular urbanization, and nitrogen fate and transport at a regional spatial scale. While much work has been done on improving knowledge of local scale transport mechanisms, understanding the factors that influence nitrogen transport at a regional scale has important implications for water resources management. Using a non-linear regression model that relates watershed characteristics to observed in-stream nitrogen loadings, SPARROW (SPAtially Referenced Regressions On Watershed Attributes), along with the National Land Cover Data (NLCD) 1992/2001 Retrofit Land Cover Change Product, we estimate the impact of land use urbanization on the fate and transport of nitrogen for the Contiguous United States. The NLCD land use change data shows an increase of 30% in urbanized area. The goal of our work is to understand how this observed increase in urbanization has impacted the delivery of nitrogen through surface water systems.
H21H-0937
Urban and Rangeland Contributions to Nonpoint Source Pollutants in the Waiulaula watershed, Hawaii
The Waiulaula is a 120 km2 watershed that encompasses rainforest in the headwaters and arid scrub at lower elevations. The stream and its tributaries contain perennial, intermittent, and ephemeral reaches. Although water quality in the upper reaches is relatively pristine, there are nevertheless concerns about the effects of urban development on both endemic freshwater fish and coral reefs present near the ocean outlet. Stormwater quality was measured using an autosampler located below the town of Waimea paired with an autosampler located above town where the stream exits the forest. Data, which were obtained for four events, consistently showed that sediment and nutrient concentrations were greater below town than above town. These increases are statistically significant at the 95% level for total dissolved nitrogen, nitrate-nitrite, and orthophosphate, and significant at the 90% level for suspended sediment, ammonium, and total phosphorus. An autosampler installed in the adjacent Pelekane watershed sampled runoff from the arid to semi-arid rangelands that are typical of the lower half of the Waiulaula watershed. This autosampler, which is located on an ephemeral stream immediately above the ocean outlet, sampled one event. In comparison to stormwater runoff from the forested portion of the Waiulaula watershed, Pelekane stormwater contained 140 times more suspended sediment, 59 times more nitrate-nitrite, 38 times more total phosphorus, 20 times more ammonium, 9 times more orthophosphate and 6.5 times more total dissolved nitrogen. Grab samples of stormwater in the Waiulaula watershed were assayed for stable isotopes of the nitrogen and oxygen present in nitrate. Nitrate concentrations were very low at all locations. Data from three events showed that isotopes at the outlet of the forest did not exhibit consistent ratios. Consistent ratios, including relatively heavy nitrogen, were found at locations receiving runoff primarily from pasture and rangelands. Although there is some tendency for nitrate ratios to become heavier with increased urban influence, this trend was not consistent, suggesting that stable isotopes are not a reliable method for detecting nitrate from urban sources in the town of Waimea.
H21H-0938
Non-point source pollution transport of a typical agricultural watershed in Taihu Basin
The generation and transport of the agricultural non-point source pollution are closely related with hydrologic processes. The mechanisms of the hydrologic responses are the basis for the research of the non-point source pollutant transport. Based on the detailed field observations, the paper studied the interacting patters between the non-point source pollution transport and the hydrologic processes. The distributed model is applied to quantify the temporal and spatial variation of flow and pollutants. Based on the field experiments, the mechanism of hydrologic responses was analyzed and the hydrologic processes were simulated in Meilin watershed. The main components and the seasonal characteristics of N and P output were investigated. Nitrate and ammonia nitrogen are the main N lost forms in Meilin watershed. Soluble N is more easily flow out with the runoff in the rainfall event, but P is more likely lose with soil particles. Although the fertilizer application is maily in summer in accordance with plant growth, but the high flow rate diluted the output concentration of N and P. Driven by the distributed hydrologic model, the non-point source pollution forecasting model is established by simplifying N and P dynamics. The simulated results compare well with observed ones and the distribution of the critical source area of N and P output are obtained for Meilin watershed.
H21H-0939
Discharge and source of particulate organic carbon and nitrogen from a small mountainous river in the Oregon Coast Range
The discharge of POC from small mountainous rivers (SMR) may contribute nearly half of the world's POC to the ocean. Understanding POC and PON quality will give us clues about the material's stability in the coastal environment and also its source within the watershed. During the 2007-2008, we collected samples from the Alsea River in the Oregon Coast Range that span a broad range of discharges that include base flow and a 5-year flood. Fine (<63 μm) and coarse (>63 μm) particulate material was analyzed for OC, N, stable isotopes (δ13C and δ15N), and cupric oxide (CuO) oxidation products (e.g. lignin). With increasing discharge the source and quality of fine POC and N appears to become more soil-like. The POC and N of the coarse particles appear to be affected by an increase in lithogenic material; however, the stable isotope and biomarker data indicate that the majority of the C and N were derived from a constant source (mainly "fresh" detritus). This study shows that OC and N quality, and source, in small mountainous rivers are controlled by discharge, which must be taken into account when studying the sources, transport and transformation of biogeochemical materials in these and other aquatic systems.
H21H-0940
Seasonal Variation of Carbon and Nutrient Transfer in a Northwestern Arkansas Cave
Carbon cycling in karst is important because of the interplay between organic and inorganic carbon pools, which controls dissolution and precipitation of carbonate minerals. However, transformation of carbon between dissolved inorganic carbon, dissolved organic carbon, and gaseous carbon dioxide are poorly defined in karst hydrologic systems; heterogeneity inherent in conduit- and fracture-dominated focused flow paths characteristic of karst introduces challenges that have caused a lag in our understanding of these systems as compared with the status of knowledge for granular media aquifers. Carbon and nutrient dynamics were investigated in a cave in northwestern Arkansas using geochemical characterization and stable isotopic analyses. Previous investigations documented total organic carbon from nearby caves ranging from 0.3 to 0.6 milligrams per liter and nitrite-nitrate-N from 0.5 to 2.0 milligrams per liter. Previous work also found that dissolved organic matter bioavailability was greatest during the spring and summer and corresponded to a nitrate loss of approximately 7 percent due to processing. Combined continuous and periodic monitoring of carbon species, nutrients, and other geochemical parameters along karst flow lines from soil to cave to springs allowed for delineation of carbon and nutrient fluxes. Monitoring targeted recharge, infiltration, unsaturated and saturated flow, and terminal discharge at springs. Seasonal variations in temperature and precipitation were shown to exercise substantial control over the carbon system in this karst setting. Isotopic signatures observed in dissolved inorganic carbon, dissolved organic carbon, and gaseous carbon dioxide identify sources and delineate carbon movement through the karst hydrologic system. Simultaneous monitoring of carbon and nutrients is revealing interactions between these species and controls from seasonal changes in temperature and precipitation. The impacts of the study include enabling better design of local watershed management and broader-scale understanding of carbon movement in a mid-latitude, terrestrial system.
H21H-0941
Nutrient and carbon cycling in the Kafue River (Zambia)
The lower Kafue River in central Zambia flows through the Kafue Flats (830 sq. km), a sensitive floodplain ecosystem (Ramsar site), and serves as an important subsistence fishery. The lower Kafue River (Qavg ~ 300 m3/s) is heavily impacted by two dams that regulate flow and flooding, and in so doing have substantially degraded the habitats. While the hydrology and ecology of the system have been studied extensively in some sections, there have been no systematic studies of the dam impacts on floodplain biogeochemistry. In May 2008 we initiated a study of C, N, and P cycling in the lower Kafue River through sampling at 10-20 km resolution along 300 km of river. Low inorganic nitrogen levels (<1 uM nitrate, <1 uM ammonium) were found along the entire river, despite it being in hydraulic communication with an extremely productive floodplain. The low inorganic N levels coincided with relatively high total nitrogen (>40 uM), suggesting that the riverine N budget is dominated by organic N. Phosphate concentrations increased by a factor of 4 along the river (0.2 uM to 0.8 uM), and transitioned from representing only a small fraction of total P upstream to accounting for nearly 100% of total P at downstream stations. Along one short (35 km; travel time ~ 16 h), relatively pristine stretch of river with a substantial flow rate (400 m3/s) and no visible tributaries, dissolved oxygen levels decreased from >5 mg/l to 1 mg/l. A drop in pH from (7.9 to 7.2) accompanied the sharp oxygen decline, consistent with respiration occurring either within the river or in the adjacent floodplain. Low dissolved oxygen levels (<2 mg/l). persisted for another 150 km downstream despite reaeration. This presentation will explore in greater detail the factors contributing to this persistent low oxygen stretch in the Kafue River, along with an exploration of N, C, and P cycling in the system
H21H-0942
Glacier runoff as a source of labile dissolved organic matter for near-shore marine ecosystems in the Gulf of Alaska
Northern rivers transport large quantities of dissolved organic matter (DOM), however this organic material is typically thought to be refractory and therefore of little significance for the biogeochemistry of downstream marine ecosystems. Recent research in both the arctic and sub-arctic has shown that terrigenous DOM may be more bioavailable than was previously thought. These findings suggest that riverine DOM has the potential to support both heterotrophic metabolism and primary productivity in near-shore marine ecosystems. Along the Gulf of Alaska (GOA), the ongoing loss of glacier ice in coastal watersheds is altering the land-to-ocean transfer of freshwater and DOM. In particular, DOM derived from glacial runoff appears to be derived largely from microbial precursor material while DOM in watersheds with little or no glacier coverage is predominantly derived from terrestrial plants. The purpose of this study was to test the hypothesis that the character and bioavailability of riverine DOM being exported to the GOA will be altered as glaciers recede and contribute less to streamflow. We sampled rivers draining 12 coastal watersheds along a 500 mile stretch of the GOA during the peak glacier runoff season in the summer of 2008. The study watersheds were typical of the thousands of moderately sized (50-450 km2) watersheds draining the coast mountains along the GOA and ranged in watershed glacier coverage from 0 to >60%. Concentrations of DOC were relatively low in all 12 watersheds ranging from 0.6-2.2 mg C L-1. However, the chemical character of DOM varied widely across the watersheds. As watershed glacial coverage increased and glacier runoff comprised a large proportion of streamflow, riverine DOM became enriched in 13C-DOC and protein content as measured by fluorescence spectroscopy. These findings are consistent with the idea that DOM in pro-glacial streams is largely derived from sub-glacial microbial populations. Moreover, incubations of riverine DOM from all twelve sites that were inoculated with near-shore marine water from the GOA showed substantial losses of DOC (23-66%) over the course of a week. The percentage of bioavailable DOC was positively correlated (p=0.01) with the percentage of watershed covered by glacier ice suggesting that glacier runoff is an important source of labile DOM for heterotrophic bacteria in near-shore marine ecosystems around the GOA. Taken together, our results suggest that changes in watershed glacial coverage may alter the magnitude and bioavailability of riverine DOM entering productive near-shore ecosystems in the GOA.
H21H-0943
Geochemistry and Reactivity of Exported Congo Riverine Dissolved Organic Matter
The Congo River basin drains the second largest area of rainforest in the world and is also the second largest river in terms of catchment size (3,680,000 km2) and freshwater discharge (42,000 m3 s- 1). Congo riverine dissolved organic carbon (DOC) export is estimated at 12.4 Tg DOC yr -1 or approximately 5 % of global riverine DOC export to the ocean. The sheer scale of this export makes further study of this system imperative for increased quantification and understanding of the terrestrial-ocean linkages in the global C cycle. Samples were collected in January and February 2008 from the Congo river- system and analyzed for a suite of compositional and degradation related measurements and its reactivity investigated. Our initial analyses of Congo riverine DOM yielded 14C ages of modern origin (fraction modern of 14C = 107.14 ± 0.50)) as expected for a large tropical river with a highly productive basin. Lignin phenol concentrations (Σ8 = 76.6 μgL-1) and compositions (Λ8 = 0.72 mg (100 mg OC)-1) are also indicative of fresh vascular plant derived DOM inputs, as are the 13C- DOC values (approximately -28.9), UV-vis spectral slope and slope ratio values and the fluorescence EEM data. Congo riverine DOM was shown to be highly photoreactive with fractional losses of DOC, colored DOM (CDOM) and lignin phenols equivalent to approximately 45 %, 95 % and 95 % respectively during the course of a 57 day irradiation experiment. CDOM spectral slope ratio values trended towards open ocean values during the irradiation (S275-295:S350-400 changed from 0.81 to 1.38). These spectral shifts have previously been shown to be indicative of a reduction in DOM aromaticity and average molecular weight. Lignin phenol data also trends towards typical carbon-normalized yield values of those observed previously in surface marine waters upon irradiation from 0.72 mg (100 mg OC)-1) at the start of the irradiation to 0.03 mg (100 mg OC)-1) after 57 days. Lignin phenols also showed increases in acid to aldehyde ratios of vanillyl and syringyl phenols and also an increase in syringyl to vanillyl phenols during the course of the irradiation. Finally, the 13C-DOC values upon irradiation shifted from approximately -28.9 to approximately -26.0 and thus the results described here highlight the reactivity of Congo riverine DOM and the potential role of photochemistry with respect to the fate of this material in the ocean.
H21H-0944
Wildfire and Salvage Logging Impacts on Stream Water Nitrogen in Southern Alberta's Rocky Mountains
Increased size and frequency of wildfires in North America has been linked to changing climate over the past 2-3 decades, raising concerns over impacts of wildfire on downstream water quality. In 2003, the Lost Creek wildfire burned more than 21,000 ha in the highest water yielding area of the Rocky Mountain region of southwestern Alberta (Crowsnest Pass). The objective of this study was to examine initial effects of the fire and post-fire salvage logging on concentrations, yield, and total export of several nitrogen (N) species, and to explore initial recovery of these effects within the first four years after the fire. Streams draining burned and post-fire salvage logged watersheds produced much higher concentrations of total nitrogen (TN), total dissolved nitrogen (TDN), and nitrate (NO3-) compared to reference streams in the first two years following the fire (p < 0.001). This resulted in average nutrient yields that were considerably greater for TN (6-fold), TDN (6-fold), and NO3- (9-fold) in burned streams than in reference streams. Salvage logging produced generally similar effects on the concentrations and yields for most N species. The temporal trend for TN, TDN, and NO3- in stream water from burned watersheds was a rapid decline in mean watershed exports over the four seasons after the fire to levels similar to those of the reference watersheds. However, exports of TN were still elevated in the fourth post-fire year in watersheds impacted by the additional disturbance of salvage logging. The effects of the burn were most noticeable (i.e., produced the greatest N concentrations, yields, and exports) during or following higher discharge periods (snowmelt freshet and storm flows) (p < 0.001). Small differences were still evident during base-flow periods, emphasizing the importance of groundwater and subsurface contributions to the headwater streams in this study.
H21H-0945
Data Collection for Investigating Hyporheic Flow and Nutrient Exchange in the Truckee River (CA-NV USA)
Hyporheic exchange is recognized for its role in influencing chemical and biological processes that contribute to lotic ecosystem health. Most research to date on hyporheic exchange has focused on relatively low order streams despite the fact that exchange in mid-order (i.e. 3-7th) river basins is becoming of increasing interest to stream ecologists and water resources managers. This study examines hyporheic flow and nutrient transport through two distinctly different reaches in the Truckee River where discharge is largely controlled. The first reach, regulated by reservoirs and more susceptible to large changes in flow from precipitation is located in a high gradient mountain ecosystem. The second reach, controlled by a series of agricultural diversions, receives less contribution from precipitation and is located in a low gradient desert ecosystem. Shallow monitoring wells, piezometers, nested mini-piezometers and temperature gradient probes were installed in multiple transects across a riffle-pool sequence. To determine hydraulic and thermal gradients in response to changes in discharge, streambed temperature and vertical hydraulic gradients are continuously monitored. Pore-water samples are collected monthly and analyzed for nutrients, anions, cations, and dissolved organic carbon. The streambed temperatures will be used in a numerical heat and solute transport model to determine the rate of hyporheic exchange under variable discharge and temperature conditions. The spatial and temporal distribution of thermal gradients and pore-water chemistry is presented along with future efforts to quantify denitrification rates using an isotopic and modeling approach.
H21H-0946
Linking Hydrologic Flow Paths and Nutrient Cycling in a Subalpine Catchment, Front Range, CO
The nutrient content of high elevation catchments in Colorado may be altered by climate change, atmospheric nitrogen deposition, and the mountain pine beetle epidemic. Previous work has speculated on mechanisms for nutrient flux out of high elevation watersheds and highlighted strong seasonal changes in C and N flux. Here we connect past work on nutrient export with hydrologic flow paths to explain C and N dynamics in Como Creek, a subalpine catchment in the Colorado Front Range, which ranges in elevation from 3560 meters to 2900 meters. We find that despite increasing N-deposition in the catchment, N fluxes have decreased over the past 30 years, while C fluxes have seen little change. Interestingly, NO3-N concentration peaks during the winter prior to snowmelt and mimics the behavior of conservative groundwater tracers. Isotopic and chemical tracers used in hydrologic mixing models indicate that groundwater is the dominant source of surface water even during snowmelt and has a residence time of two to three years. We suggest that N flux is controlled by groundwater inputs and is subject to in-stream processing during the winter months. The winter NO3-N pulse suggests a decoupling in C and N dynamics with in-stream nitrification an important component of N cycling. This highlights the importance of understanding both in-stream and ecosystem processes when studying riverine nutrient export and will be important in understanding increased nutrient export from areas afflicted by infestations of the mountain pine beetle.
H21H-0947
Hydrologic controls on the sources and dynamics of dissolved organic matter in an agricultural catchment in the Central Valley, California (U.S.A.)
The influence of agricultural practices on the dynamics of dissolved organic matter (DOM) cycling in river systems is poorly understood. We investigated molecular compositions of DOM at 14 sites in an agriculturally-impacted catchment (Willow Slough; 415 km2) under several different flow regimes over the course of two years in order to investigate the influence of sub-catchments on the biogeochemistry at the mouth of the catchment. The Willow Slough catchment area includes eastern foothills of the inner Coast Range to the alluvial plains and encompasses diverse land uses, including natural grasslands, orchards, viticulture and pasture, all draining toward the Sacramento River. Knowledge of the composition of DOM composition is crucial, as dissolved organic carbon (DOC) can form EPA-regulated carcinogenic compounds during the drinking water disinfection process and is therefore considered a drinking water constituent of concern. Willow Slough offers the opportunity to examine carbon source, cycling and transportation through multiple flowpaths and land uses that are common in Californian agricultural watersheds. As a constituent of DOM, lignin phenols provide information on the source, composition, quality and degradation state of DOM. Uniquely derived from vascular plants, lignin phenols can be used to distinguish between angiosperm and gymnosperm tissues and carbon-normalized yields can offer insight on the proportion of vascular plant-derived carbon versus in-situ production. Throughout the Willow Slough watershed, ratios of syringyl to vanillyl and cinnamyl to vanillyl lignin phenols show that the vascular plant component of DOM can be primarily attributed to non-woody angiosperm tissues. Lower lignin phenol concentrations and carbon-normalized yields were observed in the headwaters (0.1-0.6 mg/100mg OC and 2.6-33 μg/L) versus the mouth (0.7-2.0 mg/100mg OC and 25-72 μg/L), indicating that mid-catchment tributaries play important roles in determining the chemistry at the mouth. Seasonal changes observed within sub-catchments have various consequences for the DOM exported from the mouth of the watershed and highlight the impacts of diverse agricultural practices. Higher carbon-normalized lignin phenol yields were observed at sites dominated by intensive field and row crop agriculture, indicating that agriculturally influenced lands contribute more terrestrially-derived carbon to the DOM pool than the more pristine headwaters. However, mixing cannot solely explain the seasonal trends observed, suggesting that differential processing of DOM is occurring throughout the watershed. The results of this study demonstrate the importance of sub-catchments on the chemistry observed at the mouth and demonstrate that not all tributaries 'are created equal'.
H21H-0948
Factors Controlling Dissolved Organic Carbon (DOC) and Nitrogen (DON) Dynamics in Watersheds
Our previous work in a glaciated, forested watershed in western New York revealed distinct differences in the
responses of DOC and DON during storm events. We explored if these differences also occurred at an
unglaciated, forested, 12 ha watershed in the Piedmont physiographic province of Maryland. Key questions
that we addressed are: What are the sources and flowpaths for DOC and DON in the watershed? Do they
differ for DOC and DON? Does DOM quality influence the concentrations and transport of DOC and DON?
Watershed sources that are being sampled include: rainfall, throughfall, litter, soil water groundwater,
hillslope seeps and hyporheic zone water. Watershed sources are sampled manually during non-storm
periods (every two weeks) while automated samplers are used for storm events. Water samples are
analyzed for all cations, anions, DOC, DON, Silica and O18. DOM quality is characterized using specific
ultraviolet absorbance (SUVA), Fluorescence Index (FI), Excitation-Emission Matrices (EMMs), hydrophobic
and hydrophilic fractions, and concentrations of phenols, carboxylic acids, flavanoids, and amino acids.
Hydrologic monitoring includes discharge and groundwater elevations. Our initial results suggest that
differences in DOC and DON are not as large as that observed for the western New York site. Endmember
mixing analysis (EMMA) indicates that stream chemistry is regulated by seep runoff, litter, and riparian soil
water with individual endmember contributions varying with size of the storm events and wetness conditions.
Highest DOC and DON concentrations were recorded for throughfall and litter layer. SUVA values were
highest and most variable for litter leachate followed by riparian water. Concentrations of DOC and DON in
streamflow increased dramatically during events and peaked at or after the discharge peak. While SUVA
values followed a similar trend, there were slight differences among events. Additional relationships between
DOM concentrations and quality are also being investigated.
http://udel.edu/~inamdar/
H21H-0949
Impacts of Agricultural Practices on Concentrations and Fluxes of Dissolved Organic Carbon
Organic matter from the breakdown of plant and animal material is a significant concern for drinking water quality in California due to the potential formation of carcinogenic disinfection byproducts (DBPs) during water treatment with chlorine. Reducing DOC concentration at the source water is a possible management strategy being explored for the reduction of DBP precursors. We examined a variety of land use/land cover, i.e. natural grasslands and intensive agriculture in the Willow Slough Watershed (415 km2) in Yolo County, California to determine the temporal and spatial DOC dynamics. Surface water DOC concentrations ranged from 1.62 to 11.44 mg L-1 at the mouth of the watershed during the first two years, with about two times higher DOC concentrations measured downstream in an intensive agricultural subwatershed dominated by summer flood irrigation. The mean DOC yield was also the highest from the agricultural subwatershed at 0.74 g m-2 over the six months of active irrigation. Results suggest that there is a positive correlation between cropland area and DOC yield. Among many crop species examined, alfalfa showed the strongest positive linear relationship with R2 = 0.91 between the irrigation season DOC yield and percentage crop area of each subwatershed, indicating that agricultural practices such as flood irrigation have a greater impact on DOC loads than other irrigation systems. The results indicate that agricultural practices may deserve further attention for watershed management of DOC and DBP precursors and that flood irrigation practices should be targeted to reduce DOC loading within the main watershed.