Two‐Stage Ditch Floodplains Enhance N‐Removal Capacity and Reduce Turbidity and Dissolved P in Agricultural Streams

Two‐stage ditches represent an emerging management strategy in artificially drained agricultural landscapes that mimics natural floodplains and has the potential to improve water quality. We assessed the potential for the two‐stage ditch to reduce sediment and nutrient export by measuring water column turbidity, nitrate (NO₃^?), ammonium (NH₄⁺), and soluble reactive phosphorus (SRP) concentrations, and denitrification rates. During 2009‐2010, we compared reaches with two‐stage floodplains to upstream reaches with conventional trapezoid design in six agricultural streams. At base flow, these short two‐stage reaches (<600 m) reduced SRP concentrations by 3‐53%, but did not significantly reduce NO₃^? concentrations due to very high NO₃^? loads. The two‐stage also decreased turbidity by 15‐82%, suggesting reduced suspended sediment export during floodplain inundation. Reach‐scale N‐removal increased 3‐24 fold during inundation due to increased bioreactive surface area with high floodplain denitrification rates. Inundation frequency varied with bench height, with lower benches being flooded more frequently, resulting in higher annual N‐removal. We also found both soil organic matter and denitrification rates were higher on older floodplains. Finally, influence of the two‐stage varied among streams and years due to variation in stream discharge, nutrient loads, and denitrification rates, which should be considered during implementation to optimize potential water quality benefits.     

Two‐Stage Agricultural Ditch Sediments Act as Phosphorus Sinks in West Michigan

Phosphorus (P) and sediment inputs from agricultural drainage contribute to the development of hypereutrophic conditions in lakes across the world. Two‐stage (2‐S) ditches, an agricultural best management practice gaining acceptance in the Midwestern United States, increase floodplain area within drainage ditches to help capture nutrients and sediment. While denitrification has been shown to increase on 2‐S benches, less is known about their P retention ability. This study assessed the abiotic and biotic P retention of two separate 2‐S ditches compared to their corresponding traditional reaches directly upstream within the Macatawa watershed, located in West Michigan. Soluble reactive P export was significantly reduced in 2‐S baseflow of both ditch systems. Equilibrium P concentration values suggest retention of P within the 2‐S sediment. P was bound within stable fractions in both 2‐S and traditional reaches. An analysis of P stock within the ditches revealed sediment held over 96% of total P (TP) within each reach compared to <4% in bench vegetation and periphyton combined. Turbidity, but not TP, was reduced in one study ditch, whereas TP, but not turbidity, was reduced in the other study ditch. Geomorphic stability may have been responsible for differing P retention between ditches. Ability to retain P appears to be impacted by physical as well as biogeochemical characteristics; hence, structure and age of 2‐S reaches influence P retention.     

Estimating Sediment and Nutrient Loads in Four Western Lake Superior Streams

Total suspended solids (TSS) and total phosphorus (TP) have been shown to be strongly correlated with turbidity in watersheds. High‐frequency in situ turbidity can provide estimates of these potential pollutants over a wide range of hydrologic conditions. Concentrations and loads were estimated in four western Lake Superior trout streams from 2005 to 2010 using regression models relating continuous turbidity data to grab sample measures of TSS and TP during differing flow regimes. TSS loads estimated using the turbidity surrogate were compared with those made using FLUX software, a standard assessment technique based on discharge and grab sampling for TSS. More traditional rating curve methodology was not suitable because of the high variability in the particulates vs. discharge relationship. Stream‐specific turbidity and TSS data were strongly correlated (r² = 0.5 to 0.8; p < 0.05) and less so for TP (r² = 0.3 to 0.7; p < 0.05). Near‐continuous turbidity monitoring (every 15 min) provided a good method for estimating both TSS and TP concentration, providing information when manual sample collection was unlikely, and allowing for detailed analyses of short‐term responses of flashy Lake Superior tributaries to highly variable weather and hydrologic conditions while the FLUX model typically resulted in load estimates greater than those determined using the turbidity surrogate, with 17/23 stream years having greater FLUX estimates for TSS and 18/23 for TP.     

Phosphorus, sediment, and Escherichia coli loads in unfenced streams of the Georgia Piedmont, USA

Contamination of unfenced streams with P, sediments, and pathogenic bacteria from cattle (Bos taurus) activity may be affected by the availability of shade and alternative water sources. The objectives of this study were to evaluate water quality in two streams draining tall fescue (Festuca arundinacea Schreb.)-common bermudagrass (Cynodon dactylon L.) pastures with different shade distribution, and to quantify the effects of alternative water sources on stream water quality. For 3 yr, loads of dissolved reactive phosphorus (DRP), total phosphorus (TP), and total suspended solids (TSS) were measured during storm flow, and loads of DRP, TP, TSS, and Escherichia coli were measured every 14 d during base flow. We also used GPS collars to determine amount of time cattle spent in riparian areas. Our results showed that cattle-grazed pastures with unfenced streams contributed significant loads of DRP, TP, TSS, and E. coli to surface waters (p < 0.01). Time spent by cattle in riparian areas as well as storm flow loads of DRP, TP, and TSS were larger (p < 0.08) in the pasture with the smaller amount of nonriparian shade. Water trough availability decreased base flow loads of TSS and E. coli in both streams, and decreased time cattle spent in riparian areas in the pasture with the smaller amount of nonriparian shade (p < 0.08). Our results indicate that possible BMPs to reduce contamination from cattle-grazed pastures would be to develop or encourage nonriparian shade and to provide cattle with alternative water sources away from the stream.     

Estimating Suspended Solids from Turbidity in the Robeson Creek,NC Watershed

The purpose of this project was to assess the effect of estimating total suspended solids (TSS) concentrations from turbidity on TSS loads for streams in the Robeson Creek watershed. Discharge was monitored continuously and base‐flow grab and storm event composite samples were collected and analyzed for TSS and turbidity from five sites during five years of monitoring. For base‐flow samples, the TSS‐turbidity relationship for all five sites was poor indicating that TSS concentrations in base flow cannot be estimated from a TSS‐turbidity relationship. To test the effect of analyzing fewer samples, TSS from every third and the first 20 samples collected from each site was used to develop TSS‐turbidity relationships. In addition, the TSS‐turbidity relationship developed from the most downstream site was used to estimate TSS concentrations from turbidity measured at the other four sites. For four of the five sites, analyzing every third sample for TSS and using the TSS‐turbidity relationship to estimate the missing TSS concentrations would result in mean TSS loads that were not significantly different from the observed. Using the TSS‐turbidity relationship from the outlet to estimate TSS from turbidity measured at the other four sites resulted in significantly different mean TSS loads at three of the four sites. These results indicate that estimating TSS concentrations from turbidity using a TSS‐turbidity relationship developed from a subset of the overall dataset should be done with great caution.     

Floodplain Trapping and Cycling Compared to Streambank Erosion of Sediment and Nutrients in an Agricultural Watershed

Floodplains and streambanks can positively and negatively influence downstream water quality through interacting geomorphic and biogeochemical processes. Few studies have measured those processes in agricultural watersheds. We measured inputs (floodplain sedimentation and dissolved inorganic loading), cycling (floodplain soil nitrogen [N] and phosphorus [P] mineralization), and losses (bank erosion) of sediment, N, and P longitudinally in stream reaches of Smith Creek, an agricultural watershed in the Valley and Ridge physiographic province. All study reaches were net depositional (floodplain deposition > bank erosion), had high N and P sedimentation and loading rates to the floodplain, high soil concentrations of N and P, and high rates of floodplain soil N and P mineralization. High sediment, N, and P inputs to floodplains are attributed to agricultural activity in the region. Rates of P mineralization were much greater than those measured in other studies of nontidal floodplains that used the same method. Floodplain connectivity and sediment deposition decreased longitudinally, contrary to patterns in most watersheds. The net trapping function of Smith Creek floodplains indicates a benefit to water quality. Further research is needed to determine if future decreases in floodplain deposition, continued bank erosion, and the potential for nitrate leaching from nutrient‐enriched floodplain soils could pose a long‐term source of sediment and nutrients to downstream rivers.     

Estimating the Frequency and Extent of Overbank Flow of a Baseflow‐Dominated Stream Flowing through a Wetland

River floodplains provide critical habitat for a wide range of animal and plant species and reduce phosphorus and nitrogen loads in streams. It has been observed that baseflow‐dominated streams flowing through wetlands are commonly at or near bankfull and overflow their banks much more frequently than other streams. However, there is very little published quantitative support for this observation. The study focuses on a 1‐km reach of Black Earth Creek, a stream in the Midwestern United States (U.S.). We used one‐dimensional hydraulic modeling to estimate bankfull discharge at evenly spaced stream cross sections, and two‐dimensional modeling to quantitate the extent of wetland inundation as a function of discharge. We then used historical streamflow data from two U.S. Geological Survey gaging stations to quantitate the frequency of wetland inundation. For the with‐sediment case, the frequency of overbank conditions at the 38 cross sections in the wetland ranged from 3 to 85 days per year and averaged 43 days per year. Ten percent of the wetland was inundated for an average of 35 days per year. For the without‐sediment case, the frequency of overbank conditions ranged from 2.6 to 48 days per year and averaged 14 days per year. Also, 10% of the wetland was inundated for an average of 25 days per year. These unusually high rates of floodplain inundation are likely due in part to the very low stream gradient and shallow depths of overbank flow.     

Surrogate Measures for Providing High Frequency Estimates of Total Suspended Solids and Total Phosphorus Concentrations1

Spackman Jones, Amber, David K. Stevens, Jeffery S. Horsburgh, and Nancy O. Mesner, 2010. Surrogate Measures for Providing High Frequency Estimates of Total Suspended Solids and Total Phosphorus Concentrations. Journal of the American Water Resources Association (JAWRA) 1‐15. DOI: 10.1111/j.1752‐1688.2010.00505.x Abstract: Surrogate measures like turbidity, which can be observed with high frequency in situ, have potential for generating high frequency estimates of total suspended solids (TSS) and total phosphorus (TP) concentrations. In the semiarid, snowmelt‐driven, and irrigation‐regulated Little Bear River watershed of northern Utah, high frequency in situ water quality measurements were recorded in conjunction with periodic chemistry sampling. Site‐specific relationships were developed using turbidity as a surrogate for TP and TSS at two monitoring locations. Methods are presented for employing censored data and for investigating categorical explanatory variables (e.g., hydrologic conditions). Turbidity was a significant explanatory variable for TP and TSS at both sites, which differ in hydrologic and water quality characteristics. The relationship between turbidity and TP was stronger at the upper watershed site where TP is predominantly particulate. At both sites, the relationships between turbidity and TP varied between spring snowmelt and base flow conditions while the relationships between TSS and turbidity were consistent across hydrological conditions. This approach enables the calculation of high frequency time series of TP and TSS concentrations previously unavailable using traditional monitoring approaches. These methods have broad application for situations that require accurate characterization of fluxes of these constituents over a range of hydrologic conditions.     

Sources and Delivery of Nutrients to the Northwestern Gulf of Mexico from Streams in the South‐Central United States1

Rebich, Richard A., Natalie A. Houston, Scott V. Mize, Daniel K. Pearson, Patricia B. Ging, and C. Evan Hornig, 2011. Sources and Delivery of Nutrients to the Northwestern Gulf of Mexico From Streams in the South‐Central United States. Journal of the American Water Resources Association (JAWRA) 47(5):1061‐1086. DOI: 10.1111/j.1752‐1688.2011.00583.x Abstract: SPAtially Referenced Regressions On Watershed attributes (SPARROW) models were developed to estimate nutrient inputs [total nitrogen (TN) and total phosphorus (TP)] to the northwestern part of the Gulf of Mexico from streams in the South‐Central United States (U.S.). This area included drainages of the Lower Mississippi, Arkansas‐White‐Red, and Texas‐Gulf hydrologic regions. The models were standardized to reflect nutrient sources and stream conditions during 2002. Model predictions of nutrient loads (mass per time) and yields (mass per area per time) generally were greatest in streams in the eastern part of the region and along reaches near the Texas and Louisiana shoreline. The Mississippi River and Atchafalaya River watersheds, which drain nearly two‐thirds of the conterminous U.S., delivered the largest nutrient loads to the Gulf of Mexico, as expected. However, the three largest delivered TN yields were from the Trinity River/Galveston Bay, Calcasieu River, and Aransas River watersheds, while the three largest delivered TP yields were from the Calcasieu River, Mermentau River, and Trinity River/Galveston Bay watersheds. Model output indicated that the three largest sources of nitrogen from the region were atmospheric deposition (42%), commercial fertilizer (20%), and livestock manure (unconfined, 17%). The three largest sources of phosphorus were commercial fertilizer (28%), urban runoff (23%), and livestock manure (confined and unconfined, 23%).     

SPATIAL VARIATION OF NUTRIENT BALANCE IN THE TRUCKEE RIVER,CALIFORNIA‐NEVADA1

ABSTRACT: Because the Truckee River connects two lakes along the Eastern Sierra Nevada Mountains with different limiting nutrients, this research addresses whether the nitrogen:phosphorus (N:P) balance of the river ecosystem changes longitudinally. Historical (1990 to 2000) total nitro‐gen:total phosphorus (TN:TP) ratios in river water exhibited the expected gradient from high N:P ratios upstream to low N:P ratios downstream, with the major gradient of the N:P balance occurring within the transition between montane and high desert terrain. During 2001, the river contained anomalously low total nitrogen concentrations in the far upper reaches and dissolved inorganic nitrogen concentrations in the lower reaches, resulting in a less apparent longitudinal gradient of N:P ratios. Measurements of periphyton growth and physiology (nutrient bioassays and ectoenzyme activities) and stoichiometry during the summer of 2001 also exhibited a complex picture of the spatial variation of N:P balance that was not entirely consistent with a strong N:P gradient. However, the compendium of the indicators did support the overall picture of an overarching longitudinal gradient from high to low N:P ratios. The results suggest that periphyton management efforts in the Truckee River should consider the overall spatial gradient as well as the small‐scale dynamics of the stream ecosystem structure.     

NUTRIENT CONTRIBUTION OF NONPOINT SOURCE RUNOFF IN THE LAS VEGAS VALLEY1

ABSTRACT: A Geographic Information System (GIS) based non‐point source runoff model is developed for the Las Vegas Valley, Nevada, to estimate the nutrient loads during the years 2000 and 2001. The estimated nonpoint source loads are compared with current wastewater treatment facilities loads to determine the non‐point source contribution of total phosphorus (TP), total nitrogen (TN), and total suspended solids (TSS) on a monthly and annual time scale. An innovative calibration procedure is used to estimate the pollutant concentrations for different land uses based on available water quality data at the outlet. Results indicate that the pollutant concentrations are higher for the Las Vegas Valley than previous published values for semi‐arid and arid regions. The total TP and TN loads from nonpoint sources are approximately 15 percent and 4 percent, respectively, of the total load to the receiving water body, Lake Mead. The TP loads during wet periods approach the permitted loads from the wastewater treatment plants that discharge into Las Vegas Wash. In addition, the GIS model is used to track pollutant loads in the stream channels for one of the subwatersheds. This is useful for planning the location of Best Management Practices to control nonpoint pollutant loads.     

Stream Discharge and Riparian Land Use Influence In-Stream Concentrations and Loads of Phosphorus from Central Plains Watersheds

Total annual nutrient loads are a function of both watershed characteristics and the magnitude of nutrient mobilizing events. We investigated linkages among land cover, discharge and total phosphorus (TP) concentrations, and loads in 25 Kansas streams. Stream monitoring locations were selected from the Kansas Department of Health and Environment stream chemistry long-term monitoring network sites at or near U.S. Geological Survey stream gauges. We linked each sample with concurrent discharge data to improve our ability to estimate TP concentrations and loads across the full range of possible flow conditions. Median TP concentration was strongly linked (R ² = 76%) to the presence of cropland in the riparian zones of the mostly perennial streams. At baseflow, discharge data did not improve prediction of TP, but at high flows discharge was strongly linked to concentration (a threshold response occurred). Our data suggest that on average 88% of the total load occurred during the 10% of the time with the greatest discharge. Modeled reductions in peak discharges, representing increased hydrologic retention, predicted greater decreases in total annual loads than reductions of ambient concentrations because high discharge and elevated phosphorus concentrations had multiplicative effects. No measure of land use provided significant predictive power for concentrations when discharge was elevated or for concentration rise rates under increasing discharge. These results suggest that reductions of baseflow concentrations of TP in streams without wastewater dischargers may be managed by reductions of cropland uses in the riparian corridor. Additional measures may be needed to manage TP annual loads, due to the large percentage of the TP load occurring during a few high-flow events each year.     

Predicting runoff of suspended solids and particulate phosphorus for selected Louisiana soils using simple soil tests

This study was conducted to evaluate the relationships among total suspended solids (TSS) and particulate phosphorus (PP) in runoff and selected soil properties. Nine Louisiana soils were subjected to simulated rainfall events, and runoff collected and analyzed for various parameters. A highly significant relationship existed between runoff TSS and runoff turbidity. Both runoff TSS and turbidity were also significantly related to runoff PP, which on average accounted for more than 98% of total P (TP) in the runoff. Runoff TSS was closely and positively related to soil clay content in an exponential fashion (y=0.10e0.01x, R2=0.91, P<0.001) while it was inversely related to soil electrical conductivity (EC) (y=0.02 x(-3.95), R2=0.70, P<0.01). A newly-devised laboratory test, termed "soil suspension turbidity" (SST) which measures turbidity in a 1:200 soil/water suspension, exhibited highly significant linear relationships with runoff TSS (y=0.06x-4.38, R2=0.82, P<0.001) and PP (y=0.04x+2.68, R2=0.85, P<0.001). In addition, SST alone yielded similar R2 value to that of combining soil clay content and EC in a multiple regression, suggesting that SST was able to account for the integrated effect of clay content and electrolytic background on runoff TSS. The SST test could be used for assessment and management of sediment and particulate nutrient losses in surface runoff.     

Nutrient Uptake in a Large Urban River1

Abstract: Small streams have been shown to be efficient in retaining nutrients and regulating downstream nutrient fluxes, but less is known about nutrient retention in larger rivers. We quantified nutrient uptake length and uptake velocity in a regulated urban river to determine the river’s ability to retain nutrients associated with wastewater treatment plant (WWTP) effluent. We measured net uptake of soluble reactive phosphorus (SRP), dissolved organic phosphorus, ammonium (NH₄), nitrate, and dissolved organic nitrogen in the Chattahoochee River, Atlanta, GA by following the downstream decline of nutrients and fluoride from WWTP effluent on 10 dates under low flow conditions. Uptake of all nutrients was sporadic. On many dates, there was no evidence of measurable nutrient uptake lengths within the reach; indeed, on several dates release of inorganic N and P within the sample reach led to increased nutrient export downstream. When uptake occurred, SRP uptake length was negatively correlated with total suspended solids and temperature. Uptake velocities of SRP and NH₄ in the Chattahoochee River were lower than velocities in less‐modified systems, but they were similar to those measured in other WWTP impacted systems. Lower uptake velocities indicate a diminished capacity for nutrient uptake.     

Estimation of Suspended‐Sediment Concentration From Total Suspended Solids and Turbidity Data for Kentucky, 1978‐19951

Williamson, Tanja N. and Charles G. Crawford, 2011. Estimation of Suspended‐Sediment Concentration From Total Suspended Solids and Turbidity Data for Kentucky, 1978‐1995. Journal of the American Water Resources Association (JAWRA) 47(4):739‐749. DOI: 10.1111/j.1752‐1688.2011.00538.x Abstract: Suspended sediment is a constituent of water quality that is monitored because of concerns about accelerated erosion, nonpoint contamination of water resources, and degradation of aquatic environments. In order to quantify the relationship among different sediment parameters for Kentucky streams, long‐term records were obtained from the National Water Information System of the U.S. Geological Survey. Suspended‐sediment concentration (SSC), the parameter traditionally measured and reported by the U.S. Geological Survey, was statistically compared to turbidity and total suspended solids (TSS), two parameters that are considered surrogate data. A linear regression of log‐transformed observations was used to estimate SSC from TSS; 72% of TSS observations were less than coincident SSC observations; however, the estimated SSC values were almost as likely to be overestimated as underestimated. The SSC‐turbidity relationship also used log‐transformed observations, but required a nonlinear, breakpoint regression that separated turbidity observations ≤6 nephelometric turbidity units. The slope for these low turbidity values was not significantly different than zero, indicating that low turbidity observations provide no real information about SSC; in the case of the Kentucky sediment record, this accounts for 30% of the turbidity observations.     

URBAN INFLUENCE ON PHOSPHORUS AND SEDIMENT LOADING OF WEST POINT LAKE,GEORGIA1

ABSTRACT: West Point Lake is a 10,360 ha mainstream impoundment of the Chattahoochee River located 95 kilometers downstream of Atlanta, Georgia. Origins and magnitude of external total phosphorus (TP) and total suspended solids (TSS) loads from the West Point Lake basin were estimated over a one-year period. Partitioning the drainage basin allowed the sources of these loads to be determined. The upper subbasin area, from Franklin, Georgia, to the headwaters of the Chattahoochee River, contributed 96 percent of the discharge and 97 percent of the TP and TSS loads into West Point Lake. The lower subbasin area, from Franklin to West Point Lake dam, only contributed 3 percent of the TP and TSS loads. Ninety-one percent and 87 percent of the TP and TSS loads, respectively, from the upper subbasin originated from the Atlanta area. Point sources discharged 70 percent and 3 percent of the upper subbasin TP and TSS loads, respectively. A large portion (66 percent) of the TP from the upper subbasin was in the bioavailable form.     

Nutrient transport from livestock manure applied to pastureland using phosphorus-based management strategies

Land applications of manure from confined animal systems and direct deposit by grazing animals are both major sources of nutrients in streams. The objectives of this study were to determine the effects of P-based manure applications on total suspended solids (TSS) and nutrient losses from dairy manures and poultry litter surface applied to pasturelands and to compare the nutrient losses transported to the edge of the field during overland flow events. Two sets of plots were established: one set for the study of in-field release and another set for the study of edge-of-the-field nutrient transport. Release plots were constructed at three pastureland sites (previous poultry litter applications, previous liquid dairy manure application, and no prior manure application) and received four manure treatments (turkey [Meleagris gallopavo] litter, liquid dairy manure, standard cowpies, and none). Pasture plots with a history of previous manure applications released higher concentrations of TSS and higher percentages of total P (TP) in the particulate form. Transport plots were developed on pasture with no prior manure application. The average flow-weighted TP concentrations were highest in runoff samples from the plots treated with cowpies (1.57 mg L(-1)). Reducing excess P in dairy cow diets and surface applying manure to the land using P-based management practices did not increase N concentrations in runoff. This study found that nutrients are most transportable from cowpies; thus a buffer zone between pastureland and streams or other appropriate management practices are necessary to reduce nutrient losses to waterbodies.     

EFFECTS OF DAM IMPOUNDMENT ON THE FLOOD REGIME OF NATURAL FLOODPLAIN COMMUNITIES IN THE UPPER CONNECTICUT RIVER1

ABSTRACT: Understanding the effects of dams on the inundation regime of natural floodplain communities is critical for effective decision making on dam management or dam removal. To test the implications of hydrologic alteration by dams for floodplain natural communities, we conducted a combined field and modeling study along two reaches in the Connecticut River Rapids Macrosite (CRRM), one of the last remaining flowing water sections of the Upper Connecticut River. We surveyed multiple channel cross sections at both locations and concurrently identified and surveyed the elevations of important natural communities, native species of concern, and nonnative invasive species. Using a hydrologic model, HEC‐RAS, we routed estimated pre‐and post‐impoundment discharges of different design recurrence intervals (two year through 100 year floods) through each reach to establish corresponding reductions in elevation and effective wetted perimeter following post‐dam discharge reductions. By comparing (1) the frequency and duration of flooding of these surfaces before and after impoundment and (2) the total area flooded at different recurrence intervals, our goal was to derive a spatially explicit assessment of hydrologic alteration, directly relevant to natural floodplain communities. Post‐impoundment hydrologic alteration profoundly affected the subsequent inundation regime, and this impact was particularly true of higher floodplain terraces. These riparian communities, which were flooded, on average, every 20 to 100 years pre‐impoundment, were predicted to flood at 100 ? 100 year intervals, essentially isolating them completely from riverine influence. At the pre‐dam five to ten year floodplain elevations, we observed smaller differences in predicted flood frequency but substantial differences in the total area flooded and in the average flood duration. For floodplain forests in the Upper Connecticut River, this alteration by impoundment suggests that even if other stresses facing these communities (human development, invasive exotics) were alleviated, this may not be sufficient to restore intact natural communities. More generally, our approach provides a way to combine site specific variables with long term gage records in assessing the restorative potential of dam removal.     

Stormwater Runoff Quality and Quantity From Traditional and Low Impact Development Watersheds1

Abstract: The quality and quantity of residential stormwater runoff from a control, traditional, and low impact development (LID) watershed were compared in a paired watershed study. A traditional neighborhood was built using typical subdivision standards while a LID design was constructed with best management practices including grass swales, cluster housing, shared driveways, rain gardens, and a narrower pervious concrete‐paver road. Weekly, flow‐weighted, composite samples of stormwater were analyzed for nitrate + nitrite‐nitrogen (NO₃ + NO₂‐N), ammonia‐nitrogen (NH₃‐N), total Kjeldahl nitrogen (TKN), total phosphorus (TP), and total suspended solids (TSS). Monthly composite samples were analyzed for total copper (Cu), lead (Pb), and zinc (Zn). Mean weekly storm flow increased (600x) from the traditional watershed in the postconstruction period. Increased exports of TKN, NO₃ + NO₂‐N, NH₃‐N, TP, Cu, Zn, and TSS in runoff were associated with the increased storm flow. Postconstruction storm flow in the LID watershed was reduced by 42% while peak discharge did not change from preconstruction conditions. Exports were reduced from the LID watershed for NH₃‐N, TKN, Pb, and Zn, while TSS and TP exports increased.     

Non-Point-Source Impacts on Stream Nutrient Concentrations Along a Forest to Urban Gradient

We conducted statistical analyses of a 10-year record of stream nutrient and sediment concentrations for 17 streams in the greater Seattle region to determine the impact of urban non-point-source pollutants on stream water quality. These catchments are dominated by either urban (22–87%) or forest (6–73%) land cover, with no major nutrient point sources. Stream water phosphorus concentrations were moderately strongly (r²=0.58) correlated with catchment land-cover type, whereas nitrogen concentrations were weakly (r²=0.19) and nonsignificantly (at < 0.05) correlated with land cover. The most urban streams had, on average, 95% higher total phosphorus (TP) and 122% higher soluble reactive phosphorus (SRP) and 71% higher turbidity than the most forested streams. Nitrate (NO₃), ammonium (NH₄), and total suspended solids (TSS) concentrations did not vary significantly with land cover. These results suggest that urbanization markedly increased stream phosphorus concentrations and modestly increased nitrogen concentrations. However, nutrient concentrations in Seattle region urban streams are significantly less than those previously reported for agricultural area streams.