Three-dimensional numerical simulation of water quality and sediment-associated processes with application to a Mississippi Delta lake

A three-dimensional water quality model was developed for simulating temporal and spatial variations of phytoplankton, nutrients, and dissolved oxygen in freshwater bodies. Effects of suspended and bed sediment on the water quality processes were simulated. A formula was generated from field measurements to calculate the light attenuation coefficient by considering the effects of suspended sediment and chlorophyll. The processes of adsorption–desorption of nutrients by sediment were described using the Langmuir Equation. The release rates of nutrients from the bed were calculated based on the concentration gradient across the water–sediment interface and other variables including pH, temperature and dissolved oxygen concentration.The model was calibrated and validated by applying it to simulate the concentrations of chlorophyll and nutrients in a natural oxbow lake in Mississippi Delta. The simulated time series of phytoplankton (as chlorophyll) and nutrient concentrations were generally in agreement with field observations. Sensitivity analyses were conducted to demonstrate the impacts of varying suspended sediment concentration on lake chlorophyll levels.     

Sediment and phosphorus transport in irrigation furrows

Sediment and phosphorus (P) in agricultural runoff can impair water quality in streams, lakes, and rivers. We studied the factors affecting P transfer and transport in irrigated furrows in six freshly tilled fallow fields, 110 to 180 m long with 0.007 to 0.012 m m-1 slopes without the interference of raindrops or sheet flow that occur during natural or simulated rain. The soil on all fields was Portneuf silt loam (coarse-silty, mixed, superactive, mesic Durinodic Xeric Haplocalcids). Flow rate, sediment concentration, and P concentrations were monitored at four, equally spaced locations in each furrow. Flow rate decreased with distance down the furrow as water infiltrated. Sediment concentration varied with distance and time with no set pattern. Total P concentrations related directly to sediment concentrations (r2=0.75) because typically >90% of the transported P was particulate P, emphasizing the need to control erosion to reduce P loss. Dissolved reactive phosphorus (DRP) concentrations decreased with time at a specific furrow site but increased with distance down the furrow as contact time with soil and suspended sediment increased. The DRP concentration correlated better with sediment concentration than extractable furrow soil P concentration. However, suspended sediment concentration tended to not affect DRP concentration later in the irrigation (>2 h). These results indicate that the effects of soil P can be overshadowed by differences in flow hydraulics, suspended sediment loads, and non-equilibrium conditions.     

Development of Sediment and Nutrient Export Coefficients for U.S. Ecoregions

Water quality impairment due to excessive nutrients and sediment is a major problem in the United States (U.S.). An important step in the mitigation of impairment in any given water body is determination of pollutant sources and amount. The sheer number of impaired waters and limited resources makes simplistic load estimation methods such as export coefficient (EC) methods attractive. Unfortunately ECs are typically based on small watershed monitoring data, which are very limited and/or often based on data collected from distant watersheds with drastically different conditions. In this research, we seek to improve the accuracy of these nutrient export estimation methods by developing a national database of localized EC for each ecoregion in the U.S. A stochastic sampling methodology loosely based on the Monte‐Carlo technique was used to construct a database of 45 million Soil and Water Assessment Tool (SWAT) simulations. These simulations consider a variety of climate, topography, soils, weather, land use, management, and conservation implementation conditions. SWAT model simulations were successfully validated with edge‐of‐field monitoring data. Simulated nutrient ECs compared favorably with previously published studies. These ECs may be used to rapidly estimate nutrient loading for any small catchment in the U.S. provided the location, area, and land‐use distribution are known.     

上覆水营养盐浓度对底泥氮磷释放的影响

采用校园水体底泥进行上覆水营养盐浓度对底泥释放量之间的关系研究。结果表明,在本实验条件下,上覆水水质影响底泥氮、磷的释放,尤其显著影响氮、磷的初期释放;上覆水氮、磷的浓度越小,底泥氮、磷的释放量越大;上覆水氮、磷的浓度超过一定值,会抑制底泥氮、磷的释放。     

Contribution of Wastewater Treatment Plant Effluents to Nutrient Dynamics in Aquatic Systems: A Review

Excessive nutrient loading (considering nitrogen and phosphorus) is a major ongoing threat to water quality and here we review the impact of nutrient discharges from wastewater treatment plants (WWTPs) to United States (U.S.) freshwater systems. While urban and agricultural land uses are significant nonpoint nutrient contributors, effluent from point sources such as WWTPs can overwhelm receiving waters, effectively dominating hydrological characteristics and regulating instream nutrient processes. Population growth, increased wastewater volumes, and sustainability of critical water resources have all been key factors influencing the extent of wastewater treatment. Reducing nutrient concentrations in wastewater is an important aspect of water quality management because excessive nutrient concentrations often prevent water bodies from meeting designated uses. WWTPs employ numerous physical, chemical, and biological methods to improve effluent water quality but nutrient removal requires advanced treatment and infrastructure that may be economically prohibitive. Therefore, effluent nutrient concentrations vary depending on the particular processes used to treat influent wastewater. Increasingly stringent regulations regarding nutrient concentrations in discharged effluent, along with greater freshwater demand in populous areas, have led to the development of extensive water recycling programs within many U.S. regions. Reuse programs provide an opportunity to reduce or eliminate direct nutrient discharges to receiving waters while allowing for the beneficial use of reclaimed water. However, nutrients in reclaimed water can still be a concern for reuse applications, such as agricultural and landscape irrigation.     

Modeling Streamflow and Water Quality Sensitivity to Climate Change and Urban Development in 20 U.S. Watersheds

Watershed modeling in 20 large, United States (U.S.) watersheds addresses gaps in our knowledge of streamflow, nutrient (nitrogen and phosphorus), and sediment loading sensitivity to mid‐21st Century climate change and urban/residential development scenarios. Use of a consistent methodology facilitates regional scale comparisons across the study watersheds. Simulations use the Soil and Water Assessment Tool. Climate change scenarios are from the North American Regional Climate Change Assessment Program dynamically downscaled climate model output. Urban and residential development scenarios are from U.S. Environmental Protection Agency's Integrated Climate and Land Use Scenarios project. Simulations provide a plausible set of streamflow and water quality responses to mid‐21st Century climate change across the U.S. Simulated changes show a general pattern of decreasing streamflow volume in the central Rockies and Southwest, and increases on the East Coast and Northern Plains. Changes in pollutant loads follow a similar pattern but with increased variability. Ensemble mean results suggest that by the mid‐21st Century, statistically significant changes in streamflow and total suspended solids loads (relative to baseline conditions) are possible in roughly 30‐40% of study watersheds. These proportions increase to around 60% for total phosphorus and total nitrogen loads. Projected urban/residential development, and watershed responses to development, are small at the large spatial scale of modeling in this study.     

基于MIKE 21的汾河水库挟沙水流水质数值模拟研究

为了揭示悬浮泥沙(SSC)对水库水质的影响规律,对汾河水库进行样品收集和长期水质监测,采用水动力模型与泥沙转移和富营养化模型相结合的方法,将这三者关联耦合,并通过实测数据对模型进行参数率定和验证,分析含沙水和不含沙水中总氮(TN)、总磷(TP)、叶绿素a(Chla)、溶解氧(DO)四项指标,得出两者对水质造成的影响。结果表明:SSC对TN、TP的去除作用明显,对Chla、DO浓度分布影响较小,并计算了污染物的释放量以及贡献率,得出TP的负荷仅为16.47t,而贡献率高达25.25%。因此在汾河的污染控制方面应侧重削减磷,进而改善汾河地区的污染现状。     

基于MIKE 21的汾河水库挟沙水流水质数值模拟研究

为了揭示悬浮泥沙(SSC)对水库水质的影响规律,对汾河水库进行样品收集和长期水质监测,采用水动力模型与泥沙转移和富营养化模型相结合的方法,将这三者关联耦合,并通过实测数据对模型进行参数率定和验证,分析含沙水和不含沙水中总氮(TN)、总磷(TP)、叶绿素a(Chla)、溶解氧(DO)四项指标,得出两者对水质造成的影响。结果表明:SSC对TN、TP的去除作用明显,对Chla、DO浓度分布影响较小,并计算了污染物的释放量以及贡献率,得出TP的负荷仅为16.47t,而贡献率高达25.25%。因此在汾河的污染控制方面应侧重削减磷,进而改善汾河地区的污染现状。     

Quality and Quantity of Suspended Particles in Rivers: Continent-Scale Patterns in the United States

Suspended solids or sediments can be pollutants in rivers, but they are also an important component of lotic food webs. Suspended sediment data for rivers were obtained from a United States–wide water quality database for 622 stations. Data for particulate nitrogen, suspended carbon, discharge, watershed area, land use, and population were also used. Stations were classified by United States Environmental Protection Agency ecoregions to assess relationships between terrestrial habitats and the quality and quantity of total suspended solids (TSS). Results indicate that nephelometric determinations of mean turbidity can be used to estimate mean suspended sediment values to within an order of magnitude (r² = 0.89). Water quality is often considered impaired above 80 mg TSS L^–1, and 35% of the stations examined during this study had mean values exceeding this level. Forested systems had substantially lower TSS and somewhat higher carbon-to-nitrogen ratios of suspended materials. The correlation between TSS and discharge was moderately well described by an exponential relationship, with the power of the exponent indicating potential acute sediment events in rivers. Mean sediment values and power of the exponent varied significantly with ecoregion, but TSS values were also influenced by land use practices and geomorphological characteristics. Results confirm that, based on current water quality standards, excessive suspended solids impair numerous rivers in the United States.     

Magnitude,Frequency, and Duration Relations for Suspended Sediment in Stable (“Reference”) Southeastern Streams1

Abstract: Sediment is listed as one of the leading causes of water‐quality impairments in surface waters of the United States (U.S.). A water body becomes listed by a State, Territory or Tribe if its designated use is not being attained (i.e., impaired). In many cases, the prescribed designated use is aquatic health or habitat, indicating that total maximum daily loads (TMDL) targets for sediment should be functionally related to this use. TMDL targets for sediment transport have been developed for many ecoregions over the past several years using suspended‐sediment yield as a metric. Target values were based on data from “reference” streams or reaches, defined as those exhibiting geomorphic characteristics of equilibrium. This approach has proved useful to some states developing TMDLs for suspended sediment, although one cannot conclude that if a stream exceeds the target range, the aquatic ecosystem will be adversely impacted. To address this problem, historical flow‐transport and sediment‐transport data from hundreds of sites in the Southeastern U.S. were re‐examined to develop parameters (metrics) such as frequency and duration of sediment concentrations. Sites determined as geomorphically stable from field evaluations and from analysis of gauging‐station records were sorted by ecoregion. Mean‐daily flow data obtained from the U.S. Geological Survey were applied to sediment‐transport rating relations to determine suspended‐sediment load for each day of record. The frequency and duration that a given concentration was equaled or exceeded were then calculated to produce a frequency distribution for each site. “Reference” distributions were created using the stable sites in each ecoregion by averaging all of the distributions at specified exceedance intervals. As with the “reference” suspended‐sediment yields, there is a broad range of frequency and duration distributions that reflects the hydrologic and sediment‐transport regimes of the ecoregions. Ecoregions such as the Mississippi Valley Loess Plains (#74) maintain high suspended sediment concentrations for extended periods, whereas coastal plain ecoregions (#63 and 75) show much lower concentrations.     

Assessment of chlorophyll-a as a criterion for establishing nutrient standards in the streams and rivers of Illinois

Nutrient enrichment is a frequently cited cause for biotic impairment of streams and rivers in the USA. Efforts are underway to develop nutrient standards in many states, but defensible nutrient standards require an empirical relationship between nitrogen (N) or phosphorus (P) concentrations and some criterion that relates nutrient levels to the attainment of designated uses. Algal biomass, measured as chlorophyll-a (chl-a), is a commonly proposed criterion, yet nutrient-chl-a relationships have not been well documented in Illinois at a state-wide scale. We used state-wide surveys of >100 stream and river sites to assess the applicability of chl-a as a criterion for establishing nutrient standards for Illinois. Among all sites, the median total P and total N concentrations were 0.185 and 5.6 mg L(-1), respectively, during high-discharge conditions. During low-discharge conditions, median total P concentration was 0.168 mg L(-1), with 25% of sites having a total P of > or =0.326 mg L(-1). Across the state, 90% of the sites had sestonic chl-a values of < or =35 microg L(-1), and watershed area was the best predictor of sestonic chl-a. During low discharge there was a significant correlation between sestonic chl-a and total P for those sites that had canopy cover < or =25% and total P of < or =0.2 mg L(-1). Results suggest sestonic chl-a may be an appropriate criterion for the larger rivers in Illinois but is inappropriate for small rivers and streams. Coarse substrate to support benthic chl-a occurred in <50% of the sites we examined; a study using artificial substrates did not reveal a relationship between chl-a accrual and N or P concentrations. For many streams and rivers in Illinois, nutrients may not be the limiting factor for algal biomass due to the generally high nutrient concentrations and the effects of other factors, such as substrate conditions and turbidity.     

Nutrient and sediment concentrations and corresponding loads during the historic June 2008 flooding in eastern Iowa

A combination of above-normal precipitation during the winter and spring of 2007-2008 and extensive rainfall during June 2008 led to severe flooding in many parts of the midwestern United States. This resulted in transport of substantial amounts of nutrients and sediment from Iowa basins into the Mississippi River. Water samples were collected from 31 sites on six large Iowa tributaries to the Mississippi River to characterize water quality and to quantify nutrient and sediment loads during this extreme discharge event. Each sample was analyzed for total nitrogen, dissolved nitrate plus nitrite nitrogen, dissolved ammonia as nitrogen, total phosphorus, orthophosphate, and suspended sediment. Concentrations measured near peak flow in June 2008 were compared with the corresponding mean concentrations from June 1979 to 2007 using a paired t test. While there was no consistent pattern in concentrations between historical samples and those from the 2008 flood, increased flow during the flood resulted in near-peak June 2008 flood daily loads that were statistically greater (p < 0.05) than the median June 1979 to 2007 daily loads for all constituents. Estimates of loads for the 16-d period during the flood were calculated for four major tributaries and totaled 4.95 x 10(7) kg of nitrogen (N) and 2.9 x 10(6) kg of phosphorus (P) leaving Iowa, which accounted for about 22 and 46% of the total average annual nutrient yield, respectively. This study demonstrates the importance of large flood events to the total annual nutrient load in both small streams and large rivers.     

Monitored and Modeled Correlations of Sediment and Nutrients with Chesapeake Bay Water Clarity

This article analyzes the correlations of the observed and modeled light attenuation coefficient, Kd, with in situ total suspended solids (TSS) and chlorophyll‐a concentrations in Chesapeake Bay (CB) tidal waters, and with sediment and nutrient loads from the Chesapeake watershed. Light attenuation is closely related to in situ TSS and chlorophyll‐a concentrations, however, the strength of the correlation differs among the CB segments. There are distinct differences between saline and tidal fresh segments in the main Bay, but less distinction among saline and tidal fresh segments in the tidal tributaries. The correlation between Kd with sediment and nutrient loads is complicated by the lag times of TSS and the chlorophyll‐a responses to reductions in nutrient and sediment loads from the watershed, and also due to the diverse load sources. Three sets of model sensitivity scenarios were performed with: (1) differential sediment and nutrient loads; (2) selective sediment source types; and (3) geographically isolated inputs. The model results yield similar findings as those based on observed data and provide information regarding the effect of sediment on specific water bodies. Based on the model results a method was developed to determine sediment and nutrient load reductions needed to achieve the water clarity standards of the CB segments.     

基于ISM和ANP的水库水华发生影响因素分析

为提高水库水华发生预警能力,从水华形成机理着手,引入营养盐条件、水动力条件、光热条件和浮游植物生长状况,建立水库水华发生影响因素模型.首先,基于文献研究,建立水库水华发生影响因素体系,共包含10个指标;其次,利用解释结构模型(ISM)建立水库水华发生影响因素递阶结构模型;最后采用网络层次分析法(ANP)构建权重模型,并...     

OPTIMAL PROFITS UNDER ENVIRONMENTAL CONSTRAINTS: IMPLICATIONS OF NUTRIENT AND SEDIMENT STANDARDS1

To reduce nonpoint source pollution from nutrient, chemical, and sediment runoff, a number of environmental policy standards have been proposed. Such standards could be used to reduce nonpoint source pollution from nutrient, chemical, and sediment runoff to impaired water bodies. State governments can use voluntary approaches to meet nonpoint source pollution reduction goals. However, the practices that lower net returns will not be voluntarily adopted by farmers. Crop rotations and tillage practices may help producers to comply with the environmental standards while minimizing losses in farm profits. This study compares runoff from crop rotation practices and conventional continuous row cropping systems in Mississippi. The results are compared for different tillage systems in order to examine robustness of results. Nutrient runoff and sediment runoff are simulated using the Erosion Productivity Impact Calculator (EPIC). Sensitivity analysis of the sediment and nitrate reductions at 15 percent, 25 percent, and 35 percent are conducted. Under these scenarios, net returns are optimized under environmental constraints, and the marginal cost of sediment reduction ranges from US$1.61 to US$9.63 per ton depending on soil conditions, while the corresponding nitrate and phosphorus reductions costs range from US$1.21 to US$7.08 per kg and from US$0.09 to US$31.91, respectively. The empirical results from this study indicate that a nitrate reduction policy is relatively less costly than a sediment reduction policy. The results also demonstrate the importance of geophysical conditions and policy costs, which vary across regions.     

Nutrient Response Modeling in Falls of the Neuse Reservoir

In order to study system responses of Falls of the Neuse Reservoir (Falls Lake) to varied nutrient loadings, a coupled three-dimensional hydrodynamic and eutrophication model was applied. The model was calibrated using 2005 and 2006 intensive survey data, and validated using 2007 survey data. Compared with historical hydrological records, 2005 and 2007 were considered as dry years and 2006 was recognized as a normal year. Relatively higher nutrient fluxes from the sediment were specified for dry year model simulations. The differences were probably due to longer residence time and hence higher nutrient retention rate during dry years in Falls Lake. During the normal year of 2006, approximately 70% of total nitrogen (TN) and 80% of total phosphorus (TP) were delivered from the tributaries; about 20% (TN and TP) were from the sediment bottom. During the dry years of 2005 and 2007, the amount of TN released from sediment was equivalent to that introduced from the tributaries, indicating the critical role of nutrient recycling within the system in dry years. The model results also suggest that both nitrogen and phosphorus are limiting phytoplankton growth in Falls Lake. In the upper part of the lake where high turbidity was observed, nitrogen limitation appeared to dominate. Scenario model runs also suggest that great nutrient loading reductions are needed for Falls Lake to meet the water quality standard.     

Development of the Chesapeake Bay Watershed Total Maximum Daily Load Allocation

Nutrient load allocations and subsequent reductions in total nitrogen and phosphorus have been applied in the Chesapeake watershed since 1992 to reduce hypoxia and to restore living resources. In 2010, sediment allocations were established to augment nutrient allocations supporting the submerged aquatic vegetation resource. From the initial introduction of nutrient allocations in 1992 to the present, the allocations have become more completely applied to all areas and loads in the watershed and have also become more rigorously assessed and tracked. The latest 2010 application of nutrient and sediment allocations were made as part of the Chesapeake Bay total maximum daily load and covered all six states of the Chesapeake watershed. A quantitative allocation process was developed that applied principles of equity and efficiency in the watershed, while achieving all tidal water quality standards through an assessment of equitable levels of effort in reducing nutrients and sediments. The level of effort was determined through application of two key watershed scenarios: one where no action was taken in nutrient control and one where maximum nutrient control efforts were applied. Once the level of effort was determined for different jurisdictions, the overall load reduction was set watershed‐wide to achieve dissolved oxygen water quality standards. Further adjustments were made to the allocation to achieve the James River chlorophyll‐a standard.     

Rethinking the contribution of drained and undrained grasslands to sediment-related water quality problems

Grass vegetation has been recommended for use in the prevention and control of soil erosion because of its dense sward characteristics and stabilizing effect on the soil. A general assumption is that grassland environments suffer from minimal soil erosion and therefore present little threat to the water quality of surface waters in terms of sediment and sorbed contaminant pollution. Our data question this assumption, reporting results from one hydrological year of observations on a field-experiment monitoring overland flow, drain flow, fluxes of suspended solids, total phosphorus (TP), and molybdate-reactive phosphorus (<0.45 mum) in response to natural rainfall events. During individual rainfall events, 1-ha grassland lysimeters yield up to 15 kg of suspended solids, with concentrations in runoff waters of up to 400 mg L(-1). These concentrations exceed the water quality standards recommended by the European Freshwater Fisheries Directive (25 mg L(-1)) and the USEPA (80 mg L(-1)) and are beyond those reported to have caused chronic effects on freshwater aquatic organisms. Furthermore, TP concentrations in runoff waters from these field lysimeters exceeded 800 mug L(-1). These concentrations are in excess of those reported to cause eutrophication problems in rivers and lakes and contravene the ecoregional nutrient criteria in all of the USA ecoregions. This paper also examines how subsurface drainage, a common agricultural practice in intensively managed grasslands, influences the hydrology and export of sediment and nutrients from grasslands. This dataset suggests that we need to rethink the conceptual understanding of grasslands as non-erosive landscapes. Failure to acknowledge this will result in the noncompliance of surface waters to water quality standards.     

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.     

MODELING SEDIMENT AND PHOSPHORUS LOSSES IN AN AGRICULTURAL WATERSHED TO MEET TMDLs1

ABSTRACT: This paper studies the effectiveness of alternative farm management strategies at improving water quality to meet Total Maximum Daily Loads (TMDLs) in agricultural watersheds. A spatial process model was calibrated using monthly flow, sediment, and phosphorus (P) losses (1994 to 1996) from Sand Creek watershed in south‐central Minnesota. Statistical evaluation of predicted and observed data gave r² coefficients of 0.75, 0.69, and 0.49 for flow (average 4.1 m³/s), sediment load (average 0.44 ton/ha), and phosphorus load (average 0.97 kg/ha), respectively. The calibrated model was used to evaluate the effects of conservation tillage, conversion of crop land to pasture, and changes in phosphorus fertilizer application rate on pollutant loads. TMDLs were developed for sediment and P losses based on existing water quality standards and guidelines. Observed annual sediment and P losses exceeded these TMDLs by 59 percent and 83 percent, respectively. A combination of increased conservation tillage, reduced application rates of phosphorus fertilizer, and conversion of crop land to pasture could reduce sediment and phosphorus loads by 23 percent and 20 percent of existing loads, respectively. These reductions are much less than needed to meet TMDLs, suggesting that control of sediment using buffer strips and control of point sources of phosphorus are needed for the remaining reductions.