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At least 1,135 records · Page 63Linked to original sources
USGS2012

Spatial analysis of geologic and hydrologic features relating to sinkhole occurrence in Jefferson County, West Virginia

In this study the influence of geologic features related to sinkhole susceptibility was analyzed and the results were mapped for the region of Jefferson County, West Virginia. A model of sinkhole density was constructed using Geographically Weighted Regression (GWR) that estimated the relations among discrete geologic or hydrologic features and sinkhole density at each sinkhole location. Nine conditioning factors on sinkhole occurrence were considered as independent variables: distance to faults, fold axes, fracture traces oriented along bedrock strike, fracture traces oriented across bedrock strike, ponds, streams, springs, quarries, and interpolated depth to groundwater. GWR model parameter estimates for each variable were evaluated for significance, and the results were mapped. The results provide visual insight into the influence of these variables on localized sinkhole density, and can be used to provide an objective means of weighting conditioning factors in models of sinkhole susceptibility or hazard risk.

West Virginia↗
USGS2011

Cumuilative Effects of Impoundments on the Hydrology of Riparian Wetlands along the Marmaton River, west-central Missouri

The effects of proposed impoundments and resulting streamflow regulation on riparian wetlands in the Marmaton River Basin, Missouri, USA were determined using measurements and numerical simulations of wetland water budgets. Calibrated and validated Soil-Plant-Air-Water (SPAW) models were used to simulate daily water depths of four riparian wetlands for Current (model scenario of existing impoundments) and Proposed (model scenario of existing and proposed impoundments) impoundment conditions. The simulated frequency of flooding decreased 19–65% at the wetlands following the additions of proposed impoundments. The reduced flooding resulted in decreases in wetland water depths at all sites during the 10 simulated growing seasons under Proposed conditions with an average duration of continuous water-depth declines of 289 days at the upstream (most regulated) site. Downstream wetlands within the zone of least regulation had an average duration of water level decreases of about 20 days. Decreased water levels under Proposed conditions resulted in a range of 65–365 additional dry days at the study wetlands during the simulated 10-year period of Proposed conditions. The areas of the four wetlands meeting the hydrologic criteria of a formal jurisdictional wetland definition decreased ranging from zero to 31% under Proposed impoundment conditions.

Wetlands↗
USGS2015

Response of Schoenoplectus acutus and Schoenoplectus californicus at different life-history stages to hydrologic regime

For wetland restoration success to be maximized, restoration managers need better information regarding how the frequency, depth, and duration of flooding affect soil chemistry and the survival, growth, and morphology of targeted plant species. In a greenhouse study we investigated the impact of four different flooding durations (0 %, 40 %, 60 %, and 100 %) on soil physicochemistry and the responses of seedlings and adults of two species of emergent wetland macrophytes commonly used in restoration efforts ( Schoenoplectus acutus and Schoenoplectus californicus ). The longest flooding duration, which created more reducing soil conditions, resulted in significantly reduced survival of S. acutus adults (34 ± 21 % survival) and complete mortality of seedlings of both species. Schoenoplectus californicus adults exhibited higher flooding tolerance, showing little impact of flooding on morphology and physiology. A companion field study indicated that S. californicus maintained stem strength regardless of flooding duration or depth, supporting the greenhouse study results. This information serves to improve our understanding of the ecological differences between these species as well as provide restoration managers with better guidelines for targeted elevation and hydrologic regimes for these species in order to enhance the success of restoration plantings and better predict restoration site development.

Wetlands↗
USGS2016

Temperature and hydrology affect methane emissions from Prairie Pothole Wetlands

The Prairie Pothole Region (PPR) in central North America consists of millions of depressional wetlands that each have considerable potential to emit methane (CH 4 ). Changes in temperature and hydrology in the PPR from climate change may affect methane fluxes from these wetlands. To assess the potential effects of changes in climate on methane emissions, we examined the relationships between flux rates and temperature or water depth using six years of bi-weekly flux measurements during the snow-free period from six temporarily ponded and six permanently ponded wetlands in North Dakota, USA. Methane flux rates were among the highest reported for freshwater wetlands, and had considerable spatial and temporal variation. Methane flux rates increased with increasing temperature and water depth, and were especially high when conditions were warmer and wetter than average (163 ± 28 mg CH 4 m −2 h −1 ) compared to warmer and drier (37 ± 7 mg CH 4 m −2 h −1 ). Methane emission rates from permanent wetlands were less sensitive to changes in temperature and water depth compared to temporary wetlands, likely due to higher sulfate concentrations in permanent wetlands. While the predicted increase in temperature with climate change will likely increase methane emission rates from PPR wetlands, drier conditions could moderate these increases.

Wetlands↗
USGS1993

Hydrologic, chemical, and isotopic characterization of two small watersheds on Catoctin Mountain, north-central Maryland, U.S.A.

Two small (100 ha) watersheds located on Catoctin Mountain in north-central Maryland were intensively instrumented in 1990 and have been hydrologically, chemically, and isotopically monitored for 3 years. Dissolved concentrations of major ions (Ca 2+ , Mg 2+ , Na + , K + , total AI, CI - , NO 3 - , SO 4 2- , HCO 3 - , and SiO 2 ) and stable isotopic (D and 18 O) values have been analyzed for most types of water (precipitation, throughfall, two depths of soil water, shallow groundwater, and streamwater) that enter, travel through, and exit each watershed. The major objectives of the study were to characterize the chemical and isotopic signatures of all aqueous components of the watersheds and to interpret the causes of the changes in chemical and isotopic compositions of streamwater during storm runoff. This paper describes selected results of the study.

Maryland↗
USGS1994

Use of long-term tritium records from the Colorado River to determine timescales for hydrologic processes associated with irrigation in the Imperial Valley, California

Tritium records were used to study hydrologic processes associated with irrigation and drainage in the Imperial Valley, a 2000-km 2 agricultural area in the southeastern California desert. Tritium was analyzed in surface water, ground water, soil-pore water and drain water, and the results were compared to the historical record of tritium in the Colorado River. The Colorado River record was reconstructed using a simple reservoir model and precipitation data in the Colorado River Basin for the period prior to 1965, and from continuous measurements in the river for 1965–1988. This historical record is especially useful in the arid Imperial Valley because recent agricultural development has been entirely dependent on irrigation water diverted from the Colorado River and local recharge is negligible. Results indicate that it takes about 5 a for irrigation drainage to move through the soil to a depth of 2–3 m. Drainwaters have a wide range in tritium concentrations because of varying degrees of influence from ground-water intrusion, and from rapid percolation of irrigation through preferred pathways. The net result is that drainwater from about 40 fields had a range in tritium concentration similar to that of the Colorado River over the last 9 a (1980–1988), a period during which tritium concentration was declining about 15% annually in the river.

California↗
USGS2020

Quantitative paleoflood hydrology

This chapter reviews the paleohydrologic techniques and approaches used to reconstruct the magnitude and frequency of past floods using geological evidence. Quantitative paleoflood hydrology typically leads to two phases of analysis: (1) documentation and assessment of flood physical evidence (paleostage indicators), and (2) relating identified flood evidence to flood discharge, based on hydraulic calculations. Most paleoflood studies rely on stratigraphic sequences of fine-grained flood deposits found in slack-water and eddy environments in bedrock rivers to enable the estimates of paleodischarges for floods of past few centuries or millennia. Geochronology, commonly based on techniques such as optically stimulated luminescence (OSL) and radiocarbon, enable paleoflood age estimates. Such paleoflood discharge and age information can vastly improve flood frequency estimates, particularly for large and rare floods for which quantile estimates are typically poorly constrained by short historical records. The inclusion of such physical evidence of flooding into flood frequency assessments has been aided by new techniques of frequency analysis that can efficiently employ such data. Consequently, paleoflood analysis is supporting probability risk management of critical infrastructure such as nuclear facilities, dams, or bridges. Paleoflood studies also support understanding of the recurrence of geomorphically effective flows and assessment of non-stationarity in the frequency of large floods due to climate, land-use, or other environmental changes.

Llobregat River↗
USGS2006

Assimilation of snow covered area information into hydrologic and land-surface models

This paper describes a data assimilation method that uses observations of snow covered area (SCA) to update hydrologic model states in a mountainous catchment in Colorado. The assimilation method uses SCA information as part of an ensemble Kalman filter to alter the sub-basin distribution of snow as well as the basin water balance. This method permits an optimal combination of model simulations and observations, as well as propagation of information across model states. Sensitivity experiments are conducted with a fairly simple snowpack/water-balance model to evaluate effects of the data assimilation scheme on simulations of streamflow. The assimilation of SCA information results in minor improvements in the accuracy of streamflow simulations near the end of the snowmelt season. The small effect from SCA assimilation is initially surprising. It can be explained both because a substantial portion of snowmelts before any bare ground is exposed, and because the transition from 100% to 0% snow coverage occurs fairly quickly. Both of these factors are basin-dependent. Satellite SCA information is expected to be most useful in basins where snow cover is ephemeral. The data assimilation strategy presented in this study improved the accuracy of the streamflow simulation, indicating that SCA is a useful source of independent information that can be used as part of an integrated data assimilation strategy. ?? 2005 Elsevier Ltd. All rights reserved.

Advances in Water Resources↗
USGS2018

Application of hydrologic-tracer techniques to the Casargiu adit and Rio Irvi (SW-Sardinia, Italy): Using enhanced natural attenuation to reduce extreme metal loads

Hydrologic tracer techniques were applied to Rio Irvi (SW Sardinia), a stream affected by mine drainage, allowing the calculation of stream discharge and metal loads and comparison to other streams. The calculated discharge showed a continuous increase from near 21.2 L/s to 29.1 L/s. Cumulative loads of mine-related constituents, including the Casargiu adit inflow, were large, with more than 9900 kg/day of SO 4 2− , 2370 kg/day of Zn, 550 kg/day of Fe and 172 kg/day of Mn. The greatest measurable inflow source of metals, other than the Casargiu adit, was an acidic tributary (L4), but most sources of instream metal load were related to dispersed groundwater inflows. Some of those groundwater inflows were related to non-flowing tributaries. Calculations of the cumulative instream metal load, excluding the Casargiu adit inflow, indicated increases of 1250 kg/day for SO 4 2 , 858 kg/day for Zn - , 137 kg/day gain for Fe and 60 kg/day for Mn. Rio Irvi Zn load was extreme for a stream of this size and discharge. A comparison with two other mine-affected rivers in Sardinia indicated the loading in Rio Irvi was two to three orders of magnitude greater. This difference was attributed to different geochemical conditions, but also to a lack of a biogeochemical barrier like that seen to be acting along and below the riverbed in Rio San Giorgio. Several years of intense vegetation growth in the river bed of Rio San Giorgio created a biogeochemical barrier to metal loading, and the cumulative Zn load there was near 8 kg/day, despite being a drainage with a greater mass of mine wastes to contribute to the load.

Sardinia↗
USGS2010

Limited hydrologic response to Pleistocene climate change in deep vadose zones - Yucca Mountain, Nevada

Understanding the movement of water through thick vadose zones, especially on time scales encompassing long-term climate change, is increasingly important as societies utilize semi-arid environments for both water resources and sites viewed as favorable for long-term disposal or storage of hazardous waste. Hydrologic responses to Pleistocene climate change within a deep vadose zone in the eastern Mojave Desert at Yucca Mountain, Nevada, were evaluated by uranium-series dating of finely layered hyalitic opal using secondary ion mass spectrometry. Opal is present within cm-thick secondary hydrogenic mineral crusts coating floors of lithophysal cavities in fractured volcanic rocks at depths of 200 to 300 m below land surface. Uranium concentrations in opal fluctuate systematically between 5 and 550 μg/g. Age-calibrated profiles of uranium concentration correlate with regional climate records over the last 300,000 years and produce time-series spectral peaks that have distinct periodicities of 100- and 41-ka, consistent with planetary orbital parameters. These results indicate that the chemical compositions of percolating solutions varied in response to near-surface, climate-driven processes. However, slow (micrometers per thousand years), relatively uniform growth rates of secondary opal and calcite deposition spanning several glacial–interglacial climate cycles imply that water fluxes in the deep vadose zone remained low and generally buffered from the large fluctuations in available surface moisture during different climates.

Nevada↗
USGS2023

Can spatial food web subsidies associated with river hydrology and lateral connectivity be detected using stable isotopes?

During and following lateral connections, aquatic organisms residing in the river channel may assimilate material from sources imported from oxbows, and oxbow residents may consume and assimilate material imported from the channel. Hydrology, lateral connectivity, and stable isotope ratios of fishes and mussels were analyzed for evidence of spatial food web subsidies between the active channel and oxbow lakes in the floodplain of the Guadalupe River, Texas. During surveys conducted between March 2016 and April 2017, fish, mussel, periphyton, seston, and riparian plant samples were collected in and around two oxbows and adjacent channel sites for analysis of stable isotope ratios. Biplots of δ 13 C and δ 15 N were graphed for basal sources and specimens of six common fish species, four sunfish species ( Lepomis spp. combined), and two mussel species (Unionidae combined) captured from oxbows and the channel. Within each graph, polygons were drawn to indicate the space occupied by animals that could have assimilated feasible combinations of source materials originating from either oxbows or the river channel. Based on positions of animals within source polygons, riparian C4 grasses were not an important source of organic matter supporting biomass of fishes and mussels within the channel or oxbows. Overall, 84% of organisms had isotopic signatures consistent with assimilation of in situ sources, but also 76% of all organisms were inconclusive with regards to cross-habitat exchanges. Outliers that may have assimilated ex situ source material were observed for only 4% of 313 organisms from oxbows and 9% of 232 organisms from the channel, and some but not all of these cases followed high flow pulses that connected oxbows for extended periods. Several issues that compromise inferences from stable isotope analysis were identified, and estimation of spatial food web subsidies in fluvial systems could be enhanced by analyzing additional biomarkers, such as isotopic ratios of other elements and compound-specific stable isotopes, as well as additional sources, time-specific biotracers, and experimental approaches that directly track movement of sources and organisms in spatially structured food webs.

Texas↗
USGS2006

Geologic, hydrologic, and geochemical interpretations of mineral deposits as analogs for understanding transport of environmental contaminants

Base- and precious-metal mineral deposits comprise anomalous concentrations of metals and associated elements, which may be useful subjects for study as analogs for migration of environmental contaminants. In the geologic past, hydrothermal mineral deposits formed at the intersection of favorable geologic, hydrologic and geochemical gradients. In the present, weathering of these sulfide-rich deposits occurs as a result of the interplay between rates of oxygen supply versus rates of ground or surface-water flow. Transport and spatial dispersion of elements from a mineral deposit occurs as a function of competing rates of water flow versus rates of attenuation mechanisms such as adsorption, dilution, or (co)precipitation. In this paper we present several case studies from mineralized and altered sedimentary and crystalline aquifers in the western United States to illustrate the geologic control of ground-water flow and solute transport, and to demonstrate how this combined approach leads to a more complete understanding of the systems under study as well as facilitating some capability to predict major flow directions in aquifers.

Journal of Geochemical Exploration↗
USGS2023

A river basin spatial model to quantitively advance understanding of riverine tree response dynamics to water availability and hydrological management

Ecological condition continues to decline in arid and semi-arid river basins globally due to hydrological over-abstraction combined with changing climatic conditions. Whilst provision of water for the environment has been a primary approach to alleviate ecological decline, how to accurately monitor changes in riverine trees at fine spatial and temporal scales, remains a substantial challenge. This is further complicated by constantly changing water availability across expansive river basins with varying climatic zones. Within, we combine rare, fine-scale, high frequency temporal in-situ field collected data with machine learning and remote sensing, to provide a robust model that enables broadscale monitoring of physiological tree water stress response to environmental changes via actual evapotranspiration (ET). Physiological variation of Eucalyptus camaldulensis (River Red Gum) and E. largiflorens (Black Box) trees across 10 study locations in the southern Murray-Darling Basin, Australia, was captured instantaneously using sap flow sensors, substantially reducing tree response lags encountered by monitoring visual canopy changes. Actual ET measurement of both species was used to bias correct a national spatial ET product where a Random Forest model was trained using continuous timeseries of in-situ data of up to four years. Precise monthly AMLETT ( A ustralia-wide M achine L earning ET for T rees) ET outputs in 30 m pixel resolution from 2012 to 2021, were derived by incorporating additional remote sensing layers such as soil moisture, land surface temperature, radiation and EVI and NDVI in the Random Forest model. Landsat and Sentinal-2 correlation results between in-situ ET and AMLETT ET returned R 2 of 0.94 (RMSE 6.63 mm period −1 ) and 0.92 (RMSE 6.89 mm period −1 ), respectively. In comparison, correlation between in-situ ET and a national ET product returned R 2 of 0.44 (RMSE 34.08 mm period −1 ) highlighting the need for bias correction to generate accurate absolute ET values. The AMLETT method presented here, enhances environmental management in river basins worldwide. Such robust broadscale monitoring can inform water accounting and importantly, assist decisions on where to prioritize water for the environment to restore and protect key ecological assets and preserve floodplain and riparian ecological function.

The Journal of Environmental Management↗
USGS2023

Developing a stochastic hydrological model for informing lake water level drawdown management

Winter drawdown (WD) is a common lake management tool for multiple purposes such as flood control, aquatic vegetation reduction, and lake infrastructure maintenance. To minimize adverse impacts to a lake’s ecosystem, regulatory agencies may provide managers with general guidelines for drawdown and refill timing, drawdown magnitude, and outflow limitations. However, there is significant uncertainty associated with the potential to meet management targets due to variability in lake characteristics and hydrometeorology of each lake’s basin, making the use of modeling tools a necessity. In this context, we developed a hydrological modeling framework for lake water level drawdown management (HMF-Lake) and evaluated it at 15 Massachusetts lakes where WDs have been applied over multiple years for vegetation control. HMF-Lake is based on the daily lake water balance, with inflows simulated by a lumped rainfall-runoff model (Cemaneige-GR4J) and outflow rate calculated by a modified Target Storage and Release Based Method (TSRB). The model showed a satisfactory performance of simulating historical water levels (0.53 ≤ NSE ≤ 0.86), however, uncertainties from meteorological inputs and TSRB determined lake outflow rate affected the result accuracy. To account for these uncertainties, the model was executed stochastically to assess the ability of study lakes to follow the Massachusetts’ general WD guidelines: drawdown by Dec 1 and fully refilled by Apr 1. By using the stochastic HMF-Lake, the probabilities of each lake to reach the drawdown level by Dec 1 were calculated for different drawdown magnitudes (1–6 ft). The probability results suggest it was generally less possible for most of study lakes to achieve a drawdown of 3 ft or more by Dec 1. Moreover, we employed the stochastic model to derive the annual latest refill starting dates that ensure a 95 % probability of reaching the normal water level by Apr 1. We found starting a refill in March for drawdowns up to 6 ft was feasible for most of study lakes. These results provide lake managers with a quantitative understanding of the lake’s ability to follow the state guidelines. The model may be used to systematically evaluate current WD management strategies at state or regional scales and support adaptive WD management under changing climates.

Massachusetts↗
USGS2011

Quantification of a greenhouse hydrologic cycle from equatorial to polar latitudes: The mid-Cretaceous water bearer revisited

This study aims to investigate the global hydrologic cycle during the mid-Cretaceous greenhouse by utilizing the oxygen isotopic composition of pedogenic carbonates (calcite and siderite) as proxies for the oxygen isotopic composition of precipitation. The data set builds on the Aptian–Albian sphaerosiderite δ 18 O data set presented by Ufnar et al. (2002) by incorporating additional low latitude data including pedogenic and early meteoric diagenetic calcite δ 18 O. Ufnar et al. (2002) used the proxy data derived from the North American Cretaceous Western Interior Basin (KWIB) in a mass balance model to estimate precipitation–evaporation fluxes. We have revised this mass balance model to handle sphaerosiderite and calcite proxies, and to account for longitudinal travel by tropical air masses. We use empirical and general circulation model (GCM) temperature gradients for the mid-Cretaceous, and the empirically derived δ 18 O composition of groundwater as constraints in our mass balance model. Precipitation flux, evaporation flux, relative humidity, seawater composition, and continental feedback are adjusted to generate model calculated groundwater δ 18 O compositions (proxy for precipitation δ 18 O) that match the empirically-derived groundwater δ 18 O compositions to within ± 0.5‰. The model is calibrated against modern precipitation data sets. Four different Cretaceous temperature estimates were used: the leaf physiognomy estimates of Wolfe and Upchurch (1987) and Spicer and Corfield (1992), the coolest and warmest Cretaceous estimates compiled by Barron (1983) and model outputs from the GENESIS-MOM GCM by Zhou et al. (2008). Precipitation and evaporation fluxes for all the Cretaceous temperature gradients utilized in the model are greater than modern precipitation and evaporation fluxes. Balancing the model also requires relative humidity in the subtropical dry belt to be significantly reduced. As expected calculated precipitation rates are all greater than modern precipitation rates. Calculated global average precipitation rates range from 371 mm/year to 1196 mm/year greater than modern precipitation rates. Model results support the hypothesis that increased rainout produces δ 18 O-depleted precipitation. Sensitivity testing of the model indicates that the amount of water vapor in the air mass, and its origin and pathway, significantly affect the oxygen isotopic composition of precipitation. Precipitation δ 18 O is also sensitive to seawater δ 18 O and enriched tropical seawater was necessary to simulate proxy data (consistent with fossil and geologic evidence for a warmer and evaporatively enriched Tethys). Improved constraints in variables such as seawater δ 18 O can help improve boundary conditions for mid-Cretaceous climate simulations.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS2019

Effects of a changing climate on the hydrological cycle in cold desert ecosystems of the Great Basin and Columbia Plateau

Climate change is already resulting in changes in cold desert ecosystems, lending urgency to the need to understand climate change effects and develop effective adaptation strategies. In this review, we synthesize information on changes in climate and hydrologic processes during the last century for the Great Basin and Columbia Plateau, and discuss future projections for the 21st century. We develop mid-century projections of temperature and climate for the Great Basin and Columbia Plateau at timescales relevant to managers (2020 -2050), and discuss concepts and strategies for adapting to the projected changes. For the instrumented record in the Great Basin and Columbia Plateau (1985-2011), a temperature increase of 0.7°C to 1.4°C has been documented, but changes in precipitation have been relatively minor with no clear trends. Climate projections for 2020 to 2050 indicate that temperatures will continue to increase, especially in winter and during the night. Precipitation is more difficult to project and estimates range from an 11% decrease to 25% increase depending on location. Recent records indicate that the Great Basin and Columbia Plateau are becoming more arid, a trend that is projected to continue. Droughts are likely to become more frequent and last longer, invasive annual grasses are likely to continue to expand, and the duration and severity of wildfire seasons are likely to increase. Climate projections can help in developing adaptive management strategies for actual or expected changes in climate. Strategies include reducing the risks of nonnative invasive plant spread and wildfires that result in undesirable transitions, planning for drought, and where necessary, facilitating the transition of populations, communities and ecosystems to new climatic conditions. A proactive approach to planning for and adapting to climate change is needed and publicly available internet based resources on climate data and planning strategies are available to help meet that need.

California, Idaho, Nevada, Oregon, Utah, Washingto↗
USGS2020

Seasonal drivers of chemical and hydrological patterns in roadside infiltration-based green infrastructure

Infiltration-based green infrastructure has become a popular means of reducing stormwater hazards in urban areas. However, the long-term effects of green infrastructure on the geochemistry of roadside environments are poorly defined, particularly given the considerable roadside legacy metal contamination from historic industrial activity and vehicle emissions (e.g., Pb). Most current research on green infrastructure geochemistry is restricted to time periods of less than a year or limited sets of chemical species. This further limits our understanding of systems that evolve over time and are subject to seasonal variability. Between 2016 and 2018, two infiltration trenches in Pittsburgh, PA, were monitored to determine infiltration rates and dissolved nutrient and metal content. The trench water was analyzed to characterize seasonal patterns in both trench function and chemistry. Shifting patterns in infiltration rate and geochemical activity show trends corresponding with seasonal changes. Trench function is dependent on the local water table, with the highest infiltration rates occurring when evapotranspiration is active and groundwater elevation is low. Two seasonal chemical patterns were identified. The first is driven by road salt application in the winter and interaction of the salt pulse increase Pb and Cu concentrations. The second is driven by the formation of summer reducing environments that increase dissolved Fe and Mn. These findings suggest that chemical and hydrological activity in infiltration-based green infrastructure varies seasonally and may remobilize legacy contamination.

Pennsylvania↗
USGS2000

Importance of the Mississippi River Basin for investigating agricultural–chemical contamination of the hydrologic cycle

This special issue is devoted to recent and ongoing research relating to the fate and transport of agricultural chemicals in the Mississippi River Basin by the US Geological Survey Toxic Substances Hydrology (Toxics) Program. The Mississippi River Basin drains approximately 3 200 000 km 2 representing 41% of the United States. This is the largest river in the United States and the third largest in the world. The Mississippi River discharges an average of 19 920 m 3 /s of water into the Gulf of Mexico. The river is an extensively used resource, supplying drinking water to 70 cities in the United States. The Mississippi River Basin has undergone dramatic land use and cultural changes over the last 150 years. Approximately 70 million people now live within the basin, representing approximately 27% of the nation's population. This basin has also become one of the most productive agricultural regions in the world in terms of both crops and livestock grown. Approximately 65% of the nation's harvested cropland is grown in this basin, with more than 100 000 metric tons (t) of pesticides and approximately 6 500 000 t of commercial nitrogen fertilizers applied to cropland within the basin annually. The drainage of more than 20 000 000 ha within the basin has been enhanced by means of tile lines and ditches to lower the water table to make the cropland more productive. While removing the water from the soil as intended, this practice also leads to more rapid transport of contaminants to the river, and ultimately the Gulf of Mexico. Furthermore, the extensive chemical use in the Mississippi River Basin has led to the transport of pesticides and nitrate into the region’s streams, aquifers, and atmosphere. An estimated 1 000 000 t of nitrate-N is transported from the Mississippi River Basin into the Gulf of Mexico annually. The peak annual load of herbicides to the Gulf of Mexico has been documented at 1920 t. The fundamental goal of the papers presented in this volume is to provide a scientific basis for decisions necessary to promote sound and efficient agricultural practices and protect the quality of the nation's water resources.

Science of the Total Environment↗