Advertisement

  • Loading metrics

Open Access

Peer-reviewed

Research Article

Effects of return flows on stream water quality and availability in the Upper Colorado, Delaware, and Illinois River Basins

Abstract

Understanding effects of human water use and subsequent return flows on the availability and suitability of water for downstream uses is critical to efficient and effective watershed management. We compared spatially detailed estimates of stream chemistry within three watersheds in diverse settings to available standards to isolate effects of wastewater and irrigation return flows on the suitability of downstream waters for maintaining healthy aquatic ecosystems and for selected human uses. Mean-annual flow-weighted total and source-specific concentrations of nitrogen and phosphorus in individual stream reaches within the Upper Colorado, Delaware, and Illinois River Basins and of total dissolved solids within stream reaches of the Upper Colorado River Basin were estimated from previously calibrated regional watershed models. Estimated concentrations of both nitrogen and phosphorus in most stream reaches in all three watersheds (at least 78%, by length) exceed recommended standards for the protection of aquatic ecosystems, although concentrations in relatively few streams exceed such standards due to contributions from wastewater return flows, alone. Consequently, efforts to reduce wastewater nutrient effluent may provide important local downstream benefits but would likely have minimal impact on regional ecological conditions. Similarly, estimated mean-annual flow-weighted total dissolved solids concentrations in the Upper Colorado River Basin exceed standards for agricultural water use and (or) the secondary maximum contaminant level (SMCL) for drinking water in 52% of streams (by length), but rarely due to effects of irrigation return flows, alone. Dissolved solids in most tributaries of the Upper Colorado River are attributable primarily to natural sources.

Citation: Ator SW, Miller OL, Saad DA (2022) Effects of return flows on stream water quality and availability in the Upper Colorado, Delaware, and Illinois River Basins. PLOS Water 1(7): e0000030. https://doi.org/10.1371/journal.pwat.0000030

Editor: Bimlesh Kumar, Indian Institute of Technology Guwahati, INDIA

Received: January 3, 2022; Accepted: June 14, 2022; Published: July 11, 2022

This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.

Data Availability: Estimates of previously calibrated watershed models upon which results and interpretations reported herein for nitrogen and phosphorus are based are available in Miller et al. [85] for the Upper Colorado River Basin, Ator [86] for the Delaware River Basin, and Saad and Robertson [87] for the Illinois River Basin. Estimates from the previously calibrated salinity model for the Upper Colorado River Basin are available in Miller et al. [94].

Funding: This work was funded by the U.S. Geological Survey (USGS) through its Integrated Water Availability Assessment (IWAA) program. USGS officials allocating funding to the IWAA program had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Water availability for human needs and healthy ecosystems is a concern in many areas. Globally, moderate to severe water scarcity affects 4.3 billion people (71% of the world population) during at least part of the year and 0.5 billion people, year-round [1]. Similarly, aquatic habitat supported by 65% of global river discharge is under at least moderate threat [2]. Jelks et al. [3] estimated that 39% of fish species in North America are imperiled, mainly due to habitat degradation and invasive species.

Human use of freshwater is equivalent to half of global river discharge [4] and can impact the quantity, quality, and (therefore) availability and suitability of that water for reuse. A total of 388 km3 (cubic kilometers) of freshwater was withdrawn in the United States in 2015, of which 42% was used for irrigation and an additional 14% was used for public supply [5]. Globally, water scarcity is generally most prevalent in areas with high population densities and (or) irrigated agriculture [1]. Water used for public supply is commonly returned from sinks, tubs, and toilets to the water cycle through municipal wastewater treatment facilities. Effluent from these facilities can include elevated concentrations of nutrients and other contaminants such as pharmaceuticals and consumer products [6–8]. Water used for agricultural irrigation often returns to streams through natural overland runoff, along groundwater flow paths, or through ditches, tile drains, or other artificial drainage structures. Agricultural return flows can include elevated concentrations of nutrients, herbicides, insecticides, and other chemicals commonly applied to agricultural land [9, 10]. Transpiration by plants and evaporation can also increase salinity in agricultural return flows [11].

Declining water quality is often attributable to excessive nutrients. Nitrogen may affect the potability of water [12–14] and therefore the cost of drinking-water treatment [15]. Other implications of elevated nutrient concentrations in surface waters include excessive (and sometimes toxic) algal blooms, fish kills, and declines in dissolved oxygen, water clarity, biodiversity, and commercial and recreational fisheries [13, 16–21]. Phosphorus is often the limiting nutrient controlling primary production in non-tidal streams and lakes [16, 22]. Nitrogen, conversely, often limits primary production in temperate estuaries and coastal waters, although phosphorus may also be important during certain seasons and in some systems [16, 23]. Davidson et al. [24] estimated that excessive nutrients affect 66% of coastal systems, 33% of lakes, and 40% of flowing streams in the United States, and Bricker et al. [19] reported that a majority of assessed estuaries in the United States were at least moderately eutrophic. Eutrophication of surface waters has also been reported in many other areas [25], such as in parts of Africa [26, 27], Asia [28, 29], Australia [30], Europe [31–34], and South America [35].

Excessive salinity (total dissolved solids) may also contribute to declining water quality in surface waters. Economic impacts of excessive salinity in surface waters include reduced crop yields, corrosion, and obstruction of pipes or water fixtures [36]. Increasing salinity may also alter lake stratification and density gradients, stimulate algal growth, reduce denitrification, increase acidification, mobilize toxic metals, and affect the ecology of aquatic organisms [37–39]. Salinity may also affect the taste of drinking water; the U.S. Environmental Protection Agency [40] has established a secondary maximum contaminant level for total dissolved solids in drinking water of 500 mg/L (milligrams per liter). Anning and Flynn [39] estimated that mean annual flow-weighted concentrations of dissolved solids in 12.6% of stream reaches in the conterminous United States exceeded 500 mg/L in 2000. In the upper Colorado River basin in 2010, economic impacts of salinity in surface waters were estimated at $295 million [36].

Understanding the relative contributions of different sources of nutrients and dissolved solids to surface waters is fundamental to effective and efficient water-quality management and restoration. Effects of nutrients and salinity associated with wastewater discharge and agricultural return flows on the availability of water for human use and ecosystem sustainability constitute ongoing concerns [41, 42]. In addition to municipal wastewater and agriculture, common sources of nutrients to terrestrial uplands and (or) directly to surface waters include septic systems, industrial discharges, lawn fertilizers, mineral erosion (for phosphorus), and (for nitrogen) deposition or direct fixation from the atmosphere [16, 43–46]. Dissolved solids in surface waters are generally attributable to natural chemical erosion (dissolution) of minerals in geologic sources, but may be substantially increased locally through anthropogenic activities such as application of deicing chemicals or irrigation of agricultural lands [36, 37, 39, 47]. Anning and Flynn [39] estimated that the majority of dissolved solids in 89% of stream reaches in the conterminous United States is attributable to geologic sources, although deicing chemicals and agriculture are important sources in some streams, particularly in the Northeast and the West (respectively). Management challenges vary substantially among sources, particularly between point sources and diffuse (non-point) sources. Although nitrogen inputs to Chesapeake Bay are attributable primarily to non-point sources, declining nitrogen flux from the watershed to the bay in recent decades is disproportionately attributable to reductions in point sources [48–50].

The importance and impact of sources associated with water reuse and return flows to nutrient loads in the Upper Colorado, Delaware, and Illinois River Basins and to salinity in the Upper Colorado River Basin (Fig 1) are estimated and described in this paper. These estimates of the importance of water reuse to nutrients and salinity were developed from previously calibrated watershed models that illustrate and quantify the sources, fate, and transport of nutrients in watersheds of the southwestern [51], midwestern [52], and northeastern [53] United States and of dissolved solids (salinity) in the Upper Colorado River Basin [47]. Estimated mean annual flow-weighted concentrations of total nitrogen, total phosphorus, and (or) total dissolved solids (salinity) attributable to return flows in watershed streams are compared to recommended ecological, drinking-water, or agricultural water quality standards to illustrate the importance of return flows to human water use and stream ecology. This work was conducted to improve the understanding of risks and potential implications related to water reuse as part of the U.S. Geological Survey’s Integrated Water Availability and Assessments (IWAA) program [54]. The IWAA program currently includes three regional watersheds (the Upper Colorado, Delaware, and Illinois River Basins) where models, tools, and supporting data will be developed to support national water availability assessments.

thumbnail
Download:
Fig 1. Aggregated nutrient ecoregions in the Upper Colorado, Delaware, and Illinois River Basins.

https://doi.org/10.1371/journal.pwat.0000030.g001

Methods

Previously calibrated models [47, 51–53] developed using spatially referenced regression (SPARROW) [46, 55] were used to estimate total and source-specific mean-annual flow-weighted concentrations of total nitrogen, total phosphorous, and (or) total dissolved solids in streams of the Upper Colorado, Delaware, and Illinois River Basins. Estimated concentrations attributable to selected sources were compared to ecological and other standards to illustrate the potential importance of return flows to stream ecology and human water use.

The SPARROW modeling tool

SPARROW [46, 55] uses non-linear regression to relate the long-term average load of a contaminant or other water-quality constituent (such as nitrogen, phosphorus, or dissolved solids) at monitoring stations on nontidal streams (the calibration or dependent variable) to sources and landscape conditions representing contaminant fate and transport in upstream watersheds and within the stream network (explanatory or independent variables). Calibration and explanatory data for use in SPARROW models are referenced to a digital representation of hydrography (including stream reaches and associated upland catchments) within the model domain which is used to route streamflow and contaminants through the watershed of interest, to maintain mass-balance constraints, and to link each stream with its contributing watershed. Once calibrated, models can be used to estimate the total and source-specific load of contaminants generated within each individual watershed catchment and transported downstream through the stream network.

The SPARROW tool has been used for a variety of water-resources applications. SPARROW models have been developed to quantify and improve the understanding of long-term mean annual nutrient and (or) sediment sources, fate, and transport in many areas of the United States [46, 51–53, 56–70] and elsewhere [71–74]. SPARROW has also been used to model total dissolved solids [39, 47] and total organic carbon [75]. SPARROW models have been applied to predict future water quality or streamflow conditions [74, 76–78]; target management actions [79]; and to improve the understanding of temporal changes in water quality [48, 50, 80], contaminant fate and transport [81, 82], effects of management practices on water quality [83], and baseflow contributions to total streamflow [84].

Nutrient models

Concentrations and loads of nitrogen and phosphorus in streams of the Upper Colorado [85], Delaware [86], and Illinois River Basins [87] were estimated from predictions of SPARROW models calibrated for nutrients in the southwestern [51], northeastern [53], and midwestern [52] United States, respectively. Individual models were developed for nitrogen and for phosphorus within each of the three regions. The models were calibrated to long-term mean annual loads of nitrogen or phosphorus centered on 2012 that were estimated from available water-quality and streamflow data collected from watershed streams [88]. Digital stream hydrography for use in the models was modified from the National Hydrography Dataset (NHDPlus, version 2) at a resolution of 1:100,000 [89–93].

The nitrogen and phosphorus models were specified to represent the range of nutrient sources and landscape factors affecting nutrient fate and transport within each region [51–53]. Each model includes a variety of diffuse (non-point) sources and a source term representing wastewater point sources, which are return flow from diversions or extractions for municipal supply. Wastewater sources are specified to input directly to surface waters and therefore do not interact in the models with landscape factors affecting fate and transport from uplands to streams. Nutrients from wastewater point sources are affected in the models by instream processes, however, which are estimated for flowing and impounded reaches within each model. Because the models are calibrated to long term mean-annual conditions, instream nutrient losses in the models likely reflect the effects of permanent removal from or long-term storage of nutrients along stream channels (such as through denitrification or floodplain sedimentation), rather than effects of diurnal, seasonal, or other shorter-term processes [55]. Although wastewater return flows represent a minor fraction of the total nitrogen or phosphorus load in most streams in each region, they are locally important downstream of major treatment plants and (or) where other sources are limited [51–53].

Salinity model

Concentrations and loads of dissolved solids in the Upper Colorado River Basin [94] were estimated from results of a SPARROW model developed by Miller et al. [47]. The model was calibrated to mean-annual loads of dissolved solids estimated from specific conductance, evaporative residue, or dissolved solids concentrations collected from 323 monitoring locations within the watershed [95]. Calibration and explanatory data for the model were located on stream hydrography for the Upper Colorado River Basin that was modified from a stream network developed for a previous SPARROW model by Kenney et al. [96].

Sources of dissolved solids in streams of the Upper Colorado River Basin include natural erosion and return flows from irrigated agriculture [47]. Agricultural return flows estimated by the model include natural infiltration and flow through groundwater to streams as well as flow through ditches and other artificial drainage structures. Average yields of dissolved solids from mineral erosion in the watershed estimated by the model are generally less than approximately 158 kg/ha (kilograms per hectare). Agricultural areas yield an average of 525 kg/ha when irrigated by sprinklers, however, and as much as over 8,000 kg/ha under flood irrigation [47]. Although less than 2% of the Upper Colorado River Basin is used for irrigated agriculture, nearly one third (32%) of dissolved-solids loads in the watershed are attributable to irrigation return flows from agriculture [47].

Estimating the influence of return flows on water quality and suitability

The calibrated nutrient [51–53] and dissolved solids [47] models were used to estimate the mean-annual flow-weighted concentrations of total nitrogen, total phosphorus, and (or) total dissolved solids (salinity) within each stream reach in the Upper Colorado, Delaware, and Illinois River Basins. SPARROW model predictions for each stream reach include total contaminant loads as well as loads attributable to each source specified in the models [55]. These average annual total and source-specific loads of nitrogen, phosphorus, and (or) dissolved solids predicted by the models were divided by the mean annual flow within each reach to estimate the mean-annual flow-weighted concentrations [55].

Available water-quality standards were compiled for comparison to estimated salinity and nutrient concentrations within each stream reach. Estimated salinity in flowing reaches within the Upper Colorado River Basin was compared to the secondary maximum contaminant level (SMCL) for drinking water of 500 mg/L for total dissolved solids [40] and to the Utah standard of 1,200 mg/L for agricultural use [97]. Estimated nutrient concentrations in flowing reaches were compared to established or recommended State-level ecological criteria for flowing streams and rivers in Illinois, Utah, and Wisconsin, and to ecoregional criteria recommended for streams and rivers by the U.S. Environmental Protection Agency in other areas (Table 1). Nutrient criteria are often concentrations above which ecosystems can be negatively impacted by excessive algal growth, which can include certain cyanobacteria that produce toxins affecting human and animal health [98]. Because water quality often varies seasonally [99], such criteria specific to particular seasons may not be directly comparable to mean-annual flow-weighted concentrations estimated from SPARROW results.

thumbnail
Download:
Table 1. Ecological water-quality standards for nutrients for areas within the upper Colorado, Delaware, and Illinois River Basins [TN, total nitrogen; TP, total phosphorus].

https://doi.org/10.1371/journal.pwat.0000030.t001

Results

Nutrients

Nitrogen at concentrations potentially harmful to stream ecosystems is common in the Upper Colorado, Delaware, and Illinois River Basins. Mean-annual flow-weighted concentrations generally exceed recommended ecoregional criteria in streams of each watershed, although such exceedances are typically attributable to non-point sources rather than to wastewater return flows (Figs 2–4). Streams containing mean-annual flow-weighted nitrogen concentrations meeting the ecoregional criteria are particularly uncommon in the Upper Colorado River Basin and limited to 3% of stream length, mainly in larger streams in the headwaters of southwestern Wyoming (Fig 2A). Streams meeting such criteria are more common in the Delaware (22% of stream length) and Illinois (8% of stream length) River Basins, particularly in the largely forested northern portion of the Delaware River Basin in New York and Pennsylvania (Fig 3A) and in urban areas in Illinois near Chicago (Fig 4A). The spatial distribution of estimated exceedances within the Illinois River Basin, however, also reflects ecological criteria for Illinois that are greater than in other areas of the basin (Table 1). Streams containing mean-annual flow-weighted nitrogen concentrations exceeding ecoregional criteria due specifically to wastewater return flows are unusual and limited mainly to within or downstream of urban areas such as Philadelphia (Fig 3B) and Chicago (Fig 4B). Consequently, eliminating wastewater return flows entirely would reduce mean-annual flow-weighted nitrogen concentrations from above to below ecoregional criteria in relatively few streams (Figs 2C, 3C and 4C).

thumbnail
Download:
Fig 2.

Estimated mean-annual flow-weighted nitrogen concentrations in streams of the Upper Colorado River Basin, including concentrations attributable to A) all sources and B) wastewater treatment plant return-flow sources, and C) effects of eliminating wastewater sources.

https://doi.org/10.1371/journal.pwat.0000030.g002

thumbnail
Download:
Fig 3.

Estimated mean-annual flow-weighted nitrogen concentrations in streams of the Delaware River Basin, including concentrations attributable to A) all sources and B) wastewater treatment plant return-flow sources, and C) effects of eliminating wastewater sources.

https://doi.org/10.1371/journal.pwat.0000030.g003

thumbnail
Download:
Fig 4.

Estimated mean-annual flow-weighted nitrogen concentrations in streams of the Illinois River Basin, including concentrations attributable to A) all sources and B) wastewater treatment plant return-flow sources, and C) effects of eliminating wastewater sources.

https://doi.org/10.1371/journal.pwat.0000030.g004

As with nitrogen, mean-annual flow-weighted phosphorus concentrations are generally greater than ecoregional criteria in streams of the Upper Colorado, Delaware, and Illinois River Basins, although such concentrations attributable specifically to wastewater return flows are uncommon (Figs 5–7). Mean-annual flow-weighted phosphorus concentrations below ecological standards within the Upper Colorado River Basin occur in 11% of streams (by length), mostly in Utah (Fig 5A) where the standard is more than twice as high as the ecoregional nutrient criteria compared to estimated concentrations in the other watershed States (Table 1). Within the Delaware River Basin, 11% of streams (by length) meet ecological phosphorus criteria (Fig 6A), generally in similar areas to those meeting nitrogen criteria (Fig 3A). Very few streams (1%, by length) within the Illinois River Basin contain estimated mean-annual flow-weighted phosphorus concentrations meeting (below the) ecological criteria (Fig 7A). Very few streams in the Upper Colorado River Basin contain mean-annual flow-weighted phosphorus concentrations exceeding ecological criteria due specifically to wastewater return flows (Fig 5B). Such return flows, however, are likely responsible for a larger portion of streams exceeding phosphorus criteria (Figs 6B and 7B) than exceeding nitrogen criteria (Figs 3B and 4B) in the Delaware and Illinois River Basins. The impact of wastewater effluent from Chicago and other cities in the Illinois River Basin is apparent throughout the full length of the Illinois River to its outlet at the Mississippi River (Fig 7B). As with nitrogen, however, eliminating wastewater return flows entirely would reduce mean-annual flow-weighted phosphorus concentrations to levels not expected to adversely affect aquatic ecology in relatively few streams in any of the three watersheds (Figs 5C, 6C and 7C).

thumbnail
Download:
Fig 5.

Estimated mean-annual flow-weighted phosphorus concentrations in streams of the Upper Colorado River Basin, including concentrations attributable to A) all sources and B) wastewater treatment plant return-flow sources, and C) effects of eliminating wastewater sources.

https://doi.org/10.1371/journal.pwat.0000030.g005

thumbnail
Download:
Fig 6.

Estimated mean-annual flow-weighted phosphorus concentrations in streams of the Delaware River Basin, including concentrations attributable to A) all sources and B) wastewater treatment plant return-flow sources, and C) effects of eliminating wastewater sources.

https://doi.org/10.1371/journal.pwat.0000030.g006

thumbnail
Download:
Fig 7.

Estimated mean-annual flow-weighted phosphorus concentrations in streams of the Illinois River Basin, including concentrations attributable to A) all sources and B) wastewater treatment plant return-flow sources, and C) effects of eliminating wastewater sources.

https://doi.org/10.1371/journal.pwat.0000030.g007

Total dissolved solids

Estimated total dissolved solids concentrations in slightly more than half (52%, by length) of streams in the Upper Colorado River Basin exceed standards for both drinking water and agricultural use (Fig 8A). Such exceedances are less common at higher elevation in headwater reaches and in mainstem reaches that integrate water from throughout the basin. As might be expected, streams within which mean-annual flow-weighted total dissolved solids concentrations exceed drinking water or agricultural use criteria due specifically to irrigation return flows are largely confined to areas of irrigated agriculture (Fig 8B). Eliminating such return flows could reduce mean-annual flow-weighted concentrations from above to below drinking water or agricultural use criteria in some areas (Fig 8C).

thumbnail
Download:
Fig 8.

Estimated mean-annual flow-weighted concentrations of total dissolved solids in streams of the Upper Colorado River Basin, including concentrations attributable to A) all sources and B) return flows from irrigated agriculture, and C) effects of eliminating return flows from irrigated agriculture.

https://doi.org/10.1371/journal.pwat.0000030.g008

Discussion

Water quality and the importance of return flows

Water-quality impairments of sufficient magnitude to affect ecological or human water use are common in non-tidal surface waters in many areas. Total dissolved solids occur at concentrations affecting the suitability of water for human consumption or agricultural use in many areas of the Upper Colorado River Basin (Fig 8A), as predicted previously by Anning and Flynn [39] for a wider area of similarly arid climate across the central United States. Similarly, nitrogen and (or) phosphorus at concentrations potentially harmful to freshwater aquatic ecosystems are common in surface waters in areas where such nutrients are applied. Estimated mean-annual flow-weighted nutrient concentrations in the Upper Colorado, Delaware, and Illinois River Basins exceed available ecological criteria in most streams; exceptions are limited primarily to areas of relatively limited nutrient sources, including mostly forested portions of the Upper Colorado (Figs 2A and 5A) and Delaware (Figs 3A and 6A) River Basins and (for nitrogen) in some urban areas in the Illinois River Basin (Fig 4A). Although ecological impacts of nutrients on surface waters are complex [111] and controlled also by temperature and other conditions [35], the widespread nature of such elevated concentrations reflects surface-water impairments reported widely within the United States [19, 24, 112] and elsewhere [27, 29, 33].

The importance of contaminants from wastewater or irrigation return flows to water quality and suitability for certain uses is a function of their magnitude as well as of local climate and other contaminant sources. Equivalent contaminant loads may cause unacceptably high concentrations in arid areas but may be diluted to much lower concentrations in humid areas with more abundant precipitation [39]. Similarly, the relative importance of individual contaminant sources (such as wastewater or agricultural return flows) depends on the magnitude of other sources. In the Upper Colorado, Delaware, and Illinois River Basins, even relatively minor wastewater or irrigation return flows can be important to instream concentrations locally or downstream from where they occur, such as in small headwater streams (S1–S3 Figs) or even in the Illinois River downstream from Chicago (Fig 7B). Exceedances of water-quality standards for nutrients or dissolved solids are uncommon due to such sources, alone, in most areas (Figs 2B, 3B, 4B, 5B and 6B, 7B and 8B), however, particularly in the mainstem Colorado, Delaware, and (for nitrogen) Illinois Rivers (S1–S3 Figs). Nutrients in the Delaware and Illinois River and nutrients and dissolved solids in the Upper Colorado River are primarily attributable to non-point sources, although point sources are particularly important in certain tributaries (S1–S3 Figs). Natural erosion contributes most of the dissolved solids to the Upper Colorado River (S3 Fig). Nitrogen in the Illinois River is similarly attributable primarily to agriculture (S1 Fig), although wastewater and agriculture contribute approximately equivalent portions of phosphorus (S2 Fig). Effects of watershed size and other characteristics on the relative importance of nutrient sources in surface waters have been similarly demonstrated for other areas [43, 113].

Implications for management and restoration

Watershed management and restoration strategies intended to improve aquatic ecology by reducing wastewater effluent may be effective locally but have limited impact on regional water quality. Reductions in wastewater effluent have contributed to ecological improvements in some surface waters [8, 114, 115]. Few streams in the Upper Colorado, Delaware, or Illinois River Basins, however, contain nitrogen or phosphorus from wastewater at concentrations sufficient to individually affect stream ecology (Figs 2B, 3B, 4B, 5B, 6B and 7B). Although wastewater contributions may supplement inputs from other sources that would otherwise be insufficient to exceed ecological criteria, even eliminating wastewater return flows entirely would reduce mean-annual flow-weighted concentrations from above to below such criteria in few areas (Figs 2C, 3C, 4C, 5C, 6C and 7C). Such reductions may be important locally downstream of population centers such as Philadelphia and Chicago. Nitrogen sources, however, are primarily attributable to agriculture in the Illinois River, atmospheric deposition in the Upper Colorado River, and a combination of such non-point sources in the Delaware River (S1 Fig). Non-point sources similarly contribute the majority of phosphorus to each of these rivers, including more than 75% to the Delaware River and more than 90% to the Upper Colorado River (S2 Fig). Substantial future regional water-quality improvements in these watersheds will therefore likely depend on reductions in specifically non-point nutrient sources, as has been noted previously for other surface waters [16, 49, 116, 117].

Mitigating or reducing irrigation return flows may reduce concentrations of dissolved solids in agricultural streams in some areas but would likely have minimal impact on the suitability of water in many streams in the Colorado River Basin for agricultural or drinking-water use. Mean-annual flow-weighted concentrations of dissolved solids attributable to agricultural return flows are sufficient in some streams in agricultural areas of the Upper Colorado River Basin to individually affect water suitability for agricultural or drinking-water use and eliminating agricultural return flows may consequently reduce dissolved solids concentrations to within suitability standards in such streams (Fig 8). Natural erosion contributes the majority of dissolved solids to most streams in the watershed, however, and to the mainstem of the Colorado River (S3 Fig). Even if reducing irrigation return flows while retaining the viability of agriculture in such areas is feasible, consequential improvements in regional water quality in the Upper Colorado River Basin would likely be limited.

Watershed models developed for large areas can contribute substantially to regional water-quality management. Direct observations of water quality can be used to assess the suitability of water for different purposes at individual sampling locations [118]. Natural water quality [119], contaminant concentrations likely to adversely impact aquatic ecology [98], and legal or management criteria intended to protect aquatic biota (Table 1), however, vary substantially from place to place. The Delaware River Basin includes areas with five different nitrogen standards, for example, and phosphorus standards also vary across different States in the Upper Colorado and Illinois River Basins (Table 1). Water-quality conditions estimated consistently for unmonitored streams through regional watershed models can be useful for prioritizing and directing management and restoration investment over large areas of such diverse settings. Modeling approaches (such as SPARROW) that estimate source-specific (such as from wastewater or irrigated agriculture) as well as total loads and concentrations can be particularly useful for evaluating impacts of different sources or human activities on water availability and suitability.

Limitations and opportunities

Impacts of water use and related wastewater and irrigation return flows on the suitability and availability of water for future uses described in this paper are limited to selected water uses, landscape settings, contaminants, and availability needs. Other calibrated SPARROW models could be used for similar evaluations for suspended sediment in different areas [51–53, 59, 64, 70]. Such evaluations of the effects of return flows from industrial, livestock, aquaculture, mining, power generation, or other uses would likely demonstrate different impacts of nutrients, dissolved solids, and other contaminants of concern on different water needs in different hydrogeologic, soil, and other settings. A more comprehensive evaluation of effects of water reuse and return flows on water availability and other needs (as intended by the IWAA program [54]) will require sufficiently detailed monitoring and information on concentrations of relevant contaminants in water resources and of landscape sources, fate, and transport to support improved watershed models.

Impacts of water use and return flows described herein are also limited by the assumptions and simplifications inherent to the watershed modeling approach. The watershed models used in this evaluation were calibrated to long-term average conditions and are useful for estimating mean-annual flow-weighted concentrations that may be more or less relevant to the suitability of water for different uses. The SPARROW modeling approach has been calibrated over multiple time steps to evaluate temporal as well as spatial patterns in water quality [48] and could be similarly used to dynamically model contaminant loads and concentrations over annual, seasonal, monthly, or shorter time steps. Water-quality monitoring data collected currently for the purposes of estimating loads and trends as part of national, regional, state, and other programs [120] would likely be sufficient to calibrate such models. Explanatory data describing contaminant sources and landscape, climatic, management, and other conditions affecting fate and transport to and through surface waters, however, would be similarly needed at sufficient temporal and spatial detail. Some of these data are currently available [121, 122]; other data would need to be developed. Calibrating models at relatively detailed spatial and temporal resolution to support useful predictions would also require sufficient computational resources.

Further evaluation of the effects of return flows on water availability would benefit from comprehensive standards against which model-estimated water quality could be compared. Evaluations reported herein were based upon a variety of ecological nutrient standards applicable to different areas (Table 1), a secondary drinking-water standard [40], and an agricultural water-use standard from Utah [97]. Other water-quality standards, criteria, indices, thresholds, or references have been developed or proposed for drinking water [123, 124], recreation [125], aquatic or estuarine ecology [126–128], agriculture [129], natural waters [119], and for other uses [130, 131]. Future analyses of effects on water reuse of various contaminants from different sources would also benefit from these comprehensive standards.

Supporting information

S1 Fig. The relative contribution of different sources to nitrogen loads in selected stream reaches of the Upper Colorado (UCRB), Delaware (DRB), and Illinois (IRB) River Basins.

https://doi.org/10.1371/journal.pwat.0000030.s001

(TIF)

S2 Fig. The relative contribution of different sources to phosphorus loads in selected stream reaches of the Upper Colorado (UCRB), Delaware (DRB), and Illinois (IRB) River Basins.

https://doi.org/10.1371/journal.pwat.0000030.s002

(TIF)

S3 Fig. The relative contribution of different sources to dissolved solids loads in selected stream reaches of the Upper Colorado River Basin (UCRB).

https://doi.org/10.1371/journal.pwat.0000030.s003

(TIF)

Acknowledgments

This work was conducted as part of the Integrated Water Availability Assessment (IWAA) program of the U.S. Geological Survey. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

References

  1. 1. Mekonnen MM, Hoekstra AY. Four billion people facing severe water scarcity. Science Advances. 2016;2(2):e1500323. pmid:26933676
  2. 2. Vörösmarty CJ, McIntyre PB, Gessner MO, Dudgeon D, Prusevich A, Green P, et al. Global threats to human water security and river biodiversity. Nature. 2010;467(7315):555–61. pmid:20882010
  3. 3. Jelks HL, Walsh SJ, Burkhead NM, Contreras-Balderas S, Diaz-Pardo E, Hendrickson DA, et al. Conservation Status of Imperiled North American Freshwater and Diadromous Fishes. Fisheries. 2008;33(8):372–407.
  4. 4. Abbott BW, Bishop K, Zarnetske JP, Minaudo C, Chapin FS III, Krause S, et al. Human domination of the global water cycle absent from depictions and perceptions. Nature Geoscience. 2019;12(7):533–40.
  5. 5. Dieter CA, Maupin MA, Caldwell RR, Harris MA, Ivahnenko TI, Lovelace JK, et al. Estimated use of water in the United States in 2015. Circular. Reston, VA: U.S. Geological Survey, 2018 1441. https://doi.org/10.3133/cir1441
    • 6. Phillips PJ, Smith SG, Kolpin DW, Zaugg SD, Buxton HT, Furlong ET, et al. Pharmaceutical Formulation Facilities as Sources of Opioids and Other Pharmaceuticals to Wastewater Treatment Plant Effluents. Environmental Science & Technology. 2010;44(13):4910–6. pmid:20521847
    • 7. Nabi MM, Wang J, Meyer M, Croteau M-N, Ismail N, Baalousha M. Concentrations and size distribution of TiO2 and Ag engineered particles in five wastewater treatment plants in the United States. Science of The Total Environment. 2021;753:142017. pmid:32898809
    • 8. Eriksson H, Pastuszak M, Löfgren S, Mörth CM, Humborg C. Nitrogen budgets of the Polish agriculture 1960–2000: Implications for riverine nitrogen loads to the Baltic Sea from transitional countries. Biogeochemistry. 2007;85(2):153–68.
    • 9. Wagner RJ, Frans LM, Huffman RL. Occurrence, distribution, and transport of pesticides in agricultural irrigation-return flow from four drainage basins in the Columbia Basin Project, Washington, 2002–04, and comparison with historical data. Scientific Investigations Report. U.S. Geological Survey, 2006 2006–5005.
    • 10. Domagalski JL, Ator S, Coupe R, McCarthy K, Lampe D, Sandstrom M, et al. Comparative study of transport processes of nitrogen, phosphorus, and herbicides to streams in five agricultural basins, USA. Journal of Environmental Quality. 2008;37(3):1158–69. pmid:18453435
    • 11. Anning DW, Bauch NJ, Gerner SJ, Flynn ME, Hamlin SN, Moore SJ, et al. Dissolved solids in basin-fill aquifers and streams in the southwestern United States. Report. Reston, VA: 2007 2006–5315.
    • 12. Barry KH, Jones RR, Cantor KP, Beane Freeman LE, Wheeler DC, Baris D, et al. Ingested Nitrate and Nitrite and Bladder Cancer in Northern New England. Epidemiology (Cambridge, Mass). 2020;31(1):136–44. pmid:31577632
    • 13. Camargo JA, Alonso Á. Ecological and toxicological effects of inorganic nitrogen pollution in aquatic ecosystems: A global assessment. Environment International. 2006;32(6):831–49. pmid:16781774
    • 14. Van Grinsven HJ, Rabl A, De Kok TM. Estimation of incidence and social cost of colon cancer due to nitrate in drinking water in the EU: A tentative cost-benefit assessment. Environmental Health: A Global Access Science Source. 2010;9(1). pmid:20925911
    • 15. Hanson MJ, Keller A, Boland MA, Lazarus WF. The debate about farm nitrates and drinking water. Choices, a publication of the Agricultural and Applied Economics Association. 2016;31(1).
    • 16. Carpenter SR, Caraco NF, Correll DL, Howarth RW, Sharpley AN, Smith VH. Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological applications. 1998;8(3):559–68.
    • 17. Kemp WM, Boynton WR, Adolf JE, Boesch DF, Boicourt WC, Brush G, et al. Eutrophication of Chesapeake Bay—Historical trends and ecological interactions. Marine Ecology Progress Series. 2005;303:1–29.
    • 18. Hagy JD, Boynton WR, Keefe CW, Wood KV. Hypoxia in Chesapeake Bay, 1950–2001—Long-term change in relation to nutrient loading and river flow. Estuaries. 2004;27(4):634–58.
    • 19. Bricker S, Longstaff B, Dennison W, Jones A, Boicourt K, Wicks C, et al. Effects of nutrient enrichment in the Nation’s estuaries—A decade of change. Coastal Ocean Program Decision Analysis Series. NOAA, 2007 26.
      • 20. Rabalais NN, Turner RE, Díaz RJ, Justić D. Global change and eutrophication of coastal waters. ICES Journal of Marine Science. 2009;66(7):1528–37.
      • 21. Diaz RJ, Rosenberg R. Spreading dead zones and consequences for marine ecosystems. Science. 2008;321(5891):926. pmid:18703733
      • 22. Correll DL. The role of phosphorus in the eutrophication of receiving waters—A review. Journal of Environmental Quality. 1998;27(2):261–6.
      • 23. Prasad MBK, Sapiano MRP, Anderson CR, Long W, Murtugudde R. Long-term variability of nutrients and chlorophyll in the Chesapeake Bay—A retrospective analysis, 1985–2008. Estuaries and Coasts. 2010;33(5):1128–43.
      • 24. Davidson EA, David MB, Galloway JN, Goodale CL, Haeuber R, Harrison JA, et al. Excess nitrogen in the U.S. environment: Trends, risks, and solutions. Report. Ecological Society of America, 2012 15.
      • 25. McDowell RW, Noble A, Pletnyakov P, Haggard BE, Mosley LM. Global mapping of freshwater nutrient enrichment and periphyton growth potential. Scientific Reports. 2020;10(1). pmid:32107412
      • 26. Mudaly L, van der Laan M. Interactions between irrigated agriculture and surfacewater quality with a focus on phosphate and nitrate in the middle olifants catchment, South Africa. Sustainability (Switzerland). 2020;12(11).
      • 27. Ajeagah GA, Abanda WVB, Nkeng GE. An application of a water assessment and simulation model in the remediation of the eutrophication capacity of a tropical water system: Case study the Lake Obili in Yaounde (Cameroon). Journal of Water and Land Development. 2017;33(1):11–22.
      • 28. Strokal M, Kroeze C, Wang M, Ma L. Reducing future river export of nutrients to coastal waters of China in optimistic scenarios. Science of The Total Environment. 2017;579:517–28. pmid:27884528
      • 29. Wang B. Cultural eutrophication in the Changjiang (Yangtze River) plume: History and perspective. Estuarine, Coastal and Shelf Science. 2006;69(3–4):471–7.
      • 30. Huang P, Trayler K, Wang B, Saeed A, Oldham CE, Busch B, et al. An integrated modelling system for water quality forecasting in an urban eutrophic estuary: The Swan-Canning Estuary virtual observatory. Journal of Marine Systems. 2019;199.
      • 31. Artioli Y, Friedrich J, Gilbert AJ, McQuatters-Gollop A, Mee LD, Vermaat JE, et al. Nutrient budgets for European seas: A measure of the effectiveness of nutrient reduction policies. Marine Pollution Bulletin. 2008;56(9):1609–17. pmid:18649896
      • 32. Pȩdziński J, Witak M. Evidence of cultural eutrophication of the Gulf of Gdańsk based on diatom analysis. Oceanological and Hydrobiological Studies. 2019;48(3):247–61.
      • 33. Voss M, Dippner JW, Humborg C, Hürdler J, Korth F, Neumann T, et al. History and scenarios of future development of Baltic Sea eutrophication. Estuarine, Coastal and Shelf Science. 2011;92(3):307–22.
      • 34. Rosenberg R. Eutrophication-The future marine coastal nuisance? Marine Pollution Bulletin. 1985;16(6):227–31.
      • 35. Beretta-Blanco A, Carrasco-Letelier L. Relevant factors in the eutrophication of the Uruguay River and the Río Negro. Science of the Total Environment. 2021;761. pmid:33229089
      • 36. U.S. Bureau of Reclamation. Quality of water, Colorado River Basin—Progress Report No. 24 2013.
      • 37. Kaushal SS, Groffman PM, Likens GE, Belt KT, Stack WP, Kelly VR, et al. Increased salinization of fresh water in the northeastern United States. Proceedings of the National Academy of Sciences of the United States of America. 2005;102(38):13517–20. pmid:16157871
      • 38. Ramakrishna DM, Viraraghavan T. Environmental Impact of Chemical Deicers–A Review. Water, Air, and Soil Pollution. 2005;166(1):49–63.
      • 39. Anning DW, Flynn ME. Dissolved-solids sources, loads, yields, and concentrations in streams of the conterminous United States. Scientific Investigations Report. U.S. Geological Survey, 2014 2014–5012.
      • 40. Environmental Protection Agency U.S. Secondary drinking water standards: Guidance for nuisance chemicals 2021 [March 1, 2021]. Available from: https://www.epa.gov/sdwa/secondary-drinking-water-standards-guidance-nuisance-chemicals.
      • 41. van Puijenbroek PJTM, Beusen AHW, Bouwman AF. Global nitrogen and phosphorus in urban waste water based on the Shared Socio-economic pathways. Journal of Environmental Management. 2019;231:446–56. pmid:30368155
      • 42. Colorado River Basin Salinity Control Forum. 2020 Review—Water Quality Standards for Salinity—Colorado River Basin. 2020.
      • 43. Boyer EW, Goodale CL, Jaworski NA, Howarth RW. Anthropogenic nitrogen sources and relationships to riverine nitrogen export in the northeastern U.S.A. Biogeochemistry. 2002;57:137–69.
      • 44. Howarth RW, Sharpley A, Walker D. Sources of nutrient pollution to coastal waters in the United States—Implications for achieving coastal water quality goals. Estuaries. 2002;25(4 B):656–76.
      • 45. Linker LC, Dennis R, Shenk GW, Batiuk RA, Grimm J, Wang P. Computing atmospheric nutrient loads to the Chesapeake Bay watershed and tidal waters. Journal of the American Water Resources Association. 2013;49(5):1025–41.
      • 46. Smith RA, Schwarz GE, Alexander RB. Regional interpretation of water-quality monitoring data. Water Resources Research. 1997;33(12):2781–98.
      • 47. Miller MP, Buto SG, Lambert PM, Rumsey CA. Enhanced and updated spatially referenced statistical assessment of dissolved-solids load sources and transport in streams of the Upper Colorado River Basin. Report. Reston, VA: 2017 2017–5009.
      • 48. Ator SW, García AM, Schwarz GE, Blomquist JD, Sekellick AJ. Toward explaining nitrogen and phosphorus trends in Chesapeake Bay tributaries, 1992–2012. Journal of the American Water Resources Association. 2019;55(5):1149–68.
      • 49. Ator SW, Blomquist JD, Webber JS, Chanat JG. Factors driving nutrient trends in streams of the Chesapeake Bay watershed. Journal of Environmental Quality. 2020(49):812–34. pmid:33016477
      • 50. Chanat JG, Yang G. Exploring drivers of regional water-quality change using differential spatially referenced regression—A pilot study in the Chesapeake Bay watershed. Water Resources Research. 2018;54(10):8120–45.
      • 51. Wise DR, Anning DW, Miller OL. Spatially referenced models of streamflow and nitrogen, phosphorus, and suspended-sediment transport in streams of the southwestern United States. Report. Reston, VA: 2019 2019–5106.
      • 52. Robertson DM, Saad DA. Spatially referenced models of streamflow and nitrogen, phosphorus, and suspended sediment loads in streams of the midwestern United States. Scientific Investigations Report. U.S. Geological Survey, 2019 2019–5114.
      • 53. Ator SW. Spatially referenced models of streamflow and nitrogen, phosphorus, and suspended-sediment loads in streams of the Northeastern United States. Scientific Investigations Report. USGS, 2019 2019–5118.
      • 54. Miller MP, Clark BR, Eberts SM, Lambert PM, Toccalino P. Water priorities for the Nation—U.S. Geological Survey Integrated Water Availability Assessments. Fact Sheet. U.S. Geological Survey, 2020 2020–3044.
      • 55. Schwarz GE, Hoos AB, Alexander RB, Smith RA. Section 3. The SPARROW surface water-quality model: Theory, application and user documentation. Techniques and Methods. USGS, 2006 6-B3.
      • 56. Alam MJ, Goodall JL, Bowes BD, Girvetz EH. The impact of projected climate change scenarios on nitrogen yield at a regional scale for the contiguous United States. JAWRA Journal of the American Water Resources Association. 2017;53(4):854–70.
      • 57. Alexander RB, Smith RA, Schwarz GE, Boyer EW, Nolan JV, Brakebill JW. Differences in phosphorus and nitrogen delivery to the Gulf of Mexico from the Mississippi River Basin. Environmental Science & Technology. 2008;42(3):822–30. pmid:18323108
      • 58. Ator SW, Brakebill JW, Blomquist JD. Sources, fate, and transport of nitrogen and phosphorus in the Chesapeake Bay watershed—An empirical model. Scientific Investigations Report. USGS, 2011 2011–5167.
      • 59. Brakebill JW, Ator SW, Schwarz GE. Sources of suspended-sediment flux in streams of the Chesapeake Bay watershed—A regional application of the SPARROW model. Journal of the American Water Resources Association. 2010;46(4):757–76.
      • 60. Brown JB. Application of the SPARROW watershed model to describe nutrient sources and transport in the Missouri River Basin. Fact Sheet. U.S. Geological Survey, 2011 2011–3104.
      • 61. Domagalski J, Saleh D. Sources and transport of phosphorus to rivers in California and adjacent states, U.S., as determined by SPARROW modeling. Journal of the American Water Resources Association. 2015;51(6):1,463–1,86.
      • 62. García AM, Hoos AB, Terziotti S. A Regional Modeling Framework of Phosphorus Sources and Transport in Streams of the Southeastern United States. Journal of the American Water Resources Association. 2011;47(5):991–1010. pmid:22457579
      • 63. Hoos AB, Moore RB, García AM, Noe GB, Terziotti SE, Johnston CM, et al. Simulating stream transport of nutrients in the eastern United States, 2002, using a spatially-referenced regression model and 1:100,000-scale hydrography. Scientific Investigations Report. USGS, 2013 2013–5102.
      • 64. Hoos AB, Roland VLI. Spatially referenced models of streamflow and nitrogen, phosphorus, and suspended sediment loads in streams of the Southeastern United States. Scientific Investigations Report. U.S. Geological Survey, 2019 2019–5135.
      • 65. Moore RB, Johnston CM, Smith RA, Milstead B. Source and delivery of nutrients to receiving waters in the Northeastern and Mid-Atlantic regions of the United States. Journal of the American Water Resources Association. 2011;47(5):965–90. pmid:22457578
      • 66. Preston SD, Brakebill JW. Application of spatially referenced regression modeling for the evaluation of total nitrogen loading in the Chesapeake Bay watershed. Water-Resources Investigations Report. USGS, 1999 99–4054.
      • 67. Roberts AD, Prince SD. Effects of urban and non-urban land cover on nitrogen and phosphorus runoff to Chesapeake Bay. Ecological Indicators. 2010;10(2):459–74.
      • 68. Robertson DM, Saad DA. Nutrient Inputs to the Laurentian Great Lakes by Source and Watershed Estimated Using SPARROW Watershed Models. Journal of the American Water Resources Association. 2011;47(5):1011–33. pmid:22457580
      • 69. Wise DR, Johnson HM. Surface-Water Nutrient Conditions and Sources in the United States Pacific Northwest. Journal of the American Water Resources Association. 2011;47(5):1110–35. pmid:22457584
      • 70. Wise DR. Spatially referenced models of streamflow and nitrogen, phosphorus, and suspended-sediment loads in streams of the Pacific region of the United States. Report. Reston, VA: 2019 2019–5112.
      • 71. Miller MP, de Souza ML, Alexander RB, Sanisaca LG, Teixeira AD, Appling AP. Application of the RSPARROW Modeling Tool to Estimate Total Nitrogen Sources to Streams and Evaluate Source Reduction Management Scenarios in the Grande River Basin, Brazil. Water. 2020;12(10).
      • 72. Elliot AH, Alexander RB, Schwarz GE, Shankar U, Sukias JPS, McBride GB. Estimation of nutrient sources and transport for New Zealand using the hybrid mechanistic-statistical model SPARROW. Journal of Hydrology New Zealand. 2005;44(1):1–27.
      • 73. Li X, Cao FF, Chen XC, Wang ZJ, Wang YQ. Spatial source apportionment analysis of target pollutant for sensitive area—A case study in Xin’anjiang River Basin for interprovincial assessment section. Zhongguo Huanjing Kexue/China Environmental Science. 2013;33(9):1714–20.
      • 74. Morales-Marín L, Wheater H, Lindenschmidt KE. Potential changes of annual-averaged nutrient export in the South Saskatchewan River Basin under climate and land-use change scenarios. Water (Switzerland). 2018;10(10).
      • 75. Shih J-S, Alexander RB, Smith RA, Boyer EW, Shwarz GE, Chung S. An initial SPARROW model of land use and in-stream controls on total organic carbon in streams of the conterminous United States. Open-File Report. U.S. Geological Survey, 2010 2010–1276.
      • 76. Roberts AD, Prince SD, Jantz CA, Goetz SJ. Effects of projected future urban land cover on nitrogen and phosphorus runoff to Chesapeake Bay. Ecological Engineering. 2009;35(12):1758–72.
      • 77. Miller OL, Putman AL, Alder J, Miller M, Jones DK, Wise DR. Changing climate drives future streamflow declines and challenges in meeting water demand across the southwestern United States. Journal of Hydrology X. 2021;11.
      • 78. Miller MP, Capel PD, García AM, Ator SW. Response of Nitrogen Loading to the Chesapeake Bay to Source Reduction and Land Use Change Scenarios: A SPARROW‐Informed Analysis. Journal of the American Water Resources Association. 2019;56(1):100–12.
      • 79. Robertson DM, Schwarz GE, Saad DA, Alexander RB. Incorporating uncertainty into the ranking of SPARROW model nutrient yields from Mississippi/Atchafalaya River basin watersheds. Journal of the American Water Resources Association. 2009;45(2):534–49. pmid:22457567
      • 80. Alam MJ, Goodall JL. Toward disentangling the effect of hydrologic and nitrogen source changes from 1992 to 2001 on incremental nitrogen yield in the contiguous United States. Water Resour Res. 2012;48(4):W04506.
      • 81. Ator SW, García AM. Application of SPARROW modeling to understanding contaminant fate and transport from uplands to streams. Journal of the American Water Resources Association. 2016;52(3):685–704.
      • 82. Hoos AB, McMahon G. Spatial analysis of instream nitrogen loads and factors controlling nitrogen delivery to streams in the southeastern United States using spatially referenced regression on watershed attributes (SPARROW) and regional classification frameworks. Hydrological Processes. 2009;23(16):2275–94.
      • 83. García AM, Alexander RB, Arnold JG, Norfleet L, White MJ, Robertson DM, et al. Regional effects of agricultural conservation practices on nutrient transport in the Upper Mississippi River basin. Environmental Science & Technology. 2016;50:6991–7000. pmid:27243625
      • 84. Miller MP, Buto SG, Susong DD, Rumsey CA. The importance of base flow in sustaining surface water flow in the Upper Colorado River Basin. Water Resources Research. 2016;52(5):3547–62.
      • 85. Miller OL, Wise DR, Anning DW. SPARROW model inputs and simulated streamflow, nutrient and suspended-sediment loads in streams of the Southwestern United States, 2012 Base Year (version 2.0, October 2020). data release. U.S. Geological Survey, 2020.
      • 86. Ator SW. SPARROW model inputs and simulated streamflow, nutrient and suspended sediment loads in streams of the Northeastern United States, 2012 base year. Data Release. U.S. Geological Survey, 2020.
      • 87. Saad DA, Robertson DM. SPARROW model inputs and simulated streamflow, nutrient and suspended-sediment loads in streams of the Midwestern United States, 2012 Base Year. data release. U.S. Geological Survey, 2020.
      • 88. Saad DA, Schwarz GE, Argue DM, Anning DW, Ator SW, Hoos AB, et al. Estimates of long-term mean daily streamflow and annual nutrient and suspended-sediment loads considered for use in regional SPARROW models of the conterminous United States, 2012 base year. Scientific Investigations Report. U.S. Geological Survey, 2019 2019–5069.
      • 89. Brakebill JW, Schwarz GE, Wieczorek ME. An enhanced hydrologic stream network based on the NHDPlus medium resolution data set. Scientific Investigations Report. U.S. Geological Survey, 2019 2019–5127.
      • 90. Systems Horizon. NHDPlus Home: Horizon Systems; 2013 [March 18, 2013]. Available from: http://horizon-systems.com/nhdplus/.
      • 91. Schwarz GE. E2NHDPlusV2_us: Database of Ancillary Hydrologic Attributes and Modified Routing for NHDPlus Version 2.1 Flowlines. Data release. U.S. Geological Survey, 2019.
      • 92. Schwarz GE, Wieczorek ME. Database of modified routing for NHDPlus version 2.1 flowlines: ENHDPlusV2_us. data release. U.S. Geological Survey, 2018.
      • 93. U.S. Environmental Protection Agency. NHDPlus (National Hydrography Dataset Plus) 2017 [October 1, 2019]. Available from: https://19january2017snapshot.epa.gov/waterdata/nhdplus-national-hydrography-dataset-plus_.html.
        • 94. Miller MP, Buto SG, Lambert PM, Rumsey CA. SPARROW model input datasets and predictions of total dissolved solids loads in streams of the Upper Colorado River Basin watershed. Data Release. U.S. Geological Survey, 2022.
        • 95. Tillman FD, Anning DW. Updated estimates of long-term average dissolved-solids loading in streams and rivers of the Upper Colorado River Basin. Report. Reston, VA: 2014 2014–1148.
        • 96. Kenney TA, Gerner SJ, Buto SG, Spangler LE. Spatially referenced statistical assessment of dissolved-solids load sources and transport in streams of the Upper Colorado River Basin. Report. Reston, VA: 2009 2009–5007.
        • 97. State of Utah. R317. Environmental Quality, Water Quality. Rule R317-2. Standards of Quality for Waters of the State, 2021 104(5).
        • 98. U.S. Environmental Protection Agency. Nutrient criteria technical guidance manual—Rivers and streams. Report. U.S. Environmental Protection Agency, 2000 EPA-822-B-00_002.
        • 99. Hirsch RM, Moyer DL, Archfield SA. Weighted regressions on time, discharge, and season (WRTDS), with an application to Chesapeake Bay river inputs. Journal of the American Water Resources Association. 2010;46(5):857–80. pmid:22457569
        • 100. U.S. Environmental Protection Agency. Ambient water quality criteria recommendations—Information supporting the development of State and Tribal nutrient criteria—Rivers and streams in Nutrient Ecoregion III. Report. U.S. Environmental Protection Agency, 2000 EPA 822-B-00-016.
        • 101. U.S. Environmental Protection Agency. Ambient water quality criteria recommendations—Information supporting the development of State and Tribal nutrient criteria—Rivers and streams in Nutrient Ecoregion II. Report. U.S. Environmental Protection Agency, 2000 EPA 822-B-00-015.
        • 102. U.S. Environmental Protection Agency. Ambient water quality criteria recommendations—Information supporting the development of State and Tribal nutrient criteria—Rivers and streams in Nutrient Ecoregion VIII. Report. U.S. Environmental Protection Agency, 2001 EPA 822-B-01-015.
        • 103. U.S. Environmental Protection Agency. Ambient water quality criteria recommendations—Information supporting the development of State and Tribal nutrient criteria—Rivers and streams in Nutrient Ecoregion IX. Publication. U.S. Environmental Protection Agency, 2000 EPA 822-B-00-019.
        • 104. U.S. Environmental Protection Agency. Ambient water quality criteria recommendations—Information supporting the development of State and Tribal nutrient criteria—Rivers and streams in Nutrient Ecoregion XI. Report. U.S. Environmental Protection Agency, 2000 EPA 822-B-00-020.
        • 105. U.S. Environmental Protection Agency. Ambient water quality criteria recommendations—Information supporting the development of State and Tribal nutrient criteria—Rivers and streams in Nutrient Ecoregion XIV. Report. U.S. Environmental Protection Agency, 2000 EPA 822-B-00-022.
        • 106. U.S. Environmental Protection Agency. Ambient water quality criteria recommendations—Information supporting the development of State and Tribal nutrient criteria—Rivers and streams in Nutrient Ecoregion VII. Report. U.S. Environmental Protection Agency, 2000 EPA 822-B-00-018.
        • 107. U.S. Environmental Protection Agency. Ambient water quality criteria recommendations—Information supporting the development of State and Tribal nutrient criteria—Rivers and streams in Nutrient Ecoregion VI. Report. U.S. Environmental Protection Agency, 2000 EPA 822-B-00-017.
        • 108. Illinois Nutrient Science Advisory Committee. Recommendations for numeric nutrient criteria and eutrophication standards for Illinois streams and rivers. 2018.
        • 109. Baumann J, Weigel B, Callis A, Billings C, Binder M, Genskow K, et al. Wisconsin’s Nutrient Reduction Strategy. Publication Tracking EGAD. Wisconsin Department of Natural Resources, 2013 3800-3200-2013-05.
        • 110. Wisconsin State Legislature. Chapter NR 102—Water quality standards for Wisconsin surface waters. Register May 2020. 2020 No. 773.
        • 111. Suter EA, Lwiza KMM, Rose JM, Gobler C, Taylor GT. Phytoplankton assemblage changes during decadal decreases in nitrogen loadings to the urbanized Long Island Sound estuary, USA. Marine Ecology Progress Series. 2014;497:51–67.
        • 112. Fisher TR, Hagy JD III, Boynton WR, Williams MR. Cultural eutrophication in the Choptank and Patuxent estuaries of Chesapeake Bay. Limnology and Oceanography. 2006;51(1 II):435–47.
        • 113. Howarth R, Billen G, Swaney D, Townsend A, Jaworski N, Lajtha K, et al. Regional nitrogen budgets and riverine N & P fluxes for the drainages to the North Atlantic Ocean—Natural and human influences. Biogeochemistry. 1996;35(1):75–139.
        • 114. Ruhl HA, Rybicki NB. Long-term reductions in anthropogenic nutrients link to improvements in Chesapeake Bay habitat. Proceedings of the National Academy of Sciences of the United States of America. 2010;107(38):16566–70. pmid:20823243
        • 115. Boynton WR, Hodgkins CLS, O’Leary CA, Bailey EM, Bayard AR, Wainger LA. Multi-decade responses of a tidal creek system to nutrient load reductions—Mattawoman Creek, Maryland USA. Estuaries and Coasts. 2013;37(S1):111–27.
        • 116. Gakstatter JH, Bartsch AF, Callahan CA. The impact of broadly applied effluent phosphorus standards on eutrophication control. Water Resources Research. 1978;14(6):1155–8.
        • 117. Chapra S, Robertson A. Great Lakes Eutrophication: The Effect of Point Source Control of Total Phosphorus. Science (New York, NY). 1977;196:1448–50. pmid:17776924
        • 118. Qishlaqi A, Kordian S, Parsaie A. Hydrochemical evaluation of river water quality—a case study. Applied Water Science. 2017;7(5):2337–42.
        • 119. Smith RA, Alexander RB, Schwarz GE. Natural background concentrations of nutrients in streams and rivers of the conterminous United States. Environmental Science and Technology. 2003;37(14):3039–47. pmid:12901648
        • 120. Oelsner GP, Sprague LA, Murphy JC, Zuellig RE, Johnson HM, Ryberg KR, et al. Water-quality trends in the nation’s rivers and streams, 1972–2012—Data preparation, statistical methods, and trend results. Scientific Investigations Report. U.S. Geological Survey, 2017 2017–5006.
        • 121. National Atmospheric Deposition Program. Annual data summaries 2021 [November 29, 2021]. Available from: https://nadp.slh.wisc.edu/pubs/Annual-Data-Summaries/.
          • 122. Falcone JA. U.S. conterminous wall-to-wall anthropogenic land use trends (NWALT), 1974–2012. Data Series. USGS, 2015 948.
          • 123. Toccalino PL. Development and Application of Health-Based Screening Levels for Use in Water-Quality Assessments. Scientific Investigations Report. U.S. Geological Survey, 2007 2007–5106.
          • 124. World Health Organization. A global overview of national regulations and standards for drinking-water quality. 2018.
          • 125. World Health Organization. Guidelines on recreational water quality—Volume 1 Coastal and fresh waters 2021.
          • 126. U.S. Environmental Protection Agency. Ambient water quality criteria for dissolved oxygen, water clarity, and chlorophyll a for the Chesapeake Bay and its tidal tributaries. U.S. Environmental Protection Agency, 2003 EPA 903-R-03-002.
          • 127. U.S. Environmental Protection Agency. National Coastal Condition Report III. Report. U.S. Environmental Protection Agency, 2008 EPA/842-R-08-002.
          • 128. U.S. Environmental Protection Agency. National Lakes Assessment: A collaborative survey of the Nation’s lakes. Office of Water and Office of Research and Development, 2009 EPA 841-R-09-001.
          • 129. Ayers RS, Westcot DW. Water quality for agriculture. Irrigation and Drainage Paper. Food and Agriculture Organization of the United Nations, 1985 29 Rev. 1.
          • 130. National Institutes of Health. Laboratory water—Its importance and application. 2013.
          • 131. U.S. Environmental Protection Agency. Water quality standards handbook—Chapter 3—Water quality criteria. Report. Office of Water, 2017 823 B 17 001