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Effects of return flows on stream water quality and availability in the Upper Colorado, Delaware, and Illinois River Basins [1]

['Scott W. Ator', 'U.S. Geological Survey', 'Baltimore', 'Maryland', 'United States Of America', 'Olivia L. Miller', 'Salt Lake City', 'Utah', 'David A. Saad', 'Madison']

Date: 2022-08 

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.

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.

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 ].

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.

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.

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[1] Url: https://journals.plos.org/water/article?id=10.1371/journal.pwat.0000030

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