water Article Impact of Global Warming on Dissolved Oxygen and BOD Assimilative Capacity of the World’s Rivers: Modeling Analysis Steven C. Chapra 1, * , Luis A. Camacho 2 and Graham B. McBride 3   Citation: Chapra, S.C.; Camacho, L.A.; McBride, G.B. Impact of Global Warming on Dissolved Oxygen and BOD Assimilative Capacity of the World’s Rivers: Modeling Analysis. Water 2021, 13, 2408. https:// doi.org/10.3390/w13172408 Academic Editor: Chin-Pao Huang Received: 27 May 2021 Accepted: 26 August 2021 Published: 1 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Civil and Environmental Engineering, Tufts University, Medford, MA 02155, USA 2 Environmental Engineering Research Center (CIIA), Civil and Environmental Engineering Department, Universidad de los Andes, Bogotá 111111, Colombia; la.camacho@uniandes.edu.co 3 National Institute of Water and Atmospheric Research (NIWA), P.O. Box 11-115, Hamilton 3251, New Zealand; graham.mcbride@niwa.co.nz * Correspondence: steven.chapra@tufts.edu Abstract: For rivers and streams, the impact of rising water temperature on biochemical oxygen demand (BOD) assimilative capacity depends on the interplay of two independent factors: the waterbody’s dissolved oxygen (DO) saturation and its self-purification rate (i.e., the balance between BOD oxidation and reaeration). Although both processes increase with rising water temperatures, oxygen depletion due to BOD oxidation increases faster than reaeration. The net result is that rising temperatures will decrease the ability of the world’s natural waters to assimilate oxygen- demanding wastes beyond the damage due to reduced saturation alone. This effect should be worse for nitrogenous BOD than for carbonaceous BOD because of the former’s higher sensitivity to rising water temperatures. Focusing on streams and rivers, the classic Streeter–Phelps model was used to determine the magnitude of the maximum or “critical” DO deficit that can be calculated analytically as a function of the mixing-point BOD concentration, DO saturation, and the self-purification rate. The results indicate that high-velocity streams will be the most sensitive to rising temperatures. This is significant because such systems typically occur in mountainous regions where they are also subject to lower oxygen saturation due to decreased oxygen partial pressure. Further, they are dominated by salmonids and other cold-water fish that require higher oxygen levels than warm-water species. Due to their high reaeration rates, such systems typically exhibit high self-purification constants and consequently have higher assimilation capacities than slower moving lowland rivers. For slow- moving rivers, the total sustainable mixing-point concentration for CBOD is primarily dictated by saturation reductions. For faster flowing streams, the sensitivity of the total sustainable load is more equally dependent on temperature-induced reductions in both saturation and self-purification. Keywords: streams; water quality; climate change; saturation; oxygen metabolism; sustainability 1. Introduction Physicians monitor vital signs, such as body temperature, heart rate, and blood pressure, as baseline indicators of a patient’s health status. Because of its relevance to wastewater assimilation, aquatic life, taste and odor problems, and sediment–water inter- actions, dissolved oxygen (DO) concentration has been, and still is, the best “vital sign” of a waterbody’s ecosystem health. Hence, assessing how climate change might affect the oxygen content of the world’s surface waters is a critical question related to future water quality in a warming climate. In the present paper, the classic Streeter–Phelps model [1] and the concept of sustain- able assimilative capacity are used to address this question broadly and generally. After examining the direct effect of rising water temperatures on oxygen saturation, the analysis is extended to evaluate the influence of rising water temperatures on a river’s ability to break down oxygen-depleting pollutants (BOD). In addition to lowering saturation, rising water temperatures will decrease a river’s assimilative capacity by influencing its oxygen Water 2021, 13, 2408. https://doi.org/10.3390/w13172408 https://www.mdpi.com/journal/water