On the last day of my first economics course I took back in 1985, the professor wrote on the board three things he hoped we’d remember after our final exam (and the pitcher of beer that followed)– incentives work, assumptions matter, and there are opportunity costs (i.e., trade-offs) with every decision we make. These opportunity costs are often unintended, which is largely the basis for the formation of the field of environmental economics in explaining the pervasiveness of externalities and pollution in society, which goes back to the seminal work of Ayres and Kneese (1969). Another valuable nugget from Ayres and Kneese was their emphasis on considering a materials and energy balance approach in their economic models. As my career moved more into water-related research and equipped with their insight, it was natural to consider the interconnectedness of our water system when thinking about policy and how impacts in one time and place can have impacts in another time and place.
The opportunity to apply this thinking and framework to the problem of water conservation and municipal wastewater reuse, and the genesis of this article (Unintended Consequences of Water Conservation on the Use of Treated Municipal Wastewater) came about when David Jassby, then an Assistant Professor of Chemical and Environmental Engineering, called me up to discuss co-advising one of his engineering students, Dr. Quyhn Tran, in considering the broader implications of their research on wastewater treatment, specifically designing wastewater treatment systems to better mitigate the impacts of drought. Through the initial research (Tran et al. 2016; Tran et al. 2017), we built economic models of wastewater treatment systems and illustrated different strategies to both lower the costs of wastewater treatment during drought and increase the value of that treatment to end-users of the effluent (e.g., irrigated agriculture). While this initial research focused on a single plant, the question arose as to whether what we were observing at this single plant during the drought – lower flows and higher levels of salinity – were pervasive throughout California. This question was important since the State of California was requiring water agencies, and thus their customers, to reduce overall water use statewide by around 25%, and agencies were pushing conservation—both indoor and outdoor—to meet those targets. At the same time, California, along with other states and countries, was making significant investments in wastewater treatment infrastructure to treat and recycle municipal wastewater to augment water supplies since it was considered a reliable local source. Indeed, I heard numerous times something to the effect that it never stops raining at a wastewater treatment plant. While we didn’t disagree with this in general, we felt that perhaps—depending on the types of conservation employed—the rain might become a ‘drizzle,’ and the quality of the drizzle might diminish.
With the additional efforts of a Masters of Public Policy student, Refat Amin, and another economist with excellent quantitative skills, Dr. Mehdi Nemati, we collected and analyzed monthly data on effluent flows and influent/effluent salinity from over 100 WWTPs throughout California, controlling for other factors such as source water salinity, WWTPs characteristics, and water conservation efforts. This current paper focuses on around 34 WWTPs in Southern California, an area that also has numerous effluent dominated streams of significant ecological importance and which may be significantly impacted by the loss of such flows or increases in flow salinity. We feel the final product, similar to a number of other papers in Nature Sustainability (Di Baldassarre et al. 2018) and elsewhere, emphasizes the importance of recognizing the interconnected, dynamic, and complex nature of our water system to developing informed, effective, and sustainable water policies.