**1. Introduction**

Estuaries, which provide freshwater for drinking water consumption and agricultural production, have historically served as outstanding locations for human communities. Estuaries provide access to both rivers and oceans, thereby enhancing opportunities for trade and communication [1]. Because they are highly productive, estuaries have also been an important food source for human habitation [2]. In fact, the earliest civilizations in the world developed around estuaries. Many modern cities have grown near estuaries, including Jakarta, New York City, and Tokyo. Of the 32 largest cities in the world in the early 1990s, 22 were located on estuaries [3].

Estuaries, by definition, exhibit a water quality spectrum between seawater and riverine that varies with freshwater inflows and geometry [1]. The riverine ends of estuaries are often used as drinking water resources for the communities that have grown around them. However, because of the proximity to population centers, these waters tend to exhibit high concentrations of nutrients, pathogens, and other contaminants (in addition

**Citation:** Hutton, P.H.; Roy, S.B.; Krasner, S.W.; Palencia, L. The Municipal Water Quality Investigations Program: A Retrospective Overview of the Program's First Three Decades. *Water* **2022**, *14*, 3426. https://doi.org/ 10.3390/w14213426

Academic Editors: Nigel W.T. Quinn, Ariel Dinar, Iddo Kan and Vamsi Krishna Sridharan

Received: 10 September 2022 Accepted: 22 October 2022 Published: 28 October 2022

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**Copyright:** © 2022 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/).

to ocean-derived salts and organic matter from contributing watersheds), which create a challenge to drinking water suppliers that treat such waters for human consumption.

The San Francisco Estuary, including the delta formed by the Sacramento and San Joaquin Rivers (hereafter Delta) (Figure 1), plays a major role in California's prosperity and is shaped by the large population centers that have developed around it. In addition to serving as an important habitat to more than 750 animal and plant species [4], the Delta is the largest single water supply source in California and its waters are transported across river basin boundaries to support major urban and agricultural centers in the state. To accomplish these inter-basin transfers, large water projects were constructed in the 1940s by the federal government (i.e., the Central Valley Project or CVP) and in the 1960s by the State of California (i.e., the State Water Project or SWP) [5]. These projects consist of a network of dams in upper elevations of the Delta watershed, combined with aqueducts and pump stations for long-range conveyance. The CVP generally serves agricultural water users, whereas the SWP is primarily devoted to municipal supply (approximately 70%). The largest source of SWP water is the Feather River, which is impounded by Lake Oroville [5]. SWP water, exported from the Delta via the California, North Bay, and South Bay Aqueducts for irrigation and municipal use [6], is allocated through longterm contracts to 29 water agencies (termed "contractors") who are responsible for water delivery to communities and irrigation districts within their jurisdiction [5]. Allocations are developed by the California Department of Water Resources (CDWR) early in each water year (which begins on 1 October) and updated as additional hydrologic information becomes available. Releases from Lake Oroville are managed by CDWR for water exports and to meet various water quality standards in the Delta, notably standards mandated to limit the intrusion of saline water [7]. The SWP is one of the largest water conveyance systems in the world, with an average of 2.9 million acre-feet of water delivered annually in the decade ending in 2016 [5].

Collectively, Delta exports from the SWP and CVP support a \$32 billion agricultural industry and serve as an important source of drinking water to almost 27 million residents [4]. Saltwater intrusion was one of the earliest water quality concerns for human uses of Delta water. In 1920, the City of Antioch sued upstream irrigators to protect the city's intake from salinity intrusion [8,9]. In response to this lawsuit, the State of California implemented a monitoring program and published the first authoritative review of Delta salinity and its control in 1931 [10]. Early plans envisaged control of Delta salinity by means of storage regulation on the Sacramento River [10] and a saltwater barrier in the estuary near Carquinez Strait (see Figure 1) [8,11]. Later, as part of the SWP planning process, the California Water Plan [12] and subsequent investigations considered re-routing low salinity Sacramento River flows through and around the Delta (see additional discussion in [8]).

Water quality concerns broadened over time to include a suite of chemical constituents related to natural and anthropogenic sources in the Delta and its watershed. Water quality management in the Delta occurs through a complex framework of federal and state laws, such as the Clean Water Act (1973), the state Porter-Cologne Water Quality Control Act (1969), and the Safe Drinking Water Act (1974), with the goals of supporting beneficial uses for ecosystems, municipal use, and agricultural use. Here, we focus on a program that has evolved to track a subset of constituents in Delta waters that are of concern from the standpoint of drinking water supply.

Since the late 1970s and early 1980s, Delta water quality concerns have included natural organic matter (NOM) [13] and bromide [14]. NOM and bromide are persistent in Delta waters due to agricultural return flows from the region's organic peat soils and seawater intrusion, respectively. NOM promotes the formation of trihalomethanes (THMs) and other carcinogenic disinfection by-products (DBPs) when Delta waters are chlorinated during drinking water treatment [15,16]. In the presence of bromide, brominated DBPs are also formed [17]. Bromine-containing DBPs are of greater health concern than their chlorine-containing analogs [18]. In addition, the estuary receives wastewater discharges

from more than 9 million people along its periphery and other pollutant loads (including pesticides, herbicides, and nutrients) from the developed watersheds upstream [19,20].

**Figure 1.** Map of study area identifying MWQI routine discrete and real time monitoring locations (noted as RTDF stations, for real time data forecasting).

Given this confluence of factors, water quality in the estuary related to constituents of drinking water interest has been extensively studied (e.g., [13,15,21–24]). Among all state and local agencies monitoring water quality in the Delta and its tributaries, CDWR's Municipal Water Quality Investigations (MWQI) program is the most extensive and cohesive program established to investigate the quality of Delta source water with respect to its suitability for production of drinking water. MWQI program elements have evolved over three decades in response to advances in science, water treatment technology and regulations, and emerging contaminants, paralleling major investments made by local water supply agencies (Figure 2).

**Figure 2.** Timeline showing the evolution of drinking water regulations, MWQI program activities, and ozone implementation by participating MWQI water contractors.

The primary objective of this paper is to provide a retrospective overview of CDWR's MWQI program, highlighting its evolution in response to changing understanding of Delta water quality water constituents, regulatory drivers, and new technologies. It is not our intention to provide a subjective critique of the program's perceived utility, benefits, successes, and/or failures. This retrospective builds on a prior summary of the program's history [25] with a focus on program evolution and its utility to SWP operations, drinking water quality regulators, and organizations responsible for municipal water supply. Here, we describe the program, including purpose, organization and funding, and key elements. This description is followed by a historical account of the program origin, which can be traced back to federal and state regulatory activities in the 1970s and early studies by CDWR and other state and local agencies in the 1980s. We then chronicle how the program evolved from its formation in 1990 to its current configuration. We conclude this paper with a discussion of key program accomplishments and future directions. This paper is focused on program-level activities, rather than specific study results or interpretation of data collected through the program. Although this paper focuses on one region, its multi-decade interplay of science, treatment and monitoring technology, and regulations (as well as practical aspects of managing such a large-scale program) are broadly relevant to professionals engaged in drinking water quality management in other urbanized and developed regions of the world.
