Identifying untapped potential: a geospatial analysis of Florida and California ’ s 2009 recycled water production

Increased water demand attributed to population expansion and reduced freshwater availability caused by saltwater intrusion and drought, may lead to water shortages. These may be addressed, in part, by use of recycled water. Spatial patterns of recycled water use in Florida and California during 2009 were analyzed to detect gaps in distribution and identify potential areas for expansion. Databases of recycled water products and distribution centers for both states were developed by combining the 2008 Clean Water Needs Survey database with Florida ’ s 2009 Reuse Inventory and California ’ s 2009 Recycling Survey, respectively. Florida had over twice the number of distribution centers ( n ¼ 426) than California ( n ¼ 228) and produced a larger volume of recycled water (674.85 vs. 597.48 mgd (3.78 mL/d ¼ 1 mgd), respectively). Kernel Density Estimation shows the majority of distribution in central Florida (Orlando and Tampa), California ’ s Central Valley region (Fresno and Bakers ﬁ eld), and around major cities in California. Areas for growth were identi ﬁ ed in the panhandle and southern regions of Florida, and northern, southwestern, and coastal California. Recycled water is an essential component of integrated water management and broader adoption of recycled water will increase water conservation in water-stressed coastal communities by allocating the recycled water for purposes that once used potable freshwater.


INTRODUCTION
Freshwater scarcity has incentivized mitigation measures that restrict water use, generating novel ideas and innovative technologies to improve water management. One innovation to increase public water supplies is expansion of water reuse, which may assist in water mitigation strategies, specifically  Principles of epidemiology such as the dose-response assessment may be employed to assess the health risk associated with water reuse, as detection of a contaminant may not pose a significant health risk. Acceptance among the public has been found in early studies to be positively correlated with education, knowledge of the recycling process, and pro-environmental attitudes (Hui & Cain ).
However, whether or not decision-makers support supplementing public water supplies with recycled water depends on several factors including cost, availability of alternative water sources, social and legal factors, in addition to public sentiment (NRC ). Po et al. () described the 'yuck factor' as a psychological barrier of emotional discomfort because most people perceived recycled water as unclean with potential risk factors associated with the quality of recycled water. Participants of the study indicated they would rather recycled water be referred to as 'repurified water' (Po et al. ); the phrase 'toilet to tap' creates fear and revulsion that pathogens may remain in the water after processing (Hui & Cain ). Qian & Leong () found that the 'yuck factor' is the only statistically significant variable that prevents implementation for direct potable reuse. A review of perception by Dolnicar & Saunders () indicated that proper branding of recycled water could increase trust and security among the general public. To test how branding may influence acceptance, Hui & Cain () conducted a survey of willingness to use recycled water for ten applications ranging from lawn watering to clothes washing to drinking. They found that presenting recycled water use in a positive framework increased willingness to use. Interestingly, political affiliation was an important factor; Democrats were more willing than Republicans to use recycled water. In contrast to prior research, education was not a factor in how willing Californians were to use recycled water. A recent review of public responses to water reuse concluded that education on its own is not sufficient to change attitudes.
Rather, multidisciplinary efforts to address the 'yuck factor' from scientific, technological, and behavioral psychology perspectives, including risk perception, are needed (Smith et al. ).
In 2002, Singapore became the first country to blend recycled water with raw water in a reservoir to be used as recycled drinking water, called NeWater (Qian & Leong ). Similar efforts have been proposed in California and Florida, but public perception, not water quality, have halted these projects (Rodriguez et al. ). Currently, the use of recycled water as direct potable reuse is constrained by policy in most US regions (Qian & Leong ), however the Groundwater Replenishment System, a potable water reuse project in Orange County, California that injects recycled water directly into aquifers that supply local drinking water, has had wide public acceptance. Spatial analysis of recycled water products is not well represented in the literature. In Los Angeles, California, spatial modeling was used to optimize distribution of recycled water for groundwater recharge (Bradshaw & Luthy ). The only known spatial analytical study of recycled water is an econometric analysis of Florida's water reuse capacity from 1996 to 2012 (Kuwayama & Kamen ). In this study, water quality and scarcity were investigated at the county level. While water supply was found to be a driving factor for Florida's dedication to recycled water production, so too was water quality. Specifically, the authors noted that water quality in impaired streams may be improved by the addition of treated recycled water (due to dilution), especially during times of reduced precipitation. The authors also noted that regions with a large urban population have increased industrial activity with a corresponding increase in industrial recycled water production. Kuwayama & Kamen () recommended that an evaluation of recycled water production be completed at the facility level for further insight. The present research study fills this gap, by outlining a methodology to model the spatial pattern of recycled water production at the facility level to find gaps in distribution and identify potential areas for expansion of recycled water production as a way to increase public water supply. A case study of recycled water production in Florida and California is presented.
Since the 1940s, US water consumption has doubled due to population growth resulting in added stress to water Distribution of recycled municipal wastewater in California is ultimately controlled by nine Regional Water Quality Control Boards (Regional Water Boards, RWBs) assembled by the State Water Resources Control Board (State Water Board, SWB) ( Figure 2). RWBs monitor standards for constituents of emerging concern (CECs) (or chemicals of emerging concern that may impact the quality of recycled water) and work in conjunction with the SWB,   Kernel density estimation (KDE) was used to identify hotspots of water reuse. The Quartic Kernel was selected because its shape gradually reduces the influence of nearby points and it stops at the defined radius limit rather than extending to infinity, therefore, the area is limited around the point of incidence (Levine ). KDE was performed on flow and flow normalized by population served using 15 points per cluster. KDE is representative of the regional system in that every facility is accounted for in the model and production volume (flow) is used as an intensity variable to weight each facility, so that those with higher flows would contribute more to the KDE surface. All data were analyzed with CrimeStat IV (Levine ). The most common product associated with recycled water was public access area irrigation with a total distribution of 381.38 mgd (56% of the state total). Nearly 41% (154.56 mgd) of recycled irrigation water was supplied by POTWs to the South Florida WMD (Figure 4(a)). Groundwater recharge was the next largest recycled water product Each district produced recycled water for every category of product. The Suwannee River WMD was the lowest-producing district overall with a total production of 9.39 mgd (1.4% of the state total) and the lowest mean production at 0.34 mgd (per POTW), but was not significantly different from the other WMDs ( Figure 5). ANOVA results indicated significant differences in recycled water production between WMDs overall and Tukey post-hoc tests further indicated significant differences (p < 0.05) between recycled water Hot spots for flow (mgd) were located around major cities in Florida (Figure 6(a)). Dark areas have the greatest production, whereas light areas have lower production.

Of 548 POTWs in
Normalization was performed to remove the effect of population size. When flow data were normalized by population served, high per capita production was identified in Suwannee River WMD, followed by Orlando, Tampa, and Fort Myers  (Table 1) reveals that California has fewer facilities than Florida, but its facilities tend to be larger.
The most common discharge method associated with recycled water was agricultural irrigation with a total distribution of 218.33 mgd (37% of the state total). Nearly 62% (136.07 mgd) of recycled agriculture irrigation water was supplied by POTWs to the Central Valley RWB (Figure 8(a)).
Landscape irrigation was the next largest recycled water product in the state, with a total of 100.86 mgd (17% of the state total). Nearly 28% (29.05 mgd) of landscape irrigation water reuse was distributed by POTWs to users in the San Diego RWB (Figure 8    Angeles RWB at 12.91 mgd (46%) (Figure 8(g)). Moreover, recycled water used for recreational impoundment totaled 23.07 mgd (4% of the state total), with the largest portion distributed by POTWs to users in the Los Angeles RWB at 17.79 mgd (77%) (Figure 8(h)). Also, recycled water used for geothermal energy production totaled 13.34 mgd (2% of the state total), with the largest portion distributed by POTWs to users in the North Coast RWB 11.31 mgd (85%) (Figure 8(i)). Similarly, recycled water used for other purposes totaled 10.84 mgd (2% of the state total), with the largest portion distributed by POTWs to users in the San Diego RWB at 4.07 mgd (38%) (Figure 8(j)). Last, at the state level, commercial use totaled 5.70 mgd (1% of the state total), with the largest portion distributed by POTWs to users in the Los Angeles RWB at 4.07 mgd (83%) (Figure 8(k)).
While each district produces recycled water for each category of discharge method, Lahontan was the lowestproducing district overall with a total production of 11.07 mgd (1.9% of the state total) (Figure 9). ANOVA results indicated significant differences in recycled water production between RWBs. Tukey post-hoc tests further show significant differences (p < 0.05) between recycled water production in Santa Ana RWB (higher) and San Francisco Bay, Central Coast, Central Valley, and Lahontan RWBs (lower). The Central Coast RWB had the lowest per unit production at 0.64 mgd.
Hot spots for flow (mgd) are located throughout the Central Valley region and around major cities in California ( Figure 10(a)). Dark areas have the greatest production, whereas light areas have the least production and may be areas for increased production. Flow data were normalized by population served (Figure 10  These numbers indicate a growing acceptance of recycled water use, at least among uses that do not involve direct consumption.
One limitation of this study was lack of access to recycled water data more current than 2009 for California.
Once more recent recycled water data for California are released, we recommend reanalysis of California's recycled water production, with a view to assessing increases and decreases over time and space, especially considering the recent prolonged drought. Florida recycled water data are available through 2016 and a future study will analyze temporal changes from 2009 to 2015. To our knowledge, this is the first time that KDE has been applied to examine recycled water production spatially. This is an innovative application of a widely accepted analytical method, with applications to other states or production facilities. Limitations may include physical or infrastructure barriers as the KDE implies a gradual transition, however service areas for each facility have a distinct cut-off. This was modeled in the KDE using a Quartic kernel function, which has a distinct cut-off at a given distance from the facility.

CONCLUSIONS
A spatial examination of recycled water use in Florida and California is a first step toward addressing water shortages through expansion of recycled water use. Production capacity depends on a variety of factors, one of which is wastewater generation, the raw material for recycled water. Generally, wastewater increases with population, and therefore we identify high population areas with low per capita recycled water production as prime areas for expansion of water reuse. KDE is a useful method to assess the spatial patterns of recycled water production using a weighted hot spot analysis, and identify potential areas for expansion. This analysis revealed that water reuse is not balanced between Florida Water Management Districts nor California Regional Water Quality Control