Between July 1993 and April 1995, the City and County of San Francisco, Department of Public Works, conducted a Recycled Water Landscape Irrigation Pilot Study in Golden Gate Park to demonstrate the feasibility of using recycled water for landscape irrigation in San Francisco.  This study was conducted to provide information for the Recycled Water Master Plan for the City and County of San Francisco.

Matched sets of plants were sprinkler irrigated with recycled water and control water (potable or groundwater) in 1) the Golden Gate Park nursery, and 2) in a meadow area also in Golden Gate Park.  In the Nursery Test, eight replicates each of 25 woody species and four types of bedding plants were irrigated with potable water and with recycled water.  In the Meadow Test, two adjacent plots were irrigated with groundwater and with recycled water, and two types of turf and one woody species were observed on each plot.  Water quality, soil, plant tissue, physical appearance, and growth of the plants were monitored.

By the end of the study, no discernible differences were observed between plants irrigated with recycled water and those irritgated with potable or ground water.  Water quality differences between recycled water and both types of control water (potable and ground water) were associated primarily with increased salt levels in the recycled water, but recycled water quality was generally in the acceptable rance for irrigation uses based on guidelines developed at the University of California at Davis.  Chemical analyses indicated possible trends in salt levels (sodium and chloride) in soil and plant tissue for the recycled water treatments compared to the control, with salt levels increasing during the growing season but decreasing during the rainy season.  Although the recycled water treatment had higher levels of salt accumulation in soils, salt concentrations were reduced over the winter to acceptable levels, presumably due to leaching effects of winter rains.

The City's gardeners were involved in the design and conduct of the study, actively participating during periodic plant observations.  By the end of the study, they expressed satisfaction with the feasibility of using recycled water for landscape irrigation.  The study successfully demonstrated to City gardeners that recycled water is suitable for a wide range of site conditions and for 25 woody plant species in San Francisco parks.  The study also gave the Golden Gate Park gardeners hands-on experience and confidence in the use of recycled water for irrigating a variety of plants.

The following factors should be considered to ensure the feasibility of irrigating landscape plants with recycled water:  recycled water quality, salt sensitivity of plant materials, soil texture and drainage, and irrigation method and frequency.  Consideration of these factors together with sound landscape management and design practices can assure the long-term success of using recycled water for landscape irrigation throughout San Francisco.