FOLIAR DAMAGE ASSOCIATED WITH EFFLUENT USE ON MIXED LANDSCAPES
Foliar damage to landscape and agricultural plants associated with the sprinkler irrigation of poor quality water has been documented in various scientific studies (Ehlig and Bernstein, 1959, Francois, 1982, Maas and Hoffman, 1982, Bresler et al. 1982 ). A quantifiable decrease in marketable yield has been reported for different agricultural plants (United States Salinity Lab.). However, with landscape plants, yield is not the critical parameter to assess. Instead, the decline in visible quality (plant health and vigor) should be evaluated. Quality, however, is a difficult parameter to quantify. How much of a decline in quality is acceptable is generally linked directly to the cost and availability of both the municipal and wastewater sources, the degree to which quality declines in specific species and/or in all species and the general level of acceptance (golfcourse vs. park vs. athletic field).
The extent of chlorosis and necrosis occurring on the foliage of trees, shrubs and groundcover irrigated with wastewater is typically linked to a general salt effect and/or a specific ion effect (species dependent). Each species has its own threshold value with some species not showing symptoms directly linked to the foliar application but rather to the eventual rise in soil salinity. Typical wastewaters generated at the Clark County Advanced Wastewater treatment plant have a total dissolved solid content of around 1200 mg/l or an electrical conductivity of approximately 1.7 dSm-1 . Such a salt level in the irrigation water can be successfully managed with turfgrass if the water is applied in a uniform fashion with an adequate leaching fraction and with high irrigation frequency. Soil salinity levels developed under such conditions should not exceed the threshold value for most turfgrass species (bermudagrass, tall fescue). Research by Devitt et. al. (1996) indicated no decline in turf quality of either bermudagrass or tall fescue when the grasses were irrigated with shallow saline aquifer water (6.0 dSm-1 Las Vegas, NV) under best management practices (BMP's).
Urban landscapes of course are not solely turfgrass. In fact, in arid environments such as Las Vegas, educational programs have been successful in reducing the amount of turfgrass in the urban landscape. As such, the application of effluent would be applied to mostly mixed landscapes. Even in the golfcourse and park setting, trees and shrubs are still an important part of the total landscape. Attempting to restrict the application of effluent to only turfgrass areas would be difficult to accomplish and would reduce the total amount of effluent that could be utilized in a reuse program.
Information on the sensitivity of landscape species (foliar damage) to the foliar application of treated sewage effluent is meager and no such information has been generated for the plants, water quality and environmental conditions of southern Nevada. However, some information is available from several golf courses in Clark County as to the extent of damage occurring, suggesting that losses have been significant in terms of both the total number of trees affected and the total dollar value (>> $100,000, Sunrise Golf Course). Such losses have forced existing golf courses using treated effluent to restrict their choice of replacement plants and to allocate significant resources to a plant replacement program and to alterations in the irrigation systems. More importantly, it has negatively influenced the opinions of other golf course superintendents and other managers of large landscape areas (schools, parks, cemeteries, etc.) that are considering the use of effluent as an irrigation option in the future.
The research proposed herein would evaluate the sensitivity of a large number of tree species to the foliar application of sewage effluent. A plant list would be generated that would rank the various species as to their suitability in landscapes irrigated with effluent. The extent of foliar damage would be quantified (chlorosis, necrosis, % cover, leaf drop, mortality) with a complete description of the visual symptoms. More importantly, irrigation management strategies would be investigated to determine if damages could be minimized through such practices as dilution, post irrigation rinse and removal of specific ions in the effluent prior to use. Final deliverable products of this research would be 1) an extensive plant list that could be used as a selection tool for picking tree species for landscapes to be irrigated with effluent and 2) Best management practices to be incorporated into irrigation strategies to minimize the plant damage on landscapes that are currently being irrigated with sewage effluent now and/or in the future.
STATEMENT OF PROBLEM:
All Clark County Sanitation facilities are currently located on the east side of the valley. Although some reuse of effluent is occurring, it is restricted to an area adjacent to the treatment facility. Population growth continues to be high in both the southwest and northwest parts of the valley. Sewage generated in this area of the valley must be piped over 15 miles for treatment. Smaller treatment facilities are now being built for these west side areas. The master plan calls for treated effluent to be released for use on landscape areas adjacent to these facilities. Successful use of effluent on turfgrass has been well documented over the last 30 years. However, the use of effluent on mixed landscapes has been documented to cause foliar damage on a wide range of non-turfgrass species. This greater risk of foliar damage associated with effluent use on trees, shrubs and groundcovers has led many golfcourses in California to restrict the application of effluent to turfgrass areas only. This of course leads to a substantial decrease in the total amount of effluent that can be used on the landscape. Only limited information exists in the literature as to the sensitivity of landscape species to the foliar application of effluent and no such information has been generated for the planting material, wastewater and climate of southern Nevada. If a comprehensive plant list could be developed that would indicate the sensitivity of landscape species to foliar damage associated with the application of effluent, landscape areas could be planted with species that are more tolerant, thus increasing the acreage for potential effluent use. It is thus critical that this information be generated as soon as possible, such that recommendations can be made prior to planting to maximize the amount of acreage irrigated with effluent.
We propose to conduct an experiment to screen a large number of
landscape species for tolerance to the foliar application of effluent. 20 ornamental tree
species (replicated 3 times) would be selected from a plant list generated from a survey
conducted with the Clark County School District, Clark County Parks Department and the
Golf Course industry. Irrigation cells would be established to apply either CCSD effluent
or a comparable synthesized water (lacking specific toxic elements) or municipal water
(maintained as controls for comparative purposes) via a sprinkler irrigation system. Plant
material would be randomized in each cell with adequate spacing to insure uniform
application of water to all species.On a weekly basis plant material would be visually
evaluated. A rating system would be devised to assess foliar damage based on chlorosis,
necrosis and leaf drop. Developing symptoms would be photographed to document damage.
Based on the first year screening, we would select 5 species for a more intensive study.
This second study would focus on assessing the extent of foliar damage associated with a
range in effluent concentration and irrigation management strategies, such as chemical
treatment or post irrigation rinse. All results would be statistically evaluated,
published in a scientific journal and incorporated into an educational program to
disseminate the information to all potential effluent users.
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