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Recycled water causes no salinity or toxicity issues in Napa vineyards

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Authors

Edward Weber, UCANR Cooperative Extension
Stephen R. Grattan, UC Davis
Blaine R. Hanson, UC Davis
Gaetano A. Vivaldi, University of Bari
Roland D. Meyer, UC Davis
Terry Prichard, UCANR Cooperative Extension
Larry J. Schwankl, UC Kearney Agricultural Research and Extension Center

Publication Information

California Agriculture 68(3):59-67. https://doi.org/10.3733/ca.v068n03p59

Published online July 01, 2014

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Abstract

In response to Napa Sanitation District's interest in expanding its delivery of recycled water to vineyards for irrigation, we conducted a feasibility study to assess the suitability of the water for this use. We adopted two approaches: comparing the water quality characteristics of the recycled water with those of other local sources of irrigation water, and evaluating soil samples from a vineyard that was irrigated for 8 years with the recycled water. Results indicate that the quality of the recycled water is suitable for irrigation, and also that long-term accumulation of salts and toxic ions have not occurred in the vineyards studied and are unlikely to occur. Nutrients in the recycled water may be beneficial to vineyards, though the levels of nitrogen may need to be reduced by planting cover crops in some vineyards.

Full text

The use of treated municipal wastewater for irrigating crops has increased dramatically in California over the past decade and is expected to expand exponentially in the next few decades (WateReuse 2009). The California Department of Water Resources (DWR) projects that the state's population will grow to 52 million people by 2030 (DWR 2005). Treated wastewater is a necessary water source to meet the needs of this expanding population. In 2009, urban California produced about 9 million acre-feet (MAF) of urban wastewater, of which, surprisingly, only 7% (0.65 MAF) was recycled (WateReuse 2009). The state has set an ambitious goal to increase reuse of wastewater to 2.5 MAF by 2030.

With this goal in mind, the Napa Sanitation District (NSD) has developed a Recycled Water Strategic Plan to explore options to maximize water recycling in Napa County; the plan includes vineyard irrigation, in particular the nearby vineyards in the Carneros region west of the city of Napa and the Milliken-Sarco-Tulocay (MST) region east of the city. Recycling water involves the management and treatment of wastewater to produce water that can be used for irrigation and other beneficial uses (Asano et al. 2007; Vivaldi et al. 2013). Water recycling benefits the environment by limiting the discharge of treated wastewater into natural waterways and helping to preserve the supply of potable water for human consumption (DWR 2003).

A study by UC Cooperative Extension researchers found that vineyards in Napa County irrigated with reclaimed wastewater showed no buildup of salinity or ion toxicity after 8 years.

A study by UC Cooperative Extension researchers found that vineyards in Napa County irrigated with reclaimed wastewater showed no buildup of salinity or ion toxicity after 8 years.

The production of recycled water is regulated by the California Department of Health Services through Title 22 of the California Code of Regulations, which protects public health while allowing for the safe use of recycled water for agriculture. Wastewater at NSD is treated through a series of primary, secondary and tertiary processes; the steps include settling, biological oxidation, clarification, coagulation, filtration and disinfection. The resulting water is clear and colorless and may have a slight chlorine smell due to the final disinfection treatment (residual chlorine is low enough to meet irrigation water quality standards). NSD's recycled water is “disinfected tertiary quality,” the highest standard for recycled water in California.

Expanding the use of NSD recycled water has many economic and environmental advantages. It provides a reliable source of water to growers who might otherwise have no water or whose supplies diminish late in the summer and during periods of extended drought. The cost of NSD recycled water is generally less than the cost of other sources of supplemental water. Additionally, expanded use of recycled water reduces the amount of wastewater discharge to the Napa River and protects existing sources of fresh water for other uses.

Napa Sanitation District (NSD)

The NSD's National Pollutant Discharge Elimination System (NPDES) permit, issued by the San Francisco Bay Regional Water Quality Control Board, allows for the discharge of treated wastewater into the adjacent Napa River during the wet season (November through April), but during the dry season (May through October) river discharge is prohibited. During the nondischarge period, treated water is recycled for irrigation purposes or stored for wet season discharge. NSD currently delivers recycled water for irrigating vineyards, industrial landscaping and golf courses near the Soscol Water Recycling Facility.

The Carneros region has extensive plantings of vineyards, but water is often limited in this area and there is little surface water available from ponds or reservoirs. Groundwater is often limited in volume, and it may be high in salts or boron, especially from wells close to San Pablo Bay. The MST region includes considerable vineyard acreage and golf courses, which can potentially benefit from the availability of recycled water.

Aerial view of the Napa Sanitation District's recycled water use area and the location of the soil sampling site. The use of treated municipal wastewater for irrigating crops has increased in California in the past 10 years and is expected to expand exponentially in the next few decades.

Aerial view of the Napa Sanitation District's recycled water use area and the location of the soil sampling site. The use of treated municipal wastewater for irrigating crops has increased in California in the past 10 years and is expected to expand exponentially in the next few decades.

Study overview

For recycled water to be a benefit to grape growers, it needs to be suitable for vineyard irrigation and there must be no problems with it (such as high salinity or toxic constituents) that could affect the vines or soil; and growers must be confident about its quality and the effects. In 2004–2005, NSD gave UC a grant requesting a feasibility study. For this study, samples of NSD recycled water were collected in 2005 on a weekly basis during the dry season (May 1 through Oct. 31, the period when high-quality recycled water suitable for vineyard irrigation is produced by NSD). Water samples were collected from a 24-hour automatic sampler that maintains a representative sample of recycled water produced at the plant during the previous 24 hours. The sampler collects aliquots every 15 minutes. The quantity collected is based on the flow rate during that period. In this way, a truly representative, composite sample is produced. The samples were collected by NSD staff and sent to Caltest Analytical Laboratory in Napa for determination of key inorganic constituents.

Drip irrigation emitter using recycled water from the Napa Sanitation District.

Drip irrigation emitter using recycled water from the Napa Sanitation District.

To have water quality data from other local water sources to compare to the NSD samples, we also collected samples from several water sources being used for vineyard irrigation in the Carneros and MST regions. In Carneros, three wells, one surface water storage pond and a domestic tap water source from the city of Napa used for irrigating vineyards were sampled. In the MST region, three wells, one surface water storage pond, and one pond that combined surface water runoff and well water were also sampled. At each of these locations, water samples were collected in May, July and October 2005, that is, at the beginning, middle and end of the dry season, when NSD recycled water is available for irrigation.

Analyses of these samples, similar to the analyses of the NSD samples, were performed at Caltest. To meet Caltest guidelines, water was collected in three containers: one container with no preservatives added, for analysis of alkalinity, chloride (Cl), pH, electrical conductivity (EC), nitrate-nitrogen, nitrite-nitrogen, total dissolved solids (TDS), sulfate (SO4), fluoride (F) and turbidity; another container, with nitric acid as a preservative, for analysis of boron (B), iron (Fe), silica, calcium (Ca), magnesium (Mg), sodium (Na), potassium (K) and hardness; and a third container, with sulfuric acid as a preservative, for analysis of ammonia-nitrogen, organic nitrogen (N), total Kjeldahl N and phosphate. Samples were stored in coolers and transported to Caltest within hours of collection.

Irrigation water quality evaluations generally consider the water's pH, salinity hazard (which is indicated by the EC of the water and is associated with the total soluble salt content of the water), Na hazard based on the sodium adsorption ratio (SAR, the relative proportion of Na to Ca and Mg ions), alkalinity due to carbonate and bicarbonate ions, and the presence of specific ions such as B and Cl that can have toxic effects and other constituents such as N that can influence plant growth and vine vigor. All of these parameters were evaluated in this study and are presented in table 1.

Average water quality values of recycled water from NSD and water from local sources in MST and Cameras regions, 2005*

TABLE 1. Average water quality values of recycled water from NSD and water from local sources in MST and Cameras regions, 2005*

In addition to the water sampling described above, we collected soil samples in September 2005 from a vineyard that had been drip-irrigated with NSD recycled water for eight seasons (1997 to 2005). Soil samples were collected Sept. 15, 2005, at two depths. The grower typically applied 75 to 100 gallons of water per vine per season.

Because the soil samples were collected late in the growing season (but before winter rains occurred), they contained the maximum level of soil salinity likely to be found in the vineyard over the season. Soil samples were analyzed for the electrical conductivity of the saturated soil extract (ECe), saturation percentage (SP), pH, Ca, Mg, Na, Cl, bicarbonate and carbonate. Soil analyses were conducted at UC Davis Analytical Laboratory. Data were analyzed by analysis of variance (ANOVA) using R 2.15.0 software (R Foundation for Statistical Computing); standard error (SE) values were also determined.

Salinity effect on yield

Historically, salinity hazard has been assessed using yield potential as described by the Maas-Hoffman salinity coefficients (Maas and Grattan 1999). According to Maas and Hoffman (1977), as described by Ayers and Westcot (1985), salt tolerance can best be described by plotting relative yield as a continuous function of average root zone soil salinity (ECe). Maas and Hoffman proposed that this response curve could be represented by two line segments: a tolerance plateau with a zero slope, and a concentration-dependent line whose slope indicates the yield reduction per unit increase in soil salinity. For soil salinities exceeding the threshold of any given crop, relative yield (Yr), or yield potential, can be estimated using the following expression:

where a = salinity threshold soil salinity value expressed in dS/m; b = slope expressed in the percentage yield decline per dS/m increase above the threshold; and ECe = average root zone salinity in the saturated soil extract. Note that an ECe value for soil is different than the ECW value for irrigation water. The most up-to-date listing of specific values for a and b, called salinity coefficients, are found in Grieve et al. (2012). For grapes, the a and b salinity coefficients are 1.5 and 9.6. Therefore, for grapes,

Note that when the salinity of the soil (ECe) is less than the salinity threshold for grape (i.e., 1.5 dS/m), then the yield potential is 100%. This indicates that grape yields are not adversely affected by soil salinity until the seasonal average root zone salinity (ECe) exceeds 1.5 dS/m (1 dS/m = 1 mmhos/cm [millimhos per centimeter]).

Leaching, ECw and ECe results

To assess the impact on crop yield of irrigation water with a known ECw, the relationship between irrigation water salinity (ECw) and average root zone salinity (ECe) needs to be known or predicted. This relationship depends on the salinity of the irrigation water (ECw), the leaching fraction and whether the irrigation method is conventional (i.e., surface irrigation) or high frequency (e.g., drip irrigation).

The leaching fraction is the fraction (or percentage) of infiltrated water that drains below the root zone. For example, if 5 acre-inches of water were applied to 1 acre and 1 acre-inch of water drained below the root zone, the leaching fraction would be 0.20, or 20%. Soil salinity is controlled by applying sufficient quantities of irrigation water to leach salts from the root zone. The desired leaching fraction, called the leaching requirement, depends on the salinity of the irrigation water and the crop's soil salinity threshold.

Relationships between ECw and ECe at various leaching fractions under both conventional and high-frequency irrigation systems have been presented by Hanson et al. (2006). These relationships assume that water extraction by roots is proportionately higher in the upper part of the root zone, and even more so with drip irrigation. The relationships also assume steady-state conditions, in which the rate of water entering the soil surface and that draining below the root zone remains constant over time. In the case of NSD recycled water, the average ECw is 0.95 mmhos/cm. Using the high-frequency relationship proposed by Pratt and Suarez (1990) and a long-term leaching fraction of 10%, the formula becomes ECe = 1.35 × ECw. This indicates that soil salinity (ECe) over the long term will not exceed 1.3 mmhos/cm. Because this is lower than the threshold ECe value for grapes (1.5 mmhos/cm), NSD recycled water over the long term should not create salinity problems in vineyards.

This calculation, furthermore, takes no account of leaching by winter rainfall, which reduces soil salinity significantly. Leaching is particularly effective in winter, when vines are dormant and crop evapotranspiration (ETc) is essentially zero. With winter rains averaging approximately 20 inches per year in the Carneros and MST regions, the reclamation-leaching functions provided by Ayers and Westcot (1985) predict that about 80% of salts that accumulate in the top 3 feet of soil can effectively be removed each year through leaching by rain alone. The prediction assumes that the soil profile is replenished with irrigation water before winter rains occur. It is therefore advisable for growers to apply a postseason irrigation in late fall to return soil in the crop root zone to field capacity, so winter leaching is more effective. Postharvest irrigation is already a standard practice in many Napa Valley vineyards if water for irrigation is still available.

To determine whether there was any evidence of a long-term buildup of soil salinity at the vineyard where recycled water had been applied for 8 years, soil samples from the site were analyzed for ECe and saturation percentage (SP) and the results compared to the threshold ECe for grapevines. Table 2 shows ECe, SP and pH values for an average of 10 samples from two soil depths at two locations (near drip emitters and between rows). The SP values were all similar, which is typical for a sandy loam soil, indicating no major changes in soil texture between the sampling locations. The maximum ECe value among the samples was 0.79 mmhos/cm; most samples were between 0.25 and 0.5 mmhos/cm. No trends with depth or sampling location were evident. The ECe values were all less than 0.8 mmhos/cm, far below the yield threshold of 1.5 mmhos/cm. These field study results provide additional evidence that long-term salinity accumulation should not occur when using NSD recycled water.

Saturation percentage (SP), electrical conductivity of saturated soil extract (ECe) and pH of soil samples from vineyard irrigated with NSD recycled water, 1997 to 2005 (n = 10)*

TABLE 2. Saturation percentage (SP), electrical conductivity of saturated soil extract (ECe) and pH of soil samples from vineyard irrigated with NSD recycled water, 1997 to 2005 (n = 10)*

Ion toxicity results

Grapevines are sensitive to Cl and to some extent to Na in irrigation water and can develop leaf injury if concentrations exceed certain levels. Specific ion injury, if severe enough, reduces yields more than salinity (i.e., EC or TDS) alone. Although B is an essential element required for plant growth, it is nonetheless potentially toxic, should the concentration in the soil solution become too high. Threshold concentrations of Cl and B in irrigation water, above which toxicity can occur, were reported by Ayers and Westcot (1985) and updated more recently by Grieve et al. (2012).

Chloride.

Many woody species are susceptible to Cl toxicity, with variation among varieties and rootstocks within species. The degree of tolerance is often reflected in the plant's ability to restrict or retard Cl translocation from root to shoots, and particularly to leaves (Maas and Grattan 1999; Walker et al. 2004). Salt tolerance in grapes is closely related to the Cl retention properties of the rootstock, and selection of rootstocks that exclude Cl from scions avoids most Cl toxicity problems (Bernstein et al. 1969).

The maximum Cl concentrations in irrigation water that can be used by particular crops without leaf injury are reported in several references cited above, and the guidelines specific to grapes are reproduced in table 3. This list is by no means complete since data for many cultivars and rootstocks are not available, including those currently in use in the Carneros and MST regions. Original data listed by Maas and Hoffman (1977) are in relation to maximal Cl concentrations in the soil water, but data were converted to maximal tolerance in the irrigation water by assuming that EC of soil water is twice ECe and that a long-term leaching fraction of 10% is achieved using high-frequency drip irrigation. These are reasonable yet conservative assumptions.

For sensitive grape cultivars (i.e., Black Rose and Cardinal), the maximum Cl concentration of irrigation water to avoid crop injury is about 7.4 meq/L (milliequivalents per liter) (table 3). Since no tolerance data have been compiled for the predominant grape rootstocks in the Carneros and MST regions (101–14, 5C, 3309 and 110R), we took a conservative approach and selected 7.4 meq/L (262 mg/L, milligrams per liter) as an upper limit for Cl in our study. As more research is conducted on these rootstocks, the limit can be adjusted accordingly. Since the Cl content in NSD water averages 4.3 meq/L (table 1), this water will not likely cause Cl toxicity in grapes, assuming good irrigation water management. If winter leaching is also taken into consideration, the case is even stronger that the recycled water will not pose a problem for vineyard production.

Maximum chloride (Cl) concentrations in irrigation water that various rootstocks and cultivars can tolerate without developing leaf injury

TABLE 3. Maximum chloride (Cl) concentrations in irrigation water that various rootstocks and cultivars can tolerate without developing leaf injury

Sodium.

The ability of vines to tolerate Na varies considerably among rootstocks, but tolerance is also dependent upon Ca nutrition. Much of the early research on Na toxicity was done in the 1940s and ‘50s before the importance was understood of adequate Ca nutrition for maintaining ion selectivity at the root membrane level. Since then, a considerable amount of literature has indicated Na can cause indirect effects on crops, rather than toxicity exactly, either through nutritional imbalances (e.g., Na-induced Ca or K deficiency) (Grattan and Grieve 1999) or by disrupting soil physical conditions (Ayers and Westcot 1985). These indirect effects make diagnoses of Na toxicity per se very difficult. Moreover, Na toxicity is often reduced or completely overcome if sufficient Ca is made available to roots (Ayers and Westcot 1985) through the addition of gypsum or by acidifying soils high in residual lime.

Ca addition reduces the ratio of Na to Ca (Na:Ca) in the soil water, thereby reducing the SAR and exchangeable sodium percentage (ESP), resulting in both improved soil conditions and reduced Na toxicity. Ayers and Westcot (1985) indicate that there are no “restrictions on use” provided that the SAR is less than 3. They provide no concentration limits for Na above which toxicity will result, presumably because of the indirect interactions between Ca and Na mentioned above. The average Na concentration of the NSD recycled water was 5.0 meq/L and the SAR was 3.9 (table 1). Although this Na level is slightly higher than the one suggested by Ayers and Westcot, it can readily be lowered by light gypsum applications in fall. Therefore, these values indicate that Na will not be a problem over the long term provided adequate Ca nutrition and soil physical conditions are maintained.

Soil samples collected from the vineyard irrigated with NSD recycled water provide further evidence that toxicities from Na or Cl are unlikely to occur. Figure 1 shows the soluble salts extracted from the soil samples. The average Na and Cl concentrations were 1.6 meq/L and 1.2 meq/L, respectively. Cl toxicity should not be a problem unless the concentration in the saturated soil extract exceeds 10 meq/L (355 mg/L). There is no specific threshold level for Na in soils, as discussed above. The results of these soil tests indicate that toxicities from Na or Cl are not occurring at this site following long-term use of NSD recycled water.

Chemical constituents in the saturated soil extracts of samples collected in a vineyard irrigated with NSD recycled water from 1997 to 2005 (n = 10). Samples were collected at different depth intervals and distances from the emitter. Bars indicate the standard error.

Fig. 1. Chemical constituents in the saturated soil extracts of samples collected in a vineyard irrigated with NSD recycled water from 1997 to 2005 (n = 10). Samples were collected at different depth intervals and distances from the emitter. Bars indicate the standard error.

Boron.

B is an essential element for plants but has a small concentration range between levels considered deficient and those considered toxic. Grapes are particularly sensitive to B in irrigation water and can develop injury to leaves and developing shoots if concentrations exceed certain limits (Camacho-Cristobal et al. 2008). The characteristics of B injury are crop-specific and are related to a plant's ability to mobilize this element (Brown and Shelp 1997). In certain tree species (e.g., walnut and pistachio), B is immobile within the plant, and consequently it does not move out of the leaves once it has accumulated there, resulting in necrosis (burn) along the margins and tips of older leaves. In other tree species (e.g., almond, apricot, apple, nectarine, peach and plum), B is relatively mobile, and injury may not appear first on leaves but instead in young shoots as tip dieback.

Malbec vines at Trinitas Cellars vineyard in Napa.

Malbec vines at Trinitas Cellars vineyard in Napa.

Grapevines show some degree of B mobility but not to the same extent as the almond and fruit trees listed above. Threshold levels in irrigation water that produce injury are reported in Ayers and Westcot (1985). Many of these data reported by Ayers and Westcot were taken from Maas and Hoffman (1977), who extracted most of the information, including the grape data, from work conducted by Eaton (1944). When the limited data set from Eaton (1944) is examined in detail, growth of grape does not decline until B concentrations in irrigation water exceed 1 mg/L.

The guidelines for B tolerance are limited. With the exception of a few sand tank studies that provide B coefficients (i.e., threshold and slope) for some crops, most of the B classification work was conducted nearly 70 years ago by Eaton (1944). Research on common rootstocks is lacking. More importantly, the older studies defined a B tolerance limit largely on the basis of the development of incipient injury (i.e., foliar burn) or growth reduction, not on yield response under a range of B concentrations.

The average B concentration of the NSD recycled water was 0.4 mg/L (table 1), which is well below the 1 mg/L level at which grapevines have shown sensitivity. Therefore, it is highly unlikely that B will be problematic over the long term from the use of NSD recycled water. Winter rains will help in leaching soil B below the

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References

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Ayers RS, Westcot DW. Water Quality for Agriculture. FAO Irrigation and Drainage Paper 29, rev. 1. 1985. Food and Agriculture Organization of the United Nations, Rome. 174p. p. www.fao.org/DOCREP/003/T0234E/T0234E00.htm .

Bell S-J, Henschke PA. Implications of nitrogen nutrition for grapes, fermentation and wine. Aust J Grape Wine R. 2008. 11(3):242-95. DOI: 10.1111/j.1755-0238.2005.tb00028.x [CrossRef]

Bernstein L, Ehlig CF, Clark RA. Effect of grape rootstocks on chloride accumulation in leaves. J Amer Soc Hort Sci. 1969. 94:584-90.

Brown PH, Shelp BJ. Boron mobility in plants. Plant Soil. 1997. 193:85-101. DOI: 10.1023/A:1004211925160 [CrossRef]

Camacho-Cristobal JJ, Rexach J, Gonzalez-Fontes A. Boron in plants: Deficiency and toxicity. J Integr Plant Biol. 2008. 50:1247-55. DOI: 10.1111/j.1744-7909.2008.00742.x [CrossRef]

[DWR] California Department of Water Resources. Water Recycling 2030: Recommendations of California's Recycled Water Task Force June 2003. Recycled Water Task Force final report. www.water.ca.gov/pubs/use/water_recycling_2030/recycled_water_tf_report_2003.pdf

DWR. Final California Water Plan Update 2005. www.waterplan.water.ca.gov/previous/cwpu2005/index.cfm

Eaton FM. Deficiency, toxicity, and accumulation of boron in plants. J Agric Res. 1944. 69:237-77.

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Vivaldi GA, Camposeo S, Rubino P, et al. Microbial impact of different types of municipal wastewaters used to irrigate nectarines in Southern Italy. Agr Ecosyst Environ. 2013. 181:50-7. DOI: 10.1016/j.agee.2013.09.006 [CrossRef]

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Citations

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J.D. Oster et al. 2016. California Agriculture 70(2):71
http://dx.doi.org/10.3733/ca.v070n02p71

Rainfall leaching is critical for long-term use of recycled water in the Salinas Valley
Belinda E. Platts and Mark E. Grismer 2014. California Agriculture 68(3):75
http://dx.doi.org/10.3733/ca.v068n03p75

Deficit irrigation in table grape: eco-physiological basis and potential use to save water and improve quality
M. Permanhani et al. 2016. Theoretical and Experimental Plant Physiology 28(1):85
http://dx.doi.org/10.1007/s40626-016-0063-9

The effect of mineral-ion interactions on soil hydraulic conductivity
Maya C. Buelow et al. 2015. Agricultural Water Management 152:277
http://dx.doi.org/10.1016/j.agwat.2015.01.015

Recycled water causes no salinity or toxicity issues in Napa vineyards

Edward Weber, Stephen R. Grattan, Blaine R. Hanson, Gaetano A. Vivaldi, Roland D. Meyer, Terry Prichard, Larry J. Schwankl
Webmaster Email: wsuckow@ucanr.edu

Recycled water causes no salinity or toxicity issues in Napa vineyards

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Authors

Edward Weber, UCANR Cooperative Extension
Stephen R. Grattan, UC Davis
Blaine R. Hanson, UC Davis
Gaetano A. Vivaldi, University of Bari
Roland D. Meyer, UC Davis
Terry Prichard, UCANR Cooperative Extension
Larry J. Schwankl, UC Kearney Agricultural Research and Extension Center

Publication Information

California Agriculture 68(3):59-67. https://doi.org/10.3733/ca.v068n03p59

Published online July 01, 2014

PDF  |  Citation  |  Permissions  |  Cited by 4 articles

Author Affiliations show

Abstract

In response to Napa Sanitation District's interest in expanding its delivery of recycled water to vineyards for irrigation, we conducted a feasibility study to assess the suitability of the water for this use. We adopted two approaches: comparing the water quality characteristics of the recycled water with those of other local sources of irrigation water, and evaluating soil samples from a vineyard that was irrigated for 8 years with the recycled water. Results indicate that the quality of the recycled water is suitable for irrigation, and also that long-term accumulation of salts and toxic ions have not occurred in the vineyards studied and are unlikely to occur. Nutrients in the recycled water may be beneficial to vineyards, though the levels of nitrogen may need to be reduced by planting cover crops in some vineyards.

Full text

The use of treated municipal wastewater for irrigating crops has increased dramatically in California over the past decade and is expected to expand exponentially in the next few decades (WateReuse 2009). The California Department of Water Resources (DWR) projects that the state's population will grow to 52 million people by 2030 (DWR 2005). Treated wastewater is a necessary water source to meet the needs of this expanding population. In 2009, urban California produced about 9 million acre-feet (MAF) of urban wastewater, of which, surprisingly, only 7% (0.65 MAF) was recycled (WateReuse 2009). The state has set an ambitious goal to increase reuse of wastewater to 2.5 MAF by 2030.

With this goal in mind, the Napa Sanitation District (NSD) has developed a Recycled Water Strategic Plan to explore options to maximize water recycling in Napa County; the plan includes vineyard irrigation, in particular the nearby vineyards in the Carneros region west of the city of Napa and the Milliken-Sarco-Tulocay (MST) region east of the city. Recycling water involves the management and treatment of wastewater to produce water that can be used for irrigation and other beneficial uses (Asano et al. 2007; Vivaldi et al. 2013). Water recycling benefits the environment by limiting the discharge of treated wastewater into natural waterways and helping to preserve the supply of potable water for human consumption (DWR 2003).

A study by UC Cooperative Extension researchers found that vineyards in Napa County irrigated with reclaimed wastewater showed no buildup of salinity or ion toxicity after 8 years.

A study by UC Cooperative Extension researchers found that vineyards in Napa County irrigated with reclaimed wastewater showed no buildup of salinity or ion toxicity after 8 years.

The production of recycled water is regulated by the California Department of Health Services through Title 22 of the California Code of Regulations, which protects public health while allowing for the safe use of recycled water for agriculture. Wastewater at NSD is treated through a series of primary, secondary and tertiary processes; the steps include settling, biological oxidation, clarification, coagulation, filtration and disinfection. The resulting water is clear and colorless and may have a slight chlorine smell due to the final disinfection treatment (residual chlorine is low enough to meet irrigation water quality standards). NSD's recycled water is “disinfected tertiary quality,” the highest standard for recycled water in California.

Expanding the use of NSD recycled water has many economic and environmental advantages. It provides a reliable source of water to growers who might otherwise have no water or whose supplies diminish late in the summer and during periods of extended drought. The cost of NSD recycled water is generally less than the cost of other sources of supplemental water. Additionally, expanded use of recycled water reduces the amount of wastewater discharge to the Napa River and protects existing sources of fresh water for other uses.

Napa Sanitation District (NSD)

The NSD's National Pollutant Discharge Elimination System (NPDES) permit, issued by the San Francisco Bay Regional Water Quality Control Board, allows for the discharge of treated wastewater into the adjacent Napa River during the wet season (November through April), but during the dry season (May through October) river discharge is prohibited. During the nondischarge period, treated water is recycled for irrigation purposes or stored for wet season discharge. NSD currently delivers recycled water for irrigating vineyards, industrial landscaping and golf courses near the Soscol Water Recycling Facility.

The Carneros region has extensive plantings of vineyards, but water is often limited in this area and there is little surface water available from ponds or reservoirs. Groundwater is often limited in volume, and it may be high in salts or boron, especially from wells close to San Pablo Bay. The MST region includes considerable vineyard acreage and golf courses, which can potentially benefit from the availability of recycled water.

Aerial view of the Napa Sanitation District's recycled water use area and the location of the soil sampling site. The use of treated municipal wastewater for irrigating crops has increased in California in the past 10 years and is expected to expand exponentially in the next few decades.

Aerial view of the Napa Sanitation District's recycled water use area and the location of the soil sampling site. The use of treated municipal wastewater for irrigating crops has increased in California in the past 10 years and is expected to expand exponentially in the next few decades.

Study overview

For recycled water to be a benefit to grape growers, it needs to be suitable for vineyard irrigation and there must be no problems with it (such as high salinity or toxic constituents) that could affect the vines or soil; and growers must be confident about its quality and the effects. In 2004–2005, NSD gave UC a grant requesting a feasibility study. For this study, samples of NSD recycled water were collected in 2005 on a weekly basis during the dry season (May 1 through Oct. 31, the period when high-quality recycled water suitable for vineyard irrigation is produced by NSD). Water samples were collected from a 24-hour automatic sampler that maintains a representative sample of recycled water produced at the plant during the previous 24 hours. The sampler collects aliquots every 15 minutes. The quantity collected is based on the flow rate during that period. In this way, a truly representative, composite sample is produced. The samples were collected by NSD staff and sent to Caltest Analytical Laboratory in Napa for determination of key inorganic constituents.

Drip irrigation emitter using recycled water from the Napa Sanitation District.

Drip irrigation emitter using recycled water from the Napa Sanitation District.

To have water quality data from other local water sources to compare to the NSD samples, we also collected samples from several water sources being used for vineyard irrigation in the Carneros and MST regions. In Carneros, three wells, one surface water storage pond and a domestic tap water source from the city of Napa used for irrigating vineyards were sampled. In the MST region, three wells, one surface water storage pond, and one pond that combined surface water runoff and well water were also sampled. At each of these locations, water samples were collected in May, July and October 2005, that is, at the beginning, middle and end of the dry season, when NSD recycled water is available for irrigation.

Analyses of these samples, similar to the analyses of the NSD samples, were performed at Caltest. To meet Caltest guidelines, water was collected in three containers: one container with no preservatives added, for analysis of alkalinity, chloride (Cl), pH, electrical conductivity (EC), nitrate-nitrogen, nitrite-nitrogen, total dissolved solids (TDS), sulfate (SO4), fluoride (F) and turbidity; another container, with nitric acid as a preservative, for analysis of boron (B), iron (Fe), silica, calcium (Ca), magnesium (Mg), sodium (Na), potassium (K) and hardness; and a third container, with sulfuric acid as a preservative, for analysis of ammonia-nitrogen, organic nitrogen (N), total Kjeldahl N and phosphate. Samples were stored in coolers and transported to Caltest within hours of collection.

Irrigation water quality evaluations generally consider the water's pH, salinity hazard (which is indicated by the EC of the water and is associated with the total soluble salt content of the water), Na hazard based on the sodium adsorption ratio (SAR, the relative proportion of Na to Ca and Mg ions), alkalinity due to carbonate and bicarbonate ions, and the presence of specific ions such as B and Cl that can have toxic effects and other constituents such as N that can influence plant growth and vine vigor. All of these parameters were evaluated in this study and are presented in table 1.

Average water quality values of recycled water from NSD and water from local sources in MST and Cameras regions, 2005*

TABLE 1. Average water quality values of recycled water from NSD and water from local sources in MST and Cameras regions, 2005*

In addition to the water sampling described above, we collected soil samples in September 2005 from a vineyard that had been drip-irrigated with NSD recycled water for eight seasons (1997 to 2005). Soil samples were collected Sept. 15, 2005, at two depths. The grower typically applied 75 to 100 gallons of water per vine per season.

Because the soil samples were collected late in the growing season (but before winter rains occurred), they contained the maximum level of soil salinity likely to be found in the vineyard over the season. Soil samples were analyzed for the electrical conductivity of the saturated soil extract (ECe), saturation percentage (SP), pH, Ca, Mg, Na, Cl, bicarbonate and carbonate. Soil analyses were conducted at UC Davis Analytical Laboratory. Data were analyzed by analysis of variance (ANOVA) using R 2.15.0 software (R Foundation for Statistical Computing); standard error (SE) values were also determined.

Salinity effect on yield

Historically, salinity hazard has been assessed using yield potential as described by the Maas-Hoffman salinity coefficients (Maas and Grattan 1999). According to Maas and Hoffman (1977), as described by Ayers and Westcot (1985), salt tolerance can best be described by plotting relative yield as a continuous function of average root zone soil salinity (ECe). Maas and Hoffman proposed that this response curve could be represented by two line segments: a tolerance plateau with a zero slope, and a concentration-dependent line whose slope indicates the yield reduction per unit increase in soil salinity. For soil salinities exceeding the threshold of any given crop, relative yield (Yr), or yield potential, can be estimated using the following expression:

where a = salinity threshold soil salinity value expressed in dS/m; b = slope expressed in the percentage yield decline per dS/m increase above the threshold; and ECe = average root zone salinity in the saturated soil extract. Note that an ECe value for soil is different than the ECW value for irrigation water. The most up-to-date listing of specific values for a and b, called salinity coefficients, are found in Grieve et al. (2012). For grapes, the a and b salinity coefficients are 1.5 and 9.6. Therefore, for grapes,

Note that when the salinity of the soil (ECe) is less than the salinity threshold for grape (i.e., 1.5 dS/m), then the yield potential is 100%. This indicates that grape yields are not adversely affected by soil salinity until the seasonal average root zone salinity (ECe) exceeds 1.5 dS/m (1 dS/m = 1 mmhos/cm [millimhos per centimeter]).

Leaching, ECw and ECe results

To assess the impact on crop yield of irrigation water with a known ECw, the relationship between irrigation water salinity (ECw) and average root zone salinity (ECe) needs to be known or predicted. This relationship depends on the salinity of the irrigation water (ECw), the leaching fraction and whether the irrigation method is conventional (i.e., surface irrigation) or high frequency (e.g., drip irrigation).

The leaching fraction is the fraction (or percentage) of infiltrated water that drains below the root zone. For example, if 5 acre-inches of water were applied to 1 acre and 1 acre-inch of water drained below the root zone, the leaching fraction would be 0.20, or 20%. Soil salinity is controlled by applying sufficient quantities of irrigation water to leach salts from the root zone. The desired leaching fraction, called the leaching requirement, depends on the salinity of the irrigation water and the crop's soil salinity threshold.

Relationships between ECw and ECe at various leaching fractions under both conventional and high-frequency irrigation systems have been presented by Hanson et al. (2006). These relationships assume that water extraction by roots is proportionately higher in the upper part of the root zone, and even more so with drip irrigation. The relationships also assume steady-state conditions, in which the rate of water entering the soil surface and that draining below the root zone remains constant over time. In the case of NSD recycled water, the average ECw is 0.95 mmhos/cm. Using the high-frequency relationship proposed by Pratt and Suarez (1990) and a long-term leaching fraction of 10%, the formula becomes ECe = 1.35 × ECw. This indicates that soil salinity (ECe) over the long term will not exceed 1.3 mmhos/cm. Because this is lower than the threshold ECe value for grapes (1.5 mmhos/cm), NSD recycled water over the long term should not create salinity problems in vineyards.

This calculation, furthermore, takes no account of leaching by winter rainfall, which reduces soil salinity significantly. Leaching is particularly effective in winter, when vines are dormant and crop evapotranspiration (ETc) is essentially zero. With winter rains averaging approximately 20 inches per year in the Carneros and MST regions, the reclamation-leaching functions provided by Ayers and Westcot (1985) predict that about 80% of salts that accumulate in the top 3 feet of soil can effectively be removed each year through leaching by rain alone. The prediction assumes that the soil profile is replenished with irrigation water before winter rains occur. It is therefore advisable for growers to apply a postseason irrigation in late fall to return soil in the crop root zone to field capacity, so winter leaching is more effective. Postharvest irrigation is already a standard practice in many Napa Valley vineyards if water for irrigation is still available.

To determine whether there was any evidence of a long-term buildup of soil salinity at the vineyard where recycled water had been applied for 8 years, soil samples from the site were analyzed for ECe and saturation percentage (SP) and the results compared to the threshold ECe for grapevines. Table 2 shows ECe, SP and pH values for an average of 10 samples from two soil depths at two locations (near drip emitters and between rows). The SP values were all similar, which is typical for a sandy loam soil, indicating no major changes in soil texture between the sampling locations. The maximum ECe value among the samples was 0.79 mmhos/cm; most samples were between 0.25 and 0.5 mmhos/cm. No trends with depth or sampling location were evident. The ECe values were all less than 0.8 mmhos/cm, far below the yield threshold of 1.5 mmhos/cm. These field study results provide additional evidence that long-term salinity accumulation should not occur when using NSD recycled water.

Saturation percentage (SP), electrical conductivity of saturated soil extract (ECe) and pH of soil samples from vineyard irrigated with NSD recycled water, 1997 to 2005 (n = 10)*

TABLE 2. Saturation percentage (SP), electrical conductivity of saturated soil extract (ECe) and pH of soil samples from vineyard irrigated with NSD recycled water, 1997 to 2005 (n = 10)*

Ion toxicity results

Grapevines are sensitive to Cl and to some extent to Na in irrigation water and can develop leaf injury if concentrations exceed certain levels. Specific ion injury, if severe enough, reduces yields more than salinity (i.e., EC or TDS) alone. Although B is an essential element required for plant growth, it is nonetheless potentially toxic, should the concentration in the soil solution become too high. Threshold concentrations of Cl and B in irrigation water, above which toxicity can occur, were reported by Ayers and Westcot (1985) and updated more recently by Grieve et al. (2012).

Chloride.

Many woody species are susceptible to Cl toxicity, with variation among varieties and rootstocks within species. The degree of tolerance is often reflected in the plant's ability to restrict or retard Cl translocation from root to shoots, and particularly to leaves (Maas and Grattan 1999; Walker et al. 2004). Salt tolerance in grapes is closely related to the Cl retention properties of the rootstock, and selection of rootstocks that exclude Cl from scions avoids most Cl toxicity problems (Bernstein et al. 1969).

The maximum Cl concentrations in irrigation water that can be used by particular crops without leaf injury are reported in several references cited above, and the guidelines specific to grapes are reproduced in table 3. This list is by no means complete since data for many cultivars and rootstocks are not available, including those currently in use in the Carneros and MST regions. Original data listed by Maas and Hoffman (1977) are in relation to maximal Cl concentrations in the soil water, but data were converted to maximal tolerance in the irrigation water by assuming that EC of soil water is twice ECe and that a long-term leaching fraction of 10% is achieved using high-frequency drip irrigation. These are reasonable yet conservative assumptions.

For sensitive grape cultivars (i.e., Black Rose and Cardinal), the maximum Cl concentration of irrigation water to avoid crop injury is about 7.4 meq/L (milliequivalents per liter) (table 3). Since no tolerance data have been compiled for the predominant grape rootstocks in the Carneros and MST regions (101–14, 5C, 3309 and 110R), we took a conservative approach and selected 7.4 meq/L (262 mg/L, milligrams per liter) as an upper limit for Cl in our study. As more research is conducted on these rootstocks, the limit can be adjusted accordingly. Since the Cl content in NSD water averages 4.3 meq/L (table 1), this water will not likely cause Cl toxicity in grapes, assuming good irrigation water management. If winter leaching is also taken into consideration, the case is even stronger that the recycled water will not pose a problem for vineyard production.

Maximum chloride (Cl) concentrations in irrigation water that various rootstocks and cultivars can tolerate without developing leaf injury

TABLE 3. Maximum chloride (Cl) concentrations in irrigation water that various rootstocks and cultivars can tolerate without developing leaf injury

Sodium.

The ability of vines to tolerate Na varies considerably among rootstocks, but tolerance is also dependent upon Ca nutrition. Much of the early research on Na toxicity was done in the 1940s and ‘50s before the importance was understood of adequate Ca nutrition for maintaining ion selectivity at the root membrane level. Since then, a considerable amount of literature has indicated Na can cause indirect effects on crops, rather than toxicity exactly, either through nutritional imbalances (e.g., Na-induced Ca or K deficiency) (Grattan and Grieve 1999) or by disrupting soil physical conditions (Ayers and Westcot 1985). These indirect effects make diagnoses of Na toxicity per se very difficult. Moreover, Na toxicity is often reduced or completely overcome if sufficient Ca is made available to roots (Ayers and Westcot 1985) through the addition of gypsum or by acidifying soils high in residual lime.

Ca addition reduces the ratio of Na to Ca (Na:Ca) in the soil water, thereby reducing the SAR and exchangeable sodium percentage (ESP), resulting in both improved soil conditions and reduced Na toxicity. Ayers and Westcot (1985) indicate that there are no “restrictions on use” provided that the SAR is less than 3. They provide no concentration limits for Na above which toxicity will result, presumably because of the indirect interactions between Ca and Na mentioned above. The average Na concentration of the NSD recycled water was 5.0 meq/L and the SAR was 3.9 (table 1). Although this Na level is slightly higher than the one suggested by Ayers and Westcot, it can readily be lowered by light gypsum applications in fall. Therefore, these values indicate that Na will not be a problem over the long term provided adequate Ca nutrition and soil physical conditions are maintained.

Soil samples collected from the vineyard irrigated with NSD recycled water provide further evidence that toxicities from Na or Cl are unlikely to occur. Figure 1 shows the soluble salts extracted from the soil samples. The average Na and Cl concentrations were 1.6 meq/L and 1.2 meq/L, respectively. Cl toxicity should not be a problem unless the concentration in the saturated soil extract exceeds 10 meq/L (355 mg/L). There is no specific threshold level for Na in soils, as discussed above. The results of these soil tests indicate that toxicities from Na or Cl are not occurring at this site following long-term use of NSD recycled water.

Chemical constituents in the saturated soil extracts of samples collected in a vineyard irrigated with NSD recycled water from 1997 to 2005 (n = 10). Samples were collected at different depth intervals and distances from the emitter. Bars indicate the standard error.

Fig. 1. Chemical constituents in the saturated soil extracts of samples collected in a vineyard irrigated with NSD recycled water from 1997 to 2005 (n = 10). Samples were collected at different depth intervals and distances from the emitter. Bars indicate the standard error.

Boron.

B is an essential element for plants but has a small concentration range between levels considered deficient and those considered toxic. Grapes are particularly sensitive to B in irrigation water and can develop injury to leaves and developing shoots if concentrations exceed certain limits (Camacho-Cristobal et al. 2008). The characteristics of B injury are crop-specific and are related to a plant's ability to mobilize this element (Brown and Shelp 1997). In certain tree species (e.g., walnut and pistachio), B is immobile within the plant, and consequently it does not move out of the leaves once it has accumulated there, resulting in necrosis (burn) along the margins and tips of older leaves. In other tree species (e.g., almond, apricot, apple, nectarine, peach and plum), B is relatively mobile, and injury may not appear first on leaves but instead in young shoots as tip dieback.

Malbec vines at Trinitas Cellars vineyard in Napa.

Malbec vines at Trinitas Cellars vineyard in Napa.

Grapevines show some degree of B mobility but not to the same extent as the almond and fruit trees listed above. Threshold levels in irrigation water that produce injury are reported in Ayers and Westcot (1985). Many of these data reported by Ayers and Westcot were taken from Maas and Hoffman (1977), who extracted most of the information, including the grape data, from work conducted by Eaton (1944). When the limited data set from Eaton (1944) is examined in detail, growth of grape does not decline until B concentrations in irrigation water exceed 1 mg/L.

The guidelines for B tolerance are limited. With the exception of a few sand tank studies that provide B coefficients (i.e., threshold and slope) for some crops, most of the B classification work was conducted nearly 70 years ago by Eaton (1944). Research on common rootstocks is lacking. More importantly, the older studies defined a B tolerance limit largely on the basis of the development of incipient injury (i.e., foliar burn) or growth reduction, not on yield response under a range of B concentrations.

The average B concentration of the NSD recycled water was 0.4 mg/L (table 1), which is well below the 1 mg/L level at which grapevines have shown sensitivity. Therefore, it is highly unlikely that B will be problematic over the long term from the use of NSD recycled water. Winter rains will help in leaching soil B below the

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Citations

Accounting for potassium and magnesium in irrigation water quality assessment
J.D. Oster et al. 2016. California Agriculture 70(2):71
http://dx.doi.org/10.3733/ca.v070n02p71

Rainfall leaching is critical for long-term use of recycled water in the Salinas Valley
Belinda E. Platts and Mark E. Grismer 2014. California Agriculture 68(3):75
http://dx.doi.org/10.3733/ca.v068n03p75

Deficit irrigation in table grape: eco-physiological basis and potential use to save water and improve quality
M. Permanhani et al. 2016. Theoretical and Experimental Plant Physiology 28(1):85
http://dx.doi.org/10.1007/s40626-016-0063-9

The effect of mineral-ion interactions on soil hydraulic conductivity
Maya C. Buelow et al. 2015. Agricultural Water Management 152:277
http://dx.doi.org/10.1016/j.agwat.2015.01.015


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