SUITABILITY STUDY OF
NAPA SANITATION DISTRICT
RECYCLED WATER FOR VINEYARD IRRIGATION
March 6, 2006
Basically, the study confirms that the Napa Sanitation District is prohibited from discharging into the NAPA River during
the time frame it wants to use the same prohibited sewage discharge for irrigation purposes.
March 6, 2006
Prepared for the Napa Sanitation District through a grant to the University of California.
The National Organic Program (NOP) includes the Federal regulations that establish national standards for agricultural
products labeled as organic. These standards are known as the National Organic Standards. Congress authorized the
USDA to establish the NOP in the Organic Food Production Act of 1990. Organic producers in the United States must
now be certified according to the NOP Rule. Growers develop production and handling practices with one of a number
of certifiers, such as the California Certified Organic Farmers (CCOF), to qualify their produce as meeting the USDA
requirements. Individual certifiers are prohibited from making additional requirements to the USDA standards in the
The use of recycled water for vineyard irrigation is not restricted in the NOP. Therefore, organic grape growers are free
to use this water in organically certified vineyards, as long as it is agriculturally suitable for the intended use.
The Napa County Soil Survey was conducted by the United States Department of Agriculture, Soil Conservation
Service, in cooperation with the University of California Agricultural Experiment Station. It was completed in 1978.
The Ca:Mg ratio of irrigation water is similarly important because over time, soil characteristics may be changed to
reflect the characteristics of the water. If large amounts of irrigation water are applied to soils relative to the amount of
rainfall, the soil characteristics will eventually take on the irrigation water characteristics.
Tests for trace elements, [are] including heavy metals, were conducted on NSD recycled water samples per their permit
During the wet season, discharge to the Napa River is allowed, and high quality recycled water is generally not
Towards this end, NSD provided a grant to the University of California to produce this unbiased report on the suitability
of NSD recycled water for vineyard irrigation.
Recycling of wastewater has environmental benefits because it limits the discharge of treated wastewater into natural
Additionally, expanded use of recycled water reduces the amount of discharge to the Napa River and protects existing
sources of water for other uses.
During the non-discharge period, water is recycled for irrigation purposes or stored for wet season discharge.
NSD's National Pollutant Discharge Elimination System Permit issued by the San Francisco Bay Regional Water Quality
Control Board provides 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.
The use of recycled water for vineyard irrigation is not restricted in the National Organic Program standards.
Therefore, organic grape growers can use this water in organically certified vineyards.
NSD sponsored this report with a grant to the University of California in order to provide grape growers with an
unbiased source of information regarding important water quality parameters relative to vineyard irrigation.
The Napa Sanitation District (NSD) is looking to expand the use of recycled water in Napa County and has developed a
Recycled Water Strategic Plan to explore options to maximize water recycling. Included in this plan is expanded use of
recycled water for vineyard irrigation, in particular in Carneros and the Milliken-Sarco-Tulocay (MST) region east of the
City of Napa.
Based on our results, there were no salinity or toxicity issues that would limit the use of this water for vineyard irrigation.
assumes good irrigation management practices,
soil salinities (ECe) should not exceed 1.3 mmhos/cm.
should not create salinity problems in vineyards.
reclamation-leaching functions provided by Ayers and Westcot (3) predict that about 80% of the salts that accumulate
in the top three feet of soil can effectively be removed each year through natural leaching.
indicating that this water should not pose a problem regarding Cl toxicity in grapes, assuming good irrigation
Sodium levels in soils irrigated with NSD recycled water should not be a problem provided adequate calcium levels and
soil physical conditions are maintained.
Chloride toxicity should not be a problem in soils with levels below 10 meq/L.
When boron levels in irrigation water exceed 1.0 mg/L, grapevine growth and productivity may be reduced.
Potential mitigation measures for growers concerned about nitrogen in the NSD recycled water include selective use of
cover crops and having an additional source of water available for irrigation.
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.
Recent research in the San Joaquin Valley indicates that these historical approaches are not easily applied to drip
irrigation. Although the ECe-ECw-LF relationships could be used as a first approximation or guide, steady-state
conditions are never achieved under field conditions.
If irrigation water is applied in excess of the soil water holding capacity, leaching will occur but primarily under the
emitter and salts will still build up laterally.
By applying less water, salts from the irrigation water will tend to increase in the root zone as the season progresses.
We attempted to take sufficient samples with distance and depth from drip lines to show the pattern of soil salinity
around drip lines. However, a hard pan at about 18 to 24 inches deep prevented sampling at deeper depths.
Grapes are sensitive to chloride (Cl) and to some extent sodium (Na) in the irrigation water and can develop injury to
leaves if concentrations exceed certain levels. Specific ion injury, if severe enough, will reduce yields beyond that
predicted by salinity (i.e., EC or TDS) alone.
The maximum Cl concentration in irrigation water that can be used by a particular crop without leaf injury can be found
in FAO 29 (3). The guidelines for grapes are reproduced in Table 6. This list is by no means complete since data for
many cultivars and rootstocks are not available, particularly those that are currently in use in the Carneros and MST
regions. Original data listed by Maas and Hoffman (12) 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 the EC of the soil water is twice the
ECe and that a long-term leaching fraction of 10% is achieved using high frequency drip irrigation.
Much of the earlier research on sodium toxicity was done before understanding the importance of adequate calcium
nutrition for maintaining ion selectivity at the root membrane level. Since this early work, there is a considerable amount
of literature that indicates sodium can cause indirect effects on crops either through nutritional imbalances (e.g.,
sodium-induced calcium or potassium deficiency) (8), or by disrupting soil physical conditions (3). These indirect
effects by sodium make diagnoses of sodium toxicity per se very difficult.
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 the irrigation water and can develop injury to
leaves and shoots if concentrations exceed certain limits.
Threshold levels in irrigation water that produce such injury are reported in FAO 29 (3). Many of these data were taken
from Maas (10) who extracted most of the information from work conducted by Eaton (6), including grape. When the
limited data set from Eaton is examined in detail, growth of grape does not decline until B concentrations in the
irrigation water exceed 1 mg/L.
The guidelines for boron tolerance are limited. With the exception of a few sand tank studies that actually provide B
coefficients (i.e., threshold and slope) for some crops (see 10), most of the B classification has come from work
conducted over a half a century ago by Eaton (6). Research on common rootstocks is lacking.
Nitrogen is required for proper growth and development of grapevines, but high levels of nitrogen can create problems
due to excess growth and vigor. High vigor vineyards which produce large amounts of leaf growth result in shaded fruit.
This can lead to reduced yields and lowered wine quality. Fruit produced under shaded conditions is likely to be higher
in pH, lower in sugar and color, and may have herbaceous characteristics that are undesirable. In addition, high vigor
vineyards often have a greater incidence of Botrytis bunch rot and powdery mildew diseases.
Many vineyards in Carneros and the MST region are currently fertilized with nitrogen at rates approaching or
exceeding these levels,
Where there are concerns about the additional nitrogen that would be applied in conjunction with the use of NSD
recycled water, growers should consider the use of cover crops to help mitigate the potential effects on excess vigor.
Cover crops can compete with the vines and help to control their growth.
Growers concerned about nitrogen in the NSD recycled water should plant grass cover crops and avoid the use of
Another mitigation measure for growers concerned about nitrogen in the NSD recycled water is to have a secondary
source of water available for irrigation. This water could be blended with the NSD water, or used alternatively, in order
to control the amount of nitrogen applied to the vineyard during the course of the season. A secondary source of water
may also be desired for use in spray tanks later in the season once fruit is present on the vines.