Recycled Wastewater Instigates Different Responses in Turfgrass, Trees and Soils

As the population of Colorado's Front Range continues to grow, increased use of recycled wastewater (RWW, or effluent water, is viewed as one of the approaches to maximize the existing water resource and stretch Colorado's urban water supplies. While Colorado is famed for its mountains covered with snow, that does not necessarily translate into unlimited use of fresh water for golf course or landscape use.

Based on data from the Department of Public Health and the Environment's Water Quality Control Commission, there are about 10 permitted recycled wastewater facilities in Colorado that can treat and deliver about 56 million gallons of effluent water daily for reuse purposes. We conducted a survey of managers at six sites — including five golf courses and one landscape park — that use recycled wastewater.

Survey results indicate that cost is not the driving force for landscapes to use RWW. Rather, the availability and reliability of the water were rated as the two main reasons for using RWW for irrigation.

Since 2003, research was conducted at Colorado State University with two objectives:

1) to assess variability of chemical properties of recycled wastewater in the Front Range of Colorado; and

2) to evaluate landscape soils and plants that are currently under recycled wastewater irrigation.

Understanding the responses of plants and soils to recycled wastewater irrigation and identifying proper management practices are critical to the long-term success of this practice.

Water quality

Recycled wastewater samples were collected from irrigation ponds and sprinkler outlets on landscape sites. Water testing results of about 50 RWW samples collected from six landscape sites were reviewed for suitability in landscape irrigation based on irrigation water quality guidelines (Westcot and Ayers, 1985).

Quick Tip

The average electrical conductivity (EC) of over 50 recycled wastewater samples from six reuse sites was 0.84 dS/m (deci-Siemens per meter, which is the same as mmhos/cm (millimhos per centimeter) and the range was 0.47 dS/m to 1.32 dS/m. Both are measures of salinity or electrical conductivity. An electrical conductivity higher than 0.75 dS/m indicates the water can impose negative effects on salt-sensitive plants. Periodic leaching of salts is required to mitigate the potential salinity problem.

Adjusted sodium absorption ratio (SAR) of recycled wastewater from reuse sites ranged from 1.6 to 8.3. Based on the interactive effect of salinity and sodicity on soil infiltration and percolation, most of the water samples collected showed slight to moderate effects on soil infiltration and permeability (Fig. 1). Long-term and continued use of water with a high-adjusted SAR will lead to a reduction of soil infiltration and permeability. Additional management (such as a calcium product topdressing or amendments and frequent aerification) is needed to mitigate these effects.

One of the other concerns of recycled wastewater irrigation is the presence of high levels of particular ions (sodium, chloride, and boron) that are toxic to some trees and shrubs. With sprinkler irrigation, sodium and chloride frequently accumulate by direct adsorption through the leaves that are moistened. Sodium and chloride toxicity could occur on sensitive plants when their concentrations in irrigation water exceed 70 milligrams per liter (mg/L) and 100 mg/L, respectively. The average sodium concentration of over 50 water samples collected was 99 mg/L, ranging from 30 mg/L to 170 mg/L. The average chloride concentration was 95 mg/L. Chloride leaches easily through the soil profile and chloride toxicity to turf and landscape plants should be minimal if soil is well drained and salts are regularly leached. However, if the sites have poor drainage, soil percolation is impaired or limited. Or if a shallow water table is present, chloride applied over time can accumulate to a toxic level.

Figure 1: Represents relative rate of water infiltration as affected by salinity and adjusted sodium adsorption ratio of irrigation water (Adapted from Ayers and Westcot). The dots are the data points of water samples collected from Colorado water reuse sites.

In all cases, the water samples met or exceeded the regulations in regard of E. coli count as defined in the state regulations, therefore the water is suitable for landscape irrigation.

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