- Author: Michael Cahn
- Contributor: David Chambers
- Contributor: Tom Lockhart
- Contributor: Noe Cabrera
Minimizing suspended sediments in irrigation runoff is desirable for several reasons. For growers reusing tailwater for watering their crops, they must assure that the water has minimal food safety risks by testing it for generic E coli and/or treating it with chlorine. The concentration of free (or reactive) chlorine is reduced when tailwater contains a high concentration of suspended sediments. Treating a large volume of tailwater with chlorine can be a significant expense over a season so it is important to be able to remove as much of the suspended sediments as possible before treatment.
A second reason is that water quality regulations under Agriculture discharge Order 4.0 requires tailwater discharged into public water ways to not be toxic to aquatic organisms. Pesticides that strongly bind to soil, such as pyrethroids, are carried on the suspended sediments in runoff which can cause toxicity to aquatic organisms that live in creeks and rivers downstream from farms. Also, particulate forms of N and P which bind with the suspended sediments pose a water quality risk to receiving waterbodies such as the sloughs and wetlands along the coast. Both nutrients can spur algal blooms which reduces dissolved oxygen available to fish and other aquatic organisms.
In a previous article we discussed a new approach to using Polyacrylamide (PAM), an inexpensive polymer molecule for reducing soil erosion, to treat sprinkler water. This practice uses a specialized applicator (Fig. 1) to condition water flowing from a well with PAM. An advantage of this method is that the cartridges in the applicator release a small amount of PAM (1 to 2 ppm) into the irrigation water, which flocculates soil particles that could potentially become suspended and transported in runoff. Field tests using a prototype version of this applicator resulted in about 90% less suspended sediment in the tailwater when treated with PAM compared to untreated irrigation water.
Auger ditch applicator
A second approach we developed for reducing suspended sediment in runoff is to use a smart applicator that can automatically apply dry PAM to the runoff water flowing in farm ditches. This type of applicator is suspended on a platform above a ditch and uses a hopper filled with dry PAM and an auger system controlled by an electric motor and small computer to drop PAM down a tube into the flowing runoff (Fig. 2). A weir and float mechanism located upstream are used to monitor the flow rate of the runoff so that the computer can adjust the frequency that PAM is applied. A video at this link demonstrates how the auger applicator operates.
Field testing of the ditch applicator
A yearlong study at a commercial farm showed that the ditch applicator was effective in removing 98% of the suspended sediments transported in runoff (Table 1, Fig. 3). Based on the total runoff measured in a single drainage ditch during the 2022 season (21.5 acre-feet), an estimated 106 tons of sediment were removed (Fig. 4).
Turbidity in the runoff was reduced by more than 99%, and Total P and N were reduced on average by 89% and 60%, respectively, during the season (Table 1, Figs. 5 and 6). These reductions in nutrient load, suspended sediment, and turbidity could greatly improve water quality in water bodies downstream from farms that discharge irrigation runoff.
Table 1. Average concentration of N, P, and sediments carried in irrigation runoff before (upstream) and after (downstream) treatment with the PAM ditch applicator (April – October 2022). Average of 32 paired grab samples from 3 farm ditches. Downstream locations varied from 300 to 500 ft downstream from the PAM applicators.
Ditch applicator vs well applicator
Although more effective at reducing suspended sediment in runoff than the well applicator, the ditch applicator required more maintenance. PAM needed to be added to the hopper once or twice per week during the irrigation season, and sediment that settled in the ditches had to be cleaned out periodically using a backhoe. Also, removed sediment had to be spread back in the fields. The well applicator only required periodic refilling of the cartridges with PAM, and minimizes the amount of sediment that settles out in the drainage ditches.
PAM effects on chlorine requirement
To evaluate the effect of PAM on the quantity of chlorine needed to treat runoff, we performed a laboratory assay on samples of sprinkler runoff collected upstream and downstream of one of the ditch applicators. The turbidity of the upstream (untreated) and downstream samples (PAM treated) was 2276 and 9.5 NTU, respectively. The electrical conductivity of the runoff samples was 1.35 dS/m and the pH was 8.4 before adding chlorine. The main factors evaluated in the assay were sodium hypochlorite concentration and acidification with 10% sulfuric acid. Presumably, acidifying the runoff to a pH of 6.5 should increase the concentration of the more reactive form of chlorine, hypochlorous acid which is more effective as a microbial disinfectant. Residual free chlorine concentration of the treatments was evaluated 2 and 4 hours after adding 12.5% sodium hypochlorite at concentrations ranging 12.5 to 31.3 ul per liter of runoff (100 to 250 ul of 12.5% NaOCl per L of water).
The laboratory assay showed that reducing suspended sediment concentration using PAM increased the efficacy of chlorine treatment of runoff. The free chlorine concentration for PAM treated runoff was more than twice the concentration measured in the untreated runoff for all sodium hypochlorite concentrations evaluated after 2 hours and more than three times the concentration after 4 hours (Fig. 7). Free chlorine concentration in the PAM treated runoff was more than 2.5 ppm two hours after treatment at the lowest concentration of chlorine evaluated (12.5 ul/L) but was less than 0.5 ppm in the untreated runoff. To attain similar chlorine efficacy as PAM treated runoff, untreated runoff would require twice as much sodium hypochlorite (25 ul/L). These chlorine requirements would correspond to 26 and52 gallons of 12.5% sodium hypochlorite to treat and acre-foot of runoff with and without a PAM pretreatment, respectively.
Acidification of the runoff to a pH of 6.5 with sulfuric acid increased the free chlorine concentration in the PAM treated runoff at the highest concentration of sodium hypochlorite (31.3 ul/L) after 4 hours. Acidification did not have a significant effect on free chlorine concentration for the other treatments.
Summary
Both versions of the dry PAM applicators (well and ditch) show promise for greatly reducing soil erosion, as well as helping improve water quality and the efficacy of chlorine for treating tail water reused for irrigation. By considerably reducing the concentration of suspended sediment in irrigation runoff, chlorine can be more effective as a disinfection agent, and better control E. coli and other microbial pathogens that could potentially cause public health risks.
Acknowledgments: We greatly appreciate assistance in fabricating the prototype PAM applicators from RayFab. This project was funded by the California Leafy Green Research Board.
Further reading
- Author: Michael D Cahn
Erin Dicaprio, Associate Professor of Cooperative Extension, Department of Food Science and Technology, University of California Davis has summarized information and resources for assessing crop food safety after a flooding event. There are links to the Leafy Green Marketing Agreement Flood fact sheet and also a presentation made by Trevor Suslow, emeritus UC Cooperative Extension Specialist. Please follow the link below.
- Author: Michael D Cahn
- Contributor: David Chambers
- Contributor: Thomas Lockhart
- Contributor: Noe Cabrera
As the drought continues on the central coast, growers are trying to utilize water as efficiently as possible to produce their crops. Retaining and reusing sprinkler runoff, also referred to as tail water, can be an important strategy to increasing water conservation. Also, retaining runoff prevents suspended sediments, pesticides and nutrients from impairing rivers and estuaries downstream of agricultural fields.
Many ranches in the Salinas Valley have retention basins and infrastructure that can capture runoff and reuse tail water for irrigating crops. Most growers use this water during the pre-germination or the germination stages to avoid food safety risks from microbial pathogens. However, updates to the leafy green marketing agreement (LGMA) now require that water stored in open reservoirs and used for irrigating leafy greens maintain generic E. coli levels less than 10 MPN/100 ml. In most cases, tail water in open reservoirs will need to be treated with chlorine to achieve this low threshold for generic E. coli. Fine sediments suspended in the tail water can greatly reduce the effectiveness of chlorine to control bacterial growth.
Polyacrylamide (PAM), an inexpensive polymer molecule that has been used for controlling soil erosion in furrow irrigated fields since the early 1990s may be able to improve the efficacy of chlorine by reducing the suspended sediment concentration in sprinkler runoff. Additionally, if runoff is discharged from a ranch, treatment with PAM can greatly reduce the concentration of sediment-bound pesticides and nutrients that can degrade water quality downstream. Past field trials that we conducted have shown that adding PAM to irrigation water at a low concentration (< 5 ppm) is an effective way to minimize erosion in sprinkler irrigated fields and remove suspended sediments from tail water. However, for this strategy to be successful with sprinklers, we found that PAM must be injected continuously throughout each irrigation. In other words, a single application of PAM cannot control suspended sediments in runoff during subsequent irrigations.
New approaches to using PAM
Accurately injecting PAM into a pressurized irrigation system is not a simple process. Dry PAM powder becomes very gooey and viscous when moistened, and is almost impossible to uniformly dissolve into water. Emulsified oil formulations of PAM that mix up uniformly in water are available but are more costly than dry PAM products and require sophisticated pumps to meter it into a pressurized irrigation system, as well as trained staff to assure that the application rate is accurate. Another limitation of liquid PAM is that the mineral oil used to emulsify these products can be toxic to aquatic organisms. In contrast, dry PAM is less than half the cost of liquid PAM and has been shown to have no toxicity to aquatic test organisms such as Hyalella azteca and Ceriodaphnia dubia, even at concentrations 20 times greater than would be typically used for treating irrigation water. Hence, for these reasons, we have been developing and evaluating approaches of using dry PAM to control sediment in sprinkler runoff during the last several years.
Treating pressurized irrigation water with PAM
The first method that we describe in this article uses an applicator to dissolve dry PAM into pressurized irrigation water. The applicator consists of cartridges filled with PAM granules that insert into a series of cylindrical chambers (Fig. 1). A small pump can be used to divert a portion of the irrigation water from the mainline into the inlet of the applicator. PAM slowly releases from the cartridges (Fig. 2) as irrigation water streams through the space between the cartridges and the outer walls of the chambers. Vanes surrounding the cartridges increase turbulence of the flowing water to maximize the dissolution of PAM. The treated water then returns into the main line of the irrigation system where it distributes into the field through the sprinkler system.
Field-testing the dry PAM applicator
Field-testing of the prototype PAM applicator was conducted in commercial lettuce fields during 2020 and 2021. Each field test occurred during the germination phase of the crop (5 to 6 consecutive irrigations) using overhead sprinklers. The fields were divided into untreated, and PAM treated areas, where the PAM treated plots ranged from 1.9 to 4.2 acres. Soil textures at the sites varied from loam to sandy loam. A portion of the flow in the main lines was diverted through the PAM applicator. Flowmeters were used to measure the flow rate in the mainline and the inlet of the applicator. Another flowmeter monitored the volume of water applied in an adjacent untreated plot.
Flumes were installed 30 ft from the far end of the fields to measure run-off volume in the PAM treated and the untreated plots during the irrigations (Fig. 3). A stilling well and float mechanism were used to measure the height of the water in the flume. A datalogger recorded the height of the water in the flume and converted it to a flowrate using a calibration curve. The datalogger also automated sampling of run-off into collection containers using a peristaltic pump. Composite samples of run-off were collected from the plots during 5 to 6 irrigation events and analyzed for turbidity, pH, electrical conductivity, suspended sediments, total nitrogen (N), nitrate-N, total phosphate (P) and orthophosphate at the UC Davis analytical laboratory.
Results of field tests
The average concentration of suspended sediments in the untreated sprinkler runoff ranged from 466 to 1256 milligrams per liter (mg/L) during each trial (Table 2). Results of these field trials demonstrated that pretreating the irrigation water with the PAM applicator could reduce the concentration of suspended sediments carried in sprinkler runoff by 85% to 95%, depending on the soil type. The average reduction in suspended sediment concentration in the runoff was 90% across all trials. Turbidity of the runoff in the PAM treated plots was also reduced by an average of 95% across all sites (Fig. 4, Table 3). Runoff volume in the PAM treatment was reduced by an average of 26%, but the reduction in runoff volume varied from 8% to 67% depending on the site characteristics (Table 3).
Total and soluble phosphorus were reduced by an average of 65% and 14% respectively in the PAM plots compared to the untreated controls in the two trials conducted in 2021 (data not presented). Total nitrogen and nitrate nitrogen concentration in runoff from the PAM treated plots were not reduced compared to the untreated plots.
The combined effects of reduced runoff volume and suspended sediment concentration under the PAM treatment resulted in less loss of soil from these fields (Table 2). Soil erosion was reduced by an average of 93% compared to the untreated control, varying from 89% to 96% reduction in erosion among field sites. Cumulative losses of sediment during the germination phases of the crop were reduced from an average of 76 lbs per acre in the untreated plots to 5 lbs per acre in the PAM treated plots. The sediment lost in the untreated plots would equate to 15 tons for a 200-acre ranch that was planted with 2 crops of vegetables per season. This estimate is only for the germination phase of the crop and so total losses of sediment could be much higher for crops irrigated with sprinklers until harvest.
Potential soil health benefits
Improving soil structure and increasing soil organic matter may be an additional benefit of applying PAM to the field through the irrigation water and preventing the erosion of fine sediments. Much of the stable organic matter in soil is associated with the clay size particles, so preventing the erosion of these fine sediments would presumably help with retaining and building soil organic matter. Fine sediments and organic matter are also important in the development of better soil structure which can improve crop growth.
Summary
The dry PAM applicator that we field tested showed promise for greatly reducing soil erosion, as well as helping improve water quality. Presumably, by removing the sediment from the tail water, less chlorine would be required for controlling microbial pathogens. The six-unit PAM applicator tested in this study can treat up to 500 gpm. The applicator would need more mixing units to treat all the flow from a typical agricultural well in the Salinas Valley (flow rates of 1000 to 1500 gpm). To see a video showing runoff from PAM treated and untreated field plots during a sprinkler irrigation, follow this link. The second part of this article will discuss an additional method to use dry PAM to treat irrigation runoff. This second approach uses an applicator that directly treats runoff in drainage ditches.
Acknowledgments: We greatly appreciate assistance in fabricating the prototype PAM applicators from RayFab. This project was funded by the California Leafy Green Research Board.
- Author: Lennis Arriaga
Grower Series on Food Safety