In addition to concerns about food safety, microbial pathogens are considered to be among the leading causes of water quality impairment in California agricultural watersheds . Within a watershed, pathogenic bacteria and protozoa from humans, livestock, wildlife, and pets can be found in runoff and can contaminate surface water bodies . Non-point sources of pollution have become the main sources of microbial pollution in waterways, with agricultural activities, including manure application to fields, confined animal operations, pastures, and rangeland grazing, being the largest contributors . Constructed and restored wetlands have been among the few water management options proposed as being available to growers to filter and improve the quality of water in agricultural runoff that contains a wide range of contaminants . Specifically, constructed wetlands have been shown to be highly effective at removing pathogens from water . However, wetlands may also provide habitat for wildlife, including birds, livestock, deer, pigs, rodents, and amphibians, and they may in turn vector pathogens that cause human disease. These animals deposit feces and urine within the wetland, an effect that has the potential to negate any benefit from pathogen removal caused by wetland filtering . After past outbreaks of food borne illness caused by E. coli 0157:H7 borne on lettuce and spinach grown in California, some food safety guidelines have encouraged growers to reduce the presence of wildlife by minimizing non-crop vegetation, including wetlands, that could otherwise attract wildlife to farm fields growing fresh produce . In this situation, food safety guidelines may be at odds with water quality improvement measures. Many constructed and restored wetlands in California have been built with support from the USDA-NRCS through the Environmental Quality Incentives Program and the Wetland Reserve Program . Under these programs, 4×8 botanicare tray most wetland systems were initially developed to mitigate the loss of wetlands and improve wildlife habitat. A key element of the design of these systems is that they receive agricultural runoff as input flows intended to maintain the wetland’s saturated conditions .

In addition to increasing wildlife habitat, the observed water quality improvements linked with these types of wetlands have made them an attractive “best management practice” for irrigated agriculture . Our purpose in writing this publication is to show how wetlands may be used to improve water quality in agricultural settings where pathogens are a matter of concern. In addition, we will discuss wetland design and management considerations that have the potential to maximize pathogen removal and minimize microbial contamination. The following case study highlights the effectiveness of wetlands as a tool to improve water quality and demonstrates the importance of specific design characteristics. A water quality assessment of seven constructed or restored surface flow-through wetlands was conducted across the Central Valley of California. Wetlands differed in such parameters as size, age, catchment area, vegetation type and coverage, and hydrologic residence time . W-1 through W-4, located in the San Joaquin Valley and discharging into the San Joaquin River , were continuous flow wetlands. W-5 through W-7, situated in the Sacramento Valley and discharging into the Sacramento River , were flood-pulse wetlands with a water management regime consisting of flood pulses every 2 to 3 weeks, followed by drainage for 3 to 4 days prior to the next flood pulse. W-2 and W-3 shared the same input water source, and the same was the case for W-5, W-6, and W-7. Several water quality parameters were measured at input and output locations during the growing season to evaluate the systems’ ability to improve water quality. Both concentration and load are important considerations when assessing water quality constituents. Concentration represents the mass, weight, or volume of a constituent relative to the total volume of water. Load represents the cumulative mass, weight, or volume of a constituent delivered to some location.

The flow-through wetlands were most effective at reducing total nitrogen , total suspended solids , and E. coli loads , and were moderately effective at reducing total phosphorus loads. In many instances, the flood-pulse wetlands were actually a source of contaminants, as indicated in table 2 by the negative numbers they show for removal efficiency. E. coli load in outflows was significantly lower than the inflow load at all flow-through wetlands , while the flood-pulse wetlands showed significant increases in E. coli : decreases of 80 to 95% as opposed to increases in total E. coli loads, respectively. The differences in contaminant removal for flow-through versus flood-pulse wetlands can be attributed to two factors. First, the input water for the flood-pulse systems was very clean, so any introduced contaminants were readily detectable. The average E. coli concentration for input water was 62 cfu 100 ml−1 in the flood-pulse wetlands, compared to over 200 cfu 100 ml−1 in the flow-through wetlands. Second, the overly long hydrologic residence times of flood pulse systems can allow contaminants to become more concentrated through the processes of water evaporation, leaching of nutrients from soils and organic matter, and introduction of nutrients and contaminants from feces and urine deposited by wildlife that inhabit the wetlands. Enterococci and E. coli are standard federal- and state regulated constituents used as indicators of fecal contamination in water. In the flow-through wetlands , approximately 47 percent of water samples collected from irrigation return flows exceeded the EPA recreational contact water standard for E. coli of 126 cfu 100 ml−1 . In contrast, E. coli concentration in wetland outflows ranged from 0 to 300 cfu 100 ml−1. Following wetland treatment, 93 percent of wetland outflows met the California water quality standard for E. coli concentration . For enterococci, 100 percent of the input water samples exceeded the water quality standard of 33 cfu 100 ml−1.

Despite exceeding the water quality standard, the bacteria levels found here are very low when compared to other contaminated water sources, such as wastewater . Although enterococci removal efficiencies ranged from 86 percent to 94 percent , only 30 percent of the outflow enterococci concentrations met water quality standards . Results from this study indicate that by passing irrigation tail water through wetlands, a grower can significantly reduce the water’s pathogen concentration and load, as well as other water quality contaminants common to agricultural settings. Some water quality standards may never be met with wetland filtering alone, especially where the standards require extremely low values, as is the case for enterococci in irrigation water used on farms that grow produce that is intended to be consumed raw. Wetland design and management need to be considered prior to construction and throughout the life of the system. In many cases, the natural mechanisms that promote contaminant removal or retention can be manipulated through careful design, management of hydrology, and maintenance of appropriate vegetation. Natural mechanisms for reducing bacteria pathogens are not fully understood and have received only limited study in irrigated agriculture. Wetlands are known to act as bio-filters through a combination of physical , chemical , and biological factors , all of which contribute to the reduction of bacteria numbers . Where input water has a relatively low concentration , wetland background levels are so low that water passing through the wetland may actually end up with increased pathogen concentrations . As high-energy input flows disperse across the wetland, the water’s velocity decreases, and particles that had been suspended in the water settle to the bottom. The energy needed to support suspended particles in the water flow dissipates as the cross-sectional area of the wetland flow path increases, flood tables for greenhouse and vegetation reduces the water’s turbulence and velocity. The rate of sedimentation is governed by particle size, particle density, water velocity and turbulence, salinity, temperature, and wetland depth. Larger pathogens tend to settle more quickly than smaller ones. The actual removal of pathogens by means of sedimentation depends on whether the pathogens are free-floating or are attached to particles. Pathogens can be attached to suspended particles such as sand, silt, clay, or organic particulates. Microbial contaminants associated with particles, especially dense, inorganic soil particles, settle out in wetlands sooner than those in the free-floating form. Studies have shown that the rate of pathogen removal is greater in wetlands where the input waters have a high sediment load . Some wetland designs are more prone to encourage wave activity, which prevents sedimentation and encourages re-suspension of settled particulates . High wind velocities promote wave activity. Large, open-water designs are more prone to water turbulence because wind velocity increases over a large, smooth surface. Wetland vegetation can help minimize water turbulence and particle re-suspension. For example, trees planted as wind barriers surrounding the wetland decrease the amount of wind on the wetland. Emergent vegetation within the wetland can anchor sediment with its roots and can dampen the velocity of wind moving across the water surface. Dendritic wetland designs, which consist of a sinuous network of water-filled channels and small, vegetated uplands, can help reduce water turbulence associated with high winds .Vegetative cover has been shown to decrease sediment re-suspension. For example, Braskerud found that an increase in vegetative cover from less than 20 percent up to 50 percent reduced the rate of sediment re-suspension from 40 percent down to near zero. Wetland depth may also have an indirect effect on sediment retention.

The water should be deep enough to mitigate the effect of wind velocity on the underlying soil surface, but if the water is too deep, vegetation will not be able to establish and a significant increase in re-suspension of sediment will result. Water depths between 10 and 20 inches optimize conditions for plant establishment, decreased water velocity, well-anchored soil, and a short distance for particles to fall before they can settle . An excess of vegetation can significantly reduce a wetland’s capacity to retain E. coli. Maximum removal of E. coli occurs under high solar radiation and high temperature conditions , and vegetation provides shading that can greatly reduce both UV radiation and water temperatures. While vegetation can provide favorable attachment sites for E. coli, a dense foliage canopy can hinder the free exchange of oxygen between the wetland and the atmosphere. This vegetation induced barrier to free exchange of oxygen limits dissolved oxygen levels, and that in turn reduces predaceous zooplankton, further decreasing removal of microbial pathogens from the wetland environment . The plants’ uptake of pollutants, including metals and nutrients, is an important mechanism, but is not really considered a removal mechanism unless the vegetation is harvested and physically removed from the wetland. Wetland vegetation also increases the surface area of the substrate for microbial attachment and the biofilm communities that are responsible for many contaminant transformation processes. Shading from vegetation also helps reduce algae growth. However, certain types of vegetation can attract wildlife such as migrating waterfowl, which may then become a source of additional pathogens. Vegetation that serves as a food source or as roosting or nesting habitat for waterfowl may need to be reduced in some settings. Among other important considerations for vegetation coverage in wetlands, one must include total biomass and depth features. Vegetation should provide enough biomass for nutrient uptake and adsorptive surface area purposes, but must also be managed to allow sufficient light penetration to enable natural photo degradative processes and prevent accumulation of excessive plant residues, which would prevent the export of dissolved organic carbon. One way to promote this balance is to create areas of deeper water intermixed with the shallower areas. In an agricultural setting, it may be hard to establish plantings of native species within wetlands due to the large seed bank of exotic species that may be present in input waters . You can also manage the type and amount of vegetation by manipulating the timing and duration of periods of standing water in the system. In extreme instances, you can actually harvest excess biomass. In addition to managing vegetation and water depth to maximize sedimentation and pathogen photodegradation, growers can also manipulate hydrology to maximize the removal of microbial pollutants in wetlands. The importance of hydrologic residence time is apparent when you recognize that a longer HRT increases the exposure of bacteria to any removal processes such as sedimentation, adsorption, predation, impact of toxins from microorganisms or plants, and degradation by UV radiation . E. coli concentrations have been shown to increase in runoff from irrigated pastureland when the volume of runoff is increased .