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Filtering agricultural runoff with constructed and restored wetlands

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Introduction

 

Non-point source pollution (NPS) is a global problem affecting the safety of our drinking water supply and aquatic habitats. According to the 2000 National Water Quality Inventory, agriculturally derived NPS is the leading cause of water quality degradation in surface waters. Pollutants originating from agricultural runoff include sediment, nutrients (N and P), pesticides, pathogens, salts, trace elements, dissolved organic carbon and substances that contribute to biological oxygen demand (BOD). 

 

For example, discharge of nutrients into aquatic ecosystems has led to the formation of hypoxia/anoxia induced “dead zones” in more than 400 locations worldwide. Thus, new and effective management practices for agriculture must be identified, tested and monitored in order to reduce the impacts of agriculture on the sustainability of water resources.

 

Wetlands are widely advertised as critical components of our planet providing a wide variety of ecosystem services: kidneys of the hydrologic cycle by removing pollutants, biodiversity hot spots, habitats of rare and endangered species, ground water recharge zones, localized areas for flood protection, carbon sinks and aesthetic value.

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Upon settlement of the United States, the lack of understanding for the role of wetlands and drive for agricultural production resulted in a loss of over 53 percent of the nation’s wetlands. California and Texas, two states leading agricultural production in the U.S., have lost more than a combined 5 million hectares of wetlands.

 

Much of this loss was as result of programs in the U.S. such as Swamp Buster, which encouraged the conversion of marginal land (e.g. wetlands) into agricultural production. Coincident with this landscape conversion was the rise in use of agricultural chemicals. As a result, the filtration effect of wetlands has been uncoupled from riparian environments resulting in severe degradation of the nation’s (and the planet’s) water resources. Only in the last few decades have wetlands been recognized for their potential role to ameliorate NPS.

 

Traditionally, constructed and restored wetlands have been developed in agricultural settings to improve wildlife habitat, mainly through the U.S. Conservation Reserve Program and the Wetlands Reserve Program (WRP). The WRP is an outreach effort administered through the Natural Resource Conservation Service (NRCS) and its partners. It is designed to provide financial and technical assistance to landowners to restore, enhance and protect wetlands and surrounding surface waters.

 

The conversion of floodplain agroecosystems to wetlands is becoming a popular land-use practice nation-wide. In California many of these restored wetlands receive agricultural tailwaters during the growing season and little information exists to document how these wetlands filter water quality contaminants in runoff from agricultural fields.

 

There are several mechanisms acting in constructed and restored wetlands that contribute to the removal of contaminants, including: 1) sedimentation and burial (phosphorus, pesticides, particulate organic carbon, pathogens); 2) biogeochemical transformations (denitrification); 3) biotic uptake of nutrients and salts; 4) microbial degradation of pesticides and organic matter; 5) transformations affecting solubility, sorption and toxicity; 6) predation of pathogens; and, 7) photodegradation of pesticides and organic matter. As a result of these processes, it is commonly considered that wetlands have a predominantly beneficial effect on water quality

 

Researchers Toby O’Geen and Randy Dahlgren evaluated the performance of four surface flow-through wetlands located in the Central Valley of California that discharge into San Joaquin River (SJR). The goal of the study was to identify wetland design characteristics that optimize contaminant removal. We also evaluated potential adverse effects of wetlands on water quality.

 

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Approach

 

Tailwaters from flood and furrow irrigation were the main water source for all wetlands. Agricultural land use surrounding the wetlands consisted mostly of row crops (tomatoes, melons, beans and rice) and tree crops (nuts, stone fruits). Each studied wetland contained mineral dominated soils and showed no sign of peat accumulation.

 

Wetlands differed in parameters such as size, age, catchment area, vegetation type and coverage, and hydrologic residence time (HRT). Water quality monitoring was conducted at input and output locations of wetlands over the following irrigation seasons (March-August): 2004, 2005, 2007 and 2008. A suite of chemical, physical and biological water quality contaminants were measured from each sample including electrical conductivity (EC), pH, temperature, dissolved oxygen (DO), pathogens, pesticides, suspended sediment, nitrogen, phosphorus, dissolved organic carbon, and algae.

 

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Summary of Findings

 

All of the wetlands were shown to reduce most water quality contaminants. Wetlands were most effective at removing suspended sediment, nitrate-nitrogen, pathogens, and pesticides. For example, 90 to 95 percent of suspended sediment is typically removed from input waters. Approximately, 50 to 80 percent of the nitrate nitrogen is removed. Pathogen removal ranged from 79 to 90 percent.

 

Pesticide removal ranged from 50 to 100 percent. Wetlands were also effective at removing phosphorus that was bound to sediment (generally termed particulate phosphorus). Wetlands were less effective at removing dissolved phosphorus, algae, and dissolved organic carbon. In some cases wetlands actually increased the concentration of these contaminants in effluent.

 

Our findings demonstrated that large wetlands that have long (~10 days) hydrologic residence times (HRT) were very effective and removing a wide range of contaminants. Hydrologic residence time is the time it takes water to travel from input to output in a wetland. However, wetlands with long HRT also had adverse effects on water quality.

 

In particular, these systems had elevated salinity in output waters, which is an important water quality contaminant in fresh waters. Wetlands with short HRT (~1 day), were equally effective and removing suspended sediment but were slightly less effective at removing nitrogen. When considering the balance between contaminant removal and adverse effects our findings suggest that smaller wetlands (~3-5 acres) with short HRT are best for managing the wide range of contaminants in agricultural runoff.

 

There are some potentially adverse effects of wetlands that must be considered in certain regions. Areas with high background levels of mercury or selenium are of concern due to bioaccumulation and biomagnification of toxic metals within the food chain. Wetlands may also be a source of dissolved organic carbon that acts as precursors for formation of carcinogenic disinfection by-products during drinking water purification.

 

Wetlands with long HRTs can increase salinity in output waters due to evapoconcentration of salts and should be designed with short HRTS (~1 d) in areas with high salinity. Wetlands have the potential to emit potent greenhouse gasses, such as methane and N2O, thus contributing to global climate change. Wetlands may also provide breeding grounds for disease-carrying mosquitoes. Proper wetland management can greatly reduce these potentially adverse effects.

 

Wetlands can be employed as on-site or off-site BMPs to filter agricultural runoff.  To realize optimal water quality improvements at the watershed scale, wetlands should be included as part of a combination of management techniques, such as conservation tillage, improved irrigation and fertilization techniques, and vegetated filter strips. 

 

Further research evaluating the effects of wetland design and management options on NPS water quality concerns will continue to lead to enhanced wetland performance. When considering all of the ecological services provided wetlands should be promoted as an integral component of the farmscape.

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Soil Resource Specialist in Cooperative Extension


Dept. of Land, Air and Water Resources


University of California, Davis

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