The distribution coefficients for chlorpyrifos in a variety of soils range from 45 to 1300 L kg -1 . PTE is a zinc metalloenzyme that was initially isolated from strains of soil bacteria capable of hydrolyzing a variety of organophosphate compounds, including agricultural pesticides and chemical warfare agents . This enzyme is reportedly responsible for the bio-transformation of organophosphate insecticides . Phosphomonoesterase and phosphodiesterase enzymes have been shown to be involved in chlorpyrifos mineralization by making phosphorus available for uptake by microorganisms . Bacterial phosphodiesterase has been purified from a wide range of organisms, including Escherichia coliand Burkholderia caryophylli PG2982 . A novel phosphodiesterase was isolated and cloned from Delftia acidovorans which has both mono- and diesterase activity . This enzyme allows the microorganism to use diethyl phosphonate as a sole source of phosphorus under phosphoruslimiting conditions. The final enzyme in the postulated transformation pathway is alkaline phosphatase, which can hydrolyze simple monoalkyl phosphates.The most prominent of the heterocyclic metabolites from chlorpyrifos degradation in soils is 3,5,6-trichloro-2-pyridinol , the phosphotriesterase product. Other heterocyclic products have been identified, including chlorpyrifos oxon ,hydroponic nft system desethyl chlorpyrifos , and TMP ; however, TCP has been consistently found at comparatively higher concentrations . Chlorpyrifos can be transformed co-metabolically in solution by Flavobacterium sp. and Pseudomonas diminuta, strains that were initially isolated from a diazinon treated field and by parathion enrichment, respectively.

However, these microorganisms do not utilize chlorpyrifos as a source of carbon. A Micrococcus sp. isolated from a malathion enriched soil was later reported to degrade chlorpyrifos in aqueous media . Chlorpyrifos transformation rates can vary widely in different soils with half-lives ranging from 10 to 120 days . This large variation in half-lives has been attributed to different environmental factors, such as soil type, soil pH, moisture content, temperature, and organic carbon content. When soils are repeatedly exposed to chlorpyrifos, some microorganisms may exhibit an enhanced capability to transform the compound, a phenomenon called enhanced biodegradation . A possible reason for resistance of chlorpyrifos to enhanced biodegradation is the toxic effects of TCP . TCP contains three chlorine atoms on the pyridinol ring. To break the pyridinol ring of TCP, three chlorine atoms attached to the ring have to be removed. The resulting free chlorine may have toxic effects on the microorganisms . Because of this phenomenon, it has been problematic to isolate a pure bacterial strain capable of degrading chlorpyrifos from repeated treatments or enrichment of soil samples . Singh et al. suggested that chlorpyrifos is degraded by nonspecific and non-inducible enzyme systems produced in high pH soils. This suggests that chlorpyrifos is co-metabolically hydrolyzed, and enhanced degradation does not occur normally due to the toxic effects of TCP. However, Singh et al. was able to isolate an Enterobacter sp. from an Australian soil that exhibited enhanced degradation of chlorpyrifos. This bacterium degrades chlorpyrifos to DETP and TCP and utilizes DETP as a source of carbon and phosphorus . In a similar study, Yang et al. used an enrichment culture from a contaminated soil to isolate an Alcaligenes faecalis DSP3 strain which was able to degrade chlorpyrifos and use it as the sole carbon and phosphorus source. It was shown that A. faecalis DSP3 strain was able to tolerate TCP concentrations as high as 800 mg/L . Feng et al. first reported the isolation of a Pseudomonas strain ATCC 700113 that mineralized TCP in liquid medium with the concurrent evolution of chlorine.

Further studies suggested that metabolism of TCP follows two successive dechlorination steps leading to the formation first of chlorodihydro-2-pyridone and then tetra-hydro-2-pyridone . In another study, a Bacillus pumilus strain C2A1 isolated from soil was highly effective in degrading chlorpyrifos and its metabolite TCP .While a significant body of research exists on the treatment of urban storm water and municipal wastewater effluents in treatment wetlands, the application of wetlands for the treatment of agricultural drainage water has not been well studied. The applicability of first-order kinetic relationships developed in systems treating municipal wastewater to the sizing of treatment wetlands in agricultural watersheds is problematic, given large differences in hydrologic regimes, influent water quality, and the potential treatment goals for treatment wetlands in agricultural watersheds. In the case of California’s San Joaquin Valley, wetlands are being considered for mitigation of agricultural impacts, but large uncertainty in the land requirements are a major impediment to full-scale implementation. To investigate nitrate removal kinetics in wetlands receiving agricultural drainage, field studies were conducted and nitrate removal efficiencies were determined at three sites. Microcosm studies were conducted to supplement field data. The measured kinetic parameters were used to estimate the amount of land area that would need to be devoted to wetlands in order to mitigate agricultural impacts on a watershed scale.Chlorpyrifos is the most widely used organophosphate insecticide in California’s San Joaquin Valley and is widely used elsewhere. While there have been several studies evaluating the effectiveness of different BMPs on chlorpyrifos mitigation, these studies have mostly focused on sorption of chlorpyrifos to wetland sediments and soils, with removal efficiency assessed by measuring inlet and outlet concentrations in water samples.

However, it is also important to elucidate the fate of the chlorpyrifos in a wetland system. This is particularly important because, in the long term, particle-associated pesticides stored in the sediments can be transported via runoff and other processes where they could impact downstream surface water systems. Therefore, it is important to quantify the ultimate fate and transformation rate of the chlorpyrifos within the system before reaching conclusions on how effective a natural treatment system is with respect to contaminant attenuation. To investigate the factors controlling biodegradation of chlorpyrifos, four agricultural watersheds were selected and transformation rates were measured for a 24-month period in sediment deposits from agricultural drains and wetlands receiving agricultural return flows. To explain the observed temporal and spatial variation in the biotransformation rates among the study sites and to gain insight into wetland management for the mitigation of organophosphate pesticides, several factors such as sediment characteristics, redox conditions and exposure histories were investigated. To evaluate the effect of wetland management practices under different scenarios, a GIS based water quality model WARMF was used. The model has a specific interface, “The San Joaquin River Modeling Interface” designed for the region, and it is capable of simulating hydrology and non-point source of pollutants from various land uses including the non-point loads due to fertilizer and pesticide applications. Utilizing the model’s capabilities and the kinetic parameters obtained from Objective 1, nitrate removal was assessed under different land use scenarios based on the amount of land that would be allocated to wetlands; and using kinetic parameters obtained from Objective 2 for different wetland management strategies, chlorpyrifos mitigation was evaluated based on chlorpyrifos application trends in the region. High flux of nitrate and other sources of nitrogen from a watershed are an indication that the level of functional ecosystem services in that watershed is low . The San Joaquin River in California is a highly engineered and therefore highly impaired water body that is the focus of a number of major engineered management and restoration efforts, including a new effort directed at the reestablishment of flows to support extinct salmon runs . Current restoration plans do not explicitly include the restoration of general ecosystem services,nft channel in part because engineering criteria for restoration of such services are not well developed.The San Joaquin Basin is one of the most productive agricultural regions in the world and agricultural runoff has been identified as an important factor contributing to the eutrophication of the San Joaquin River . Agricultural runoff entering the San Joaquin River is characterized by high concentrations of nutrients, including nitrate as the predominant form of nitrogen . There has been a long-standing interest in improving water quality in the San Joaquin Basin and mitigating the impact of agricultural runoff on the San Joaquin River and the Bay-Delta Estuary . Wetlands included in this study are seasonal wetlands that were constructed for purposes other than pollutant treatment, including fish farming and wildlife habitat . These and other wetlands in the region currently receive inputs made up almost entirely of return flows from irrigated agriculture . Treatment wetlands have been designed, and their performances evaluated based on first order degradation models, including newer models such as the P-k-C* model, typically limited by a residual outlet concentration . In part, the uncertainty of the first-order approach is compensated for by drawing on a large body of experience using wetlands for the treatment of municipal waste waters and databases are available for meta-analysis of wetland performance in the treatment of these waste waters . There are a number of potential drawbacks to this approach, including questions about the applicability of data collected in one wetland for use in designing subsequent wetlands .

The applicability of first-order kinetic relationships developed in systems treating municipal wastewater to the sizing of treatment wetlands in agricultural watersheds is problematic, given large differences in hydrologic regimes, influent water quality, and the potential treatment goals for treatment wetlands in agricultural watersheds. In the case of the San Joaquin Valley, construction of wetlands to mitigate agricultural impacts are being considered, but large uncertainty in the land requirements are a major impediment to full-scale implementation. In this chapter, kinetics of nitrate removal in wetlands located in the San Joaquin River Valley that are receiving agricultural drainage from irrigated cropland was investigated. Saturation kinetics for nitrate removal in wetland sediments was evaluated and the measured kinetic parameters were used to estimate the amount of land area that would need to be devoted to wetlands in order to mitigate agricultural impacts on a watershed scale. The target effluent nitrate-nitrogen concentration was chosen as 0.5 mg L-1 which is the proposed limiting concentration for control of nuisance algae.Three field sites, Ramona Lake , the San Joaquin River National Wildlife Refuge , and Miller Lake were included in this study . All three field sites are located in Stanislaus County, CA. Miller Lake is located on the eastside of the San Joaquin River and receives operational spill water from the Modesto Irrigation Districts Main Drain Canal. In 2005, Miller Lake was drained, dredged, and replanted to rehabilitate the lake for aesthetics and wildlife habitat. The dominant vegetation at Miller Lake includes tule , water primrose , pennywort , native willows , and oak . Miller Lake has a surface area of approximately 206,539 m2 with an average depth of 0.6 m and it has a drainage area of 56.25 km2 . Seventeen water samples of the Miller Lake inlet and outlet were collected between April and October 2008. Ramona Lake and the SJRNWR are located on the west side of the San Joaquin River and, like Miller Lake; they receive variable pulse in-flows of agricultural drainage during irrigation periods . Ramona Lake is a late succession, culturally eutrophic lake. Ramona Lake was originally constructed to capture irrigation drainage as part of a fish farming operation in the 1960s. Since then, it has become silted and is now predominantly a marsh ecosystem. It is occasionally utilized as a reservoir for the reuse of irrigation drainage by local growers. The dominant vegetation at Ramona Lake consists of tule and cattail . Ramona Lake receives drainage from 20.23 km2 of farmland and has a surface area of approximately 80,937 m2 with an average depth of 0.6 m. Fourteen water samples of the Ramona Lake inlet and outlet were collected between April and August of 2008. The SJRNWR study site is former farmland that is being restored to a managed riparian wetland by the U.S. Fish and Wildlife Service. The SJRNWR study site is a 270,010 m2 , riparian wetland with an average depth of approximately 0.6 m. It has a drainage area of 38.45 km2 . Dominant vegetation at the SJRNWR include native willows , annual fire weed , and bermuda grass . Fourteen water samples of the SJRNWR inlet and outlet were collected between April and October of 2008. All sample collection, data evaluation, and analysis in this study were completed in accordance with rigorous QA/QC procedures . Flow rates were measured at weir structures installed at the inlet and outlets of the wetlands.