Modeling pollutant removal using the P-k-C* model in a proposed surface-flow treatment wetland to treat and reuse agricultural runoff in Newman, California

Naivy Dennise Rodal Morales

University of Califorina, Merced

Co-Authors: M. W. Beutel

Constructed wetlands use vegetation, sediment and biological processes to reduce pollutants in water which make them an effective treatment system. As an environmentally sustainability technique to improve water quality, the City of Newman, located in California’s Central Valley, aims to construct a 12-hectare surface-flow treatment wetland to treat irrigation runoff from surrounding agricultural fields. Our project involved modeling seasonal patterns of nutrient removal (total phosphorous (TP) and nitrate), and changes in salinity, in the proposed treatment wetland. Salinity is a main project concern as the City intends to reuse outflow from the wetland to dilute high-salinity water used to irrigate fodder crops. Hydrologic and water quality data in a “normal” water year were used to make water and mass balance calculations based on the P-k-C* model for the outlet concentration predictions of the wetland. The presentation discusses model development and the results of monthly outlet nutrient and salinity concentrations. In summary, nitrate concentration decreased on average 60%, but higher reduction was observed during the summer months of almost 90%. TP concentration decreased 20-35% and had two peaks in removal, one in April-May and another in September-October. The seasonality of the model results highlights the fact that nitrate is microbially mediated and temperature dependent (higher values in summer), while TP is agronomically mediated (peaks in spring and fall). Finally, salinity increased due to evapotranspiration by around 30% and mostly during warm summer months. The presentation concludes with a discussion of factors that play an important role in the effective operation of the wetland (e.g., residence time, number of cells, layout of cells) and a recommended wetland layout.

Author E-mail
nrodalmorales@ucmerced.edu

Please post comments and questions for the author below.

11 thoughts on “Modeling pollutant removal using the P-k-C* model in a proposed surface-flow treatment wetland to treat and reuse agricultural runoff in Newman, California

  1. Hi there! Treatment wetlands are such a cool way to improve water quality, so thank you for taking the time to talk about your research! I have a couple questions:
    What are fodder crops?
    Because salinity was higher coming out of the wetland, do you think wetlands should still be used? Should they be combined with other treatment options?
    I really like the idea of using agricultural runoff to continue irrigating agriculture! I’m surprised I haven’t heard this idea before! Thank you!

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    1. Hi, thank you for commenting! Wetlands are a natural and effective way to improve water quality, mostly when having a big area of land.
      Fodder crops are the ones used for animal feed, not for human consumption.
      Answering your second question, we still consider the wetland to be a good option to treat the water quality for the purposes of the City. The increment in salinity (~15%) was not as high as expected despite the climate and the fact that the water has high salinity since the beginning. However, we are suggesting Newman city other actions to avoid having a huge increment. First, the wetland should have a design that allows the water to be continuously flowing; subsequently, the water should not be stored in a tank for its future use because it can evaporate and concentrate the salts. We also suggest mixing the water with storm/irrigation water for dilution, and finally, we expect the vegetation in the wetland (halophytes) can help retain some of the salts in its structure decreasing the increment in salinity in the effluent of the wetland. However, we are still doing some research to find new alternatives to treat salinity (if you have any suggestions, please, feel free to mention it!)

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      1. Oh, I love the idea of using vegetation adapted to saline conditions to help improve the salinity! Thank you so much for your thoughtful response!

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  2. Interesting work Naivy, seems like an exciting project you were able to work on. I was wondering if you are planning on testing the uncertainty associated with your choice of k values. The range provided in the Kadlec book is for a large number of systems worldwide, and it seems to me like you could use values more specific to California.

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    1. Hi Mauricio, thank you for your comment, it is an interesting project! As a matter of fact, when we were developing the model for phosphorus removal, we were concern with which k-value to use. As you mentioned, the value the book provides is not specific for California´s region, which tends to have warmer temperatures that increase the k-value. One way we will address the uncertainty is to present the results using confidence intervals for the rate constant, and calculate the 95 percentile in the outlet concentration (for variability due to stochastic events). However, as these results are preliminary, the wetland is not already built, we used the mean value presented in the book, which is a little bit low, to measure the possible effectiveness of the wetland for the City purposes under a low-rate constant. We are doing some literature review to find some studies of wetlands in California or areas with similar climate conditions to have a better sense of which k-value to use in the future.

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  3. Excellent work Naivy.
    I have a small question, could it be possible to develop a similar idea but in a city that doesn’t have agriculture?

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    1. Thank you, Gustavo! Yes! Treatment wetlands are an effective technique to treat wastewaters due to their high rate of biological activity and processes that allow the transformation of pollutants into less harmful substances and at a low cost. Although, the transformations also depend on the wetland size, therefore, most of the wetlands are built outside the cities, but treat their waters, because of the area of land needed.
      The type of wetland modeled is called Free Water Surface wetland (FWS) which is similar to natural wetlands, and is commonly used for urban, agricultural, and industrial stormwater, because of their ability to deal with pulse flows and changing water levels. They are also a frequent choice for treatment of mine waters, and for groundwater remediation and leachate treatment due to its adaptability to multiple climates, among other characteristics (Kadlec and Wallace, 2008).

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