Monitoring Data from a Pilot-Scale Horizontal Levee

Data were collected from a pilot-scale horizontal levee at the Oro Loma Sanitary District wastewater treatment facility in San Lorenzo, CA. The experimental system was constructed in 2017 to test this new type of treatment wetland. Horizontal levees, which are situated between coastal storm control levees and tidal mud flats, were first proposed for use in San Francisco Bay to remove contaminants from wastewater effluent in subsurface treatment layers while simultaneously providing flood control benefits by protecting coastal levees against wave action and storm surges. Additional benefits, such as the establishment of valuable habitat and recreational opportunities, may also be afforded. The experimental constructed wetland consisted of a set of twelve separate wetland cells that were designed with various "treatments" based on differences in their plant community, soil type and topography. Data were collected to: (1) assess the relative influence of design parameters on water quality improvements; (2) evaluate the importance of various mechanisms to contaminant transformation and removal; and, (3) develop new methods for monitoring nature-based treatment systems. Measurements of water quality parameters, contaminant concentrations, and flows, characterizations of mineral formation and microbial communities in soils, and plant biomass growth measurements and satellite imagery are provided within this dataset. The results of a few of the models used to interpret these data are also provided. Metadata are provided within the spreadsheets or in separate text files to facilitate data interpretation. References are provided where data are related to other spreadsheets, datasets or specific publications. We expected design parameters to have a significant impact on contaminant removal, but design did not impact fractional removal of contaminants (i.e., nutrients, trace organic contaminants and pathogen indicators). Rather, hydrological conditions controlled fractional removal, with design parameters having an impact on the total mass of contaminants removed. For example, willows increased subsurface flow rates and the mass of contaminants removed in those cells, although the fraction of applied contaminants removed remained the same. We also hypothesized denitrification would be the primary removal mechanism for nitrate. As expected, nitrate removal occurred primarily through microbial processes in the saturated subsurface and plant uptake contributed only 8% o