The Fate of Recycled Water in Restored Wetlands

This research investigates the hydrological fate of treated wastewater or “recycled water” used for irrigation at a restored wetland site (Thomas Dairy) near Tigard, Oregon, that is managed by Clean Water Services (CWS). CWS is interested in understanding how treated wastewater moves through plants and soil in ecosystems that might be targeted for recycled water applications. Using a water balance approach, we evaluated irrigation inputs, soil infiltration dynamics, and hydraulic conductivity measurements across three soil types with different textures and other properties. Field measurements were collected using advanced sensors (TEROS and ATMOS devices) and laboratory tools (KSAT) to quantify soil hydraulic properties and assess infiltration rates. The water balance framework was developed to assess rainfall and irrigation inputs, as well as the infiltration capacity of three soil types. Additionally, to estimate outputs, evapotranspiration (ET) was calculated using the FAO Penman–Monteith method. We found significant variability in hydraulic conductivity among the three main soil types found at the site. Cove Clay exhibited the lowest hydraulic conductivity, with slower water movement due to finer particles, while McBee silty clay loam soils had higher hydraulic conductivity due to coarser soil texture. Trends in matric potential and water content illustrate the dynamic nature of soil hydrology, with shallow soils responding faster to irrigation and environmental changes compared to deeper soils. During the primary summer analysis period (19 July–22 August 2024), estimates of evapotranspiration, soil infiltration, and available water retention capacity in the three soil types indicate that groundwater depth is the primary control on soil water storage and the soil’s ability to maintain moisture availability for wetland vegetation and resist drying stress during periods of high evapotranspiration specially in dry summers at each site. These results provide critical insights into the fate of recycled water in these restored wetlands, emphasizing the influence of soil characteristics on water storage and movement. These hydrologic assessments will help managers understand the likelihood of groundwater recharge with different irrigation rates at Thomas Dairy, with the potential to impact water management practices relating to water reuse. In the long run, this assessment will help CWS evaluate sites for water reuse, supporting more sustainable water management and better irrigation practices.