Water balance of municipal wastewater irrigation in a coastal forested watershed
In the southeastern United States, coastal communities face challenges for water resources and wastewater treatment capacity. In North Carolina, 51 municipalities irrigate forests with municipal wastewater to absorb nutrients, reduce direct effluent discharge to surface waters, and recharge groundwater. Most facilities have landapplied wastewater for decades, but there are no quantitative studies on the hydrologic impacts of this practice. This study developed a simulated water balance for the largest forest land-application system in North Carolina which treats wastewater daily by irrigating 30 km2 of a mixed hardwood-loblolly pine forest. A distributed hydrological model (MIKE SHE) was adapted to simulate 20 years of watershed evapotranspiration (ET) and water table depth (WTD) under irrigated and nonirrigated conditions. We found that irrigation impact to annual and monthly WTD was negligible in years with average and above average rainfall. For wet years, drainage increased with irrigation while ET and WTD remained similar to nonirrigated conditions. In dry years, ET was 31 to 39 mm higher in irrigated forest than nonirrigated forest though the change in groundwater storage remained close to zero annually. Our simulation study suggested that the drivers of on-site drainage were predominantly rainfall and irrigation, and the annual watershed drainage increased in volumes equal to 93%–100% of the added annual irrigation input. This study offers insights to water balance dynamics in irrigated forests and coastal forest resiliency to variable wastewater hydraulic loading.