A watershed model to understand groundwater and surface water interactions to support sewer utility resilience at the Jacksonville N.C. forest water re-use facility

Abstract

The goal of this project was to develop a comprehensive, deterministic, distributed and physically-based hydrologic model (MIKE-SHE) to provide the City of Jacksonville (NC) with historical and current visualizations of how their current municipal wastewater treatment system, a forest land application site (FWR), functions hydrologically among the seasons in response to weather, forest age, and forest management. Project objectives were to use the model to (1) forecast FWR response under different scenarios of weather extremes and (2) to forecast FWR response under different regimens of water reuse or forest composition using a water balance approach. The City of Jacksonville is exploring strategies to increase FWR capacity for future demand. Simulated evapotranspiration (ET) and water table depth (WTD), using MIKE-SHE and twenty years of measured precipitation and irrigation data from the land treatment facility, were used to calculate drainage (runoff and lateral flow) across the site. Irrigation impacted ET and WTD to the greatest extent for forest areas surrounded by irrigation fields but caused little change in annual ET. Forest water use was relatively unchanged by irrigation, and annual watershed drainage increased proportionally to irrigation input. The drivers of on-site drainage were rainfall and the amount of irrigation. In wet years, ET and groundwater levels remained constant while drainage increased in response to rainfall and irrigation. WTD in wells surrounded by wastewater irrigation remained consistently closer to the surface than wells with only partial irrigation nearby. The model under-predicted WTD for wells on the site’s exterior during below average rainfall periods. Groundwater withdrawal for agricultural use by adjacent landowners may explain this discrepancy. Extreme rainfall events, such as Hurricane Florence, resulted in high volumes of drainage but rapid recovery of groundwater storage in the FWR. The model provided insight to management practices that could increase FWR efficiency and flexibility for managing variable weather with climate change. One observation of the model was that current increases in irrigation volumes from winter application volumes to higher summer application volumes lag behind ET. Hence, one management option to increase irrigation capacity is to increase irrigation volume earlier in March rather than May. A sensitivity analysis of rooting depth and leaf area index to water use showed that rooting depth mattered more for water use than LAI. Forest management practices such as bedding for replanting would improve rooting depth of trees. Nancy Gibson presented project results to the City of Jacksonville in March 2019. COJ was very excited that project results support current operation perspectives that the FWR could treat more wastewater if allowed more flexibility to land apply when conditions are optimal rather than prescribed volumes per week. Model observations that irrigation does not limit FWR ET and that rainfall drives FWR export of water were key outcomes that resonated with city officials and operators. Project results will be provided as an executive summary for the City of Jacksonville personnel to use in discussions with NCDEQ as both organizations discuss revised permits to avoid emergency spraying for extreme storms. This study has shown that these unique forest systems offer insights to water balance dynamics in irrigated forests and forest resiliency to extreme hydraulic loading that can be of use to regional wastewater land treatment systems for North Carolina.