The goal of this project is to develop a three-dimensional (3D) numerical groundwater flow model to evaluate the potential impacts to surface- and groundwater resulting from the disposal of treated wastewater in a portion of the Inland Bays drainage basin. The work will be accomplished with in-house software (ArcGIS, Surfer, and Visual MODFlow).
By developing a sub-regional, fresh, groundwater flow model and analyzing results, several issues will be addressed that are related to state policy, regulation revision, and proposed projects associated with land-based wastewater disposal (LBWD) in Sussex County.
The study area is bounded on the south by Indian River, on the north by Herring Creek, on the east by Rehoboth Bay, and the west by Cow Bridge Branch, in eastern Sussex County, with an approximate area of 64 mi2. This area is of interest because it includes two existing large (> 1 MGD, Inland Bays and Mountaire) and one small (<50 KGD, Baywood Greens) spray irrigation facilities, and an existing large (>500 KGD), but underutilized (<20 KGD) rapid infiltration basin disposal site (Stonewater Creek). Water supply is largely served by individual domestic wells with additional spatially dispersed public and agricultural wells.
Research for this project was completed in late 2012. Conclusions are as follows:
We conducted a groundwater flow model around two major wastewater irrigation facilities located in Sussex County, Delaware. The model was constructed using Visual Modflow 2011 to simulate the groundwater flow and NO3- movement in the shallow aquifers. The application of wastewater irrigation was simulated using Hydrus-1D to incorporate both evaporation and plant-water uptake. The leached wastewater was then applied to the original flow and transport model to study the impacts on the groundwater conditions. Some conclusions based on modeling results are summarized below:
1. The water table closely mimics the land surface topography. The residence time of groundwater has a bimodal distribution, with times less than 10 years as the largest grouping. The spatial distribution of residence times along with a median groundwater residence time of 29 years indicate that several decades are required to flush the majority of the water now in the aquifer with new water.
2. The long-term operation of wastewater spray irrigation increases the water-table elevations. The amount of increase is dependent on different irrigation practices, and can significantly change flow patterns around the treatment facilities.
3. The DNF constant is a sensitive parameter for modeling NO3- movement and transformation in both the unsaturated and saturated zones. More field data such as DOC, solid phase organic carbon, concentrations of dissolved gasses, and types and concentrations of organisms should be collected to obtain an accurate estimate of the parameter.
4. Simply reducing the effluent concentration and increasing the irrigation rate may facilitate the migration of the NO3- plume to deeper aquifers and further down gradient, but the high concentration area of nitrate could be smaller. An optimal practice of wastewater irrigation can be achieved by adjusting the irrigation rate and effluent concentration.
5. The accuracy of model predictions is highly dependent on the accuracy and completeness of the input data to construct, calibrate, and validate the model. Several data deficiencies limit the predictive power of the model used in this study. Improved data for groundwater levels and streamflow would permit better model calibration. More complete records of water use would permit more realistic predictions of the hydrogeological effects of pumping wells. Better data on wastewater discharge rates and effluent would improve the accuracy of the model predictions.
The complete report has been approved for release (August 2015).
He, C., and Andres, A.S., in press, Simulation of groundwater flow and contaminant transport in eastern Sussex County, Delaware with emphasis on impacts of spray irrigation of treated wastewater: Delaware Geological Survey Report of Investigations