Development of Algorithms for Modeling Onsite Wastewater Systems within SWAT

Abstract

Onsite wastewater systems (OWSs) are a significant source of nonpoint-source pollution to surface and groundwater in both rural and suburban settings. Methods to quantify their effect are therefore important. The mechanics of OWS biogeochemical processes are well studied. However, tools for their assessment, especially at the watershed scale, are limited. As part of this work, modeling capabilities were developed within the Soil Water Assessment Tool (SWAT) such that OWSs and their subsequent environmental impacts can be evaluated A case study was initiated on the Hoods Creek watershed in North Carolina to test the new SWAT algorithms. Included were: (1) field-scale simulations of groundwater quantity (water table height) and quality (N, P), (2) Monte Carlo evaluations of OWS service life to evaluate suggested calibration parameters, and (3) assessments of watershed-scale pollutant loadings within the model. Results were then analyzed at both the field and watershed scales. The model performed well in predicting both site groundwater table levels (R 2 = 0.82 and PBIAS = -0.8%) and NO 3 -N concentration in the groundwater (R 2 = 0.76, PBIAS = 2.5%). However, the performance for PO 4 -P simulations was less reliable due to difficulty in representing the mobility of soluble P in the soil. An advanced P algorithm is recommended to address the sophisticated physiochemical properties of soil particles and improve the model's performance.n rural areas, it is often inefficient to operate centralized wastewater systems for the purpose of domestic sewage treatment due to the sparse residential densities, uneven terrain, and/or limited water or energy supplies. In such instances, decentralized types of wastewater treatment systems (i.e., onsite wastewater systems) are typically used. In fact, more than 25% of existing homes and 37% of new developments in the U.S. use onsite wastewater systems (OWSs) for wastewater disposal (USEPA, 1997). The U.S. Census Bureau (1999) estimates that more than 60 million people depend on decentralized systems for their treatment needs. Hence, understanding the biophysical processes within OWSs, and the environmental impacts thereof, is of great importance to scientists, watershed managers, and regulatory agencies.

Assessment of Nitrogen Inputs into Hunt River by Onsite Wastewater Treatment Systems via SWAT Simulation

“…According to Jeong et al [11] and Hoghooghi et al [14], the contribution of groundwater to streamflow is relatively high during dry years, similar to the research findings of this study.…”

“…By representing the area under the septic system, using available sewer data and TN loads data, a more accurate model of anthropogenic nitrogen inputs into the Hunt River Watershed was able to be developed. This study showed the importance of groundwater contributions to streamflow and nitrogen loading and the findings are corroborated by research conducted by Jeong et al and Hoghooghi et al [11,14]. …”

“…In this study, the BFlow separation tool was used to determine the initial Alpha_BF [31]. As shown in both Tables 2 and 3, a higher ALPHA_BF value indicates higher base flows and lower GWQMN resulted in lower base flows [11,16]. Finding optimal values for ALPHA_BF and GWQMN ensures that the streamflow is separated well to account for runoff and baseflow.…”

“…For mitigation purposes, it is helpful to model these inputs on local watershed scales to identify potential areas of high N inputs to target for reduction through policies and infrastructure development. Few studies have been conducted modeling nutrient inputs on the watershed scale using the Soil Water Assessment Tool (SWAT) due to complexities stemming from the uncertainty of differences in subsurface hydrology, soil and water dynamics, and the often lack of detailed information about OWTS [11]. Increased densities of OWTS have been shown to increase baseflow within a watershed system [12][13][14], which can have the effect of increasing nutrient loading into ground and surface waters, eventually entering into coastal waters.…”

Development and Application of Algorithms for Simulating Terraces within SWAT

“…The SWAT model was selected because of its use of readily available input data, process-based simulation algorithms, and its open source code (Easton et al, 2008). Many previous successful modifications of the SWAT model, conducted by developers all over the world, have contributed to the model's widespread use and flexibility (e.g., Easton et al, 2008;Gassman et al, 2007;Jeong et al, 2011aJeong et al, , 2011bMoriasi et al, 2009Moriasi et al, , 2011.…”