Analysis of Septic-Tank Density for Rockville, Washington County, Utah
Authors: Schlossnagle Trevor H. , Duncan Torri 
Published: September 2024

Rockville is a small rural town in southwestern Utah that is experiencing an increase in residential development. New developments in rural areas often use septic tank soilabsorption systems for wastewater disposal, although there is potential to utilize an existing community sewer system. Because potential future septic-tank systems may overlie the principal drinking water aquifer for Rockville, city off icials asked the Utah Geological Survey to conduct a septictank density analysis. The purpose of our study is to provide tools for water-resource management and land-use planning. In this study we (1) characterize the groundwater quality of Rockville with an emphasis on nitrate, and (2) provide a mass-balance analysis for three subdomains (Virgin River corridor, South Mesa, South Mesa subdivision) based on numbers of septic-tank systems, groundwater flow available for mixing, and baseline nitrate concentrations, and thereby determine appropriate septic-system density requirements to limit water-quality degradation. There are two aquifers commonly utilized in Rockville: The Shinarump Conglomerate (primary aquifer) and the Shnabkaib Member of the Moenkopi Formation and overlying alluvial deposits along the Virgin River (secondary aquifer). Using 11 groundwater samples taken from water wells and a spring, we established that baseline nitrate concentrations for these two Rockville aquifers are low, at 0.1 mg/L for the primary aquifer and 0.26 mg/L for the secondary aquifer. To determine the ideal septic-tank density, we employed a mass-balance approach using existing septic systems and baseline nitrate concentrations. Nitrogen in the form of nitrate is one of the principal indicators of pollution from septic tank soil-absorption systems. For the mass-balance approach, the nitrogen mass from projected additional septic tanks is added to the current nitrogen mass and then diluted with groundwater flow available for mixing plus the water added by the septic-tank systems themselves. We used an allowable degradation of 1 mg/L with respect to nitrate. Groundwater flow volume available for mixing was calculated from existing hydrogeologic data. We used data from aquifer tests and specific capacity data from water well logs to derive hydraulic conductivity for Rockville’s aquifers. Existing publications and potentiometric surface datasets were used to determine groundwater flow directions and hydraulic gradients. Our results using the mass balance approach indicate that the most conservative septic-tank densities for the Virgin River corridor, South Mesa, and South Mesa subdivision subdomains are 11 acres per system, 13 acres per system, and 8 acres per system, respectively. Due to the low baseline nitrate concentrations, higher septic-tank densities may still be conservative enough to protect water quality. These determinations are based on hydrogeologic parameters used to estimate groundwater flow volume. Connecting to existing community sewer systems may be a safer alternative to septic-tank systems where feasible.