SITE SPECIFIC SEPTIC TANK SOIL-ABSORPTION SYSTEM DENSITY/LOT-SIZE RECOMMENDATIONS, CEDAR VALLEY, SOUTHWESTERN UTAH

Septic tank soil-absorption systems are the primary means of wastewater disposal in Cedar Valley, Utah, where development is proceeding at a rapid rate and ground water from the unconsolidated valley-fill aquifer is the primary source of drinking water. Nitrate is one indicator of pollution from septic-tank systems, and can aid in determining any deleterious effects of development on ground-water quality. We performed site-specific mass-balance evaluations for three areas in Cedar Valley to be used as models for proposed subdivisions to determine recommended septic-system density/lot size. The aquifer in each area has different hydrogeologic properties and hydraulic gradients, ranging from leaky confined to confined conditions and recharge to possible discharge areas; these characteristics control the amount of ground water available for mixing with septic effluent, and thus yield recommendations for lot-size development unique to each subdivision. Background nitrate concentrations vary for each area, from 0.6 to 2.15 mg/L, but the amount of ground-water flow available for mixing is the major control on projected nitrate concentrations.

Calculations show (assuming a degradation limit of 1 mg/L in water quality from nitrate) recommended average lot size on an alluvial-fan recharge-area development in southwestern Cedar Valley may be as small as 5.6 acres, while in the drier western part of the basin located in a secondary recharge area, average lot size should be no smaller than 54 acres. In the basin center, a possible discharge area, the amount of effluent water produced from existing septic systems exceeds the amount of natural ground-water flow in the aquifer available for mixing. Because of this condition and an apparent upward vertical-head gradient in the area, we believe the mass-balance approach is not the best land-use management tool, and recommend managing ground-water withdrawals to maintain the upward gradient; relatively thick, continuous clay layers that are prevalent in this area produce confined conditions that may protect the aquifer from contamination associated with septic systems.