Hydrologic Assessment for Wastewater Land Disposal.

Proper field assessment of soil hydrological properties is the most important part of any soil/site assessment and design for wastewater disposal/dispersal systems.  Whether for large or small systems, any wastewater application to land at a given site is in addition to the water that enters the site through natural precipitation, surface flow and subsurface flow.  Therefore, a hydrologic assessment of the ability of soil and the site to assimilate the added water must be performed.  In general, for small systems (e.g., single family housing), this hydrologic assessment is performed indirectly by determining an appropriate long-term acceptance rate (LTAR) through morphological characterization or in-situ measurement of the soil percolation rate.  For large systems, however, a more elaborate assessment of soil hydrologic properties and modeling water flow from the system/site are needed.  For most practical applications, the hydrologic properties are limited to the depth to the water table (i.e., thickness of the vadose zone), thickness of the aquifer, and hydraulic conductivity within the vadose zone and the aquifer.  Due to difficulties associated with determining the unsaturated hydraulic conductivity curve at different depths and locations within the vadose zone, the measured saturated hydraulic conductivity (K_sat) is used for modeling purposes.  Although the procedures for determining K_sat of both saturated and unsaturated zones are readily available, some consultants and regulatory officials dealing with land application of wastewater do not follow the proper steps or the most appropriate methodology for a given site.  In this paper proper procedures for determining K_sat of the vadose zone by constant head well permeameter method and the saturated zone by slug test will be presented.  Specifically, field data collection and the use of an appropriate model for determining K_sat will be discussed.