2019_KittitasCountyHMP_Volume 1_For Adoption

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CHAPTER 4. KITTITAS COUNTY PROFILE 4-5 The load-bearing capacity of soil, its hydric properties, erosion potential, and other characteristics all play a significant role in the development of land. Hydric properties in soils indicate the existence of wetlands and signal the potential for other environmental concerns. Soil suitability for structural support and stability is also important in determining the potential for development. Area soil types vary considerably: • Soils in western mountainous portion of the county are more suited for growing forest products than food-crop farming. These soils are strongly acidic, gravelly or rocky, saturated most of the year, and occur in steep areas at high elevations. • Soils in the foothill areas with streams are ideal for growing native trees. • Soils in the Yakima River Valley are more suited to agriculture and those on the south slopes of the valley are used for extensive fruit growing. • Other areas have been designated as critical areas due to erosion and landslide potentials. Suitability for Septic Tanks and Drainfields For developments dependent on septic tank systems for on-site wastewater treatment, soils are important in determining the degree of development feasible without contaminating groundwater and surface water supplies. Areas well-suited for liquid waste disposal contain gravelly, sandy soils approximately 4 to 6 feet in depth sitting over an impermeable layer, such as till. Several factors severely limit septic tank use in Kittitas County: • Shallow soils cover much of the western portion of the county. If soils are too shallow, the decomposition process of septic tank effluent does not proceed far enough to avoid contaminating surface water or groundwater. • High water tables exist in river valley areas, rendering the underground reservoirs susceptible to contamination from failing septic systems. • Rainfall varies widely from one end of the county to the other. Depth-to-Seasonal Water Table Depth to seasonal water table is a measurement from the surface to the water ta ble during the wet months of the year. A shallow depth between the ground surface and the water table may cause both foundation and septic tank effluent disposal problems. A high seasonal water table may inhibit septic tank effluent from being properly treated in the soil. It may also cause foundations to “float” on their footings, resulting in structural damage to buildings. Glacially cemented hardpan layers and shallow depth to bedrock account for portions of the county having a shallow depth-to-seasonal water table, (0 to 3 feet below the ground level.) These areas are not perched water tables. They can be either level or sloped areas with a hardpan layer underneath. Aquifer Recharge Potential Aquifer recharge potential is the relative ability of the soil and underlying geology to transport rainwater into underground aquifers. This classification considers the water-intake rate of topsoil and the permeability of subsoils and parent materials. While it is not known if water falling on these areas actually r eaches the aquifers, it is not unreasonable to assume that these areas do play a role in recharging underground water reservoirs. Aquifer recharge areas contain some of the most permeable soils. Conflicts persist regarding appropriate land use consistent with soil capabilities for on-site wastewater treatment. Rooftops, driveways, walkways, and frontage roads all reduce the amount of land surface available to receive rainwater. In areas of extreme permeability, septic tank effluent may percolate faster