Description of the ground-water flow system in the Portland Basin, Oregon and Washington - Softcover

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1996-06 edition. Excerpt: ...detention storage, climatic, and panevaporation data, it was found that about 74 percent of the average annual precipitation falling on impervious surfaces would run off to drywells (where drywells were used). In urban areas where drywells are present, the estimated average volume rate of recharge from drywells is 61.7 cubic feet per second, of which nearly one-half, or 27.1 cubic feet per second, occurs in Clark County. Rates of drywell recharge in the 3,000 foot ground-water model grid cells range from 0.1 to 26 inches per year, with a mean of 9.4 inches per year (Snyder and others, 1994). On-site waste-disposal systems (septic tanks and cesspools) are used in many parts of the study area to dispose of commercial and domestic wastewater. Effluent from these systems, like surface runoff shunted to drywells, bypasses most or all of the nearsurface, unsaturated-zone processes of evapotranspiration, filtration, and bio-retardation. This makes these on-site waste-disposal systems an efficient avenue for recharge to the aquifer system in local areas. On-site waste-disposal systems are concentrated mostly in urban areas, whereas recharge from these systems in rural areas is less significant. The midMultnomah County, Oregon, area has the highest density of on-site waste-disposal systems in the Portland Basin, with approximately 50,000 residential and commercial systems within a 39-square-mile area. This area is currently being sewered to eliminate onsite waste-disposal systems by the year 2003. Another large unsewered urban area is located in the vicinity of Burnt Bridge Creek in Clark County, Washington, which is similar in size (approximately 7,500 systems) to the unsewered area in Multnomah County. Annual recharge volume for each residential...

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