ISSN:
1745-6584
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Energy, Environment Protection, Nuclear Power Engineering
,
Geosciences
Notes:
Over the past 30 years East Portland and central Multnomah County have metamorphosed from a rural-suburban to a locally urban community. Services, including community water and sewer have been extended to most of the area. However, a 30-square-mile (80 km2) area within central Multnomah County remains unsewered today. This area reportedly disposes of 8 to 10 mgd (34,400 m3/day to 38,000 m3/day) sewage via subsurface systems, i. e. cesspools, seepage beds, and drainfields. These methods of waste disposal have resulted in the degradation of the ground-water resource within the study area.Most of the developed area is located on a relatively level terrace made up of Pleistocene fluviolacustrine sediments. Partially cemented gravels of the Pliocene Troutdale Formation underlie the terrace deposits. Both of these units are generally excellent aquifers where saturated. The depth to water in the unsewered area ranges from about 100-200 feet (30 to 60 m) in the southern terraced area to less than 10 feet (3 m) in much of the northern area underlain by younger, floodplain, terraces adjacent to the Columbia River.Central Multnomah County is situated within a regional ground-water discharge zone. It receives ground-water recharge from the Cascade Mountains to the east and intermediate recharge from the Cascade foothills and other isolated hills bordering and within the study area. The major surface drains receiving ground water from the regional and intermediate flow systems are the Willamette, Clackamas, and Columbia Rivers.The fluviolacustrine terraces constitute a local recharge zone. The primary ground-water recharge source is infiltrating precipitation as evidenced by the paucity of natural surface drainage, although the area receives in excess of 40 inches (100 cm) of precipitation per year. However, due to development and its attendant reduction in area for infiltration, e.g. paving and building, there has been a decrease in natural recharge. The estimated 8 to 10 mgd (34,400 m3/day to 38,000 m3/day) of domestic waste which is disposed of via the subsurface is thus introduced as supplemental local recharge.Infiltrating precipitation and sewage effluent migrates downwards through the water table. The depth to which the local recharge can penetrate the water table is limited by its hydraulic potential and the vertical hydraulic conductivity of the substrata. Therefore, the NO3-N contaminated recharge is effectively buoyed up and migrates laterally along the upper portion of the water table to its eventual surface drain, Columbia Slough South Arm.Water samples from wells developing water in adjacent or upgradient sewered areas and/or from deeper aquifers within the unsewered area generally have NO3-N concentrations of less than 1 mg/1. Shallower wells and springs within the unsewered area and South Arm Slough, downgradient grom the unsewered area, had NO3-N concentrations ranging from 4.7 to 11.86 mg/1, with a mean value of 7.74 mg/1 in July 1974.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1745-6584.1974.tb03045.x
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