Modeling Sediment Accumulation and Soil Erosion with 137Cs and 210Pb in the Ala Wai Canal and Central Honolulu Watershed, Hawai'i

McMurtry, Gary M.; Snidvongs, Anond; Glenn, Craig R.

Modeling Sediment Accumulation and Soil Erosion with 137Cs and 210Pb in the Ala Wai Canal and Central Honolulu Watershed, Hawai'i

Files

v49n4-412-451.pdf (11.82 MB)

Date

1995-10

Authors

McMurtry, Gary M.

Snidvongs, Anond

Glenn, Craig R.

Publisher

University of Hawaii Press

Abstract

Radiochemical studies of sediments from the Ala Wai Canal, an urban estuary in Honolulu, and of soils and stream sediments from the central Honolulu watershed were undertaken to investigate the sediment accumulation history and estimate the sediment yield and denudation rate of the watershed. Modern high-purity Ge gamma spectrometry techniques were used to assess the activities of U-series and 137Cs radioisotopes in stratigraphic subsamples of three 1- to 2-m-long sediment cores, 14 watershed soil horizons, and grab samples of seven tributary stream sediments. Geochronology based on excess 210Pb, using either steady-state constant flux or constant activity models, yields ages that exceed the known age of the Ala Wai Canal. Geochronology based on a nonsteady-state, two-box, erosion/redeposition model of fallout 137CS yields sedimentation rates for the canal of between ca. 2 and 22 cm yr-1. These rates generally exceed those based upon excess 210Pb by more than a factor of two and agree with the known age of the canal and with sedimentation rate estimates based upon bathymetry changes. Based on the 137Cs-model chronology from 1957 to 1991, the Ala Wai Canal collects bulk sediment at a mean rate of ca. 3100 tons annually. About 80% of the sediment is detrital clays from erosion of the central Honolulu watershed, whereas about 20% of the sediment is composed of marine authigenic and biogenous phases. The sediment yield for the central Honolulu watershed of ca. 60 metric tons km-2 yr-1 equates to a physical denudation rate of ca. 6 mg cm-2 yr-1 --at the low end of the range of physical denudation rate estimates for the island of O'ahu. Based on the mean 137Cs sedimentation rates and an average canal water depth of 2 m, the average time to fill the canal is about 60 yr, assuming that little sediment escapes. The mean fill time is only about 40 yr for the middle canal segment, which receives most sill development from the Manoa-Palolo Stream drainage canal, whereas for the outer and inner canal sediments, mean fill times are about 70 yr. Fallout 137Cs-derived sedimentation rates for each 4-cm interval range from <0.1 to >1.0 g cm-2 month-1 and reveal three episodes of relatively high sediment accumulation in the canal over the ca. 35-yr period before 1991: 1957-1967, 1979-1982, and 1986-1991. The two earlier episodes appear to coincide with periods of high rainfall, but are generally preceded by dry periods where accumulation of marine authigenic phases are high. The most recent high sediment accumulation episode does not appear to correlate with high rainfall, although the annual rainfall trend has increased toward 1990 from a low in 1983. For the Ala Wai Canal, the flux of excess 210Pb generally follows the sedimentation rate and is not constant with time. Two possible causes of higher excess 210Pb fluxes than those expected from a linear relationship are nonsteady-state atmospheric input to the Hawaiian Islands from 222Rn_rich air masses that originate in Asia, and 222Rn from local volcanic eruptions. The variable excess 210Pb flux into the canal sediments may, however, be related to a complex mechanism of soil erosion.

Citation

McMurtry GM, Snidvongs A, Glenn CR. 1995. Modeling sediment accumulation and soil erosion with 137Cs and 210Pb in the Ala Wai Canal and central Honolulu watershed, Hawai'i. Pac Sci 49(4): 412-451.

URI

http://hdl.handle.net/10125/2333

Collections

Pacific Science Volume 49, Number 4, 1995

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