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

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Title:Modeling Sediment Accumulation and Soil Erosion with 137Cs and 210Pb in the Ala Wai Canal and Central Honolulu Watershed, Hawai'i
Authors:McMurtry, Gary M.
Snidvongs, Anond
Glenn, Craig R.
Date Issued:Oct 1995
Publisher:University of Hawaii Press
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.
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.
Appears in Collections: Pacific Science Volume 49, Number 4, 1995

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