harry williams, geomorphology1 cesium-137 released into atmosphere by above ground atomic blasts

Harry Williams, Geomorphology 1

Cesium-137 released into atmosphere by above ground atomic blasts.


Assessing the impact of weir construction on recent sedimentation using cesium-137.Cesium-137 is a man-made radionuclide globally dispersed as fallout since the onset of atmospheric nuclear weapons testing in the 1950's. The shape of the cesium-137 profile (cesium-137 concentration versus sediment depth) can be used to identify two marker horizons: 1954*, the base of cesium-137 activity, corresponding to the onset of atmospheric nuclear weapons testing; and, 1963, the peak of cesium-137 deposition after which atmospheric fallout declined due to the Nuclear Test Ban Treaty of 1964. (* I’m now using 1950 – there’s uncertainty over when this boundary should be; I’m now taking a more conservative approach).


Mad Island Lake is a small fresh to brackish-water lake located on the shores of Matagorda Bay, Texas.


Mad Island Slough and Northeast Slough are the two largest streams entering Mad Island Lake. Mad Island Slough, a former extension of the west arm of the lake, was impounded by a weir constructed in 1948.


An understanding of sediment supply and sedimentation within the lake is important for management of marsh and lake habitats. Sedimentation at the lake periphery, for example, is required for the maintenance and growth of marsh habitats; sedimentation on the lake bed may reduce the lake's storage capacity, affect water levels in and around the lake, and result in the loss of aquatic habitats. Sediment supply is likely to respond to changes in land use within the lake's watershed, much of which is beyond the boundaries and control of the preserve. Mad Island Slough and Northeast Slough are probably the major sediment sources for the lake.


Emplacement of the weir across Mad Island Slough caused permanent flooding of a large area upstream and created a marshy wetland. The flooded area has undergone sedimentation due to trapping of sediment inflow by the weir (i.e. a change in base level, followed by aggradation). The objectives of this study were to use cesium-137 dating to assess recent sedimentation behind the weir and the impact of the weir on sedimentation in Mad Island Lake.


Ten cores were obtained from the marsh area upstream of the weir. The cores were obtained by pushing 5 cm diameter PVC tubing as far as possible into the marsh sediments. The tubes were cut lengthwise on a table saw and the intact sediment cores extracted. Visible downcore changes in sediment characteristics were recorded. Each core was divided into 4 cm segments and bagged for cesium-137 analysis. Cores from Mad Island Lake had been collected and analyzed in a previous study.


Unlike the cesium-137 profiles obtained from the bed of Mad Island Lake, none of the 10 cores analyzed for this study has a well-defined cesium-137 peak concentration and therefore the depth to the 1963 marker horizon could not be determined. However, in 9 cores, cesium-137 concentrations are relatively high in near-surface sediments and fall to zero or near zero values at depth, enabling the 1954 base to cesium-137 activity to be identified.


Initially, the PVC tubes penetrated the sediments fairly easily, but in each case coring was stopped when resistance was encountered at relatively shallow depth. Examination of the cores indicated that the increase in resistance corresponded to an abrupt change in sediment character, from soft dark organic-rich mud in the upper part of cores, to more compact less organic-rich sandy deposits at depths ranging from 18 to 36 cm.

B horizon?


Core 18 contained a 2 cm thick sand layer, with a sharp base, at a depth of 16-18 cm. The sand layer was enclosed by dark organic-rich mud.

TOP BOTTOM


Marker HorizonsThe presence of cesium-137 in deposits behind the weir attests to recent sedimentation. Relatively little sedimentation has occurred since 1954. This may explain why the 1963 peak could not be observed in the cesium-137 profiles; the samplinginterval (4 cm) may be too coarse to resolve a peak concentration over such a small thickness of sediment. It may also be that some mixing of the sediments has occurred due to bioturbation (plant roots, burrowing organisms). However, the 1954 marker horizon is recognizable on the cesium-137 profiles; the depth to the base of cesium-137 activity in each core provides a measurement of the thickness of post-1954 sediment accumulation.


A second marker horizon is based on the core lithology; the abrupt transition from deeper sandy deposits to overlying organic-rich muddy deposits is interpreted as coinciding with construction of the weir in 1948. The abrupt change to finer-grained sediment and abundant organic matter above this transition is consistent with the rapid development of a low-energy, flooded, densely-vegetated marsh environment. The position of this transition, below the 1954 marker horizon in each core, is also consistent with this interpretation. The underlying sandy, organic-poor deposits may represent the former floodplain and channel deposits of Mad Island Slough.


A tentative interpretation is also suggested for the thin sand layer between 16 and 18 cm depth in core 18. This sand may represent a deposit resulting from the drainage of flood waters following the impact of Hurricane Carla in 1961. The position of the sand layer above the 1954 marker horizon is consistent with this interpretation. The absence of other sand layers within the post-1948 deposits (the dark organic-rich mud), in this and the other 9 cores, suggests that deposition of the sand was a rare event and that sedimentation occurred only in isolated areas.


Core lithologic profiles and the 1954 marker horizons from cesium-137 analysis.


Sedimentation RatesSedimentation rates are derived from the 1954 and the 1948

marker horizons (Table 1):


Two significant findings are:•relatively little sediment has accumulated behind the weir since 1948 (an average of only 27 cm, based on the core data) •sedimentation rates declined substantially in 1954-1994 compared to 1948-1954 (from an average of 2.4 cm/year to 0.32 cm/year - Table 1).

The reason(s) for the decline in sedimentation remain speculative; possible reasons include: •less sediment is being eroded from the surface of the Mad Island Slough watershed (presumably due to land use changes)•more sediment is being trapped within the watershed by barriers such as levees and dikes•less of the sediment flowing through Mad Island Slough is being trapped behind the weir.


Effect of the weir on sedimentation in Mad Island Lake Sedimentation data is available from core 14 from the west arm of Mad Island Lake, below the outlet of the weir. Although the time frame covered by this data is not identical to that of the present study, a similar pattern of declining sedimentation is apparent: in the period 1954-1963, sedimentation at this core site averaged about 6.7 cm/yr; during 1963-1993, this rate fell to 0.83 cm/yr (Table 1). Cores from the east arm of the lake show the same trend of declining sedimentation (Williams 1994 Table 5). These results suggest that both systems (the slough and the lake) are responding to a reduction in sediment supply. The possibility that the same amount of sediment is flowing through Mad Island Slough, but less is being trapped by the weir can be discounted, since this does not explain why sedimentation has declined in the lake.


The grain size of sediment must be sufficiently fine (mostly silt and clay, based on field observations) and the flow of water sufficiently turbulent, that a considerable amount of sediment is carried over the weir in suspension, rather than settling out in the ponded area upstream.

Given that the evidence suggests that sediment supply has declined, it seems likely that land use changes within the lake's watershed are responsible. Either less sediment is being eroded from the surface of the watershed and/or more sediment is being trapped within the watershed. It is known that wide-spread rice production was initiated in the mid-1950's. Because rice requires ponded water, levelling of the surface and the construction of levees is usually required. Both these practices are likely to reduce sediment supply; levelling, by reducing surface run-off, and levee construction, by trapping sediment within the watershed.


Conclusions1. Surprisingly little sediment (averaging 27 cm) has accumulated behind the weir since 1948. Sedimentation has been consistently higher in the lake, compared to the area above the weir. Clearly, the weir has not prevented a large amount of sediment from entering the lake. The flow through the slough must be sufficiently turbulent and the sediment sufficiently fine-grained that much sediment passes over the weir in suspension.

2. Both the weir and the lake exhibit the same trend of declining sedimentation in recent decades. This suggests the supply of sediment from the lake's watershed has declined, possibly due to the initiation of rice production in the 1950's.

3. Two significant findings, in regard to management strategies on the preserve, are that land use practices beyond the boundaries of the preserve are the major control on sedimentation within the preserve, and that physical barriers, such as the weir across Mad Island Slough, do not act as particularly efficient sediment traps in this watershed.

harry williams, geomorphology1 cesium-137 released into atmosphere by above ground atomic blasts

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