On a perpendicular shoreline transect in the middle zone of Xuan Thuy National Park, five sediment cores were collected (Fig. 1). Prior to sampling, the coring locations were determined by observing the mean lower low water (MLLW) level in 2020–2021. Core A was taken in the middle of a semi flooded swamp in Con Ngan, with an average elevation of 0.6 m above the MLLW level. Core B is in the middle of a flooded swamp in Con Ngan, on the right side of the Tra river, with an average elevation at the MLLW level. Core C is in the middle of the Tra river where the sediment bed is 1.5 m below the MLLW level. The other cores D and E were taken in flooded swamps in Con Lu, on the left side of the Tra river, with an average elevation at the MLLW level.
A stainlesssteel corer was used to take sediment cores A, B, D, E and a gravity corer was used to take sediment core C. In the field, the cores were taken by manually pushing the corer into the subsurface. The corer length was chosen as 100 cm to cover the sedimentation period of recent 100–120 years, based on previous findings that the average sedimentation rate in the area was 0.77 cm year− 1 (Nhon Dang Hoai et al. 2019). Each undisturbed sediment core of 5.9 cm diameter was wrapped in plastic foil and placed in two halved PVC tubes for transporting to the laboratory. The cores were placed in a freezer in the lab to harden them before being cut. Sediment core C was cut into slices of 2 cm thickness and the other cores A, B, D, and E were cut into slices of 4 cm thickness. Each slice was air dried in a ceramic bowl for further analysis. The cores’ location, elevation vs MLLW level, taken date, core length and number of slices are summarized in Table 1.
The total 210Pb concentration in each sediment slice was determined by alpha spectrometry through its daughter 210Po when equilibrium was guaranteed. The radiochemical procedure of 210Po determination includes the followings: 1 g of sediment is decomposed, then the sample is centrifuged and evaporated with 2 mol dm− 1 HCl, 210Po is then settled to deposited on a silver disc, and the prepared source is measured in an alpha spectrometer. In every analysis, the tracer 209Po is used for the determination of chemical yield (Martin and Hancock 1992; Szarlowicz 2019).
226Ra (supported 210Pb) concentration was determined by the gamma spectrometer CANBERRA BEGE6530. The spectrometer is calibrated using IAEA reference materials (RGU–1, RGTh–1, RGK–1 and Soil6) to construct the detector efficiency curve. Concentration of 226Ra is calculated directly from the 186.2 keV peak by separating the 186.2 keV peak of 226Ra and the 185.7 keV peak of 235U (De Corte et al. 2005).
Table 1
Core’s location, elevation vs MLLW level, taken date, core length and number of slices
Name  Location  Elevation vs MLLW level (m)  Taken date  Core length (cm)  Number of slices 
Latitude (N)  Longitude (E) 
Core A  20.23512  106.56488  0.6  25/02/2021  96  24 
Core B  20.23265  106.56715  0  05/07/2021  92  23 
Core C  20.22957  106.56907  1.5  23/05/2020  84  42 
Core D  20.22614  106.57164  0  05/07/2021  100  25 
Core E  20.22227  106.57308  0  05/07/2021  92  23 
The excess concentration of 210Pb, or 210\({Pb}_{ex}\) (210\({Pb}_{ex}\) = 210Pb concentration – 226Ra concentration), for each sediment slice was determined. 210\({Pb}_{ex}\) was used to calculate the age of all slices using sediment dating models (Appleby 2001; SanchezCabeza and RuizFernandez 2012) by the following steps:
1. Use the Constant Flux Constant Sedimentation (CFCS) model to determine the average sedimentation rate.
CFCS model hypothesis assumes that 210\({Pb}_{ex}\) concentration would decrease exponentially whith increasing mass depth by the following equation (SanchezCabeza and RuizFernandez 2012):
ln \({C}_{i}\) = ln \({C}_{0}\)\(\)\(\frac{\lambda }{r} {m}_{i}\) (1)
Where:

\({C}_{i}\) : 210Pbex concentration in layer i (Bq kg1);

\({C}_{0}\) : initial 210Pbex concentration when layer i is formed (Bq kg1);

λ: 210Pb disintegration constant and equals to 0.03118 year1;

\(r\) : mass accumulation rate (kg m2 year1);

\({m}_{i}\) : mean mass depth of layer i (kg m2).
By drawing in MS Excel the graph of ln(210\({Pb}_{ex}\)) against accumulated mass depth, the trend line equation in the form of y = a + bx gives \({C}_{0}\) = ea, and \(r\) = λ/b. The CFCS model is applicable if the graph has linear form. If the graph has more than 2 linear segments with different slope, the average \(r\) could be determined for each segment (SanchezCabeza and RuizFernandez 2012).
2. Use the Constant Supply Rate (CRS) model.
The CRS model is used to determine the age of the sediment slices by the following equation (Appleby 2001; SanchezCabeza and RuizFernandez 2012):
t(i) = \(\frac{1}{\lambda }\text{ln}\frac{A\left(0\right)}{A\left(i\right)}\) (2)
Where:
In case of incomplete coring, when the excess 210Pb concentration of the bottom slide does not approach the background level (the 210\({Pb}_{ex}\) detectable limit), the total 210\({Pb}_{ex}\) inventory of the sediment core A(0) could be determined by applying the CFCS model for the bottom linear segment to estimate the 210\({Pb}_{ex}\) inventory of the missing core section (SanchezCabeza and RuizFernandez 2012).
3. Calculating MAR and SAR
The sedimentation mass accumulation rate (MAR) of the ith sediment slice (\({r}_{i}\), kg m2 year1) could be calculated from the age of the sediment slice by the following equation (SanchezCabeza and RuizFernandez 2012):
\({r}_{i}\) = \(\frac{\lambda A\left(i\right)}{C\left(i\right)}\)= \(\frac{\varDelta m\left(i\right)}{\varDelta t\left(i\right)}\) (3)
And the sedimentation accumulation rate (SAR) of the ith sediment slice (s, m year1) could be determined by the following equation (SanchezCabeza and RuizFernandez, 2012):
\({s}_{i}\) = \(\frac{\varDelta z\left(i\right)}{\varDelta t\left(i\right)}\) (4)

\(\varDelta m\left(i\right):\) mass of the sediment slice (kg m2);

\(\varDelta z\left(i\right):\) thickness of the sediment slice (m);

\(\varDelta t\left(i\right):\) duration the sediment slice was formed (year), calculated from the age of the sediment slice.