Analysis of selected soil physical and chemical properties in peak ground acceleration zones within the Talar Watershed, Mazandaran Province

 
ABSTRACT
Soil erosion is one of the basic problems of watersheds. Soil erodibility is related to factors such as physical and chemical properties of soil. Talar watershed is one of the tectonically active basins in Mazandaran province. The study aims to conduct a detailed analysis of the relationship between ground acceleration and soil properties. In order to compare the effect of acceleration on the soil, the ranges of high acceleration (0.5-0.6) g and low (0.3-0.4) g were determined in sub-basins 1 and 2 of Talar basin. Then, 40 soil samples were collected from different sub-basins, soil texture, organic carbon, and liquid limit tests were conducted, and the relationship between ground acceleration and soil physical properties was investigated using linear regression analysis. The results showed that the highest correlation coefficients in the high acceleration of sub-basin 1 and 2 for the sand variables were -0.97 and -0.95, and the silt variables were -0.97, 0.81, and 0.77, respectively. At low acceleration, sub-basin 1 sand is -0.93, silt 0.84, clay 0.78, organic carbon 0.72, liquid limit 0.74, and sub-basin 2 sand is -0.94. The relationship between the acceleration factor and the physical and chemical properties of the soil is not always linear and is highly dependent on the natural characteristics of the watershed. The analysis of the variables in the acceleration ranges showed that in the high acceleration areas (0.5-0.6) g, the peak acceleration factor has a direct effect on the transfer and displacement of soil particles (sand and silt). These results will benefit environmental managers and decision-makers by creating a scientific basis for planning to reduce erosion.
Extended Abstract
Introduction
Soil erosion is widely regarded as a primary cause of soil degradation, primarily due to its removal of topsoil and essential organic matter, which are vital for plant growth. One significant factor influencing erosion is peak ground acceleration (PGA), a measure of the potential ground acceleration resulting from an earthquake. Here, the peak ground acceleration accurately measures an area's seismicity status since it depends strongly on the frequency of large and small earthquakes. PGA serves as a direct indicator of tectonic activity. Various physical and chemical properties, including soil texture, organic matter content, pH, and permeability, influence soil erodibility. Given that the Talar drainage basin is a seismically active area with notable erosion and sedimentation, this study aims to examine changes in the selected soil physical and chemical properties at varying levels of PGA to understand its impact on sediment transport and erosion within the Talar drainage basin.
 
Methodology
The probabilistic seismic hazard assessment (PSHA) method was used to determine the level of ground motion at a given place. To analyze and assess seismic hazards and determine the response of each basin area, it is necessary to study the seismicity trend in the basin in question. This study used the zoning map of peak ground acceleration (PGA), obtained through seismic hazard analysis, as an erosion control factor. To assess physical soil properties, such as soil texture, soil organic carbon, and liquid limit, 40 soil samples were collected from the Talar drainage basin as 20 from sub-basin 1 and 20 from sub-basin 2. Sampling was conducted in areas with high (0.5–0.6 g) and low (0.3–0.4 g) PGA, with 10 samples collected at each acceleration range from 0–20 cm depth in each sub-basin. Soil texture was analyzed using the hydrometer method to determine sand, silt, and clay content. Soil organic carbon was measured via the loss-on-ignition (LOI) method, as described by Nelson and Sommer (1982). The liquid limit, which is relevant for fine-grained soils, was tested using the Casagrande cup. In this analysis, PGA served as the independent variable. At the same time, soil properties such as sand, silt, clay, organic carbon, and liquid limit were treated as dependent variables in sub-basins 1 and 2 across high and low acceleration zones.
The Talar drainage basin is situated along the Qaimshahr-Tehran axis, spanning geographic coordinates from 52˚ 35' 22'' to 53˚ 23' 34'' east longitude and 35˚ 44' 23'' to 36˚ 19' 01'' north latitude.  The main channel is 100 km long.  The main and active faults in the Talar watershed, are IRQ112 and IRQ 357. the main formations in the study area are Shemshak, Elika, Karaj, Lar with sandstone, conglomerate, dolomitic limestone, marl and shale lithologies. The main land use types comprise cultivated lands and orchards (80.8 km2, 3.9%), rangelands (730.9 km2, 34.7%), forests (1280.5 km2, 60.8%), and residential areas (12.8 km2, 0.6%). Important cities and villages in the basin can be mentioned as Pol Sefid, Alasht, Doab, Zirab.
 
Results and discussion
In sub-basin 1, soil textures include sandy loam, loam, and occasionally clay-sandy loam, while in sub-basin 2, the texture is predominantly sandy loam. In high-acceleration zones, the correlation coefficients of sand, silt, clay, organic carbon, and liquid limit with peak ground acceleration (PGA) in sub-basin 1 are -0.97, 0.77, 0.55, 0.33, and 0.46, respectively. Sub-basin 2 coefficients are -0.95, 0.81, 0.64, 0.31, and 0.68, respectively. For low-acceleration zones in sub-basin 1, the correlations of PGA with sand, silt, clay, organic carbon, and liquid limit are -0.93, 0.84, 0.78, 0.72, and 0.74, respectively, while in sub-basin 2, the values are -0.94, 0.30, 0.46, 0.43, and 0.46, respectively. In sub-basin 1, when sand and silt were entered as dependent variables, the PGA showed significant correlations (p=0.01), with correlation coefficients of r=0.97 and r=0.77, respectively. The resulting regression models are:
Sand = 134.5 - 144.8 * PGA                  Silt = -21.8 + 88 * PGA
In low-acceleration areas of sub-basin 1, PGA was significantly correlated (p<0.05) with sand (p=0.01), silt (p=0.002), clay (p=0.008), organic carbon (p=0.019), and liquid limit (p=0.015), with correlation coefficients of 0.93, 0.84, 0.78, 0.72, and 0.74, respectively. The regression equations for these variables are:
  Sand = 140.3-222.6 * PGA    Silt = -13.9 +110.5 * PGA    
 Clay = -26.4 + 112.1 * PGA Organic carbon = -28.6+124.4 * PGA      Liquid limit = -6.2 +20.7 * PGA
In high-acceleration areas of sub-basin 2, the correlation coefficients for sand, silt, and liquid limit with PGA are 0.95, 0.81, and 0.68, respectively, with regression equations as follows:
Sand = 175.6-209.1 * PGA       Silt = -45.2 +127.3 * PGA     Liquid limit = -41 +105.6 * PGA;
For low-acceleration areas in sub-basin 2, the PGA’s correlation with sand was 0.94 (p=0.01), with the regression model:  Sand = 95.31-92.1 * PGA.
Overall, a non-linear correlation exists between PGA and soil erosion, influenced by the basin's unique characteristics. Key factors include the minimum PGA threshold required to initiate soil particle displacement, increased erosion with higher PGA, and the effects of earthquake frequency, magnitude, and duration. Extended shaking destabilizes particles, particularly in soils with low cohesion and high moisture content. PGA influences soil cohesion, pore pressure, and displacement, particularly on steep slopes where gravitational forces amplify erosion. The findings indicate that high-acceleration areas significantly impact soil particle transport (e.g., sand and silt) in both sub-basins, underscoring the role of PGA in erosion processes.
 
Conclusion
The values of PGA vary in the Talar drainage basin. Due to several active faults in the region, the highest value of PGA (0.6 g) exists in sub-basins 1 and 2. The placement of erosion-prone formations in peak ground acceleration zones accelerates sediment yield and erosion. High-acceleration areas strongly influence PGA on soil erosion, particle movement, and sediment yield. Analysis of soil properties of sand, silt, clay, organic carbon, and liquid limit in high- and low-acceleration sub-basins 1 and 2 highlights PGA's direct impact on soil particle displacement and movement.
 
Funding
We acknowledge the support of Grant number 600.871 funded by the research council of Shahid Beheshti University, Tehran, Iran.
 
Authors’ Contribution
All of the authors approved thecontent of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgment
We are grateful to all the scientific consultants of this paper.