Variations of Sediment Organic Matter and Nutrients in Nam Oun Dam Reservoir, Sakon Nakhon Province

Abstract

Background and Objectives : Nam Oun Dam is a large reservoir, serves water storage, irrigation, flood control and drainage. Nam Oun Dam Reservoir covers the areas of many districts in Sakon Nakhon province. Land use in the areas surrounding the reservoir, such as residential communities, tourist sites, fisheries, agriculture (crop cultivation and livestock farming), fisheries conservation zones, and water inflow and outflow areas. These areas are important for the generation of sediments and nutrients within the reservoir. This reservoir is currently facing the problem of Giant Salvinia infestation (Salvinia molesta D.S. Mitchell), which creates a risk to accelerate the accumulation of sediments and may affect the water quality in the long-term of the reservoir. This study aimed to investigate the spatial and seasonal variations of organic matter and nutrient contents in the sediments of Nam Oun Dam Reservoir, including sediment water content and particle size.

Methodology : Study sites in Nam Oun Dam Reservoir were 10 stations (S1-S10) such as S1 (water outlet-left gate), S2 (water outlet-right gate), S3 (tourist attraction and community), S4 (raft-ecotourist attraction and fisheries conservation zone), S5 (pumping station for tap water production and agriculture areas), S6 (Huai Lek Fai inlet and community areas), S7 (fisheries conservation zone), S8-S9 (fisheries and agriculture area) and S10 (Nam Oun River inlet area). Surface sediment samples were collected using an Ekman Grab sampler (15x15 cm) from 10 stations (S1-S10) located in areas with different land uses surrounding the reservoir. Sampling was conducted seasonally within a year, three sampling periods were summer (late March 2023), rainy (mid- August 2023) and winter (early January 2024). The sediment samples were analyzed to determine the contents of organic carbon (OC), organic matter (OM), ammonium-nitrogen (NH₄⁺-N), nitrate-nitrogen (NO₃^--N), phosphate-phosphorous (PO₄^3--P), sediment water content (WC) and percentage (%) of sediment particles (clay, silt and sand). The data were statically analyzed using One-Way Analysis of Variance (ANOVA), the averages were compared by Duncan’s Multiple Range Test, and the relationships between parameters were analyzed by Pearson’s Correlation.

Main Results :The contents of OC and OM ranged from 0.85-4.93% and 1.46-8.50%, with the averages of 2.22±0.97% and 3.83±1.66%, respectively. There were not significant seasonal differences (p>0.05), whereas significant spatial differences were found among the study sites (p<0.05). Site S7 was the highest OC and OM contents, while the lowest contents were found at S1, S2, S3, S6, S9 and S10. The averages of OM content in most areas of the reservoir indicated a low level of organic matter in the sediments. The averages of NH₄⁺-N, NO₃^--N and PO₄^3--P contents ranged from 3.68-58.69 mg/kg, 10.94-160.23 mg/kg and 0.07-0.72 mg/kg, with the averages of 17.55±12.99 mg/kg, 50.17±40.73 mg/kg and 0.36±0.20 mg/kg, respectively. There were not significant spatial differences for nutrients (p>0.05). However, significant seasonal differences were observed for NO₃^--N and PO₄^3--P (p<0.05), with the highest averages recorded in winter and rainy seasons, respectively. In contrast, NH₄⁺-N was not significant seasonal difference (p>0.05). The NH₄⁺-N content was significantly correlated with OM content (p<0.05, r=0.41) and highly significantly correlated with NO₃^--N content (p<0.001, r=0.68). Furthermore, PO₄^3--P content was also found to be very significantly correlated with OM content (p<0.01, r=0.49). These indicated that the organic matter content in the sediments had positive effect on the nutrient contents. WC ranged from 41.47-77.11%, with the averages of 58.45±9.03%. WC was not significant differences between seasons (p>0.05), but significant difference was found among study sites (p<0.05). The highest contents were found at S7 and S8, with the lowest content was found at S3. WC showed highly significant positive correlation with the contents of OM (p<0.001, r=0.83), very significant positive correlation with NH₄⁺-N (p<0.01, r=0.48), and PO₄^3--P (p<0.01, r=0.55) and significant positive correlation with NO₃^--N (p<0.05, r=0.39). Therefore, WC could be the important parameter for preliminary indicator of the organic matter and nutrient conditions in the sediments. The results of the sediment particle size were found that clay, silt and sand ranged from 19.96-54.27%, 14.72-44.57% and 22.63-62.09%, with the averages of 29.06±8.50%, 26.83±8.28% and 44.17±11.60%, respectively. Particle size was not statically significant seasonal difference (p>0.05), but it was statically significant spatial difference (p<0.05). The results of sediment texture analysis showed that sandy clay loam was mostly found in the study areas of the reservoir (S2, S4, S7 and S8), followed by loam (S1, S3 and S5), clay loam (S6 and S9) and clay (S10), respectively. This research did not detect a correlation in the particle size and the contents of organic matter and nutrients.

Conclusions : The land use areas surrounding the Nam Oun Dam Reservoir had a significant effect (p<0.05) on the variations in sediment organic matter contents (OC and OM), water content (WC) and Particle size (clay, silt and sand) within the reservoir, but had no significant effect (p>0.05) on the sediment nutrient contents. In contrast, the season had a significant effect (p<0.05) on the variations of OC, OM and particle size, but had no significant effect (p>0.05) on the variations of most sediment nutrients. Further study on water current velocity could provide more explanation of sediment transport direction. The data from this research can be used as baseline for sustainable management of Nam Oun Dam Reservoir.